Test and analysis of power supply ripple based on oscilloscope

1. What is ripple?

Ripple is defined as the AC component superimposed on the DC stable quantity in the DC voltage or current.

It has the following main disadvantages:

1.1. It is easy to generate harmonics on electrical appliances, and harmonics will cause more harm;

1.2. It reduces the efficiency of the power supply;

1.3. Strong ripples will cause surge voltage or current, leading to burning of electrical appliances;

1.4. It will interfere with the logical relationship of digital circuits and affect their normal operation;

1.5. It will cause noise interference, making the image equipment and audio equipment unable to work normally

2. Ripple and ripple coefficient representation method It

can be expressed by effective value or peak value, or by absolute quantity or relative quantity;

the unit is usually: mV

A power supply works in a stable voltage state, its output is 12V5A, and the effective value of the ripple is measured to be 10mV. This 10mV is the absolute amount of the ripple, and the relative amount, that is, the ripple coefficient = ripple voltage/output voltage = 10mv/12V = 0.12%.

3. Ripple test method

3.1. With 20M oscilloscope bandwidth as the limit standard, the voltage is set to PK-PK (effective value is also measured), remove the clip and ground wire on the oscilloscope control head (because the clip and ground wire will form a loop, like an antenna receiving noise, introducing some unnecessary noise), use a ground ring (it is also possible not to use a ground ring, but the error it produces must be considered), connect a 10UF electrolytic capacitor and a 0.1UF ceramic capacitor in parallel on the probe, and test directly with the oscilloscope probe; if the oscilloscope probe is not directly in contact with the output point, it should be measured with a twisted pair or a 50Ω coaxial cable.

4. Main classification of switching power supply ripple

The output ripple of the switching power supply mainly comes from five aspects:

4.1. Input low-frequency ripple;

4.2. High-frequency ripple;

4.3. Common-mode ripple noise caused by parasitic parameters;

4.4. Ultra-high frequency resonant noise generated during the switching process of the power device;

4.5. Ripple noise caused by closed-loop regulation control.

5. Power supply ripple test

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 for special purposes, such as laser power supplies, ripple is one of its fatal flaws. Therefore, the test of power supply ripple is extremely important.

There are roughly two methods for measuring power supply ripple: one is the voltage signal measurement method; the other is the current signal measurement method.

Generally, for constant voltage sources or constant current sources with low ripple performance requirements, the voltage signal measurement method can be used. For constant current sources with high ripple performance requirements, it is best to use the current signal measurement method. Voltage

signal measurement of ripple refers to the use of an oscilloscope to measure the AC ripple voltage signal superimposed on the DC voltage signal. For constant voltage sources, the test can directly use a voltage probe to measure the voltage signal output to the load. For the test of constant current sources, it is generally done by using a voltage probe to measure the voltage waveform across the sampling resistor. During the entire test process, the setting of the oscilloscope is the key to whether the real signal can be sampled.

The instrument used is: TDS1012B oscilloscope equipped with a voltage measurement probe.

The following settings need to be made before measurement.

1. Channel settings:

Coupling: that is, the selection 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 select the voltage probe method. Then select the attenuation ratio of the probe. It must be consistent with the attenuation ratio of the actual probe used, so that the number read from the oscilloscope is the real data. For example, if the voltage probe used is placed in the ×10 position, then at this time, the probe option here must also be set to the ×10 position.

2. Trigger settings:

Type: Edge

Source: The actual selected channel, for example, if you plan to use the CH1 channel for testing, then CH1 should be selected here.

Slope: Rise. [page]

Trigger mode: If you are observing the ripple signal in real time, select 'Auto' trigger. The oscilloscope will automatically follow the changes of the actual measured signal and display it. At this time, you can also set the measurement button to display the measured value you need in real time. 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 time, you also need to set 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 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 (seconds/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 actual signal can be seen, and the real actual signal cannot be obtained. Therefore, in actual measurement, you need to rotate the button back and forth and observe carefully until the displayed waveform is a real and complete waveform.

