UC3842-UC3845 and company: parallel mosfets?

[eccerr0r]

Wondering if anyone has tried using a lot of MOSFETs in parallel with the UC3842-UC3845 series chips, how many transistors/how much gate capacitance can it drive before it becomes a low pass filter?

Wondering how much I need to buffer the UC384x output to drive a bunch of mosfets in parallel, if at all. I suspect it also depends on the switching frequency, so let's say, 50KHz. My back of envelope calculation seems to imply I can hook a small handful of 60N05's in parallel at a cost of edge rate, which is quite important too. Then again it'd probably reduce the amount of RF noise the PSU generates...

Most of the existing UC384x designs I see only drive one MOSFET, which I calculate is not quite enough for what I need, any ideas?

[CapLeaker]

Re: UC3842-UC3845 and company: parallel mosfets?

What on earth are you trying to do?

[eccerr0r]

Re: UC3842-UC3845 and company: parallel mosfets?

More current of course Not sure I can get a single mosfet that can deal with the amount of current I want to draw - several amperes....

Just wondering how much current I can control with these things without buffering.

[budm]

Re: UC3842-UC3845 and company: parallel mosfets?

You also need to take care of Gate discharge and also balancing the paralleled MOSFET.

[eccerr0r]

Re: UC3842-UC3845 and company: parallel mosfets?

The output of the 384x were designed for MOSFETs to begin with, it can charge and discharge, but the question still remains: how many... anyone come across multiple mosfets being driven by these chips? All the ones I've seen so far are single device, which isn't quite enough current. Just wondering how many I can run before needing drastic measures... Hoping I can run several nF of gate cap...

[budm]

Re: UC3842-UC3845 and company: parallel mosfets?

Notes about paralleling MOSFET's.
https://cdn.badcaps-static.com/pdfs/...3a40ec90e2.pdf

[eccerr0r]

Re: UC3842-UC3845 and company: parallel mosfets?

But nothing specifically about the 3842?

[eccerr0r]

Re: UC3842-UC3845 and company: parallel mosfets?

Hmm... I was doing some more back of envelope calculations and found out that charging and discharging a single MOSFET gate can many milliwatts at 40KHz if I got the math right. Where is that power dissipating?

probably cooking the 384x. Makes me want to go back to DIP8 because the SOIC8 cannot handle nearly as much power as the DIP8.

Hmm. I guess I will try cooking a IRF640 first since I half cooked one already, with a few 1nF capacitors hooked to the gate to see what happens...