Push-pull converter

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A push-pull converter is a type of DC to DC converter that uses a transformer to step the voltage of a DC power supply. Since it uses a transformer, its ratio is arbitrary but fixed. The primary advantages of push-pull converters are their simplicity and ability to scale up to high power throughput, earning them a place in industrial DC power applications.

The push-pull converter is similar to the flyback converter and especially the forward converter.

1 Circuit operation
2 Transistors
3 Timing
4 See also
5 External links

[edit] Circuit operation

The push-pull is an extremely simple circuit. The switches alternate the voltage across the supply side of the transformer, causing the transformer to function as it would for AC power and produce a voltage on its output side. The output is then rectified and sent to the load. Capacitors are often included at the output to buffer against the inevitable switching noise.

In practice, it is necessary to allow a small interval between powering the transformer one way and powering it the other: the “switches” are usually pairs of transistors (or similar devices), and were the two transistors in the pair to switch simultaneously there would be a risk of shorting out the power supply. Hence, a small wait.

[edit] Transistors

Usually the signal for switching is on some logic levels (lets say between 0 and 3 Volts) and can supply very little current.

The power-transistors are supplied with 24 Volt, for example.

N-type and p-type power transistors can be used. This is like CMOS is constructed. The gate (base) of the power transistors is tied via a resistor to one of the supply voltages. A p-type transistor is used to pull up the n-type power transistor gate (common source). A n-type transistor is used to pull down the p-type power transistor.

All power transistors can be n-type (often 3 times the gain of p-type). This is like TTL is constructed. Then the n-type transistor, which replaced the p-type has to be driven this way: The voltage is amplified by one p-transistor and one n-transistor in common base configuration to rail-to-tail amplitude. Then the power transistor is driven in common drain configuration to amplify the current.

[edit] Timing

If both transistors are open, this is a short circuit. If both transistors are closed, high voltage peaks due to back EMF appear.

If the driver for transistor is powerful and fast enough, the back EMF has no time to charge the capacity of the windings and of the body-diode of the mosfets to high voltages.

If a microcontroller is used, it could measure the peak voltage and digitally adjust the timing for the transistors, so that the peak just appears (coming from no peak, starting from cold transistors in warm-up / boot-phase).