Measures to reduce ripple and noise voltage in switching power supplies
Compared with linear power supplies, switching power supplies (including AC/DC converters, DC/DC converters, AC/DC modules, and DC/DC modules) have the most prominent advantage of high conversion efficiency, which can generally reach 80% to 85%, and up to 90% to 97%; Secondly, the switching power supply uses high-frequency transformers instead of bulky power frequency transformers, which not only reduces weight but also reduces volume, resulting in an increasingly wide range of applications. However, the disadvantage of switching power supply is that its switch transistor operates in a high-frequency switching state, and the output ripple and noise voltage are relatively large, usually around 1% of the output voltage (the low is about 0.5% of the output voltage). The best product also has a ripple and noise voltage of tens of mV; The adjustment tube of the linear power supply operates in a linear state, without ripple voltage, and the output noise voltage is also small, with the unit of μ V.
This article briefly introduces the causes and measurement methods of ripple and noise generated by switching power supplies, measurement devices, measurement standards, and measures to reduce ripple and noise.
1、 Reasons for ripple and noise generation:
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.
2、 Measures to reduce ripple and noise voltage:
In addition to switching noise, the switching power supply input in the AC/DC converter undergoes full wave rectification and capacitor filtering, and the current waveform is pulse, as shown in Figure 17 (Figure a shows the full wave rectification and filtering circuit, and b shows the voltage and current waveforms). There are higher-order harmonics in the current waveform, which will increase the noise output. A good switching power supply (AC/DC converter) has added a power factor correction (PFC) circuit in the circuit, making the output current approximate sine wave, reducing high-order harmonics, and increasing the power factor to around 0.95, reducing pollution to the power grid.
