Switching power supplies have become the protagonist of our circuit design, and can even be said to have become an inseparable part of the development of the industry. Compared with the traditional linear power supply, the cost of the linear power supply at a certain output power point is higher than that of the switching power supply. The common switching power supply can be divided into two types, isolated and non-isolated.
In this article, we will mainly discuss the topological form of isolated switching power supplies. Therefore, in the following article, if there is no special explanation, the power supply mentioned in the article will refer to the isolated power supply. Isolated power supplies can be divided into two broad categories according to their structural form: forward and flyback. The flyback means that the secondary side is cut off when the primary side of the transformer is turned 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, the conventional flyback power supply has a single tube and the double tube is not common. The forward type refers to the conduction of the primary side of the transformer while the secondary side induces the corresponding voltage output to the load, and the energy is directly transmitted through the transformer. According to the specifications, it can be divided into regular positive, including single tube forward and double tube forward. Half-bridge and bridge circuits are all forward circuits.
Both forward and flyback circuits have their own characteristics, and they can be used flexibly in order to achieve optimal cost performance in the process of designing circuits. Flyback is usually available in low power applications. A slightly larger one can use a single-tube forward circuit, a medium power can use a double-tube forward circuit or a half-bridge circuit, and a low voltage voltage push-pull circuit is used, which is the same as the half-bridge operation state. High-power output, generally adopts bridge circuit, and low-voltage can also adopt push-pull circuit.
Because of its simple structure, the flyback power supply eliminates an inductor similar in size to the transformer, and is widely used in small and medium power supplies. In some introductions, the flyback power supply can only achieve tens of watts, and the output power exceeds 100 watts, which has no advantage and is difficult to implement. I think this is the case in general, but it cannot be generalized. PI's TOP chip can achieve 300 watts. There is an article about the flyback power supply that can achieve kilowatts, but I have never seen anything in kind. The output power is related to the output voltage level.
The leakage inductance of the flyback power transformer 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, it is generally necessary to open the air gap in the magnetic circuit, the purpose of which is to change the core hysteresis. The slope of the return line enables the transformer to withstand large pulse current surges, so that the core enters a saturated non-linear state, the air gap in the magnetic circuit is in a high reluctance state, and the leakage flux in the magnetic circuit is much larger than the fully closed magnetic circuit. .
The pulse voltage connection is as short as possible, in which the input switch tube is connected to the transformer and the output transformer is connected to the rectifier connection line. The pulse current loop is as small as possible as the input filter capacitor is positive to the return capacitance of the transformer to the switch. Output part of the transformer output to the rectifier to the output inductor to the output capacitor return transformer circuit X capacitor should be as close as possible to the input end of the switching power supply, the input line should avoid parallel with other circuits, should be avoided. The Y capacitor should be placed at the chassis ground terminal or FG connector. The total inductance is kept at a certain distance from the transformer to avoid magnetic coupling.
The output capacitor can generally be used with two ones close to the rectifier and the other should be close to the output terminal, which can affect the output ripple index of the power supply. The parallel effect of two small-capacity capacitors should be better than using a large-capacity capacitor. The heating device should be kept at a certain distance from the electrolytic capacitor to extend the life of the whole machine. The electrolytic capacitor is the bottle strength of the switching power supply life. For example, the transformer, the power tube, the high-power resistor should be kept away from the electrolysis, and the heat dissipation space must be left between the electrolysis. , conditions allow it to be placed at the air inlet.
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