Pulse-powered current limiter

Piotr Wyderski · 2022-02-18T21:29:11+00:00

Hi,I am experimenting with white LED COBs and wanted to power them from a constant current source while still having the PWM dimming capability.The current limiter should be located next to the diode and the lamp "interface" should be regular two wires. In other words, the current limiter should be capable of being powered from a +12V/open drain source with the PWM frequency of ~400Hz.I started my experiments with a boring two-transistor series MOSFET/BJT current limiter. The voltage at the MOSFET source turns on a BJT that steals the MOSFET gate voltage, stabilizing the drain current. It works, but the V_BE tempco makes the current drift way beyond my comfort zone.OTOH, an opamp-based current limiter + a decent reference would keep the current rock-solid, but it can go nuts during the PWM edge transients.Any suggestion on how to make a limiter capable of being PWM-powered and maintaining stable current during the ON phase down to, say, 3% accuracy over the 11-15V input voltage range and 25-80 degrees C temperature would be appreciated. The nominal LED voltage is 9V and the required current is ~500mA. Are there any particularly forgiving opamps? Best regards, Piotr

Feb 18, 2022 Last reply: 4 years ago 23 Replies

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Jasen Betts 4 years ago

Putting two high impedances in series is asking for trouble.

perhaps shunt the LED current instead of cutting the power to the current regulator, or arrange the current limiter to tune its limit according to a PWM input

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Piotr Wyderski 4 years ago

The trouble can be caused only by energy stored in stray inductance or an EMP from a nearby lightning. TVSes on both ends will handle both cases nicely.

Best regards, Piotr

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Piotr Wyderski 4 years ago

Fine is an understatement. I didn't expect this sort of performance from a couple of dirt cheap components. It works perfectly well even at 10V and there is no (reasonable) upper limit - just the regular cooling constraints apply. Selected for implementation. :)

Thank you all for your help.

Best regards, Piotr

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piglet 4 years ago

You are very welcome, I enjoyed it and it is good to have on-topic posts to read!

The 431 packs a lot of gain into a dirt cheap part, folk even use them as op-amps with a well defined input offset voltage!

piglet

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