smps problem

Hi;

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

Reply to
ksoner
Loading thread data ...

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, Peter

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!

Reply to
Bushy Pete

ElectronDepot website is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.