24
\$\begingroup\$

I have designed and printed a 4-layer PCB that accommodates 91 infrared LEDs in a 7x13 rectangular layout. This will be used as a backlight for a machine vision project. I am having a problem where individual LEDs are burning out or perhaps becoming disconnected from the circuit in some way. I suspect the heat dissipation may be the cause of the problem.

Image

Front of array Back of array

PCB Layout

Circuit diagram

Each row of 7 LEDs (green LED text) is wired in series. The 12V supply (VCC powerplane) connects to the first LED. The next 6 are wired in series. Finally a current-limiting resistor (green R text) connects the last LED to the ground plane.

Specifications:

  • VCC plane: 12V, 2A supply
  • LED: TSHG6200. 100mA maximum rated current.
  • Current limiting resistor: 20 ohms
  • Solder: Thermoflow Sn60/PB40
  • Total estimated power dissipation: 12V * 0.1A per row * 13 rows = 15.6W.
  • Size of array: 13 rows of 7 LEDs, approximately 7cm x 6cm

Measurements

With a 12V power supply, there is about 1.45V over each LED, and about 2.0V over the current limiting resistor, meaning a current of 100mA. Because this is right at the maximum allowable current, I put a big high-power potentiometer between the power supply and the VCC plane, and used this to regulate the input voltage to be slightly lower (11.5V or so). This gets the current safely below the maximum allowable amount.

I am also using a Darlington pair to control the backlight with an Arduino. The backlight is on almost all the time, and occasionally is pulsed off for about 30ms. I don't think this is relevant to the problem but can provide more details if necessary.

Problem

After about 10-30 minutes of use, one or more of the rows of LEDs will go out. If I measure the voltage across each LED in the broken row, most LEDs are at about 0.8V and one has about 8.0V across it. No current is flowing. Sometimes resoldering the pins or tapping the LED fixes this. Sometimes it has to be replaced. In any case I only get another 10-30 minutes of use before another one goes out.

Another observation is that the whole back side of the board is kind of sticky. You can see this in the picture above. I wonder if it is getting too hot and the solder is becoming compromised (perhaps exuding flux??).

Question

What should I try to improve the reliability? I've already tried running it at a lower voltage to get the current safely below the rated maximum. I wonder if I need to use a different kind of solder? Or some kind of heat sink? The LEDs get hot to the touch but not unbearably so.

Edit, after trying suggestions

Thanks everyone for the tips! I did something quite simple -- pointed a computer fan to blow air across the array -- and it worked fantastically! I guess this is really obvious to many of you but I was surprised at how enormous the difference was.

Without fan:

  • 25mA per row -> 39C
  • 33mA per row -> 41C
  • 40mA per row -> 48C
  • 55mA per row -> 52C

So we get into the "danger zone" of temperature well before reaching the maximum current per LED.

With fan:

  • 35mA per row -> 26C
  • 60mA per row -> 30C
  • 90mA per row -> 34C

I ran it at 90mA per row and 34C for over an hour with no problems. Great!

\$\endgroup\$
  • 1
    \$\begingroup\$ @Gilad I purchased them from Mouser \$\endgroup\$ – cxrodgers Apr 7 '16 at 19:34
  • 2
    \$\begingroup\$ its a PCB with an alu alloy instead of fiber-glass, for heat intensive applications: electropages.com/wp-content/uploads/article_images/large/… \$\endgroup\$ – Wesley Lee Apr 7 '16 at 20:16
  • 2
    \$\begingroup\$ As others have said, the main problem is undoubtedly thermal dissipation. One additional issue is that the pads look a bit small. They might be fine for flow soldering, but for hand soldered stuff a larger pad size would help, and would probably also improve thermal dissipation. You have plenty of space for bigger pads- use it. \$\endgroup\$ – JavaLatte Apr 7 '16 at 20:35
  • 2
    \$\begingroup\$ May I suggest that you'd probably be much better off using something like this: aliexpress.com/item/… \$\endgroup\$ – Jules Apr 8 '16 at 14:27
  • 2
    \$\begingroup\$ @user6030: There is one resistor per series chain of LEDs as is standard practice. Each LED in the chain will have the same current. \$\endgroup\$ – Transistor Apr 9 '16 at 13:04
22
\$\begingroup\$

You have already hit on the answer: your LEDs are getting hot. 15 watts may not sound like much, but it's building up and killing your LEDs. I suggest you get a thermistor and attach it to the center of the board, then monitor the temperature as the system operates. Even better, attach it to the body of one of the LEDs.

