Ripple test - the problem you have neglected in power supply testing (2)

For DC power supply, the purity of DC is evaluated, so there is a ripple test .

Voltage fluctuations are generally divided into two types: ripple and noise. Generally speaking, fluctuations with a frequency lower than 500KHz are called ripples, and those with a frequency higher than 20MHz are generally not tested (they belong to the category of interference).

When testing ripple , are you using your oscilloscope correctly for the following points?

✔The bandwidth of the oscilloscope is limited to 20M, which is necessary. Coupling mode: AC coupling

✔The oscilloscope must be tested in a floating state, that is, the ground wire of the oscilloscope's power triangle plug must be removed and not connected to the ground wire of the plug board. Be especially careful about this, especially when the power supply voltage is high.

✔ Use the probe position X1, you can check the probe settings during your test. Even if the power supply output voltage is high.

✔ Use the near-ground end of the probe instead of the standard one with a clip. Generally, a line no longer than 2CM is drawn from the near-ground end of the probe for testing.

✔When necessary, connect resistors and capacitors in parallel at both ends of the probe to filter out high-frequency signals.

How to solve the problem of excessive ripple?

✔Adjust the LC filter circuit and increase the switching frequency. Disadvantages: L cannot be very large, and when C reaches a certain level, the filtering improvement effect will deteriorate. High switching frequency will increase switching losses and reduce conversion efficiency.

✔ Add a secondary filter LC. Advantages: You can choose according to the frequency, and the filtering effect is good. Disadvantages: The voltage feedback point should be selected later. And the introduction of a new LC circuit will affect the system stability of the power supply.

✔ Add LDO filtering, which has good effect, high cost and reduced conversion efficiency.

✔ The simplest and most effective method: connect a diode in parallel with a capacitor, or connect a resistor and a capacitor in parallel, or connect an inductor in series.

The most valuable things are often at the end. I hope you didn’t miss it. If you are interested, you can give it a try.