Solid State Tesla Coils

Some SSTC basics to introduce them. Solid State Tesla Coils, in their earlier stage (see my "Old Projects") were basically just a power amplifier driving a secondary coil at resonance. The most common method for switching power into the resonator is a half-bridge or full-bridge of transistors (typically MOSFETs or IGBTs for their high power capabilities). Basically, line voltage is rectified (and sometimes filtered) to get a DC power supply of a few hundred volts. This DC is then fed into the bridge and comes out as an AC square wave into the TC's primary coil. Switching frequency can be determined by a simple oscillator, or by using feedback from the secondary coil itself. I have always preferred the later method as no tuning is required.

From here the DRSSTC was developed. The idea is to make not only the secondary a tuned circuit, but to also have a tuned primary circuit. There are 2 great advantages here. Firstly, resonant voltage rise allows us to achieve several kilovolts on the primary coil, which gives a much better impedance match to the secondary resonator. Also, by the fact that there is a greater voltage across the tank circuit, this implies that the current must also be much greater to support this voltage. A typical DRSSTC running around 1kw of input power might generate around 6kv and 600amps, peak, within the tank circuit. To keep the H-bridge from failing during these stressful conditions, we operate the coil in "bursts" of RF. A typical burst may last 200us, and transfer 10J or so into the secondary coil during this period. Duty cycles are typically less than 5%. It is important to mention that IGBTs must be used to handle these great currents. IGBTs do not have an ON-state resistance, rather they have a voltage drop, similar to a diode. IGBTs generally are much better for high current applications where I*I*R losses would burn up most MOSFETs.