Flux Doubling - Powering Silicon Valley San Jose State University通量加倍-硅谷圣何塞州立大学供电

• 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.
Summary
• Current present in
secondary winding through D6 and L1
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.
References
• Hart, Daniel W. Introduction to Power
Electronics. Upper Saddle River, NJ 2019
• Billings, Keith. Switchmode Power Supply
Handbook. 2nd Edition. McGraw Hall.
saturating for the first half cycle
Solutions for Flux Doubling
• Initial choice of working flux density swing
must be lower than B to reduce utility of the core
Schematic of Push Pull Converter
Q1 on, Q2 off Operation Mode
• Primary voltage
placed across P1a
• All windings will go
positive
• Collector of Q2 will
have 2Vcc
• Clo halves of the
primary winding to prevent excessive voltage overshoot on the collectors of the transistors during the turnoff transient
• 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
Duty Ratio Controlled Push Pull Converter
By Rafael Lim
Overview
• Operation of Push Pull Converter • Staircase Saturation • Flux Doubling • Design Considerations
• This will cause the core to quickly staircase into saturation
with successive cycles
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
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.
• 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
• Have the control circuit recognize the flux
doubling and reduce the pulse width until proper working conditions become established
Design Considerations
• D5 and D6 are on • Rectifier diodes have
forward voltages that are equal by opposite across secondary winding
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