Hello people.

I have a project building a Half bridge motor controller. I've done some testing and its looking good, but now I want to scale up the project for much more voltage/current. I would like to get a bit of assistance on my calculations as I'm fairly new to mosfets (been using bi-polar for a long time).

Doing some initial calculations on the size of Mosfet driver output I need I came up with a number much smaller then I initially expected.

I have a Astable 555 set at ~16khz which I will use to drive a mosfet driver.
I was thinking of using 12x IRFB3077PbF mosfets. 6 for the high side and 6 for the low side (non-reversing controller) 75V 210A.
Qg = 160typ, 220max nC.
Qrss = 350 pF

Derating them to 60v and 150Amps I calculated the turn on current at around 6.5Amps in 25 nanoseconds.
At 16kHZ and 60v 150A I come up with a power dissipation below 40 watts. With some of the math saying only 3 or 5 watts. I was just punching them in the calculator and not sure if I was calculating right so can only give estimates on the results I got.

So since my frequency is not terribly high, and I probably do not need a perfect 25nS switching slope it was looking as though a 6amp driver would be plenty for 12 of these mosfet. The switching should still be under 150nS and power dissipation would not exceed 40watts per fet

First does this sound right? or was I doing all this math incorrectly?

I was considering using the mosfet driver to drive a pre-output stage using a couple HRF3205s that I have (100Amp 55V).

So with all that can someone recommend me a driver to use or confirm if my rough estimations on the math is correct? Do I require a second stage or not (trying to keep a simple design).

OR if I'm completely off base, let me know, I'm a man I can take constructive criticism.

Another side note. Can mosfets be used in series to raise the total voltage handling? say 2x 75v mosfets in series to handle 150v peak? I did this with other transistors but under if it works for fets.

-TriX-

 

Can't help with the maths but wondered why, on a non-reversing controller, you need high side mosfets?

Mike.

 

I haven't thought of that too much yet. But I thought it was something to do with the free wheeling of the motor.

3 modes, Motor on, Motor Flywheel, and Motor Braking.
Is that the proper function of an Half Bridge?

My testing right now is with just a smaller motor that I don't even have a flywheel connected to test. So I've been using up to this point a single mosfet and a reverse voltage diode.

Then I drove one mosfet from my 555, and used the Drain connected through a 10 ohm 1o watt resistor with the gate of the next stage mosfet connected to the drain of the first. It worked but I didn't hook the scope up to that point since the 10 watt resistor got hot pretty fast with my 19V test supply.

This is when I came to the conclusion if I replace the high side resistor with a high side mosfet, It would provide a good source/sink to both ground and Vcc
with less waste dissipation.

This was before I did the math to find out a single driver might be able to run the parallel output stage without needed a pre-driver, hence my post asking if my math is close.

-TriX-

 

Has anyone used Driver chips and know if I can drive these 12 Mosfet in parallel at 16khz with reasonable losses from a single mosfet driver?