"Flyback" Driver - Why are my MOSFETS hot? (fwd)

[Steven Roys <sroys@xxxxxxxx>]

---------- Forwarded message ----------
Date: Mon, 28 Jun 2004 17:28:13 +0930
From: Matthew Smith <matt@xxxxxxxxxx>
To: High Voltage list <hvlist@xxxxxxxxxx>
Subject: "Flyback" Driver - Why are my MOSFETS hot?

Hi All

I have a 555 based oscillator running at 20kHz driving a totem pole arrangement
in turn driving the gates of five IRF620s in parallel.  This is driving my
"flyback" transformer.

Supply voltage to the MOSFETs is 24V (control is 5V).

Inductance of transformer primary is 200uH.

That gives a reactance of about 25 Ohms.

Current measured at around 1A - everything adds up so far.

The MOSFETs have an Rdson of 0.8 Ohms.  Assuming (!) that each MOSFET is taking
one-fifth of the current, we should be dissipating (1/5)^2 . 0.8 = 32mW.

Why are the (small) heatsinks getting so hot?  (Please note that it's mid-winter
here and the ambient temperature is below twenty degrees Celcius/sixty something
of your Earth Farenheits.)

I tried disconnecting my snubber (which was getting really hot) and it didn't
seem to make any difference.  Snubber is  Rsnub=2.7k Ohms in parallel with
Csnub=11nF, all in series with MUR4100.

I know I'm missing something fundamental here - having a head full of cold
doesn't help.

Cheers

M

-- 
Matthew Smith
Kadina Business Consultancy
South Australia