Structural features and applications of high-brightness LEDs
Although LEDs are current devices—high-brightness LEDs are no exception, applications such as automotive taillights, brakes, and turn signal lighting can still benefit from voltage driver structures.
When more efficient LEDs are available, LED indoor lighting in retail stores and homes will likely appear soon. LED manufacturers are just beginning to solve the problem of high color temperature light sources.
Due to the progress of high-brightness LED manufacturing process, device design, and assembly technology, the performance of LED light emitters has been improving, its cost has been decreasing, and the speed of performance improvement and cost reduction is unforgettable. The PN junction design, re-radiation phosphor and lens structure all help to increase the efficiency and therefore also the available light output (attachment "LED in the laboratory"). For high-output white LEDs, the improvement in broad-spectrum performance gives hope for low-maintenance, high-efficiency light sources for general lighting.
Although it will take some time to achieve LED efficiencies comparable to standard fluorescent lamps, as stated by Yung S Liu, Chairman of Semiconductor LighTIng Industry AssociaTIon: "LED lamps are also more environmentally friendly products because they are compatible with fluorescent lamps. The lights are different and no mercury is used. "
The environmental advantages of solid-state lighting in terms of composition and working efficiency are not currently the main market drivers, but they do give this technology and its suppliers a good image.
At the same time, OEM designers and salespeople working in various fields have been expanding the practical application of solid-state lighting, and have been closely watching the market acceptance. However, the cost-benefit experience of end users in the life of solid lighting equipment varies, which is very different from traditional lighting equipment. This fact complicates market values. Compared with tungsten filament bulbs and fluorescent bulbs, the use and maintenance costs of high-brightness LEDs are much lower, which can offset the higher initial cost of LEDs. Although the above discussion may be very attractive, it makes it difficult to promote sales in the consumer market where the idea of ​​"price first, other second" tends to dominate.
Lamp fixture manufacturers have historically not considered the thermal management of the bulb in their respective designs, but only provided sufficient convection to ensure that the high operating temperature of the tungsten filament lamp does not pose a risk of fire from surrounding materials or burns by fixture operators. This fact complicates the mass production of high-output solid-state lighting equipment. However, if the final design is to optimize the light output and working life of the LED, the fixture of the high-brightness LED requires a certain thermal design.
Therefore, although high-brightness LEDs will not soon be seen squeezing out traditional tungsten or fluorescent lights from shelves in hardware stores and home centers, these devices are entering the market segments such as automobiles, traffic control, and external signs. Because in all these areas, the high efficiency and long life of the lamp will add obvious value.
High-brightness LEDs are striking. In fact, few people hear the phrase "early adopter" and "automotive market segment" in the same sentence. Some people may assert that this kind of juxtaposition will prevail in contradiction modification. However, high-brightness LEDs have brought several compelling features to car manufacturers, and, although this application is relatively new, most of their basic characteristics are derived from the manufacture of LED indicators--much older than them and already The same principles and similar processes that have been well proven for similar products.
LED car taillights, turn signal lights, work lights, brake lights can overcome several shortcomings inherent in tungsten filament incandescent lamps. The moderate shocks and vibrations often encountered by automobiles will shorten the filament life. Similarly, the instantaneous surge current caused by the positive temperature coefficient of the filament resistance will accelerate the destruction of the bulb. Thermal loop-an important feature of brake light operation, often shortens the life of incandescent lamps.
The instantaneous surge current of the incandescent bulb also complicates the tasks of circuit protection and fault detection. Automobile manufacturers must set the fuse rating and fault detection threshold to a sufficiently large current value to adapt to the magnitude and duration of the inrush current without the occurrence of a blown fuse fault or the detection of false faults.
In contrast, LEDs are more durable than filaments when the car is subjected to shock and vibration in the typical amplitude and frequency range. The LGD structure is light in weight and small in size, which can reduce the mechanical torque caused by shock and vibration. The small size of LEDs also allows automotive designers to design the lighting lamps to be smaller and design them to better meet the overall design requirements of the car. For example, some cars do not install the CHMSL (middle high brake light) module on the rear cover, but use the small size of the LED to include this function in the trunk cover (Figure 1).
