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

[Steven Roys <sroys@xxxxxxxx>]

---------- Forwarded message ----------
Date: Sat, 03 Jul 2004 10:17:47 +0930
From: Matthew Smith <matt@xxxxxxxxxx>
To: High Voltage list <hvlist@xxxxxxxxxx>
Subject: Re: "Flyback" Driver - Why are my MOSFETS hot? (fwd)

Malcolm Watts replies to Matthew's Message - Matthew's reply interspersed:

> 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?
I sketched a schematic, but found that my scanner is currently hors de combat -
too much furniture to move to get to the cables!

Basically, 555 drives totem pole via resistor, both powered by common 5V supply.
  Totem pole drives MOSFET gates via parallel resistor and diode (diode points
back towards totem pole).  MOSFETS have their own 24V supply sharing common
ground with the 5V stuff.

<http://www.mss.cx/tmp/scopeshot1.jpg> shows the circuit running with the 24V
supply turned off.  Waveform is taken between totem pole and MOSFET gate
resistor.  Lower trace (straight line) is taken from a resistor divider (n/2)
connected between ground and MOSFET common drain line.

<http://www.mss.cx/tmp/scopeshot2.jpg> shows same hook-up, but with 24V turned on.

Images are reduced to about 30kb for painless viewing ;-)

> Some quick and dirty calcs:
>
> ASSUMPTION: duty cycle is 50%
Yep - I've tweaked it so that it is :-)
>
> Ipk = V*Ton/Lp = 24*25e-6/2e-4 = 3A   which sounds OK ASSUMING the
> core is gapped.
It is - there are mica (?) shims 0.3mm thick.

> How many turns on the primary?
26 - as near as I can see.

 > What is the effective pole area (Ae) of the core?
150mm sq, based on the legs on which the windings sit and deducting the cutaway
that accommodates the tie bolts.

> 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.

Thanks for your help!

Cheers

M

-- 
Matthew Smith
Kadina Business Consultancy
South Australia