hexfet mosfet get hot too fast

Hello Terry,

The main job in driving a FET is to swing the voltage fast and hold the gate after a rising edge (when Cgd wants to push it right back down). If a decent PWM chip is out of budget I prefer to do that with CMOS logic. For really large FETs with a pnp/npn follower. That way the total for the semiconductors sans power FET typically stays below 10c.

It is amazing how much waste and heat shallow slopes can cause. Even with a low switcher frequency of 20kHz a few hundred nsec combined on and off can easily cost a percent, dissipated as heat in the power transistor.

Regards, Joerg

and the pnp can be tied to the source, rather than 0V, so the turn-off gate current spike doesnt upset the current comparator :). Then bung 10R in series with npn c, and 100n from npn c to pnp c. That way, the turn-on gate current spike doesnt make it thru the current sense resistor either (although the DC does).

this ones not great, 71% efficiency. it switches at 250kHz....

BUT we need 3 big fat resistors, so we provide enough load to the source, lest it think there is a fault.

so I dont care about efficiency, in fact I want the converse.

Cheers Terry

Hello Terry,

IIRC the OP wanted to run at 20kHz. I think the 494 can do up to 300kHz but up there the losses due to slow transitions would be really bad.

Regards, Joerg

Hello Tim,

I had only one. But upon runaway voltage it probably won't make a difference, except that if you have five caps you'd have five rockets instead of one. Meaning more "snow" and five dents to spackle...

Regards, Joerg

Hi Joerg,

YDRC, and yes its lossy at 250kHz; the level-shifted gatedrive doesnt help, either. But the loss is in the FET, which has a nice heatsink, and I need the losses anyway.

BTW thanks for the WDF references.

Cheers Terry

Hello Terry,

Hmm, no idea what the acronym YDRC could mean. Sometimes these are like the custom license plates in the US where you can pick your letter/number combo (for a fee, of course...) and then the other drivers have to guess what it means.

Need losses anyway? Are you guys already preparing for winter? ;-)

Most welcome.

Regards, Joerg

IIRC -> YDRC (You Do...)

monitoring circuit gets unhappy if power too low. forces a maximum efficiency of 60% or so, which I make up with a dump resistor :(

Cheers Terry