smps problem

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hope the questions is relevant with the group:

I am designing a battery charger which gives 13.8V 7A or 27.6v 3.5A.
The topology is flyback and I decide the output(27V or 13V) with two
jumpers. the problem is that when I switch from 13.8 to 27.6V, VCC also
increases ()and I dont want VCC to increase above 30V because the FPS
of fairchild shuts down. So I put a 22V 1W zener diode and a resistor
to limit the current of zener on VCC circuitry. but those dissipate too
much power and zener live on the limits. Is there a nice method(fast
and cheap) to limit the voltage on VCC rather than using a
zener+resistor. thanks in advance

Re: smps problem

If I understand your circuit from your description, you are disconnecting
one jumper and inserting it on a second jumper. Your problem is occurring
while the jumper is disconnected.

Change your system to one jumper and with the jumper removed you get one
voltage and the with the jumper connected you get the other.

Use two resistors that will give you the alternate resistance in parallel,
or the other resistance with just the other one. I'm sure you can cope with
resistors in parallel calculations!

This way, your jumpers are never both disconnected and you don't have to
worry about unconnected voltages.....

And you will be able to get rid of the "Brother-in-Law Diode"

Hope this helps,

p.s. A "Brother-in-Law Diode" is one that is in circuit for no other reason
than the designer's Brother-in-Law owns the diode factory!

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