Oscilloscope test switching power supply

In practical applications, engineers often encounter scenarios where power measurement is required. In addition to a special power analyzer that can complete the measurement, the oscilloscope used daily can also be used for it.
Theoretically, power is equal to voltage multiplied by current, and an oscilloscope is a voltage response instrument. How to perform power analysis? After the oscilloscope is equipped with a current probe, the current signal can be converted into a voltage signal through the current probe to achieve the purpose of measuring current, so the oscilloscope can measure power.
First, let us clarify what functions the oscilloscope power analysis can do:
1. Analyze overall harmonic distortion, effective power, apparent power, power factor, crest factor
2. Conduct current harmonic test according to IEC61000-3-2 standard
3. Measure the switching loss and conduction loss of the switchgear.
4. Analyze the conversion rate of current and voltage dl/dt and dV/dt
5. Automatically set the oscilloscope ripple measurement
6. Analyze pulse width modulation
From traditional analog power supplies to high-efficiency switching power supplies, the types and sizes of power supplies vary greatly. They all face complex and dynamic working environments. Equipment load and demand may change greatly in an instant. Even a "daily-used" switching power supply must be able to withstand instantaneous peaks that far exceed its average operating level. Engineers who design power supplies or systems to use power supplies need to understand how the power supplies work under static and poor conditions.
In the past, to describe the behavioral characteristics of a power supply meant using a digital multimeter to measure the quiescent current and voltage, and using a calculator or PC for painstaking calculations. Today, most engineers turn to oscilloscopes as their power measurement platform. Modern oscilloscopes can be equipped with integrated power measurement and analysis software, simplifying settings and making dynamic measurements easier. Users can customize key parameters, automatically calculate, and see the results within seconds, not just the raw data.
Power supply design issues and measurement requirements
Ideally, each power supply should work like a mathematical model designed for it. But in the real world, the components are defective, the load will change, the power supply may be distorted, and environmental changes will change the performance. Moreover, changing performance and cost requirements also make power supply design more complex. Consider these questions:
How many watts of power can the power supply maintain beyond the rated power? How long can it last? How much heat does the power supply emit? What happens when it overheats? How much cooling air flow does it need? What happens when the load current increases significantly? Can the equipment maintain the rated output voltage? How does the power supply deal with a complete short circuit at the output end? 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 stricter EMI/EMC standards. Only a strict measurement system can enable engineers to achieve these goals.
Oscilloscope and power measurement
For those who are accustomed to making high-bandwidth measurements with an oscilloscope, power measurement may be simple because of its relatively low frequency. In fact, there are many challenges in power measurement that high-speed circuit designers never have to face.
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. Must capture and analyze the waveform truthfully, find the abnormality of the waveform. This is demanding on 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 very different amplitudes in the acquisition.
Basics of Switching Power Supply
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 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.
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 the 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 (MOSFET) and insulated gate bipolar transistors (IGBT). These devices have short switching times and can withstand unstable voltage spikes. It is also important that they consume very little energy, high efficiency and low heat, regardless of whether they are in the on or off state. 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.

Figure 1. Simplified schematic diagram of switching power supply.

Figure 2. MOSFET switching device, showing measurement points.
Prepare for power measurement
When preparing for the measurement of the switching power supply, you must choose the appropriate tools and set up these tools so that they can work accurately and repeatably. Of course, the oscilloscope must have a basic bandwidth and sampling rate to adapt to the switching frequency of the SMPS. Power measurement requires less than 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 on and off voltages of switching devices in the same acquisition? The ratio of these signals may reach 100,000:1.
Are there reliable and accurate voltage probes and current probes? Is there an effective way to correct their different delays?
Is there an effective way to reduce the static noise of the probe?
Can the instrument be equipped with enough record length to capture long complete power frequency waveforms at a high sampling rate?
These characteristics are the basis for meaningful and effective power supply design measurements.
Measure the 100 volt and 100 millivolt voltage in the acquisition
To measure the switching loss and average power loss of the switching device, the oscilloscope must first determine the voltage on the switching device when it is off and on.
In AC/DC converters, the voltage dynamic range on the switching device is very large. The voltage passed on the switching device in the on state depends on the type of the 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 mainly depends on the saturation turn-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 80 Vrms and 264 Vrms.
The off-state voltage (between TP1 and TP2) on the switching device under the input voltage may be as high as 750 V. In the on state, the voltage between the same terminals may range from a few millivolts to about 1 volt. Figure 3 shows the typical signal characteristics of the switching device.

