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

[Steven Roys <sroys@xxxxxxxx>]

---------- Forwarded message ----------
Date: Fri, 02 Jul 2004 09:34:44 -0600
From: robert heidlebaugh <rheidlebaugh@xxxxxxxxxxxxxx>
To: High Voltage list <hvlist@xxxxxxxxxx>
Subject: Re: "Flyback" Driver - Why are my MOSFETS hot?

The usual cause of overheating in your type of circuit is the circuit is
turned  ON durring off time. . A diode in the drain circuit or in the 555
output will provide a switching delay to allow shut off time.
   Robert   H
-- 


> From: "High Voltage list" <hvlist@xxxxxxxxxx>
> Date: Thu, 1 Jul 2004 19:48:39 -0600 (MDT)
> To: <hvlist@xxxxxxxxxx>
> Subject: "Flyback" Driver - Why are my MOSFETS hot? (fwd)
> Resent-From: hvlist@xxxxxxxxxx
> Resent-Date: Thu, 1 Jul 2004 19:49:16 -0600
>
> 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.
>
> Cheers
>
> M
>
> --
> Matthew Smith
> Kadina Business Consultancy
> South Australia
>
>