How To Properly Design A Switching Power Supply

- Dec 28, 2019-

Switching power supplies are divided into two forms of isolation and non-isolation. Here we mainly talk about the topological form of isolated switching power supply. In the following, unless otherwise specified, it refers to isolated power supply. Isolated power supplies can be divided into two categories according to their structural forms: forward and flyback. Flyback means that the secondary side is cut off when the primary side of the transformer is on, and the transformer stores energy. When the primary side is turned off, the secondary side is turned on, and the energy is released to the working state of the load. Generally, there are many single-tubes in the conventional flyback power supply, and double-tubes are not common. The forward type refers to when the primary side of the transformer is turned on and the secondary side induces a corresponding voltage to output to the load, and the energy is directly transmitted through the transformer. According to the specifications, it can be divided into conventional forward excitation, including single-tube forward excitation and double-tube forward excitation. Half-bridge and bridge circuits are all forward circuits.


The forward and flyback circuits each have their own characteristics. In order to achieve the best cost performance in the design of the circuit, they can be used flexibly. Generally, flyback can be used in low power situations. Slightly larger can use a single tube forward circuit, medium power can use a double tube forward circuit or half-bridge circuit, low-voltage push-pull circuit, the same as the half-bridge working state. High power output, generally use bridge circuit, low voltage can also use push-pull circuit.


Because of its simple structure, the flyback power supply saves an inductor with a size similar to that of a transformer, and is widely used in small and medium power supplies. In some introductions, it is mentioned that the power of the flyback power supply can only be tens of watts, and there is no advantage when the output power exceeds 100 watts, which is difficult to achieve. I think this is the case in general, but it can't be generalized. The TOP chip of PI company can achieve 300 watts. There is an article that describes that the flyback power can reach kW, but I have not seen the real thing. The output power is related to the output voltage.


Flyback power transformer leakage inductance is a very critical parameter. Since the flyback power supply requires the transformer to store energy, in order to make full use of the transformer core, an air gap is generally required in the magnetic circuit. The purpose is to change the core hysteresis. The slope of the return line enables the transformer to withstand large pulse current impacts without the iron core entering a saturated non-linear state. The air gap in the magnetic circuit is in a high reluctance state. The magnetic leakage generated in the magnetic circuit is much greater than that of a fully closed magnetic circuit .


The pulse voltage connection is as short as possible, where the input switch tube is connected to the transformer and the output transformer is connected to the rectifier tube. The pulse current loop is as small as possible, such as the input filter capacitor is positive to the transformer to the switch tube and the return capacitor is negative. In the output part of the transformer, to the rectifier tube, to the output inductor, to the output capacitor and return to the transformer circuit, the X capacitor should be as close as possible to the input terminal of the switching power supply. The input line should be avoided parallel to other circuits and should be avoided. The Y capacitor should be placed on the chassis ground terminal or FG connection terminal. Keep the electric induction and the transformer at a certain distance to avoid magnetic coupling.