Blue LED Premature Failure

Thread Starter

justinvil1103

Joined Apr 6, 2016
41
Hi All,

We are using a blue LED in our design From Rohm and We have been having premature LED failure within a month from the field. I am not sure what's going with the LED driver. I have attached the driver circuit/datasheet for your review and help would be greatly appreciated.

Thanks,
WJ
 

Attachments

panic mode

Joined Oct 10, 2011
2,669
your LED shows typical Vf of 3.3V at 20mA which is max current you should ever allow through it continuously.
assuming your driver circuit is continuously on, you only have 121+10=131 Ohm in series with LED and that is connected to 12V.

so if we take Vf away, resistors will be taking 12V-3.3V=8.7V

and therefore LED current is 8.7V/131 Ohm = 0.0664 Amp or 66.4mA.
that is more than DOUBLE of what is stated as MAXIMUM ALLOWED current (30mA).
no wonder they blow up....

that as just quick and dirty sanity check.
of course OpAmp max voltage and Vbe of the NPN transistor will bring the voltage across resistor a bit down and make things better... but not much.
call it another 0.7V reduction, and you get 8V/131=61mA

also the 10 Ohm resistor will drop a bit more voltage since current through it is for several LEDs
but at any rate, if you wish to make the LEDs survive, you should change the 121 resistor to 330 Ohm
 
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dl324

Joined Mar 30, 2015
16,672
I have attached the driver circuit/datasheet for your review and help would be greatly appreciated.
That's a terrible datasheet. It doesn't show the maximum continuous forward current and you don't give enough information regarding how the LEDs are being driven (continuous, pulsed, duty cycle).

Assuming it's 20mA, you're potentially driving it at a current that's too high. LM324 with a 12V positive rail voltage can probably get to 10V. Dropping 0.7V for the transistor junction, you could have about 50mA. That's well above the 30mA absolute maximum current.
 

MisterBill2

Joined Jan 23, 2018
17,783
The drawing gives no hint as to the number of LEDs in series.It could be one or several. And even if the LED is separated. that could be shown in the drawing, which will be a help to any person servicing the system. Just show the LED part as a separate assembly, but with the interconnection details shown.
 

Irving

Joined Jan 30, 2016
3,793
That's a terrible datasheet. It doesn't show the maximum continuous forward current and you don't give enough information regarding how the LEDs are being driven (continuous, pulsed, duty cycle).
Its on the full datasheet: 30mA which corresponds to Vf of 3.5v, of a Pd of 105mW (120mW abs max on datasheet)

1707565274257.png
Plotting in Excel and taking an 2nd order poly trendline gives:

\[ i = 25.152v^2 - 121.08v + 145.78 \]

And a good LTSpice model for that is:

.model rohm580B d(Ron=.4 Vfwd =2.4 Ilimit=0.1 epsilon=2)

1707574085558.png

Putting that into a simulation where the lower MOSFET is ON (Vgs = 4.5v, Rds = 7ohm) shows the LED will have a high chance of failing if LEDadj > 7.7v for any period of time:

1707574860552.png

Alternately, using the lower MOSFET as a PWM, at 1kHz, duty cycle 70%, LEDadj = 10v, shows a peak current of 47mA and a power dissipation of 128mW - again a failure.

1707575812265.png


While we don't know all of the TS' operating parameters, including the # of LEDs in series (but I'm sure its only 1 from the naming conventions), the above shows the circuit is quite capable of destroying Blue LEDs without difficulty! By comparison the Red and Yellow versions are much more robust.
 

Ron H

Joined Apr 14, 2005
7,056
Its on the full datasheet: 30mA which corresponds to Vf of 3.5v, of a Pd of 105mW (120mW abs max on datasheet)

View attachment 314901
Plotting in Excel and taking an 2nd order poly trendline gives:

\[ i = 25.152v^2 - 121.08v + 145.78 \]

And a good LTSpice model for that is:

.model rohm580B d(Ron=.4 Vfwd =2.4 Ilimit=0.1 epsilon=2)

View attachment 314908

Putting that into a simulation where the lower MOSFET is ON (Vgs = 4.5v, Rds = 7ohm) shows the LED will have a high chance of failing if LEDadj > 7.7v for any period of time:

View attachment 314909

Alternately, using the lower MOSFET as a PWM, at 1kHz, duty cycle 70%, LEDadj = 10v, shows a peak current of 47mA and a power dissipation of 128mW - again a failure.

