How to suppress ripple noise in switching power supplies
In general, the operating frequency of a switching power supply is very high, and it is extremely important to suppress ripple and reduce frequency noise. Electronic components can be added to hardware to suppress ripple
Low frequency ripple
Low frequency ripple is related to the filtering capacitor capacity of the output circuit. Due to the limitation of the volume of switch mode power supplies, the capacity of electrolytic capacitors cannot be increased indefinitely, resulting in residual low-frequency ripple in the output. The frequency of this output ripple varies depending on the rectification circuit method.
For a regular 24V power supply, the ripple suppression ratio of a voltage controlled DC/DC converter is generally 45-50dB, and the effective value of the low-frequency AC ripple at its output terminal is 60-120mV. The ripple suppression ratio of the current controlled DC/DC converter has been slightly improved, but the low-frequency AC ripple at its output terminal is still relatively large. To achieve low ripple output of switching power supply, filtering measures must be taken for low-frequency power supply ripple. Pre stabilization of the front-end and increasing the closed-loop gain of the DC/DC converter can be used to eliminate it.
Suppression of low-frequency ripple
a, Increase the inductance of the output low-frequency filter, increase the capacitance parameter, and reduce the low-frequency ripple to the desired level.
b, Adopting feedforward control method to reduce low-frequency ripple components.
High frequency ripple
High frequency ripple noise originates from high-frequency power switching conversion circuits. In the circuit, the input DC voltage is high-frequency switched by power devices, rectified and filtered to achieve stable output. The output terminal contains high-frequency ripple with the same frequency as the switching operating frequency. Its impact 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, the working frequency of the power converter should be increased as much as possible to reduce the filtering requirements for high-frequency switching ripple.
Suppression of high-frequency ripple
a, Raising the operating frequency of the switching power supply to increase the high-frequency ripple frequency is beneficial for suppressing the output high-frequency ripple.
b, Increasing the output high-frequency filter can suppress the output high-frequency ripple.
c, Adopting a multi-stage filter.
Common mode ripple noise
Due to the parasitic capacitance between the power device and the radiator base plate, as well as the primary and secondary sides of the transformer, and the parasitic inductance of the wires, when a rectangular wave voltage is applied to the power device, common mode ripple noise will be generated at the output terminal of the switching power supply. Reducing and controlling the parasitic capacitance between power devices, transformers, and casings, and adding common mode rejection inductors and capacitors on the output side can reduce the common mode ripple noise of the output.
