connecting LEDs in series and parallel + creating a characteristic curve

[stepper]

 

[Tony Stewart]

In my experience, using better matched LED's e.g. +/- 0.1V in parallel will not result in thermal runaway, whereas std tolerances +/- 0.3 or more may cause failures.
But when a series string of identical LEDs with the same std tolerances tend to have lower deviation within a batch, you can safely parallel series strings without having to add bulk resistors to dominate the internal bulk resistance in the LED. My experience showed me that the number N LEDs in series needed to avoid adding series R has reduced in time as LED process controls have improved the quality over the last 4 decades. At one time N=1 meaning every single LED needed at least some external Rs to avoid current hogging or current sharing and thermal runaway issues. Today it is possible to have tolerances within a batch low enough to do this without adding Rs if you know what the tolerances are. I think there is a TI technical bulletin which supports this characteristic. Also for thermal assistance, https://www.wikiwand.com/en/Thermal_management_of_high-power_LEDs

This critical thermal runaway threshold margin drops as you increase the current to maximum so you need to understand the thermal resistance- tempco -4mV/'C and power dissipation to determine when you need to add Rs to a series string for shunting in parallel.

Typically I look at the rated power dissipation of the device (LED or Diode or Vce(sat))*Ic or case package and use Rs=k/Pmax for k= 0.25 to 1 then add a bulk Rs in series that is >= to this value in series as the minimum to prevent thermal runaway at rated current and slightly more to share current equally within low %. This is why you see ganged audio drivers with 0.1 ohm more or less on emitters of high power audiuo amps using ganged transistors (N=1) .

A power 3V nom white LED will have an internal Rs of 0.25 to 1/ Pd max resistance.

eg. 1W LED 1 ohm max, 10W LED 0.1 Ohm max @ 10W as the incremental resistance.

Thus to make an array of say 100W from 12.5V using 10W chips you need to know the junction rise of each chip. This is why traffic street lights in early designs often failed with one or more shorted LEDs. The designer failed to recognize LEDs near the centre were hotter and thus lower Vf and would start to hog more current and fail in parallel.

How many street lights have you seen with burnout chips in RYG stop lights? they do not all fail from thermal runaway, but it is a common root cause.

2W White LED Ri = 0.5 Ohm, some better quality ones are 0.25 ohm.

 

[Tony Stewart]

Arhennuis Law predicts the MTBF from overcurrent from heat rise.
Typically 1/2 MTBF for every 10'C rise of junction from 50khr.
So a 100'C rise of junction is 1/ (2^10) lower MTBF at 125'C is often used for relative MTBF.

You can also use any diode or LED to measure its junction temperature by pulsing off current and measure Vf rise from room temp. Themal matching is essential to reduce thermal runaway when parallel diodes are used then Ri or Rs or ESR internal incremental resistance determines the heat loss variations since they share a match theoretical threshold Vt at low current. (1% of rated)

 

[stepper]

Hi Tony, thanks for the interesting information there. I think it's currently going a little over my head but I realize that what you say about thermal management of LEDs is important. That reminds me of something curious I observed only recently when removing some small status LEDs from step-up converters to save some current for output. When I desoldered them they emitted visible amounts of light. Seems like the temperature difference between the junctions also caused a difference in potential.

 

[Tony Stewart]

. When I desoldered them they emitted visible amounts of light.

if you used a soldering gun (?) the transformer reduces line voltage to create high current and this voltage is enough to stimulate Vf and illuminate with a common mode voltage and some stray capacitance to create a differential voltage.

or if you used an ungrounded iron tip, the stray e-field is enough to induce xxx uA of current. (Although ESD protected irons only have 1Meg to PE gnd)

The same is true for unpowered strings of LEDs outside can pickup grid EMI and induce < 1mA to dim LEDs. by induction of E-fields.

i.e. it is not a Peltier or thermal effect.

 

[stepper]

or if you used an ungrounded iron tip, the stray e-field is enough to induce xxx uA of current. (Although ESD protected irons only have 1Meg to PE gnd)

probably so, it's a HANDSKIT Digital Soldering Iron.

i.e. it is not a Peltier or thermal effect.

that was what I was wondering about. Thanks for the explanation.