Switch mode power supply measurement method with digital oscilloscope
In order to accurately measure the power supply of switching devices, it is necessary to first measure the off and on voltages. However, the dynamic range of a typical 8-bit digital oscilloscope is not sufficient to accurately capture both millivolt level signals during the turn-on period and high voltages during the turn 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 1000V, allowing for the acquisition of 700V signals without overloading the oscilloscope. The problem with using this setting is that the maximum sensitivity (the minimum signal amplitude that can be resolved) has become 1000/256, which is approximately 4V.
To use a digital oscilloscope for power measurement, it is necessary to measure the voltage and current between the drain and source of MOSFET switching devices (as shown in Figure 2), or the voltage between the collector and emitter of IGBT. This task requires two different probes: a high-voltage differential probe and a current probe. The latter is usually a non insertable Hall effect probe. These two probes each have their own unique transmission delay. The difference between these two delays (known as time deviation) can result in inaccurate amplitude measurements and time related measurements. It is important to understand the impact of probe transmission delay on the measurement of maximum peak power and area. After all, power is the product of voltage and current. If two multiplied variables are not properly corrected, the result will be incorrect. When the probe is not correctly calibrated for time deviation, the accuracy of measurements such as switch losses will be affected.
Actual oscilloscope screen diagram showing the impact of probe delay. It uses differential probes and current probes connected to the DUT. Voltage and current signals are provided through calibration fixtures. Figure 6 illustrates the time delay between the voltage probe and the current probe, while Figure 7 shows the measurement results obtained without correcting the time delay of both probes (6.059mW). Figure 8 shows the effect of correcting probe delay. The overlap of two reference curves indicates that the delay has been compensated for. The measurement results in Figure 9 indicate the importance of correctly correcting time delays. This example demonstrates that time delay introduces a measurement error of 6%. Accurately correcting the time delay reduces the measurement error of peak to peak power loss.
Some power measurement software can automatically correct the time deviation of the selected probe combination. Software controls the oscilloscope and adjusts the delay between voltage and current channels through real-time current and voltage signals to eliminate the difference in transmission delay between voltage and current probes.
A static correction time deviation function can also be used, provided that specific voltage and current probes have constant and repeatable transmission delays. The function of static correction time deviation automatically adjusts the delay between the selected voltage and current channels for the selected probe based on a built-in transmission schedule. This technology provides a fast and convenient method to minimize time deviations.
