# Issues with driving LEDs with a CC driver

My circuit consists of 16 MP-3030-1100-40-90 connected in series which I'm driving with this CC driver @ Vs = 48V.

I'm setting the current by changing the voltage on the control pin.

I'm having trouble with the LEDs overheating:

@ If = 50mA (1/3 of Itest), they reach a stable temperature of 85°C.
@ If = 100mA (2/3 of Itest), they go >150°C.


The PCB they're mounted on is 1.6mm FR4 with minimal copper coverage.

Questions:

1. Is the overheating caused by a lack of cooling? Are manufacturer's specs measured with in some kind of active cooling setup?
2. It was my understanding that a CC driver keeps the current constant and the dimming happens by changing the voltage supplied to the LEDs. That's not what I'm seeing. Why?
3. My plan for driving the LEDs was to use an accordingly sized CC driver. Since I can't seem to be able to drive these LEDs safely at If > 50mA, how do I prevent them from going over that number in hardware?

Here's my attempt to calculate a current-limiting resistor based on this tutorial:

If = 50 mA @ Vf = 2.53 V (measured)
R = (Vs - V) / If = (48 V - 2.53 V * 16) / 50 mA = 160 Ohm


However, the maximum current I'm seeing after connecting this resistor in series is 31.3 mA. What gives?

• Take the resistor out of the equation, take two wires out of the place of the sense resistor and measure with a digital multi meter. Aug 20, 2019 at 22:27
• Thermal resistance is 11deg typical. How is the soldering pads of the LEDs. Can you share it? It doesn't match Aug 20, 2019 at 22:33
• What is the input voltage? How are you setting the LED current? Aug 20, 2019 at 22:40
• @andrey, a tip: Link to the datasheets, not the catalogue page. It saves an additional click per link. Aug 20, 2019 at 22:55

My circuit consists of 16 MP-3030-1100-40-90 connected in series which I'm driving with this CC driver @ Vs = 48V.

The LED datasheet says Vf can be up to 3.4 V at 150 mA so that's 16 x 3.4 = 54.4 V so your converter hasn't enough voltage headroom.

1. Is the overheating caused by a lack of cooling? Are manufacturer's specs measured with in some kind of active cooling setup?

Yes. At maximum Vf, 3.4 V x 150 mA gives 0.51 W per LED to dissipate. That's easily doable with enough spacing and enough copper.

The PCB they're mounted on is 1.6mm FR4 with minimal copper coverage.

That's your problem. You should design for minimum copper removal. You're paying for the copper anyway.

1. It was my understanding that a CC driver keeps the current constant and the dimming happens by changing the voltage supplied to the LEDs.

Almost. The constant current driver just controls the current. The voltage across the output terminals is just whatever the sum of the LED forward voltages works out to be. A CC (constant current) driver is complimentary to a CV (constant voltage) driver. The CV supplies whatever current is required at that voltage. The CC supplies whatever voltage is required to drive the required current.

That's not what I'm seeing. Why?

It's not clear from your question what you are seeing.

1. My plan for driving the LEDs was to use an accordingly sized CC driver. Since I can't seem to be able to drive these LEDs safely at If > 50mA, how do I prevent them from going over that number in hardware?

Figure 1. Output Current Adjustment by Variable Resistor. Source: LDU08-48.

See the datasheet. You can use a fixed or trimmer potentiometer.

However, the maximum current I'm seeing after connecting this resistor in series is 31.3 mA. What gives?

• Your resistor calculation looks OK.
• Measure the CC output voltage and double check that the voltage is still 48 V on load.
• Use the built-in current control feature instead.
• Short out half of the LEDs to see if the system works with a lower overall voltage requirement.

Is the overheating caused by a lack of cooling? Are manufacturer's specs measured with in some kind of active cooling setup?

Probably, but if you really are only using a current of 0.1A then thats 3V through the LED and approximately 300mW being dissipated. With an 11C/W junction temp, thats only a 3C temperature rise. I think your current measurement is suspect.

You also need a decent heat thermal heat pathway out of the pads, so if you only have a small pad and trace, then there will be nowhere for the heat to go.

It was my understanding that a CC driver keeps the current constant and the dimming happens by changing the voltage supplied to the LEDs. That's not what I'm seeing. Why?

If you measure the voltage across the anode of the first LED and ground, you should see the voltage vary. Constant current drivers keep the current constant, but they do this by varying the voltage.

However, the maximum current I'm seeing after connecting this resistor in series is 31.3 mA. What gives?

Verify the current with a meter in series

• "With an 11C/W junction temp" -- it's not specified, but that has to be junction to pad (probably the cathode pad, since it's biggest). If the board crowds a bunch of them together, that'll just make things worse. Aug 20, 2019 at 23:17
• It was probably tested with a heatsink, 300mW shouldn't send the temp through the roof Aug 21, 2019 at 3:23