Method for testing switching power supply with digital oscilloscope

From traditional analog power supplies to efficient switching power supplies, the types and sizes of power supplies vary widely. They all have to face a complex and dynamic working environment. Equipment load and demand may change greatly in an instant. Even "daily use" switching power supplies must be able to withstand instantaneous peaks that far exceed their average operating level. The engineer who designs the power supply or the power supply to be used in the system needs to understand the working condition of the power supply under the static condition and the worst condition.

In the past, to describe the behavioral characteristics of a power supply meant that a digital multimeter was used to measure the quiescent current and voltage, and a calculator or PC was used to make the hard calculation. Today, most engineers turn to oscilloscopes as their preferred power measurement platform. Modern oscilloscopes can be equipped with integrated power measurement and analysis software, which simplifies setup and makes dynamic measurements easier. Users can customize key parameters, calculate automatically, and see the results in seconds, not just raw data.

Power supply design problems and measurement requirements

Ideally, each power supply should work like a mathematical model designed for it. But in the real world, components are defective, the load will change, the power supply may be distorted, and environmental changes will change performance. Moreover, ever-changing performance and cost requirements also make power supply design more complicated. Consider these issues:

How many watts can the power supply maintain beyond the rated power? How long can it last? How much heat does the power supply dissipate? What happens when it overheats? How much cooling airflow does it need? What happens when the load current increases significantly? Can the device maintain the rated output voltage? How does the power supply respond to a complete short circuit at the output? What happens when the input voltage of the power supply changes?

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

Oscilloscope and power measurement

For those who are accustomed to high-bandwidth measurements with an oscilloscope, power measurement may be simple because of its relatively low frequency. In fact, there are many challenges that high-speed circuit designers never have to face in power measurement.

The voltage of the entire switchgear may be very high and it is "floating", that is, it is not grounded. The pulse width, period, frequency, and duty cycle of the signal will vary. The waveform must be truthfully captured and analyzed to find the abnormality of the waveform. This is demanding on the oscilloscope. Multiple probes-single-ended probes, differential probes, and current probes are also required. The instrument must have a large memory to provide space for recording long-term low-frequency acquisition results. And it may be required to capture different signals with very different amplitudes in one acquisition.

Switching power supply basics

The mainstream DC power supply architecture in most modern systems is the switching power supply (SMPS), which is well known for its ability to effectively cope with changing loads. The electrical energy 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.

SMPS equipment also has a control part, which includes pulse width modulation regulator pulse frequency modulation regulator and feedback loop 1 and other components. The control part may have its own power supply. Figure 1 is a simplified schematic diagram of SMPS, which shows the power conversion part, including active devices, passive devices and magnetic components.

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

Prepare for power measurement

When preparing for the measurement of switching power supplies, be sure to select the appropriate tools and set these tools so that they can work accurately and repeatedly. Of course, the oscilloscope must have basic bandwidth and sampling rate to adapt to the SMPS switching frequency. Power measurement requires at least two channels, one for voltage and one for current. Some facilities are equally important. They can make power measurement easier and more reliable. Here are some things to consider:

Can the instrument handle the turn-on and turn-off voltage of the switching device in the same acquisition? The ratio of these signals may reach 100,000: 1.

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

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

Can the instrument be equipped with a sufficient record length to capture a long complete power frequency waveform at a high sampling rate?

These characteristics are the basis for meaningful and effective power supply design measurements.

Measure 100 volts and 100 millivolts in one acquisition

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

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

At the highest input voltage, the off-state voltage (between TP1 and TP2) on the switching device may be as high as 750V. In the on state, the voltage between the same terminals may be between a few millivolts and about 1 volt. Figure 3 shows the typical signal characteristics of the switching device.

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

The Tektronix DPOPWR software solves this problem. The user can enter the RDSON or VCEsat value in the device technical data into the measurement menu shown in Figure 4. If the measured voltage is within the sensitivity range of the oscilloscope, DPOPWR can also use the collected data for calculation instead of using manually entered values.

