LeCroy Oscilloscope Basic Application Series VI - Testing of Switching Power Devices

Although most engineers design or test products that are not power devices, almost every engineer must have encountered the need to verify the quality of the power supply components of the product. This technical article provides some test suggestions on power device design issues and black box testing of power devices. The discussion includes the response of switching power supplies to changes in load or transmission lines, and verifying the SOA (safe operating area) of power devices.

Most of the power supplies used in the electronics industry today are switching power supplies. The output power in a switching power supply is controlled by a series of gate drive pulses. When the load increases and more power output is required, the feedback loop that controls the output voltage will adjust the width of the gate drive pulse (the pulse width becomes wider); on the contrary, when the load decreases, the pulse width becomes narrower. In order to verify the performance of the power supply equipment, engineers will perform a series of tests, mainly including safe operating area test (SOA), power loss, upper gate measurement, dynamic impedance analysis, control loop response, power supply output ripple, line current harmonics, power factor, actual/apparent power, etc.

Measuring the response of a power supply to changes in load

Whether it is a power supply design engineer or an engineer who uses power supply components in their products, the first question they care about is, how does the power supply respond to changes in load? Figure 1 shows the analysis of the change in power supply performance when the load is reduced, which is similar to the situation when the load is increased. The time base label in the lower right corner shows that the current time base range is 1ms/division, so the total time length of the signal displayed on the screen is 10ms, and the sampling rate is 1.0 GS/s. This sampling rate is sufficient for switching power supply signals. The number of sampling points displayed on the screen is 10MSamples (the label shows "10MS").

Figure 1: The top grid captures a 10ms gate drive pulse waveform. The second waveform shows the tracked values ​​for all pulse widths of the gate drive pulse. The third and fourth waveforms are zoomed-in waveforms of the gate drive pulse signal under different load conditions.

The top grid in Figure 1 shows the gate drive pulse captured by the oscilloscope. The time before the load change is 1ms, and the time after the change is about 9ms. The small yellow triangle mark below the grid indicates the trigger position. There are two parts of the highlighted waveform about half a grid before and after the trigger position. The enlarged waveform of the red highlighted part is shown in the third grid, and the purple highlighted waveform is shown in the bottom grid. The two enlarged waveforms show the gate drive pulse width before the load change (red) and after the load change of about 500us (purple). In the enlarged waveform, we can see with the naked eye that the power supply outputs more power (wider pulse width) when the load is large, and the output power is less (reduced pulse width) when the load is reduced.

The above discussion is based on visual observation. The response of the power supply output power to load changes can be analyzed more deeply from the second blue curve in Figure 1. This curve is a tracking curve for the gate-level drive pulse width value. Digital oscilloscopes are best at displaying a series of time-based values, which do not necessarily have to be voltage vs. time or current vs. time. In the above power supply test case, the engineer is actually most concerned about: "How does the control loop of the power supply respond to changes in the external load?" Based on this, the blue tracking curve can better illustrate the problem. In the first 1ms, the gate drive pulse width is large, and then due to the load change, the pulse width decreases sharply, then increases again to appear a small spike, then falls back a little, then slowly increases, and then stabilizes at the new load value. Power supply design engineers can use this test function of the digital oscilloscope. For example, they can also track the fall time of the pulse edge, the minimum value of the waveform, etc. If the engineer wants to debug the response characteristics of the control loop, he can first save this waveform, then improve the power supply control loop design, and then repeat the measurement, compare the two measurement results, and verify the debugging effect. If necessary, the two tracking curves can be subtracted to see the difference more intuitively. This function of the digital oscilloscope can visualize the step load change response of the switching power supply control loop, and perform precise measurements to verify the power supply characteristics.

Measuring the Power Supply Safe Operating Area (SOA)

Whether it is the designer of the power supply or the engineer who integrates the power supply device into the product, they need to ensure that the power supply can operate within the safe zone under different working conditions. In addition, the power supply is also required not to output excessively high voltage/current pulses. Engineers also need to ensure that the buffer diode can work properly. When measuring SOA, one channel of the oscilloscope measures the voltage value of the switching power supply device, and the other channel tests the current flowing through the line. Then, based on the captured voltage vs time waveform and current vs time waveform, an XY graph of voltage vs current is drawn. Note that the time base label in Figure 2 indicates that the current acquisition time is 2 msec/div, the sampling rate is 100 MS/s, and the total sampling points are 2 MS (megasamples).

Figure 2: In the figure above, the yellow curve is voltage vs time. The blue curve is the current curve. The horizontal axis of the XY graph is the voltage value, and the vertical axis is the current value.

When performing an SOA test, the "unsafe" part is located at the sampling points in the upper right corner of the XY graph. These sampling points have high voltage and current values. When using a LeCroy oscilloscope for SOA testing, the user can place a cursor on the XY graph, move the cursor to the sampling point position of interest, and the voltage and current vs time values ​​at the cursor position will be displayed in the lower right corner of the screen. This function is very convenient for users to perform SOA testing. The unsafe sampling points of SOA collected by the instrument at the beginning may mean a surge current problem, or the unsafe sampling point problem encountered later may mean a step response problem to a sudden load change.

Other measurements

This article is unlikely to cover all switching power supply measurements. If you want to learn more about measurement examples, please visit www.lecroy.com. The measurement examples on the website include instantaneous power, power loss, dynamic resistance value, real power and apparent power, line current harmonics, how to use pulse width modulation analysis (Figure 1) to test the power supply soft start process and some other types of measurements.

All of these measurement cases are based on measuring the voltage and current values ​​of power devices. Among them, the most critical link is to use the most appropriate probe for safe and accurate measurement. LeCroy provides a large number of optional solutions, current probes (up to 500A) and voltage probes (up to 20kV). In some applications, it is necessary to measure the floating voltage. You can use the ADP300 (20MHz bandwidth) and ADP305 (100MHz) high-voltage differential probes to measure the maximum 1.4kV differential voltage value. In some other test occasions, engineers often need to test the small signal value superimposed on the large voltage. In these cases, the front-end differential amplifier can be used. LeCroy DA1855A provides industry-leading 100,000:1 CMRR and ultra-fast overload recovery capability.

Summarize

There are many test items required for designing power devices, and integrated power devices also require testing of the power supply system. Using the above test items and functions of LeCroy oscilloscope can greatly shorten the measurement time, simplify the measurement process, speed up the product design cycle, and not miss any power supply defects, thereby increasing the reliability of the design.