What is the method of testing switching power supply with digital oscilloscope?

Digital oscilloscope is one of many types of oscilloscopes. It is easy to use, reliable, durable, and has a long service life. It is widely used in many industries. Digital oscilloscopes have many functions and can also test switching power supplies. So what is the method of using a digital oscilloscope to test a switching power supply? Let me introduce it to you in detail.

Power supplies come in a wide variety of types and sizes, from traditional analog power supplies to highly efficient switching power supplies. They all face complex, dynamic operating environments. Equipment loads and demands can change dramatically in an instant. Even "daily" switching power supplies must be able to withstand instantaneous peaks that far exceed their average operating levels. Engineers who design power supplies or use power supplies in systems need to understand how the power supply operates under static conditions as well as worst-case conditions.

In the past, characterizing the behavior of a power supply meant measuring quiescent current and voltage with a digital multimeter and performing painstaking calculations with a calculator or PC. Today, most engineers have turned to oscilloscopes as their preferred power measurement platform. Modern oscilloscopes can be equipped with integrated power measurement and analysis software, simplifying setup and making dynamic measurements easier. Users can customize key parameters, automatically calculate, and see results in seconds, not just raw data.

Power Supply Design Issues and Measurement Needs

Ideally, every power supply should work just like the mathematical model it was designed for. But in the real world, components are imperfect, loads vary, power supplies can be distorted, and environmental changes can alter performance. And changing performance and cost requirements make power supply design even more complex. Consider these questions:

How many watts can the power supply sustain beyond its rated power? For how long? How much heat does the power supply dissipate? What happens when it overheats? How much cooling airflow does it require? What happens when the load current increases significantly? Can the device maintain the rated output voltage? How does the power supply handle a dead short at the output? What happens when the input voltage to the power supply varies?

Designers need to develop power supplies that take up less space, reduce heat, cut manufacturing costs, and meet more stringent EMI/EMC standards. Only a rigorous measurement system can enable engineers to achieve these goals.

Oscilloscope and Power Supply Measurements

To those accustomed to making high-bandwidth measurements with an oscilloscope, power measurements may seem simple because of their relatively low frequencies. In reality, power measurements present many challenges that high-speed circuit designers never have to face.

The voltage across the switching device may be high and "floating", that is, not grounded. The pulse width, period, frequency, and duty cycle of the signal may vary. The waveform must be captured and analyzed faithfully to detect anomalies. This is demanding for the oscilloscope. Multiple probes - single-ended probes, differential probes, and current probes are required at the same time. The instrument must have a large memory to provide recording space for long-term low-frequency acquisition results. And it may be required to capture different signals with greatly different amplitudes in a single acquisition.

Switching Power Supply Basics

The dominant DC power architecture in most modern systems is the switch mode power supply (SMPS), which is well known for its ability to efficiently handle varying loads. The power signal path of a typical SMPS includes passive devices, active devices, and magnetic components. SMPS uses as few lossy components as possible (such as resistors and linear transistors) and mainly uses (ideally) lossless components: switching transistors, capacitors, and magnetic components.

The SMPS device also has a control section, which includes components such as a pulse width modulation regulator, a pulse frequency modulation regulator, and a feedback loop 1. The control section may have its own power supply. Figure 1 is a simplified schematic diagram of an SMPS, showing the power conversion section, including active devices, passive devices, and magnetic components.

SMPS technology uses power semiconductor switching devices such as metal oxide field effect transistors (MOSFET) and insulated gate bipolar transistors (IGBT). These devices have fast switching times and can withstand unstable voltage spikes. Equally important, they consume very little energy whether they are on or off, with high efficiency and low heat generation. The switching device largely determines the overall performance of the SMPS. The main measurements of the switching device include: switching loss, average power loss, safe operating area, and others.

