Download Presentation
## Power Electronics

- - - - - - - - - - - - - - - - - - - - - - - - - - - E N D - - - - - - - - - - - - - - - - - - - - - - - - - - -

**PowerElectronics**Chapter 8 Soft-Switching Techniques**Pursuing of higher switching frequency**• Better waveform • PWM waveform will be closer to expected waveform • Harmonics is easier to be filtered Benefits • Smaller volume and weight • 4.44fNf =4.44fNAB Faster response**Disadvantages of higher switching frequency**Disadvantages • Higher power losses on power semiconductor devices • Power loss=f x Energy loss at each switching • More severe electromagnetic interference (EMI) • Steeper edges introduce more noises • Easier to be radiated Solution • Soft-switching techniques**Outline**8.1 Basic concepts on soft-switching 8.2 Classification of soft-switching techniques 8.3 Typical soft-switching circuits and techniques**8.1 Basic concepts on soft-switching**Hard-switching b) Turning-off a) Turning-on The process of power semiconductor device hard-switching**The concept of soft-switching**Soft-switching b) Turning-off a) Turning-on The process of power semiconductor device soft-switching**Two types of soft-switching**ZVS—Zero-voltage switching • Specifically means zero-voltage turn-on, i.e., the voltage across the device is reduced to zero before the current increases ZCS—Zero-current switching • Specifically means zero-current turn-off, i.e., the current flowing through the device is reduced to zero before the voltage increases**8.2 Classification of soft-switching techniques**• Quasi-resonant soft-switching • ZVS PWM (zero-voltage-switching PWM) and ZCS PWM (zero-current-switching PWM) • ZVT PWM (zero-voltage-transition PWM) and ZCT PWM (zero-current-transition PWM)**Quasi-Resonant Converter—QRC**• ZVS QRC • ZCS QRC • ZVS MRC (multi-resonant converter) • Resonant DC link converter Basic switching cells for QRC**ZVS PWM converter and ZCS PWM converter**Feature: use of auxiliary switch Basic switching cells for a) ZVS PWM and b) ZCS PWM**ZVT PWM converter and ZCT PWM converter**Feature: auxiliary switch is in parallel with main switch Basic switching cells for a) ZVT PWM and b) ZCT PWM**8.3 Typical soft-switching circuits and techniques**• ZVS QRC • Resonant DC link converter • Phase-shift full bridge ZVS PWM converter • ZVT PWM converter**to ~ t1 t1 ~ t2**S t O u ( u ) S C r t O i S t O i L r t O u VD t O t t t t t t t t 0 1 2 3 4 6 5 0 ZVS QRC**Resonant DC link converter**Circuit Equivalent circuit to RDCL**Summary of QRC**• Disadvantages • Voltage stress increased due to the resonant peak • Current RMS value increased due to large circulating energy • Variable switching frequency due to pulse-frequency modulation (PFM) control**Circuit**Phase-shift full-bridge ZVS PWM converter**Equivalent circuit during**t1 ~ t2 Equivalent circuit during t3 ~ t4 Phase-shift full-bridge ZVS PWM converter**Summary of ZVS or ZCS PWM converter**• Improvement over QRC • Voltage and current are basically square-wave, therefore stresses are greatly reduced • Constant switching frequency due to pulse-width modulation (PWM) control**Circuit**ZVT PWM converter Equivalent circuit during t1 ~ t2**Summary of ZVT or ZCT PWM converter**• Improvement over ZVS or ZCS PWM converter • Soft-switching can be achieved in a wider range of input voltage and load current • Circulating energy is reduced to minimum so that efficiency is increased