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Posted on 20 April 2020

Power Electronic Switches: Zero Voltage Switching and Zero Voltage Resonant Switches

 

 

 

 

 

 

 

Switching sequences in power electronics are either inductive (current-impressed) or capacitive (voltage-impressed).

Zero-voltage switches (ZVS) can be actively turned off. They are passively turned on in the zero-crossing of the switch voltage (Vsub>S= 0). Active turn-off produces very low losses if a sufficiently high parallel capacitance is selected. These lower switching losses allow for higher switching frequencies compared to hard switching.

The possibility to control switching that now remains calls for the use of "pulse shift modulation" (PSM). In our case of circuits with zero-voltage switches, this control process is also known as "phase-angle control". A typical example is a voltage-impressed parallel resonant converter.

Zero-voltage resonant switches (ZVRS) are to be considered a borderline case of the ZVS. A ZVRS actively turns off exactly at the point of zero-crossing of the applied alternating commutation voltage. This alternating voltage ensures current commutation between two switches. Even for a very low capacitance in the commutation circuit, the switching losses are lower than for the ZVS.

This effect occurs in combination with the loss of a further control option, because the turn-off moments are determined by the zero-voltage-crossing given by the outer circuit. In circuits with ZVRS, only indirect control of the energy flow is possible, where switches conduct and block currents across several periods of alternating current. This is referred to as "pulse density modulation" (PDM) or even "pulse group modula­tion".

Conclusion: The price for reducing switching losses is a loss of control options. The turn-on losses of zero-voltage resonant switches are lower than in zero-voltage switches. They are caused by the change in charge of the junction capacitances in the power semiconductors involved.

 

For more information, please read:

Four Types of Switching Processes

Power Electronic Switches: Neutral Switches

An Advantage of Hard Switching

 

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