component heat

What's the gate drive: voltage, frequency?

John

Thanks for the responses!

The IRFZ44 gate is driven by a ULN2003A against a 10K pullup to the same

Take a look at the gate risetime with a scope. Fets like that have a lot of capacitance, so the fet may be turning on very slowly when pulled up by a wimpy 10k resistor. Try a lower gate pullup resistor or an active pullup.

I bet the gate waveform looks like...

----------- 13v / / / / / ________/ / /

and the plateau is a killer for dissipation.

But that wouldn't very well explain fet-to-fet differences. So see if the gate drives look different some other way. Check the heatsinking, too.

John

Could well be; that might explain the differences. A small (47 ohms, maybe) resistor in series with the gate lead, real close, usually fixes that.

John

That fet is certainly big enough to handle the 4 amp load without much heat if it is fully on. Either it is not being turned completely on with enough gate voltage, it is transitioning between on and off too slowly, or it is on the verge of oscillating.

Then there is a fourth possibility, in addition to the three I listed. The darlington transistors do not pull all the way to zero when they are on, so if some mosfets have a lower turn on threshold than others, they may leak a considerable current in the off state (when the darlington pulls down) and dissipate heat. You can test this by forcing the darlington on, continuously (disconnect the processor line) and see if the transistor still gets warm.

the

duty

OR some of them might oscillate due to layout, this being continuously in the analog region