Measuring power circuits: viewing switching waveforms safely
Measuring the switching node and phase currents with a differential probe, a current probe and a short ground. Telling apart the false signals that the measurement creates.
Differential probe
Used to measure between two points that are not ground, such as the high-side gate-source voltage or the switching node. Subtracting two ordinary probes (A − B) also works, but the error grows when the common-mode voltage is large. For a high-voltage differential probe, check the rated voltage and bandwidth.
Current probe
| Type | Characteristics |
|---|---|
| Clamp-on Hall current probe | DC to several MHz, without breaking the circuit |
| Rogowski coil | AC only, wide bandwidth, flexible |
| Shunt + differential probe | Cheap, wide bandwidth, has to be inserted in the circuit |
Zero (degauss) a current probe before measuring.
RoadmapSee where this note sits in Power Electronics→False signals from the ground loop
Measure a switching node with a long ground clip and the clip acts as an antenna, showing more ringing than is really there. Keep the ground within 1 cm using a spring tip and you see the real waveform.
What to look at
- Gate voltage: time to cross the threshold, the Miller plateau, ringing
- Switching node: is the overshoot voltage within the MOSFET rating?
- Diode conduction during dead time
- Phase current waveform: ripple, and its relation to the sampling instant
The measurements lead on to changes in switching speed (gate resistor), snubbers and layout.