Causes of ripple and noise in switching power supplies
The output of the switching power supply is not a pure DC voltage, but contains some AC components, which is caused by ripple and noise. Ripple is the fluctuation of the output DC voltage, which is related to the switching action of the switching power supply. During each opening and closing process, electric energy is "pumped" from the input end to the output end, forming a charging and discharging process, which causes the output voltage to fluctuate with the same frequency as the switching frequency. The ripple voltage is the peak-to-peak value between the peak and the trough of the ripple. Its size is related to the capacity and quality of the input capacitor and output capacitor of the switching power supply.
There are two causes of noise, one is generated by the switching power supply itself; the other is external electromagnetic field interference (EMI), which can enter the switching power supply through radiation or input the switching power supply through the power line.
The noise generated by the switching power supply itself is a high-frequency pulse train, which is caused by the sharp pulses generated at the moment when the switch is turned on and off. It is also called switching noise. The frequency of the noise pulse train is much higher than the switching frequency, and the noise voltage is its peak-to-peak value. The amplitude of the noise voltage is largely related to the topology of the switching power supply, the parasitic state in the circuit and the design of the PCB.
Use an oscilloscope to see the ripple and noise waveforms. The frequency of the ripple is the same as the frequency of the switching tube, while the frequency of the noise is twice that of the switching tube. The sum of the peak-to-peak value of the ripple voltage and the peak-to-peak value of the noise voltage is the ripple and noise voltage, and its unit is mVp-p.
Ripple and Noise Measurement Methods
Ripple and noise voltage are one of the main performance parameters of switching power supplies, so how to accurately measure them is a very important issue. The current method of measuring ripple and noise voltage is to use a wide-band oscilloscope, which can accurately measure ripple and noise voltage values.
Since there are many types of switching power supplies (with different topologies, operating frequencies, output powers, different technical requirements, etc.), all manufacturers use oscilloscope measurement methods. Only the measurement devices are not exactly the same. Therefore, each manufacturer has different requirements for different switching power supplies. All measurements have their own standards, namely corporate standards.
Block diagram of a setup for measuring ripple and noise with an oscilloscope. It consists of the tested switching power supply, load, oscilloscope and measurement connections. Some measuring devices also have inductors, capacitors, resistors and other components welded on them.
