1. Ripple test method

The bandwidth of the oscilloscope is set to 20M, the peak-to-peak value is selected for reading, the clip and ground wire on the oscilloscope probe are removed (because the clip and ground wire themselves will form a loop, like an antenna receiving noise, introducing some unnecessary noise), a ground ring is used (it is also possible not to use a ground ring, but the error it causes must be considered), a 10UF electrolytic capacitor and a 0.1UF ceramic capacitor are connected in parallel to the probe, and the test is performed directly with the oscilloscope probe; if the oscilloscope probe is not in direct contact with the output point, it should be measured with a twisted pair or a 50Ω coaxial cable.

two,Opening power supplyMain categories of ripple

The output ripple of the switching power supply mainly comes from five aspects:

(1) Input low frequency ripple;

(2) High frequency ripple;

(3) Common mode caused by parasitic parametersRipple Noise;

(4) Ultra-high frequency resonant noise generated during the switching process of power devices;

(5) Ripple noise caused by closed-loop regulation control.

3. Improvement methods

(1) Low-frequency ripple is related to the filter capacitor capacity of the output circuit. Common methods to suppress low-frequency ripple: increase the inductance and capacitance parameters of the output low-frequency filter.

• The ripple on the capacitor has two components, one is the voltage rise and fall during charging and discharging, and the other is the I*R voltage drop on the ESR when the current enters and exits the capacitor;
• From the relationship between output ripple and output capacitance: Vripple = Imax/(Co×f), it can be seen that increasing the output capacitance value can reduce the ripple;
• Use parallel connection to reduce the ESR value, or use low ESR capacitors.

(2) High-frequency ripple noise is mainly caused by the mismatch of component parameters or parasitic effects in the internal circuit of the switching power supply. For example, the parasitic inductance of capacitors and the parasitic capacitance of resistors will generate high-frequency ripple noise. This noise usually has a higher frequency but a relatively small amplitude. Commonly used suppression methods:

• Use multi-stage filtering;
• Increase the output high frequency filter
• Increase the operating frequency of the switching power supply to increase the high-frequency ripple frequency

(3) Common-mode ripple noise is generated due to the potential difference between components in the internal circuit of the switching power supply. When the switching power supply operates at a high frequency, the potential difference causes current to flow between the components, thus generating common-mode ripple noise. This noise has a significant impact on circuit performance and stability, and measures need to be taken to suppress it.

• Use specially designed EMI filters;
• Reduce switching glitch amplitude

(4) Ultra-high frequency resonance noise is caused by the resonance effect between components in the internal circuit of the switching power supply. When the switching power supply operates at high frequency, the resonance effect between components will cause fluctuations in current and voltage, thereby generating ultra-high frequency resonance noise. This noise has a significant impact on circuit performance and stability, and measures need to be taken to suppress it.

• Adopt soft recovery characteristic diode;
• Use a switch tube with small junction capacitance;
• Reduce wiring length.