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Using Arduino => General Electronics => Topic started by: localbroadcast on Feb 04, 2015, 05:22 am

Title: Understanding MOSFET datasheets
Post by: localbroadcast on Feb 04, 2015, 05:22 am
Hello.

I am planning on purchasing some mosfets for a number of projects I have on the go at the moment.  I want to make sure that I am buying the right mosfets for the job, because these things can be rather expensive.

I want to make sure that I am reading and understanding the datasheets, and was hoping some of you people could clarify things for me.

The basics that I need to know are the following:
Maximum continuous voltage the mosfet will allow from Source to Drain
Maximum continuous current the mosfet will allow from Source to Drain
The voltage / current needed to fully turn the mosfet ON
The turn on / turn off time

Do mosfets work strictly fully ON or fully off?  Or do they operate like a transister, where there is a region that it is only slightly on, depending on the gate current / voltage?

Also, will the mosfet turn off when the gate voltage is dropped to 0?  Or does the source voltage have to be 0 as well for the mosfet to turn off?  I remember there is some type of device that only needs a pulse at the gate to be turned on, and won't turn off until the source voltage = 0..  I just don't remember if this is how a mosfet works too.

Here is a mosfet I was thinking of buying.. If anyone could tell me the vital information that I need to know about it, that would be very helpful.

IRFP460 mosfet (http://ca.mouser.com/ProductDetail/Vishay-Siliconix/IRFP460/?qs=zorda86t5M%252bnyKjlDVoLdg%3D%3D)

Here is a link to the DATA SHEET (http://www.mouser.com/ds/2/427/91237-86272.pdf)

Thanks again!
Title: Re: Understanding MOSFET datasheets
Post by: weedpharma on Feb 04, 2015, 06:12 am
The maximum specifications are just that, maximum that can't be exceeded. They are considerably higher than what it will run with continuously.

MOSFETS are like transistors in that they have part on areas. Read the data to see what the series resistance and possible current flow at a particular gate voltage.

These are curves. EG, at 2 volts on the gate the current is very low. At 5 volts it may be 5A while at 10 volts it may be 50A. It varies according to the device.

With the arduino, you need is a logic level gate mosfet. The gate needs to be fully on or the resistance of the drain/source is not at its least so causes heat.

My favourite small mosfet is NTD5867NL. It works with 5v on the gate and handles a few amps with ease. They are cheap. I bought mine from RS Components.

There are others but make sure they are fully on at 5v on the gate.

When you remove the gate voltage, there is a time taken to discharge the gate capacitor (internal). Therefore it is best to have a discharge resistor from gate to Gnd. This is important especially when you use PWM as it will get excessively hot without the R.

Weedpharma
Title: Re: Understanding MOSFET datasheets
Post by: JimboZA on Feb 04, 2015, 06:36 am
Do mosfets ... operate like a transister
MOSFETS are like transistors
Not to be too picky, but mosfets are transistors  ;)
Title: Re: Understanding MOSFET datasheets
Post by: localbroadcast on Feb 04, 2015, 07:53 am
Yea.. I've been doing some reading, and I think for my purposes I'll be needing a mosfet driver.  There's so many different drivers to choose from.. it's hard to decide!  Any suggestions on a driver that works well with the arduino outputs?
Title: Re: Understanding MOSFET datasheets
Post by: weedpharma on Feb 04, 2015, 08:15 am
Not to be too picky, but mosfets are transistors  ;)

Good to meet a fellow pedant!!  ;)

Weedpharma
Title: Re: Understanding MOSFET datasheets
Post by: localbroadcast on Feb 04, 2015, 08:57 am
Not to be too picky, but mosfets are transistors  ;)

yea.. yur right.. but I hope you all understood what I meant by what I said.. I was just comparing mosfets to yur standard sized bipolar transistors..

So what value in the datasheet shows the voltage that needs to be applied to the gate in order to turn the mosfet completely ON?

After doing some reading.. it sems like the gate pin operates like a capacitor.. and the mosfet turn on time is dependent on how quickly the gate is charged.. so a higher current applied to the gate results in a quicker turn on time.  This is the Q value in the datasheet, given as a value measured in nC.  Q = IT, where T is the turn on time in seconds, I is the current applied to the gate, and Q is the value in the datasheet given by
Qg or Qtot.. Some datasheets give more than one value for Q, which is confusing.. Qtot Qgs and Qgd... I'm not sure what the purpose of all 3 is yet.. still need to figure that out..
Title: Re: Understanding MOSFET datasheets
Post by: michinyon on Feb 04, 2015, 11:03 am
That capacitance stuff matters mostly when you are going to use a very high PWM frequency -  like megahertz.

It's less important at lower frequencies typically used for driving robot motors  and that the arduino can practically generate.

Quote
So what value in the datasheet shows the voltage that needs to be applied to the gate in order to turn the mosfet completely ON?
That mosfet is not a great choice.  Look at Fig 1 in the first post you linked.    If your gate voltage is 4.5 volts,    you will only get 4 amps through the circuit,  even for large Vds.    This implies a fairly high apparent resistance and therefore heating.     To turn that on,   as far as it will go,   you need a gate voltage of around 7 or 8 volts.

If you look at Fig 8,   that fet looks good for controlling voltages around 100-200 volts  but not so good for controlling 12 V or 24 V to a robot because the on-resistance is rather high.



Title: Re: Understanding MOSFET datasheets
Post by: michinyon on Feb 04, 2015, 11:16 am
The   Rds resistance of your mosfet -  even with 10 volts on the gate voltage,  is 270 milliohms at 12 A load current. 

The Rds for the one weedpharma prefers,  with only 4.5 V on the gate and a 10 A load current,   is only 50 milliohms.

That's 80% less heat you are generating,  right there.       And the performance of your mosfet will be even worse than that,  if you can only drive the gate to 5 volts.
Title: Re: Understanding MOSFET datasheets
Post by: MarkT on Feb 04, 2015, 01:20 pm
That capacitance stuff matters mostly when you are going to use a very high PWM frequency -  like megahertz.

It's less important at lower frequencies typically used for driving robot motors  and that the arduino can practically generate.

Except that it matters from a few kHz upwards...  Switching losses for a high power load
will dominate, and to keep them under control you normally want the switching time to be
< 1% of the PWM cycle time, so for 8kHz PWM that's 1us or better.  That usually means
needing 10's to 1000's of mA drive to charge the gate capacitance (for instance a gate
capacitange of 2nF charges to 10V in 1us with 20mA or more, but a large MOSFET with
20nF needs 0.2A
Title: Re: Understanding MOSFET datasheets
Post by: MarkT on Feb 04, 2015, 01:24 pm
The key parameters you need to look at in a datasheet are:

Vds(max)
Rds(on)  and the Vgs at which this is achieved
Qg (total gate charge) for the gate voltage you use.

You may also be interested in the body-diode's performance and the Vgs(max) and
the plateau voltage (usually only hinted at in graphs).  Its best to use a gate drive
voltage about twice the plateau voltage as this gives roughly symmetrical switching
times.

You don't need to look at the max current because if you do your sums right
with Rds(on) you'll never be anywhere near this, its an extreme thermal limit.
And if you have an Internation Rectifier datasheet they lie blatantly about
max current anyway.
Title: Re: Understanding MOSFET datasheets
Post by: localbroadcast on Feb 04, 2015, 01:48 pm
well.. I'm not using the mosfets to drive 24 volt robot motors.. I'm using them for a load at 100v - 150v at 10amps.  So I need the bigger mosfets.

