an innovation by nucon.de
LEDs have a much longer life time than other light sources, when keeping their temperature and their current below certain limits. Reduction of brightness or a color shift is very small when LEDs are driven for maximum life time but it is there.
When having a look on an LED installation of cheap flood lights in a riders hall, where 50% of the devices are out of service after a few years of operationial time only, you can see that the remaining devices have a different color temperature to each other.
The white LEDs are blue LEDs having a yellow phosphor converter layer on it, which is changing its transmission and converting performance over the life time. There is always a slight blue shift followed by a yellow back shift. If in case there is lot of heat impact on LED devices the blue shift in the beginning is very small followed by a strong yellow shift.
If in case there is a strong yellow shift prersent and the life time is below the promissed operational time it is a strong indication, that the LEDs had been operated too hot.
The cause of color shift due to aging was reported by Davis, Mills, Yaga, Johnson.
On car head lights a shift of the color temperature is a very unwanted behavior especially if you can nor replace LEDs of such reflectors to obtain a symmetrical again. There are cases reported that one side of the head light was replaced due to a crash and the rplaced head light does not match the color of the old one. Low beam and matric LED hign beam does not match anymore to each other , see the following link: low beam yellow high beam white!
If in case your LED head light shows a color shift to yellow and the involved unit does not offer a service of the installed LEDs insight such head lamp, the conclusion must be that the car manufacturer has to replace such a unit for free no matter if a warranty has already timed out or not since such system was designed and offered to last a car life time. Such head lights costs so much money in the case of a replacement, that a customer can expect, that it was designed to avoid color shifts. The question is, how this goal can be achieved if head light manufacturers design the life time of such head lights only to last 8000 hours. Is a too hot operation not already tolerated in such designs to provoke color shift of a number of LEDs?
What are the system advantages LED-Temperature-Protection L-T-P in comparison with NTC sensor:
There are more computer operations with L-P-T involved to obtain the system benefits. In these modern times of embedded micro controllers this is not a problem anymore. Even with small microcontrollers of the 8 bit class the method can be used, depending of how many system features the software shall perform.
The L-T-P method can reduce by software costs and component space while increasing the reliability.
The reliability is much better, because there is no external influence possible to cheat the system by it's direct analyse. The NTC sensor itself can be easily displaced or changed in value to cheat the system. The NTC sensor can be changed in value by dirt or water and there is no way to decide if the change of value is by temperature or other reasons.
Use your available resources of software power to save money and parts - even more parts can be saved by embedded processors they may even replace analogue ICs. The more know how you have inside your software the less easy is to copy your product.
Boost Buck driver for automotive front light modules
This is an automotive headlight LED schematic design example using SPI bus controlled NXP ASL4500SHN and ASL3416SHNY LED buck driver chips. The hysteretic buck DC-to-DC topology of the buck regulator grants LED drive currents with low ripple noise and high accuracy of the current itself. The main software development work was done on NXP demo board and can be ordered over standard distributors: ASL45XASLX41
the board with the attached FRDMKEAZ128Q80 based on CortexM0+
on the left there is a 4 channel matrix driver board:
This board demonstrates that matrix driving can be combined with sensorless TJ LED measurements.
Voltage and current of a LED string.
A special phase shifting PWM pattern lets you analyse the specific LED channel, when it is active for a short moment only.
The micro controller is able to determine the temperature of the LED, dimming the LED and even to determine the type of LED with a view pulses before turning the LED to full brightness. The PIC16F1765 is able to generate a PWM and combine it in a logic cell with the signal from the comparator op amp section, which is sensing the shunt. Such a combination of digital with analogue features is ideal to change drive current and PWM to drive all kinds of LEDs with the same hardware.
LED Temperature Protection shall keep the LED alive when there is high temperature and you may exceed the junction temperature.
So if you power an LED which has no cooling active - for example if it was forgotten to mount it on a cooling block or the thermal contact was removed, like it is shown below - it will soon overheat under high power and the LED will be damaged.
Automotive OSRAM LED with a mounted reference K-thermal sensor on top right.
This LED was operated with no cooling mounted.
But not if you have our method integrated into your design as the following chart is proving:
LED temperature protection regulates back the LED at about 153°C junction temperature.
The temperature protection is effective even if an LED is connected, which was not callibrated with an external reference:
There are - due to manufacturing tolerences of the LED chips and possible aging effects - different forward voltage classes within a LED series present.
If such different LEDs are connected to the system there are due to the voltage class different temperature offsets present!
The problem is solved by analysing the relative rate of the voltage decrease at the measruement point. This value is constant and not changed at different temperatures.
So the method can sense at any time what type of LED is currently in operation and adopt automatically the calculation parameters on it.
This clears the way for automotive and consumer applications to hook "unknown" LEDs to a driver and use them as a sensor.
The benefit for the OEM is the cost reduction inside automotive head light reflectors of about 25 cents for each NTC channel.
Depending on the platform the OEM can safe up to 2$ on each car or even more including tail lights !
The end customer gets a more reliable system.
Our main focus is to sell our method by a license model that a customer can integrate it under it's quality requirements in his own designs.
The method works in combination with NXP automotive LED drivers - this is recommended not mandatory. A demo board is orderable for instant at Mousers: ASL45XASLX41
Another option is that Nucon GbR is helping you by designing parts of a software or complete LED driver power stage using the thermal sensor-less method.
You can also buy from us a PIC16f1765 single buck driver demo board
This regulator is a current driven switcher at about 300kHz using the comparator and op amp out of the applied micro controller, for instance set to 1A drive current.
It can be used in standalone applications or be used connected to a serial terminal. In the portable application on the right side it is powered by 8 AAA cells to provide you a superior spot with a long running time included.