Since the "20 kHz" revolution was achieved in switching power supplies in the 1970s, although improvements and enhancements were made in circuit technology, due to the development level of the semiconductor device industry at that time, the main parameters of the high-power switching tubes, which are the main components of switching power supplies, could not meet the design requirements of higher-frequency and higher-power switching power supplies.

What are power transistors used in high-frequency switching rectifiers

Since the 1980s, when high-power and high-reverse-voltage insulated gate field-effect power transistors were introduced, along with the application of dedicated integrated control circuits for switching power supplies, the operating frequency of switching power supplies has gradually been raised to over 100kHz, and even reaching 500 to 700kHz. The volume and weight of the power supply have been greatly reduced, and the single-unit output power has developed from several hundred watts in the past to several kilowatts now.

The characteristics and main parameters of MOSFET power transistors

Metal-oxide-semiconductor field-effect transistors, abbreviated as MOSFET, are a type of high-speed, high-power, and high-voltage switching device that has developed in the past decade or so. Due to the fact that this switching power transistor is voltage-controlled, the gate control current is very small during steady-state operation, thus it has a very high current gain and input impedance. Moreover, the junction capacitance is very small, and there is almost no storage time.

(a) and (b) are the representation symbols for NPN bipolar transistors and N-channel MOSFETs, respectively.

In (a), the three poles of a bipolar transistor are the collector (C), the base (B), and the emitter (E), which is what we generally call a triode. The three corresponding terminals of a MOSFET are the drain (D), the gate (G), and the source (S). When an N-channel MOSFET is in operation, the current flow direction is the same as that of an NPN transistor. This determines that the connection method of a MOSFET in a circuit is consistent with that of an NPN transistor, but they have significant differences in structure and working methods. The fundamental difference is that MOSFET tubes are semiconductor devices that conduct electricity through the movement of majority carriers, while transistors are semiconductor devices that conduct electricity by using both majority and minority carriers.

The static operating characteristics of MOSFET tubes are very similar to those of bipolar transistors, and the output characteristic curve of MOSFET tubes.