How to suppress the ripple noise of switching power supply
1. Capacitor filtering,
2. LC filter,
3. How is LDO filtering realized? The following 5 forms can be selected according to the actual situation.
low frequency ripple
The low-frequency ripple is related to the filter capacitor capacity of the output circuit. Due to the limitation of the size of the switching power supply, the capacity of the electrolytic capacitor cannot be increased without limit, resulting in the residual of the output low-frequency ripple. The frequency of the output ripple varies with the rectification circuit mode.
Suppression of low frequency ripple
a. Increase the inductance of the output low-frequency filter, increase the capacitance parameters, and reduce the low-frequency ripple to the required index.
b. Adopt feed-forward control method to reduce low-frequency ripple components.
High frequency ripple
The high-frequency ripple noise comes from the high-frequency power switching conversion circuit. In the circuit, the input DC voltage is converted by high-frequency switching through the power device, and then rectified and filtered to realize the stable voltage output. The high-frequency ripple of the frequency, its influence on the external circuit is mainly related to the conversion frequency of the switching power supply, the structure and parameters of the output filter, and the operating frequency of the power converter should be increased as much as possible in the design to reduce the impact on the high-frequency switching ripple. filtering requirements.
Suppression of high frequency ripple
a. Increase the operating frequency of the switching power supply to increase the high-frequency ripple frequency, which is beneficial to suppress the output high-frequency ripple.
b. Enlarging the output high-frequency filter can suppress the output high-frequency ripple.
c, using multi-stage filters.
Common Mode Ripple Noise
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, 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 generate common-mode ripple noise. Reduce and control the parasitic capacitance between power devices, transformers and casings, and add common-mode suppression inductors and capacitors on the output side to reduce output common-mode ripple noise.
a. The output adopts a specially designed EMI filter
b. Reduce switching glitch amplitude
UHF resonance noise
Ultra-high-frequency resonance noise mainly comes from the resonance of diode junction capacitance during reverse recovery of high-frequency rectifier diodes, power device junction capacitance and line parasitic inductance when power devices are switched, and the frequency is generally 1-10MHz. Measures such as switching tubes with small capacitance and reducing wiring length can reduce ultra-high frequency resonance noise.
Suppression of UHF Resonant Noise
Ultra-high-frequency resonance noise can be reduced by selecting diodes with soft recovery characteristics, switching tubes with small junction capacitance, and reducing wiring length.
Ripple noise caused by closed-loop regulation control
All switching power supplies require closed-loop control of the output voltage, and improper design of regulator parameters will also cause ripples. When the output fluctuates, it enters the regulator loop through the feedback network, which may cause the self-excited oscillation of the regulator, causing additional ripple. This ripple voltage generally has no fixed frequency.
Suppression of Ripple Noise Caused by Closed-loop Adjustment Control
In the switching DC power supply, the output ripple is often increased due to improper selection of regulator parameters, and this part of the ripple can be suppressed by the following methods.
a. Add a compensation network to the ground at the output of the regulator, and the compensation of the regulator can suppress the increase of the ripple caused by the self-excitation of the regulator.
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, sometimes it will cause oscillation or self-excitation of the regulator, which will increase the content of the output pattern. If the open-loop magnification is too small The stability of the output voltage will deteriorate and the ripple content will increase, so the open-loop magnification of the regulator and the parameters of the closed-loop regulator should be selected reasonably, and adjusted according to the load condition during debugging.
c. No pure lag filtering link is added in the feedback channel to minimize the delay and lag, so as to increase the rapidity and timeliness of the closed-loop adjustment, which is beneficial to suppress the output voltage ripple
