From: email@example.com (Bob Wilson)
Subject: Re: My flyback experiment is on the web
Date: Mon, 14 Oct 2002 02:30:15 -0000
Organization: Your Organization
X-Newsreader: WinVN 0.99.9 (Released Version) (x86 32bit)
In article <3DAA0563.9040507@BOGUS.earthlink.net>, crobc@BOGUS.earthlink.net
>Here is the URL to my web page showing the results of my most recent
>experiments with a flyback converter. I still haven't gotten a PWM chip
>hooked up yet, as my first goal was to buy some inductors that look like
>they are designed for switching supplies, and see if they'd work better
>than the hand wound transformer I made from a PC power supply core.
>Actually, the inductor seems a bit worse :-(
>All the details are on the web page.
>My main question is why does the inductor get so warm? I figured I
>wasn't really pushing it too hard, so it shouldn't loose much power.
>but it seems most of my losses are occurring in the inductor. I was
>hoping to avoid complex topologies such as a two transistor flyback,
>which requires a transformer, but lends itself to easily shunting the
>remaining core energy back to the input rail. Can that be done with the
>simple flyback that I am working with? I figured I could get 75% or
>better, so I'm not really far off. But I don't like heating the
>inductor, and I want to push up the power another 25% from what I'm
>testing at before I finish.
>I have a smaller inductor, and a larger one too, but all of the toroidal
>type, designed for 150kHz switchers.
>Comments appreciated on what steps I might take to make this simple
>circuit move in the direction of something worth committing to FR-4.
>I will put more on the web page when I get to testing out the PWM chips,
Your inductor gets warm for one of two reasons:
1. It is an el-cheapo, lossy iron core inductor (these are very common),
which was never designed for what you are using it for. These types of
inductors are designed for power filtering, where there is a relatively
small amount of current ripple, superimposed on DC. Their hysteresis losses
at any reaonable frequency are far to high to me useful as a SMPS inductor.
2. Your copper losses are too high (wire gauge is too small).
The problem is undoubtably due to 1. above. Using an inductor that "looks
like it is designed for switching supplies" is probably your big problem.
You need to DESIGN the thing, use core material that is intended for what
you want it to do. Also, forget about using a toroidal inductor, It isn't
worth the time and expense. It makes far more sense to design an inductor
around a ferrite E-E. or RM, or ETD core, whose type of ferrite does what
you want. Those cheap iron-core toriods are useless for what you are doing.
A simple flyback cannot return leakage energy back to the bulk storage cap,
like the 2-transistor flyback can. It has to be dissipated in snubbers. But
big deal! All you will lose is a couple of percent of efficiency if you
design the thing right, so it is hardly worth worrying about.