Re: flyback converter driving

On Oct 21, 2:31 pm, Jamie Morken <jmor...@xxxxxxx> wrote:
Hi,

I tested the circuit from 20V input to 450V input,
the circuit starts to operate at about 30V (below
that it just doesn't start up) and the startup
time is about 550ms at 30V input. The startup time
at 450V input is about 18ms, measured by when the
input switch shuts off. So using an input switch
series resistor of 20k, this resistor is dissipating
about 10watts for 18ms, and then 0watts once the
circuit is started. Is that safe to use a 1206 package
or even smaller resistor for this?

Also the fet dissipates about 1.5watts over the whole
input voltage range, and at 450V input, there are very
short 700watt power spikes going through the fet. Is
this ok for this fet to handle?

http://parts.digikey.com/1/parts/1015931-mosfet-nch-900v-5a-to220sis-...

It says 5amp continuous current, but 700watt spikes seem high :)

I think I could increase the switching frequency and/or increase
the inductance to reduce these spikes but the low voltage operation
may be effected by this. Right now at 30Volts input, the circuit
is only outputting about 8.5volts, I would like to increase this
to at least 10V if possible to do without increasing the heat
dissipation too much at 450V.

Are these 3 coil ferrite transformers an off the shelf part?
I would rather modify the circuit to work with an off the
shelf transformer than to get one custom wound to my trial
and error "specifications" :)

cheers,
Jamie

Ouch, 1.5 watts dissipation in a part in a supply that's only
supplying a watt?? Something needs to change there, I think. I don't
have time to run the simulation and analyze it, but you should
probably look more at the max current in the MOSFET. If it stays
within the rating, you should be OK.

You're asking for a huge input voltage range. Why so large? Most
commercial switching modules operate over a 2:1 range or a little
more, and they are designed by people with quite a bit of experience.
It's not that it can't be done, but you need to make tradeoffs that
will almost certainly cause you to pick parts that will lower the
efficiency: you have to handle higher currents at low input voltage,
and still handle high voltages. Back when you were suggesting 100 to
400 volts, that seemed at least feasible, but now you're talking about
going to 30 volts on the low end?

You MAY be able to find an off-the-shelf transformer, but especially
since you want such a high ratio of max:min input voltages, the design
of the transformer is going to be a lot touchier than if you are doing
a narrow range. I've pretty much always just wound my own for protos,
and/or contracted with a transformer manufacturer. I notice CoilCraft
has a few stock switcher transformers, but I have no idea if any will
be even close to what you need.

I'm going to need to bow out of this for a while. I hope you're able
to get to where you want with this, but I suspect you'll need to make
some compromises to get something workable. I still think for such
low power that one of the chips designed for use in compact off-line
switchers, with built-in mosfet switch, will be a good solution. I
think they've thought through many of the things you've been
struggling with. EVEN IF you have to put a circuit in front of it to
boost low input voltages a bit, it will likely be a simpler and more
efficient solution.

Cheers,
Tom

.

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