Oscilloscope test switching power supply
Figure 3. Typical signal of switchgear
In order to accurately measure the power supply of the switching device, the disconnection and opening voltage must be measured first. However, the dynamic range of a typical 8-bit digital oscilloscope is not enough to accurately collect both the millivolt 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. Under this setting, the oscilloscope can accept voltages up to 1000 V, so that 700 V signals can be collected without overloading the oscilloscope. The problem with this setting is that the sensitivity (the signal amplitude that can be resolved) becomes 1000/256, which is about 4 V.
Tektronix DPOPWR software solves this problem. The user can input the RDSON or VCEsat value in the equipment 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 manually inputting the value.
Figure 4. The DPOPWR input page allows the user to input the technical data values of RDSON and VCEsat.

Oscilloscope test switching power supply
Figure 4. The influence of propagation delay on power measurement
Eliminate the time deviation between the voltage probe and the current probe
To use a digital oscilloscope for power measurement, it is necessary to 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 collector and emitter of the 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 probe. Each of these two types of probes has its own unique transmission delay. The difference between these two delays (called time deviation) will cause inaccurate amplitude measurements and time-related measurements. It is important to understand the impact of probe transmission delay on peak power and area measurements. After all, power is the product of voltage and current. If the two multiplied variables are not well corrected, the result will be wrong. When the probe is not correctly "time offset correction", the accuracy of measurement such as switching loss will be affected.
The test setup shown in Figure 5 compares the signal at the probe tip (lower trace display) and the signal at the front panel of the oscilloscope after transmission delay (upper display).
Figure 6-Figure 9 are actual oscilloscope screen shots showing the effect of probe time lag. It uses Tektronix P5205 1.3 kV differential probe and TCP0030AC/DC current probe to connect to the DUT. The voltage and current signals are provided by the calibration fixture. Figure 6 illustrates the time lag between the voltage probe and the current probe, and Figure 7 shows the measurement result (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. This example shows that the time lag introduces a measurement error of 6%. Accurately correcting the time lag reduces the peak-to-peak power loss measurement error.

Figure 5. The effect of propagation delay on power measurement.

Oscilloscope test switching power supply
Figure 7. The peak amplitude and area measurement display when there is a time deviation is 6.059 watts.
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 function of statically correcting time deviation, but the premise is that the specific voltage probe and current probe have a constant and repeatable transmission delay. The static correction time offset function 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 technology provides a quick and convenient method to reduce the time deviation.
Eliminate probe offset and noise
Differential probes and current probes may have a small offset. This offset should be eliminated before measurement because it will affect measurement accuracy. Some probes use built-in automatic methods to eliminate offsets, while other probes require manual elimination of offsets.

Figure 8. Voltage and current signals after correcting the time deviation.

Automatically eliminate offset
The combination of a probe equipped with a TekVPITM probe interface and an oscilloscope can eliminate any DC offset errors that occur in the signal path. Press the Menu button on the TekVPITM probe, the Probe Controls box appears on the oscilloscope, showing the AutoZero function. Selecting the AutoZero option will automatically clear any DC offset errors 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 in the measurement system.
Manually remove the offset
Most differential voltage probes have built-in DC zero offset trimming control, which makes eliminating the zero offset a relatively simple step: After the preparation is completed, the next step is:
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 0 V (or as close as possible to 0 V).
Similarly, the current probe must be adjusted before measurement. After eliminating the zero offset:
Set the oscilloscope sensitivity to the value that will be used in the actual measurement;
Close the current probe without signal;
Adjust the DC balance to zero;
Adjust the intermediate value to 0 A or as close to 0 A as possible;
Note that these probes are all active devices, even in static state, there will always be some low-level noise. This noise may affect measurements that rely on both voltage and current waveform data. The DPOPWR software package contains a signal conditioning function (Figure 10) to reduce the effects of inherent probe noise.
The role of record length in power measurement
The ability of the oscilloscope to capture events over a period of time depends on the sampling rate used and the depth (record length) of the memory in which the acquired signal samples are stored. The speed of memory filling is proportional to the sampling rate. If the sampling rate is set high in order to provide a detailed high-resolution signal, the memory will quickly fill up.
For many SMPS power measurements, it is necessary to capture a quarter cycle or half a 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 real Ton and Toff conversion
In order to determine the loss in the switching conversion, the oscillation 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. This large-scale oscillation is caused by parasitic elements in the circuit when the SMPS switches between discontinuous current mode (DCM) and continuous current mode (CCM).
Figure 11 shows a switch signal in simplified form. This oscillation makes it difficult for the oscilloscope to recognize 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. Or, you can specify some other neat signal.
in some openIn power-off 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 begins 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 (ie, the clock signal in Figure 1 and Figure 2) can determine the true turn-on and turn-off transition, as shown in Figure 13. In this way, it is only necessary to appropriately set the reference level and the hysteresis level of the gate drive signal.
Oscilloscope test switching power supply
Figure 11. The gate signal Vg used to identify Ton and Toff transitions
Oscilloscope test switching power supply
Figure 12. Typical signal characteristics of switching devices.