Reduce the switching power supply output "ripple and noise" 10 big tricks!

Ripple is mainly in five areas: 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 the power device, and ripple noise caused by closed-loop regulation and control.

■ Low frequency ripple is related to the capacity of the filter capacitors of the output circuit. The capacity of the capacitor cannot be increased indefinitely, resulting in the residual output low-frequency ripple. After the AC ripple is attenuated by the DC/DC converter, it manifests itself as low-frequency noise at the output of the switching power supply, the magnitude of which is determined by the 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 the output is still large. To realize the low ripple output of the switching power supply, filtering measures must be taken for the low-frequency power supply ripple. This can be eliminated by pre-regulating the voltage in the front stage and increasing the closed-loop gain of the DC/DC converter.

     Low-frequency ripple suppression of several commonly used methods:

 ⒈.Increase the output low-frequency filtering inductance, capacitance parameters, so that the low-frequency ripple is reduced to the desired target.

 ⒉. Adopt feed-forward control method to reduce the low-frequency ripple component.

■ High-frequency ripple noise from high-frequency power switching converter circuit, in the circuit, through the power device on the input DC voltage high-frequency switching converter and then rectifier filtering and then realize the regulated output, the output side of the high-frequency ripple with the same frequency as the switching frequency, the size of its impact on the external circuit is mainly and the switching power supply converter frequency, the structure of the output filter and the parameters of the relevant, the design try to improve the The operating frequency of the power converter can be designed to minimize the filtering requirements for high-frequency switching ripple.

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

⒈. Increase the operating frequency of the switching power supply to increase the high-frequency ripple frequency, which is conducive to the suppression of output high-frequency ripple

⒉. Increase the output high-frequency filter, can suppress the output high-frequency ripple.

⒊. The use of multi-stage filtering.

■ Due to the existence of parasitic capacitance between the power device and the heatsink base plate and the transformer primary and secondary edges, and parasitic inductance in the wires, the output of the switching power supply will generate common mode ripple noise when the rectangular wave voltage is applied to the power device. Reduce and control the parasitic capacitance between the power device, transformer and chassis ground, and add common mode suppression inductor and capacitor on the output side, can reduce the output common mode ripple noise.

  Common methods to reduce the output common mode ripple noise:

⒈. Use specially designed EMI filters for the output.

⒉. Reduce the amplitude of switching burrs.

■ Ultra-high frequency resonance noise mainly from high frequency rectifier diode reverse recovery diode junction capacitance, power device switching power device junction capacitance and line parasitic inductance resonance, the frequency is generally 1-10MHz, through the selection of soft recovery characteristics of the diode, the junction capacitance of the switching tube and reduce the length of the wiring and other measures to reduce ultra-high frequency resonance noise. Switching power supply are required for closed-loop control of the output voltage, regulator parameters are not properly designed will also cause ripple. When the output fluctuations through the feedback network into the regulator loop, may lead to self-excited oscillation of the regulator, causing additional ripple. This ripple voltage generally has no fixed frequency. In switching DC power supply, often due to the regulator parameter selection is not appropriate will cause the output ripple increase.

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

⒈.In the regulator output to increase the compensation network to the ground, regulator compensation can inhibit the regulator self-excitation caused by the increase in ripple.

⒉.Reasonable selection of the closed-loop regulator open-loop amplification and closed-loop regulator parameters, open-loop amplification is too large will sometimes cause oscillation or self-excitation of the regulator, so that the output ripple content increases, too small an open-loop amplification to make the output voltage stability deterioration and ripple content increases. Therefore, the regulator's open-loop amplification and the parameters of the closed-loop regulator should be selected reasonably, and should be adjusted according to the load conditions in the debugging.

⒊.In the feedback channel does not increase the pure hysteresis filter link. To minimize the delay hysteresis. To increase the rapidity and timeliness of closed-loop regulation, it is beneficial to suppress the output voltage ripple.