From: "Christopher R. Carlen"
Subject: Re: My flyback experiment is on the web
Date: Tue, 15 Oct 2002 08:12:20 -0700
Organization: Sandia National Laboratories, Albuquerque, NM USA
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NNTP-Posting-Date: Tue, 15 Oct 2002 15:11:17 +0000 (UTC)
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Robert Baer wrote:
> Inductor heating usually is due to IR losses; you are running too much
> current thru the wire.
> If it has a core, it is also possible that overdrive (saturation) and
> frequency in the loss ergion of the core are other possible loss aeras
> (mentioned in order).
> Toroids are not for flyback supplies; get an E-I, U-I or pot core and
> gap it with grocery bag plastic (0.5 mil is thinnest) or other plastic
> sheet (up to 3 mils).
In this case the inductor is wound with pretty heavy wire (haven't
measured it exactly) but the DC resistance is 0.1ohm. It is getting a
lot hotter than the FET, which is 1ohm. Saturation is not occurring, as
evidenced by the waveforms, perhaps you didn't see them, they are at:
Thus, is is clearly core loss.
Can you tell me why it would be advisable to gap a core for a flyback,
as opposed to having an ungapped core? I'm just entering the field of
SMPS, so this is all new to me, and I haven't delved into rigorous study
of the magnetic design, though I should be getting to it soon.
But I would have thought that for a flyback it is wise to have as little
winding capacitance as possible, so as to make it possible to discharge
as much magnetic energy as possible into the output circuit on flyback.
With a lot of winding capacitance, this adds to the FET's output
capacitance, which creates the residual energy that rings-down in the
drain+winding capacitance/inductor tank circuit after the output diode
turns off. This residual energy is to be minimized.
Since a gapped core needs more windings to get a given inductance, it
has more capacitance, hence is bad for a flyback. Also, the increased
windings have more IR loss, and this is particularly to be avoided in a
flyback since it operates at twice the peak currents of converters that
don't use the magnetic core to store energy. I also thought that a gap
is for avoiding flux imbalance conditions that arise in push-pull
converters, and to a lesser degree in some bridge converters.
Christopher R. Carlen
Principal Laser/Optical Technologist
Sandia National Laboratories CA USA