Understanding MOSFET datasheets

[MarkT]

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.

[polymorph]

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.

[rockwallaby]

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:

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

[localbroadcast]

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.
 

[polymorph]

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.

[localbroadcast]

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.

[polymorph]

Nope, I'm just looking forward to a video of the resulting explosion. That'll be appropriate recompense for the insults I've received.

[localbroadcast]

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.

[polymorph]

I absolutely have NOT posted incorrect information. Insults again. Very classy.

[michinyon]

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.

[michinyon]

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.

[rockwallaby]

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  

[localbroadcast]

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

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.

[polymorph]

Do the math.

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

[MarkT]

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.