Power Electroni cs Chapter 7 Soft-Switching Techniques Power E l e ct r o n i cs 2 Pursuing of higher switching frequency Better waveform ? PWM waveform will be closer to expected waveform ? Harmonics is easier to be filtered Faster response Smaller volume and weight ?4.44fNφ =4.44fNAB ? LC f π2 1 = Benefits Power E l e ct r o n i cs 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 E l e ct r o n i cs 4 Outline 7.1 Basic concepts on soft-switching 7.2 Classification of soft-switching techniques 7.3 Typical soft-switching circuits and techniques Power E l e ct r o n i cs 5 7.1 Basic concepts on soft-switching Hard-switching t V J V J t V J V J t Q t Q 0 0 0 0 a) Turning-on b) Turning-off The process of power semiconductor device hard-switching Power E l e ct r o n i cs 6 The concept of soft-switching Soft-switching a) Turning-on b) Turning-off t V J VJ t V J V J t Q t Q 0 0 0 0 The process of power semiconductor device soft-switching Power E l e ct r o n i cs 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 E l e ct r o n i cs 8 7.2 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) Power E l e ct r o n i cs 9 The concept of basic switch cell VD S L a) L S VD L S VD c) d) VD S L b) Power E l e ct r o n i cs 10 Quasi-Resonant Converter—QRC ZVS QRC ZCS QRC ZVS MRC (multi-resonant converter) Resonant DC link converter S VD L r L C r S VD L r L C r1 C r2 S L r C r VD L a) ZVS QRC b) ZCS QRC c) ZVS MRC Basic switching cells for QRC Power E l e ct r o n i cs 11 ZVS PWM converter and ZCS PWM converter Feature: use of auxiliary switch S L r C r VD L S 1 S VD L r L C r S 1 a) b) Basic switching cells for a) ZVS PWM and b) ZCS PWM Power E l e ct r o n i cs 12 ZVT PWM converter and ZCT PWM converter Feature: auxiliary switch is in parallel with main switch S L r VD L S 1 C r VD 1 L r C r S 1 S VD VD 1 L a) b) Basic switching cells for a) ZVT PWM and b) ZCT PWM Power E l e ct r o n i cs 13 7.3 Typical soft-switching circuits and techniques ZVS QRC Resonant DC link converter Phase-shift full bridge ZVS PWM converter ZVT PWM converter Power E l e ct r o n i cs 14 ZVS QRC U i C r S VD S L r VD L C A R S u S (u Cr ) i S i Lr u VD t 0 t 1 t 2 t 3 t 4 t 6 t 0 t t t t t t 5 O O O O O C r U i + u Cr I L A C r U i + u Cr i Lr t o ~ t 1 t 1 ~ t 2 Power E l e ct r o n i cs 15 Resonant DC link converter Circuit L r U i S C r VD S t 0 t 1 t 2 t 3 t 4 t 0 i Lr u Cr U i I L t t O O Equivalent circuit to RDCL L r U i S C r VD S L i Lr + u Cr I L R Power E l e ct r o n i cs 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 E l e ct r o n i cs 17 Phase-shift full-bridge ZVS PWM converter Circuit S 1 S 2 S 3 S 4 C S1 C S4 C S2 C S3 VD 2 VD 1 L r L A B U i u R C R +  S 1 S 3 S 4 S 2 u AB u Lr i Lr u T1 u R i VD1 i VD2 i L t 0 t 1 t 2 t 3 t 4 t 5 t 6 t 7 t 8 t 9 t 0 t 9 t 8 t t t t t t t t t t t t O O O O O O O O O O O O Power E l e ct r o n i cs 18 Phase-shift full-bridge ZVS PWM converter i Lr i L k T :1 C S1 S 4 L r L VD 1 U i U o + C S2 VD S2 A R i Lr i L C S3 S 2 L r L VD 1 U i U o + C S4 VD S3 VD 2 B R Equivalent circuit during t 3 ~ t 4 Equivalent circuit during t 1 ~ t 2 Power E l e ct r o n i cs 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 E l e ct r o n i cs 20 ZVT PWM converter Circuit L S C r S 1 L r VD 1 VD C U i + U o I L i Lr i VD VD S R Equivalent circuit during t 1 ~ t 2 S S 1 u S i Lr i S1 u S1 i D i S I L t 0 t 1 t 2 t 3 t 4 t 5 t t t t t t t t O O O O O O O O L S 1 L r i Lr U i I L C rVD S Power E l e ct r o n i cs 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