1. Power Electronics Notes 07B Some Real-World Issues in DC/DC Converters
Marc T. Thompson, Ph.D.
Thompson Consulting, Inc.
9 Jacob Gates Road
Harvard, MA
Phone: (978)
Fax: (888)
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© Marc Thompson,

2. Summary Efficiency Component selection (inductors, capacitors)
Off-the-shelf ICs
Electromagnetic interference (EMI) and EMI testing

3. Basic Buck Converter --- Efficiency
This converter suffers from low efficiency at low output voltage, due to finite diode drop
For 1.2V output, a 0.4V diode drop results in 19% loss of efficiency for Vin = 4V
Power loss in diode is (1-D)VDIL assuming constant VD

4. Synchronous Buck Converter --- Efficiency
This is the basic buck with an additional controlled switch to lower losses when diode is on
Higher efficiency than standard buck; more complicated control; typically >10% more efficiency than standard buck
Need to be very mindful of timing in the two switches (shoot–through)
Synchronous
switch

5. Synchronous Buck Converter --- PSIM
“Dead time” circuit ensures that Q1 and Q2 are never on at the same time
Note: 0.5 microsecond dead time is probably excessive

6. Synchronous Buck Converter --- PSIM
Note that after turnon of the diode/switch the voltage drop decreases from 0.5V to 0V. The MOSFET drop is lower than the diode drop
Diode on
Sync. switch on

7. Other Losses in DC/DC Converters
Switch ON resistance (conduction loss)
Turn on/off switching loss
Diode ON voltage
Diode reverse recovery (if any)
Diode reverse current
Inductor ESR
Capacitor ESR
Losses in MOSFET gate driver
PWM and control system power dissipation
Etc., etc.

8. Boost Converter Efficiency
Assume lossy inductor

9. Boost Converter Efficiency
Assume lossy inductor

10. Boost Converter Efficiency vs. Duty Cycle
Calculate efficiency; assume Rind/RL = 0.1

11. Boost Converter Output Voltage vs. D
Calculate effect on output voltage assume Rind/RL = 0.1
Ideal boost
Nonideal boost

12. Pulse-Width Modulation (PWM) in DC-DC Converters

13. Converter Input Filter
Added to reduce ripple component drawn from input source
Reference: Kassakian, et. al., Principles of Power Electronics, pp

14. Converter Input Filter (cont.)
If RC >> T, almost all the ripple current passes through C
Reference: Kassakian, et. al., Principles of Power Electronics, pp

15. Converter Input Filter (cont.)
Reference: Kassakian, et. al., Principles of Power Electronics, pp

16. Example 1: 170W Buck Converter Filter Caps
Po = 170 Watts. Average input current = 10A
Let’s look at the switch current and the RMS current rating on the input filter capacitor (Cf) and output filter capacitor (Cload)
Main output
filter capacitor
(330 F)
Main input
filter capacitor
(22 F) 16

17. Example 1: Buck Converter Input Filter
Switch current is very discontinuous; average = 10.3A, RMS = 12.35A
Look at input capacitor current; RMS capacitor current = 4.8A
Need to spec cap with this RMS value
Switch current
Input capacitor current 17

18. Example 1: Buck Converter Output Filter
RMS current in Cload = 0.27A
Output capacitor current 18

19. Example 2: Capacitor ESR
Real-world capacitors have equivalent series resistance (ESR)
This ESR may have dominant effect on output voltage ripple
Note: sometimes we need to worry about the equivalent series inductance as well

20. Example 2: Effects of Capacitor ESR
Without ESR, output ripple is 24 mV-pp
ESR has increased ripple to approximately 30 mV-pp

21. Impedance of Capacitor

22. Impedance of Capacitor
Resonance

23. Key Things to Specify when Selecting a Capacitor
Temperature coefficient of capacitance
Equivalent series resistance (ESR)
Voltage rating
Ripple current
Series inductance
Polarized or non-polarized
Leakage/dielectric loss
Physical size

24. Electrolytic Capacitors
25V rating

25. Tantalum Capacitors
15F, 35V = 9.2 mJoules; 4.7F, 16V = 0.6 mJoules

26. Ceramic Capacitors
Used mainly for bypassing

27. Mica Capacitor
Low capacitance, low loss

28. Film Capacitor

29. Inductors
Toroids
Planar spiral
Surface-mount

30. Real-World Inductors
Things to spec: inductance, RMS current, peak current, SRF, ESR, package type/dimensions
Reference:

