What are the top five reasons why switching power supply' output ripples?
With the 20M oscilloscope bandwidth as the limit standard, the voltage is set to PK-PK (effective value is also measured), and the clip and ground wire on the oscilloscope control head are removed (because the clip and the ground wire will form a loop, like an antenna receiving Noise, introduce some unnecessary noise), use a grounding ring (it is also possible not to use a grounding ring, but the error generated by it should be considered), connect a 10UF electrolytic capacitor and a 0.1UF ceramic capacitor in parallel on the probe, and use an oscilloscope The probe of the oscilloscope should be tested directly; if the oscilloscope probe is not directly in contact with the output point, twisted pair or 50Ω coaxial cable should be used for measurement.
The output ripple of switching power supply mainly comes from five aspects: input low-frequency ripple; high-frequency ripple; common-mode ripple noise caused by parasitic parameters; ultra-high-frequency resonance noise generated during switching of power devices; ripple noise.
Ripple is an AC interference signal superimposed on a DC signal, and is a very important criterion in power supply testing. Especially for power supplies for special purposes, such as laser power supplies, ripple is one of its fatal points. Therefore, the test of power ripple is extremely important.
The measurement method of power supply ripple is roughly divided into two types: one is the voltage signal measurement method; the other is the current signal measurement method.
Generally, the voltage signal measurement method can be used for constant voltage sources or constant current sources that do not require much ripple performance. For a constant current source with high requirements for ripple performance, it is best to use the current signal measurement method.
Voltage signal measurement ripple refers to measuring the AC ripple voltage signal superimposed on the DC voltage signal with an oscilloscope. For a constant voltage source, the test can directly use a voltage probe to measure the voltage signal output to the load. For the test of the constant current source, the voltage waveform at both ends of the sampling resistor is generally measured by using a voltage probe. Throughout the test process, the setting of the oscilloscope is the key to whether the real signal can be sampled.
The following settings are required before measurement.
1. Channel settings:
Coupling: the choice of channel coupling mode. Ripple is an AC signal superimposed on a DC signal, so if we want to test the ripple signal, we can remove the DC signal and directly measure the superimposed AC signal.
Bandwidth Limit: Off
Probe: First choose the voltage probe method. Then select the attenuation ratio of the probe. It must be consistent with the attenuation ratio of the probe actually used, so that the number read from the oscilloscope is the real data. For example, if the voltage probe used is set to ×10, then at this time, the option of the probe here must also be set to ×10.
2. Trigger settings:
Type: Edge
Source: the channel actually selected, for example, CH1 channel is going to be used for testing, then CH1 should be selected here.
Slope: up.
Trigger mode: If you are observing the ripple signal in real time, select 'Auto' to trigger. The oscilloscope will automatically follow the changes of the actual measured signal and display it. At this time, you can also display the measured value you need in real time by setting the measurement button. However, if you want to capture the signal waveform during a certain measurement, you need to set the trigger mode to 'normal' trigger. At this point, it is also necessary 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 signal you are measuring. If not known, the trigger level can be set slightly lower.
Coupling: DC or AC..., usually AC coupling.
3. Sampling length (second/grid):
The setting of the sampling length determines whether the required data can be sampled. When the set sampling length is too large, the high-frequency components in the actual signal will be missed; when the set sampling length is too small, only part of the measured actual signal can be seen, and the real actual signal cannot be obtained. Therefore, in the actual measurement, it is necessary to rotate the button back and forth and observe carefully until the displayed waveform is a real and complete waveform.
4. Sampling method:
It can be set according to actual needs. For example, if it is required to measure the P-P value of the ripple, it is best to choose the peak measurement method. The number of sampling 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. At the same time, you can also select the frequency, maximum value, root mean square value, etc. of the corresponding channel.
Through reasonable setting and standardized operation of the oscilloscope, the required ripple signal can be obtained. However, during the measurement process, care must be taken to prevent other signals from interfering with the oscilloscope probe itself, lest the measured signal is not true enough.
To measure the ripple value by the current signal measurement method means to measure the AC ripple current signal superimposed on the DC current signal. For a constant current source with a relatively high requirement for ripple index, that is, a constant current source that requires a relatively small ripple, a more realistic ripple signal can be obtained by using the direct measurement method of the current signal. Unlike the voltage measurement method, a current probe is also used here. For example, continue with the oscilloscope described above, and add a current amplifier and a current probe. At this point, just use the current probe to clamp the current signal output to the load, and the current measurement method can be used to measure the ripple signal of the output current. Like the voltage measurement method, the setting of the oscilloscope and current amplifier is the key to sampling real signals during the whole test.
In fact, when measuring with this method, the basic settings and usage of the oscilloscope are the same as above. The difference is that the probe settings in the channel settings are different. Here, you need to choose the mode of the current probe. Then, choose the ratio of the probe, which must be the same as the ratio set by the amplifier, so that the reading from the oscilloscope is the real data. For example, if the ratio of the amplifier used is set to 5A/V, then this item of the oscilloscope also needs to be set to 5A/V. As for the coupling mode of the current amplifier, when the channel coupling of the oscilloscope has been selected as AC coupling, you can choose AC or DC here.
