Yesterday I explained that the higher the incoming AC frequency to a power transformer, the smaller that transformer can be to deliver energy. In fact, get the frequency high enough and you can reduce a 50 pound transformer the size of a breadbox to several ounces and size of a pack of playing cards.
But wait a minute! Our AC in the wall is only 60Hz, meaning it is changing from plus to minus sixty times a second, and to reduce our power transformer to the size of a matchbook we need to increase that by a factor of 1000. How is that possible?
Here's where we learn where a Switch Mode Power Supply got its name. Imagine placing an on/off switch at the output of your AC receptacle and this switch is turning on and off 100,000 times a second. Whatever you connected to this AC receptacle would then be receiving the power in the wall in short littlebursts - on and off - which is all you really need to make a magnetic field go on and off in a coil of wire (remembering that putting power into a coil of wire turns it into a magnet). Our transformer is only two coils of wire: one connected to this switched AC and the other close by converting the magnetic field back into power again and bingo! You have transferred energy safely and efficiently.
So this input switch, sometimes called a chopper (because it chops up the power), is nothing more than a transistor turning on and off. Simple switchers aren't all that tricky and are part of the reason why SMPS have a bad reputation (more about that shortly).
Now that we have reduced the size of our transformer by chopping up the incoming AC, pretty much everything else after the transformer is the same old technology used in a linear supply - converting the AC to DC and storing some energy to smooth out all these nasty transitions.
So that's how these babies work. But to make one work well is quite another matter.
You can imagine that chopping up the AC causes all sorts of nasty issues on the power line itself which is sensitive to noises andinterruptionsin current flow. In fact, think about how crazy we as Audiophiles get with a little bit of cell phone and RF noises on our power lines - we spend thousands of dollars cleaning these up with everything from Power Plants, Noise Harvesters and power conditioners of all kinds. Then, think about just how brutal one of these switch mode power supplies is to the line!
So tomorrow, let's start looking at updating some of our well founded fears about these amazing little supplies.
