Example demonstration, take you to understand the switching power supply test in depth

　　In recent years, the relationship between power electronic equipment and people's work and life has become increasingly close. Program-controlled switches, communications, electronic equipment, control equipment, etc. have widely used switching power supplies, which has greatly promoted the rapid development of switching power supply technology. While switching power supplies are developing in the direction of high frequency, high reliability, low consumption, low noise, anti-interference and modularization, more stringent requirements are also put forward for product design verification and functional testing. This article will take the products of RIGOL (Beijing Puyuan Precision Technology Co., Ltd.) as an example to introduce some common test schemes for switching power supplies. The instruments used in this test scheme are RIGOL DS1302CA digital oscilloscope, DM3064 digital multimeter and DG series function/arbitrary waveform signal generator.

　　Digital Oscilloscope Application Solution

　　Transient response signal measurement

　　Load transient time is a dynamic time. It is the time it takes for the output voltage of the switching power supply to stabilize within the predetermined stable band after the load current transient. In the process of measuring the signal, the oscilloscope must have sufficient sampling rate and waveform capture rate to effectively capture the required waveform. The response we usually measure is in the order of μs or ms, while RIGOL's DS1302CA can measure the minimum rise time of 1.2ns, which can fully capture the transient signal. Figure 2 reflects the transient change of voltage during the process of current from 0.1mA to 65mA.

　　Detect harmful waveform signals in time

　　The power supply start-up delay is the time from applying the AC input to the output reaching its adjustment range. If a harmful waveform is generated when the switching power supply is turned on, it is a current spike that may damage the switching transistor. For this harmful waveform, it can be eliminated by suppressing the circuit and other methods. By measuring with an oscilloscope, harmful waveforms can be found in time, and the generation of harmful signals can be effectively suppressed at the moment the power supply is turned on, which indirectly improves the working efficiency of the power supply. Figure 3 is the waveform captured by DS1302CA.

　　Reproduce the signal in the system

　　If you are not satisfied with the simple analysis and calculation of the incidental and transient signals after capturing them, but also want to reproduce the signals in the system, then the seamless interconnection between RIGOL digital oscilloscope and function/arbitrary waveform signal generator can provide a perfect solution. The DG3121A signal generator can directly access the waveform memory of the DS1302CA through a dedicated interface, so as to reproduce the previously captured burrs or incidental signal waveforms on the signal generator. In this way, the signal can be reproduced, making the test measurement more convenient.

　　Simultaneous measurement of input and output signals

　　In the switching power supply test, the input and output of the circuit are often measured at the same time, but the large difference between the input and output voltages brings certain difficulties to the operation. The alternating trigger of DS1302A can measure and analyze the input and output signals at the same time, solving the operational difficulties. In the past, in mid-range oscilloscopes, the time base was shared when the two channels were displayed simultaneously, but now the products can use alternating triggering to achieve that the two channels each have their own time base, and different trigger modes can be selected on each channel. As shown in Figure 5, two different signals, one is a Vpp=188V signal, and the other is a Vpp=102mV signal, and their frequencies are 50Hz and 20kHz respectively.

　　Capture ripple and analyze noise

　　Ripple and noise are the periodic and random deviations of the DC output voltage from its average value within a specified bandwidth under the condition that all other parameters remain constant. It represents the unwanted AC and noise components remaining in the DC output voltage after the adjustment and filtering circuit. Ripple and noise can be measured by effective value or peak-to-peak value. The peak-to-peak value can provide information about high-amplitude, short-duration spikes, while the effective value is helpful in determining the expected signal-to-noise ratio. Specifically, ripple is a component that appears between the output terminals that is synchronized with the input frequency and the switching frequency. It is expressed in peak-to-peak values ​​and is generally less than 0.5% of the output voltage. Noise is a high-frequency component other than ripple that appears between the output terminals. It is also expressed in peak-to-peak values ​​and is generally around 1% of the output voltage. Ripple noise is a combination of the two, expressed in peak-to-peak values, and is generally less than 2% of the output voltage. Figure 6 is the waveform of the ripple signal observed by the DS1302CA. [page]

　　Switching Power Supply Drift Measurements

　　Drift mainly refers to the periodic and random deviation of the power supply output current or voltage. The drift measurement of the switching power supply takes a long time and requires high measurement accuracy. In actual measurement, we usually make a statistics of all the measured data, and then calculate its PPM value based on this statistical value, and the total time is about 24 hours. This test process is not only boring but also prone to errors. In order to make the measurement more convenient for engineers and minimize the measurement error, RIGOL provides a solution of DM3064 multimeter plus PC control.

　　Usually during experiments, we must not only meet the high requirements of test accuracy and test speed, but also ensure a certain amount of test time. In this way, if we are on duty for a long time, measurement errors are inevitable. To solve this problem, we can control the measurement through computer software, allowing the multimeter to collect voltage and current at regular intervals, and engineers can also set the size of the time period arbitrarily, truly realizing the easy collection of data through the multimeter. DM3064 can collect up to 2 million data in one channel. If you want to store more data, you can also choose to store it directly in a USB flash drive or in a computer. The convenience of using DM3064 is that after data collection, its built-in software can directly save the data and save the data as a txt or mdb database file, so that the data can be directly analyzed on the computer with the help of mathematical analysis tools.

　　Multi-channel inspection measurement

　　If we are no longer satisfied with single-channel measurement and want to achieve multi-channel simultaneous measurement, we can choose the multi-channel data inspection function of DM3064, which can measure up to 16 signals at a time. After measuring the current and voltage of each output module at the same time, the required data can be counted through computer software, and then each channel of data can be analyzed. If we want to see the waveform of the data during the inspection, we can also turn on the waveform display and observe the data change curve during the data inspection. In the waveform chart, we can use Ultralogger software to intuitively see the voltage and current change curve on the PC. RIGOL multimeter's data inspection and collection functions can quickly realize current and voltage inspection and data collection. In addition, Ultralogger software can realize on-site control through GPIB and USB buses, and can also realize remote control through LAN, and can regularly return the data report of each workstation to the remote server.

　　Temperature measurement

　　Temperature measurement is one of the most important links in the power supply product testing process. Today's power supply products must meet FEC, VDE, UL, CSA, FCC and other temperature measurement standards, so temperature is also one of the important measurement parameters for power supply. With the advancement of power supply technology, the product's requirements for temperature are getting higher and higher. During the measurement process, you can use the DM3064 multimeter plus a temperature sensor to convert the collected physical signal into an electrical signal, and then convert it into the corresponding temperature value. Similarly, if other applications are required, you can also use the corresponding sensor for measurement, and select the acquisition function to achieve the measurement function of fast sensing.

　　Other application measurements

　　In the switching power supply industry, it is necessary to simulate the switching frequency and synchronization frequency of the switching tube for measurement. In this type of product testing, a signal source can be added to simulate the operating frequency of the switching power supply in different states. The signal emitted by the signal generator can also be used to simulate the original control signal. In addition to the DG3121A mentioned above, you can also choose the signal source of the DG2000 series and DG1000 series. In addition, the above instruments can be integrated into the test system, and the signal source, multimeter, oscilloscope, power meter, etc. can be bound together. Through software control, different performance and input and output characteristics can be tested, and the production line can be automated, as shown in Figure 10.