Preparing for Power Measurements

When preparing to make measurements on switching power supplies, it is important to select the right tools and set them up so that they work accurately and repeatably. Of course, the oscilloscope must have the basic bandwidth and sampling rate to accommodate the switching frequency of the SMPS. Power supply measurements require at least two channels, one for voltage and one for current. Equally important are some facilities that can make power supply measurements easier and more reliable. Here are some things to consider:

Can the instrument handle both the turn-on and turn-off voltages of a switching device in the same acquisition? The ratio of these signals can be as high as 100,000:1.

Are there reliable, accurate voltage and current probes? Is there an effective way to correct for their different delays?

Is there an effective way to minimize the static noise of the probe?

Does the instrument have sufficient record length to capture a long, complete line frequency waveform at a high sampling rate?

These characteristics are fundamental to making meaningful and effective power supply design measurements.

Measure 100V and 100mV in one acquisition

To measure the switching losses and average power loss of a switching device, the oscilloscope must first determine the voltage across the switching device when it is turned off and on, respectively.

In AC/DC converters, the voltage dynamic range across the switching device is very large. The voltage across the switch device in the on state depends on the type of switch device. In the MOSFET tube shown in Figure 2, the turn-on voltage is the product of the on-resistance and the current. In bipolar junction transistor (BJT) and IGBT devices, this voltage mainly depends on the saturation conduction voltage (VCEsat). The voltage in the off state depends on the operating input voltage and the topology of the switching converter. Typical DC power supplies designed for computing equipment use a universal mains voltage between 80Vrms and 264Vrms.

The off-state voltage across the switching device (between TP1 and TP2) can be as high as 750 V at full input voltage. In the on-state, the voltage between the same terminals can be between a few millivolts to about 1 V. Figure 3 shows the typical signal characteristics of a switching device.

To accurately measure the power supply of a switching device, the off and on voltages must first be measured. However, the dynamic range of a typical 8-bit digital oscilloscope is not sufficient to accurately acquire both the millivolt-level signal during the on period and the high voltage that occurs during the off period in the same acquisition cycle. To capture this signal, the vertical range of the oscilloscope should be set to 100 volts per division. At this setting, the oscilloscope can accept voltages up to 1000V, so a 700V signal can be acquired without overloading the oscilloscope. The problem with using this setting is that the maximum sensitivity (the smallest signal amplitude that can be resolved) becomes 1000/256, which is about 4V.

Tektronix DPOPWR software solves this problem by allowing users to enter the RDSON or VCEsat value from the device's technical data into the measurement menu. If the voltage being measured is within the oscilloscope's sensitivity range, DPOPWR can also use the acquired data for calculations instead of using manually entered values.

Eliminate timing deskew between voltage and current probes

To make power measurements with a digital oscilloscope, you must measure the voltage and current between the drain and source of a MOSFET switching device (as shown in Figure 2), or the voltage between the collector and emitter of an IGBT. This task requires two different probes: a high-voltage differential probe and a current probe. The latter is usually a non-intrusive Hall-effect type probe. Each of these two probes has its own unique propagation delay. The difference between these two delays, called time skew, can cause inaccuracies in amplitude measurements and time-related measurements. It is important to understand the impact of probe propagation delay on maximum peak power and area measurements. After all, power is the product of voltage and current. If two variables that are multiplied are not well calibrated, the result will be wrong. When the probe is not properly "time-skewed", the accuracy of measurements such as switching loss will be affected.

Actual oscilloscope screen shots showing the effects of probe skew. It uses a Tektronix P5205 1.3kV differential probe and a TCP0030 AC/DC current probe connected to the DUT. The voltage and current signals are provided through a calibration fixture. Figure 6 illustrates the skew between the voltage and current probes, and Figure 7 shows the measurement result (6.059mW) obtained without correcting the skew between the two probes. Figure 8 shows the effect of correcting for probe skew. The two reference traces are overlaid, indicating that the delay has been compensated. The measurement results in Figure 9 show the importance of correcting for skew correctly. This example shows that skew introduces a 6% measurement error. Correcting for skew accurately reduces the peak-to-peak power loss measurement error.

