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　　Sharing the filter circuit design of a small switching power supply with a power of 30W

　　In today's solution sharing, this article will continue to summarize and briefly analyze the filter circuit design of this small switching power supply. Let us take a look together.

　　Low pass filter circuit design

　　In this solution, the low-pass filter circuit design of this small switching power supply with a power of 30W is designed in this article, as shown in Figure 1 below. The low-pass filter circuit in this switching power supply system can effectively prevent noise interference from the input power supply, and at the same time suppress the entry of surge voltages and peak voltages. It can also prevent and limit the noise and high-frequency electromagnetic interference generated by the switching power supply. The signal is fed back into the power grid through input wires. Because the inductive reactance generated by electromagnetic waves in different bands is different, the capacitors and inductors selected for the design circuit are also different. According to the requirements of the calculation formula, the reactance of the circuit is the vector sum of impedance, inductive reactance and capacitive reactance, that is:

　　If you want the electromagnetic interference of the designed low-pass filter circuit to be no greater than 8dB/txV, then you must choose appropriate inductors and capacitors to maximize the circuit's ability to resist interference.

　　Rectifier and filter circuit design

　　In this solution, we designed a small switching power supply with an output power of 30W. Its internal rectification and filtering circuit design is shown in Figure 2 below. In the design process of the rectifier and filter circuit of the switching power supply, I believe many engineers are very clear that the input capacitor is determined by the output holding time and the ripple size required by the DC input voltage, and the ripple flowing through the capacitor is within the allowable value of the capacitor. within the range. Since most filter capacitors use electrolytic aluminum as the electrolyte, the ripple current has a great impact on the life of the capacitor.

　　In this solution, the rectifier bridge of the low-power switching power supply we designed is composed of four diodes. Each diode is connected in series to complete half-cycle rectification of the voltage. Therefore, in the actual test, the current flowing through each diode in the rectifier filter circuit is only half of the average value of the entire current, and the voltage endured by each diode is half of the maximum reverse voltage. The peak value of the input circuit is 0.5A. The maximum input peak DC voltage of the input circuit is 750V.

　　Absorption loop design

　　In the plan, the design of this low-power switching power supply absorption loop system is shown in Figure 3 above. It can be seen that the absorption loop shown in Figure 3 is a clamping circuit composed of resistors, capacitors and blocking diodes, which can effectively protect the switching power tube from damage. When the switching power tube is turned on, the magnetic flux of the transformer increases, and electric energy is accumulated. When the switching power tube is turned off, the accumulated electric energy is released, and residual magnetism is generated in the primary winding of the transformer, which is fed back to the secondary through D5. After the residual magnetism is released, the voltage VP of the primary winding NP is 120V~340V. The maximum peak voltage that the power tube can withstand is 650V. The minimum absorption time is 21xs, then the resistance of the absorption loop can be calculated as:

　　It should be noted during the calculation process that the time constant RC is usually much larger than the period T, so it is generally taken to be about 5 times during calculation. Because the circuit works in a high-frequency state, an ultra-fast recovery diode must be used. The diode here is Motorola's MUR460, which has a withstand voltage of more than 600V and a current value of more than 4A.

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