The automotive taillight lighting and control system raises several interesting issues, and these problems also appear in other systems where the control device and the controlled device are far away from each other. LEDs are essentially current devices. The electron-hole pairs recombine within the electroluminescent compound and emit photons when they recombine. The increase in current will increase the composite speed and luminous flux output accordingly. The efficiency of this process is not 100% (almost less than 100%), so the increase in current will also increase the self-heating of the device through 1-h power consumption. Unless working conditions are poor, LEDs generally do not experience catastrophic failures like tungsten filament lamps, but they tend to dim due to aging. Many device designers define the end-of-life of an LED as the time it takes for the light output to fall to 50% of its initial value.
Overcurrent and overheating conditions will accelerate the end of LED life, so most device manufacturers recommend that OEMs carefully control LED energy.
These characteristics imply that in order to achieve the 11-year life expectancy of LEDs in automotive CHMSL or taillight assemblies, automotive body control modules should operate each device with a constant current. However, as Bill Reidel, an automotive market expert at Analog Devices, said, the constant current design complicates the wiring between the car body control module and the light element, and drives designers to remove the power control IC from the car body control module. Put it in the lamp housing. The constant voltage drive can keep the control IC in the control module that needs the control IC fault detection status information, and can reduce the number of external components (ie fuses) and the number of wiring between the control module and the lamp housing in the same design.
Keith Wolford, an automotive application engineer at Texas Instruments, agrees: "One of the functions of an LED control IC is the function of a fuse. If you place the LED driver in the lamp housing, you have to send electricity to that location and give it to the LED driver. Fuse ... And if you have a central lighting module, all you have to do is fuse the power feeder connected to the module. With the diagnostic function of the LED driver, if the wire connected to a lamp housing is shorted, you It can be protected with electronic equipment without having to install a fuse for each lamp housing wiring. "
Analog Devices' AD8240 LED driver / monitor is a concrete manifestation of this approach. The device's operating current is 300mA, and the supply voltage range is 9V ~ 27V. The PWM input controls the brightness of the lamp, thereby achieving different minimum brightness levels during the day and night that comply with the automotive regulations. The overcurrent detection circuit consists of an external high-voltage side shunt resistor and an on-chip comparator. If the voltage drop across the shunt resistor exceeds the reference voltage (typically 5V), the overcurrent detection circuit locks the output drive signal. The latch is reset after every PWM cycle.
Shunt resistors and external PNP transmission elements limit the maximum load current. The manufacturer's recommended 0.1Ω ~ 0.5Ω shunt resistance range corresponds to a maximum load current of 2A ~ 0.4A. The microcontroller of the control module can read the reading of the IC detection pin through an ADC input channel to monitor the load current. The AD8240, priced at $ 1.15 (1,000 pieces in batches), can detect open-circuit loads, short circuits, and local faults, such as a short circuit in one LED in a series of LEDs. This driver / monitor IC is packaged in MSOP-8.
In designs that require a low-side controller, designers can consider using Melexis ’MLX10801 because the MLX 10801 is packaged in an SO-8 and can absorb 550 mA absolute maximum peak current and 400 mA absolute maximum without an external transmission device Average current. A package option with a trailing code A is packaged in MLPD-8 with thermal pads, and the bare die used are unchanged, thereby reducing RΘJA from 120K / W to 37K / W. This package improvement can increase the absolute maximum peak current and absolute maximum average current to 1.2A and 750mA, respectively.
A diagnostic pin enables the local microcontroller to monitor the load current through an ADC channel. Those designs with more driver / monitor chips than ADC channels can sum the ground current and monitor the total ground current with an analog input pin (Figure 2).
Melexis' MLX10801 is characterized by a set of transient pulses, 40V load dump, and abnormally induced undervoltage conditions, which are all expected non-standard operating conditions that the device must withstand. A programmable non-volatile data latch enables OEMs to perform temperature measurements through an on-chip detection diode or an external detection diode. A control input pin can realize PWM dimming, which is a common feature of LED drivers. Keeping this control input pin low for more than 32 milliseconds will force the driver into sleep mode, reducing its quiescent current from 2mA to 105mA. Keep this control input pin high for 8mS to start a wake-up sequence that lasts only 300mS.
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