The output ripple of switch power supply mainly comes from five aspects: low frequency ripple, high frequency ripple, common mode ripple noise caused by parasitic parameters, ultra-high frequency resonant noise generated during power device switching, and closed-loop modulation control. Wave noise.
First, the low frequency ripple is related to the filter capacitance capacity of the output circuit. Capacitors 'capacity can not be increased indefinitely, resulting in the output of low-frequency textures. After the AC ripple is attenuated by the DC/DC converter, the low-frequency noise at the output of the switch power supply 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 higher than that of the voltage type. However, the low-frequency AC ripple at its output is still large. If we want to realize the low ripple output of the switch power supply, we must take filtering measures to the low frequency power supply ripple. The elimination can be achieved by using the front stage prestabilizer and increasing the closed-loop gain of the DC/DC converter.
Several commonly used methods of ripple suppression:
Low frequency a, increase the output of low frequency filter inductors, capacitance parameters, low frequency ripple reduced to the required indicators.
B, adopt feedforward control method to reduce the low frequency ripple component.
Second, the high-frequency ripple noise comes from the high-frequency power switch transformation circuit. In the circuit, the power device performs a high-frequency switch transformation of the input DC voltage and then rectifies the filter to achieve the voltage stabilization output. The output end contains high-frequency ripple waves with the same frequency as the switching operating frequency. The influence of the external circuit is mainly related to the conversion frequency of the switching power supply, the structure of the output filter, and the parameters of the output filter. The power converter is designed to maximize the operating frequency. It can reduce the filtering requirements for high-frequency switching ripple.
The purpose of high-frequency ripple suppression is to provide access to high-frequency ripple. The commonly used methods are as follows:
A) Improve the operating frequency of the switch power supply to increase the high-frequency ripple frequency, which is conducive to suppressing the output high-frequency ripple
B, increase output high frequency filter, can suppress output high frequency ripple.
C, use multi-level filtering.
Third, because there is a parasitic capacitor between the power device and the radiator base plate and the original and secondary sides of the transformer, there is a parasitic inductance in the wire. Therefore, when the rectangular wave voltage acts on the power device, the output end of the switch power supply will thus produce a common mode ripple noise. By reducing the parasitic capacitance between the power device, Transformer and the shell, and adding the common mode to the output side to suppress the inductance and capacitance, the common mode ripple noise of the output can be reduced.
Commonly used methods for reducing output common mode wave noise:
A, the output uses a specially designed EMI filter.
B, reduce the switch burr range.
Ultra high frequency resonant noise is mainly derived from the resonance of the diode junction capacitor and the power device junction capacitor and the line parasitic inductor at the reverse recovery of the high-frequency rectifier diode. The frequency is generally 1-10 MHz. Ultra high frequency resonant noise can be reduced by using soft recovery diode, switch tube with small junction capacitance and reducing wiring length.
Switching power supply needs to control the output voltage in a closed-loop way. Improper design of regulator parameters can also cause ripple. When the output is fluctuating, it enters the regulator loop through the feedback network, which may cause the regulator's self-excited oscillation and cause additional ripple. This ripple voltage generally has no fixed frequency.
In switching DC power supply, the output ripple increase is often caused by improper parameter selection of regulator.
This part of the ripple can be suppressed by the following methods:
A, in the regulator output to increase the compensation network to the ground, the regulator's compensation can inhibit the regulator self-excitation caused by the increase of the ripple.
B) reasonably choose the open-loop magnification of the closed-loop regulator and the parameters of the closed-loop regulator. If the open-loop magnification is too large, it will sometimes cause the regulator's oscillation or self-excitation, so that the output grain content will increase. The output voltage stability is reduced and the ripple content is increased by the small open-loop magnification. Therefore, the parameters of the open-loop magnification and closed-loop regulator of the regulator should be reasonably selected, and the adjustment should be made according to the load condition during debugging.
C, do not add pure lag filter loop in feedback channel. So that the delay lag is minimized. In order to increase the speed and timeliness of closed-loop regulation, it is beneficial to suppress the output voltage ripple.
Win
2019-03-26
Disclaimer: This article comes from individual, KRECO has the right of final interpretation.
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