View attachment 314910


While we don't know all of the TS' operating parameters, including the # of LEDs in series (but I'm sure its only 1 from the naming conventions), the above shows the circuit is quite capable of destroying Blue LEDs without difficulty! By comparison the Red and Yellow versions are much more robust.
Your schematic has an error. The feedback resistor should connect to the cathode of the LED. You have it connected to the anode.
As I reported, but then deleted because I thought I was wrong (but wasn't), the LED current is v(LED_ADJ)/(121+Rds(ON)).
Feeding back from the anode yields an adjustable voltage source with a series limiting resistance. Feeding back from the cathode yields a true voltage-controlled current source, which is largely independent of the voltage drop across the LED (due to temperature, device-to-device variations, etc.).
 
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Irving

Joined Jan 30, 2016
3,793
Your schematic has an error. The feedback resistor should connect to the cathode of the LED. You have it connected to the anode.
As I reported, but then deleted because I thought I was wrong (but wasn't), the LED current is v(LED_ADJ)/(121+Rds(ON)).
Feeding back from the anode yields an adjustable voltage source with a series limiting resistance. Feeding back from the cathode yields a true voltage-controlled current source, which is largely independent of the voltage drop across the LED (due to temperature, device-to-device variations, etc.).
Hi @Ron H, thanks for pointing that out. If anything it makes it worse! In the static case, if LED_adj is >4.1V the LED will over-heat:

1707741996196.png

While the dynamic case is about the same:

1707743041144.png
 

Thread Starter

justinvil1103

Joined Apr 6, 2016
41
Hi All,

We are using a blue LED in our design From Rohm and We have been having premature LED failure within a month from the field. I am not sure what's going with the LED driver. I have attached the driver circuit/datasheet for your review and help would be greatly appreciated.

Thanks,
WJ
Its on the full datasheet: 30mA which corresponds to Vf of 3.5v, of a Pd of 105mW (120mW abs max on datasheet)

View attachment 314901
Plotting in Excel and taking an 2nd order poly trendline gives:

\[ i = 25.152v^2 - 121.08v + 145.78 \]

And a good LTSpice model for that is:

.model rohm580B d(Ron=.4 Vfwd =2.4 Ilimit=0.1 epsilon=2)

View attachment 314908

Putting that into a simulation where the lower MOSFET is ON (Vgs = 4.5v, Rds = 7ohm) shows the LED will have a high chance of failing if LEDadj > 7.7v for any period of time:

View attachment 314909

Alternately, using the lower MOSFET as a PWM, at 1kHz, duty cycle 70%, LEDadj = 10v, shows a peak current of 47mA and a power dissipation of 128mW - again a failure.

View attachment 314910


While we don't know all of the TS' operating parameters, including the # of LEDs in series (but I'm sure its only 1 from the naming conventions), the above shows the circuit is quite capable of destroying Blue LEDs without difficulty! By comparison the Red and Yellow versions are much more robust.
Thank you Irving
 

Thread Starter

justinvil1103

Joined Apr 6, 2016
41
Your schematic has an error. The feedback resistor should connect to the cathode of the LED. You have it connected to the anode.
As I reported, but then deleted because I thought I was wrong (but wasn't), the LED current is v(LED_ADJ)/(121+Rds(ON)).
Feeding back from the anode yields an adjustable voltage source with a series limiting resistance. Feeding back from the cathode yields a true voltage-controlled current source, which is largely independent of the voltage drop across the LED (due to temperature, device-to-device variations, etc.).
Thank you Ron H.
 

