Is Driver circuit is necessary for driving mosfet in flyback

[kalaianand]

hi friends
i am trying to design 12V isolated output from 24V constant DC source using flyback configuratiom. i am using ST3524 for pulse generation. operating at 40kHz, switching device is IRF840(500V,8A).as my input is constant i am using openloop configuration to obtain output, no closedloop feedback. I am getting my desird output voltage but Mosfet is Heating up quickly. dont know why. i am using Gate resister of 4.7ohm and Gate-Source reisister of 2.2kohm.
please suggest me the solution for this problem friends
thank you

 

[flatulent]

Re: Is Driver circuit is necessary for driving mosfet in fly

The switching speed of the FET is slow and so there is an extended period where there is voltage across it while current flows through it. This is probably caused by the limited drive current sent to the capacitive gate-source part of the FET.

I would suggest that you use and IC specifically designed to drive FET gates. They commonly have several A drive capability.

 

[kalaianand]

Re: Is Driver circuit is necessary for driving mosfet in fly

ok friend, then which IC i have to use, please suggest. IC is mosfet or DRIVER?

 

[dick_freebird]

You want to snap that FET gate off, hard, to get all the inductor
energy out as voltage and not in the FET as heat. As fast as
will let you pass EMI testing. Probably also a snubber network
on the drain node to protect the FET and lessen EMI.

Lots of FET drivers out there, you should work backward
from "what gate rise/fall time do I need to get within 2%
of best efficiency" to "what peak current does that turn out
to be" and then you will know what driver output current
rating to search for.

 

[Tahmid]

Hi,
Just as a starter, you could take a look at IR4427, TC4427. You could also use BJTs in totem-pole configuration to work as drivers.
Instead of SG3524, you could use SG3525 or SG3526 which have MOSFET drivers incorporated in them with drive capabilities of 400mA peak.

Hope this helps.
Tahmid.