Recovery of the phototransistor after big IR pulse

photodiode saturation recovery

Colleagues,

I’m working on a small device, whose purpose is to produce 8W IR pulses with IR LEDs. Each pulse is 100us long. The pulses are produced only when the device is in proximity of the target. The proximity is established by the IR proximity sensors* (SFH9201 made by Osram). The proximity sensor consists of a small IR LED, a phototransistor and a 20k pull-up connected to the collector. Part of the 8W IR reflects into the phototransistor and saturates it. The problem is that it takes the transistor more than 1ms to come out of saturation, after the 8W IR is turned off, and I can’t make proximity detection during this time. To deal with this problem, I’ve connected the pull-up to the digital output of the PIC and drove the pull-up low, when 8W IR was on, then drove it high to make a proximity detection. The phototransistor was still saturated for 1ms. Why does it take so long to recover? Why does it saturate when the collector is pulled to the ground?

I can upload the schematics and waveform, if anyone wants to take a look.

Thanks,
- Nick

* This part of the design was inherited. If the device will be redesigned, it will probably get a capacitive proximity sensor.

phototransistor pulsed

Well I'm not a engineer, but maybe try reduces the value of the 20K collector resistor, for example 1K looking good. My suspicion is that the load capacitors is charge by a this 20K collector resistor and that maybe cause the problem.

phototransistor pullup value

I dont know the schematic, but high saturation recovery time for bjt is connected with charges in the base which does not have path to flow out. You can see how this problem has been solved for TTL logic(one switch from base to ground which should work for high currents). Also you could use some agc that turns on only for high input currents.

phototransistor with agc

A phototransistor with unconnected base (as in SFH9201) has limited options to recover rapidly from overload. As you already found out, the saturation also occurs without an external voltage during the light pulse.

The only option is to switch a low pullup-resistance temporarily, but it may be still insufficient to achieve an acceptable recovery behaviour. In this case, you would have to use a different sensor, e. g. with a photodiode.