Chop, chop

Back to how switching power supplies work. I explained in my previous post why conventional power transformers are so big. Now it's time to find out how a switching power supply can become small. And, like a good Agatha Christie mystery, we already know the answer: higher frequency. But, as in the same author's mysteries, we just don't know how to get those higher frequencies until we get to the end of the story and find out who dunnit. What comes out of our wall sockets are slow moving AC at 50 or 60 times a second. What we need is 50,000 times a second. Then we'd be working with minuscule transformers. I previously described to you that our new fangled switching power supply has the same three basic elements to it as all other power supplies: transformer (isolate and voltage change), diode bridge (convert AC to DC) and capacitors (gives us smoother DC). What's different is a 4th element, called a chopper, placed in front of everything else. A chopper does exactly what it sounds like. It chops. Chop, chop. It's really nothing more than a high speed on/off switch that turns the incoming power on and off as often as you like. In the case of a switch mode power supply, we like 50,000 times a second, maybe even faster. Chop, chop, chop. Our chopping switch is fed DC through a simple arrangement of yet another diode bridge in front of this switch. It does not have a transformer. So, in theory, it is pretty simple and now you can follow the path: Incoming AC-> AC/DC converter-> chopper-> small transformer-> AC/DC converter-> capacitor-> clean DC out That's it. And yet. Designing and building a SMPS is one of the most challenging engineering tasks I can think of. There's elements of black magic involved in their design. So simple in concept, so difficult in execution. I'll try and cover some reasons why tomorrow.