My POWER MOSFETS end up in smoke!!! Any idea?

[borescopeit]

My POWER MOSFETS end up in smoke!!! Any idea?

Here is my setup:
I run a dry cell capable of consuming 350 amps, feed it with 14.2VDC from 220AMP Delco heavy duty alternator through #4 OFC wires that handle around 70 amp (this is the amps the cell runs at). Will upgrade my wires to #1 welding cables (will carry through up to 150 amps).

My KOH e-lyte is at 21%.
POWER MOSFET is ON Semiconductor NTB125N02R capable of driving 125A 24VDC @ 25C.

When I run it at 100% duty @ around 20KHz (it is that fast to handle up to 5 MHz) everything is just fine, being cooled by 12V 2" fan stays at 35C, but when I lower its duty cycle it immediately heats up to 102C and if I do not shut it off on time it ends up in a garbage bin.

Does anybody experienced the same sh1t?
Should I try lower switching frequency while lowering its duty cycle? I use microcontroller to drive the FET so I can preset any frequencies. Best freq. I found to be for my cell is around 3.6 & 23 Khz.

Also, I am playing with alternating two different freqs. every X seconds.

Any advice is appreciated!

[bobsbbq]

My POWER MOSFETS end up in smoke!!! Any idea?

Here is my setup:
I run a dry cell capable of consuming 350 amps, feed it with 14.2VDC from 220AMP Delco heavy duty alternator through #4 OFC wires that handle around 70 amp (this is the amps the cell runs at). Will upgrade my wires to #1 welding cables (will carry through up to 150 amps).

My KOH e-lyte is at 21%.
POWER MOSFET is ON Semiconductor NTB125N02R capable of driving 125A 24VDC @ 25C.

When I run it at 100% duty @ around 20KHz (it is that fast to handle up to 5 MHz) everything is just fine, being cooled by 12V 2" fan stays at 35C, but when I lower its duty cycle it immediately heats up to 102C and if I do not shut it off on time it ends up in a garbage bin.

Does anybody experienced the same sh1t?
Should I try lower switching frequency while lowering its duty cycle? I use microcontroller to drive the FET so I can preset any frequencies. Best freq. I found to be for my cell is around 3.6 & 23 Khz.

Also, I am playing with alternating two different freqs. every X seconds.

Any advice is appreciated!

Are you running the Mosfets at 24v as they are rated or at 12v? When running at 12 volts they will have half the current capacity so they are more like 62.5 amps. The lower the duty cycle the harder the mosfets are working.

[borescopeit]

Are you running the Mosfets at 24v as they are rated or at 12v? When running at 12 volts they will have half the current capacity so they are more like 62.5 amps. The lower the duty cycle the harder the mosfets are working.

No, I run these mosfets at 14V. Can you please prove your statement? I never never assumed that running lower voltage through the PFETs would derate amperage rating!

[bobsbbq]

Well I can't say with actual hands on experience, but as far a voltage and current goes there is nothing to prove.

Double the voltage half the current/ Half the voltage double the current. This is true is house wiring such as 110v vs 220v as well.

So without seeing the data information on the mosfets and their operating parameters I'm not sure. But I suspect if they are made for and rated at 24v input then the current is also rated at 24v input. If this is the case then you are cutting the voltage in half and effectively raising the current or in this case the current load of the mosfet is decreased.

I hope this makes sense.

[borescopeit]

Well I can't say with actual hands on experience, but as far a voltage and current goes there is nothing to prove.

Double the voltage half the current/ Half the voltage double the current. This is true is house wiring such as 110v vs 220v as well.

So without seeing the data information on the mosfets and their operating parameters I'm not sure. But I suspect if they are made for and rated at 24v input then the current is also rated at 24v input. If this is the case then you are cutting the voltage in half and effectively raising the current or in this case the current load of the mosfet is decreased.

I hope this makes sense.

All you are saying makes a lot of sense. This is exactly how Ohm's law states: I=V/R so R=V/I and V=I*R. P=V*I (I=Amps, V=volts, P= power in watts, R=resistance in ohms).

What I have discovered for myself from looking through a dozen of different power mosfet datasheets is that "Continuous Drain Current, VDS @ xxV" and "Power Dissipation, PD @ TC = xx°C" and "Static Drain-to-Source On-Resistance, RDS" are a few of most important parameters to take into account.

So, with my PFET (ON Semiconductor NTB125N02R: 125 AMPERES, 24 VOLTS, RDS(on) = 3.7M
(Typ)) I will take the following data into account:
VDS = 24V, PD @ 25°C = 113.6W, RDS = 3.7M, ID = 125A

Will run 14V @ 25°C, will not go over safely dissipated heat (Watt) to find out what load I can put on one mosfet:
ID = 113.6/14 = 8.11A (to make the FET stay cold)

If I want to run lets say 50 AMP through it, my heat amount will be at P = 50*14 = 700W (will cool with liquid hydrogen ). If I fail to cool down this much heat the FET will be kaput.

So, what I see from these calculations, probably the safest amps I can run through my FETs is around 65 AMP (100% duty cycle) with good 10000 rpm cooling fan. If I lover duty cycle the heat generation goes up x2 or even x3.
This is where my FETs start frying. In this case I will lower PWM frequency (on the fly) to ease up switching load on FETs...

[R&D4me]

Chiming in here, your title caught my eye.

The P-MOSs you're running have an 'avalance' (Drain time) that is between your ON/OF duty cycle. If you'res is 100% than you are emiting NO pulse at all.100% is 100% ON ALL the time no?

Also, the drain side of the FETs (Going to your electrolyzer) may be receiving some stored inductance (parasitic inductance possibly from the flyback diodes... ) that needs to be 'releived off' so to speak during each pulse cycle to lower their heat.

The Avalance is a property of the type of FET you're using; obviously without looking at the datasheet for your type, they have to be high-speed switching type. (No brainer excuse me if I don't check)

I was told by a smart guy...."an rc snubber to catch anything left over.
the resistance should be about twice the resistance of the load(YIKES!) you are switching, and the capacitor size determined by watching the switching waveform with an oscope."

I am currently working out a solution due to an increase in repairing them for friend with same problem. Here is my design, a modification of a practical kit I bought. I'd REALLY like to know what your solution was.....



This one should be good for a 60 continuous amp draw.(Fingers crossed) On the OScope 70~90% duty looks great. Will post the screenshot if anyone wants - it's avalance looks super clean but we'll see after hours/days of use the truth will always appear!

FYI- Just got this off Wiki: RE- {Flyback diodes}"The voltage spike across the switch (not the voltage spike across inductor) is usually the biggest problem in real circuits. So if you decide to draw only one voltage graph, this is the one I would prefer. Many transistor switches come with a built-in diode. (Yours does) Too often people incorrectly believe that diode can be used as the flyback diode in this kind of circuit. The "voltage across the switch" graph is the only one that clearly shows why they need *another* diode.
The graph of the voltage across the switch, on the other hand, is always positive."