Push-pull, flyback, and forward converters are DC-DC converters that use push-pull switching devices and transformers to efficiently convert DC voltage. They can be used in various applications, including TV sets and electronic equipment, and can operate with single or dual power supplies.
A push-pull converter is a converter circuit that uses push and pull switching devices, which are usually bipolar junction transistors (BJTs), field effect transistors (FETs), or silicon controlled rectifiers (SCRs). Push-pull is a term usually associated with two switches, with each switch connected to either a positive or negative leg of a direct current (DC) power supply. The push-pull converter is a very common DC-DC converter due to the high efficiency of the circuit which is mainly determined by the power lost at the main terminals of the push-pull device and the energy efficiency of the transformer used. Typically, the push-pull converter uses highly efficient ferrite core transformers that operate in the audio frequency range up to the highest audible frequencies. The push-pull converter also uses duty cycle control to produce a desired output voltage under carrier 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 sets (TVs) uses the horizontal deflection signal to produce the high positive voltage required at the anode of cathode ray tube televisions (CRT) and some plasma displays. From a signal rise and fall time perspective, the rapid drop-off of the horizontal deflection output as the CRT beam retraces to the start of the next video line is crucial to generating high voltage at the flyback transformer output. The resulting high voltage is rectified and filtered to obtain the anode power, which is typically greater 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 high frequency oscillators and transformers that use push-pull output to regenerate the DC voltage to the required level. Electronic equipment usually has a minimum input voltage below which the equipment stops functioning. The forward converter can be used to allow supply voltages lower than the previous minimum to still be useful. This feature is very useful for field and rescue equipment that needs all the energy that can be put to use under special conditions such as extended power outages.
The push-pull output can be derived from 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 one positive or “+” and one negative or “–” output including a common line. Different circuit configurations allow for single or dual supply configurations to generate the desired output.
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