I have HF H-bridge resonant inverter with phase-shift control applied in IPT system. The current from the inverter depends on the resonant network. What are the required conditions to apply soft switching for the inverter?
The idea behind soft switching is to control the switch when the voltage across or the current through the switch is at zero. The provides the least stress on the switch and can, likely, improve harmonics. The trick is to have a converter where the power varies and crosses zero, like in a SMPS AC-DC converter.
At the switching point, the voltage and power might not be in sync, so you get to pick which to target in the soft switch, voltage or current, and this depends on the converter design. For example, if you can track and switch the voltage as it passes through zero, the power will also be zero because of V*I.
The closer you can switch at a zero crossing, the less switch capacitance (and perhaps inductance) you have to dissipate, the less heat to dissipate and the higher the efficiency of the converter.
Thank you very much Jon for your answer. Is the shape of the current waveform helps in implementing the soft switching. For example if the current is sinusoidal or not, is it make difference during the implementation.
The waveform of the current does not directly make a difference. Skipping the theory about what happens inside the switch for the moment, the control of when to switch is critical to soft switching. As a practical matter, you need to figure out when, in time, the current (or voltage) will transition zero. With that prediction, you then need to determine when to send the gate signal to switch the gate. This will include control delays, wire length delays and the switch (gate) and often the gate driver circuit.
In soft switching, ideally, you can predict then the signal will cross zero and knowing the delays in the control circuits, switch the gate on a cycle by cycle basis. Remember, the goal in soft switching is switching the gate when it is conducting zero power, and that is when the current or voltage (or both) are crossing zero.
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