Full-Bridge Push-Pull Converter

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How to use the program

Reference: The shapes of current and voltage curves are calculated using Faraday's Law. They do not represent an incremental simulation like it is done normally by programs like P-Spice. In the calculations the forward voltages of the diodes are considered with VF = 0.7V, and the transistors are interpreted as ideal switches.

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The Full-Bridge Push-Pull Converter belongs to the primary switched converter family since there is isolation between input and output. It is suitable for very high output powers.

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Function principals

Illustration 1: Full-Bridge Push-Pull Converter

For the following analysis it will be assumed that the transistor is simplified as an ideal switch and the diode has no forward voltage drop. In the program itself, the diode will take into account a forward voltage drop VF = 0.7V.

The Push-Pull converter drives the high-frequency transformer with an AC voltage, where the negative as well as the positive half swing transfers energy.
The primary transformer voltage V1 can be +Vin, -Vin or zero depending on which pair of transistors (T1,T4 or T2,T3) are turned on or off.
On the secondary side, the AC voltage is rectified, so that V3 is a pulse-width-modulated control voltage which switches between Vin.(N2/N1) and zero. Due to the rectification, the pulse-frequency of V3 is equal to 2· f .
The Low-Pass filter, formed by the inductor L and the output capacitor Cout, produces the average value of V3. For continuous mode (IL never becomes zero) this leads to:

Eqn 1

The Duty cycle of this converter may theoretically increase to 100%. In practice this is not possible because the serial connected transistors T1 and T2, have to be switched with a time difference to avoid a short circuit of the input supply.

Due to the fact that the duty cycle t1/T can theoretically increase to 100%, a condition for the turns ratio emerges:

Eqn 2

In the program, this value is multiplied by a factor of 0.95, so that the proposed value for N1/N2 includes a small margin which guarantees the demagnetisation of the core, when the input voltage is minimal, (remember: at minimum input voltage the duty cycle reaches its maximum).

For the allocation of the inductor L, the same rules as for the Buck Converter can be used. One also distinguishes between discontinuous and continuous mode, depending on whether or not the inductor current falls to zero during the on-time of the transistor.

During continuous operation:

Eqn 3

The inductor current IL has a triangular shape and its average value is determined by the load. The change in inductor current ΔIL is dependent on L and can be calculated with the help of Faraday's Law.
During continuous mode, with Vout = Vin · (N2/N1) · t1/T and a chosen switching frequency  f  it can be shown that:

Eqn 4

For a small load current, namely if Iout < ΔIL/2, the current will fall to zero during every period. This is what is known as discontinuous mode. In this case the calculations stated above are no longer valid.
In that moment, when the inductor current becomes zero, the voltage V3 jumps to the value of Vout. The diode junction capacitance of the secondary rectifier forms a resonant circuit with the inductance, which is activated by the voltage jump at the rectifier. The voltage V3 then oscillates and fades away.

Continuous Mode

Discontinuous Mode

Illustration 2: Operating modes of the Full-Bridge Push-Pull Converter

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Mathematics used in the program

The following parameters must be entered into the input fields:

Vin_min, Vin_max, Vout, Iout and  f 

Using these parameters, the program produces a proposal for N1/N2 and L:

For the calculation of the curve-shapes, and also for the calculation of "ΔIL for Vin_max", two cases have to be distinguished, i.e. continuous mode and discontinuous mode:

Eqn 5.3

Eqn 5.4

From this it follows that:

  1. For ΔIL< 2Iout the converter is in continuous mode and it follows that:

    Eqn 6.0

    Eqn 6.1

    Eqn 6.2

  2. For ΔIL> 2Iout the converter is in discontinuous mode and it follows that:

    Eqn 6.3

    Eqn 6.4

    Eqn 6.5

Main page | How to use the program | Function principals | Mathematics used in the program | Help for HF transformer
Top of page | Application            | Tips              | Literature Notes                | Help for choking coils