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SMPS - forward mode


I would like to make power supply with galvanic isolation and output voltage 325V.
By time I will post several queistions here.

# 1) output capacitors on secondary side will be charged with much higher voltage. What ratio of turns it should be compared to primary side (325V)? I am thinking about 1:3 to 1:4? It should deliver power around 3kW (version D) and another version B - 200W.

Image is taken from danyk.cz widely know as DiodeGoneWild. I tried to contact him cause I am czech too but it seems his mail is old.



Active Member
If the output voltage is equal to the input voltage, then a turns ratio of 1:1 should be good enough. If you have feedback from secondary back to primary and a 1:1 turns ratio then the duty cycle should settle down at approximately 50%. If the turns ratio is higher (1:2) then the duty cycle will change to accommodate this.

There are 2 things to consider: if you have high turns on the primary and low turns on the secondary, the secondary side experiences a high current, but the primary FET experiences a low voltage stress. If the turns ratio is low on the primary and high on the secondary, the primary current is high, but the voltage stress on the secondary rectifier circuit is low.

Having a duty cycle equal to Vin:Vout, balances the current and voltage stresses.

Now, this is true for flybacks. It has been ages since I designed a forward converter, but I think the theory is true for this

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