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Structural features and applications of high-brightness LEDs

Although LEDs are current devices - high-brightness LEDs are no exception, but automotive taillights, brakes, turn signal lighting and other applications can still benefit from the voltage driver structure.

LED interior lighting in retail stores and homes is likely to emerge soon when more efficient LEDs are available. LED manufacturers are just beginning to address the issue of high color temperature sources.

Due to advances in high-brightness LED manufacturing processes, device design, and assembly technology, the performance of LED illuminators has been increasing, the cost has been decreasing, and the speed of performance improvement and cost reduction are unforgettable. The PN junction design, re-radiation phosphor, and lens structure all contribute to efficiency and therefore help to increase the available light output (Attachment "LEDs in the Laboratory"). For high-output white LEDs, the wide spectral performance improvement promises a low-maintenance, high-efficiency light source for general lighting.

Although it will take some time to achieve LED efficiency comparable to standard fluorescent lamps, as the chairman of the Semiconductor Lighting Industry Association, Yung S Liu said: "LED lights are also environmentally friendly because they are not the same as fluorescent lamps, not used. mercury."

The environmental advantages of solid-state lighting in terms of composition and work 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's acceptance. However, end users experience different cost benefits over the life of solid lighting equipment, which is quite different from traditional lighting equipment. This fact complicates the values of the market. Compared to tungsten and fluorescent bulbs, high-brightness LEDs have much lower cost of ownership and maintenance, which offsets the higher initial cost of LEDs. Although the above discussion may be very attractive, it makes it difficult to sell in the consumer market dominated by the "price first, other second" thinking.

Lamp fixture manufacturers have traditionally not considered the thermal management of the bulb in their respective designs, but provided sufficient convection to ensure that the high operating temperature of the tungsten filament does not pose a fire hazard to surrounding materials or a burn hazard to the fixture operator. This fact complicates mass production of high output solid state lighting equipment. However, if the final design is to optimize the light output and operating life of the LED, the fixture of the high-brightness LED requires a certain thermal design.

Therefore, although high-brightness LEDs will not be quickly seen to extrude traditional tungsten or fluorescent lamps from hardware stores and home center shelves, these devices are entering the market segment of automobiles, traffic control, and external signs. Because in all of these areas, the high efficiency and long life of the lamp will add significant value.

High-brightness LED

In fact, it is rare to hear people use the phrases "early adopter" and "auto market segment" in the same sentence. Some people may assert that this juxtaposition will prevail in contradictory modification. However, high-brightness LEDs have brought several compelling features to automakers, and although they are relatively new, their basic features are mostly derived from the manufacture of LED indicators - much older than they are The same principles and similar processes have been obtained for similar products that are well proven.

LED car taillights, turn signals, work lights, brake lights can overcome the inherent shortcomings of tungsten incandescent lamps. Moderate shocks and vibrations that are often experienced by cars can shorten filament life. Similarly, an instantaneous surge current caused by the positive temperature coefficient of the filament resistor accelerates the destruction of the bulb. Thermal Loop - An important feature of brake light operation that tends to shorten the life of incandescent lamps.

The instantaneous inrush current of incandescent bulbs also complicates the task of circuit protection and fault detection. The car manufacturer must set the fuse rating and fault detection threshold to a sufficiently large current value to accommodate the surge current amplitude and duration without a blown fuse or a false fault.

In contrast, LEDs are more robust and durable than filaments in the event of shock and vibration in a typical amplitude and frequency range. The LGD structure is lightweight and small in size, reducing the mechanical torque generated by shock and vibration. The small size of the LEDs also allows automotive designers to design the lights to be smaller and to be more compliant with the overall design of the car. For example, some cars do not mount the CHMSL (intermediate high-position brake light) module on the rear cover, but instead use the small required size of the LED to include this function in the trunk lid.

Automotive taillight lighting and control systems present several interesting questions that can also arise in other systems where the control device and the controlled device are far apart from each other. LEDs are essentially current devices. Electron-hole pairs recombine within the electroluminescent compound and emit photons upon recombination. An 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 increase the self-heating of the device through 1-h power consumption. Unless the working conditions are bad, LEDs generally do not experience catastrophic failure like tungsten lamps, but they tend to darken due to aging. Many device designers define the end of life of an LED as the time it takes for the light output to drop to 50% of its initial value.

Overcurrent and overheat conditions will accelerate the end of LED life, so most device manufacturers recommend that OEMs carefully control the energy of the LEDs.

These characteristics suggest 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 at a constant current. However, as Bill Reidel, an automotive market expert at Analog Devices, said, the constant current design complicates the wiring between the body control module and the lamp components and drives the designer to remove the power control IC from the body control module. Put it in the lamp housing. The constant voltage drive keeps the control IC in the control module that needs to control the IC fault detection status information, and can reduce the number of external components (ie, fuses) and the number of wires between the control module and the lamp housing in the same design.

Keith Wolford, automotive applications engineer at Texas Instruments, agrees: "One of the functions of the LED control IC is the function of the fuse. If you place the LED driver in the lamp housing, you must transfer the electricity to that location and give the LED driver Fuse...and if you have a central lighting module, all you have to do is fuse the power feeder that connects the module. With the diagnostics of the LED driver, if the wire connecting a lamp housing is shorted, you It can be protected with electronic equipment without having to fuse each lamp housing."

Analog Devices' AD8240 LED driver/monitor is the embodiment of this approach. The device operates at 300mA and has a supply voltage range of 9V to 27V. The PWM input controls the brightness of the lamp to achieve the lowest brightness level for day and night in compliance with automotive regulations. The overcurrent detection circuit consists of an external high 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 each PWM cycle.

The shunt resistor and the external PNP transfer element limit the maximum load current. The manufacturer's recommended 0.1Ω ~ 0.5Ω shunt resistor range corresponds to a maximum load current of 2A ~ 0.4A. The microcontroller of the control module monitors the load current by reading the read value of the IC sense pin through an ADC input channel. The AD8240, which sells for $1.15 (1000-piece quantities), can detect open-circuit loads, short circuits, and local faults, such as a short-circuit in one of a series of LEDs. This driver/monitor IC is available in an MSOP-8 package.

In designs that require a low-side controller, designers can consider using Melexis' MLX10801 because the MLX 10801 is packaged in an SO-8 package that absorbs 550 mA absolute maximum peak current and 400 mA absolute maximum without external transfer devices. Average current. A package option with a tail code A is packaged in a MLPD-8 with thermal padding, while the bare die used is unchanged, reducing RΘJA from 120K/W to 37K/W. This package improvement increases the absolute maximum peak current and absolute maximum average current to 1.2A and 750mA, respectively.

A diagnostic pin allows the local microcontroller to monitor the load current through an ADC channel. The design of those driver/monitor wafers over the ADC channel can be used to find the sum of the ground currents and monitor the total ground current with an analog input pin (Figure 2).

Melexis' MLX10801 features a set of transient pulses, 40V load dumps, and abnormally induced undervoltage conditions that are expected to be unacceptable for the device. A programmable non-volatile data latch allows OEMs to perform temperature measurements with an on-chip sense diode or external sense diode. A control input pin enables PWM dimming, a common feature of LED drivers. Keeping this control input pin low for more than 32 milliseconds forces the driver into sleep mode, reducing its quiescent current from 2mA to 105mA. Keeping this control input pin high for 8mS starts a wake-up sequence that lasts only 300mS.

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