Line filter before or after diode bridge?

Posted by: m2babaey on 03/05/2014

I have tried designing various power supplies using PI expert suit with TOPSwitch-GX

I have noticed that when I request for a design for a 13V 1A output, The software suggest a 6mH coupled inductors (elf18n016) and 2 capacitors after diode bridge

When I want a higher power (like 24V 1 A or 13V 2A) the software places this filter before diode bridge

I'd like to understand more about the reason and differences between these 2 solutions.

Any help would be extremely appreciated

Comments

There are not a solid rule for where to place the input filter if before or after the rectifier

One reason to place the inductor before the rectifier is to reduce the current when a surge in voltage apeard at the line voltage. if you please post both schematics I may give you more reasons why

Thanks for your response

Here are the 2 schematics:

(atached)

Thanks again for your comments

By my experience, when you are adjusting your filter to pass EMI specification, you need to play with the placement of the filter becasue you do not know which solution is better for you.

I sumbited your entry to the software department and lets wait for they answer.

I will let you know as soon as I have an answer

Answer from software team

The decision about the position of the filter is based on empirical data took over many designs. For designs with POUT < 20 watts the software selects a PI filter. For designs over 20 watts , the software choose an inductor at the input.