Electronic – LED current source circuit design concerns

Assumption: LED Driver used is PDA012A-350S-R, from link provided in question. Assumption: A single LED is being driven, based on "flashing the LED about 3 times ..."

The constant current driver is going into overload, due to being forced to dissipate power far over its design limits...

From the datasheet of the LED, it has a forward voltage of 3.2 Volts. The LED driver has a minimum DC output voltage of 17 Volts. When it starts powering the LED, the 350 mA current limit is reached with the LED dissipating just about 3.2 Volts. Thus, the LED driver needs to dissipate (17 - 3.2) x 0.35 = 4.83 Watts at a minimum, to supply the 350 mA design current.

The LED driver is likely to enter self-protection reset loops, either due to this dissipation causing an overheat, or due to short-circuit protection kicking in - since the output will not see the designed voltage across it when nominal current is supplied.

Solution: Use the constant current driver with a string of between 6 and 10 of the selected LEDs, in series. This will provide the required forward voltage range, between 17 and 34 Volts.

If you assume that the junctions are the same (patently false, and this dubious assumption must be stated), then the current through R2 ~= current through R1 since \$\beta>>1\$ . You know the voltage at the base of the transistor within a reasonable margin, so you can calculate the current through R1. The value of R2 is not important unless you take 2nd-order effects such as the Early voltage into account. To see how far off the initial assumption is in reality, let's simulate it with a 100K resistor:

^{simulate this circuit – Schematic created using CircuitLab}

The current through R1 is 145uA and the current through R2 is 360uA according to the simulation. So the error is about 2:1 and there's really no point bothering about the 0.3% or whatever due to hFE.

This error originates from the difference in saturation currents of the two junctions and also the ideality factor of the diode junction, which is 1.45 in the above simulation. If I replace the diode with a diode-connected 2N2222, the currents will be much closer:

^{simulate this circuit}

Now the currents are 145uA and 169ua so the error is only about 15%, but hFE is still an effect of little significance. The error at this point is mostly due to the Early voltage.

Note that you have to use a reasonably accurate transistor model to get reasonably accurate results. In this case Circuitlab (and also the free LTSpice) use good models (Ebers-Moll) with good default parameters and give consistent results showing about 15-20% error.