Measuring junction temperature of LED tape?

Question

My boss at work wants me to figure out a method of measuring the junction temperature of an LED that is part of a reel of tape. Unfortunately, there is no datasheet for the LED modules themselves, just a datasheet for the tape, which is lacking in the information I imagined was essential (junction to solder/ambient thermal resistance etc.).

I was wondering if there is a method, using a thermocouple and a multi-meter, that I can measure the junction temperature of the LED?

Cheers for taking the time to answer.

Answer 1

The standard method to measure temperature using a diode, is to measure the voltage at two (or three) different currents. If you use the LED, then you are measuring the junction temperature.

For example at currents of 50mA and 5mA. The difference in the diode voltage at these two currents is a fundamental function of the diode curve, and the ratio of currents (e,g. 10x), and is quite interchangeable between devices of the same type. (whereas the simple voltage-temperature curve is not).

The trick with LED's is switching the currents quickly from the running current (which causes heating) to the measurement currents, fast enough that junction temperature is unchanged. You can use capacitors to sample and hold the voltage difference quickly, and measure this more slowly with an ADC.

^{simulate this circuit – Schematic created using CircuitLab}

From memory, the system cycled RunCurrent(1ms) I1(20us) RunCurrent(1ms) I2(20uS). SW1,2 are analog switches or sample and hold amplifiers. The fet gate drive invertors are running from 12V. The measurement currents are switched into D1,2 when not used. This allows continuous current flow in the constant current sources, so giving fast accurate currents compared to turning the constant current generators on and off.

Calibration can be done with runcurrent=0, so you measure temp of water bath or hot air.

The schematic was for a single led measuring arrangement. If I was doing it for a led strip, and needing to individually measure all leds, then the current switches would be arranged to control the whole led string, and the led voltage sampling would be separate and isolated, so you can probe across the individual leds.

TonyEE can probably comment on the usable current range where modern GAN leds behave as diodes. I have only used this for older types of heterojunction leds.

Extending this to using 3 currents eliminates resistances, but LEDs are known to behave peculiarly at low currents, so this might not work.

Some leds have a reverse protection diode, and this could be used as a package temperature sensor instead.

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A variation on this principle would be to vary the run current by a small amount, e.g. 10% i.e. if LEDs are normally running at 200mA, then switching them 190-210mA @ 50% duty cycle. Now the change in voltage across the leds is very small, but it is a simple AC signal that can be measured with a good meter or amplified easily. The drawback of this method, is that calibration will be difficult, as it is being done at full led current.

Answer 2

I created a simple test process that I suspect is pretty common to some.

All you need is a calibrated forced air oven, a pulsed transistor switch, a resistor and a DSO scope.

• Measure the strip Vf vs Temp with a current limiting resistor.
• Chose a value to reduce the current to < 1% such that self-heating is negligible

• the NTC thermal coefficient is recorded by the oven temperature (‘C)

• these results are compared during pulsed off to 1% current at room temp.

• the temperature is matched with the results from the oven test

Details to follow later....if there are any questions.

All diodes can produce a linear drop in x mV / deg C, White LEDs are ~-3mV+-1 /‘C . The Rja is about 0.2’C/mW rise with power dissipation so using about 2mA @2.85 or 5mW causes only 1 deg rise in the oven which is acceptable. Measuring 3 LEDs in series on 1 string is also acceptable. From 12V you can measure the current of 2ma by a series drop R to the reel multiplied by the number of strings so if normally 2A from 12V for 5m reel you can easily choose R added to the ground side. Then use any power Nfet to shunt the R for normal use and pulse off to measure the V+ - V(R) = Vf *3 on the oven for say temperatures 60, 90, 120’C and curve fit on a spreadsheet with formula displayed.

The y=aT+b gives “a” as your Tempco coefficient (x3)

Now you can compute any junction Temp rise with voltage measured.

The thermal time constant depends if you have a metal substrate (heat sink) in your application but will be around 1 second so turn off pulse voltage after inductive transients will decay so after this microsecond interval.

Measure the voltage within 1 millisecond to capture a reading before temp decays or a duty cycle of 1% about 1k to 10kHz. (10ms to 1 ms off time)

Then repeat results for T rise vs Voltage to 14.2V which most stripleds can handle since there are current limiting R’s on tape.

Any questions?

If supply voltage is not chosen yet and you want to measure Tjcn at different Supply voltages from 12 to 14.2V then consider that the I minimum using a fixed current limiter will change. So a Constant current sink is better. Here I chose a JFET with IDSS<2mA.

For convenience, I displayed only one LED voltage, current and power so you can see max/min on scope. ( autorange)

Your values may differ at room temp for your selected stripled reel. You can scale everything for 5m or cut-off 1 string to achieve the minimum current for the diode low-self-power Forward voltage vs Temp.

The sine modulated supply would be done manually but here shows the sensitivity to I max and almost none to I min, whereas a fixed R of 1k or so would change with V+.

It is important that this minimum current be constant thruout your tests from oven calibration with MOSFET OFF and constant 1~2mA current to calibrate Vf/ vs. Temp.

Then in ambient temp with MOSFET pulse generator at 99% or off 1% of the time.

Now you have a test jig that can be used for any LEDs.