Ferrite Transformer

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When I started this project I didn't understand how one determined the number of turns to put on the primary coupling transformer. There are several factors to consider. First, the wire needs to be able to handle the current. If you are dealing with high frequencies, the majority of the current is conducted on the surface. This is the skin effect. You will need to have several insulated strands to increase the surface area; these strands will need to be twisted in order to reduce eddy currents. As you pack more wire into the space, heating becomes more significant. If your wire is not robust enough you might need a cooling system.

The power to your system has a voltage and a current. If you have the means to run high voltages, you can adjust your windings to keep the primary current low enough to reduce the heating of your transformer and switches. If I want to keep the primary current low I need more turns on the primary. As long as I have enough voltage, the same primary current will yield a much larger secondary current. Let's go over an example:

 

My transformer has 10 turns on the primary and one on the secondary (this is the resonant tank). Let's assume that the load across the secondary is 1 ohm. If I have 100v on the primary, a 10:1 transformer gives us 10v on the secondary. 10A of secondary current requires 1A of primary current. The power draw is 100W. If I want to draw less current I can wind a 20:1 transformer. Now, 200v on the primary results in 10v on the secondary. The current is still 10A on the secondary, but it is 0.5A on the primary. This means that as long as I have a higher voltage supply, I can reduce the current my inverter requires, and still maintain the same power to my workpiece. If I have 400v available, I can draw the same 1A on the primary, but have 20A available on the secondary.

When heating small pieces of metal with small coils, the current demand will go up quickly as there is little material to quench the tank. You want a lot of turns on the primary in order to keep the current draw low while still supplying a lot of current to the tank. If you plan on heating large pieces of metal, the tank gets quenched and the current draw will be too low for effective heating. You need less turns on the primary in order to provide a higher excitation voltage to the tank.

Let's look at another example where the workpiece is quenching the tank. In this case you don't have enough voltage to get an adequate current to flow in the tank. If you have 200v on a 20:1 transformer you will have 10v on the secondary. If the load is 1R you will have 10A on the secondary and 0.5A in the primary. If our maximum voltage is 200v we need to draw more current, making sure our switches can handle this of course. By changing to a 10:1 transformer we get 20v @ 20A on the secondary; the primary we have 200v @ 2A. We are drawing more power and we have doubled the output current at the expense of needing to deal with four times the primary current. As long as the primary circuit can handle this we have solved the problem. As you go lower on the turns you need to make sure you do not saturate the core. Also remember that a small amount of the total primary current is magnetization current.