Losses due to dissipation in switching devices such as MOSFET's and IGBT's contribute significantly to the overall losses in switch mode power supplies. This application note explains how to measure switching loss and conduction loss in power supply switches using an oscilloscope equipped with a current probe and differential voltage probe. It covers both manual and automated techniques, along with guidance on making connections.
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Measuring Power Supply Switching Loss with an Oscilloscope

Measuring Power Supply Switching
Loss with an Oscilloscope
APPLICATION NOTE
Application Note
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Figure 1. Simplified switch mode power supply switching circuit.
Introduction
With the demand for improving power efficiency and extending the operating time of
This application note will provide a quick overview of these measurements and some tips for making better, more repeatable measurements with oscilloscopes and probes.
A typical
Of this loss, a significant portion is dissipated in the switching devices, usually MOSFETs or IGBTs.
Ideally, the switching device is either “on” or “off” like a light switch, and instantaneously switches between these states. In the “on” state, the impedance of the switch is zero and no power is dissipated in the switch, no matter how much current is flowing through it. In the “off” state, the impedance of the switch is infinite and zero current is flowing, so no power is dissipated.
Measuring Power Supply Switching Loss with an Oscilloscope
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Figure 2. Multiplying instantaneous voltage across and current through a switching device gives instantaneous power throughout the switching cycle.
Figure 3. A: How the switch appears on the schematic, and B: How the circuit sees the switch.
In practice, some power is dissipated during the “on” (conduction) state, and, often, significantly more power is dissipated during the transitions between “on” and “off”
These
Application Note
Conduction Loss
In the conduction state, the switch does have a small resistance and voltage drop across it, and the switch dissipates power as a function of the current flowing through it.
For a MOSFET, this power is typically modeled as:
P = ID2 * RDSon = ID * VDS
-where ID is the Drain current,
-RDSon is the dynamic
-VDS is the saturation voltage between the Drain and the Source, often < 1 V
For an IGBT or BJT, this power is typically modeled as:
P = IC * VCEsat
-where IC is the Collector current and
-VCEsat is the saturation voltage between the Collector and the Emitter, often < 1 V
During
For a MOSFET, during
P = ID * VDS
-where ID is the Drain current and
-VDS is the voltage between the Drain and the Source
For an IGBT or BJT, during
P = IC * VCE
-where IC is the Collector current and
-VCE is the Collector to Emitter voltage
In a similar manner, during
Measuring Power Supply Switching Loss with an Oscilloscope
Measuring Switching Loss
There are two approaches to measuring switching loss: it can be measured using manual setups and
Probing and Measurement Setup
Before discussing the specific power measurements, there are six key steps to making accurate and repeatable measurements:
1.Remove voltage offset errors: The amplifiers in differential probes may have a slight DC voltage offset which will affect measurement accuracy. With the inputs shorted and no signals applied, automatically or manually adjust the DC offsets in the probe to zero.
2.Remove current offset errors: Current probes may also exhibit DC offset errors due to residual magnetism in probe, as well as amplifier offsets. With the jaws closed and no signals applied, automatically or manually null out the DC offsets in the probe.
3.Remove timing errors: Because instantaneous power measurements are calculated based on multiple signals, it is important that the signals be properly
may be significantly different, leading to measurement errors. Good results are generally possible by adjusting the
4.Optimizing
5.Signal conditioning: Measurement quality can also be improved by conditioning the input signals. Bandwidth limiting can be used to selectively reduce noise above the frequencies of interest, and averaging can be used to reduce uncorrelated or random noise on the signal. High Res acquisition mode provides bandwidth limiting and noise reduction, increased vertical resolution, and it even works on signals acquired in single shot mode.
6.Accuracy and safety: For best accuracy, be sure to use the equipment within the normal operating range and below the peak ratings. And, for your safety, always stay well within the equipment’s absolute maximum specifications and follow manufacturer’s instructions for use.
Application Note
Figure 4. Switching power loss measurement using waveform multiplication and mean measurement on the power data over the whole acquisition. This technique relies on manual setup, using standard capabilities of this oscilloscope.
