Low frequency ripple

Low frequency ripple is related to the filter capacitance of the output circuit. Due to the size limitation of switching power supply, the capacity of electrolytic capacitor can not be increased without limitation, resulting in the residual low-frequency output ripple, which varies with the rectifier circuit.

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Low frequency ripple

For example, for ordinary 24 V power supply, the ripple rejection ratio of voltage controlled DC / DC converter is generally 45 ~ 50 dB, and the effective value of low frequency AC ripple at its output is 60 ~ 120 MV. The ripple rejection ratio of current mode controlled DC / DC converter is slightly improved, but the low frequency AC ripple of its output is still large. In order to achieve low ripple output of switching power supply, it is necessary to take filtering measures for low-frequency power supply ripple. It can be eliminated by pre stabilizing and increasing the closed-loop gain of DC / DC converter.

Suppression of 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. Feedforward control method is used to reduce low frequency ripple.

High frequency ripple

The 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 switch through the power device, and then rectified and filtered to achieve the output voltage stabilization. The output end contains the high-frequency ripple with the same frequency as the switch working frequency. Its influence on the external circuit is mainly related to the conversion frequency of the switching power supply and the structure of the output filter It is related to the parameters. In the design, increasing the working frequency of the power converter as much as possible can reduce the filtering requirements of high frequency switching ripple.

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High frequency ripple

Suppression of high frequency ripple

a. Increasing the working frequency of the switching power supply can improve the high-frequency ripple frequency, which is beneficial to suppress the output high-frequency ripple.

b. The output high frequency ripple can be suppressed by increasing the output high frequency filter.

c. Multistage filtering is used.

Common mode ripple noise

Due to the parasitic capacitance between the power device and the radiator bottom plate 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 of the switching power supply will produce common mode ripple noise. The output common mode ripple noise can be reduced by reducing and controlling the parasitic capacitance between power device, transformer and enclosure ground, and adding common mode suppression inductor and capacitor at the output side.

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Common mode ripple noise

a. The output adopts specially designed EMI filter

b. Reduce switch burr

UHF resonant noise

The UHF resonant noise mainly comes from the resonance between the diode junction capacitance and the parasitic inductance of the line when the high-frequency rectifying diode reversely recovers, and the power device junction capacitance and the line parasitic inductance when the power device switches. The frequency is generally 1 ~ 10MHz. The UHF resonant noise can be reduced by selecting the diode with soft recovery characteristics, switching transistor with small junction capacitance and reducing the wiring length.

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UHF resonant noise

Suppression of UHF resonant noise

The UHF resonant noise can be reduced by selecting soft recovery diode, switching transistor with small junction capacitance and reducing wiring length.

Ripple noise caused by closed loop control

The output voltage of switching power supply must be controlled by closed-loop control, and the improper design of regulator parameters will also cause ripple. When the output end fluctuates, it enters the regulator loop through the feedback network, which may cause the self-excited oscillation of the regulator and cause additional ripple. This ripple voltage generally has no fixed frequency.

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Ripple noise caused by closed loop control

Suppression of ripple noise caused by closed loop control

In switched DC power supply, the output ripple is often increased due to the improper selection of regulator parameters, which can be suppressed by the following methods.

a. 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 self excitation of the regulator.

b. The open-loop magnification and the parameters of the closed-loop regulator should be selected reasonably. If the open-loop magnification is too large, it will cause the oscillation or self excitation of the regulator and increase the output ripple content. If the open-loop magnification is too small, the stability of the output voltage will become worse and the ripple content will increase. Therefore, the open-loop magnification and the parameters of the closed-loop regulator should be selected reasonably, During debugging, it should be adjusted according to the load condition.

c. In order to increase the speed and timeliness of the closed-loop regulation, it is beneficial to suppress the output voltage ripple

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