1. Duty Ratio Controlled Push Pull Converter
By Rafael Lim

2. Overview Operation of Push Pull Converter Staircase Saturation
Flux Doubling
Design Considerations

3. Schematic of Push Pull Converter

4. Q1 on, Q2 off Operation Mode
Primary voltage placed across P1a
All windings will go positive
Collector of Q2 will have 2Vcc
Current present in secondary winding through D6 and L1

5. Q1 off, Q2 off
L1 maintains current flow in D5, D6, C7, and output load
The current returns through the center tap of the secondary windings through D5 and D6.
D5 and D6 are on
Rectifier diodes have forward voltages that are equal by opposite across secondary winding

6. Staircase Saturation
Push pull converter vulnerable to saturation because there is a direct DC path through main transformer primary and primary transistors
If average volt-seconds across transformer when Q1 is on is not equal to the average volt-seconds across transformer when Q2 is on, a net DC polarizing component will exist
This will cause the core to quickly staircase into saturation with successive cycles

7. Solutions
Imbalances can be caused by variations in saturation voltage and storage times between transistors, by differences in voltage drops of output rectifier diodes, or by differences in the winding resistance between the two halves of the primary or secondary winding. This will cause a DC bias towards saturation. To help reduce these problems need careful matching of drive and output components and introducing an air gap in the core.
However, air gap reduces permeability, increasing magnetization currents

8. Flux Doubling
Under steady state operation, the starting position for the flux at the beginning of each half cycle will be either +B or –B.
Maximum flux density swing during steady state half cycles will be 2B.
Starting point for initial flux excursion will almost be zero
Sudden flux excursion of 2B would result in core saturating for the first half cycle

9. Solutions for Flux Doubling
Initial choice of working flux density swing must be lower than B to reduce utility of the core
Have the control circuit recognize the flux doubling and reduce the pulse width until proper working conditions become established

10. Design Considerations
Close coupling is required between the two halves of the primary winding to prevent excessive voltage overshoot on the collectors of the transistors during the turnoff transient
The two halves of the primary should be bifilar wound to minimize leakage inductance
Push pull is favored for low voltage DC to DC converters.

11. Summary Push pull is favored towards low voltage DC to DC converters
Issues with staircase saturation and flux doubling must be considered is designing push pull converter

12. References
Hart, Daniel W. Introduction to Power Electronics. Upper Saddle River, NJ 1997
Billings, Keith. Switchmode Power Supply Handbook. 2nd Edition. McGraw Hall.