This document describes the operation and modeling of one-switch and two-switch flyback converters. It discusses the ideal and non-ideal cases, comparing the circuits with and without parasitic components. The one-switch converter has issues with resonance from leakage inductance, while the two-switch topology clamps voltages and recycles leakage energy. Simulation models of the circuits are presented, showing the effects of parasitic capacitances and inductances. Component mismatches and their impacts are also analyzed. Finally, losses including conduction, forward voltage, and switching are calculated and compared between the converter designs.

1. Development of SiC-Based PEBB 1000
July 28, 2016
PWM DC-DC
Flyback Converters
Pedro Campos Fernandes
Jun Wang

2. 1. One-Switch Flyback Converter
 Advantages
 Simplicity: fewer semiconductor and magnetic components
 Low cost
 Disadvantages
 Resonance caused by the leakage inductance and the device junction capacitances
 High-frequency ringing and EMI
July 28, 2016 2Development of SiC-Based PEBB 1000

3. 2. Ideal One-Switch Flyback Converter
 Circuit components:
 Q: Ideal MOSFET Switch
 D: Ideal Rectifier Diode
 C: Output Capacitance
 RL: Load Resistance
 Vin: Input Voltage Source
 I1: Input Current (primary)
 LM: Magnetizing Inductance
 IM: Magnetizing Current
 I2: Diode Current (secondary)
 T: Ideal Flyback Transformer
July 28, 2016 3Development of SiC-Based PEBB 1000

4. 2. Ideal One-Switch Flyback Converter
 Simulation model:
July 28, 2016 4Development of SiC-Based PEBB 1000

5. 2.1. CCM Operation
2.1.1. First Stage: DTS
 Q is ON
 D is OFF
 Energy from the DC source
is stored in LM
July 28, 2016 5Development of SiC-Based PEBB 1000

6. 2.1. CCM Operation
2.1.2. Second Stage: (1-D)TS
 Q is OFF
 D is ON
 Transformer voltage reverses
forward-biasing the rectifier diode
and delivering energy to the output
July 28, 2016 6Development of SiC-Based PEBB 1000

7. 3. Non-Ideal One-Switch Flyback Converter
 Circuit components:
 Q: MOSFET Switch
 D: Rectifier Diode
 C: Output Capacitance
 RL: Load Resistance
 Vin: Input Voltage Source
 I1: Input Current (primary)
 LM: Magnetizing Inductance
 IM: Magnetizing Current
 I2: Diode Current (secondary)
 T: Flyback Transformer
 CJ: Diode Junction Capacitance
 CDS: Drain-Source Capacitance
 Lleak: Leakage Inductance
July 28, 2016 7Development of SiC-Based PEBB 1000

8. 3. Non-Ideal One-Switch Flyback Converter
 Simulation model:
July 28, 2016 8Development of SiC-Based PEBB 1000

9. 3.1. CCM Operation
3.1.1. First Stage: DTS
 Subinterval 1:
 Q is switching from OFF to ON
 D is switching from ON to OFF
 Switch transient ringing
July 28, 2016 9Development of SiC-Based PEBB 1000

10. 3.1. CCM Operation
3.1.1. First Stage: DTS
 Subinterval 2:
 Q is effectively ON
 D is effectively OFF
 No switch transient ringing
July 28, 2016 10Development of SiC-Based PEBB 1000

11. 3.1. CCM Operation
3.1.2. Second Stage: (1-D)TS
 Subinterval 3:
 Q is switching from ON to OFF
 D is switching from OFF to ON
 Switch transient ringing
July 28, 2016 11Development of SiC-Based PEBB 1000

12. 3.1. CCM Operation
3.1.2. Second Stage: (1-D)TS
 Subinterval 4:
 Q is effectively OFF
 D is effectively ON
 No switch transient ringing
July 28, 2016 12Development of SiC-Based PEBB 1000

13. 3.2. Ideal Case vs. Parasitic Case
July 28, 2016 Development of SiC-Based PEBB 1000 13

14. 3.2. Ideal Case vs. Parasitic Case
July 28, 2016 Development of SiC-Based PEBB 1000 14

15. 4. Two-Switch Flyback Converter
 Advantages
 Maximum switch voltage is clamped to the DC input voltage Vin
 Leakage inductance energy is also clamped and recycled back to the DC
input source (improve efficiency)
 Reduced switching and conduction losses
July 28, 2016 Development of SiC-Based PEBB 1000 15

