1. EE 136 Project Presentation
The Diagonal Half-Bridge Flyback Converter
Non-isolated Self-Oscillating Flyback Converter with Primary Current-Mode Control
Isolated Self-Oscillating Flyback Converter with Primary Current-Mode Control
Presented By: Latt M. Tee Naw Sandar Shwe

2. The Diagonal Half-Bridge Flyback Converter
This converter is suitable for power FETs operation
Same design procedure applies as a single-ended flyback converter

3. Circuit Analysis for Half-Bridge Flyback Converter
Switches is driven by control circuit, both “on” and “off” together
Flyback action takes place during “off” period
Cross-connected diodes D1 and D2 return excess flyback energy to supply lines
D1 and D2 eliminates the need of an energy recovery winding or large snubbing components.

4. Schematic Diagram for Half-Bridge Flyback Converter

5. The Circuit Diagram When Switches ‘On’ & ‘Off’ and Voltage and Current Waveform

6. Schematic Diagram Voltage and Current waveforms (using Simplorer Software)

7. Advantages of this converter
Using the diodes D1 and D2 is good for over-voltage stress.
Flyback action returns the stored energy to the supply line.
Do not need bifilar-wound energy recovery winding
Reduce the cost and eliminate unreliable source.

8. Self-oscillating flyback converter
Low component count without loss of performance
Widely used in low power applications such as computers, video display terminals and so on

9. Frequency variation as a function of load Non-isolated self-oscillating flyback converter
Type C operation
Variable ‘on’ time, ‘off’ time, and repetition rate (freq.)
Has a more desirable characteristic

10. Circuit Diagram of non-isolated flyback converter

11. Base drive current waveform of self-oscillating converter

12. Schematic and waveforms using Simplorer software

13. Isolated self-oscillating flyback converter
The input and output voltage does not depend on each other
Optocoupler is used as a feedback loop
Transistor Q2 and R4 control the current-mode
Input voltage +300 V DC and output voltage +12V is applied to the circuit for self-oscillating

14. Non-isolated self oscillating circuit

15. The wave form of Q1 and Q2 Q1 Ic increase Ie increase Vbe 0.6V
Q2 turn on

16. Simulation result

17. Advantages
The system as if the primary is a high-impedance current source.
Results in a first order transfer function.
The control circuit may have high frequency response.
Line ripple rejection and stability improved.
Primary current limiting is provided.

18. Conclusion Low cost Fewer components need
Widely used in low power application