Four ways to reduce the "ripple and noise" of switching power supply output!

Ripple mainly exists in 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 ripple noise caused by closed-loop regulation control.

01 Low-frequency ripple is related to the capacity of the filter capacitor of the output circuit. The capacity of the capacitor cannot be increased indefinitely, resulting in residual output low-frequency ripple. After the AC ripple is attenuated by the DC/DC converter, it appears as 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-type control DC/DC converter is slightly better than that of the voltage type. However, the low-frequency AC ripple at its output end is still large. If you want to achieve low-ripple output of the switching power supply, you must take filtering measures for the low-frequency power supply ripple. It can be eliminated by using pre-stage pre-stabilization and increasing the closed-loop gain of the DC/DC converter.

Several common methods for suppressing low-frequency ripple:

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

b. Use feedforward control method to reduce the low-frequency ripple component.

02 High-frequency ripple noise comes from the high-frequency power switch conversion circuit. In the circuit, the input DC voltage is converted by high-frequency switching through power devices, and then rectified and filtered to achieve a regulated output. The output end contains high-frequency ripples with the same frequency as the switch operating frequency. The impact of the external circuit is mainly related to the switching frequency of the switching power supply, the structure and parameters of the output filter. In the design, the operating frequency of the power converter is increased as much as possible to reduce the filtering requirements for high-frequency switching ripples.

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

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

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

c. Use multi-stage filtering.

03Due to the parasitic capacitance between the power device and the bottom plate of the radiator 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. Reducing and controlling the parasitic capacitance between the power device, the transformer and the chassis ground, and adding common-mode suppression inductors and capacitors on the output side can reduce the output common-mode ripple noise.

Common methods to reduce output common-mode ripple noise:

a. Use a specially designed EMI filter for the output.

b. Reduce the amplitude of switching glitches.

04Ultra-high frequency resonant noise mainly comes from the resonance of the diode junction capacitance during the reverse recovery of the high-frequency rectifier diode, the junction capacitance of the power device and the parasitic inductance of the line during the power device switching, and the frequency is generally 1-10MHz. The ultra-high frequency resonant noise can be reduced by selecting soft recovery diodes, switching tubes with small junction capacitance, and reducing the wiring length.

Switching power supplies all need to perform closed-loop control on the output voltage, and improper design of regulator parameters will also cause ripples. When the output fluctuates, it enters the regulator loop through the feedback network, which may cause the regulator to oscillate self-excitedly and cause additional ripple. This ripple voltage generally does not have a fixed frequency. In a switching DC power supply, the output ripple often increases due to inappropriate selection of regulator parameters.

This part of the ripple can be suppressed by the following methods:

a. Add a compensation network to the ground at the output of the regulator. The compensation of the regulator can suppress the increase of ripple caused by the self-excitation of the regulator.

b. Reasonably select the open-loop gain of the closed-loop regulator and the parameters of the closed-loop regulator. If the open-loop gain is too large, it may sometimes cause the regulator to oscillate or self-excite, increasing the output ripple content. If the open-loop gain is too small, the output voltage stability will deteriorate and the ripple content will increase. Therefore, the open-loop gain of the regulator and the parameters of the closed-loop regulator should be reasonably selected, and they should be adjusted according to the load conditions during debugging.

c. Do not add a pure lag filter link in the feedback channel. Minimize the delay lag to increase the rapidity and timeliness of the closed-loop regulation, which is beneficial to suppressing the output voltage ripple.