Switching Power Supply Technical Questions and Answers

    Answer: The switching power supply is divided into single-excitation switching power supply and dual-excitation switching power supply, which refers to the magnetization curve (magnetization process) of the switching power supply transformer core. If the magnetic field strength and magnetic induction strength (magnetic flux density) of the switching transformer core change only in value but not in direction during the magnetization process, this switching transformer is called a single-polarization switching transformer, and the switching power supply using this single-polarization switching transformer is called a single-shot switching power supply; if the magnetic field strength and magnetic induction strength (magnetic flux density) of the switching transformer core change not only in value but also in direction during the magnetization process, this switching transformer is called a dual-polarization switching transformer, and the switching power supply using this dual-polarization switching transformer is called a dual-excitation switching power supply.

    In the switching power supply circuit, single excitation and dual excitation respectively represent single excitation (single drive) and dual excitation (dual drive). Single excitation means that only one switch tube is used to control the on/off of the switching transformer and the input power supply; dual excitation means that two switch tubes are used to control the on/off of the switching transformer and the input power supply in turn. The circuit of single-excitation switching power supply is simple, but the output power is relatively small; the circuit of dual-excitation switching power supply is relatively complex, but the output power is relatively large. The output power of dual-excitation switching power supply is about 4 times that of single-excitation switching power supply.

    In the switching power supply circuit, when the power switch tube just connects the primary coil of the switching transformer to the input power supply, the secondary coil of the switching transformer has power output. This switching power supply is called a forward switching power supply; and when the power switch tube just connects the primary coil of the switching transformer to the input power supply, the secondary coil of the switching transformer has no power output. At this time, the switching transformer only stores energy (magnetic energy). Only when the power switch tube just connects the primary coil of the switching transformer to the input power supply from on to off, the secondary coil of the switching transformer has power output. This switching power supply is called a flyback switching power supply.

    The load characteristics of the forward switching power supply are better than those of the flyback switching power supply, but the forward switching power supply circuit is relatively complex, and its output voltage is generally taken from the average value of its output pulse voltage. After rectification, an LC filter with a relatively large time constant is required to take out the average value of the pulse voltage, which is relatively expensive; while the output voltage of the flyback switching power supply is taken from the half-wave average value of its output pulse voltage, and after rectification, only a filter capacitor is needed to take out the voltage.

    At present, most of the switching power supply products sold on the market (such as power adapters or chargers for portable digital products) are single-excitation and flyback transformer switching power supplies.