1. Power Electronics
Chapter 5
DC to DC Converters
(Choppers)

2. 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 )

3. 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

4. 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

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

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

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

8. 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

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

10. 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:

11. Buck converter analysis: inductor current waveform

12. Inductor voltage and current subinterval 1: switch in position 1

13. Inductor voltage and current subinterval 2: switch in position 2

14. Inductor voltage and current waveforms

15. Determination of inductor current ripple magnitude

16. Inductor current waveform during start-up transient

17. The principle of inductor volt-second balance: Derivation

18. Inductor volt-second balance: Buck converter example

19. The principle of capacitor charge balance: Derivation

20. Boost converter example

21. Boost converter analysis

22. Subinterval 1: switch in position 1

23. Subinterval 2: switch in position 2

24. Inductor voltage and capacitor current waveforms

25. Inductor volt-second balance

26. Conversion ratio M(D) of the boost converter

27. Determination of inductor current dc component

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

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

30. 5.2 Composite DC/DC converters and connection of multiple DC/DC converters
A current-reversible chopper
Bridge chopper (H-bridge DC/DC converter)
Multi-phase multi-channel DC/DC converters

31. 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

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

33. 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.

34. 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

35. 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

36. 5.3.1 Forward converter Simple, low cost
Uni-polar transformer current, low power applications

37. 5.3.2 Flyback converter Simple, low cost
Uni-polar transformer current, low power applications

38. 5.3.3 Half bridge converter
Cost higher than forward and flyback converter
Bi-polar transformer current, up to several kilowatts

39. 5.3.4 Push-pull converter
Cost higher than forward and flyback converter
Center-tapped transformer

40. 5.3.5 Full-bridge converter
Cost is even higher
Bi-polar transformer current, up to several hundreds of kilowatts

41. 5.3.6 Rectifier circuits in the isolated DC to DC converters
Full-wave rectifier
Full-bridge rectifier
Synchronous rectifier

42. 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