16. 5. Non-Ideal Two-Switch Flyback Converter
 Circuit components:
 Q1,2: Symmetrical MOSFET Switches
 D1,2: Symmetrical Clamping Diodes
 D: Rectifier Diode
 C: Output Capacitance
 RL: Load Resistance
 Vin: Input Voltage Source
 I1: Input Current (primary)
 LM: Magnetizing Inductance
 IM: Magnetizing Current
 I2: Diode Current (secondary)
 T: Flyback Transformer
 CJ: Diode Junction Capacitance
 CDS1,2: Drain-Source Capacitances
 Lleak: Leakage Inductance
July 28, 2016 16Development of SiC-Based PEBB 1000

17. 5. Non-Ideal Two-Switch Flyback Converter
 Simulation model:
July 28, 2016 17Development of SiC-Based PEBB 1000

18. 5.1. CCM Operation
5.1.1. First Stage: DTS
 Subinterval 1:
 Q1, Q2 are switching from OFF to ON
 D is switching from ON to OFF
 D1, D2 are OFF
 Switch transient ringing
July 28, 2016 18Development of SiC-Based PEBB 1000

19. 5.1. CCM Operation
5.1.1. First Stage: DTS
 Subinterval 2:
 Q1, Q2 are effectively ON
 D is effectively OFF
 D1, D2 are OFF
 No transient ringing
July 28, 2016 19Development of SiC-Based PEBB 1000

20. 5.1. CCM Operation
5.1.2. Second Stage: (1-D)TS
 Subinterval 3:
 Q1, Q2 are switching from ON to OFF
 D is switching from OFF to ON
 D1, D2 are OFF
 Voltage spike
July 28, 2016 20Development of SiC-Based PEBB 1000

21. 5.1. CCM Operation
5.1.2. Second Stage: (1-D)TS
 Subinterval 4:
 Q1, Q2 are switching from ON to OFF
 D is effectively ON
 D1, D2 are ON
 Switch voltages VDS1, VDS2 are clamped
to Vin
July 28, 2016 21Development of SiC-Based PEBB 1000

22. 5.1. CCM Operation
5.1.2. First Stage: DTS
 Subinterval 5:
 Q1, Q2 are switching from ON to OFF
 D is ON
 D1, D2 are switching from ON to OFF
 Switch transient ringing
July 28, 2016 22Development of SiC-Based PEBB 1000

23. 5.1. CCM Operation
5.1.2. First Stage: DTS
 Subinterval 6
 Q1, Q2 are effectively OFF
 D is ON
 D1, D2 are effectively OFF
 No transient ringing
July 28, 2016 23Development of SiC-Based PEBB 1000

24. 5.2. One-Switch vs. Two-Switch
July 28, 2016 Development of SiC-Based PEBB 1000 24

25. 5.2. One-Switch x Two-Switch
July 28, 2016 Development of SiC-Based PEBB 1000 25

26. 5.3. Component Mismatches
 Real applications do not provide perfect symmetry
between junction capacitances and gate driver signals
 There are mismatches between these variables and
they lead to different behaviors during the converter
operation
July 28, 2016 Development of SiC-Based PEBB 1000 26

27. 5.3. Component Mismatches
 Two types of mismatch will be covered:
 20% mismatch on drain-source capacitance of the low side
MOSFET switch Q2 given the high side MOSFET switch Q1
as reference
 5% delay (given the period as reference) on the gate driver of
the low side MOSFET switch Q2
July 28, 2016 Development of SiC-Based PEBB 1000 27

28. 5.3. Component Mismatches
5.3.1. Capacitance Mismatch
 CDS1 = 120 pF and CDS2 = 144 pF
 Q1 is clamped earlier than Q2, i.e.,
D1 starts conducting earlier than D2
 Q1, Q2 voltages reach different
steady values after the ringing dies
July 28, 2016 Development of SiC-Based PEBB 1000 28

29. 5.3. Component Mismatches
5.3.2. Gate Drive Delay Mismatch
 DelayQ1 = 0 s and DelayQ2 = 0.17 us
 Q2 turns ON later, so Q1 will be clamped
earlier
 Q1 turns ON earlier, leading to another
clamping action during the delayed turn ON
of Q2
July 28, 2016 Development of SiC-Based PEBB 1000 29

30. 5.3. Component Mismatches
5.3.3. Merged Mismatch
July 28, 2016 Development of SiC-Based PEBB 1000 30

31. 5.3. Component Mismatches
 Possible Solutions
 Design an integrated solution with complete control circuit and
gate drive for both high side (Q1) and low side (Q2) switches
 Work with safety margins so that the circuit can still present
good performance for a certain percentage of mismatch
July 28, 2016 Development of SiC-Based PEBB 1000 31

