From: John Popelish
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Subject: Re: 12 vdc to 6vdc @ 30amps
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Date: Mon, 16 Sep 2002 11:37:22 GMT
NNTP-Posting-Date: Mon, 16 Sep 2002 07:37:22 EDT
Tony Williams wrote:
> 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
> at changeover.
> > 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.
Take another look at the circuit. It is sort of unique in not having
the tapped inductor tied to any particular voltage source any any
point, yet, the gate drive is strictly contained between 12 and zero
by two diodes (the mosfet body diode in the negative direction, and
the schottky in the positive one.) And the switching transitions are
softened by the inductance of the load.