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

[Steven Roys <sroys@xxxxxxxx>]

---------- Forwarded message ----------
Date: Fri, 2 Jul 2004 16:39:15 +1200
From: Malcolm Watts <m.j.watts@xxxxxxxxxxxx>
To: High Voltage list <hvlist@xxxxxxxxxx>
Subject: Re: "Flyback" Driver - Why are my MOSFETS hot? (fwd)

Hi Matt,

On 1 Jul 2004, at 19:48, High Voltage list wrote:

> Original poster: 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.

Is the core gapped? The average current doesn't give a good idea of
the peak currents. What is the duty cycle of the driving waveform? Do
you have a circuit you can post? Better still, an oscillogram of the
waveforms across the MOSFETs?

Some quick and dirty calcs:

ASSUMPTION: duty cycle is 50%

Ipk = V*Ton/Lp = 24*25e-6/2e-4 = 3A   which sounds OK ASSUMING the
core is gapped.

How many turns on the primary? What is the effective pole area (Ae)
of the core? From that information I can calculate the core flux and
determine whether B (flux density) is being kept to a reasonable
level. You should be able to do this all with a single MOSFET and I
speak from experience.

If the core is not gapped, you may be running into trouble.

Malcolm