SDG #279 Simple Common Base MOSFET driver

This build follows on from an earlier gate driver design, swapping a dedicated driver IC for a single NPN transistor wired as a common base stage. The appeal is straightforward: the topology provides voltage gain for level shifting a 3.3V logic signal up to a full 12V gate drive, and it does so without inverting the signal, which avoids the failure mode where a glitching or resetting microcontroller unintentionally switches a heavy load on.

The trade off is current gain below unity. Turning the MOSFET on means turning the transistor off and letting a 3k3 pull up supply only three to four milliamps of gate current; turning it off means whatever drives the emitter has to sink that current. The board is laid out with selectable zero ohm links so both cases can be compared: driven through a buffer, and driven directly from a PIC microcontroller pin.

Scope captures show the buffered version reaching roughly 11.76V at the gate with a turn on time near five microseconds. Driving the pin directly adds a visible hump and stretches turn on toward fifteen microseconds, but the load waveform at around four amps stays clean and thermal imaging shows no measurable MOSFET heating in either configuration, which suggests switching losses remain trivial at 200Hz.

A useful side lesson is the ringing diagnosis: an electrolytic across the supply rail kills the supply side dip caused by long leads, and a 100nF cap across the load absorbs the inductive turn off spike. Worth noting the design needs a separate rail above the load supply, or zener clamping if the supply exceeds the MOSFET’s Vgs rating.