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Posted on 19 April 2019

Loss Reduction in Power Electronics Switches by Soft Switching

 

 

 

 

 

 

 

Soft switching is a possible way of reducing losses in power electronic switches. The expression "soft switching" actually refers to the operation of power electronic switches as zero-voltage switches (ZVS) or zero-current switches (ZCS). The many different converter circuits that working according to these principles are generally assigned to resonance or quasi-resonance technology.

Zero Voltage Switching (ZVS)

Operating point characteristic of switch current and voltage during soft/resonant switching as ZVS

Figure 1Operating point characteristic of switch current and voltage during soft/resonant switching as ZVS

  • The commutation process is started by active turn-off, switching losses are reduced thanks to parallel connection of commutation capacitance CK
  • The commutation process is completed by passive, low-loss turn-on at a switch voltage vs ≈ 0
  • Before next commutation, the direction of current flow changes in the switch that is turned on with impressed di/dt
  • Inductance in commutation circuit LK should be at the minimum

Sample circuit diagrams for commutation circuits with ZVS and a series resonant converter

Figure 2. Sample circuit diagrams for commutation circuits with ZVS and a series resonant converter

Zero Current Switching (ZCS)

Operating point characteristic of switch current and voltage during soft/resonant switching as ZCS

Figure 3. Operating point characteristic of switch current and voltage during soft/resonant switching as ZCS

  • The commutation process is started by active turn-on, switching losses are reduced thanks to series connection of commutation inductance LK
  • The commutation process is completed by passive, low-loss turn-off at a switch current iS ≈ 0,
  • Before next commutation, the voltage direction changes in the switch that is turned on with impressed di/dt
  • Capacitance in commutation circuit CK should be at the minimum

Sample circuit diagrams for commutation circuits with ZCS and a parallel resonant converter

Figure 4. Sample circuit diagrams for commutation circuits with ZCS and a parallel resonant converter

Continuous soft switching is based on the condition that only one kind of commutation process - either inductive commutation/ZCS or capacitive commutation/ZVS – takes place in the commutation circuit of the converter. Owing to this restriction, the loss of one control possibility as compared with hard switching has to be accepted.

This is only achieved if the polarities of the driving commutation voltage vK or the commutated output current iL are reversed between two identical commutation processes.

Operating point characteristic of switch current and voltage for hard switching (IGBT, MOSFET) and typical circuit diagram for voltage source inverter

Figure 5. Operating point characteristic of switch current and voltage for hard switching (IGBT, MOSFET) and typical circuit diagram for voltage source inverter

The IGBT, MOSFET and diodes available today were developed and optimized almost exclusively for hard switch applications and display comparable features for this area of application (Figure 5).

Extensive investigations over the past years, however, ([1], [2], [3]) have demonstrated that the different component structures and technologies behave very differently during soft switching. And yet the datasheets available at the moment do not make these differences apparent to the user.

A comparison of the track of the operating points in figures 1, 3, and 5 shows that the area encircled by the track is different in size. The area is a relative measure of the power losses that occur during switching. During hard switching (Figure 5) the area is at its maximum. In the case of ideal resonant switching, the track of the operating point would go along the axes of the coordinate system. In real resonant or quasi-resonant switching and commutation processes, the resulting path will lie somewhere between the two aforementioned ideal conditions (Figures 1 and 3).

References:
[1] Helsper, M.: "Analyse und Verbesserung des Verhaltens von Planar- und Trench-IGBT Modulen in hart bzw. weich schaltenden Applikationen", diss., Christian-Albrechts-Universität Kiel, 2003, Shaker Verlag, Aachen, 2004, ISBN 3-8322-2621-4
[2] Reimann, T.: "Verhalten abschaltbarer Leistungshalbleiterbauelemente im ZVS-Mode", diss., TU Ilmenau, 1994, Verlag Shaker, Aachen, 1995, ISBN 3-8265-0538-7
[3] Bernet, S.: "Leistungshalbleiter als Nullstromschalter in Stromrichtern mit weichen Schaltvorgängen", diss., TU Ilmenau, 1995, Verlag Shaker, Aachen, 1995, ISBN 3-8265- 0762-2

 

For more information, please read:

Switching Loss Reduction Networks - Snubber Circuits

Basic Operating Modes of Power Semiconductors

Four Types of Switching Processes

 

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