Switching power supply ripple source and control

　　The output ripple of switching power supply mainly comes from five aspects: input low-frequency ripple, high-frequency ripple, common-mode ripple noise caused by parasitic parameters, ultra-high-frequency resonance noise generated during the switching process of power devices, and closed-loop regulation control. ripple noise.

　　1. The low frequency ripple is related to the filter capacitor capacity of the output circuit. The capacity of the capacitor cannot be increased indefinitely, resulting in residual low frequency ripple in the output. After the AC ripple is attenuated by the DC/DC converter, it appears as a low-frequency noise at the output end of the switching power supply, and its magnitude is determined by the transformation ratio of the DC/DC converter and the gain of the control system. The ripple suppression of the current mode control DC/DC converter is slightly improved than that of the voltage mode. However, the low-frequency AC ripple at the output is still relatively large. In order to realize the low ripple output of the switching power supply, filtering measures must be taken for the low frequency power supply ripple. It can be eliminated by pre-regulating and increasing the closed-loop gain of the DC/DC converter.

　　Several common methods of low frequency ripple suppression:

　　a. Increase the inductance and capacitance parameters of the output low-frequency filter to reduce the low-frequency ripple to the required index.

　　b. The feedforward control method is adopted to reduce the low frequency ripple component.

　　2. The high-frequency ripple noise comes from the high-frequency power switching conversion circuit. In the circuit, the input DC voltage is subjected to high-frequency switching conversion through power devices, and then rectified and filtered to achieve regulated output. The high-frequency ripple with the same frequency and the same frequency, its influence on the external circuit is mainly related to the conversion frequency of the switching power supply, the structure and parameters of the output filter. In the design, try to increase the operating frequency of the power converter, which can reduce the impact on the high-frequency switching ripple. wave filtering requirements.

　　The purpose of high-frequency ripple suppression is to provide a path for high-frequency ripple. The commonly used methods are as follows:

　　a. Increase the operating frequency of the switching power supply to increase the high-frequency ripple frequency, which is conducive to suppressing the output high-frequency ripple

　　b. Increase the output high-frequency filter to suppress the output high-frequency ripple.

　　c. Multi-stage filtering is used.

　　3. Due to the parasitic capacitance between the power device and the radiator bottom plate and the primary and secondary sides of the transformer, and the parasitic inductance of the wire, when the rectangular wave voltage acts on the power device, the output end of the switching power supply will generate common mode ripple noise. . Reduce and control the parasitic capacitance between power devices, transformers and chassis ground, and add common mode suppression inductance and capacitance on the output side to reduce the output common mode ripple noise.

　　Common methods to reduce output common mode ripple noise:

　　a. The output adopts specially designed EMI filter.

　　b. Reduce the switching glitch amplitude.

　　Fourth, the ultra-high frequency resonance noise mainly comes from the diode junction capacitance during reverse recovery of the high-frequency rectifier diode, and the resonance of the power device junction capacitance and the line parasitic inductance when the power device switches. The frequency is generally 1-10MHz. Measures such as switching tubes with small junction capacitance and reducing wiring length can reduce ultra-high frequency resonance noise.

　　Switching power supplies all need closed-loop control of the output voltage, and the inappropriate design of the regulator parameters will also cause ripples. When the output end fluctuates, it enters the regulator loop through the feedback network, which may lead to self-oscillation of the regulator and cause additional ripple. This ripple voltage generally does not have a fixed frequency.

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