Pwm MOSFET driver

[jackbob]

Yes it's a trade off for switching losses. Every snubber I have seen has caps at least 1kv. You have a good point though. It shouldn't reach higher than what I'm putting into the motor.  I have not ordered snubber caps yet (they are very expensive for capacitors) and have a few nice ac caps rated at 275v. Will those do?

[amspire]

You cannot let the drain voltage rise above 200V - unless you get higher voltage MOSFETs.

Higher voltage caps are nice, but if the voltage on the snubber caps every gets to 250V, then you probably have 20 blown mosfets.

Richard.

[jackbob]

Yes so the higher voltage is just extra room for the caps. It won't ever get that high. So I will just use 275v caps I won't ever be running higher than 160v on the input. I ordered all the parts to build this speed control. The only thing I'm still working on is how I will do safety. I mentioned it above and am looking for ways to make it safe as possible and how to stop a failure if it happens. Mcu monitoring seems best but I am just getting
into mcu's and am not experienced enough to do that. It should probably be a topic under microcontrollers if I want to get into that.

Jacob

[Zero999]

Wow that's a little more advanced. I'm just beginningin mcu's. I understand what you are saying but have not a clue where to start as far as setting up a mcu to do that. I also decided to add a snubber circuit of resistors and 1800v caps between the Gate and source to prevent MOSFET failure from high voltage spikes generated by the motor.

There's no nee for that, just connect a diode in reverse parallel with the motor.

If you know how to set up that mcu to monitor the mosfets I would greatly appreciate your help in doing it. I would prefer to use an atmega 328 or 316 if possible. I am not the best at programming them and I have only used arduino programming. I am currently using a 328 and all the outputs are used now.

It's actually very simple. When the MOSFET is on, the drain voltage should be near 0V and when it's off it will be near the supply voltage minus the motor's EMF. All you need to do is monitor the MOSFET to verifiy it's off when it's supposed to be and if it's not turn the main contactor off. The reason why I suggested using another MCU is because it can supervise the other MCU to ensure it's behaving as expected.

[jackbob]

Do you know how to set this up though. Would I have a seprate mcu monitoring the outputs of the pwm mcu then comparing it to the drain voltage. How would it be set up. I don't know how to monitor a mcu with annother mcu.
Thanks,
Jacob.

[Zero999]

The simplest way would be just to measure the drain voltages and configure the MCU so the duty cycle never gets to 100% so the MOSFETs are still being periodically turned off to test they still work. You don't even need another MCU, a simple missing pulse detector will probably work. The most common way to make this is a 555 timer but you could also use a comparator such as the LM393 or even a simple MCU.

[jackbob]

Ah a missing pulse detector is what it's called. Thanks I will look into it, there is a lot about them on the Internet.

Thanks again,

Jacob.

[jackbob]

I made a small quick prototype of the pwm controller and tested it on 12v running a little rc motor and it works great. It's using irfp150's in the prototype because I'm only running 12v. I realized just how important the diodes in parallel with the motor are. Without it while it's not fully on or off the mosfets won't last a minute without catching fire, I felt them and they were cookin. So I put the diode in and the fets go all night not even slightly warm. I need some really good diodes in the final project.  Bad news is I had a bad diode at first and blew up one of my tc4422 but it was for sure the diode that caused it. I will need to order more of those but they come very quick so I'm not worried.

[amspire]

Sounds odd that you are getting so much heat, and one diode has blown. What frequency are you switching the MOSFET's at?

[jackbob]

It didn't blow the diode, the first diode I tried was already blown just a bad one I porbably put back in the drawer on accident. I am switching at 500 Hz and yes without the diode the fets will fry eggs. But just a little 3 amp diode in parallel with the motor and bingo the fets don't even seem to get warm and it runs great. Keep in mind I have no heatsinks on them now but even if I did the fets get way too hot. But as Long as the diode is there with the motor everything runs great. A quick look on the oscilloscope and there are very high voltage peaks
at the motor without the diode and those probably make the fets hot. With the diode the output at the motor looks great.

[amspire]

It would be good if you can get an oscilloscope to look at the waveforms on the drain.

It sound like when the MOSFETs turned off, the inductance of the motor winding caused a transient spike that exceeded 200V.  This caused the MOSFETs to go into avalanche breakdown (like a zener diode) which is not destructive, but dissipates a lot of heat.  If you were using a higher frequency, the MOSFETs probably would not have survived.

Richard

[jackbob]

Yes that's what it is the peaks are pretty high and I have done this at higher frequencies before with transformers and it just blows the fets. When I build the final project how many amps in diodes should I have and what should they be rated at like 300v?

[amspire]

It is very hard to know until you have found the characteristics of the motor.

If you have a digital oscilloscope, what you probably want to do, if possible is have the motor and the 150V and a big  150V DC switch (has to be a genuine DC switch, no an AC rated switch).  Across the motor, put a hefty 400V reverse biased diode - say 30A - in series with a power resistor. Something like 1 ohm, 20 watts.

Run the motor and capture the waveform across the motor when you switch it off. Try it unloaded and loaded.

From the captured waveforms, you can get an idea as to how much energy will need to be absorbed from the motor inductance.  And from that you can calculate whether you can keep the voltage below 200V or not.

It may be that the motor's manufacturer already has good specs on the motor.

Richard.

[jackbob]

Is it possible to over correct the issue too? Can I have too many diodes or would it be ok
if I had extra. I see what you mean by testing and looking at the waveforms, but I don't want to generate too high of peaks during testing and blow up the fets then. I don't want to hook it up for testing at 150v without diodes and hope it doesn't blow my fets and I can see the waveform, you see the problem there?  Thanks,

Jacob.

[amspire]

That is why I was suggesting using a switch to do the initial testing. you only need to capture the waveform of one switch off.

If you don't have a switch, arrange so you can yank one of the wires off the motor and capture the waveform.

The point is it is best to know real data before you buy your mosfets, high current switching diodes, power resistors and maybe big capacitors. If you want to be safe, you could get 600V MOSFETS, but you will need a lot of them.

With your slow switching speed, you could look at one IGBT instead of 20 mosfets. Probably worth a look.

Richard

[jackbob]

If the transistors give me problems or they give me a fireworks show I will go for igbt's. So far on small scale everything is good but that may change on the real thing.

[papanasos]

Could you please explain how did you calculate the additional current (0.4A) required to discharge the gate source capacitance?