On the resonant PWM techniques
9:17 PM
There are two kinds of resonant switching schemes known as zero voltage switching (ZVS) and zero current switching (ZCS). The former makes the voltage across the switching device zero before the switching transition and the later makes the current through the switching device zero. They are highly efficient because by forcing the voltage or current zero makes the power product in each switching transient zero. The voltage and/or current are made zero by using an additional LC tank. This means that any conventional hard-switched power electronic circuit can be operated at resonant conditions with LC tank circuit.These converters are also categorized on the basis of resonance cycle therefore, the one in which a complete cycle of resonance is allowed is known as a full-wave circuit and the circuit in which only positive half cycle of resonance is allowed is called as a half-wave mode. In the half wave resonant circuit the negative cycle of resonance is blocked using a diode[2].
The resonant converters require high voltage/ current stress, therefore the device rating for such converters is higher than those of hard switched converters. Also, the resonant converters have more components requirement compared with the conventional converters.
Points to ponder about ZVS
- In ZVS a capacitor is connected in shunt with the active switch. In full wave operation, the same is connected across the switch diode combination.
- They are naturally recommended for metal oxide semiconductor MOSFET up to kW power rating[1].
- It eliminates the turn on losses (caused by the energy stored in the drain-source capacitance that is dissipated in the switch during turn on) but offers high turn-off losses (caused by the switch having simultaneous non-zero current and voltage while it is turning off).
- ZVS is dependent on load current and input voltage.
- The operation of ZVS is difficult at low load and high input voltage. This is because the energy stored in the inductor at light load is not sufficient to discharge the resonant capacitor before the active switch is turned on[2].
- Compared with ZCS it has high voltage stress proportional to load voltage[2].
- The auxiliary switch in a duty cycle controlled ZVS converter will experience ZCS [2].
- In this method, an inductor is connected in series with the active switch.
- This method is preferred for IGBT because of the low turn-off losses.
- This method eliminates the turn-off losses.
- In the half-wave mode, a diode is connected in series with the switch to make the power flow unidirectional [2].
- It has high current stress and capacitive turn-on the loss.
- The auxiliary switch in a duty cycle controlled ZCS converter will experience ZVS [2].
[1] Canesin, C.A. and, Barbi, I., 1999. A novel single-phase ZCS-PWM high-power-factor boost rectifier. IEEE Transactions on Power Electronics, 14(4), pp.629-635.
[2] Divakar, B.P. and Sutanto, D., 1999. Optimum buck converter with a single switch. IEEE Transactions on Power Electronics, 14(4), pp.636-642.
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