Because you're using this as a backlight, don't use narrow-beam LEDs. Use relatively wide beam units, and space them apart so air can flow through. If you can find a source of, let's say, 35 degree LEDs, install only every other one in a checkerboard pattern, soldering jumper as necessary. You'll only get half the total brightness, but that's barely perceptible, and the improved airflow should be a big help. You may also need to provide a fan with some ducting to keep the air flow through the array adequate.

And always include a temperature monitor. While not directly applicable, this YouTube video shows the principles of cooling. In your case, since you've got a forest of vertically standing LEDs, it is important not to let the LEDs touch each other, since this will block the flow of air.

\$\endgroup\$
  • \$\begingroup\$ I will try to install a thermistor as you suggest. I wonder if a fan or a heatsink would be easier for me to get working. I'll check out the video. And point taken about the greater spacing. \$\endgroup\$ – cxrodgers Apr 7 '16 at 19:39
  • 3
    \$\begingroup\$ For what you're doing, heatsinks just don't work. There's no way to attach them. SMD LEDs can be made to work, though. \$\endgroup\$ – WhatRoughBeast Apr 7 '16 at 20:08
  • 1
    \$\begingroup\$ Get it working again, and point a good-powered fan at it. See what that does to the operating time. I concur, 15W doesn't sound like much, but that's enough to get the tip of a soldering iron to 650°F in 5 minutes. \$\endgroup\$ – rdtsc Apr 7 '16 at 23:19
  • 1
    \$\begingroup\$ Problem is, if it's a backlight, it will probably be sandwiched behind a display of some sort, so only cross ventilation will work. Airflow will be much less than if you're just blowing a fan across the array. \$\endgroup\$ – WhatRoughBeast Apr 7 '16 at 23:26
29
\$\begingroup\$

From the datasheet:

enter image description here

Figure 1. Absolute maximum forward current.

and further on:

enter image description here

Figure 2. Derating LED current at increased temperature.

Current and temperature are your problems. You are running at absolute max current with no wriggle-room and you are allowing the temperature to rise. At 60° ambient the max current allowed falls off dramatically.

\$\endgroup\$
  • 1
    \$\begingroup\$ I accepted the other answer because of some of the additional info in it but I just wanted to say this graph was really helpful for me to understand the problem \$\endgroup\$ – cxrodgers Apr 8 '16 at 1:10
  • 2
    \$\begingroup\$ It is worth mentioning that the graph does not say once the LED has reached 60 degrees C, the current must be derated. It's not. It is saying if the ambient temperature of the environment the LED is shedding heat into is 60 degrees, it must be derated. Now, this is more or less the case as each LED is surrounded by a bunch of other hot LEDs, which will do a great job of simulating a hot ambient environment. It's a distinction worth making, however. \$\endgroup\$ – metacollin May 8 '16 at 13:00
  • \$\begingroup\$ Good point. I've added the word "ambient" into the last sentence to clarify. \$\endgroup\$ – Transistor May 8 '16 at 13:37
1
\$\begingroup\$

A quick google for "LED heatsink" will find you a lot of suitable solutions for cooling down your burning-out LEDs. My initial recommendation though would be to turn the top of the PCB into a heatsink layer and use thermal adhesive to stick the LEDs to it. I'm afraid that's more soldering, but hey ho.

For more info, I've found that Wikipedia has a link to various LED cooling tech which may be of interest.

\$\endgroup\$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.