Rate of change

If we're going to understand how a SMPS (Switch Mode Power Supply) can be so small we must first comprehend why conventional supplies are so big. If you have ever looked inside a power amplifier you'll note just how big, big is. Take a look at the enclosed photo. This is the inside of the new BHK Signature power amplifier we're releasing in April. I put an arrow pointing towards the transformer. This beast weighs 35 pounds and consumes 3/4 of the inside of the chassis. It is the definition of big. For reference, look at the tubes on the other end of the photo. We could replace this massive hunk of metal with a SMPS weighing less than a couple of pounds and consuming no more than 1/10th the real estate. Why we chose not to is another story, of which we'll get to when the time is appropriate. First we need to understand what these things are and how they work. Why are conventional transformers so big? The answer is deceptively simple. Frequency. What comes out of our home's wall sockets is AC (Alternating Current). AC is voltage that alternates between plus and minus, 50 or 60 times per second, in the same way as flipping a battery around that many times. Because we, like most manufacturers, build our products to work around the world and don't want to have multiple models, all our transformers are 25% larger than they would be if we sold only to 60Hz countries. Indeed, the difference in transformer size between a 50Hz country and a 60Hz country is almost 25% greater. Let's see why. Remember we talked about one of my hero's, Michael Faraday? He's one of the stars of my video, Coal to Coltrane of several years past (available as a free DVD in 5.1 if you're interested). One of his many accomplishments was to quantify the parameters of converting electricity to magnetic energy and back again; the basis of how a transformer works. Faraday's Law describes this interesting fact: the amount of energy transferred from input to output in a transformer is proportional to the rate of change. Ok, that sounds fancy. What's it mean? Remember we are putting AC into our transformer? And that AC means changing from plus to minus at a rate of either 50 or 60 times per second? That is our rate of change, referred to in Faraday's Law. The faster the AC, the more energy is transferred. The opposite is also true. Let's put this in very simple terms. Buckets. Filling and emptying a bucket 50 times a second will net you 50 buckets worth of water every second. Filling the same bucket 500 times a second will net you 10 times more water. Therefore, if you only needed 50 buckets worth of water within a 1 second period, you could make the bucket 10 times smaller if you filled it 500 times. Or 100 times smaller if you filled it 5,000 times. Make sense? In a transformer, our bucket is called a core and is one of three basic elements: an input and output coil of wire with a lump of iron in the middle. The iron is made in sheets (big stand up transformer) or wound in a circle (toroid, like what's shown in the picture of the amp). This iron is like a magnetic capacitor. It stores energy and releases energy, providing a link coupling the input coil of wire with the output coil of wire. The lower the frequency of the incoming AC, the bigger our core (bucket) has to be. The converse is true. So that's why a conventional transformer is so big. If only we could figure out a way to make the incoming AC frequency faster...