Re: +/-45V PWM supply - terrible inefficiency and feedback inaccuracy
- From: "Bob Eldred" <nsmontassoc@xxxxxxxxx>
- Date: Sat, 21 Jan 2006 15:40:04 GMT
"mw" <mw@xxxxxxxxxxxxx> wrote in message
news:9CqAf.12853$H71.11530@xxxxxxxxxxxxxxxxxxxxxxxxxxxxx
> Ref: my Dec 05 post, "transformers for cheap low current flyback supply,
> +/- 45V ? "
>
> After holidays I was able to devote some time to this and I have a pair
> of independent supplies built. They basically work but there are two
> problems and they are likely related.
>
> 1) Low efficiency, GIANT current spike at turn on, immediately followed
> by a sag in the sense resistor current, then it rises back up at a more
> expected ramp. Snip.......
A high turn on current transient may be due to too long of a an on time when
the the circuit comes up before the oscillator stabilizes. You need a
storage or digital scope to capture and observe the turn on switching events
to see what's happening. Low efficiency means power is being dissipated
somewhere, most likely in the power switch (FET). Again a careful analysis
of the switching waveforms will be required to discover the problem. I
suspect that the switch comes on too early or goes off too late while high
currents with high voltage are present. Both at the same time causes
excessive dissipation. It's best to switch when both current and voltage are
low or zero. Depending on the design that may not be possible. I can't speak
for the transformer but one thing to look for is winding capacitance. It can
cause resonant waveforms that are quite different than expected causing
excessive dissipation. Also be sure the transformer gets nowhere near
saturation during the on time current run up. What is the frequency, what is
the max on time, what is the max flux density, what is the core material.
What are the core losses? Many questions. Power supply design, while simple
in concept is complex and difficult in practice.
Bob
.
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