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Registered Member #1103
Joined: Mon Nov 05 2007, 06:02PM
Location: Houston
Posts: 80
I'm having some issues with a flyback driver I built. I used a SE555P to feed a 15-16khz square wave signal into a UCC37321P chip. This chip is driving a single IRFP460A MOSFET. The supply voltage is 12V @ 4.5A from an AT power supply. The MOSFET only gets warm after long use, same with the driver chip.
The problem involves the MOSFET driver chip. It seems to be the most bothersome part of the driver. I've blown at least 5 chips trying to get this working properly. I might be drawing too much current from it, but I'm not sure. I've got a 7ohm resistor from pin 6+7 of the driver chip to the gate of the MOSFET. I don't know really, after all it might be voltage spikes. I think the driver chip is decoupled properly, but what do you guys recommend for decoupling?
Registered Member #1211
Joined: Fri Jan 04 2008, 11:20AM
Location: England - UK
Posts: 19
Make sure the driver is well decoupled - place a 0.1uf (100nf) capacitor directly across the driver supply pins, along with a small electrolytic capacitor. Your supply rails should be very well decoupled anyway, preferably with a number of different value caps. The 555 should also have a 0.1uf accross & close to it's supply pins - they are prone to oscillate at very high frequencies un-decoupled.
Is your FET protected ? - add a 1W 10/15V zener diode between gate & source - cathode to the gate. Add a fast diode - BVY29 etc between drain & source - cathode to drain - it will kill back any back EMF from the flyback. A snubber network - 5W low value 100R-470R resistor in series with a capacitor from drain to source. A good quality 1nf 1KV+ rated ceramic disc capacitor, (blue type) on it's own, is usually enough.
Registered Member #1103
Joined: Mon Nov 05 2007, 06:02PM
Location: Houston
Posts: 80
The driver is decoupled properly I think. I have a 0.0022uF cap and an Aerovox AFPS 3uF 400vdc capacitor across the supply rails. I tried to use a 50vdc 4.4uF electrolytic at first, but it proved to heat up rather quickly. I will add a .1uF across it to be safe.
I will be sure to add the decoupling to the 555. I never knew they would act up without decoupling.
The FET isn't protected at the moment. I thought about adding a decent .015uF ceramic disk capacitor across the drain to source pins, but never really got around to it. I haven't blown a FET yet, which is pretty interesting since I've destroyed quite a few driver chips. I will be sure to add the zener diode and a fast diode to protect it.
I will be sure to get some pictures soon, but first I must get some more driver chips :(. I might replace the drivers with a totem pole or darlington pair and be done with it. I'd like to get a full bridge going sometime.
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Flyback Driver Questions
