There are five main aspects of ripple: input low-frequency ripple, high-frequency ripple, common ode ripple noise caused by parasitic parameters, ultra high-frequency resonant noise generated in the switching process of power devices and ripple noise caused by closed-loop regulation control.
Low frequency ripple is related to the filter capacitance capacity of the output circuit.
The capacity of the capacitor cannot be increased without limit, resulting in the output of low-frequency ripple residue, AC ripple attenuated by the DC/DC converter, at the power adapter output as low-frequency noise, which is determined by the ratio of the DC/DC converter and the gain of the control system.
The ripple suppression of current-controlled DC/DC converter is slightly improved than that of voltage control DC/DC converter, but the low-frequency AC ripple at the output end is still large. In order to achieve low ripple output of power adapter, the heart must take filtering measures for low-frequency power supply ripple. This can be eliminated by stabilizing the voltage in the front stage and increasing the closed-loop gain of the DC/DC converter.
Several common methods of low-frequency ripple suppression:
a. Increase the inductance and capacitance parameters of the output low-frequency filter to reduce the low-frequency ripple to the required index.
b. Adopt feedforward control method to reduce the low frequency ripple component.
High-frequency ripple noise comes from high-frequency power switching circuit.
In the circuit, the input DC voltage is transformed by high-frequency switching through the power device and then rectified and filtered to achieve voltage stable output, and the output end contains high-frequency ripple of the same frequency as the switch working frequency. Its influence on the external circuit is mainly related to the conversion frequency of the power adapter, the structure and parameters of the output filter. In the design, the working frequency of the power converter is increased as far as possible, which can reduce the filtering requirements of the high-frequency switching ripple.
The purpose of high-frequency ripple suppression is to provide a path for high-frequency ripple. The commonly used methods are as follows:
a. Increase the working frequency of the power adapter to improve the high-frequency ripple frequency, which is conducive to suppressing the output high-frequency ripple
b. Increase the output high-frequency filter to suppress the output high-frequency ripple.
c. Multistage filtering is adopted.
Common ode ripple noise caused by parasitic parameters
Due to the existence of parasitic inductance between the power device and the radiator baseplate and the primary side and secondary side of the transformer, as well as the existence of parasitic inductance in the wire, when the rectangular wave voltage is applied to the power device, the output end of the power adapter will generate common-mode ripple noise. The output common-mode ripple noise can be reduced by reducing and controlling the parasitic capacitance between the power device, transformer and housing, and adding common-mode suppression inductance and capacitance to the output side.
Common methods to reduce output common-mode ripple noise:
A. The output adopts specially designed EMI filter.
B. Reduce the burr amplitude of the switch.
Ultra high-frequency resonant noise generated in the switching process of power devices and ripple noise caused by closed-loop regulation control
The ultra-high-frequency resonant noise mainly comes from the diode junction capacitance during the reverse recovery of the high-frequency rectifier diode, the power device switching, and the resonance between the capacitor and the parasitic inductor of the line. The frequency is generally 1-10MHz. The ultra-high-frequency resonant noise can be reduced by using soft recovery diodes, switching tubes with small capacitance and reducing the length of wiring.
All power adapters need to control the output voltage in a closed loop. Improper design of regulator parameters may also cause ripple. When the output fluctuates, it is introduced into the regulator loop through the feedback network, which may lead to self-excited oscillation of the regulator and cause additional ripple. The ripple voltage generally has no fixed frequency. In switched DC power supply, the increase of output ripple is often caused by improper selection of regulator parameters.
This part of the ripple can be suppressed by:
A. Add a compensation network to ground in the output of the regulator, and the compensation of the regulator can suppress the increase of ripple caused by the self-excitation of the regulator.
b. Reasonable selection of open-loop magnification and closed-loop regulator parameters. Too large open-loop magnification will sometimes cause oscillation or self-excitation of the regulator and increase the output ripple content; too small open-loop magnification will lead to poor stability of the output electric bed and increase the ripple content. Debugging should be adjusted according to the load condition,
C. No pure hysteresis filtering is added in the feedback channel to minimize the delay lag, so as to increase the rapidity and timeliness of closed-loop regulation, which is beneficial to the suppression of output voltage ripple.
JPTPOWER produces power adapter ripple control within 120mvp-p. Good ripple helps to reduce the interference to the end product. If you have this problem, please feel free to contact our sales. They can provide you with the perfect solution.
