Power supply ripple test
Using a 20MHz oscilloscope bandwidth as the limiting standard, set the voltage to PK-PK (or measure the RMS value). Remove the clips and ground wire from the oscilloscope control head (because these clips and ground wire can form a loop, acting like an antenna to receive noise and introduce unwanted noise). Use a grounding loop (it's possible to not use a grounding loop, but the resulting error must be considered). Connect a 10µF electrolytic capacitor and a 0.1µF ceramic capacitor in parallel to the probe, and test directly with the oscilloscope probes. If the oscilloscope probe is not in direct contact with the output point, a twisted pair cable or a 50Ω coaxial cable should be used for measurement.
Ripple is an AC interference signal superimposed on a DC signal and is a very important standard in power supply testing. Especially for power supplies used in special applications, such as laser power supplies, ripple is one of their fatal flaws. Therefore, power supply ripple testing is extremely important.
There are two main methods for measuring power supply ripple: one is the voltage signal measurement method, and the other is the current signal measurement method.
For constant voltage sources or constant current sources with low ripple performance requirements, voltage signal measurement methods can generally be used. However, for constant current sources with high ripple performance requirements, current signal measurement methods are preferable.
Voltage signal ripple measurement refers to measuring the AC ripple voltage signal superimposed on a DC voltage signal using an oscilloscope. For a constant voltage source, the voltage signal output to the load can be measured directly using a voltage probe. For a constant current source, the voltage waveform across a sampling resistor is typically measured using a voltage probe. Throughout the testing process, the oscilloscope settings are crucial for successfully sampling the true signal.
The instrument used is a TDS1012B oscilloscope equipped with a voltage measurement probe. The following settings are required before measurement.
1. Channel settings:
Coupling: This refers to the selection of the channel coupling method. Ripple is an AC signal superimposed on a DC signal. Therefore, to test the ripple signal, we can remove the DC signal and directly measure the superimposed AC signal.
Broadband restrictions: Off
Probe: First, select a voltage probe. Then, select the probe's attenuation ratio. This must match the attenuation ratio of the actual probe used; otherwise, the data read from the oscilloscope will be the accurate data. For example, if the voltage probe is set to ×10, then the probe option here must also be set to ×10.
2. Trigger settings:
Type: Edge
Source: The actual channel selected. For example, if you plan to use channel CH1 for testing, then CH1 should be selected here.
Slope: Rising.
Triggering Mode: If you are observing the ripple signal in real time, select 'Auto' triggering. The oscilloscope will automatically follow and display the changes in the measured signal. At this time, you can also use the measurement button to display the measured value in real time. However, if you want to capture the signal waveform during a specific measurement, you need to set the triggering mode to 'Normal' triggering. In this case, you also need to set the trigger level. Generally, if you know the peak value of the signal you are measuring, set the trigger level to 1/3 of the peak value. If you don't know the peak value, you can set the trigger level slightly lower.
Coupling: DC or AC...? (Seems to make little difference)
3. Sampling length (seconds/division):
The sampling length setting determines whether the required data can be sampled. If the sampling length is set too large, high-frequency components of the actual signal will be missed; if the sampling length is set too small, only a portion of the measured signal will be visible, and the true actual signal cannot be obtained. Therefore, during actual measurement, the button needs to be rotated back and forth, and careful observation is required until the displayed waveform is the true and complete waveform.
4. Sampling method:
The settings can be configured according to actual needs. For example, if the PP value of ripple needs to be measured, the peak value measurement method is the best choice. The number of samples can also be set according to actual needs, which is related to the sampling frequency and sampling length.
5. Measurement:
By selecting the peak value measurement for the corresponding channel, the oscilloscope can display the necessary data in a timely manner. You can also select the frequency, maximum value, and RMS value for the corresponding channel.
By properly setting and operating the oscilloscope, the desired ripple signal can certainly be obtained. However, during the measurement process, it is crucial to prevent interference from other signals to the oscilloscope probe itself, otherwise the measured signal may not be accurate.
Measuring ripple using the current signal measurement method refers to measuring the AC ripple current signal superimposed on a DC current signal. For constant current sources with high ripple requirements, i.e., those requiring low ripple, the direct current signal measurement method can obtain a more accurate ripple signal. Unlike the voltage measurement method, a current probe is also used here. For example, continuing with the oscilloscope mentioned above, add a current amplifier and a current probe. Now, simply clamp the current probe onto the current signal output to the load to measure the ripple signal of the output current using the current measurement method. Similar to the voltage measurement method, the settings of the oscilloscope and current amplifier are crucial for sampling the true signal throughout the entire testing process.
In fact, the basic settings and usage of the oscilloscope are the same as described above when using this method. The difference lies in the probe settings within the channel settings. Here, you need to select the current probe mode. Then, the probe ratio must be the same as the ratio set on the amplifier to ensure that the data read from the oscilloscope is accurate. For example, if the amplifier's ratio is set to 5A/V, then this setting on the oscilloscope must also be set to 5A/V. As for the current amplifier's coupling mode, if the oscilloscope's channel coupling is already selected as AC coupling, then either AC or DC can be selected here. It is important to note that when using this method, you must turn on the oscilloscope first, and then turn on the current amplifier. Also, remember to demagnetize the current probe before use.
How to accurately measure power supply ripple?
In the example shown in Figure 1, a junior engineer used an oscilloscope completely incorrectly. His first mistake was using an oscilloscope probe with a long ground lead; his second mistake was placing the probe loop and the ground lead near the power transformer and switching elements; his final mistake was allowing excess inductance between the oscilloscope probe and the output capacitor. This problem manifests as high-frequency pickup in the ripple waveform. In the power supply, there are numerous high-speed, large-signal voltage and current waveforms that can easily couple to the probe, including the magnetic field coupled from the power transformer, the electric field coupled from the switching nodes, and the common-mode current generated by the transformer's interconnected capacitance.
1. An oscilloscope probe with a very long ground wire was used.
2. Bring the loop formed by the probe and grounding wire close to the power transformer and switching elements.
3. Allows for the formation of additional inductance between the oscilloscope probes and the output capacitor.
