SDG #251 IGBT's, MOSFETs, Trailing Edge Dimmers and Micsig CP210A current probe

This episode sits somewhere between a design post mortem and a probe demonstration. The project under test is a mains trailing edge dimmer, built in two flavours so the switching devices can be compared directly: one board using MOSFETs and another using IGBTs. Along the way there is a genuinely useful refresher on why an IGBT behaves like a hybrid of a BJT and a MOSFET, where the collector saturation voltage penalty bites, and why that penalty matters less as voltage and current climb.

The schematics are walked through before any power is applied, which is the right order for anyone planning to replicate the circuit. Attention then turns to measurement: capturing gate drive and load current on a live mains circuit demands proper isolation, so a differential probe handles voltage while the Micsig CP210A current probe takes care of current without inserting a shunt into the path. That combination, feeding a PicoScope 5000 series, is what makes the switching behaviour visible rather than guessed at.

The waveform comparison is where the real value lies. Turn on and turn off edges, tail current behaviour and the differences in dissipation between the two device families show up clearly, and the maths channel functions in the early access PicoScope 7 build are used to derive instantaneous power from the voltage and current traces. That is the sort of workflow worth borrowing whether or not you ever build a dimmer.

The closing section is honest about what did not work as intended and what would change on a revised board, which makes this more instructive than a straightforward success story. Anyone specifying switching devices for mains loads, or weighing up a clamp style current probe for switching measurements, will find plenty to take away.