Introduction to Ripple and Noise of Power Supply
Ripple and noise are: the fluctuation of the same frequency as the switching frequency of the power supply superimposed on the output of the DC power supply is ripple, and the high-frequency noise is noise. Specifically, as shown in Figure 1, the low frequency and regular fluctuations are ripples, and the peak part is noise.
Figure 1 Power supply ripple noise
Irregular Ripple Test
A 3.3V power supply signal is connected to the oscilloscope, and the probe gear is X10 to measure the power ripple. After clicking[Auto Setup]after adjusting the horizontal time base, vertical gear and vertical offset, the following figure can be obtained. 2 shown.
Figure 2 Irregular ripple test
It can be seen from the figure that the measured waveform is mixed with a lot of noise and clutter, and the DC and AC waveforms are mixed together. Many engineers measure ripple this way because they are not using a standard ripple measurement method.
Standardized Ripple Test Method
1. The probe should choose the appropriate gear. If the voltage is relatively large, or if the bandwidth requirement is relatively high, the X10 gear can be used. In normal cases, it is recommended to use the X1 gear to avoid unnecessary noise attenuation affecting the ripple measurement.
Figure 3 Probe gear selection
2. Ripple is an AC component, so the “channel coupling” method can use the “AC” method to limit the input of the DC signal, as shown in Figure 4.
3. The “bandwidth limit” function can be used appropriately, and the “20MHz” bandwidth limit can be selected to filter out unnecessary high-frequency noise, as shown in Figure 4.
Figure 4 Channel interface parameter settings
4. Because there will be a lot of EMI (electromagnetic interference) in the test environment, the oscilloscope is a high-impedance device, which will introduce EMI signals into the test results, causing the measurement results to be too large. The correct measurement method should be grounded with a grounding spring to reduce the introduction of electromagnetic interference. The comparison diagram of grounding with a grounding spring and grounding with an alligator clip is shown below:
Figure 5 Comparison of grounding methods
5. The trigger mode can be edge trigger, and the trigger mode can be Auto/Normal.
6. Properly adjust the horizontal time base, vertical scale and vertical offset, so that the waveform signal can be displayed in the center of the screen with a better effect. The specific captured ripple is shown in Figure 6 below.
Figure 6 Ripple capture
The correct ripple measurement method can clearly capture the correct ripple and reduce the influence of unnecessary noise and clutter on the ripple. The test result is a clean ripple. On this basis, the power supply ripple is measured. measurement, the ripple value can be measured more accurately, and the power quality can be more accurately estimated.
7. Measurement analysis. Ripple test is generally expressed by peak-to-peak value, which can be automatically measured by using[measure]. ZDS series oscilloscopes support 51 real parameter measurement and statistical functions, and measure ripple parameters based on the full storage depth, or you can also use “One key cursor” for manual measurement, as shown in Figure 7 below.
Figure 7 Results analysis
It can be seen from the measurement that the peak-to-peak value of the power supply ripple is 18mv. Intel stipulates in the ATX12V specification that the peak-to-peak value of the +12V output ripple shall not exceed 120mv, and the peak-to-peak value of the +3.3V and +5V ripple shall not exceed 50mv. The smaller the power quality, the better.
Three steps to complete the ripple noise test method
The ZDS5054A oscilloscope has a complete power supply analysis function, which can easily measure the power supply output ripple and noise.
Step 1: After selecting the appropriate gear probe, click[Auto Setup]to capture the waveform signal;
Step 2: Click[Channel 1]to enter the channel interface, and change the channel coupling mode to[AC];
Step 3: Turn on the power analysis function, select[Output Ripple]the test type can be selected from “Ripple” and “Noise”. Test out the ripple measurement results.
Figure 8 Ripple automatic test function
Note: There are 4 items in the measurement results table:
Ripple/noise voltage: the rms value of the filtered voltage;
Maximum peak value: the maximum peak value of the filtered voltage;
Minimum peak value: the minimum peak value of the filtered voltage;
Frequency: The frequency of the filtered signal.
The Links: NL3224AC35-13 VI-BW0-CV