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. Alternatively, all power transistors can be N-type, which offer around three times the gain of their P-type equivalents. In this alternative the N-type transistor used in place of the P-type has to be driven in this way: The voltage is amplified by one P-type transistor and one N-type 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. Because the transistors operate in an alternating fashion, the device is called a push-pull converter. Timing If both transistors are in their on state, a short circuit results. On the other hand, if both transistors are in their off state, high voltage peaks appear due to back EMF. If the driver for the transistors 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 can be used to measure the peak voltage and digitally adjust the timing for the transistors, so that the peak only just appears. This is especially useful when the transistors are starting from cold with no peaks, and are in their boot phase. The cycle starts with no voltage and no current. Then one transistor turns on, 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 turned off, 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 turns on. 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 turns on 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 Class B or AB push–pull Rectifier External links Switchmode PSU