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Please help noob with simple mosfet driver

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Bender

Thu Jan 03 2008, 10:30PM

Registered Member #1027 Joined: Tue Sept 25 2007, 01:44PM
Location: Poland
Posts: 15

Im trying to make simple TL494 driven single mosfet flyback driver but it isn't looking good.

Here is schematic as it is:

I borrowed o-scope from friend and mosfet gate sees this kind of waveform (those unwanted? oscillations move around when pulling arc)

Problem is that mosfet heats up quite fast and badly (soldered wires fall off!) and it is mounted on heatsink and i even tried fan without success.

Mosfet is 500V 20A 0.27ohm RDSon (IRFP460A)

It is pretty sure that its 100% my fault, but im out of ideas so little help would be nice :)

cjk2

Fri Jan 04 2008, 04:44AM

Registered Member #51 Joined: Thu Feb 09 2006, 04:17AM
Location:
Posts: 263

Use a TVS from drain to source on your mosfet, or try a better snubber. The mosfet is likely in avalenche mode and is heating as a result. Your circuit looks ok to me, make sure you have the proper pullup/down resistor on the output of the 494. I think your largest problem is not using a snubber.

Dr. Dark Current

Fri Jan 04 2008, 06:07AM

Registered Member #152 Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384

This always works for me: Increase the 7R gate resistor to ~200R. Put a fast diode- anode to Gate cathode to UCC3721 output. Now solder a 0.1-0.47uF film/foil capacitor (find out which value works the best) of at least 400V rating across D-S o the MOSFET.
This always drastically decreases the FET heating for me.

For your transistor (500V) you could probably safely go over 50V supply voltage (gate drive on 15V), this also helps efficiency.

Thomas F

Sun Jan 06 2008, 05:19PM

Registered Member #503 Joined: Sun Nov 19 2006, 06:37PM
Location:
Posts: 59

Hello,
I am facing a similar kind of problem with my setup too.I have put an RC network as a snubber,but it still heats up.
I am using a 30Hz pulse with a 10% ON cycle.
Can anyone give some ideas about how an RC snubber would work ? My feeling is that the cap would charge up to sec voltage (30kV ) / 100 ( turns ratio ).What would dissipate the charge in the capacitor so that it can snub again in the next pulse. As far as I can see it would have to be the MOSFET, which would thus see the high voltage(300V) the snubber is supposed to protect it from anyway.
How would the diode across gate drive resistor protect from over heating ? It would only turn off the gate faster than it gets turned on.
The MOSFET is IRFP250N and the gate driver is 4420.

Thomas
]1199639769_503_FT36872_circuit.pdf[/file]

cjk2

Sun Jan 06 2008, 06:26PM

Registered Member #51 Joined: Thu Feb 09 2006, 04:17AM
Location:
Posts: 263

Everything about your driver looks fine except for the snubber cap value. try something like 1uf. Someone smarter than me may correct me, but the cap should get discharged every cycle because it will be shorted through the resistor during the on cycle of the FET.

Steve Ward

Sun Jan 06 2008, 06:53PM

Registered Member #146 Joined: Sun Feb 12 2006, 04:21AM
Location: Austin Tx
Posts: 1055

For the original schematic, the 50% duty cycle is almost always going to be too high. You are likely saturating the core of the flyback. Cut it back to about 25% duty cycle so that the flyback core has some more time to reset rather than overloading it with DC.

Thomas F

Mon Jan 07 2008, 03:12PM

Registered Member #503 Joined: Sun Nov 19 2006, 06:37PM
Location:
Posts: 59

Hello,
I think I made a mistake intertwining my query here :). Apologies to Bender.

@ CJK2, my assumption was that the snubber RC would protect the MOSFET from the high voltage induced in the primary during the OFF phase of the MOSFET which would not happen if the cap which is charged to this voltage is discharged by the same MOSFET.

Thomas

Dr. Dark Current

Mon Jan 07 2008, 04:03PM

Registered Member #152 Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384

Try it the way I described. Using this method I can push over 100W into a flyback with the MOSFET just on a little heatsink. Also very high voltages can be generated, I had one diodesplit flyback output cca. 100KV (arcs staring at 10cm) with my circuit.

Thomas F

Mon Jan 07 2008, 08:38PM

Registered Member #503 Joined: Sun Nov 19 2006, 06:37PM
Location:
Posts: 59

Hi JMartis,

Whats the frequency and duty cycle you were using ?

Dr. Dark Current

Mon Jan 07 2008, 08:52PM

Registered Member #152 Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384

Thomas F wrote ...

Hi JMartis,

Whats the frequency and duty cycle you were using ?

IIRC duty cycle was a little below 50% and frequency 19-23kHz

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