Switching power supply high frequency is the direction of its development, high frequency makes the switching power supply miniaturization, and makes the switching power supply into a wider range of applications, especially in the field of high and new technology applications, promote the development of the switching power supply, and toward light, small, thin, low noise, high reliability, anti-interference direction.Switching power supply can be divided into AC/DC and DC/DC two categories, DC/DC converter has been modular;The modularization of AC/DC, because of its own characteristics makes in the process of modularization, encountered more complex technology and process manufacturing problems.In addition, the development and application of switching power supply is of great significance in energy saving, resource saving and environmental protection.

But switching power supply also has its disadvantages: because the switch tube works in the high frequency switching state, the output ripple and noise voltage is large, generally about 1% of the output voltage (low output voltage about 0.5%), the best product ripple and noise voltage also has dozens of mV, so its ripple and noise is how to generate?

Generation of ripple of switching power supply:

Switching power supply output is not the pure DC voltage, each open, close process, electric energy from the input end is "pumped to" the output end, forming a process of charging and discharging, resulting in the output voltage fluctuations, fluctuation frequency and switch frequency is the same.Ripple voltage is the peak value between the peak and the trough of the ripple, and its size is related to the capacity and quality of the input capacitance and output capacitance of the switching power supply.

With the SWITCH SWITCH, the current in the inductor L also fluctuates around the effective value of the output current.Therefore, there will also be a ripple of the same frequency as the SWITcH at the output end, which is generally referred to as the ripple.It is related to the capacity of the output capacitor and ESR.The frequency of this ripple is the same as that of the switching power supply, ranging from tens to hundreds of kHz.

Harm of ripple to switching power supply:

1. Easy to produce harmonics in electrical appliances, and harmonics will produce more harm;

2. The efficiency of power supply is reduced;

3. Strong ripple will cause the generation of surge voltage or current, resulting in the burning of electrical appliances;

4. It will interfere with the logical relationship of digital circuit and affect its normal work;

5. It will bring noise interference, so that the image equipment and audio equipment can not work properly;

How to suppress switching power ripple and noise:

1.Increase the filter of inductance and output capacitance: according to the formula of switching power supply, the current fluctuation in the inductance is inversely proportional to the value of inductance, and the output ripple is inversely proportional to the output capacitance. Therefore, the ripple can be reduced by increasing the value of inductance and output capacitance. 

2. The two-stage filter is equivalent to adding a one-stage LC filter: the LC filter has an obvious inhibition effect on the noise ripple. The filter circuit can be formed by selecting appropriate inductors and capacitors according to the ripple frequency to be removed, which can generally well reduce the ripple.

3. After the output of switching power supply, connect LDO filter: this method is the most effective to reduce ripple and noise. It does not need to change the original feedback system, but it has high cost and high power consumption. 

4. In the diode and capacitor C or RC: diode high-speed conduction cutoff, parasitic parameters should be considered.During diode reverse recovery, the equivalent inductance and equivalent capacitance become a RC oscillator, producing high frequency oscillations.To suppress this high frequency oscillation, a capacitor C or RC buffer network is needed in parallel at both ends of the diode.

5.Diode back inductor (EMI filter) : according to the frequency of noise generation, the selection of appropriate inductor can also effectively suppress noise.It should be noted that the rated current of the inductor should meet the actual requirements. 

Switching power supply in addition to switching noise, the input of the mains in the AC/DC converter through full-wave rectification and capacitor filtering, the current waveform is pulse, the current waveform has high order harmonics, it will increase the noise output.Good switching power supply (AC/DC converter) in the circuit to increase the power factor correction (PFC) circuit, so that the output current approximate sine wave, reduce the high order harmonics, power factor increased to about 0.95, reduce the pollution to the power grid.

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