I am thinking of driving the mosfet gate using a push-pull setup with npn pnp BJT transistors.. This way the arduino can trigger the transistors, which can provide a high current to the mosfet gate, allowing for fast gate charge / discharge of the mosfet.

I am trying to find a 3 or 4 amp transistor that the arduino can turn on.. and from what I'm reading, the arduino can only provide about 5 volts @ 20mA to do this..

I was looking at the MJE243 MJE253 npn pnp transistor set.. but I'm not sure the arduino can provide enough current to completely turn it ON.. Here's the datasheet stats for those transistors:

Collector−Emitter Saturation Voltage = 0.3v , 0.6v
(IC= 500 mAdc, IB= 50 mAdc)
(IC= 1.0 Adc, IB= 100 mAdc)

Base−Emitter Saturation Voltage = 1.8V
(IC= 2.0 Adc, IB= 200 mAdc)

Base−Emitter On Voltage = 1.5V
(IC= 500 mAdc, VCE= 1.0 Vdc)

From this information.. what is the current required to fully turn this transistor on?
Title: Re: Understanding MOSFET datasheets
Post by: rockwallaby on Feb 04, 2015, 02:06 pm
I'm sure we've been here before with this, but I'll suggest these drivers again to you from that other thread we were having discussions.

Quote
And to drive them nice and cleanly with these UCC27424P MOSFET drivers
The drivers are high speed dual drivers with a paralleled MOSFET and bipolar transistor complimentary output for good sourcing and sinking of main power MOSFET gate charge.
This driver can be driven directly by logic, I'll be connecting some of these to a DUE soon.

They also have a nice feature, with an enable pin, which you can connect back into some other safety cut-out system if you need.

I think I paid AUD$1.35 each from element14 and recieved another tube of them today. UCC27424P (http://au.element14.com/texas-instruments/ucc27424p/driver-mosfet-dual-27424-pdip8/dp/8462739)

These drivers have an output stage that has both bipolar and MOSFET devices in parallel.
Check out the datasheet for the specs.

Surely, it's got to be easier to use such a device as this that is designed and proven to work with power MOSFETS, than to mess about with discrete components.
_____
Paul
Title: Re: Understanding MOSFET datasheets
Post by: localbroadcast on Feb 04, 2015, 02:34 pm
rockwallaby.. yes I ordered a handful of those drivers.  So they work fine with the pwm from the arduino?  Also.. I hope they work with the mosfets I ordered. 

IRFP460 20A 500V N Channel MOSFET Transistor

Any advice how to wire this driver up with my arduino and this mosfet?
Title: Re: Understanding MOSFET datasheets
Post by: MarkT on Feb 04, 2015, 05:36 pm
well.. I'm not using the mosfets to drive 24 volt robot motors.. I'm using them for a load at 100v - 150v at 10amps.  So I need the bigger mosfets.
Then you should seriously consider changing over to IGBTs as a possibility.
Title: Re: Understanding MOSFET datasheets
Post by: MarkT on Feb 04, 2015, 05:47 pm
I am thinking of driving the mosfet gate using a push-pull setup with npn pnp BJT transistors.. This way the arduino can trigger the transistors, which can provide a high current to the mosfet gate, allowing for fast gate charge / discharge of the mosfet.
No, use proper MOSFET driver chips, they work :)

Quote
I am trying to find a 3 or 4 amp transistor that the arduino can turn on.. and from what I'm reading, the arduino can only provide about 5 volts @ 20mA to do this..
Not likely, you need fast switching so darlingtons are out of the question, and
single BJTs don't have the gain.  The only option is a pair of MOSFETs to drive
the gate on the big MOSFET. Funnily enough that's exactly what you get in a MOSFET
driver chip....

Quote
I was looking at the MJE243 MJE253 npn pnp transistor set.. but I'm not sure the arduino can provide enough current to completely turn it ON.. Here's the datasheet stats for those transistors:


Collector−Emitter Saturation Voltage = 0.3v , 0.6v
(IC= 500 mAdc, IB= 50 mAdc)
(IC= 1.0 Adc, IB= 100 mAdc)

Base−Emitter Saturation Voltage = 1.8V
(IC= 2.0 Adc, IB= 200 mAdc)

Base−Emitter On Voltage = 1.5V
(IC= 500 mAdc, VCE= 1.0 Vdc)

From this information.. what is the current required to fully turn this transistor on?
Undefined, you haven't said what collector current or what power loss you can
tolerate.

Just get a MIC4422 and some good ceramic decoupling for it.  At that power level
(> 1kW load) you need really clean fast switching - during switching transients the
peak power dissipation in your MOSFETs will be similar to the load.  You keep
the switching time down to 100ns or so to prevent nasty things happening.  The
MIC4422 can drive many amps if needed. 

Use a fast opto-coupler on the input to the MIC4422 (with that power level if
you blow a MOSFET the MIC4422 will explode too, you don't want to take out
everything else and give yourself a lethal shock either).  Something like ACSL64000
series?

If you are PWM'ing the load you'll need to calculate your losses back-of-the-envelope
style...
Title: Re: Understanding MOSFET datasheets
Post by: rockwallaby on Feb 04, 2015, 11:29 pm
MarkT, why suggest using IGBT without giving us a valid reason over using MOSFETS.
I think we all have some agreeance with a MOSFET being a suitable device for the project.
As with standard transistors, you can can't simply parallel IGBTs as you do MOSFETS.

Also, MarkT, it appears that you haven't read the post prior to yours, where I mention the use of a MOSFET driver, specifically the UCC27424P, which is based on the basic variety of XX4422 style drivers, but is way better.

The one I specified has the extra driver enable pin as well, it can take logic inputs direct from as low as 3.3v.

localbroadcast, yes with what I have seen, this driver will work fine with your MOSFET.
What I suggest you try to do is to use a drive voltage of higher than 5.0 volts, say something closer to 10v or 12v. The reason is that, even though you may have a nice chunky 5.0v source, any MOSFET driver will not swing its output rail to rail. It will be some point below 5.0v, and too close to the gate threshold voltage of the power MOSFET.

If you have as part of your design a 12v supply, then use this as the Vcc of the driver chip to give the a good gate voltage on the main power MOSFET. The driver chip will handle the higher charge and discharge of the power MOSFET quite well.

Using 12 volts on the driver chip will still allow you to connect the Aruino directly to the input pin.
These drivers have two separate drivers within them, so you can use two power MOSFETS, or, if you want, you can parallel the two drivers stages foe even greater current source/sinking for really demanding power MOSFETS.

MarkT suggests the use of an opto-coupler, I wouldn't suggest this, just connect your Arduino output pin directly to the input pin of the driver you have. Adding more circuitry for extra protection can be worthwhile sometimes, but can actually cause more nuisance other times with extra complexity and reducing performance.

Do put a fuse or some form of safety circuit in the high voltage side.

With any MOSFET, be careful with static when handling the device before placing it in circuit, the gate insulation layer is susceptible to damage. So don't walk around in synthetic clothing rubbing your feet on the ground.
____
Paul
Title: Re: Understanding MOSFET datasheets
Post by: localbroadcast on Feb 05, 2015, 12:17 am
The voltage the MOSFET will be switching on / off will be about 115Vdc.  The load current will be about 6 amps. I want the load to see 36Vdc, so the 115Vdc output pulses from the MOSFET will be filtered, giving hopefully a smooth 36Vdc.

So now I understand what the Max power dissipation means.. It's basically the maximum allowable power loss due to the Drain-source resistance Rds(on).

Obviously I want this loss to be as low as possible.  From what I can tell, this loss can be lowered by choosing a MOSFET with a lower Rds(on).  Also, keeping the MOSFET as cool as possible during operation will help because the Rds(on) increases with temperature.  I can't think of any other ways to minimize this loss... anyone have any other ways to lower this loss?

