From: "Christopher R. Carlen"
Subject: Re: Core gapping techniques and general SMPS magnetics stuff
Date: Mon, 16 Dec 2002 09:29:12 -0800
Organization: Sandia National Laboratories, Albuquerque, NM USA
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NNTP-Posting-Date: Mon, 16 Dec 2002 16:27:53 +0000 (UTC)
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> I didn't see anything about your input voltage, but assuming that
> it's relatively low, like 12 volts, one reason to use a transformer
> rather than a single inductor is that for large step up ratios the
> current pulses during the off time of your switch get very large at
> the small pulse widths when you use a single inductor. With a
> transformer you can get some of the step up from the turns ratio and a
> more favorable duty cycle; the output current pulse magnitude is
> proportionately smaller. But, at low power levels, this is quite
> doable even with a single inductor.
But those current pulses are going through the diode and into the output
capacitor, not through the switch. So I think it bears upon ripple in
the output, but not upon the stresses faced by the MOSFET, right?
The other aspect is that with a transformer, there is a leakage spike
across the MOSFET, forcing me to either use a rather over-rated voltage
MOSFET anyway, or to snub, which I really don't want to do. The
inductor avoids this entirely.
Christopher R. Carlen
Principal Laser/Optical Technologist
Sandia National Laboratories CA USA