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Archive for May 22nd, 2014

Hall Effect LED Current Control: Switched MOSFET Gates

The rise and fall times for the LEDs on the over-the-top blinky taillight left a bit to be desired, at least if you were interested in short pulses:

VG 1193 mV - ID 50 mA-div - 1 ms PWM filter - overview

VG 1193 mV – ID 50 mA-div – 1 ms PWM filter – overview

So I spliced an analog switch between the PWM filter and the gate, with inputs selecting ground and the PWM drive voltage:

Switched MOSFET gate - analog switch schematic

Switched MOSFET gate – analog switch schematic

This being a prototype, that involved epoxying a SOT23-6 package atop the MOSFET, with flying wires all over the PCB:

Hall Effect PCB - MOSFET gate switch

Hall Effect PCB – MOSFET gate switch

It works pretty well:

VDS ILED 50 mA div - 200 50 150 25 mA

VDS ILED 50 mA div – 200 50 150 25 mA

The top trace is the drain voltage and the bottom trace is the LED current at 50 mA/div.

The current transitions come out vertical at any sweep speed, even through the Tek Hall effect current probe. The pulses would be rectangular if I weren’t changing the current for each one.

The current feedback loop closes through the Arduino’s main loop, which includes a 1 ms delay after each PWM output change before measuring the LED current. As a result, each loop takes just under 2 ms, but, fortunately, ramping from 25 mA (in the last pulse) to 200 mA (in the first pulse) requires less than 10 PWM increments; you can see the stairsteps if you squint.

Next up: bump the PWM to 64 kHz (from 32 kHz) and rip out the pull-down resistor that used to hold the gate near ground when the output floated as an input. That should improve the output ripple caused by the MOSFET’s bias as a perfectly serviceable linear amplifier.

The control loop now fetches durations & currents from an array-of-structs, so I can customize each pulse. An obvious enhancement: remember the gate drive that produced a given current, then restore the corresponding value as the starting PWM setting for each pulse, so the loop begins closer to reality. The gate drive varies with temperature and suchlike, so it can’t be a compile-time constant, but maybe the startup routine could preload the array / list / cache with values taken from a “lamp test”.

Four close-together flashes repeating at about 2 Hz, even with two runt pulses, turn the LEDs into a real eye magnet…

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