Re: flyback converter driving

On Oct 19, 4:32 pm, Jamie Morken <jmor...@xxxxxxx> wrote:
Tom Bruhns wrote:

As drawn, it is, I believe, a forward converter, not a flyback. You'd
need to reverse the polarity of one of the windings for it to be
flyback. The 'acid test' is to note when the output capacitor is
charging. If it is charging when the FET on the primary side is
conducting, then it's a forward converter; if it's charging when the
FET turns off, it's a flyback. The fact that it steps the voltage
down doesn't matter.

Since you only need low power, I'd suggest that you look into a
switching regulator part that's designed to do as much of the whole
job as you reasonably can. A lot of small "wall wart" power supplies
these days will supply a few watts happily, and do it from AC inputs
from under 100V to 250V, which translates to perhaps 130VDC to about
350VDC after rectification. I know there are ICs that implement
practically everything, including directly driving the transformer
primary. The applications notes will show you just how to use them.
You can probably take a reference design and make very minor
modifications to get the output voltage you want. The 400V input may
be a little high for them, but you may well find one that can handle
that. Seems worth a look. Maybe Linear Technology, but I'd look at
NXP, ST and some of the others that are more oriented toward consumer
electronics.

I found the LM5020 at national.com:http://www.national.com/pf/LM/LM5020.html

Is this the type of IC that would be best to use? I will need to power
it with the high resistance series resistor and storage cap from the
400VDC rail as you suggested I think. Now I just need to find its spice
model..

cheers,
Jamie

Actually, I was thinking more like the ST Electronics VIPer22A,
http://www.st.com/stonline/products/literature/ds/12050.pdf
It has a 400V max rating; at the low power you'll be running and
because of the simplicity of the design using such a part, it could be
worth adding a simple circuit to limit the maximum input rail to, say,
350 volts to give a little headroom. There well may be other such
parts from other vendors, or even others from ST; that just happens to
be one I know of. Do a search for things like "off line flyback ..."
where ... might be "regulator" or "driver" or "controller".

On the other hand, since limiting the max input to a lower voltage
would require some parts, it may be just as efficient to use a flyback
controller IC driving a high voltage mosfet. LOTS of manufacturers
make those, including Linear Technology (which means LTSpice will have
models already built in). And if you want to use a National part, you
can just use their "WebBench" simulation tool.

An alternative way of doing all this that may ultimately be simpler:
International Rectifier makes a self-oscillating half-bridge driver
that I'm pretty sure supports voltages to over 400. You could use it
in a dog-simple circuit to drive a transformer to get to a much lower
voltage, say 0.12 times the input DC voltage (i.e., 12 to 48 volts
out) and use something like a National SimpleSwitcher buck regulator
to get from that down to your desired 10V output. It's kind of a wide
input voltage range, but seems like it should be possible.

I'm gathering that simplicity is what you're after, but I don't think
you really ever did say much about your goals. If you're
uncomfortable working with stuff like this, I'd highly recommend
getting a local guru that can help you a bit in person. I have the
feeling that you're on a steep learning curve, and at times like that,
you're likely going to miss some fairly important point. 400V is
nothing to take lightly.

Cheers,
Tom

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