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Can you help with these waveforms? (fwd)

Original poster: List moderator <mod1@xxxxxxxxxx>

---------- Forwarded message ----------
Date: Wed, 30 May 2007 11:57:52 +1000
From: Mr Gregory Peters <s371034@xxxxxxxxxxxxxxxxx>
To: Tesla list <tesla@xxxxxxxxxx>
Cc: greg.peters@xxxxxxxxxxxxxxxxxxx
Subject: Can you help with these waveforms?

Hi All
I'm trying to make what is essentially a single ended HF (40kHz or so) 
transformer and power supply using a homewound transformer on a 
ferrite core, and am hoping someone may be able to offer some advice.
The circuit is essentially identical to the single ended flyback 
transformer driver circuits all over the web, with a single driver 
mosfet on the primary being pulsed from a totem pole circuit and 555 
Anyway, I'm wondering if you can help explain why I seem to be getting 
the massive voltage spikes across the transformer primary shown in the 
image at:


Please note that the input voltage is about 12v and the image is 
10v/division, so the spikes are huge, and the peak voltage of the 
square part of the wave itself is at least 20v! 
Could it be because my mosfet reverse recovery diode is not 
functioning properly? I tried substituting it for a super fast 3A 
diode, but maybe that is not quick enough either? The frequency here 
is about 40kHz
Also, what do the waveforms suggest about this core's suitability for 
use at this frequency? I note that apart from the actual voltage 
spike, the rise and fall times seem quite square??

Many thanks for your advice,