The gate threshold voltage VGS(TH) and the gate source Voltage for ON state VGS are different values.
For the MOSFET 2SK3502-01MR:
VGS(TH) = 5V
VGS(ON) = 10V

For example:
if VGS = 7.5V
and the full current potential form the drain to source = 6A
and the full voltage potential from the drain to source = 50V

So if VGS is between 5V and 10V, then the Drain-source current / voltage is affected..
Will the full 50V still be applied to the load, and the load current is reduced to something like 3 Amps?  Or will the full 6 Amps make it through the load and the voltage is reduced to something like 25 Volts?
Or will both the current and the voltage be reduced by some amount proportional to VGS?

Since I want the MOSFETs to work strictly as switches, I want the gate voltage to always be 10V.  Does the output of the MOSFET driver MIC4421 you mentioned provide this 10V?  Some MOSFETs have 15v gates.. Does the same driver work for those as well? I guess my question is.. How do you get the proper gate voltage from the driver?  Do you need to provide a separate voltage source for the driver in order to provide the voltage and current needed?  I know that the arduino PWM outputs will not provide enough power to do the job.

Also.. do you know the part number for an equivalent HIGH-side mosfet driver??  I prefer to have the MOSFET before the load..  
Title: Re: Understanding MOSFET datasheets
Post by: rockwallaby on Feb 05, 2015, 12:36 am
Sounds like you've been doing a lot of reading and making good sense of it all.  ;D

As far as the driver output voltage, like I mentioned, you need to supply this, as a good solid supply, so, if you have 12Vdc available, and it has good current capacity, then let this be the supply to the Vcc of the driver chips, which will then be the output swing of the driver, ~0V to ~12V, which will be near to ideal for the gate on the power MOSFET.

The Arduino will interface nicely into the input of those UCC27424 driver chips you have bought.

If you want high side MOSFET, then you need to get P-Channel MOSFETs and also high side drivers.
Is there any technical reason you need to have high side control, or is it simply a personal preference?
Low side driver and N-Channel is more typically used.

Remember, the electrons flow from the negative potential to the more positive potential, so, what we often think as 'hot side' being the more positive end of the wire is the end where electrons flow into. Not that it really matters in this design.  :D
____
Paul
Title: Re: Understanding MOSFET datasheets
Post by: polymorph on Feb 05, 2015, 01:03 am
Please have something capturing video when you fire this up the first time.
Title: Re: Understanding MOSFET datasheets
Post by: rockwallaby on Feb 05, 2015, 01:22 am
polymorph wrote:
Quote
when you fire this up the first time
was the word 'fire' used consciously I wonder, do you have no faith?  :smiley-twist:

Actually, we should have a section in this forum for such things, we have the gallery where people proudly show off their projects. We can still learn a lot when projects go sideways and smoke is let loose.

I even learn from these threads where others have a different point of view or experience.

I think localbroadcast will make this work, he's reading up a lot and asking the right questions.
____
Paul
Title: Re: Understanding MOSFET datasheets
Post by: localbroadcast on Feb 05, 2015, 02:04 am
mkay so I just ordered a bunch of different mosfets, a bunch of different drivers.. and a bunch of different sockets and adapters for the different IC pin configurations so they can fit in a bread board..  It will be a month or so before all the parts get here and I can "fire" my switch regulator up.

It's hard to order some of the parts that I'd like because of shipping restrictions..  I'm in Canada, and a lot of the amazon and ebay sellers only ship to USA.  I think I got what I need though.. plus a handful of extras because I'm sure I'll burn up a few IC's along the way.

Ohya, the question I was going to ask..

Why wouldn't an N-channel mosfet work on the high side of a load?  P channel has current flowing from source to drain, and N channel has current flowing from drain to source..  I don't understand why you couldn't put the load on whatever side of the N-channel mosfet you want and have it work.. weather the load is connected to the drain or source.. the only way for current to flow through the load is to have the gate triggered.  Can someone explain this why you need to have the load on a specific side of the mosfet? 

Thanks for everyone's help this far along.  You know the saying... everyone has a dick.. except for girls..
Title: Re: Understanding MOSFET datasheets
Post by: rockwallaby on Feb 05, 2015, 02:10 am
Hopefully this tutorial might shed some light on the matter, regarding N-Channel and P-Channel anda few other factors about FETs in general.

http://www.electronics-tutorials.ws/transistor/tran_6.html (http://www.electronics-tutorials.ws/transistor/tran_6.html)
Title: Re: Understanding MOSFET datasheets
Post by: localbroadcast on Feb 05, 2015, 10:41 am
I just ordered these bad a$$ MOSFETS...

SiR872ADP Made by Vishay Siliconix.  Ordered through Digikey at $2.40 each.
VDS = 150V
RDS(ON) = 0.0180 Ohms at VGS = 10 V
ID = 53.7A continuous
QG = 22.8 nC
Turn on delay = 10ns
Turn off delay = 15ns

This MOSFET is much better suited for my purposes in this circuit.  I think when you go overboard on the voltage and current limits, you sacrifice the drain source resistance, gate capacitance, price, etc.

The first mosfet I ordered (IRFP460) has a d-s resistance of 0.270 Ohms!  This MOSFET has a much much much lower drain source resistance at 0.0180 Ohms.  This is going to make the mosfet run cooler, and use less power.  The QG value at 22.8 nC is relatively low, which will give fast gate charge / discharge rates, and the turn on / turn off delays are very fast too!

It took a long time to find this mosfet, but I think I will be happy with the results.  Only downside is I couldn't find an equivalent mosfet with a better package... This one is PowerPAK® SO-8.. So I will need to order an adapter board or socket so I can solder this thing to my circuit board.

Any thoughts?
datasheet attached.
Title: Re: Understanding MOSFET datasheets
Post by: MarkT on Feb 05, 2015, 02:52 pm
115V between two legs of a SO-8 package?  You'll need a conformal coating to prevent
flashover.

I still vote for IGBT, 100+V with MOSFETs is fraught with issues - good protection
circuitry is everything.  IGBTs are far ruggeder at high voltage.

Anyway I have a question:

You are using PWM to switch 6A at 115V and derive 36V from it?  You mention "filtering",
but I wonder what you mean - its sounds like you may be making an ad-hoc switch-mode
supply - which is ambitious.
Title: Re: Understanding MOSFET datasheets
Post by: rockwallaby on Feb 05, 2015, 02:58 pm
MarkT, yes, localbroadcast is making such a device.
You might like to look over this thread (http://forum.arduino.cc/index.php?topic=293926.0) for the more specific details.
Title: Re: Understanding MOSFET datasheets
Post by: JimboZA on Feb 05, 2015, 03:00 pm
You are using PWM to switch 6A at 115V and derive 36V from it?  You mention "filtering",
but I wonder what you mean - its sounds like you may be making an ad-hoc switch-mode
supply - which is ambitious.
Mark you had the honour of being the first to respond on the OP's thread (http://forum.arduino.cc/index.php?topic=293926) where that whole approach is discussed to death...
Title: Re: Understanding MOSFET datasheets
Post by: polymorph on Feb 05, 2015, 04:04 pm
Video!
Title: Re: Understanding MOSFET datasheets
Post by: MarkT on Feb 05, 2015, 05:02 pm
MarkT, yes, localbroadcast is making such a device.
You might like to look over this thread (http://forum.arduino.cc/index.php?topic=293926.0) for the more specific details.
Ah - a mains induction motor with PM rotor replacement?  That opens up
whole sets of new issues methinks.  Mains induction motors are (unsurprisingly)
highly inductive, which is not the usual setup with a buck converter's power
source.   Is this 1-phase or 3-phase?  How is it being rectified?
Title: Re: Understanding MOSFET datasheets
Post by: localbroadcast on Feb 05, 2015, 06:25 pm
115V between two legs of a SO-8 package?  You'll need a conformal coating to prevent
flashover.

