Re: flyback converter driving

Tom Bruhns wrote:
On Oct 19, 2:12 am, Jamie Morken <jmor...@xxxxxxx> wrote:
Hi,

I have a flyback converter that takes an input of 100VDC to 400VDC and
outputs ~15V at 100mA max. I am using ltspice to simulate it and
it works fine, but I am not sure now how to drive the fet as the
only available power supply is the 100V-400V input signal. I would be
ok with using a fixed dutycycle driving the fet, but would prefer to use
an IC like the MB3800 to drive the fet. Any ideas on how to get this
flyback started so that it can power itself? :)

cheers,
Jamie

A typical way is to use a circuit with a low-voltage lockout; in
lockout, it draws only very low current. A capacitance on its power
supply pin is charged from the DC input (100--400V) through a large
resistance so power dissipation in that resistance is negligible.
When the converter power input reaches a high enough voltage, it turns
on, and the cap holds the voltage up long enough to get things
started. Once started, the converter, through another winding on the
core, provides power to run the circuit. Under some circumstances,
that winding can also be used to sense the output voltage and no other
feedback from the output side is necessary to maintain acceptable
regulation. Tight control of the output may require other techniques.

I added the big series resistor and capacitor for this startup function,
but I would have to use a 10K resistor or so to get it working. I think
the problem is that the IC Vin pin is drawing over 200uA, even during
shutdown mode, when the data*** says its should be drawing only 15uA
in shutdown mode.. so I think it could just be a simulator error but it
is hard to know. I tied the shutdown pin to ground and the IC is still
drawing 200uA into the Vin pin, I checked with a 1 ohm series resistor
on it. Any ideas on this, if I should just ignore the 200uA and assume
it is really only drawing 15uA? Dangerous assumption maybe..

cheers,
Jamie



Cheers,
Tom

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