SMPS questions

[sakistsalikis]

I am going to design a new off line half bridge switch mode power supply. The input voltage will be 230V AC and the output will be +-20V
8A. I would appreciate if you answered some questions:

1) How can i calculate the output inductors?
2) How can i calculate the output capacitors?
3) Is there a way to calculate an RC snubber for the primary side to protect mosfets from voltage spikes?
4) How can i choose the right mosfets for this smps? I mean: are 3A/40W mosfets suitable or they will burn out?

[luka]

Hi

I clearly see that you didn't search much, or you would find this thread.
Here you have all you need to make one.

[sakistsalikis]

I searched the entire topic but i only found answear to question 1.
I did not find something about 2,3,4.

[cerrem]

Originally posted by sakistsalikis


1) How can i calculate the output inductors?

First need to know the topology....
Output L is calculated based on the requirement for output current ripple....

2) How can i calculate the output capacitors?

Usually this is calculated based on the requirement for output voltage ripple.... at higher switching frequencies the ESR dominates over the C value....then usually low ESR, ceramics are used for bypass of the larger capacitor, since a larger cap is needed for placing the POLE in the loop response...


3) Is there a way to calculate an RC snubber for the primary side to protect mosfets from voltage spikes?

Yes...... but requires knowledge of the transformer parasitics..if you designed the transformer, then you can calculate these things..plus the FET capacitance....it is quicker to simply start out with a bit more C than needed and work your way down in the lab till you get the transient under control but still have a descent waveform....

4) How can i choose the right mosfets for this smps? I mean: are 3A/40W mosfets suitable or they will burn out?

It is imperative to know the topology being used, this way you are aware of the stresses that will be presented...then in addition there are some transients and also you should consider worst case voltages..... Choose the FET for the current capabilty as well as give yourself some safe headroom above the voltage stresses......

Chris