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How to reduce the output ripple and noise of switching power supplies?

Article Source：Kinri Energy | Author：Kinri Energy | Issuing Time：2024.06.05
The 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 power device switching process, and ripple noise caused by closed-loop regulation control.


250W Switching Power Supply

1. Low frequency ripple is related to the filtering capacitor capacity of the output circuit. The capacity of a capacitor cannot increase indefinitely, resulting in residual low-frequency ripple output. After being attenuated by a DC/DC converter, the AC ripple exhibits low-frequency noise at the output of the switching power supply, which is determined by the ratio of the DC/DC converter and the gain of the control system. The ripple suppression of current controlled DC/DC converters is slightly improved compared to voltage controlled ones. But the low-frequency AC ripple at its output end is still relatively large. To achieve low ripple output of switching power supplies, filtering measures must be taken for low-frequency power supply ripple. It can be eliminated by using pre stabilization and increasing the closed-loop gain of the DC/DC converter.

Several commonly used methods for low-frequency ripple suppression:

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

● Adopting feedforward control method to reduce low-frequency ripple components.

2. High frequency ripple noise comes from high-frequency power switch conversion circuits. In the circuit, the input DC voltage is high-frequency switched through power devices, then rectified and filtered to achieve stable output. The output end contains high-frequency ripple with the same frequency as the switching frequency. The impact on 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 working frequency of the power converter should be increased as much as possible to reduce the filtering requirements for high-frequency switching ripple.

The purpose of high-frequency ripple suppression is to provide a pathway for high-frequency ripple, and commonly used methods include the following:

●  Increasing the operating frequency of the switching power supply to increase the high-frequency ripple frequency is beneficial for suppressing the output high-frequency ripple.

● Increasing the output high-frequency filter can suppress high-frequency ripple.

●  Adopting multi-level filtering.

3. Due to the presence of parasitic capacitance and parasitic inductance in the wires between the power device, the heat sink base plate, and the primary and secondary sides of the transformer, when a rectangular wave voltage acts on the power device, common mode ripple noise will be generated at the output end of the switching power supply. Reducing and controlling the parasitic capacitance between power devices, transformers, and casing ground, and adding common mode suppression inductance and capacitance on the output side can reduce the common mode ripple noise of the output.

Common methods to reduce output common mode ripple noise:

● The output adopts a specially designed EMI filter.

● Reduce the amplitude of switch burrs.

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

Switching power supplies require closed-loop control of the output voltage, and improper design of regulator parameters can also cause ripple. When the output fluctuates and enters the regulator circuit through the feedback network, it may cause self-excited oscillation of the regulator, causing additional ripple. This ripple voltage generally does not have a fixed frequency. In switching DC power supplies, improper selection of regulator parameters often leads to an increase in output ripple.

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

● By adding a compensation network to the ground at the output of the regulator, the compensation of the regulator can suppress the ripple increase caused by the regulator's self-excitation.

● Reasonably select the open-loop amplification factor and parameters of the closed-loop regulator. Excessive open-loop amplification factor can sometimes cause oscillation or self-excitation of the regulator, leading to an increase in output ripple content. A too small open-loop amplification factor can lead to a decrease in output voltage stability and an increase in ripple content. 

Therefore, the open-loop amplification factor and parameters of the closed-loop regulator should be reasonably selected, and adjustments should be made according to load conditions during debugging.

● Not adding a pure lag filtering link in the feedback channel to minimize the delay lag, in order to increase the speed and timeliness of closed-loop regulation, which is beneficial for suppressing output voltage ripple.
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