Automatic measurement system of power supply ripple based on RIGOL digital oscilloscope

Introduction

At present, AC/DC and DC/DC converters have developed into important components of various electronic equipment and automatic control equipment, and their quality directly affects the reliability and stability of the entire equipment. This requires power module manufacturers to strictly control the design, process, manufacturing, testing and other links. Strict testing of products before leaving the factory is a very important link in the production process of power products.

The domestic power supply industry has generally adopted automatic measurement systems for product testing. The entire system will include a variety of measurement items, and most tests can use automatic measurement (using programmable measurement and testing equipment). However, ripple noise is a special case. Many power supply companies still measure ripple noise through manual operation, which greatly affects production efficiency. The solution introduced in this article is based on the RIGOL programmable digital oscilloscope to achieve fast and reliable measurement of power supply ripple.

What does ripple and noise mean? It

can be said that any DC power supply contains ripple and noise, as shown in Figure 1:

A. Noise: Usually we use Vpp to represent noise, which is the difference between the highest point and the lowest point of the waveform (the Vpp value can be obtained by enabling the Vpp measurement in the automatic measurement function of the oscilloscope);

B. Ripple: The generation of ripple mainly comes from the switching and rectification process. Usually we use RMS (root mean square value) to represent ripple, and generally use an RMS RF voltmeter to measure it (adjust the time base of the oscilloscope and enable the average value measurement in the automatic measurement function of the oscilloscope to get an approximate result).

Figure 2 is a true description of the noise and ripple of the switching power supply.

Engineers often see the term PARD (periodic and random deviation), which actually means ripple and noise. The bandwidth of ripple and noise signals is generally distributed between 20Hz-20MHz. Noise can be filtered out by selecting appropriate capacitors, but the effect of ripple filter capacitors is not very obvious.

How to measure noise and ripple

There are many methods for measuring ripple and noise at present, and the industry has not yet formed a unified standard. Different test methods will result in large deviations in test results.

From the test results, the test results obtained by the method introduced below have a high degree of credibility.

A. Oscilloscope settings: Set the probe input ratio to 1:1; select "AC" for coupling mode; if the oscilloscope input bandwidth is greater than 20MHz, turn on the oscilloscope bandwidth limit function.

B. Probe selection: As shown in Figure 3, the original ground wire of the probe is replaced by a ground spring; a fixed test socket can be used on the factory test line, and the probe is directly inserted into the test socket.

C. Measurement: Direct contact or as close to the test point as possible. If the power supply to be tested needs to be loaded, the wiring should be as short as possible to reduce the noise introduced from the outside world to the system under test. At the same time, different test environments also have a great impact on the test results.

Automatic test system

Take a company that produces power modules as an example. It mainly produces power supply voltage regulator modules with one input and one or two outputs. Taking a certain model of the company as an example, the component under test is a 16-32V DC input and a 20V DC-DC regulated DC output. The measured waveform, that is, the ripple, has a peak-to-peak value of about 55mV and a frequency of 255Hz, as shown in Figure 4.

On the production line, each device needs to be tested, and during the measurement of each device, multiple sets of data (varying between 16-32V) are connected to the device input, and loads with different resistance values ​​are connected to the output. In this way, there are multiple permutations and combinations. The ripple of each combination needs to be measured, and the maximum peak-to-peak value of the ripple needs to be counted. If the measurement is done manually, the test process is too complicated and the test time is long, which of course also results in low production efficiency. Therefore, an automatic measurement system is very necessary.

Figure 5 is a block diagram of the automatic measurement system. The main thing related to the oscilloscope DS5000 is the control of the oscilloscope by PC software and the communication between the PC software and the microcontroller. The following content describes in detail the functions implemented by each part of the block diagram.

1. Input source, a voltage source with variable voltage within a certain range (its output value can be controlled by a computer)
2. Variable electrical load, a load with variable resistance within a certain range (its resistance value can be controlled by a computer)
3. DS5000, measures the ripple of the output voltage, collects the peak-to-peak value of the ripple through computer control, and uploads the collected peak-to-peak value to the PC.
4. Microcontroller, the PC receives the measurement command from the microcontroller, and sends the peak-to-peak value uploaded by DS5000 to the microcontroller through the PC. The microcontroller compares it with the set value to draw a conclusion on whether the product is qualified. The conclusion is displayed to the workers, and the result is fed back to the PC.
5. PC, completes communication with various measuring devices and is the core of the automatic measurement system.
During the test process, the input source will have a set of changing values, and the variable electrical load will have a set of changing values, so there will be multiple combinations. Measuring the peak-to-peak value of the device output voltage ripple under each combination is what DS5000 does. The maximum ripple value of each measured device is counted through the PC software and fed back to the microcontroller. The program is shown in Figure 6.