1. 7. Introduction to DC/DC Converters
Marc T. Thompson, Ph.D.
Adjunct Associate Professor of Electrical Engineering
Worcester Polytechnic Institute
Thompson Consulting, Inc.
9 Jacob Gates Road
Harvard, MA
Phone: (978)
Website:
Portions of these notes excerpted from the CD ROM accompanying Mohan, Undeland and Robbins, Power Electronics Converters, Applications and Design, 3d edition, John Wiley 2003
Power Electronics
Chapter 7 Introduction to DC/DC Converters

2. Chapter 7 Introduction to DC/DC Converters
Summary
Non-isolated (i.e. no transformer) DC/DC converters
Power Electronics
Chapter 7 Introduction to DC/DC Converters

3. Block Diagram of Typical AC Input, Regulated DC Output System
Power Electronics
Chapter 7 Introduction to DC/DC Converters

4. Stepping Down a DC Voltage
In this example, the average value of the output voltage = DVin where D is the DUTY CYCLE in PWM (pulse-width modulation) control
D = ton/Ts
Power Electronics
Chapter 7 Introduction to DC/DC Converters

5. Chapter 7 Introduction to DC/DC Converters
Step-Down (Buck) DC-DC Converter
Add LC filter to reduce switching ripple
Flyback diode also needed
Power Electronics
Chapter 7 Introduction to DC/DC Converters

6. Buck Converter: Waveforms
Steady state; inductor current flows continuously
Waveform below for buck in continuous conduction mode
Power Electronics
Chapter 7 Introduction to DC/DC Converters

7. Buck Converter: SPICE Circuit
Circuit shown: fsw = 200 kHz, D = 0.5
Power Electronics
Chapter 7 Introduction to DC/DC Converters

8. Buck Converter: Startup Waveforms
Power Electronics
Chapter 7 Introduction to DC/DC Converters

9. Analysis for DC/DC Converter in Continuous Conduction and Steady State
In steady state, the inductor current returns to the same value every switching cycle, or every T seconds
Therefore, the inductor ripple current UP equals ripple DOWN
Several assumptions to simplify analysis:
Periodic steady state --- all startup transients have died out
Small ripple --- ripple is small compared to average values
Power Electronics
Chapter 7 Introduction to DC/DC Converters

10. Buck Converter in Continuous Conduction
In continuous conduction, buck converter has 2 states --- switch OPEN and switch CLOSED
Switch closed (for time DT)
Switch open (for time (1-D)T)
Power Electronics
Chapter 7 Introduction to DC/DC Converters

11. Buck Converter in Continuous Conduction
The inductor ripple current UP equals ripple DOWN
We already knew this result from first principles, but this methodology of inductor Volt-second balance can be used to evaluate other more complicated DC/DC converters
Power Electronics
Chapter 7 Introduction to DC/DC Converters

12. Buck Converter: Waveforms at the Boundary of Cont./Discont. Conduction
ILB = critical current below which inductor current becomes discontinuous
Power Electronics
Chapter 7 Introduction to DC/DC Converters

13. Buck Converter: Discontinuous Conduction Mode
Steady state; inductor current discontinuous (i.e. it goes zero for a time)
Note that output voltage depends on load current
Power Electronics
Chapter 7 Introduction to DC/DC Converters

14. Buck: Limits of Discontinuous Conduction
The duty-ratio of 0.5 has the highest value of the critical current
For low output current, buck goes discontinuous
Power Electronics
Chapter 7 Introduction to DC/DC Converters

15. Buck: Limits of Cont./Discont. Conduction
In regulated power supply, Vd may fluctuate but Vo is kept constant by control of D
Power Electronics
Chapter 7 Introduction to DC/DC Converters

16. Buck Conv.: Output Voltage Ripple
ESR is assumed to be zero; continuous conduction mode
Power Electronics
Chapter 7 Introduction to DC/DC Converters

17. Buck Conv.: Output Voltage Ripple
ESR is assumed to be zero
Power Electronics
Chapter 7 Introduction to DC/DC Converters

18. Buck Conv.: Calculations
Shown for SPICE example with fsw = 200 kHz, D = 0.5, L = 33 µH, C = 10 µF, Io = 1A
Power Electronics
Chapter 7 Introduction to DC/DC Converters

19. Buck: SPICE Result in Periodic Steady State
Analysis shows inductor ripple = 0.38 A-pp, output voltage ripple = 24 mV-pp, confirmed by SPICE
Power Electronics
Chapter 7 Introduction to DC/DC Converters

20. Pulse-Width Modulation (PWM) in DC-DC Converters
Power Electronics
Chapter 7 Introduction to DC/DC Converters

21. Step-Up (Boost) DC-DC Converter
Output voltage must be greater than the input
Power Electronics
Chapter 7 Introduction to DC/DC Converters

22. Boost Converter Waveforms
Continuous current conduction mode
Switch closed:
Switch open:
Inductor Volt-second balance:
Power Electronics
Chapter 7 Introduction to DC/DC Converters

