1. What is ripple?
The definition of ripple (ripple) refers to the AC component superimposed on the DC stable quantity in the DC voltage or current.
It mainly has the following disadvantages:
1.1. It is easy to generate harmonics on electrical appliances, and harmonics will cause more harm;
1.2. reduces the efficiency of the power supply;
1.3. Strong ripple will cause surge voltage or current, resulting in burning electrical appliances;
1.4. It will interfere with the logic relationship of the digital circuit and affect its normal operation;
1.5. It will cause noise interference, so that the image equipment and audio equipment cannot work normally.
Second, the main source of switching power supply ripple
2.1. Input low frequency ripple;
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 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. To realize the low ripple output of the switching power supply, filtering measures must be taken for the low frequency power supply ripple.
2.2. High frequency ripple:
High-frequency ripple noise comes from high-frequency power switching circuit
In the circuit, the input DC voltage is rectified and filtered by the power device to perform high-frequency switching and then rectified and filtered to realize the regulated output. The output terminal contains high-frequency ripple of the same frequency as the switching operating frequency, and its influence on the external circuit is mainly It is related to the conversion frequency of the switching power supply, the structure and parameters of the output filter;
In the design, the operating frequency of the power converter should be increased as much as possible, which can reduce the filtering requirements for the high-frequency switching ripple.
2.3. Common mode ripple noise caused by parasitic parameters:
Due to the parasitic capacitance between the power device and the bottom plate of the radiator and the primary and secondary sides of the transformer, the wires have parasitic inductance, reduce and control the parasitic capacitance between the power device, the transformer and the chassis ground, and effectively ground the radiator (according to In different cases, you can choose to ground through a capacitor or a capacitor series resistor), and add a common-mode inductor and capacitor on the output side to reduce the common-mode ripple noise of the output.
Therefore, 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.
2.4. Ultra-high frequency resonance noise generated during switching of power devices;
UHF resonance noise mainly comes from:
※ Diode junction capacitance and power device switching during reverse recovery of high-frequency rectifier diodes
※ Resonance between the junction capacitance of the power device and the parasitic inductance of the line;
※ The frequency is generally 1-10MHz;