I still vote for IGBT, 100+V with MOSFETs is fraught with issues - good protection
circuitry is everything.  IGBTs are far ruggeder at high voltage.

Anyway I have a question:

You are using PWM to switch 6A at 115V and derive 36V from it?  You mention "filtering",
but I wonder what you mean - its sounds like you may be making an ad-hoc switch-mode
supply - which is ambitious.
Yes, I am making a switching voltage regulator.  This circuit needs 36 volts, but I will be programming it so that it can easily be changed to different voltages if needed in the future.
Conformal coating does sound like a good idea.  Once dust starts piling up, an arc can happen pretty easy.  Thankfully the Drain pins are all on the one side of the device and the source and gate pins are on the other side of the device.. so yea the 115v drain pin won't be right next to the source pin.

Is it really that hard to turn a PWM pulsing 115V to a smooth DC voltage?  From what I've been reading, it doesn't take much.. check out THIS CALCULATOR (http://sim.okawa-denshi.jp/en/PWMtool.php) it even gives you a graph of the resulting signal.  Or THIS PAGE (http://electronics.stackexchange.com/questions/34843/how-determine-the-rc-time-constant-in-pwm-digital-to-analog-low-pass-filter) has a pretty good discussion on the matter.

The generator I'm using is a 3 phase induction motor being spun at slightly above synchronous speed.  Does not have any extra permanent magnets added or anything at all really.. Pretty much unmodified.  Don't really want to get into all the fine details about my project.. lets keep this thread to mosfets please?

 
Title: Re: Understanding MOSFET datasheets
Post by: polymorph on Feb 05, 2015, 08:38 pm
If you smooth a power PWM with an RC filter, you've just lost all the advantage in efficiency.
Title: Re: Understanding MOSFET datasheets
Post by: MarkT on Feb 05, 2015, 09:48 pm
The point is most switching designs assume a constant voltage source, not a highly inductive
dynamo winding / rectifier network.  You probably need to establish a good DC rail with
substantial capacitance on the input side rated for lots of ripple current.

You do know unmodified induction motors can work as generators?  You need external
power to bootstrap, but otherwise they can be used this way with a smart controller.
Title: Re: Understanding MOSFET datasheets
Post by: polymorph on Feb 05, 2015, 11:07 pm
Actually, the point is that a switch mode regulator -requires- an inductance.

It sounds to me like perhaps what he needs is more akin to a current regulated switch mode regulator, like what is used in a chopper stepper driver. In which case you -don't- smooth the PWM output.

But he clearly thinks he knows much more than any of us, so I just want to see the video of the first time this is "fired" up.
Title: Re: Understanding MOSFET datasheets
Post by: rockwallaby on Feb 06, 2015, 12:06 am
I know localbroadcast asked us to only talk about MOSFETS, and I'd like to respect that, but, if I could just quickly say  :)

MarkT wrote:
Quote
You do know unmodified induction motors can work as generators?  You need external
power to bootstrap, but otherwise they can be used this way with a smart controller
Yes, they are generally called IMAG, (Induction Motor As Generator)

Maybe nobody picked it up inthe other thread, but that is what I have.

Each and every electron I use is generated from a 3 phase induction motor acting as a generator.
What you are reading right now was generated from such a device as I'm not connected to any grid at all.

What I do have is a water stream on my property that has around 37m of static head.
I have the induction motor at the bottom of my property with a 100mm pelton wheel on it and is being spun at around 1400rpm by water passing through an adjustable spear valve, so I can adjust the power the turbine generates.

I have 3 phase armoured VSD cable connected directly to the terminals on the unit coming back up 150m to where I am. Then I have two capacitors, connected across in a Delta configuration. The capacitors are high voltage 660Vac with one being 2C.

To start up this thing, I need to make sure there is residual magnetisium in the rotor, and if not, I zap a 12 volt battery across one of the windings for a few seconds while the turbine is stopped to induce a good magnet field into the rotor.

Then open up the water gate and bring it up to speed. There is a point that suddenly it will load down, this is when the voltage generated is high enough that the capacitors kick in and the whole unit is then self excited and power is being generated.

I simply then pull power from one of the phases and into a very large transformer and then the rest is boring.

I use an Arduino to monitor my output volts after the large transformer and bridge rectifier, which is essentially my battery volts.

The Arduino then uses a PID function to shunt any excess turbine energy directly into a 240Vac load, thereby keeping the volts at the transformer output clamped at a constant upper limit value, which I can adjust remotely via the HTTP interface.

Generally, such power generating units have quiet a steep voltage vs rpm curve, where it doesn't take much in the way of rotational speed to change the outut volts.

These things have been used all over the world where grid power is not available, especially in poor countries.

Sorry for the length of the post, just couldn't help myself  :-[

I know localbroadcast said his use was to use the induction motor as generator, though I think he mentioned from a wind turbine.

Like I said before, my bet is that he'll get it working, as he seems dead keen and determined, which is good to see.

Plus, it is an interesting project, but I'll keep out of the politics on this one and try to help where I can.
____
Paul
Title: Re: Understanding MOSFET datasheets
Post by: localbroadcast on Feb 06, 2015, 02:53 am
The point is most switching designs assume a constant voltage source, not a highly inductive
dynamo winding / rectifier network.  You probably need to establish a good DC rail with
substantial capacitance on the input side rated for lots of ripple current.

You do know unmodified induction motors can work as generators?  You need external
power to bootstrap, but otherwise they can be used this way with a smart controller.
Yes, that is how my generator works.  unmodified 3 phase induction motor.  If it is not self-exciting on startup because the residual magnetism has somehow disappeared, it is automatically sensed that no output voltage is happening and a DC pulse is applied to magnetize the rotor.  With some capacitors connected to the windings, once it gets going, it self-excites and outputs a good amount of power without any issues.  I'm using water power from a stream.  Since the stream is always running, and the speed of the water is pretty consistent, the only time I need to re-magnetize  the rotor is if the thing gets clogged up by twigs or something.. which isn't often.  By the sounds of it, mine and rockwallaby's systems sound to be pretty similar.  Great minds think alike.

Actually, the point is that a switch mode regulator -requires- an inductance.

It sounds to me like perhaps what he needs is more akin to a current regulated switch mode regulator, like what is used in a chopper stepper driver. In which case you -don't- smooth the PWM output.

But he clearly thinks he knows much more than any of us, so I just want to see the video of the first time this is "fired" up.
Hmm.. I dunno man.  I've been looking all over, and it's pretty consistent..  Everyone is advising the use of a low pass RC filter to convert PWM to an analog voltage level (digital to analog conversion, DAC).

You are correct, the use of a resistor in the filter will cause losses.  I haven't calculated the capacitor and resistor values that I will need just yet.. but when I get around to it, I'm hoping the resistor value isn't too bad.

Either way, I know for a fact that this loss will be much much less than the losses you get using a standard linear voltage regulator circuit, such as a zener diode regulator or a linear voltage regulator IC.. Both of these just burn up the excess voltage as heat loss.

If you have a better, more efficient suggestion for getting the stable 36Vdc, let me hear it.  I'm all ears.

I've attached some filter schematics for your amusement.
 