31. Real-World Inductors
Reference:

32. Real-World Inductors
Reference:

33. Real-World Inductors
Reference:

34. Real-World Inductors
Reference:

35. Buck Regulator ICs
Reference:

36. Buck Regulator Internal (integrated) power switch
Reference:

37. Another Buck Regulator (Linear Technology)
External MOSFET switches
Reference:

38. Interleaved Buck Regulator (Fairchild)
Fairchild FAN5090
Reference:

39. Multiphase Buck Regulator (Fairchild)
Fairchild FAN5094
Reference:

40. Boost Regulator (Linear Technology)
Internal switch
Reference:

41. Another Boost Regulator (Linear Technology)
Interleaved outputs
Reference:

42. SEPIC (Linear Technology)
Reference:

43. Cuk (Linear Technology)
Reference:

44. Buck/Boost (Linear Technology)
Reference:

45. Buck/Boost (Linear Technology)
Reference:

46. EMI Let’s look at electromagnetic interference (EMI)
We’ll focus on the switch current isw in the boost converter
Reference: Alexander Kusko and Marc Thompson, Power Quality in Electrical Systems, McGraw-Hill, 2007

47. EMI
Reference: Alexander Kusko and Marc Thompson, Power Quality in Electrical Systems, McGraw-Hill, 2007

48. Example 3: EMI
Find the spectrum of the drain current for a boost converter operating at a switching frequency fsw = 500 kHz, a duty cycle D = 0.5, and with a switch risetime and falltime tr = 25 nanoseconds.
Reference: Alexander Kusko and Marc Thompson, Power Quality in Electrical Systems, McGraw-Hill, 2007

49. Testing for Conducted EMI
Line impedance stabilizing network (LISN)
Reference: Alexander Kusko and Marc Thompson, Power Quality in Electrical Systems, McGraw-Hill, 2007

50. LISN
Reference: Keith Billings, Switchmode Power Supply Handbook, McGraw-Hill, 1999

51. EMI Filter
Reference: Alexander Kusko and Marc Thompson, Power Quality in Electrical Systems, McGraw-Hill, 2007

52. EMI Filter
Reference: On Semiconductor

53. Typical EMI Spectrum from a DC/DC Converter
In this case, fsw ~ 300 kHz DC
1st
2nd
3rd
Reference: Power Integrations, Inc., “Techniques for EMI and Safety,” Application Note AN-15, June 1996 ,

54. Conducted EMI Standards
Covers 150 kHz – 30 MHz
Reference: Power Integrations, Inc., “Techniques for EMI and Safety,” Application Note AN-15, June 1996 ,

55. Conducted EMI Testing Differential mode capacitor
Reference: T. Curatolo and S. Cogger “Enhancing a Power Supply to Ensure EMI Compliance,” EDN, Feb. 17, 2005, pp

56. Conducted EMI Testing Add bypass capacitors
Reference: T. Curatolo and S. Cogger “Enhancing a Power Supply to Ensure EMI Compliance,” EDN, Feb. 17, 2005, pp

57. Conducted EMI Testing Differential-mode choke
Reference: T. Curatolo and S. Cogger “Enhancing a Power Supply to Ensure EMI Compliance,” EDN, Feb. 17, 2005, pp

58. Conducted EMI Testing Common-mode filter
Reference: T. Curatolo and S. Cogger “Enhancing a Power Supply to Ensure EMI Compliance,” EDN, Feb. 17, 2005, pp

59. Radiated EMI Standards
There are also radiated EMI standards
Radiated EMI is tested up to 1000 MHz
Reference: Texas Instruments, Inc.,

60. EMI Testing Lab EMI testing is often done In-house (pre-certification)
At specialty EMI testing labs (Chomerics, Curtis-Straus, etc.)

61. Example 4: Non-Ideal Buck Simulation

62. Example 4: Buck Simulation Results

63. Example 4: Buck Simulation Results
Switch current
In this case, switch current = supply current

64. Example 4: Buck Simulation Results
Switch current spectrum; fsw = 200 kHz
1st
3rd
2nd

65. Example 5: Non-Ideal Buck Simulation with Input Filter

66. Example 5: Non-Ideal Buck Simulation with Input Filter
Switch current
Supply current
Capacitor current

67. Example 5: Buck Simulation Results
Switch current spectrum; fsw = 200 kHz
Switch current
Supply current
Capacitor current