Measuring Switch Loss – Manual Setup and
One way to measure
VDS is acquired with a differential voltage probe and is shown in yellow in Figure 4. The Drain current is acquired with an AC/DC current probe and is shown in cyan. The vertical sensitivity and offset of each channel is adjusted so the signals occupy more than half of the vertical range, but without extending beyond the top and bottom of the graticule.
A stable display is important for visual analysis, so the oscilloscope’s edge trigger is set to the 50% point on the voltage waveform. Then the sample rate is set to assure adequate timing resolution on the signals’ edges. In this case, a sample rate of 6.25 GS/s results in many sample points on each edge of the switching waveform. Finally, High Res acquisition mode is enabled to increase the vertical resolution to 16 bits.
Waveform math is then used to multiply the current by the voltage to create the orange instantaneous power waveform. An automated measurement is used to measure the average or mean value of the power waveform.
In this example, the engineer manually adjusted the oscilloscope to optimize the quality of the switching loss measurement. At a later date, this engineer or another engineer would likely set up the measurement slightly differently, resulting in different measurement results. Automating the measurement through power analysis software removes many of the sources of variation.
Measuring Power Supply Switching Loss with an Oscilloscope
Figure 5. Automated switching loss measurement determines power and energy loss during
Measuring Switch Loss – Automated Using Power Analysis Software
To consistently optimize the setup and improve measurement repeatability, a power measurement application can be useful. In this case, the PWR Advanced Power Analysis application provides a custom autoset for the Switching Loss measurement and then, with the push of a button, makes the full suite of switching loss power and energy measurements.
Slew Rate and Switching Loss
As expected from inspecting the instantaneous power waveform and as indicated by the switching loss measurement values in Figure 5, the
Application Note
Figure 6. A trajectory plot shows voltage versus current during
A slew rate measurement is the change of voltage in a given time interval (usually between the 10% and 90% points on an edge) and has the units of volts/second. Because the mathematical derivative is inherently a
Slew rate measurements can be made manually with cursors by placing one waveform cursor at the 10% point of the signal edge and the other cursor at the 90% of the waveform edge.
The slew rate is then calculated by dividing the difference between the voltage measurements by the time difference between the cursors. This technique requires the user to estimate the 10% and 90% points on the waveform and calculate the result.
Measuring Power Supply Switching Loss with an Oscilloscope
Figure 7. Automated slew rate measurement on a MOSFET gate signal.
Many oscilloscopes can improve this process with automatic measurements. Automated amplitude and
However, power analysis software makes slew rate measure- ment setup easy, and it reduces variation in measurement results as the design engineer adjusts component values in the circuit.
A
Application Note
Figure 8. Automated switching loss measurement, showing significant improvement.
The exponential decay, shown between the vertical cursors in Figure 7, is a function of the output impedance of the gate drive circuit, the parasitic gate capacitances within the switching MOSFET device, and the circuit board capacitances at the gate. When the speed of the drive signal was increased, by reducing the gate drive output impedance and the capacitance at the gate node, the switching loss was improved by almost 30%, as shown in Figure 8.
Switching loss measurements are a critical part of optimizing the efficiency of switch mode power supplies. By using good measurement techniques and automating the power measurements, it is easy make a series of complex switching loss measurements, quickly and repeatably.
Measuring Power Supply Switching Loss with an Oscilloscope
Several Tektronix oscilloscope series offer automated switching loss measurements. Consult www.TEK.COM for information on specific instruments. The measurements shown in this application note were made with the following equipment:
5 Series MSO
Analysis Application
TDP1000 1 GHz Differential Probe
TCP0030A 120 MHz AC/DC Current Probe
Copyright © Tektronix. All rights reserved. Tektronix products are covered by U.S. and foreign patents, issued and pending. Information in this publication supersedes that in all previously published material. Specification and price change privileges reserved. TEKTRONIX and TEK are registered trademarks of Tektronix, Inc. All other trade names referenced are the service marks, trademarks or registered trademarks of their respective companies.
09/19 EA