Figure 1: Setup for analysis of switching losses
in a switch-mode power supply's MOSFET
Most modern digital oscilloscopes will provide the requisite capabilities to analyze such devices, but some are particularly well suited for the task due to dedicated software and UIs tailored to the job at hand. Teledyne LeCroy's Power Analyzer software is one such example; some of the discussion to follow keys in on that software.
Figure 1 depicts a detail of the setup for measuring the source-to-drain voltage of the MOSFET switch. A current probe is also attached at the location of the top probe to measure current as well as for calculating power.
|Figure 2: Voltage, current, and power traces are shown|
for a switch-mode power supply
One may also have the losses displayed in Joules or energy. The software performs an integration of the power waveform for each state to calculate their respective energy losses. Dividing those energy loss figures by their duration in time yields the power losses for each state. These figures provide a window into the power supply's efficiency. Not only does it give the total loss per switching cycle, but it also gives the losses for each state. It shows exactly where in the switching cycle excessive losses are incurred.
|Figure 3: Using a differential amplifier with fast overdrive|
recovery makes for accurate on-resistance measurements
As a result, you can achieve very good detail on saturation voltage. Referencing Figure 3, with current and voltage measurements in hand, calculating RDSon is a simple Ohm's Law problem. The Power Analyzer software automatically calculates the on-resistance, and in the case shown here, the result is 724 mΩ.