Useful Information | Why is your power supply ripple so large?

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Since DC stable power supply is generally formed by AC power supply through rectification and voltage stabilization, it is inevitable that there will be some AC components in the DC stable quantity. This AC component superimposed on the DC stable quantity is called ripple. The components of ripple are relatively complex. Its form is generally a harmonic similar to a sine wave with a frequency higher than the power frequency, and the other is a pulse wave with a very narrow width. For different occasions, the requirements for ripple are different.

Ripple can be expressed in terms of effective value or peak value, in absolute value or in relative value. For example, a power supply is working in a regulated state, and its output is 100V 5A. The effective value of the ripple is 10mV. This 10mV is the absolute value of the ripple, and the relative value is the ripple coefficient = ripple voltage/output voltage = 10mv/100V = 0.01%, which is equal to one ten-thousandth.

When a user used a 500MHz bandwidth oscilloscope to test the ripple of the 5V signal output by his switching power supply, he found that the peak-to-peak value of the ripple and noise reached more than 900mV (as shown in the figure below), while the nominal peak-to-peak value of the ripple of his switching power supply was <20mv. Although the user's circuit board has an LDO in the back stage to stabilize the output of the switching power supply, the user believes that the measured result is too large and unreliable, and hopes to find out the problem.

The problem of excessive power ripple test is usually related to the probe used and the connection method of the front end. First, the user probe connection method was checked and it was found that the long alligator clip ground wire was used as shown in the left figure below, and the ground point was clamped on the fixing screw of the single board, and the entire ground loop was relatively large. Since a large ground loop will introduce more spatial electromagnetic radiation noise and ground loop noise caused by the switching power supply, it was replaced with a short ground spring pin as shown in the right figure below.

After actual testing, it was found that the peak-to-peak value of the measured ripple noise has been greatly improved, as shown in the figure below. However, the peak-to-peak value of the ripple noise is still more than 40mV, which is still quite different from the nominal <20mV of the switching power supply manufacturer.

After further checking the probe model used by the user, it was found that the user was using the 10:1 passive probe that comes standard with the oscilloscope, as shown in the figure below.

The 10:1 probe will attenuate the measured signal by 10 times before sending it to the oscilloscope, and then the oscilloscope will perform 10 times mathematical amplification on the measured signal. The advantage of this probe is that the probe bandwidth can be increased to several hundred MHz through the matching circuit in front, and the range of the oscilloscope is extended, but it is not particularly beneficial for measuring small signals. If the amplitude of the measured signal is small, it may be submerged in the bottom noise of the oscilloscope after being attenuated 10 times. Even if it is mathematically amplified 10 times, the signal-to-noise ratio itself will not be improved. Therefore, for the measurement of power supply ripple noise, a probe with a small attenuation ratio should be used as much as possible, such as a 1:1 probe. So I found another 1:1 passive probe. Although this 1:1 passive probe has a low bandwidth (usually tens of MHz), it has a small attenuation ratio and is very suitable for small signal testing.

The following figure is a comparison test result of using a 1:1 passive probe and a 10:1 probe under different bandwidth limits. It can be seen that after using a 1:1 probe and setting a 20MHz bandwidth limit, the peak-to-peak value of the measured ripple noise is less than 10mV, which is much better than the test result of the 10:1 probe. From the test result of the 1:1 probe, a clear ripple waveform can be seen, and it meets the user's expectation that the power supply ripple noise is <20mV. In addition, we can also see that bandwidth limitation also has a certain improvement effect on the peak-to-peak value of noise.

This is a typical problem in power ripple testing. We have greatly improved the test results of ripple noise by using short ground wire connections, switching to low attenuation ratio probes, and bandwidth limiting functions. Generally speaking, the factors that affect the power ripple test results are mainly the following in order of importance:

1. The length of the front-end connection line and the ground loop: A long ground loop will pick up more electromagnetic radiation and ground noise from the switching power supply, so it is necessary to use the shortest possible ground connection.

2. Probe attenuation ratio: A probe with a large attenuation ratio will make the small signal amplitude even weaker, or even submerged in the oscilloscope bottom noise, so you should try to use a probe with a 1:1 attenuation ratio.

3. Bandwidth limitation: Many electromagnetic noises and the bottom noise of the oscilloscope are broadband. Setting a suitable bandwidth limit can filter out additional noise. Many power supply ripple noise test occasions use a 20MHz bandwidth limit, and some chips require measurement to 80MHz or 200MHz.

4. Measurement range: Usually the power supply ripple test is performed at a small range (such as 10mv/grid or 20mv/grid). The larger the range, the higher the bottom noise of the oscilloscope. However, some oscilloscopes have limited offset ranges, and may not be able to pull the measured DC voltage signal back to the center of the screen for measurement at a small range. Therefore, the AC coupling function of the oscilloscope is often used to isolate the DC before performing ripple noise testing.

5. Input impedance: Many oscilloscopes have 50 ohm and 1M ohm input impedance options. Usually, the oscilloscope has lower noise floor under 50 ohm input impedance. However, when the oscilloscope is connected to most passive probes, the impedance will automatically switch to 1M ohm. It can only be set to 50 ohm input impedance when connected to an active probe or coaxial cable.

Before conducting actual tests, a good habit is to check the system noise floor under the current equipment and settings. The five waveforms in the figure below are the noise floor results of using a 500M S series oscilloscope with different probe and bandwidth settings. The waveforms from top to bottom are: 50 ohm input impedance, 1:1 probe, 500MHz bandwidth; 1M ohm input impedance, 1:1 probe, 20MHz bandwidth; 1M ohm input impedance, 1:1 probe, 500MHz bandwidth; 1M ohm input impedance, 10:1 probe, 20MHz bandwidth; 1M ohm input impedance, 10:1 probe, 500MHz bandwidth. The peak-to-peak value of the noise floor ranges from less than 1mV to nearly 30mV, which shows the importance of probe, bandwidth, and input impedance settings in the test.

If you don't have a suitable low attenuation ratio probe, you can also make a probe with a 50-ohm coaxial cable in the following way. In fact, one end of the cable is connected to the oscilloscope, and the oscilloscope is set to 50 ohm input impedance; the other end of the cable is stripped, the shielding layer is welded to the ground of the circuit under test, and the center conductor is connected to the power signal under test through a DC blocking capacitor. The advantages of this method are low cost and low attenuation ratio, and the disadvantages are poor consistency, and the parameters and bandwidth of the DC blocking capacitor are difficult to control. In addition, in recent years, oscilloscope manufacturers have also launched probes designed specifically for power ripple testing, which combine low attenuation ratio (1.1:1), high bandwidth (hardware 2GHz, bandwidth limit can be set by software), impedance matching that takes into account measurement needs and noise (the DC input impedance of the probe itself is 50k ohms, but the oscilloscope end is a 50 ohm input impedance spectrum), short ground wire (providing a very low loop inductance welding front end), large offset range (up to ±24V), and can test ripple and DC voltage at the same time. It is suitable for users with high requirements for power ripple measurement.

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