Title: Re: Understanding MOSFET datasheets
Post by: polymorph on Feb 06, 2015, 03:53 pm
The links you gave earlier were for RC lowpass. An RC lowpass filter on PWM will give -just- as much in losses as a linear regulator.
Title: Re: Understanding MOSFET datasheets
Post by: localbroadcast on Feb 07, 2015, 03:08 am
The links you gave earlier were for RC lowpass. An RC lowpass filter on PWM will give -just- as much in losses as a linear regulator.
Ya?  Hmm.. Well, lets say my supply voltage was 120v, and the zener diode is rated for 36v.. This means that 84v is dropped across the resistor 100% of the time.  This resistor will have the same current running through it as what the load is drawing, plus the current through the zener diode.  My load is 3 amps, so that would mean the resistor would be consuming at least 84v x 3A = 252w.  That's a whole lot of power to just dump as waste heat.

If I used an RC lowpass filter, then the resistive portion of it would be consuming the same 3 amps because it's in series with the load, but it would not be on 100% of the time.  It would probably be on around 40% of the time.  I don't know the voltage that would drop across it without calculating the size needed for the filter to operate correctly.. so lets exaggerate and say it would drop 100 volts.  There's no way it would ever drop that much voltage, but let's just say it does for arguments sake.  100v x 3A x 40% = 120watts.  That's less than half of the wasted power of the zener regulator, and realistically the resistor voltage is probably nowhere near 100v.

Anyways.. I don't think a resistor is necessarily crucial for the operation of the filter.. check out all the inductor / capacitor combinations I posted as attachments in my previous post.  In that case, the wasted power would be even less.

Polymorph.. I'm not saying that I know for sure that I will be able to perfectly filter the PWM output.. but it's what I'm going with until I hear of a better option..  If you can suggest one I'm more than willing to consider it.
Title: Re: Understanding MOSFET datasheets
Post by: polymorph on Feb 07, 2015, 07:36 am
Nope, I'm just looking forward to a video of the resulting explosion. That'll be appropriate recompense for the insults I've received.
Title: Re: Understanding MOSFET datasheets
Post by: localbroadcast on Feb 12, 2015, 12:45 am
Nope, I'm just looking forward to a video of the resulting explosion. That'll be appropriate recompense for the insults I've received.
So because you think you have been insulted in the past causes you to post incorrect and inaccurate information for everyone to read?  Strange modality.  Strange indeed.  Please don't post in my threads anymore.  Your BS is not wanted and is not asked for.


The SO-8 package is extremely small!  I got some of the mosfets in the mail today and wow they are small!  I ordered some adapters / sockets for these tiny IC packages so they can more easily be soldered to my circuits.

I am still waiting on my 100w LED arrays to arrive.. So until then, I can't really put anything together just yet.  I'm also still waiting on the mosfet drivers.

Title: Re: Understanding MOSFET datasheets
Post by: polymorph on Feb 12, 2015, 01:56 am
I absolutely have NOT posted incorrect information. Insults again. Very classy.
Title: Re: Understanding MOSFET datasheets
Post by: michinyon on Feb 12, 2015, 02:03 am
Quote
Why wouldn't an N-channel mosfet work on the high side of a load?  P channel has current flowing from source to drain, and N channel has current flowing from drain to source..  I don't understand why you couldn't put the load on whatever side of the N-channel mosfet you want and have it work.. weather the load is connected to the drain or source.. the only way for current to flow through the load is to have the gate triggered.  Can someone explain this why you need to have the load on a specific side of the mosfet?
Yeah,  you could have N-type mosfets on both the low side and high side of the load.

But here's the problem.  The N-type mosfet works by manipulating the voltage between the gate and the source.  On the low side,   when the mosfet is being used as a switch,  or in an H-bridge type circuit,  the source is the ground level or low-side DC level.    It is easy to organise the gate voltage Vgs in relation to the low-side source potential.

If you put the N-type mosfet on the high side,   then the source voltage of the mosfet, is what ?   Varies all the time.   It's hard to set a gate voltage relative to it.   Possible,  but difficult.

The P-type mosfet has it's gate voltage set with respect to the source,   which, in the high side position,  is the high voltage dc supply.   Setting the gate voltage with respect to that,  is more straightforward.
Title: Re: Understanding MOSFET datasheets
Post by: michinyon on Feb 12, 2015, 02:11 am
Quote
These things have been used all over the world where grid power is not available, especially in poor countries.
Poor countries like Norway and Vancouver Island and Fiordland and anywhere else you got permanent water flowing off a cliff.
Title: Re: Understanding MOSFET datasheets
Post by: rockwallaby on Feb 12, 2015, 04:25 am
Quote
Quote
These things have been used all over the world where grid power is not available, especially in poor countries.
Poor countries like Norway and Vancouver Island and Fiordland and anywhere else you got permanent water flowing off a cliff.
Not as widespead due to the fact there are many other available options in terms of availablility of different and better generation systems and the cost is not such a driving factor for using an industrial 3 phase motor as generator, where it is in these traditionally poorer countries.

Countries like Napal, in the mountainous regions of southern west China and Peru often use the simple 3 phase induction motor as generator IMAG, as they are robust and reliable with only two bearings to maintain, easy to carry into these places and easy to install.

A simple IMAG controller maintains the shunt load to mainatain operational voltage and also to keep the frequency as best as possible to within working parameters.

Then there are folks like localbroacast and myself who also see the virtue in these sort of generating systems. There are many others who also make use of IMAG in more developed countries who know a thing or two about micro-hydro generation  :D
Title: Re: Understanding MOSFET datasheets
Post by: localbroadcast on Feb 12, 2015, 08:20 am
check out this filter designer app I found direct from texas instruments.. I think they offer it free so that you will buy their op-amps and other components.

http://www.ti.com/lsds/ti/analog/webench/webench-filters.page?DCMP=sva-web-filter-en&HQS=sva-web-filter-powerhouse-20140204-lp-en (http://www.ti.com/lsds/ti/analog/webench/webench-filters.page?DCMP=sva-web-filter-en&HQS=sva-web-filter-powerhouse-20140204-lp-en)

And ya polymorph... When you said that the resistors in linear regulators and the resistors in the filter of a switch mode regulator would waste the same amount of power as heat... that was false information.
Title: Re: Understanding MOSFET datasheets
Post by: polymorph on Feb 12, 2015, 08:50 pm
Do the math.

Using an RC filter with PWM is not really a switch mode regulator.
Title: Re: Understanding MOSFET datasheets
Post by: MarkT on Feb 13, 2015, 03:00 pm
Yeah,  you could have N-type mosfets on both the low side and high side of the load.

But here's the problem.  The N-type mosfet works by manipulating the voltage between the gate and the source.  On the low side,   when the mosfet is being used as a switch,  or in an H-bridge type circuit,  the source is the ground level or low-side DC level.    It is easy to organise the gate voltage Vgs in relation to the low-side source potential.

If you put the N-type mosfet on the high side,   then the source voltage of the mosfet, is what ?   Varies all the time.   It's hard to set a gate voltage relative to it.   Possible,  but difficult.

The P-type mosfet has it's gate voltage set with respect to the source,   which, in the high side position,  is the high voltage dc supply.   Setting the gate voltage with respect to that,  is more straightforward.
Untrue - driving p-MOSFETs like this is a right-royal pain(*), but driving
all-n-channel bridges is trivial since the driver chips to do this that bootstrap the
high-side gate voltage are plentiful and cheap and work upto 600V...