23. Boost: Limits of Cont./Discont. Conduction
The output voltage is held constant
For low load current, current conduction becomes discontinuous
Power Electronics
Chapter 7 Introduction to DC/DC Converters

24. Boost Converter: Discont. Conduction
Occurs at light loads
Power Electronics
Chapter 7 Introduction to DC/DC Converters

25. Boost: Limits of Cont./Discont. Conduction
The output voltage is held constant
Power Electronics
Chapter 7 Introduction to DC/DC Converters

26. Boost Converter: Effect of Parasitics
The duty-ratio D is generally limited before the parasitic effects become significant
Power Electronics
Chapter 7 Introduction to DC/DC Converters

27. Boost Converter Output Ripple
ESR is assumed to be zero
Assume that all the ripple component of diode current flows through capacitor; DC component flows through resistor
Power Electronics
Chapter 7 Introduction to DC/DC Converters

28. Step-Down/Up (Buck-Boost) Converter
The output voltage can be higher or lower than the input voltage
Note output phase inversion
Power Electronics
Chapter 7 Introduction to DC/DC Converters

29. Buck-Boost Converter: Waveforms
Continuation conduction mode
Switch closed:
Switch open:
Inductor Volt-second balance:
Power Electronics
Chapter 7 Introduction to DC/DC Converters

30. Buck-Boost: Limits of Cont./Discont. Conduction
The output voltage is held constant
Power Electronics
Chapter 7 Introduction to DC/DC Converters

31. Buck-Boost: Discontinuous Conduction
This occurs at light loads
Power Electronics
Chapter 7 Introduction to DC/DC Converters

32. Buck-Boost Converter: Limits of Cont./Discont. Conduction
The output voltage is held constant
Power Electronics
Chapter 7 Introduction to DC/DC Converters

33. Buck-Boost Converter: Effect of Parasitics
The duty-ratio is limited to avoid these parasitic effects from becoming significant
Power Electronics
Chapter 7 Introduction to DC/DC Converters

34. Buck-boost Converter: Output Voltage Ripple
ESR is assumed to be zero
Power Electronics
Chapter 7 Introduction to DC/DC Converters

35. Chapter 7 Introduction to DC/DC Converters
Cuk DC-DC Converter
The output voltage can be higher or lower than the input voltage
Capacitor C1 is primary means of storing and transferring energy from input to output
When switch is ON, C1 discharges through the switch and transfers energy to the output
When switch is OFF, capacitor C1 is charged through the diode by energy from the input and L1
Power Electronics
Chapter 7 Introduction to DC/DC Converters

36. Cuk DC-DC Converter: Waveforms
The capacitor voltage is assumed constant (very large)
Note phase inversion at the output
Power Electronics
Chapter 7 Introduction to DC/DC Converters

37. Chapter 7 Introduction to DC/DC Converters
SEPIC Converter
Single-ended primary inductance converter (SEPIC)
Can buck or boost the voltage
Note that output is similar to buck-boost, but without a phase inversion
Power Electronics
Chapter 7 Introduction to DC/DC Converters

38. Converter for DC-Motor Drives
Four quadrant operation is possible
For:
DC motor drives
DC to AC inverters for UPS
Power Electronics
Chapter 7 Introduction to DC/DC Converters

39. Chapter 7 Introduction to DC/DC Converters
Converter Waveforms
Bi-polar voltage switching
Power Electronics
Chapter 7 Introduction to DC/DC Converters

40. Chapter 7 Introduction to DC/DC Converters
Converter Waveforms
Uni-polar voltage switching
Power Electronics
Chapter 7 Introduction to DC/DC Converters

41. Output Ripple in Converters for DC-Motor Drives
Bi-polar and uni-polar voltage switching
Power Electronics
Chapter 7 Introduction to DC/DC Converters

42. Switch Utilization in DC-DC Converters
It varies significantly in various converters
PT = VTIT where VT and IT are peak switch voltage and current
In direct converters (buck and boost) switch utilization is good; in indirect converter (buck-boost and Cuk) switch utilization is poor
Power Electronics
Chapter 7 Introduction to DC/DC Converters

43. Equivalent Circuits in DC-DC Converters
Replacing inductors and capacitors by current and voltage sources, respectively
Power Electronics
Chapter 7 Introduction to DC/DC Converters

44. Reversing the Power Flow in DC-DC Conv.
For power flow from right to left, the input current direction should also reverse
Power Electronics
Chapter 7 Introduction to DC/DC Converters

45. Real-World Issue: Capacitor ESR
Real-world capacitors have equivalent series resistance (ESR)
This ESR may have dominant effect on output ripple
Power Electronics
Chapter 7 Introduction to DC/DC Converters

46. Effects of Capacitor ESR
Without ESR, output ripple is 24 mV-pp
ESR has increased ripple to approximately 30 mV-pp
Power Electronics
Chapter 7 Introduction to DC/DC Converters