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

[Steven Roys <sroys@xxxxxxxx>]

---------- Forwarded message ----------
Date: Thu, 1 Jul 2004 21:12:43 -0500
From: Dr. Resonance <resonance@xxxxxxxxxx>
To: High Voltage list <hvlist@xxxxxxxxxx>
Subject: Re: "Flyback" Driver - Why are my MOSFETS hot? (fwd)


If your system is out of resonance with the flyback's normal resonance
point, then the energy in the primary has no where to go, thus creating
extra heat and other losses.

Dr. Resonance

>
>
>
> ---------- 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
>
>
>