1. POWER ELECTRONICS DC-DC CONVERTERS (CHOPPERS) PART 1
SITI ZARINA BINTI MD NAZIRI TMT 404 ADVANCED ENGINEERING 4

2. Schedule Class/Week Topic Reading Assessment Lecture 1
DC-DC Converters (Choppers)
(Part 1)
Daniel W. Hart
(Chapter 6)
Quiz
Lecture 2
(Part 2)
(Chapter 7)
Lecture 3
DC-AC Converters (Inverters)
(Chapter 8) -
Lecture 4
Presentation (?)
A schedule design for optional periods of time/objectives.

3. Content Content Introduction Bulk Converter Boost Converter
Buck-Boost Converter
Questions/Discussions
(provided in separate sheet)
Introductory notes.
SOURCES: “Power Electronics”, Daniel W. Hart, Mc-Graw Hill, “Power Electronics & Drives”, Lecture Notes, Dr. Zainal Salam, UTM

4. DC-DC CONVERTER (CHOPPERS)
DEFINITION:
Converting the unregulated DC input to a controlled DC output with a desired voltage level.
General block diagram:
APPLICATIONS:
Switched-mode power supply (SMPS), DC motor control, battery chargers

5. Linear Regulator Transistor is operated in linear (active) mode.
Output voltage:
The transistor can be conveniently modeled by an equivalent variable resistor, as shown.
Power loss is high at high current due to:

6. Switching Regulator Power loss is zero (for ideal switch):
when switch is open, no current flow in it,
when switch is closed no voltage drop across it.
Since power is a product of voltage and current, no losses occurs in the switch.
Power is 100% transferred from source to load.
Switching regulator is the basis of all DC-DC converters

7. Buck Converter
Circuit
Switch closed,
diode OFF
Switch open,
diode ON

8. Bulk Converter Waveforms closed open Inductor voltage Inductor current
Capacitor
current

9. Bulk Converter
Switch closed, diode off

10. Bulk Converter
Switch open, diode on

11. Bulk Converter Steady-state Operation
Steady-state operation requires that iL at the end of switching cycle is the same at the beginning of next cycle.
The change of iL over one period is zero, i.e:

12. Bulk Converter
Average, Max & Min Inductor Current

13. Bulk Converter

14. Bulk Converter
Output Voltage Ripple

15. Bulk Converter (Design)
Calculate D to obtain required output voltage.
Select a particular switching frequency:
preferably >20KHz for negligible acoustic noise
higher fs results in smaller L, but higher device losses. Thus lowering efficiency and larger heat sink. Also C is reduced.
Possible devices: MOSFET, IGBT and BJT. Low power MOSFET can reach MHz range.

16. Bulk Converter (Design)
Determine Lmin. Increase Lmin by about 10 times to ensure full continuous mode.
Calculate C for ripple factor requirement.
Capacitor ratings:
must withstand peak output voltage
must carry required RMS current. Note RMS current for triangular w/f is Ip/3, where Ipis the peak capacitor current given by ΔiL/2
Wire size consideration:
Normally rated in RMS. But iL is known as peak. RMS value for iL is given as:

17. Bulk Converter
Equivalent Series Resistance (ESR)

18. Boost (Step-up) Converter
Circuit
Switch closed,
diode OFF
Switch open,
diode ON

19. Boost Converter Waveforms Inductor voltage Diode current
Inductor current
Capacitor current

20. Boost Converter
Switch closed, diode OFF

21. Boost Converter
Switch open, diode ON

22. Boost Converter Steady-state Operation
Boost converter produces output voltage that is greater or equal to the input voltage.
Alternative explanation:
when switch is closed, diode is reversed. Thus output is isolated. The input supplies energy to inductor.
When switch is opened, the output stage receives energy from the input as well as from the inductor. Hence output is large.
Output voltage is maintained constant by virtue of large C.

23. Boost Converter
Output Voltage Ripple

24. Boost Converter
Average, Max & Min Inductor Current

25. Boost Converter
For continuous operation,

26. Buck-Boost Converter Circuit Switch closed, diode OFF Switch open,
diode ON

27. Buck-Boost Converter Waveforms Inductor current Inductor voltage Diode
Capacitor
current

28. Bulk-Boost Converter
Switch closed, diode OFF

29. Bulk-Boost Converter
Switch open, diode ON

30. Bulk-Boost Converter
Steady-State Operation

31. Bulk-Boost Converter Output Voltage
Output of a buck-boost converter either be higher or lower than the source voltage.
If D>0.5, output is higher
If D<0.5, output is lower
Output voltage is always negative
Note that output is never directly connected to load. Energy is stored in inductor when switch is closed and transferred to load when switch is opened.

32. Bulk-Boost Converter Inductor current Inductor voltage Diode current
Capacitor current

33. Bulk-Boost Converter Average Inductor Current
Assuming no power loss in the converter, power absorbed by the load must equal power supplied the by source, i.e.
But average source current is related to average inductor current as:
Substituting for Vo:

34. Bulk-Boost Converter Max & Min Inductor Current
For continuous current,

35. Bulk-Boost Converter
Output Voltage Ripple