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LED Driver

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Identifying the originating power source is the first step to power management while designing and LED driver circuit. The power source may vary from AC to DC, low voltage or high, depending on the environment and availability of electrical power. For example, consider two custom LED lighting designs, one on board a large transport aircraft, the other onboard a smaller single engine aircraft. The three-phase AC power system onboard the transport aircraft might require some relatively complex LED driver circuitry. However, the low voltage DC power system found onboard the smaller single engine aircraft might only require simple constant current source LED drivers.

Power LED Drivers

Many custom LED lighting designs operate from a high voltage AC power source. Utilizing a standard AC to DC power supply to achieve a DC source voltage is often the most cost efficient and reliable LED lighting solution. To ensure proper LED light operation, external DC-to-DC LED driver circuitry is always required in conjunction with the primary power supply. Power supply availability may range from one watt to several hundred watts.

Constant Current Source LED Drivers

Variations in source voltage can cause LED driver currents to fluctuate dramatically. In addition, the forward voltage drop can cause drive current to fluctuate within the power LED drivers. Subsequent to the light emitting diode manufacturing process, the LED binning process ensures separation of LEDs with significantly dissimilar forward voltage ratings. However, the binning process does not ensure that LED lamps common to a bin will share an identical forward voltage rating. Additionally, it is often difficult if not impossible to order from the same bin. These inconsistencies are among a few responsible for LED driver current fluctuations. The constant current source resolves all associated current fluctuations by supplying a continuous, pre-defined current under varying conditions. As a result, all members of an LED array exhibit a more desirable, LED optical consistency. In addition, the risk of thermal runaway is nearly illuminated. Applications integrating high power LEDs always require a constant current source to ensure efficient and reliable operation. These special power LED drivers ensure this type of operation.

LED Resistor Circuits

A series resistor circuit is an extremely simple yet effective method of driving an LED light bar. The simple drive circuit can minimize overall design and production costs. However, disadvantages include minimal efficiently and the need for various resistor values based on forward voltage ratings. Increasing the total number of series LEDs per resistor can result in greater efficiency and sensitivity to varying source voltages. Decreasing the total number of series LEDs per resistor will tend to decrease efficiency, but can provide a benefit by decreasing sensitivity to varying source voltages. Simple resistor LED driver circuitry remains among the most popular and dependable methods utilized in modern LED lighting designs.

Thermal Runaway

Thermal runaway can occur within the PN junction of an LED light or other semiconductor device. The danger is most commonly associated with poorly designed LED driver circuitry, or circuitry intended to overdrive LED lights and achieve an increased luminous output. A dramatic increase in ambient temperature can also promote thermal runaway. Exceeding the maximum forward current rating can cause excessive heat dissipation within the PN junction. As current increases, temperature also increases. When generated heat exceeds the normal rate of heat dissipation, thermal runaway is imminent. The vicious cycle normally ends when catastrophic failure occurs on a device or system level.

LED Pulse Driving

Pulse driving consists of an extremely high rate of continuous electrical pulsing. The LED array operates intermittently as the drive circuitry produces a series of rapid pulses. Utilizing the correct frequency can eliminate all visible flicker and the LEDs appear to maintain a continuous state of operation. Simple pulse driving commonly employed in multiplexed drive circuitry allows multiple LED driver circuits to operate using a single LED driver chip. A control circuit must provide switching between circuits, allowing each LED cluster or group to draw current from the LED drive circuitry in a sequential reoccurring manor. As a result, each individual LED cluster exhibits pulse driving, rather than continuous driving. In some cases, LED overdriving can help compensate for loses in luminous flux, because of a decreased duty cycle.

LED Pulse Width Modulation (PWM)

At low currents, LED lights tend to exhibit significant variations in optical wavelength as well as luminous output. This is why dimming operations are among the most common applications for pulse driven LED circuits. Varying the pulse width can achieve dimming, which engineers commonly referred to a pulse width modulation, or PWM. The PWM circuitry prevents color matching complications that would otherwise occur using a standard DC drive circuit. This also makes PWM ideal for color mixing applications. In most cases, an embedded microcontroller provides the necessary output signal for the LED PWM circuitry.

Example LED Application

For demonstrational purposes, let us consider an LED lighting application designed specifically for aquarium lighting. The client an aquarium manufacture and wishes to integrate an LED array into a new fish tank design. The client has already designed the fish tank itself, and has carefully incorporated a half-inch channel into the plastic frame located on the top of the aquarium. The objective is to design integrate an LED array so that it fits perfecting into the per-existing cavity. In this case, the LED light integration is feasible considering the use of a low-profile surface mount LED. However, the fish tank designer probably should have consulted with Lunar Accents Design prior to initiating his or her new aquarium design. This would permit the flexibly to design the lighting cavity with respect to the physical dimensions of more appropriate LED lights.

Since the intension is home-use, the fish tank lighting application must be compatible with a standard 120-volt AC power source. The LED array will consist of 33 high intensity surface mount LED lights, with a combined maximum current draw of 220 mA. Based on the LED circuit architecture, a 9-volt DC is power source is going to be most suitable. A database search yields a relatively low cost, 300 mA switching power supply. The power supply is appealing due to its compact design and incredibly small physical dimensions. A detachable AC power cord connects the power supply with the AC wall outlet. A low voltage DC wire enable a remote connection with the LEDs located within the fish tank itself. This modular design is beneficial because it separates the high voltage from the water in the fish tank.

The surface mount LED lamps selected for this application work best in conjunction with a constant current source. The 9-volt power wire will enter through the top of the tank and connect with the inner circuit board. The constant current source circuitry and LEDs share a common circuit board. This design is appealing due to its compact nature as well as the advantage of decreased assembly costs. The circuit architecture consists of eleven groups of three LEDs. Each group sources its current directly for the integrated constant current source. The constant current source circuitry consists of a single IC chip and several small external components. Altogether, the circuit occupies an area equal to only three of the thirty-three installed LEDs.