Oscilloscope-based power supply ripple test

1. What is ripple?

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

It has the following main disadvantages:

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

1.2. Reduced power efficiency;

1.3. Strong ripples can 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 properly

2. Expression methods of ripple and ripple coefficient

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

The unit is usually: mV

A power supply works in a regulated state, and its output is 12V5A. The measured effective value of the ripple is 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

[page]3.1. Take the 20M oscilloscope bandwidth as the limit standard, set the voltage to PK-PK (also measure the effective value), remove the clip and ground wire on the oscilloscope control head (because the clip and ground wire will form a loop, like an antenna to receive 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 to the probe, and use the oscilloscope probe to test directly; if the oscilloscope probe is not directly in contact with the output point, it should be measured using 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 power devices;

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 used for special purposes, such as laser power supplies, ripple is one of its fatal weaknesses. 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, the voltage signal measurement method can be used for constant voltage sources or constant current sources with low ripple performance requirements. However, the current signal measurement method is best used for constant current sources with high ripple performance requirements.

Measuring ripple of voltage signal means using an oscilloscope to measure the AC ripple voltage signal superimposed on the DC voltage signal. For constant voltage source, the voltage signal output to the load can be directly measured with a voltage probe. For constant current source testing, the voltage waveform across the sampling resistor is generally measured by using a voltage probe. 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 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.

[page]Broadband limit: Off

Probe: First, select the voltage probe. Then select the probe attenuation ratio. It must be consistent with the actual probe attenuation ratio, so that the data read from the oscilloscope is real 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 CH1 for testing, you should select CH1 here.

Slope: Ascending.

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 AC coupling is used.

3. Sampling length (seconds/grid):

The setting of sampling length determines whether the required data can be sampled. When the sampling length is set too long, the high-frequency components in the actual signal will be missed; when the sampling length is set too short, only part of the actual signal can be seen, and the real actual signal cannot be obtained. Therefore, when measuring, you need 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 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 value measurement of the corresponding channel, the oscilloscope can help you display the required data in a timely manner. You can also select the frequency, maximum value, RMS value, etc. of the corresponding channel.

By properly setting up and operating the oscilloscope in a standardized manner, 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, so as to avoid the measured signal being unrealistic.

Measuring the ripple value by the current signal measurement method means 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, a more realistic ripple signal can be obtained by using the current signal direct measurement method. Unlike the voltage measurement method, a current probe is also used here. For example, continue to use the above oscilloscope, and add a current amplifier and a current probe. At this point, 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 ripple signal of the output current. As with the voltage measurement method, during the entire test process, the settings of the oscilloscope and the current amplifier are the key to whether the real signal can be sampled.

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 select the current probe mode. Then, select the probe ratio, which 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.

[page]It should be noted that when using this method, you need to turn on the oscilloscope first, and then turn on the current amplifier. Also, 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. In this example, several mistakes were made. First, an oscilloscope probe with a long ground wire was used; second, the loop formed by the probe and the ground wire was allowed to be close to the power transformer and switching components; and finally, additional inductance was allowed to form between the oscilloscope probe and the output capacitor. The resulting problem is that the high-frequency components picked up are carried in the measured ripple waveform.

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

Figure 1: Improper ripple measurement yields poor results.

The correct measurement technique can effectively improve the results of ripple measurement. First, the bandwidth upper limit of the ripple is usually specified to avoid picking up high-frequency noise that exceeds the ripple bandwidth upper limit. The oscilloscope used for measurement should be set with an appropriate bandwidth upper limit. Secondly, the antenna formed by the long ground lead can be removed by taking off the "hat" 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, careful consideration of common-mode filtering is required in the power supply design.

[page] In addition, this current can be reduced by wrapping the oscilloscope leads around the core, because this creates a common-mode inductance that does not affect the differential voltage measurement, but reduces 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, when the power supply is integrated into the system, the power supply ripple performance is even better. There is almost always a certain amount of inductance between the power supply and the rest of the system. The inductance may be formed by wires or etched lines on the printed circuit board, and there are always additional bypass capacitors near the chip as a power supply load, which form a low-pass filtering effect and further reduce 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 effectively reduce the ripple. Smart engineers should try to take advantage of it during testing.