A push-pull converter is a converter circuit that uses push-type and pull-type switching devices, which are usually bipolar junction transistors (BJTs), field-effect transistors (FETs), or silicon-controlled rectifiers (SCRs). Push-pull is a term that is usually associated with two switches, with each switch connected to either a positive or a negative leg of a direct current (DC) power supply. The push-pull converter is a DC-to-DC converter that is very common due to the high circuit efficiency that is determined mostly by the power lost at the main terminals of the push-pull device and the power efficiency of the transformer used. Usually, the push-pull converter takes advantage of high-efficiency ferrite core transformers that operate in the audio frequency range to the higher audible frequencies. The push-pull converter also makes use of duty cycle control to produce a desired output voltage under carrying load conditions.
Like the push-pull converter, the flyback converter is also a DC-to-DC converter, although it can also be used in alternating current (AC)-to-DC power conversion. The flyback converter on television (TV) sets makes use of the horizontal deflection signal to produce the high positive voltage required at the anode of cathode ray tube (CRT) TVs and some plasma sets. From a signal rise-and-fall-times point of view, the rapid drop in horizontal deflection output as the CRT beam retraces to the start of the next video line is crucial in generating high voltage in the output of the flyback transformer. The resulting high voltage is rectified and filtered to obtain the anode supply, which is typically more than 20,000 volts direct current (VDC) for small CRTs.
The forward converter inputs DC and usually outputs a higher DC voltage. Forward converters have oscillators and high-frequency transformers that use push-pull output to regenerate DC voltage at the required level. Electronic equipment usually has a minimum input voltage below which the equipment stops working. The forward converter may be used to allow supply voltages below the previous minimum to still be useful. This feature is very useful for field and rescue equipment that need all energy that can be tapped given special conditions like prolonged power outages.
Push-pull output may be derived from either a single power supply or a dual power supply. The 12 VDC car voltage is a single-ended power output. There is only one hot line, which is +12 VDC; the other line is the return or common. Other DC power supplies may have a positive or “+” and a negative or “–“ output including a common line. Different circuit configurations allow single or dual power supply configurations to generate the desired output.