4. Sampling mode:

It can be set according to actual needs. For example, if you need to measure the PP value of the ripple, it is best to choose the peak measurement method. The number of 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 a timely manner. At the same time, you can also select the frequency, maximum value, RMS value, etc. of the corresponding channel.

By setting the oscilloscope reasonably and operating it in a standardized manner, you can definitely 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 signal being not real enough.

Measuring the ripple value by the current signal measurement method refers to measuring the AC ripple current signal superimposed on the DC current signal. For constant current sources with relatively high ripple index requirements, that is, constant current sources with relatively small ripples, the current signal direct measurement method can obtain a more realistic ripple signal. Unlike the voltage measurement method, a current probe is also used here. For example, continue to use the above oscilloscope, add a current amplifier and a current probe. At this time, you only need to clamp the current signal output to the load with the current probe, and you can use the current measurement method to measure the output current ripple signal. As with the voltage measurement method, the settings of the oscilloscope and current amplifier are the key to whether the real signal can be sampled during the entire test process.

In fact, when using this method to measure, the basic settings and usage of the oscilloscope are the same as above. The difference is that the settings of the probe in the channel settings are different. Here, you need to select the current probe method. Then, the ratio of the probe must be the same as the ratio set by the amplifier, so that the number read 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 must also 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.

It should be noted that when using this method, you need to turn on the oscilloscope first, and then turn on the current amplifier. And remember to demagnetize the current probe before use.

In addition, measuring power supply ripple itself has certain skills. Figure 1 shows an example of improper use of an oscilloscope to measure power supply ripple. Several mistakes were made in this example. The first was to use an oscilloscope probe with a long ground lead. The second was to allow the loop formed by the probe and ground lead to be close to the power transformer and switching elements. The last was to allow additional inductance to form between the oscilloscope probe and the output capacitor. The resulting problem is that the measured ripple waveform carries the picked-up high-frequency components.

There are many high-speed, high-voltage and current signal waveforms in the power supply that can easily couple into the probe. These include magnetic field coupling from the power transformer, electric field coupling from the switch node, and common-mode currents generated by the transformer interwinding capacitance.

Figure 1: Improper ripple measurement leads to poor results.

Using the right measurement technique can really improve the results of ripple measurements. First, the bandwidth limit of the ripple is usually specified to avoid picking up high-frequency noise that exceeds the ripple bandwidth limit. The oscilloscope used for measurement should be set with an appropriate bandwidth limit. Second, the antenna formed by the long ground lead can be removed by removing the "cap" of the probe. As shown in Figure 2, we wrap a short wire around the probe ground lead and connect it to the power supply ground. This has the added benefit of shortening the probe length exposed to the high-intensity electromagnetic radiation near the power supply, thereby further reducing high-frequency pickup.

Finally, in an isolated power supply, the real common-mode current is generated by the current flowing in the probe ground lead, which causes a voltage drop between the power supply ground and the oscilloscope ground, which appears as ripple. To suppress this ripple, common-mode filtering needs to be carefully considered in the power supply design.

In addition, wrapping the oscilloscope leads around the core can reduce this current because it will form a common-mode inductance that does not affect the differential voltage measurement but can reduce the measurement error caused by the common-mode current. Figure 2 shows the ripple voltage measurement results of the same circuit using the improved measurement technique. It can be seen that the high-frequency spikes have been almost eliminated.

Figure 2: Four simple improvements greatly improve measurement results.

In fact, power supply ripple performance can be even better once the power supply is integrated into the system. There is almost always some amount of inductance between the power supply and the rest of the system. The inductance may be formed by wire or etched lines on the printed circuit board, and there is always additional bypass capacitance near the chip as the power supply load, which creates a low-pass filtering effect and further reduces the power supply ripple and/or high-frequency noise.

As an extreme example, a filter consisting of a one-inch short wire with an inductance of 15nH and a bypass capacitor with a capacitance of 10μF has a cutoff frequency of 400kHz. This example means that high-frequency noise can be greatly reduced. The cutoff frequency of this filter is many times lower than the power supply ripple frequency, which can actually reduce the ripple. Smart engineers should try to take advantage of this during testing.