Solid state tesla coils - full-bridge design help

 

Get the Excel full-bridge SSTC design sheet. There's more info on the full-bridge switch mode power supply topology below.

Update: the design sheet might be erroneus in the minimum number of pri turns, off by a factor of 2. I'll check it some time later...

 



This is an ugly schematic of the high-voltage (well, ok, mains voltage...;o) side of a typical full-bridge switching power supply. There are four MOSFETs that act as switches.

 

Full-bridge operation: opposing switches QA and QD are turned on, which makes the left lead of the primary jump to +Vin and the right to -Vin. After some microseconds, QA and QD are turned off, and the other switch pair formed by QB and QC is turned on. This now causes the left lead of the primary to go to -Vin and the right one to +Vin, i.e. the voltage seen by the primary reverses.

Thus the transformer primary sees an alternating voltage of first +Vin..-Vin and then -Vin..+Vin, that is, a square wave of amplitude Vin (the peak-to-peak voltage is 2*Vin). In other words the switches are turned on in such a way that the transformer primary gets driven with a square wave.

Another way of picturing this is that when QA and QD are on, current flows downwards through the transformer. Then you toggle the switches. QB and QC are now on, and current flows upwards through the transformer. It is like you'd connect the primary windings to a car battery, then quickly flip over the leads, flip over again, flip... etc.

 

In the center of everything is the transformer primary that is being driven by this setup. It has a series capacitor (MKP or FKP type polypropylene capacitor) which helps to block out transformer-unhealthy DC voltage.

Note the schottky diodes that are in series with the mosfets. Also note the diodes in parallel with this schottky+mosfet string - these have to be fast-recovery diodes with reverse recovery times of less than 135ns. All this extra trouble is necessary to disable the intrinsic diode of the mosfets, i.e. bypass it so that no current will ever flow through it.

This "diode" is actually a NPN bipolar transistor, but the mosfet source metallization is shorting the base of this transistor to its emitter. The trouble is that a high reverse turn-off current on this diode, especially at a raising but low drain-source voltage on the mosfet, can cause charge to accumulate on the base-emitter junction, thus turning on this intrinsic bipolar transistor at a time when it really should be off. This is not very nice, because if the opposing mosfet in the bridge is turned on at the same time, the first mosfet's intrinsic transistor and the next mosfet will place a short circuit accross the mains. Probably both mosfets will blow up.