Push–pull converter

A push–pull converter is a type of DC to DC converter that uses a transformer to change the voltage of a DC power supply. The transformer's ratio is arbitrary but fixed; however, in many circuit implementations the duty cycle of the switching action can be varied to effect a range of voltage ratios. 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.

Circuit operation

The term "push–pull" is sometimes used to generally refer to any converter with bidirectional excitation of the transformer. For example, in a full-bridge converter, the switches (connected as an H-bridge) 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.

However, "push–pull" more commonly refers to a two-switch topology with a split primary winding.

In any case, 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 is needed to avoid this problem.

Transistors

N-type and p-type power transistors can be used. Power MOSFETs are often chosen for this role due to their high current switching capability and their inherently low ON resistance. The gates (base) of the power transistors are 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) and an n-type transistor is used to pull down the p-type power transistor gate.

All power transistors can be n-type (often 3 times the gain of p-type). 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-rail amplitude.Then the power transistor is driven in common drain configuration to amplify the current.

In high frequency applications both transistors are driven with common source.In fact they are both pushing, pulling is done by a low pass filter (coil) in general and by a center tap of the transformer in the converter application.Because the transistors push alternating this device is also called a push–pull converter.

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).

The cycle starts with no voltage and no current. Then one transistor opens, a constant voltage is applied to the primary, current increases linearly, and a constant voltage is induced in the secondary. After some time T the transistor is closed, the parasitic capacities of the transistors and the transformer and the inductance of the transformer form an LC circuit which swings to the opposite polarity. Then the other transistor opens. For the same time T charge flows back into the storage capacitor, then changes the direction automatically, and for another time T the charge flows in the transformer. Then again the first transistor opens until the current is stopped. Then the cycle is finished, another cycle can start anytime later. The S-shaped current is needed to improve over the simpler converters and deal efficiently with remanence.

See also

* Inverter (electrical)
* Push–pull output
* Rectifier

External links

* [http://www.fbe.fh-darmstadt.de/team/hschmidtwalter/snt/snt_eng/snteng4a.pdf Push–Pull Converters] -- a more detailed explanation of the full-bridge converter, described here as "push pull" in the general sense of the term. Includes helpful graphs.

* [http://sound.westhost.com/project89.htm Switchmode PSU for car audio] 12V to symmetric output push–pull converter used for powering car audio amplifiers. This is a true push–pull topology with two switches and a center-tapped transformer.