32. 6. Power Losses on Flyback Converters
 Design considerations:
 Zero winding resistances (rprimary = rsecondary = 0)
 Zero leakage resistance
 The MOSFET switches and clamping diodes are considered
symmetrical to each other
 Two-Switch topology - switches model: IRF510
 100 V, 5 A, ron,max = 0.85 Ω and CDS = 60 pF
 One-Switch topology - switch model: IRF840
 500 V, 8 A, ron,max = 0.54 Ω and CDS = 120 pF
 Rectifier Diode model: MBR10100
 100 V, 10 A, VF = 0.65 V and rF = 20 mΩ with CJ = 200 pF
 Clamping Diodes model: MBR10100
 100 V, 10 A, VF = 0.65 V and rF = 20 mΩ with CJ = 0 F
July 28, 2016 Development of SiC-Based PEBB 1000 32

33. 6. Power Losses on Flyback Converters
 Losses presented by the design:
 Conduction Losses
 Forward Voltage Losses
 Switching Losses
July 28, 2016 Development of SiC-Based PEBB 1000 33

34. 6.1. Conduction Losses
6.1.1. MOSFET Switches Q1, Q2
 Since Q1, Q2 are symmetrical to each other, conduction losses will be given by:
𝑃𝑐𝑜𝑛𝑑,𝑄1 = 𝑃𝑐𝑜𝑛𝑑,𝑄2 = 𝑟𝑂𝑁,𝑄 𝐼 𝑅𝑀𝑆,𝑄
2
= 𝑟𝑂𝑁,𝑄 𝐼 𝑅𝑀𝑆,1
2
6.1.2. Rectifier Diode D3
𝑃𝑐𝑜𝑛𝑑,𝐷3 = 𝑟𝑂𝑁,𝐷3 𝐼 𝑅𝑀𝑆,𝐷3
2
= 𝑟𝑂𝑁,𝑄 𝐼 𝑅𝑀𝑆,2
2
6.1.3. Clamping Diodes D1, D2
 Since Q1, Q2 are symmetrical to each other, conduction losses will be given by:
𝑃𝑐𝑜𝑛𝑑,𝐷1 = 𝑟𝑂𝑁,𝐷1 𝐼 𝑅𝑀𝑆,𝐷1
2
𝑃𝑐𝑜𝑛𝑑,𝐷2 = 𝑟𝑂𝑁,𝐷2 𝐼 𝑅𝑀𝑆,𝐷2
2
And
𝑃𝑐𝑜𝑛𝑑,𝐷1 = 𝑃𝑐𝑜𝑛𝑑,𝐷2
July 28, 2016 Development of SiC-Based PEBB 1000 34

35. 6.2. Forward Voltage Losses
6.2.1. Rectifier Diode D3
 The average power dissipated by the forward voltage across the ON stage rectifier
diode is given by:
𝑃𝑓𝑜𝑟,𝐷3 = 𝑉𝑓𝑜𝑟,𝐷3 𝐼 𝑎𝑣𝑔,𝐷3
6.2.2. Clamping Diodes D1, D2
 The average power dissipated by the forward voltage across the ON stage
clamping diodes is given by:
𝑃𝑓𝑜𝑟,𝐷1 = 𝑉𝑓𝑜𝑟,𝐷1 𝐼 𝑎𝑣𝑔,𝐷1
𝑃𝑓𝑜𝑟,𝐷2 = 𝑉𝑓𝑜𝑟,𝐷2 𝐼 𝑎𝑣𝑔,𝐷2
And
𝑃𝑓𝑜𝑟,𝐷1 = 𝑃𝑓𝑜𝑟,𝐷2
July 28, 2016 Development of SiC-Based PEBB 1000 35

36. 6.3. Switching Losses
 Switching Losses on the MOSFET switches can be obtained by
the simplified formulation presented on [4]:
𝑃𝑠𝑤,𝑄1 =
1
2
𝑓𝑠𝑤 𝐶 𝐷𝑆1 𝑉𝐷𝑆1
2
𝑃𝑠𝑤,𝑄2 =
1
2
𝑓𝑠𝑤 𝐶 𝐷𝑆2 𝑉𝐷𝑆2
2
And
𝑃𝑠𝑤,𝑄1 = 𝑃𝑠𝑤,𝑄2 = 𝑃𝑠𝑤
July 28, 2016 Development of SiC-Based PEBB 1000 36

