From: Tony Williams
Subject: Re: 12 vdc to 6vdc @ 30amps
Date: Mon, 16 Sep 2002 08:09:31 +0100
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NNTP-Posting-Date: Mon, 16 Sep 2002 07:09:27 +0000 (UTC)
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In article ,
Winfield Hill wrote:
> Very interesting. Are you sure this will work as drawn with
> FETs? There's no equivalent to a capacitor paralleling a base
> resistor... Talk us through determination of the oscillation
> frequency, as well as a transition from one FET full ON to the
> other full ON.
Yes, I would be nervous about the MOSFET version.
The key to self-saturating oscillators is getting
the device OFF, pdq, as the inductor saturates.
Transistors only need a small dip in Vbe to start
the OFF-going process. MOSFETs are another matter.
Having said that, I have to confess to doing a
saturating dc-dc converter, with a pair of VN10KM,
gates cross-coupled in exactly the same way, and
with a square loop core. It oscillated fine, but
I didn't dare think about the transient dissipation
> Q. Is the transformer's leakage inductance our friend, or enemy?
Your bosom pal (with transistors), because the only
free end of the inductor is connected to the
collector of the transistor that is going to come
ON. The flyback is in a direction that forces that
collector down (below 0v), which (in turn) pulls
it's base ON, early.
That's not my original thoughts, it comes from
Thomas Roddam who is quoting from a paper by
Meyerhoff and Tillman, Trans I.R.E CT-4, 1957.
The accompanying picture shows a smooth square
wave at 700KHz oscillation frequency.
There is also a reference to a 'wearing-out' of
transistors used with saturating inductors.
Significant changes in current-gain, Vce(sat)
and Vbe(sat) after 5000 hours of operation.