# Basic knowledge of circuit testing: power supply ripple analysis and testing with an oscilloscope

First, what is ripple?

Ripple is defined as the AC component superimposed on the DC stabilized amount in a DC voltage or current.

It has the following main disadvantages:

1.1. It is easy to generate harmonics on the electrical appliances, and harmonics will cause more harm;

1.2. Reduce the efficiency of the power supply;

1.3. Strong ripples can cause surge voltage or current, resulting in burning electrical appliances;

1.4. Will interfere with the logical relationship of the digital circuit, affecting its normal operation;

1.5. Will cause noise interference, making image equipment and audio equipment not working properly

Second, the representation of ripple and ripple coefficients

It can be expressed by a rms value or a peak value, or by an absolute quantity or a relative quantity;

The unit is usually: mV

A power supply operates in a regulated state, its output is 12V5A, and the effective value of the measured ripple is 10mV. This 10mV is the absolute amount of ripple, and the relative amount, that is, the ripple coefficient = ripple voltage / output voltage = 10mv / 12V = 0.12%.

Third, the ripple test method

3.1. With 20M oscilloscope bandwidth as the limit standard, the voltage is set to PK-PK (also measured RMS), remove the clip and ground wire on the oscilloscope control head (because this clip and ground line will form a loop, like an antenna to receive Noise, introduce some unnecessary noise), use the grounding ring (can not use the grounding ring, but consider the error), connect a 10UF electrolytic capacitor and a 0.1UF ceramic capacitor in parallel with the probe, with an oscilloscope The probe is tested directly; if the oscilloscope probe is not in direct contact with the output point, it should be measured with a twisted pair or 50Î© coaxial cable.

Fourth, the main classification of switching power supply ripple

Switching power supply output ripple mainly comes from five aspects:

4.1. Input low frequency ripple;

4.2. High frequency ripple;

4.3. Common mode ripple noise caused by parasitic parameters;

4.4. Ultra-high frequency resonant noise generated during power device switching;

4.5. Ripple noise caused by closed loop regulation control.

Five, power ripple test

Ripple is an AC interference signal superimposed on a DC signal and is a very important criterion in power supply testing. Especially for special-purpose power supplies, such as laser power supplies, ripple is one of its deadliest. Therefore, the test of power supply ripple is extremely important.

The measurement method of power supply ripple is roughly divided into two types: one is voltage signal measurement method; the other is current signal measurement method.

Generally, for constant current sources with constant voltage source or ripple performance, voltage signal measurement can be used. For constant current sources with high ripple performance, current signal measurement is preferred.

Voltage Signal Measurement Ripple is the measurement of an AC ripple voltage signal superimposed on a DC voltage signal with an oscilloscope. For constant voltage sources, the test can directly measure the voltage signal output to the load with a voltage probe. For the constant current source test, the voltage waveform across the sampling resistor is generally measured by using a voltage probe. The oscilloscope settings are key to the ability to sample real signals throughout the test.

The instrument used was a TDS1012B oscilloscope with a voltage measuring probe.

The following settings are required before measurement.

1. Channel setting:

Coupling: The choice of channel coupling mode. Ripple is an AC signal superimposed on a DC signal. Therefore, we can remove the DC signal by testing the ripple signal and directly measure the superimposed AC signal.

Probe: The way to use the voltage probe first. Then select the attenuation ratio of the probe. It must be consistent with the attenuation ratio of the actual probe used, so that the number read from the oscilloscope is the real data. For example, if the voltage probe used is placed in &TImes; 10, then the probe options here must also be set to &TImes; 10 files.

2. Trigger settings:

Type: Edge

Source: The actual selected channel, for example, if you are ready to test with the CH1 channel, you should select CH1 here.

Slope: Rise.

Trigger mode: If you are observing the ripple signal in real time, select 'Auto' trigger. The oscilloscope will automatically follow the changes in the actual measured signal and display it. At this time, you can also display the value of the measurement you need in real time by setting the measurement button. However, if you want to capture the signal waveform for a measurement, you need to set the trigger mode to 'normal' trigger. At this point, you also need to set the size of the trigger level. Generally, when you know the peak value of the signal you are measuring, set the trigger level to 1/3 of the peak value of the measured signal. If you don't know, the trigger level can be set slightly smaller.

Coupling: DC or AC..., generally AC coupled.

3. Sampling length (seconds/division):

The sample length setting determines whether the required data can be sampled. When the set sampling length is too large, the high frequency component in the actual signal will be missed; when the set sampling length is too small, only the local part of the measured actual signal can be seen, and the real actual signal can not be obtained. Therefore, in the actual measurement, you need to rotate the button back and forth, and carefully observe until the displayed waveform is a true complete waveform.

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