Solid state tesla coils - general notes

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5. Switch dead time and SSTCs

In typical push-pull, half-bridge, and full-bridge SMPS designs it is very desirable to add some extra delay between one mosfet turning off and  the next one turning on. This extra time ensures that the first mosfet is really fully off when the other one starts to turn on, so that the accident of two mosfets being on simultaneously, thus shorting out the mains, can not happen.
The extra time is also called "dead time", and is automatically added to the drive signals in SMPS controllers ICs like LM3524, UC3846, and the like. 


The ferrite core transformer primary voltage waveform when driving the secondary base at the
TC resonant frequency with a dead time of about 10%.

The above picture shows the effect of dead time on the transformer primary voltage waveform of a SSTC. The "jumps" in the picture occur during the dead time, when all mosfets are off and the primary is not connected to the supply rail(s). I.e. during this time the primary is floating freely (at least for a short instant...)

The jumps in pri voltage are mainly caused by the TC secondary coil. Even though current flow in the primary coil was interrupted by switching off the mosfets, the TC secondary will continue oscillating and feed the transformer secondary winding.

This causes the voltage accross the primary to rise towards the supply rails, until the voltage is high enough (350V peak) to force the freewheeling diodes of the bridge to conduct. When the diodes are conducting, they are hooking up the primary to the supply rails and will support the current flow into the bridge. => the TC secondary is driving the bridge, and there will be conduction losses in the freewheeling diodes (~ V_forward * I_conducted)..

The longer the dead time of the driver IC (or the shorter the drive signal duty cycle), the longer will be the time during which the TC resonantor current that is mirrored to the transformer primary side is flowing through the freewheeling diodes. The current is still sinusoidal, so the longer the dead time or the shorter the duty cycle, the higher the current will be at the instant the mosfets are turned off. This is also the case later as the mosfets are turned on again.  This amounts to increased power dissipations in the freewheeling diodes, which can be rather bad unless you use TO-220 packaged diodes which can be mounted on their own heatsinks.
A larger diode conduction current has the bad effect of increasing the reverse recovery time, i.e. turning off of the diodes gets slower.

There's even more trouble ahead, because switching the mosfets off at  high currents (hard switching) causes not only higher switching losses and larger voltage spikes in stray inductances, but also nasty high frequency ringing in the bridge and radiated HF power => possible RF interference problems with other equipment. Using close to 50% duty and switching in-tune with the TC secondary i.e. at zero current (soft switching) will minimize spikes L * dI/dt and ringing, so there are almost no RFI problems caused by the bridge.

The final thing to note here is that a longer dead time, which is equivalent to shorter on-times / smaller drive signal duty cycle, will lead to less energy transferred to the secondary per cycle because power isn't applied as long as it could be. Controlling the max power and energy transfer to the load is desireable for regulated switch mode power supplies to restrict output voltage and current and do some other things too, but, for SSTCs, one'd probably want max power transfer and nevermind mosfet current ratings, so this would be a reason more to keep dead time at a minimum and the drive signal close to 50% duty cycle.

 

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