Four ways to reduce the output "ripple and noise" of a switching power supply
Ripple is mainly represented by five aspects: input low-frequency ripple, high-frequency ripple, common-mode ripple noise caused by parasitic parameters, microwave resonant noise caused by switching power devices, and ripple caused by closed-loop control waveform noise.
Low frequency ripple is related to filter capacitance of output circuit. Capacitor capacitance cannot be increased indefinitely, resulting in residual low-frequency ripple at output. After AC ripple is attenuated by DC/DC converter, it appears as low-frequency noise at output of switching power supply, and its magnitude is determined by transformation ratio of DC/DC converter and gain of regulator. system. The ripple suppression of DC/DC converter controlled in current mode is somewhat improved compared to voltage mode. But low-frequency ripple of alternating current at its output is still relatively large. To realize low ripple at output of switching power supply, measures must be taken to filter out low-frequency ripple of power supply. This can be eliminated by using prestage preconditioning and increasing closed loop gain of DC/DC converter.
A few common methods for reducing low frequency ripple:
a.Increase parameters of inductance and capacitance of output low-pass filter to reduce low-frequency ripple to required level.
b. Use feed-forward control method to reduce low frequency ripple.
High-frequency ripple noise comes from a high-frequency power conversion circuit. In this circuit, input DC voltage is converted by high-frequency switching through a power device, and then rectified and filtered to obtain a stable output signal. , output terminal contains and switch high-frequency ripple at same frequency as operating frequency, its effect on external circuit is mainly due to switching frequency of switching power supply, structure and parameters of output filter, and design should try to increase operating frequency of power converter to reduce impact on filtering requirements of high-frequency switching ripples.
The purpose of high frequency ripple suppression is to provide a path for high frequency ripple. Commonly used methods are:
a. Increase operating frequency of switching power supply to increase frequency of high-frequency ripple, which is conducive to suppression.
b. Increasing output high-pass filter can suppress high-frequency output ripple.
c using multi-stage filtering.
Due to parasitic capacitance between power unit and bottomThe heat sink and primary and secondary sides of transformer, as well as parasitic inductance of wire, when a square-wave voltage is applied to power device, common mode ripple will be generated at output of switching power supply. Reduce and control parasitic capacitance between power devices, transformers, and chassis ground, and add inductors and common-mode suppression capacitors on output side to reduce output common-mode ripple noise.
Common methods for reducing output common-mode ripple noise:
a. The output uses a specially designed EMI filter.
b.Reduce amplitude of switching noise.
UHF resonant noise is mainly due to resonance of diode junction capacitance during reverse recovery of high frequency rectifier diode, power device junction capacitance, and line parasitic inductance during power off. The device is switched The frequency is typically 1-10 MHz By selecting a soft recovery characteristic Measures such as diodes, switching tubes with low junction capacitance, and reduced wire length can reduce UHF resonant noise.
Switching power supplies require closed-loop output voltage regulation, and miscalculation of regulator parameters also causes ripple. When output signal fluctuates, it enters regulator loop through feedback circuit, which can cause regulator to self-oscillate, causing additional ripple. This pulsating voltage usually does not have a fixed frequency. In a pulsed DC source, an increase in output ripple is often caused by improper selection of controller parameters.
This part of ripple can be suppressed by following methods:
a. Add a compensation network to ground at output of regulator, and regulator compensation can suppress increase in ripple caused by regulator self-excitation.
b. Wisely select open-loop gain of closed-loop controller and parameters of closed-loop controller. If open-loop gain is too high, it can sometimes cause regulator to oscillate or self-excite. , which will increase ripple content of output. If opening is too small Increasing loop degrades stability of output voltage and increases ripple content. Therefore, gain of open-loop controller and closed-loop parameters of controller should be chosen wisely, and during debugging, adjustments should be made according to load conditions .
c. No net delay filtering is added to feedback channel. Latency is minimized to increase speed and timeliness of feedback adjustment, which is useful for suppressing output voltage ripple.
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