Discuss how to reduce the noise of switching power supply

　　Power supply devices are indispensable components in electronic and electrical equipment. Switching power supplies are favored by related industries for their high efficiency, small size, light weight, and good voltage adaptability. However, the current disadvantage is that the electromagnetic interference is large, which has an adverse effect on the environment or other equipment. At present, for switching power supplies with variable loads, the lowest output noise voltage of the products that the author has learned is also above 70 mV. Designing a switching power supply with low electromagnetic interference has become the hope of many designers, and various methods have been proposed for this purpose.

　　Switching power supply circuit structure and noise reduction principle

　　The design goal of this switching power supply is to stabilize the 20 V output and the output current is variable from 0 to 2 A for use in audio systems. In order to highlight the processing technology for reducing electromagnetic noise and simplify the circuit, the monolithic switching power supply chip TOP224Y is used for design. TOP224Y already contains all the circuits required for PWM modulation and the output of the excitation tube, which excites the transformer. The switching frequency is 100 kHz, the withstand voltage of the internal MOS excitation tube is 700 V, and the output power is less than 45 W. The circuit is shown in Figure 1. This circuit can obtain a higher output power by simply changing some components. The circuit R1, L1, D1, C1 to C7 on the left side of Figure 1 is a conventional common-mode filtering and rectification circuit, which obtains a DC voltage of about 300 V for use in the DC-DC conversion circuit; the circuit L5, C11, etc. on the far right are ordinary LC filtering circuits; IC2, D8, R9, R10 form a voltage feedback circuit to form a closed-loop structure to stabilize the power supply output voltage; the middle part is a DC-DC converter, and the key to noise reduction is to properly process this part of the circuit.

　　Figure 1: Schematic diagram of a low-noise switching power supply

　　For the middle circuit, TOP224Y is used as PWM control and excitation, which are all conventional treatments. The working voltage of the control terminal C is taken from the reverse excitation voltage of the transformer, where D3 is a rectifier tube and D4 is a light-emitting diode, which is used as a guide light. The feedback signal of the C terminal comes from the output of IC2. The drain output terminal D of the chip is connected to the transformer and R1, D2, where R1 is a semiconductor varistor, and together with D2, it forms a chip voltage limiting protection circuit to prevent the chip from breaking down due to overvoltage. The excitation method of this circuit adopts a positive and negative mixed excitation type with positive excitation as the main method. The transformer has 4 windings, 2 of which are basically similar output windings n3 and n4, and its same-name terminal relationship is shown in Figure 2.

　　Figure 2: Circuit freewheeling path

　　Three rectifier tubes are used after DC-DC conversion: D5, D6 and D7. There is no independent freewheeling diode, which is different from other power supply circuits . D5 is a multiplexed diode set for freewheeling, D6 and are positive excitation pulse rectifier diodes, and D7 is a reverse excitation voltage rectifier diode. L4 is the first-stage filter inductor after DC-DC conversion. During the positive excitation period, the transformer output winding n3 outputs current through D6 and L4, and the current i4 in the first-stage filter inductor L4 increases. At the same time, the excitation magnetic current i1 of the transformer's own interest is also increasing.

　　When the positive excitation ends, the reverse excitation stage will begin immediately. The current i4 in the filter inductor L4 will gradually decrease from the original value. The transformer will also maintain the excitation current, but it is a multi-winding structure. The excitation current can appear in any winding, and the direction of each current is based on maintaining the original magnetic field direction. If the filter inductor current i4>n1i1/n4 at that time is controlled, the excitation current in the transformer core can be transferred to the n4 winding. That is, the current i4 flows through the transformer output winding n4. In addition to maintaining the magnetic field of the transformer core, there is still excess, and the remaining amount is distributed between n4 and n3 according to the turns ratio. At this time, diode D5 is immediately turned on, diode D6 continues to be turned on, and diode D7 is still turned off. There is no induced voltage in the transformer winding, and the magnetic field energy is not released. With the release of the energy stored in the filter inductor, the current i4 gradually decreases until i4=n1i1/n4, and D6 enters the cut-off state. It can be seen that D6 is not forced to be cut off by the dividend. If properly handled, its turn-off noise can be eliminated. Then, the transformer starts to generate reverse excitation electromotive force and releases stored energy, diode D7 starts to conduct, and the reverse excitation voltage of the transformer is limited until the transformer energy storage is fully released and waits for the next cycle of excitation.

　　According to this method, the hard turn-off noise of the rectifier diode D6 can be eliminated, but the hard turn-off noise of the chip excitation tube caused by the transformer leakage inductance still exists, and the auxiliary winding here can play a certain role in absorption. For the hard turn-on noise of the rectifier diode, the RC circuit is still used to absorb energy and reduce noise, such as the R7, C10 circuit in Figure 1.