Eliminate the time deviation between voltage probe and current probe

To use a digital oscilloscope for power measurement, you must measure the voltage and current between the drain and source of the MOSFET switching device (as shown in Figure 2), or the voltage between the IGBT collector and emitter. This task requires two different probes: a high-voltage differential probe and a current probe. The latter is usually a non-plug-in Hall effect probe. Each of these two probes has its own unique transmission delay. The difference between these two delays (called the time deviation) will cause inaccurate amplitude measurements and time-related measurements. It is important to understand the effect of probe propagation delay on maximum peak power and area measurements. After all, power is the product of voltage and current. If the two multiplied variables are not corrected well, the result will be wrong. When the probe does not perform the "time deviation correction" correctly, the accuracy of the measurement such as switching loss will be affected.

The actual oscilloscope screen showing the effect of probe time lag. It uses a Tektronix P52051.3kV differential probe and a TCP0030AC / DC current probe to connect to the DUT. The voltage and current signals are provided through a calibration fixture. Figure 6 illustrates the time lag between the voltage probe and the current probe, and Figure 7 shows the measurement results (6.059mW) obtained without correcting the time lag between the two probes. Figure 8 shows the effect of calibrating the probe time lag. The two reference curves overlap, indicating that the delay has been compensated. The measurement results in Figure 9 show the importance of correcting the time lag correctly. This example shows that the time lag introduces a 6% measurement error. Accurately correcting the time lag reduces the peak-to-peak power loss measurement error.

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

You can also use a static correction function for time deviations, but only if certain voltage and current probes have constant, repeatable propagation delays. The function of statically correcting time deviation automatically adjusts the delay between the selected voltage and current channels for the selected probe (such as the Tektronix probe discussed in this document) according to a built-in transmission schedule. This technique provides a quick and convenient method to minimize time deviations.

Eliminate probe bias and noise

Differential and current probes may have small offsets. This offset should be eliminated before the measurement, because it will affect the measurement accuracy. Some probes use built-in automatic methods to eliminate offsets, others require manual offset removal.

Automatically eliminate bias

A probe equipped with a TekVPITM probe interface is combined with an oscilloscope to eliminate any DC offset errors that occur in the signal path. Press the Menu button on the TekVPITM probe, and the ProbeControls box appears on the oscilloscope, showing the AutoZero function. Selecting the AutoZero option will automatically clear any DC offset errors present 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 present in the measurement system.

Manually eliminate bias

Most differential voltage probes have built-in DC zero bias trimming control, which makes eliminating zero bias a relatively simple step: after preparation is complete, 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 adding a signal, adjust the trimmer to zero and make the average level 0V (or as close to 0V as possible).

Similarly, the current probe must be adjusted before measurement. After eliminating the work offset:

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

Turn off the current probe without signal;

Adjust the DC balance to zero;

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

Note that these probes are active devices, even in static, there will always be some low-level noise. This noise may affect measurements that depend on both voltage and current waveform data. The DPOPWR software package includes a signal conditioning function (Figure 10) that minimizes the effects of inherent probe noise.

The role of record length in power measurement

The oscilloscope's ability to capture events over a period of time depends on the sampling rate used and the depth (record length) of the memory that stores the collected signal samples. The speed of memory filling is proportional to the sampling rate. If the sampling rate is set high to provide a detailed high-resolution signal, the memory will quickly fill up.

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

Identify the true Ton and Toff conversion

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 switching. This large-scale oscillation is caused by parasitic elements in the circuit when 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. According to different signal complexity and measurement requirements, the measured signal itself can also be used as the signal source of the edge level. Alternatively, some other neat signals can be specified.

In some switching power supply designs (such as active power factor correction converters), the oscillation may be much more severe. The DCM mode greatly enhances the oscillation because the switched capacitor starts to resonate with the filter inductor. Merely setting the reference level and hysteresis level may not be enough to identify the true transition.

In this case, the gate drive signal of the switching device can determine the true on and off transition, as shown in Figure 13. In this way, it is only necessary to appropriately set the reference level and hysteresis level of the gate drive signal.

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