37. 6.4. Total Power Loss
 The total power loss of the circuit is given by:
𝑃𝑙𝑜𝑠𝑠 = 𝑃𝑐𝑜𝑛𝑑 + 𝑃𝑓𝑜𝑟 + 𝑃𝑠𝑤
With
𝑃𝑐𝑜𝑛𝑑 = 𝑃𝑐𝑜𝑛𝑑,𝑄1 + 𝑃𝑐𝑜𝑛𝑑,𝑄2 + 𝑃𝑐𝑜𝑛𝑑,𝐷3 + 𝑃𝑐𝑜𝑛𝑑,𝐷1 + 𝑃 𝑐𝑜𝑛𝑑,𝐷2
𝑃𝑓𝑜𝑟 = 𝑃𝑓𝑜𝑟,𝐷3 + 𝑃𝑓𝑜𝑟,𝐷1 + 𝑃𝑓𝑜𝑟,𝐷2
July 28, 2016 Development of SiC-Based PEBB 1000 37

38. 6.5. Results
 Initial Considerations
 The simulation time covered 0 to 0.05s
 The samples were saved on a .mat file and a reused on a MATLAB
script in order to compute the losses
 The RMS and average currents were computed considering one
switching cycle only at steady state
July 28, 2016 Development of SiC-Based PEBB 1000 38

39. 6.5. Results
July 28, 2016 Development of SiC-Based PEBB 1000 39
0.064
0.353
0.433
0.849
0.874
0.074
0.355
0.042
0.470
0.924
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
Condcution Losses Forward Voltage Losses Switching Losses Total Losses Efficiency
Power Losses at CCM (W)
One-Switch Two-Switch

40. 6.5. Results
 Comparison of the performance of the converters in
CCM:
 The conduction losses slightly increased due to the presence
of more components on the two-switch topology
 The switching losses were drastically reduced
 The efficiency increased 5 %
July 28, 2016 Development of SiC-Based PEBB 1000 40

41. 6.5. Results
July 28, 2016 Development of SiC-Based PEBB 1000 41
0.064
0.353
0.433
0.849
0.874
0.084
0.367
0.140
0.592
0.912
0.074
0.355
0.042
0.470
0.924
0.102
0.365
0.011
0.477
0.927
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
Conduction Losses Forward Voltage Losses Switching Losses Total Losses Efficency
Power Losses at CCM and BCM (W)
One-Switch CCM One-Switch BCM Two-Switch CCM Two-Switch BCM

42. 6.5. Results
 Comparison of the performance of the converters in
CCM vs. BCM:
 Higher conduction losses at BCM
 Lower switching losses for both topologies at BCM
 Higher efficiency for both topologies at BCM
July 28, 2016 Development of SiC-Based PEBB 1000 42

43. 7. Conclusion
 Does the Two-Switch Flyback Converter present a
better performance when compared to the One-Switch
topology?
Voltage across the MOSFET switches is clamped to Vin (no
high-voltage spikes)
Lower ringing effect
Lower switching losses
Higher efficiency
July 28, 2016 Development of SiC-Based PEBB 1000 43

44. 8. References
[1] “Improving the Performance of Traditional Flyback-Topology With Two-Switch –Approach”, J.
Pesonen; Texas Instruments
[2] “Understand Two-Switch Forward/Flyback Converters”, Y. Xi, R. Bell; National Semiconductor
[3] “Hard-Switching and Soft-Switching Two-Switch Flyback PWM DC-DC Converters and Winding
Loss due to Harmonics in High-Frequency Transformers”, D. M. Bellur, Wright State University
[4] “Two-Switch Flyback PWM DC-DC Converter in Continuous-Conduction Mode”, D. M. Bellur, M.
K. Kazimierczuk, Wright State University
[5] “Fundamentals of Power Electronics”, R. W. Erickson, D. Maksimovic, University of Colorado
Boulder
[6] “Characterization and Modeling of High-Switching-Speed Behavior of SiC Active Devices”,
Zheng Chen; Virginia Polytechnic Institute and State University
[7] “AN-9010 MOSFET Basics”, Fairchild Semiconductor
[8] “Analysis of SiC MOSFETs under Hard and Soft-Switching”, M. R. Ahmed, R. Todd, A. J.
Forsyth, The University of Manchester, UK
[9] “Power Electronics - A First Course”, N. Mohan, University of Minnesota
[10] “Development of an Isolated Flyback Converter Employing Boundary-Mode Operation and
Magnetic Flux Sensing Feedback”, M. V. Kenia, Massachusetts Institute of Technology
July 28, 2016 Development of SiC-Based PEBB 1000 44