Push Pull Converter

The push pull converter belongs to the feed forward converter family. With reference to the diagram above, when Q1 switches on, current flows through the 'upper' half of T1's primary and the magnetic field in T1 expands. The expanding magnetic field in T1 induces a voltage across T1 secondary, the polarity is such that D2 is forward biased and D1 reverse biased. D2 conducts and charges the output capacitor C2 via L1. L1 and C2 form an LC filter network. When Q1 turns off, the magnetic field in T1 collapses, and after a period of dead time (dependent on the duty cycle of the PWM drive signal), Q2 conducts, current flows through the 'lower' half of T1's primary and the magnetic field in T1 expands. Now the direction of the magnetic flux is opposite to that produced when Q1 conducted. The expanding magnetic field induces a voltage across T1 secondary, the polarity is such that D1 is forward biased and D2 reverse biased. D1 conducts and charges the output capacitor C2 via L1.

After a period of dead time, Q1 conducts and the cycle repeats.

There are two important considerations with the push pull converter:

  1. Both transistors must not conduct together, as this would effectively short circuit the supply. Which means that the conduction time of each transistor must not exceed half of the total period for one complete cycle, otherwise conduction will overlap.
  2. The magnetic behaviour of the circuit must be uniform, otherwise the transformer may saturate, and this would cause destruction of Q1 and Q2. This requires that the individual conduction times of Q1 and Q2 be exactly equal and the two halves of the centre-tapped transformer primary be magnetically identical.

These criteria must be satisfied by the control and drive circuit and the transformer.

The output voltage Vout equals the average of the waveform applied to the LC filter:

Vout = Vin x (n2/n1) x f x (Ton,q1 + Ton,q2)

where:

Vout=Average output voltage - Volts

Vin=Supply Voltage - Volts

n2=half of total number of secondary turns

n1=half of total number of primary turns

f = frequency of operation - Hertz

Ton,q1 = time period of Q1 conduction - Seconds

Ton,q2 = time period of Q2 conduction - Seconds

The control circuit monitors Vout and controls the duty cycle of the drive waveforms to Q1 and Q2.

If Vin increases, the control circuit will reduce the duty cycle accordingly, so as to maintain a constant output. Likewise if the load is reduced and Vout rises the control circuit will act in the same way. Conversely, a decrease in Vin or increase in load, will cause the duty cycle to be increased. The diagram below shows associated waveforms from the push pull converter.

Push Pull Converter Waveforms

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