What are the five most common sources of output ripple in a switching power supply?
Switching power supply output ripple mainly from five sources: input low-frequency ripple; high-frequency ripple; parasitic parameters caused by the common mode ripple noise; power device switching process generated by the ultra-high frequency resonance noise; closed-loop regulation and control caused by the ripple noise.
Ripple is an AC interference signal superimposed on the DC signal, which is a very important criterion in power supply testing. Especially for special-purpose power supplies, such as laser power supply, ripple is one of the fatal key. Therefore, the test of power supply ripple is extremely important.
Power supply ripple measurement method is broadly divided into two kinds: one is the voltage signal measurement method; another is the current signal measurement method.
Generally for the constant voltage source or ripple performance requirements of the constant current source, can use the voltage signal measurement method. For high ripple performance requirements of the constant current source is best to use the current signal measurement method.
Voltage signal measurement of ripple means that an oscilloscope is used to measure the AC ripple voltage signal superimposed on the DC voltage signal. For constant voltage sources, the test can be directly with a voltage probe to measure the output voltage signal to the load. For constant current sources, the test is generally through the use of voltage probes, measuring the voltage waveform at the ends of the sampling resistor. Throughout the test process, the oscilloscope settings are the key to sampling the real signal.
The following settings are required before measurement.
1. Channel settings:
Coupling: that is, the choice of channel coupling method. Ripple is an AC signal superimposed on a DC signal, so we want to test the ripple signal can remove the DC signal, and directly measure the superimposed AC signal is good.
Wideband limit: Off
Probe: first choose the way of voltage probe. Then choose the attenuation ratio of the probe. Must be consistent with the actual attenuation ratio of the probe used, so that the number read from the oscilloscope is the real data. For example, the voltage probe used is placed in the × 10 gear, then at this time, the options for the probe here must also be set to × 10 gear.
2. Trigger settings:
Type: edge
Source: the actual channel selected, such as, ready to use the CH1 channel for testing, then here should be selected as CH1.
Slope: rising.
Trigger Mode: If the ripple signal is being observed in real time, select 'Auto' trigger. The oscilloscope will automatically follow the actual measured signal and display it. At this time, you can also set the Measurement button to display the value of your desired measurement in real time. However, if you want to capture the signal waveform during a particular measurement, you need to set the trigger method to 'Normal' trigger. In this case, you also need to set the magnitude of the trigger level. Generally when you know the peak value of the signal you are measuring, set the trigger level at 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 use AC coupling.
3. Sampling length (sec/g):
The setting of the sampling length determines whether the required data can be sampled. When the set sampling length is too large, it will miss the high-frequency components of the actual signal; when the set sampling length is too small, you can only see the actual signal measured locally, the same can not get the real actual signal. Therefore, in the actual measurement, you need to rotate the button back and forth, carefully observe, until the displayed waveform is a real complete waveform.
4. Sampling mode:
Can be set according to the actual need. For example, if you want to measure the P-P value of the ripple, it is better to choose the peak measurement method. Sampling times can also be set according to actual needs, which is related to the sampling frequency and sampling length.
5. Measurement:
By selecting the peak measurement of the corresponding channel, the oscilloscope can help you display the required data in time. You can also select the frequency, maximum value and root mean square value of the corresponding channel.
Through reasonable settings and standardized operation of the oscilloscope, you can surely get the required ripple signal. However, during the measurement process, you must pay attention to prevent other signals from interfering with the oscilloscope probe itself, so as to avoid the measured signals are not real enough.
