Analysis and introduction of the use of multi-purpose oscilloscope in LLC testing

Keysight Technologies DSOX1204A/G 4-channel oscilloscope

Complements the 1000X Series Oscilloscope Family

The 1000X Series digital oscilloscopes are Keysight's entry-level oscilloscopes.

Although the DSOX1204A/G 4-channel oscilloscope is positioned as an entry-level product, it is feature-rich, with a maximum sampling rate of 2GSa/s, a maximum bandwidth of 200MHz, and a capture rate of 50,000 waveforms per second. In addition to the reliable oscilloscope functions, it is also equipped with advanced analysis tools such as frequency response analysis, mask testing, protocol decoding, FFT, and segmented storage to obtain more in-depth test results.

Figure 1 DSOX1204G oscilloscope

These high-end analysis functions, coupled with Keysight's 60 years of oscilloscope research and development, can help small and medium-sized enterprises, students and enthusiasts complete various product tests and instrument learning within limited budget conditions.

LLC Power Supply Measurement     

Now we have found a 200W voltage-type LLC switching power supply and tested it using the newly released DSOX1204G oscilloscope from Keysight Technologies, which has a starting price of less than 10,000 yuan.

First of all, engineers are most concerned about the loop response test. The loop response test can accurately evaluate the stability of the negative feedback loop, which can greatly improve product quality. It can be said to be one of the most important test items for switching power supplies. In the past, it took more than 100,000 yuan to buy an expensive loop analyzer, but now it can be done with an oscilloscope of a few thousand yuan (as shown in Figure 2).

▲ Figure 2 Loop analysis test

The waveform stored in the oscilloscope is as follows (Figure 3):

▲ Figure 3 Loop analysis diagram

From the Bode plot test results, we can see that the LLC line crossover frequency is about 79.4HZ, the phase margin is 83.11 degrees, and within the 10-fold frequency range near the crossover frequency, the gain is 18.1DB, and the gain slope is around -1, which is a single-pole crossover and meets the requirements of loop stability. It also confirms that the LLC circuit generally has a low crossover frequency.

Since it is an LLC power supply, the resonant waveform must be measured (as shown in Figure 4):

▲ Figure 4 LLC switching waveform test

Among them: 1, 2, 4 channels are measured with active differential probes , 1 and 4 channels (yellow and red) are set to 500X ratio, respectively measuring the VDS waveforms of the lower tube and upper tube of the half-bridge LLC, 2 channels (green) are set to 50X ratio, measuring the VGS waveform of the lower tube drive of the half-bridge LLC, and 3 channels (blue) use active current probes set to 1A/V ratio to measure LLC primary resonant current Ir waveform. The measured waveforms are stored as shown in Figure 5.

▲ Figure 5 LLC switching waveform

Finally, of course, we are concerned about the output ripple. The output low-frequency ripple current of the voltage-mode LLC has always been a concern for everyone. Let’s use DSOX1204G to test the effect (as shown in Figure 6):

▲ Figure 6 Ripple test

▲ Figure 7 Oscilloscope probe operation method

Before testing the ripple, it is necessary to talk about a problem that many beginners tend to overlook, that is, the operation of the probe. Since the use of the ground clamp that comes standard with the oscilloscope probe will result in a larger loop area when wiring, which will cause other noise to couple in, so the loop area needs to be minimized, remove the probe hook and ground clamp, use the ground spring that comes standard with the oscilloscope probe to connect to the front end of the probe, and at the same time, set the oscilloscope's attenuation ratio to X1 (as shown in Figure 7), and then connect it to the output end of the power supply (as shown in Figure 8)

▲ Figure 8 Probe connection to measure ripple

Then set the oscilloscope to AC coupling, set the probe attenuation ratio to 1X, and enable the 20M bandwidth limit to perform measurements. The measurement results of DSOX1204G are shown in Figure 9.

▲ Figure 9 Output ripple

It can be seen from the figure that low-frequency ripple does exist, and the peak-to-peak value of the ripple is as high as 1V. For 12V output, the ripple ratio is 8.3%, which is indeed a bit high.

Of course, there are many more items that need to be tested using an oscilloscope, such as MOSFET  transient voltage, loss, output voltage overshoot, etc. These items can be tested using DSOX1204G, so I will not list them one by one here.

Finally, how to process the report more quickly after the test is completed? This requires the Web server function of Keysight instruments. As long as the computer and oscilloscope are connected to the same LAN or WAN, the oscilloscope IP address can be entered in the browser to achieve remote control of the instrument and save waveforms, which is very convenient for engineers to process reports!

Remote connection and control