Power Electronics Chapter 5 DC to DC Converters (Choppers)

Outline connection of multiple DC/DC converters 5.1 Basic DC to DC converters 5.2 Composite DC/DC converters and connection of multiple DC/DC converters 5.3 Isolated DC to DC converters (Indirect DC to DC converters )

5.1 Basic DC to DC converters 5.1.1 Buck converter (Step-down converter) 5.1.2 Boost converter (Step-up converter) 5.1.3 Buck-Boost converter (Step-down/step- up converter) and Cuk converter 5.1.4 Sepic converter and Zeta converter

5.1 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

Dc component of switch output voltage Fourier analysis: Dc component = average value

Insertion of low-pass filter to remove switching harmonics and pass only dc component

Basic operation principle of buck converter Buck converter with ideal switch Realization using power MOSFET and diode

Thought process in analyzing basic DC/DC converters Basic operation principle (qualitative analysis) How does current flow 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

Actual output voltage waveform of buck converter Buck converter containing practical low-pass filter Actual output voltage waveform v(t) = V + vripple(t)

The small ripple approximation v(t) = V + vripple(t) In a well-designed converter, the output voltage ripple is small. Hence, the waveforms can be easily determined by ignoring the ripple:

Buck converter analysis: inductor current waveform

Inductor voltage and current subinterval 1: switch in position 1

Inductor voltage and current subinterval 2: switch in position 2

Inductor voltage and current waveforms

Determination of inductor current ripple magnitude

Inductor current waveform during start-up transient

The principle of inductor volt-second balance: Derivation

Inductor volt-second balance: Buck converter example

The principle of capacitor charge balance: Derivation

Boost converter example

Boost converter analysis

Subinterval 1: switch in position 1

Subinterval 2: switch in position 2

Inductor voltage and capacitor current waveforms

Inductor volt-second balance

Conversion ratio M(D) of the boost converter

Determination of inductor current dc component

Continuous-Conduction-Mode (CCM) and Discontinuous-Conduction-Mode (DCM) of buck V + - M R L VD i o u G Electronics Power 28

Continuous-Conduction-Mode (CCM) and Discontinuous-Conduction-Mode (DCM) of boost Electronics Power 29

5.2 Composite DC/DC converters and connection of multiple DC/DC converters 5.2.1 A current-reversible chopper 5.2.2 Bridge chopper (H-bridge DC/DC converter) 5.2.3 Multi-phase multi-channel DC/DC converters

5.2.1 A current reversible chopper Electronics 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 Power 31

Bridge chopper (H-bridge chopper) Can be considered as the combination of two current-reversible choppers. Can realize 4-quadrant operation of DC motor.

Multi-phase multi-channel DC/DC converter 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.

5.3 Isolated DC to DC converters (Indirect DC to DC converters ) Inverter Transformer Rectifier Filter DC input 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 Power semiconductor devices usually used Inverter part: Power MOSFETs, IGBTs Rectifier part: Fast recovery diodes, Schottky diodes, Synchronous rectifiers

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

5.3.1 Forward converter Simple, low cost Uni-polar transformer current, low power applications

5.3.2 Flyback converter Simple, low cost Uni-polar transformer current, low power applications

5.3.3 Half bridge converter Cost higher than forward and flyback converter Bi-polar transformer current, up to several kilowatts

5.3.4 Push-pull converter Cost higher than forward and flyback converter Center-tapped transformer

5.3.5 Full-bridge converter Cost is even higher Bi-polar transformer current, up to several hundreds of kilowatts

5.3.6 Rectifier circuits in the isolated DC to DC converters Full-wave rectifier Full-bridge rectifier Synchronous rectifier

5.3.7 Configuration of switching power supply Linear power supply Line frequency AC input Rectifier Filter Series Pass Regulator Transformer DC Regulated DC output Line frequency Isolation Switching power supply High frequency AC High frequency Line frequency AC input Regulated DC output AC Rectifier Filter Inverter Transformer Rectifier Filter DC Isolation Indirect DC to DC converter 42