(*) Because unless the supply voltage happens to be about 12V (or 5V for logic-level)
you have to level shift, but the easy circuit for level shifting involves a fairly high value
resistor on the p-channel gate meaning it cannot be PWM'd very fast.  Also the
whole bridge will respond to a brown-out by popping MOSFETs, which is
simply unacceptable - so you need extra protection circuitry to detect brown out,
it all gets ugly fast.

Just sticking a HIP4081 or FAN7388 in the circuit is so simple by comparison, and
this approach decouples the drive supply completely from the motor/load supply
(you need 12V for the driver, the supply can be anything from 0V to 100's of volts
depending on the abs max of the driver - you can power up driver and load indepdently
without anything bad happening.
Title: Re: Understanding MOSFET datasheets
Post by: Boardburner2 on Feb 13, 2015, 04:51 pm
While I favour the video approach.
There is something sad about a circuit that just gives off a whisp of smoke.
With proper design there is a big flash and bang to be had here.

Can I suggest you post your circuit here before applying power.

You may find a critique by more experienced arsonists to be helpful.
Title: Re: Understanding MOSFET datasheets
Post by: localbroadcast on Feb 14, 2015, 02:49 am
Do the math.

Using an RC filter with PWM is not really a switch mode regulator.

Yes.. I have done the math.  And yes, a switch mode voltage regulator can be had by controlling a MOSFET with PWM and a gate driver, with the output filtered any number of ways.  I have attached a PDF document that outlines the topologies of pretty much every switch mode power supply imaginable.  It's a great document that is well worth the read.  It outlines the operation of the different switch mode regulator designs, as well as outlines the equations used for design calculations.  Also helps engineers decide which type of switch mode regulator to use for their specific needs.

In many situations, a simple lowpass RC filter is sufficient.  The specs of the resistor and capacitor can be calculated to provide the proper cutoff frequency, desired amount of output ripple, etc.  The order of filtration will determine the quality of filtration achieved.  A simple RC filter would be a 1st order passive filter.  Of course, we can make things more complex and achieve an even smoother output by increasing the filter order, using an inductor as well as capacitor and resistor, or even using active filtration with op amps instead of just passive filtration.  You will have to decide yourself what filter design is best suited for your needs.  I think a first order RLC filter will suffice.  Follow the links at the bottom for more information on PWM filter design.  These are very informative PDF files that really help wrap your head around how different filters work, and how to achieve the best efficiency for your system.

One of the more important facts that I came across when researching filtration was this: The higher the PWM frequency used, the smaller the capacitance will need to be, since capacitor reactance is inversely proportional to the frequency.
This is nice to know, because it tells us that the higher the PWM frequency we can attain, the more efficient we can make our filter, since we can minimize the capacitor and resistor sizes.  Of course, the frequency is limited by the switching speed of the MOSFET and the micro-controller being used..  So now we know why it would be a good idea to aim for a MOSFET with the best switching speed characteristics possible, as well as a low Rds(on) value as always.

Enjoy the reading material!

Filtering PWM signals by Jim Wagner (http://ltwiki.org/images/8/82/PWM_Filters.pdf)
TI - FilterPro Users Guide (http://www.ti.com/lit/an/sbfa001c/sbfa001c.pdf)

Polymorph.. I hope this clues you into what you haven't quite been understanding.  Please refrain yourself from posting anything else in my thread unless you know for sure that your information is accurate.  I hate to see so much false information being spread around.. It confuses everyone.. Especially people just starting out in electronics.
Title: Re: Understanding MOSFET datasheets
Post by: JimboZA on Feb 14, 2015, 04:22 am
my thread
That's an interesting take on things... worthy of discussion somewhere one day.
Title: Re: Understanding MOSFET datasheets
Post by: polymorph on Feb 18, 2015, 12:10 am
Quote
I hate to see so much false information being spread around.. It confuses everyone.. Especially people just starting out in electronics.
How ironic.
Title: Re: Understanding MOSFET datasheets
Post by: localbroadcast on Feb 18, 2015, 12:15 pm
The links you gave earlier were for RC lowpass. An RC lowpass filter on PWM will give -just- as much in losses as a linear regulator.
Do the math.

Using an RC filter with PWM is not really a switch mode regulator.

I dunno.. but I doubt you can back these claims up...
Title: Re: Understanding MOSFET datasheets
Post by: MarkT on Feb 18, 2015, 03:35 pm
600V 20A IGBT 3-phase bridge with gate drivers and protection circuitry: job done.

http://uk.farnell.com/fairchild-semiconductor/fsbb20ch60f/ipm-igbt-20a-600v-spmca-027/dp/2322642 (http://uk.farnell.com/fairchild-semiconductor/fsbb20ch60f/ipm-igbt-20a-600v-spmca-027/dp/2322642)

[ well I think its a more rational approach to the problem - these modules are extremely
handy ]
Title: Re: Understanding MOSFET datasheets
Post by: polymorph on Feb 18, 2015, 11:24 pm
Read a book, in between insulting me. Perhaps you can show me in one of these documents where a switch mode power supply uses a resistor as part of the smoothing network.

http://www.st.com/web/en/resource/technical/document/application_note/CD00003910.pdf

http://ww1.microchip.com/downloads/en/AppNotes/01114A.pdf

Quote
In SMPS, the series element, RS, is replaced by a
semiconductor switch, which offers very low resistance
at the ON state (minimizing conduction loss), and very
high resistance at the OFF state (blocking the
conduction). A low-pass filter using non-dissipative
passive components such as inductors and capacitors
is placed after the semiconductor switch, to provide
constant DC output voltage.
http://www.smps.us/topologies.html

http://www.onsemi.com/pub_link/Collateral/SMPSRM-D.PDF

http://www.nxp.com/documents/application_note/APPCHP2.pdf

http://www.element14.com/community/servlet/JiveServlet/previewBody/27614-102-1-76799/01207A.pdf

You are on your own.
Title: Re: Understanding MOSFET datasheets
Post by: rockwallaby on Feb 19, 2015, 12:24 am
Localbroadcast, I think there is going to be a problem with the overall concept as I understand it which I now recall I had as well.

First to confirm, you intend to run the 3 phase induction motor as generator, IMAG, with the correct capacitors across the windings to provide the excitation flux to be able to self generate AC power.

Then to this, you intend to connect a switchmode power supply across either one, two or all three phases to then produce the 36 volts at some current.

Is this correct?

If that is correct, then there will be a problem if you place any form of rectification directly across any of the generator phases and you draw current.

The rectification of the generator AC will distort the generated waveform so badly that is will cause the self excitation to stop.

An IMAG will work reliably with a low frequency transformer based system across its phases and then into rectification at the secondary voltage.

If you place a capacitor in series with such a transformer primary winding, then you can reliably start and stop your turbine as the reactance of the capacitor will be high at low turbine speeds, thereby making the transformer out of circuit.  When the turbine gets up to speed, the capacitor reactance becomes low and then the transformer will then be in circuit. The capacitor will in theory limit the current that can flow through the transformer primary, but in reality this should not be a problem for these smaller power systems.

I only recall now that I found this to be the case when I placed a standard industrial SMPS across one of the phases of my turbine. As soon as I started to draw any significant current, I would loose all generator excitation.

Not sure how this effects your plans or how best you can work around it?

What size induction motor do you have for this, is its capacity around the 500W to 1kW mark?
Is it a 2 pole or 4 pole or 6 pole motor?
____
Paul
Title: Re: Understanding MOSFET datasheets
Post by: localbroadcast on Feb 19, 2015, 02:54 am
Read a book, in between insulting me. Perhaps you can show me in one of these documents where a switch mode power supply uses a resistor as part of the smoothing network.
I already posted tons of links for you to peruse on the matter of filtering.  I've got enough information to work with, thanks for trying to help tho.

