500W SMPS topology question.

I read in sci.electronics.design that Pat wrote (in ) about '500W SMPS topology question.', on Sat, 29 Jan 2005:

It would certainly be that! An unpleasant one. Switching power supplies are DIFFICULT, and the higher the power the more difficult. By all means learn how to design them, but start with something far less ambitious,

You haven't told us the input and output voltages or whether it needs to be regulated or not. This is important to the advice you will get.

An unregulated 12V to unregulated 120V 500W DC-DC converter is within the grasp of a beginer to DC-DC but perhaps not to a beginer in electronics.

Everything else is a bad place to start learning.

If you are making a supply this large finding the transformer can be hard. You may have to wind your own or have it wound. This is no fun for the first timer. You could use the transformer from something like a battery charger and perhaps run at as much as 100Hz.

If the input side is at 12V, or so, the current in the transistors is so high that you are forced into using several in parallel. This makes mistakes a lot more costly. It also make for more mistakes. The more transistors you parallel the bigger the trouble with current hogging. (even with mosfets see below).

If the input side is running from rectified mains voltage, I say "stop right now and don't even think about trying this". Rectified mains voltage can killy very dead. A switcher running from it can kill you even deader because it makes about twice the voltage on the primary.

MOSFETs and current hogging:

The temp-co of the on resistance of a MOSFET is positive so they share current well in the DC case.

The threshold voltage has a negitive temp-co. When you parallel MOSFETs and try to switch them, one of them tends to turn on first. If the loading is capacitive in nature, that transistor has much higher switching losses than the rest. This can make for a thermal run-away.

The silly bit is that even if you get it right the first time, buying