Power Electronics Chapter 8 Soft-Switching Techniques

Power Electronics 2 Pursuing of higher switching frequency Benefits Better waveform PWM waveform will be closer to expected waveform Harmonics is easier to be filtered Faster response Smaller volume and weight 4.44fN  fNAB

Power Electronics 3 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

Power Electronics 4 Outline 8.1 Basic concepts on soft-switching 8.2 Classification of soft-switching techniques 8.3 Typical soft-switching circuits and techniques

Power Electronics Basic concepts on soft-switching Hard-switching a) Turning-on b) Turning-off The process of power semiconductor device hard-switching

Power Electronics 6 The concept of soft-switching Soft-switching a) Turning-on b) Turning-off The process of power semiconductor device soft-switching

Power Electronics 7 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

Power Electronics 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)

Power Electronics 9 The concept of basic switch cell

Power Electronics 10 Quasi-Resonant Converter—QRC ZVS QRC ZCS QRC ZVS MRC (multi-resonant converter) Resonant DC link converter Basic switching cells for QRC

Power Electronics 11 ZVS PWM converter and ZCS PWM converter Feature: use of auxiliary switch Basic switching cells for a) ZVS PWM and b) ZCS PWM

Power Electronics 12 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

Power Electronics Typical soft-switching circuits and techniques ZVS QRC Resonant DC link converter Phase-shift full bridge ZVS PWM converter ZVT PWM converter

Power Electronics 14 ZVS QRC t o ~ t 1 t 1 ~ t 2 t o ~ t 1 t 1 ~ t 2

Power Electronics 15 Resonant DC link converter Circuit Equivalent circuit to RDCL

Power Electronics 16 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

Power Electronics 17 Phase-shift full-bridge ZVS PWM converter Circuit

Power Electronics 18 Phase-shift full-bridge ZVS PWM converter Equivalent circuit during t 1 ~ t 2 Equivalent circuit during t 3 ~ t 4

Power Electronics 19 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

Power Electronics 20 ZVT PWM converter Circuit Equivalent circuit during t 1 ~ t 2

Power Electronics 21 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