Duty Ratio Controlled Push Pull Converter

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Duty Ratio Controlled Push Pull Converter

• By Rafael Lim

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Overview

• Operation of Push Pull Converter
• Staircase Saturation
• Flux Doubling
• Design Considerations

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Schematic of Push Pull Converter
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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

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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

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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

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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

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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

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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

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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.

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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

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References

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