The Causes of Ripples and Noise in Switching Power Supplies
The output of the switching power supply is not pure DC voltage, but there are some AC components inside, which are 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. In each opening and closing process, electrical energy is "pumped" from the input end to the output end, forming a process of charging and discharging, resulting in fluctuations in the output voltage, with a frequency similar to the frequency of the switch. Ripple voltage is the peak to peak value between the peaks and valleys of the ripple, and its size is related to the capacity and quality of the input and output capacitors of the switching power supply.
There are two reasons for noise generation: one is generated by the switching power supply itself; Another type is interference from external electromagnetic fields (EMI), which can enter the switching power supply through radiation or input through power lines.
The noise generated by the switching power supply itself is a high-frequency pulse train caused by sharp pulses generated at the moment of switch conduction and cutoff, also known as 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 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.
The waveform of ripple and noise can be seen using an oscilloscope, as shown in Figure 1. The frequency of the ripple is the same as the frequency of the switch tube, while the frequency of the noise is twice that of the switch tube. The sum of the peak to peak values of ripple voltage and noise voltage is the ripple and noise voltage, expressed in mVp-p.
Measurement methods for ripple and noise
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. At present, the method of measuring ripple and noise voltage is using a broadband oscilloscope, which can accurately measure the ripple and noise voltage values.
Due to the wide variety of switch power supplies (with different topologies, operating frequencies, output power, and technical requirements), various manufacturers use oscilloscope measurement methods, and the measurement devices are not completely the same. Therefore, each factory has its own standard for measuring different switch power supplies, which is the enterprise standard.
The block diagram of the device for measuring ripple and noise using an oscilloscope is shown in Figure 2. It consists of the tested switch power supply, load, oscilloscope, and measurement wiring. Some measuring devices also have components such as inductors, capacitors, and resistors soldered onto them.
