Power bridges ready. Or maybe not.

Posted on September 12, 2014


Njay: Hi, something happened. No, I didn’t burn the bridges. 🙂 But I goofed up again.

Before handing them over to you I wanted to make a specific measurement now that I’ve got a 4 channel oscilloscope: to see the Vgs & Vds voltage wave forms in 2 parallel FETs. It’s a tough one to do, mechanically. While preparing to do it, here’s what I found out:

  1. the bridge I’ve been using all this time for development and testing only had 1 of the N-FETs connected on the side that I’ve been using for PWM! Doh!! I had removed the gate resistor from the other FET, probably a long time ago for some other measurement. Well, if 1 FET can take the load all alone, things are looking better than I thought. 🙂
  2. I went ahead and soldered the 10Ohm resistor to activate the 2nd FET. When verifying it, I found that the gate resistor of the other FET was 100Ohm instead of 10Ohm… which is probably also a leftover from another experimental measurement a long time ago. Now, the bridge you sent back to me has the proper drive circuit, both FETs are connected and the resistors are 10Ohm. I did notice the difference while running a test at 15 to 17A for a couple of minutes, the FET drivers  got warmer than expected. At the time I was worried, the drivers reached about 65ºC and were climbing. Now I know why, the 10Ohm resistors feeding 2 FETs in parallel draw a lot more current from the driver circuit.

Vnevoa: Yeah, well, we pretty much abandoned the project about 2 years ago… this kind of thing was bound to happen. 🙂

Njay: The funny thing is that I did a 20A double pulse test and the FET commutation time seemed perfectly consistent with what I remembered from the beginning, so I didn’t think much of it. In fact the test result is pretty decent, there’s only a 5V spike for the 25V feed (and the FETs can take 55V). 🙂

The issue is: lower resistor values on the FET gates will give more current to the FET and therefore faster switching, which reduces power MOSFET heat losses; but will also raise heat losses in the driver circuit. So now I have to think about it… I’m inclined to use 100Ohm on all of them to keep the drivers safe and connect all the FETs, so I can’t give you the bridges today. We can come back to this point later on and optimize.

What do you think?

(to be continued…)Lisbon_MMF_logos_Banner_big