Understanding MOSFET datasheets

[rockwallaby]

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

[localbroadcast]

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 R_ds(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 R_ds(on).  Also, keeping the MOSFET as cool as possible during operation will help because the R_ds(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 V_GS(TH) and the gate source Voltage for ON state V_GS are different values.
For the MOSFET 2SK3502-01MR:
V_GS(TH) = 5V
V_GS(ON) = 10V

For example:
if V_GS = 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 V_GS 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 V_GS?

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..  

[rockwallaby]

Sounds like you've been doing a lot of reading and making good sense of it all.  

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.  
____
Paul

[polymorph]

Please have something capturing video when you fire this up the first time.

[rockwallaby]

polymorph wrote:

when you fire this up the first time

was the word 'fire' used consciously I wonder, do you have no faith? 

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

[localbroadcast]

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..

[rockwallaby]

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

[localbroadcast]

I just ordered these bad a$$ MOSFETS...

SiR872ADP Made by Vishay Siliconix.  Ordered through Digikey at $2.40 each.
V_DS = 150V
R_DS(ON) = 0.0180 Ohms at V_GS = 10 V
I_D = 53.7A continuous
Q_G = 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 Q_G 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.

[MarkT]

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.

[rockwallaby]

MarkT, yes, localbroadcast is making such a device.
You might like to look over this thread for the more specific details.

[JimboZA]

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 where that whole approach is discussed to death...

[polymorph]

Video!

[MarkT]

MarkT, yes, localbroadcast is making such a device.
You might like to look over this thread 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?

[localbroadcast]

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 it even gives you a graph of the resulting signal.  Or THIS PAGE 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?

 

[polymorph]

If you smooth a power PWM with an RC filter, you've just lost all the advantage in efficiency.