DPOPWR power measurement software can automatically correct the time deskew of the selected probe combination. The software controls the oscilloscope and adjusts the delay between the voltage channel and the current channel using real-time current and voltage signals to remove the difference in transmission delay between the voltage probe and the current probe.

A static deskew feature is also available, provided that specific voltage and current probes have constant, repeatable propagation delays. The static deskew feature automatically adjusts the delays between selected voltage and current channels for selected probes (such as the Tektronix probes discussed in this document) based on a built-in propagation time table. This technique provides a quick and convenient way to minimize skew.

Eliminate probe offset and noise

Differential and current probes may have a small offset. This offset should be removed before making a measurement because it affects the measurement accuracy. Some probes have a built-in automatic method for removing the offset, while other probes require manual removal of the offset.

Automatic offset removal

Probes equipped with the TekVPITM probe interface, combined with an oscilloscope, can eliminate any DC offset errors that occur in the signal path. Press the Menu button on the TekVPITM probe and the ProbeControls box will appear on the oscilloscope, showing the AutoZero function. Selecting the AutoZero option will automatically eliminate any DC offset errors that exist in the measurement system. The TekVPITM current probe also has a Degauss/AutoZero button on the probe body. Pressing the AutoZero button will eliminate any DC offset errors that exist in the measurement system.

Manual offset removal

Most differential voltage probes have built-in DC offset trim controls, which makes de-offsetting a relatively simple procedure: Once you have done the preparation, next:

Set the oscilloscope to measure the average value of the voltage waveform;

Select the sensitivity (vertical) setting that will be used in the actual measurement;

Without applying a signal, adjust the trimmer to zero and make the average level 0V (or as close to 0V as possible).

Similarly, current probes must be adjusted before measurements can be made. After de-biasing:

Set the oscilloscope sensitivity to the value that will be used in the actual measurement;

Turn off the current probe with no signal;

Adjust the DC balance to zero;

Adjust the middle value to 0A or as close to 0A as possible;

Note that these probes are active devices and will always have some low-level noise even when quiescent. This noise can affect measurements that rely on both voltage and current waveform data. The DPOPWR software package includes a signal conditioning function (Figure 10) that can minimize the effects of inherent probe noise.

The Role of Record Length in Power Measurements

The ability of an oscilloscope to capture events over a period of time depends on the sampling rate used, as well as the depth of the memory (record length) used to store the acquired signal samples. The speed at which the memory fills is directly proportional to the sampling rate. If the sampling rate is set high in order to provide a detailed, high-resolution signal, the memory will fill quickly.

For many SMPS power supply measurements, it is necessary to capture a quarter or half cycle (90 or 180 degrees) of the power frequency signal, and some even require a full cycle. This is to accumulate enough signal data to offset the impact of power frequency voltage fluctuations in the calculation.

Identify true Ton and Toff transitions

In order to accurately determine the losses in the switching transition, the oscillations in the switching signal must first be filtered out. Oscillations in the switching voltage signal can easily be mistaken for turn-on or turn-off transitions. Such large amplitude oscillations are caused by parasitic elements in the circuit when the SMPS switches between discontinuous current mode (DCM) and continuous current mode (CCM).

The simplified form shows a switching signal. This oscillation makes it difficult for the oscilloscope to identify the true turn-on or turn-off transition. One solution is to predefine a signal source for edge recognition, a reference level, and a hysteresis level, as shown in Figure 12. Depending on the signal complexity and measurement requirements, the measured signal itself can also be used as the signal source for the edge level. Alternatively, some other clean signal can be specified.

In some switching power supply designs (such as active power factor correction converters), the oscillations can be much more severe. The DCM mode greatly enhances the oscillations because the switch capacitors start to resonate with the filter inductors. Simply setting the reference level and hysteresis level may not be enough to identify the true transitions.

In this case, the gate drive signal of the switching device can determine the actual turn-on and turn-off transitions, as shown in Figure 13. In this case, only the reference level and hysteresis level of the gate drive signal need to be set appropriately.