Power Electroni cs Chapter 3 DC to DC Converters (Choppers) Power E l e ct r o n i cs 2 Outline 3.1 Basic DC to DC converters 3.1.1 Buck converter (Step-down converter) 3.1.2 Boost converter (Step-up converter) 3.1.3 Buck-Boost converter (Step-down/step-up converter) and Cuk converter 3.1.4 Sepic converter and Zeta converter 3.2 Composite DC/DC converters and connection of multiple DC/DC converters 3.2.1 A current-reversible chopper 3.2.2 Bridge chopper (H-bridge DC/DC converter)bridge DC/D 3.2.3 Multi-phase multi-channel DC/DC converters Power E l e ct r o n i cs 3 3.1 Basic DC to DC converters Introduction—Buck converter SPDT switch changes dc component Switch output voltage waveform Duty cycle D: 0 ≤ D ≤ 1 complement D’: D’ = 1 - D Power E l e ct r o n i cs 4 Dc component of switch output voltage Fourier analysis: Dc component = average value Power E l e ct r o n i cs 5 Insertion of low-pass filter to remove switching harmonics and pass only dc component Power E l e ct r o n i cs 6 Basic operation principle of buck converter Buck converter with ideal switch Realization using power MOSFET and diode Power E l e ct r o n i cs 7 Thought process in analyzing basic DC/DC converters Basic operation principle (qualitative analysis) – How does current flows during different switching states – How is energy transferred during different switching states Verification of small ripple approximation Derivation of inductor voltage waveform during different switching states Quantitative analysis according to inductor volt-second balance or capacitor charge balance Power E l e ct r o n i cs 8 Actual output voltage waveform of buck converter Buck converter containing practical low-pass filter Actual output voltage waveform v(t) = V + v ripple (t) Power E l e ct r o n i cs 9 The small ripple approximation v(t) = V + v ripple (t) In a well-designed converter, the output voltage ripple is small. Hence, the waveforms can be easily determined by ignoring the ripple: Power E l e ct r o n i cs 10 Buck converter analysis: inductor current waveform Power E l e ct r o n i cs 11 Inductor voltage and current subinterval 1: switch in position 1 Power E l e ct r o n i cs 12 Inductor voltage and current subinterval 2: switch in position 2 Power E l e ct r o n i cs 13 Inductor voltage and current waveforms Power E l e ct r o n i cs 14 Determination of inductor current ripple magnitude Power E l e ct r o n i cs 15 Inductor current waveform during start-up transient Power E l e ct r o n i cs 16 The principle of inductor volt-second balance: Derivation Power E l e ct r o n i cs 17 Inductor volt-second balance: Buck converter example Power E l e ct r o n i cs 18 The principle of capacitor charge balance: Derivation Power E l e ct r o n i cs 19 Boost converter example Power E l e ct r o n i cs 20 Boost converter analysis Power E l e ct r o n i cs 21 Subinterval 1: switch in position 1 Power E l e ct r o n i cs 22 Subinterval 2: switch in position 2 Power E l e ct r o n i cs 23 Inductor voltage and capacitor current waveforms Power E l e ct r o n i cs 24 Inductor volt-second balance Power E l e ct r o n i cs 25 Conversion ratio M(D) of the boost converter Power E l e ct r o n i cs 26 Determination of inductor current dc component E V +  M R L VD i o E M u o i G t t t O O O CCM mode T E i G t on t off i o i 1 i 2 I 10 I 20 t 1 u o O O O t t t T EE DCM mode i G i G t on t off i o t x i 1 i 2 I 20 t 1 t 2 u o E M Power E l e ct r o n i cs 27 Continuous-Conduction-Mode (CCM) and Discontinuous-Conduction-Mode (DCM) of buck Power E l e ct r o n i cs t t T E i O O b) t O T O E t c) i 1 i 2 I 10 I 20 I 10 t on t off u o i o i 1 i 2 t 1 t 2 t x t on t off I 20 u o CCM mode DCM mode E M VDL V a) E M u o Continuous-Conduction-Mode (CCM) and Discontinuous-Conduction-Mode (DCM) of boost 28 E L V 1 VD 1 u o i o V 2 VD 2 E M M R 29 Power E l e ct r o n i cs Can be considered as a combination of a Buck and a Boost Can realize two-quadrant ( I & II) operation of DC motor: forward motoring, forward braking 3.2 Composite DC/DC converters and ```` ` connection of multiple DC/DC converters A current reversible chopper Power E l e ct r o n i cs 30 Bridge chopper (H-bridge chopper) E L R +  V 1 VD 1 u o V 3 E M V 2 VD 2 i o V 4 VD 3 VD 4 M Can be considered as the combination of two current-reversible choppers. Can realize 4-quadrant operation of DC motor. Power E l e ct r o n i cs 31 Multi-phase multi-channel DC/DC converter tO t t t t t t t O O O O O O O u 1 u 2 u 3 u o i 1 i 2 i 3 i o C L E M V 1 V 2 V 3 i 1 i 2 i 3 VD 3 VD 2 L 1 L 2 L 3 i o u o u 1 u 2 u 3 VD 1 Current output capability is increased due to multi-channel paralleling. Ripple in the output voltage and current is reduced due to multi- channel paralleling. Ripple in the input current is reduced due to multi-phase paralleling.