1. POWER ELECTRONICS DC-DC CONVERTERS (CHOPPERS) PART 2
SITI ZARINA BINTI MD NAZIRI TMT 404 ADVANCED ENGINEERING 4

2. Content Content Switch-mode Power Supply (SMPS) Transformer Models
Flyback Converter
Full-bridge Converter
Half-bridge 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

3. Switch-mode Power Supply
Advantages over linear power
Efficient (70-95%)
Weight and size reduction
Disadvantages
Complex design
Electromagnetic interference (EMI) problems
However above certain ratings, SMPS is the only feasible choice
Types of SMPS
Flyback
Forward
Push-pull
Bridge (half and full)

4. Switch-mode Power Supply
Block diagrams
Basic Block diagram of linear power supply
Basic Block diagram of SMPS

5. Switch-mode Power Supply
Isolated topologies
All isolated topologies include a transformer, and thus can produce an output of higher or lower voltage than the input by adjusting the turns ratio.
For some topologies, multiple windings can be placed on the transformer to produce multiple output voltages.
Some converters use the transformer for energy storage, while others use a separate inductor.

6. Switch-mode Power Supply
Isolated types
Source: Wikipedia

7. Transformer Models 2 basic functions:
I/O electrical isolation
Step-up/step-down time varying voltages & currents
I/O relationship of idealized model:
Models:
Ideal model
Model used in most PE circuits

8. Flyback Converter
Flyback converter is a buck-boost converter with the inductor split to form a transformer  voltage ratio are multiplied with additional advantage of isolation
Circuit
Equivalent circuit includes magnetizing inductance

9. Flyback Converter
Switch closed
Switch open

10. Flyback Converter Magnetizing inductance current Source current
Diode current
Capacitor current
Transformer primary voltage

11. Flyback Converter
Switch closed

12. Flyback Converter
Switch open

13. Flyback Converter Steady-state Operation Note:
I/O relationship is similar to buck-boost converter, with the present of transformer ratio
Output depends to value of D (greater or less than input)

14. Flyback Converter
Average Inductor Current

15. Flyback Converter Min & Max Inductor Current
For continuous operation, ILm, min = 0

16. Flyback Converter Output Voltage Ripple Equivalent series resistance
Same as buck-boost converter
Equivalent series resistance

17. Full-bridge Converter
D – duty ratio of each switch pair
Circuit

18. Full-bridge Converter
Operation
Switch “pair”: [S1 & S2]; [S3 & S4]
Each switch pair turn on at a time as shown. The other pair is off.
When [S1 & S2] is closed – voltage across transformer primary is Vs
When [S3 & S4] is closed – voltage across transformer primary is -Vs
“AC voltage” is developed across the primary. Then transferred to secondary via high frequency transformers.
On secondary side, diode pair is “high frequency full wave rectification”.
The choke (L) and (C) acts like the “buck converter” circuit.

19. Full-bridge Converter
Waveforms
Switching sequence
Voltage on transformer primary
Voltage vx

20. Half-bridge Converter

21. Half-bridge Converter
Waveforms
Switching sequence
Voltage on transformer primary
Voltage vx

22. Questions/Discussions
Flyback Converter
The flyback converter has the following parameters: Vs = 36 V, D = 0.4, N1/N2 = 2, R = 20 Ω, Lm = 100 μH, C = 50 μF, and the switching frequency is 100 kHz. Determine:
the output voltage
the maximum and minimum inductor current
the output voltage ripple
A flyback converter has an input of 44 V, an output of 3 V, a duty ratio of 0.32, and a switching frequency of 300 kHz. The load resistor is 1 Ω. Determine:
the transformer turns ratio
the transformer magnetizing inductance Lm such the minimum inductor current is 40% of the average.