Power E l e ct r o n i cs 1 Chapter 8 Composite Converters Power Electroni cs Power E l e ct r o n i cs 2 The concept of composite converter Composite converter: Combination of two or more converters in cascaded connection Indirect AC to AC converters (AC-DC-AC converters) Composite converters Indirect DC to DC converters (Isolated DC to DC converters) Power E l e ct r o n i cs 3 8.1 Indirect AC to AC converters (AC-DC-AC converters) Classifications According to type of the DC links: Voltage-source type AC-DC-AC converters Current-source type Variable voltage variable frequency (VVVF) (AC-DC-AC frequency converters) According to whether output voltage and frequency is variable: AC-DC-AC converters Constant voltage constant frequency (CVCF) Power E l e ct r o n i cs 4 8.1.1 Configurations of AC-DC-AC converters Configurations with one-direction power flow AC -PBE AC 4PVSDF AC -PBE AC 4PVSDF Voltage-source type Current-source type Power E l e ct r o n i cs 5 Configuration with regenerative energy dissipating circuit AC -PBE AC 4PVSDF V 0 R 0 Braking transistor and braking resistor Power E l e ct r o n i cs 6 Configurations with regenerative power feedback through inversion-mode thyristor rectifier circuit AC -PBE AC 4PVSDF AC -PBE AC 4PVSDF I d U d U L Voltage-source type Current-source type AC -PBE AC 4PVSDF U V W a b c 4PVSDF -PBE Power E l e ct r o n i cs Configurations realizing bi-directional power flow through double-sided PWM converters Voltage-source type Current-source type 7 Power E l e ct r o n i cs 8 8.1.2 Major Applications of AC-DC-AC frequency converters (VVVF converters) Adjustable speed AC motor drives Advantages of AC motors over DC motors Energy saving on AC motors High-performance AC motor drives Control of AC motors driven by AC-DC-AC frequency converters (VVVF converters) Constant voltage frequency ratio control Slip frequency control Vector control Direct torque control Power E l e ct r o n i cs 9 8.1.3 Major Applications of CVCF converters Uninterruptible power supplies (UPS) Basic configuration of UPS Source Electricity utility Rectifier Inverter Load Power E l e ct r o n i cs 10 UPS with back-up energy source Source Electricity utility Rectifier Inverter Load Diesel Engine S 1 2 Power E l e ct r o n i cs 11 UPS with back-up energy source and bypass lines Source Electricity utility Rectifier Inverter Load Diesel Engine 1 2 3 4 Bypass lines S 1 S 2 Power E l e ct r o n i cs 12 8.2 Indirect DC to DC converters ` (isolated DC to DC converters) Inverter Transformer Rectifier Filter DC input AC AC DC output High frequency Isolation Reasons to use indirect DC to DC structure Necessary isolation between input and output In some cases isolated multiple outputs are needed The ratio of input and output voltage is far away from 1 Reduction of transformer and filter volume and weight Power semiconductor devices usually used Inverter part: Power MOSFETs, IGBTs Rectifier part: Fast recovery diodes, Schottky diodes, Synchronous rectifiers Power E l e ct r o n i cs 13 Classification of isolated DC to DC converters According to whether transformer current is uni-direction or bi-directional Single-ended converters ? Forward converter ? Flyback converter Isolated DC to DC converters Double-ended converters ? Half bridge ? Push-pull ? Full bridge Power E l e ct r o n i cs 14 8.2.1 Forward converter Simple, low cost Uni-polar transformer current, low power applications S u S i L i S O t t t t U i O O O + + U i S VD 1 VD 2 L N 1 N 3 N 2 VD 3 U o W 2 W 1 W 3 T t N N U U on 1 2 i o = Power E l e ct r o n i cs 15 8.2.2 Flyback converter Simple, low cost Uni-polar transformer current, low power applications + + U i S VD N 1 N 2 U o W 1 W 2 S u S i S i VD t on t off t t t t U i O O O O off on 1 2 i o t t N N U U = Power E l e ct r o n i cs 16 8.2.3 Half bridge converter Cost higher than forward and flyback converter Bi-polar transformer current, up to several kilowatts + + S 1 S 2 VD 1 VD 2 L U i N 1 N 2 N 3 + u d U o + C 1 C 2 W 1 W 3 W 2 S 1 S 2 u S1 u S2 i S1 i S2 i D1 i S2 t T t t t t t t t t on U i U i i L i L O O O O O O O O T t N N U U on 1 2 i o = Power E l e ct r o n i cs 17 8.2.4 Push-pull converter ++ S 1 S 2 VD 1 VD 2 L U i N 1 N 1 ' N 2 N 2 ' U o S 1 S 2 u S1 u S2 i S1 i S2 i D1 i S2 t on T t t t t t t t t 2U i 2U i i L i L O O O O O O O O T t N N U U on 1 2 i o 2 = Cost higher than forward and flyback converter Center-tapped transformer Power E l e ct r o n i cs 18 8.2.5 Full-bridge converter Cost is even higher Bi-polar transformer current, up to several hundreds of kilowatts + + N 1 S 2 S 3 S 4 VD 1 VD 2 VD 4 VD 3 L S 1 N 2 +  +  u d u T U i U o W 2 W 1 S 1 (S 4 ) S 2 (S 3 ) u S1 (u S4 ) u S2 (u S3 ) i S1 (i S4 ) i S2 (i S3 ) i D1 (i D4 ) i S2 (i S3 ) t on T t t t t t t t t U i U i i L i L O O O O O O O O T t N N U U on 1 2 i o 2 = Power E l e ct r o n i cs 19 8.2.6 Rectifier circuits in the isolated DC to DC converters + VD 1 VD 2 L + VD 1 L VD 3 VD 2 VD 4 + V 1 V 2 L Full-bridge rectifier Synchronous rectifier Full-wave rectifier Linear power supply ? Switching power supply 20 Line frequency AC input Rectifier Filter Series Pass Regulator Regulated DC output Transformer DC InverterFilter Transformer High frequency AC Rectifier Rectifier Filter AC High frequency Regulated DC output Isolation Indirect DC to DC converter Line frequency Isolation 8.2.7 Configuration of witching power supply Line frequency AC input DC