I plan to test the performance of a couple different filter designs.  One will be a simple straight forward RC single pole filter.  resistor in series with load, capacitor in parallel.

PWM frequency = 10kHz
R1 = 1 ohm
C1 = 10000uF

from my calculations, here's the results I'll get..

settling time = 23 ms roughly
peak-peak ripple voltage = 0.3v roughly
cut-off frequency = 16hz roughly
With a 3 amp load, I'll be losing 3 watts from the resistor.  Not too bad for a 3 amp load.

Transfer function:
G(s) = 100 / s+100

The other filter I was going to try looked more like this:

Inductor in series with load
Capacitor in parallel with load

L1 = 16 uH
C1 = 10000uF

This will give me a reactance from L1 of 1 ohm, so hopefully similar losses as the RC filter, close to 3 watts roughly.. give or take of course.


The final (and probably most successful) filter I will try will be more like this:

Inductor and resistor in series with load
Capacitor in parallel with load

L1 = 42 uH
C1 = 10000uF
R1 = 0.1 ohm

I'll be running through testing with all these different filter combinations to figure out the actual losses developed in real time, not just theoretical models.  Also want to take a look at the wave forms produced on the oscilloscope.  Should be very interesting!
Title: Re: Understanding MOSFET datasheets
Post by: Boardburner2 on Feb 19, 2015, 11:27 am
The rectification of the generator AC will distort the generated waveform so badly that is will cause the self excitation to stop.

____
Paul
At last.

Its one of the reasons that small wind turbines for charging batteries use permanent magnet generators.
The other, variable speed does not apply here.
Title: Re: Understanding MOSFET datasheets
Post by: Boardburner2 on Feb 19, 2015, 02:27 pm
The rectification of the generator AC will distort the generated waveform so badly that is will cause the self excitation to stop.

____
Paul
At last.

Its one of the reasons that small wind turbines for charging batteries use permanent magnet generators.
The other, variable speed does not apply here.

With some capacitors connected to the windings, once it gets going, it self-excites and outputs a good amount of power without any issues.  I'm


When you rectify that the smoothing cap on the other side is effectively in parallel with the cap used for the excitation field for part of the cycle.

Have you considered what the effect of the pwm load will be ON THE MOTOR/generator.

IMAG do not like capacitive loads as Rockwallaby has discovered
Title: Re: Understanding MOSFET datasheets
Post by: rockwallaby on Feb 19, 2015, 04:06 pm
Quote
IMAG do not like capacitive loads as Rockwallaby has discovered
You do need the excitation caps in there and I do also have caps in series with certain loads, even my main shunt load of around 300Watts has a cap of around 45uF at 380VAC to give DC isolation when I start or stop the turbine.

Having these capacitors in series with resistive loads keeps the residual magnetism in the IMAG.

Placing more capacitors in parallel with the excitation caps will change the reactance and hence the volts you get and also unbalance the whole thing, but generally, it doesn't have a drastic effect such that rectifiers do across any phase or phases.

It is the forward conduction of the rectifier that causes the colapses the excitation flux, as the current at this point is relatively high and it is being effectively shorted out.

Most times, you also kill the residual megnetism in the rotor as well, which can be brought back by putting a good old 12volt battery across a phase for a few seconds while the turbine is not rotating.
Sorry for spelling, it's after 02h here  :smiley-sleep:
____
Paul
Title: Re: Understanding MOSFET datasheets
Post by: Boardburner2 on Feb 19, 2015, 04:21 pm
It is the forward conduction of the rectifier that causes the colapses the excitation flux, as the current at this point is relatively high and it is being effectively shorted out.

____
Paul
Yes the capacitor has to recharge over a very short part of the cycle.
it results In a very high Ipk.

Once charged it has no effect on operation as the motor does not see it until current is drawn.

I wondered if using an inductive filter would solve it but did not get around to experimenting.

I am thinking that what is required is a current to voltage topology.
AFAIK there are no topologies that can do that in a single stage.
Title: Re: Understanding MOSFET datasheets
Post by: alka on Feb 20, 2015, 12:10 am
i just wanted to understand mosfet datasheets...
Title: Re: Understanding MOSFET datasheets
Post by: Boardburner2 on Feb 20, 2015, 12:54 am
i just wanted to understand mosfet datasheets...
My apologies for hijacking your thread.

I have in the past done similar with smaller similar setups.

When it comes to switch mode supplies there is a lot to be considered and it can get complex.

The buck converter mentioned works well with a voltage  source.
As already mentioned a voltage source from imag could result in loss of excitation.

A generator is often considered to be a voltage source especially the infinite grid at fixed frequency.

However a generator can be a current source also and your previous mention of experience with variable frequency drives makes me wonder if that has a bearing on what you are trying to do.

A previous argument elsewhere (in depth over many years)indicate that the self excited imag generator is a special case.
I did not understand the maths.
The previous argument involved a proposed current to voltage convertor but the proponent who successfuly did it got the huff with us and would not describe how it worked.

I just used a permanent magnet which worked fine.


Experimenting with pwm regulation could well give you some very difficult to understand results ,some of which could result in flash and bang rather than a whisp of smoke as I previously posted.

There are some here who may be able to help but you would have to post more detail.

A mosfet is a fantastic device for switching large currents with low resistance loss .
When used in switched power supplies there is more to be considered.
I do not think anyone here is being unhelpful but on its own the mosfet is not the only thing to be considered.

Often when people ask for help the assumption is they ask for help because they assume that people here know more , then argue.
It could be that in this instance you are the professor and we just do not know.


edit
I just reread that.
Style was not as intended.
Don't want to sound pompous or so.
But when considering switch mode supplies the power source has to be considered as well as the output. 
Title: Re: Understanding MOSFET datasheets
Post by: Boardburner2 on Feb 20, 2015, 01:40 am
I just noticed you are not the op.
wrong thread for understanding dadtshets.

Start your own please.
Title: Re: Understanding MOSFET datasheets
Post by: localbroadcast on Feb 22, 2015, 10:09 am
lol??? He's not the op, but the thread IS called undestanding mosfet datasheets..

Since the generator voltage output is so similar to AC mains from your standard house wall plug of 120vrms, I am basically building this power supply to work when plugged into the wall.  I'll get it working off of this power source, and then will deal with any issues that comes from powering it with my generator.  The generator as the power source is the end objective, but I'm going to get it running off of standard 120vac first.. just to keep things simple.

I really didn't want to use a transformer to step down the voltage.. I didn't want to have the losses associated with it.  I wanted to use strictly pwm to do it, but it seems that the size of capacitor needed to filter such a gap in voltages would be unrealistic.. It wouldn't be impossible, just too large.  So I think I'm going to cut the voltage in half by using a 2:1 wound transformer to step down from 120vac to 60vac, rectify and filter to a nice smooth DC, PWM and filter to a nice smooth 36vdc, feed the load.

caps and trannys on the way.  already got my mosfets and leds.
Title: Re: Understanding MOSFET datasheets
Post by: Boardburner2 on Feb 22, 2015, 10:35 am
This may be of interest
Lots of fets there


http://www.ijcee.org/papers/278-E747.pdf (http://www.ijcee.org/papers/278-E747.pdf)

Some ambiguities though.

Some important details missing also

What it does is convert the output from a voltage to a current scource which makes the switching easier
Title: Re: Understanding MOSFET datasheets
Post by: Boardburner2 on Feb 22, 2015, 11:04 am

http://forum.arduino.cc/index.php?topic=302331.0 (http://forum.arduino.cc/index.php?topic=302331.0)

An ac wall outlet is normally considered to be a voltage source.