Thread Starter

justinvil1103

Joined Apr 6, 2016
41
That's a terrible datasheet. It doesn't show the maximum continuous forward current and you don't give enough information regarding how the LEDs are being driven (continuous, pulsed, duty cycle).

Assuming it's 20mA, you're potentially driving it at a current that's too high. LM324 with a 12V positive rail voltage can probably get to 10V. Dropping 0.7V for the transistor junction, you could have about 50mA. That's well above the 30mA absolute maximum current.
Thank you Dennis
 

MisterBill2

Joined Jan 23, 2018
17,783
Certainly the data sheet does not match the format of most data sheets. And in addition, as already pointed out, important data is missing.
THERE IS A GOOD REASON TO FOLLOW THE COMMON FORMAT, which is because the data is in the right place, and missing data becomes obvious early in the preparation. Also, speaking the same language reduces the chance of translation errors. Many times that is useful.
 

Irving

Joined Jan 30, 2016
3,793
Certainly the data sheet does not match the format of most data sheets. And in addition, as already pointed out, important data is missing.
THERE IS A GOOD REASON TO FOLLOW THE COMMON FORMAT, which is because the data is in the right place, and missing data becomes obvious early in the preparation. Also, speaking the same language reduces the chance of translation errors. Many times that is useful.
Is there a common format for LEDs? In any case, the information is all there, but you have to read the whole data sheet, not just the table the TS posted.
 

tonyStewart

Joined May 8, 2012
127
I used to sell 5 mm LEDs custom made to binned specs for Iv > 16 Cd and Vf =3.05 +/-0.5 in 50k qty per order for 10 yrs.
These Rohm LEDs are unbinned and low quality as such. They also need magnification to 10 deg to achieve 4Cd to 27 Cd max. Mine were 30 deg.

What are your specs? why so many drivers?
Are they ESD protected?
Is there a flyback voltage on turn off > -5V per LED?
How many in the field? Future volume?
These should last >> 10 yrs.
I have fixed many factory problems with their designs and processing issues. Diode protected LEDs were essential due to flyback and ESD handling errors.
If you want my assistance, let me know. e.g. 20 to 30 Cd @ 20 mA 10 deg or 16~24 Cd @25 deg or anything you need.

My guess is that there is an ESD issue if you are using 20 mA with Vctrl = 2.45V regardless of # of LEDs that are in series.

Please confirm your thermal resistance or show photos and Control voltage
 
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Irving

Joined Jan 30, 2016
3,793
Just by way of comparison, here's the same results for the Red LED:

The static case shows the Red LED has a greater than 40% wider operating margin over the blue LED (6v v 3.5v for max current, 7.3v v 4.1v on power dissipation) and dynamically under the same conditions the Red LED is running at 98mW compared to the 128mW for the Blue LED.

1707764518259.png

1707762907805.png
 

tonyStewart

Joined May 8, 2012
127
So far we have a lot of design suggestions but no specs for " 6 LED_ADJ".

So far there is no obvious fault unless above signal exceeds 2.45V for 20 mA and no specs for PWM which cannot should be used to exceed 30 mA.

Did I miss something? ( IF only we knew the purpose of this design ... hint )
 

Irving

Joined Jan 30, 2016
3,793
So far we have a lot of design suggestions but no specs for " 6 LED_ADJ".

So far there is no obvious fault unless above signal exceeds 2.45V for 20 mA and no specs for PWM which cannot should be used to exceed 30 mA.

Did I miss something?
Nope, the TS hasn't stated the expected operating conditions. My simulations merely demonstrate the potential for failure, and show the relative fragility of the Blue LED compared to the Red.
 

Thread Starter

justinvil1103

Joined Apr 6, 2016
41
Hi All,

We are using a blue LED in our design From Rohm and We have been having premature LED failure within a month from the field. I am not sure what's going with the LED driver. I have attached the driver circuit/datasheet for your review and help would be greatly appreciated.

Thanks,
WJ
Notes: New Information: LED Pulsing at 300ns and LED_ADJ ranges (0-5V) signal and Rgs(ON) = 5-6 ohms
 
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