Steps for Oscilloscope Power Supply Test

In the past, people used to use multimeters to test power supplies, which was very troublesome when there were many test parameters. Now, oscilloscopes provide many automatic measurement functions, which can be used to easily implement amplitude measurements (amplitude, high, low, maximum, minimum, RMS, peak-to-peak, positive/negative overshoot, average, cycle average, cycle RMS), timing measurements (period, frequency, rise/fall time, positive/negative duty cycle, positive/negative pulse width, burst width, delay, phase), and comprehensive measurements. In practice, many engineers are not very clear about the key points of using oscilloscopes for power supply testing. Here are some steps and key points for your reference. (The statements here are based on the Tektronix mixed signal oscilloscope MSO4000 series (MSO4034) and Tektronix probe configurations that I use. Different oscilloscopes and probes may be slightly different.)

Several key points in choosing an oscilloscope

1. Recording length and analysis tools

For many power supply measurements, it is necessary to capture 1/4 cycle or 1/2 cycle (90 degrees or 180 degrees) of the power frequency signal. Some measurements even require capturing the entire cycle, which requires the oscilloscope to have sufficient record length to meet the requirements (MSO4034 has a record depth of 10M, which is sufficient for general power supply tests).

Even more important than long record lengths is having the tools (such as Tektronix's Wave Inspector) to make use of all this data. Otherwise dealing with record lengths of millions of points, or thousands of screens of signal activity, is like looking for a needle in a haystack.

2. Time lag between voltage probe and current probe

Each voltage and current probe has its own characteristic propagation delay. The difference in delay between the current and voltage probes is called skew, and can cause inaccuracies in amplitude and timing measurements. When the probes are not properly "deskewed," measurement accuracy can be degraded, such as switching losses. When the Tektronix TekVPI probes I use are connected to a Tektronix 4000 Series oscilloscope, they automatically set the appropriate deskew value to achieve maximum accuracy in power measurements.

3. Probe offset

Differential probes typically have a small voltage offset. This can affect accuracy and must be removed before continuing measurements. Most differential voltage probes have built-in DC offset adjustment controls that make removing the offset relatively simple.

Some probes have an auto-degauss/auto-zero routine built in, such as with the TekVPI probes, which can be done by simply pressing a button on the probe "comp" box.

Safely and accurately test voltage and current waveforms

When using a digital oscilloscope to perform power supply measurements, it is necessary to measure the voltage and current in the device. This requires the use of two different probes: a voltage probe (usually a high-voltage differential probe) and a current probe.

Measuring the current through a MOSFET is relatively simple and can be done with many different Hall-effect current probes, such as the TCP0030. Measuring voltage is more problematic. The MOSFET is not connected to the AC power ground or the circuit output ground. Therefore, it is not possible to make ground-referenced voltage measurements using an oscilloscope, because connecting the probe ground lead to any of the MOSFET terminals would short the circuit through the oscilloscope ground.

The best way to measure the voltage of a MOSFET is to make a differential measurement. In a differential measurement, you measure the drain-to-source voltage, which is the voltage across the drain and source terminals of the MOSFET. The drain-to-source voltage can be on the top of a few tens of volts to a few hundred volts, depending on the range of the power supply.

Measuring instantaneous power

Characterizing the instantaneous power dissipation in switching transistors is part of almost every power supply design project. It is critical to select components that can operate economically and reliably at the worst-case operating limits. Current and voltage probing solutions from some manufacturers are ideal for these measurements. In addition to providing a safe measurement solution, they also provide a very simple time-delay correction function. Automatically set the appropriate time-delay correction value to achieve maximum accuracy in power supply measurements. Provide correct scale and units for voltage and current waveforms and calculated waveforms in watts. The following simple steps for measurement using a Tektronix 4000 Series oscilloscope are as follows: Connect the probe; Press Autoset, the oscilloscope automatically adjusts the vertical setting, horizontal setting; trigger setting to view the waveform; define the calculated waveform as Ch1 * Ch2; Turn on Area measurement to measure the area (energy) under the curve; Cursor readout indicates the instantaneous power. By using measurement gating, we can limit the Area measurement to a specific area and view the power loss associated with the turn-on time (Ton) and turn-off time (Toff) of the MOSFET.