A generator can be either but in the particular case of imag is best used as a current scource to avoid excitation collapse.

This is why standard buck converters (voltage source ) cause collapse as mentioned by rockwallaby.
Title: Re: Understanding MOSFET datasheets
Post by: localbroadcast on Feb 22, 2015, 03:27 pm
Hmm..
Boardburner2...

Your post about current sources and voltage sources intrigues me.. I have heard mentions of a difference between constant current output and constant voltage output of power supplies.. and current sources instead of voltage sources.. but I never really took the time to actually research the meaning of the two.

I've always assumed that a circuit will have a source voltage.. determined by the voltage of batteries used, or the voltage of the wall outlet...  And based on this voltage, the circuit will draw the amount of current that it needs based on the resistance, reactance, etc. of the combined total of all components of the circuit.  More resistance, more amps.  As long as the power source can provide the amperage being drawn, there's no issues.  If the power supply is capable of providing more current, it won't just put more current into the circuit... it only provides the amount of current that the circuit draws, or "asks politely for".

Always using this method of thinking to understand how circuits work, it kind of stresses out my brain to think of a circuit who's power supply provides a constant current, and the voltage used depends on what the components of the circuit draw, or "ask for politely".

Maybe I'm not understanding it quite correctly.. maybe you can clue me in to what you're talking about with the difference of a current source vs. voltage source...

As for if my generator will be able to handle the load and still keep on generating...
I haven't really felt like discussing it here or arguing the point of if it will do the job or not, because I'm going to get it working off of 120vac 60hz from the wall first.. and then tweak it to run on my generator output.

I am confident in my generator as the supply for a couple reasons..  The main reason is that it's not wired quite the same as what most people imagine a self excited induction generator to be wired like.. most assume they only work at a constant speed, or they suck balls at any other speed.. mine works quite well at a range of speeds.... but since it's fed by a stream that speed is pretty consistent.  Attached is a diagram of BASICALLY how my generator is rigged up.  Not exactly connected this way, but almost exactly like this..

I think you guys would get a kick out of seeing one of the schematics from when I built VFD's.. I'm not sure if I'm allowed to show other people the schematics, but since I haven't been employed there for over 5 years and the customers are given a copy when they buy the vfd, I don't see there being any risk.  Just let me know if it's worth scanning... I won't do it unless someone is interested in seeing it.

Title: Re: Understanding MOSFET datasheets
Post by: localbroadcast on Feb 22, 2015, 03:43 pm
Here is a PDF that describes in more detail how my generator is wired up..  I used this document as reference when building my generator system.  I don't really wish to discuss it anymore because I have it working fine and I need more assistance with the power supply project I am working on at the moment.



Title: Re: Understanding MOSFET datasheets
Post by: Boardburner2 on Feb 22, 2015, 08:31 pm
I spent a couple of days researching terminology to try and explain .
This is the best I could find but it takes some digesting.

http://cds.cern.ch/record/987498/files/p13.pdfhttp://cds.cern.ch/record/987498/files/p13.pdf (http://cds.cern.ch/record/987498/files/p13.pdf)

The pdf you posted appears to be the same I posted in my earlier link so it appears we are both working to the same hymn sheet.

Using a imag connected to a transformer then a Buck convertor works.

However it can be very lossy and I was using a motor which I think was too small.

Using a series inductance works but only for a given frequency which is not much good for a wind generator.

I thought as you clearly do that a switching method was the solution but I lacked the experience at the time when building a dc dc convertor was adventurous and building an ac to dc invertor was for the brave.

I looked up Z invertor which was a new term for me but. quote

The source can be either a voltage source or a current source. The DC source of a ZSI can either be a battery, a diode rectifier or a thyristor converter, a fuel cell stack or a combination of these.
The main circuit of a ZSI can either be the traditional VSI or the traditional CSI.
Works as a buck-boost inverter.
The load of a ZSC can either be inductive or capacitive or another Z-Source network.

unquote.

I think you need the current scource type.  

That pdf omits some things.

It does not explore the performance wrt frequency of the input signal which is strange considering it is for wind power.

For the dc link L4 is shown but the text describes an inductive capacitive circuit without explaining detail.

Also it does not describe the size of motor and inductor but the text mentions it does not work well for small motors without qualifying it.
I think this is probably where I fell down.

It also mentions torque discontinuities without elaboration which I take to mean whine or vibration.
I think that may be the onset of excitation failure but that's a guess.


Hmm..


I've always assumed that a circuit will have a source voltage.. determined by the voltage of batteries used, or the voltage of the wall outlet...  And based on this voltage, the circuit will draw the amount of current that it needs based on the resistance, reactance, etc. of the combined total of all components of the circuit.  More resistance, more amps.  As long as the power source can provide the amperage being drawn, there's no issues.  If the power supply is capable of providing more current, it won't just put more current into the circuit... it only provides the amount of current that the circuit draws, or "asks politely for".




That's true of the grid supply which to all intents and purposes is infinite.
IMAG however has its own ideas.

A generator can be considered either a voltage source (fixed frequency) or a current scource (variable frequency).
In the case of IMAG it can be both.

I believe it defies conventional analysis requiring iterative means to describe its behaviour.



it kind of stresses out my brain to think of

Oh yes.

I tried thinking what does the motor 'see' and what does the input circuit 'see'

Went around in circles for months there
Title: Re: Understanding MOSFET datasheets
Post by: Boardburner2 on Feb 22, 2015, 08:37 pm
It appears you are taking up where I left off.
I will be very interested to see what your results and conclusions are.

I will attempt to answer any specific questions if I can but I think I have explained my experience as best I can

Edit

My suspicion is that you will encounter a problem between the uncontrolled rectifier and the Z input convertor depending on weather the input to the Z convertor is capacitive, inductive or resistive.

I think it will work off AC mains in any case but not off an IMAG source.

I would be delighted if you could prove me wrong but I do not have sufficient hardware to test myself.
I think I would need several sizes of motor and inductors to test and they are plain expensive.
Title: Re: Understanding MOSFET datasheets
Post by: localbroadcast on Feb 23, 2015, 12:46 pm
Well.. Like I said... that diagram is a basic starting point of how I designed my generator system.. The actual way that I built it was not a "z source invertor".. As I don't know what that is either.. But it's a simple 6 mosfet inverter with the gates triggered with PWM to create 120vAC at 60hz.  You can buy motor control ICs that have all 6 of these mosfets already built in as one chip.  Between the rectifier and the inverter is what's called the DC BUS.  Its a simple capacitor in parallel and inductor in series which makes what's coming out of the rectifier a pretty smooth DC signal.

It's the basic design of a VFD but the rectifier and the inverter are reversed. 
Title: Re: Understanding MOSFET datasheets
Post by: localbroadcast on Mar 05, 2015, 01:57 am
I'm still working on this project, it's coming along slowly but surely.
Here's something I would like to have clarified...
In the attached circuit diagram, I would like to switch the diode "D" with a mosfet in order to improve efficiency, making it a synchronous constant current DC - DC converter.

Should I use an N-type mosfet or a P-type mosfet for this?  How should it be triggered ON / OFF?

If I use an N-type mosfet here, when the main mosfet 'M' is OFF, if I apply a +10vdc signal at the gate pin to turn it on, the inductor will be producing a voltage which will be entering the SOURCE pin of this mosfet.  Since VGS needs to be about +10vdc to turn the mosfet on, if I have a voltage at the gate pin from the inductor 'freewheeling', then my +10vdc at the gate pin isn't going to turn on the mosfet.

Any help is much appreciated!