Automotive LED lighting design

Light-emitting diode (LED) lighting opens up new horizons for unconventional, comfortable and customizable. These design opportunities are rapidly increasing the level of application and speed of LEDs in the car. There are several methods and design techniques to choose from when using LEDs in the car, in front of the car, and in the rear of the car.

For automotive LEDs, the relative anti-vibration, long life, high energy efficiency and fine control of the light source are key factors. Compared to incandescent bulbs, LEDs are not sensitive to mechanical shock, but require a drive circuit. In general, automotive electrical power systems use a lead-acid battery that is charged by the engine through a mechanically driven alternator/regulator. Such a system is suitable for old-fashioned incandescent bulbs, but not for LEDs. In order to achieve the best performance of LEDs, a precise constant current source is required.

In order to properly drive the LED, the current needs to be controlled regardless of the voltage. The light output is basically dependent on the current and not the voltage. In theory, each electron is converted into a photon, and a fixed proportion of the photons that escape the LED become the light we see.

If the voltage is constant, only one resistor is needed to achieve a low quality solution. It should be noted that when the LED and the resistor are simply connected in series, the LED itself is self-regulating to some extent. If the temperature rises, the efficiency and brightness of the LED decrease, and the forward voltage drop decreases simultaneously. The reduced forward voltage drop in turn causes an increase in current, which somewhat compensates for the decrease in brightness due to temperature rise. As long as the battery voltage is constant, the series resistor scheme is sufficient for computer and instrumentation applications. However, the automotive industry mandates that equipment be able to withstand battery variations between 8V and 18V and tolerate peaks of 80V. In addition, high-brightness LEDs generate a lot of heat on the resistors. This makes thermal design more difficult.

A better but not the best alternative is to use a dc-dc voltage converter to generate a suitable regulated voltage and then combine this with a resistor. This solution works if you already have a dc-dc converter that powers your computer or other electronic device. In addition, this method may be the most common method of driving LEDs.

However, it is a better solution to drive the LED with a voltage-independent constant current regulator. Energy consumption and energy conversion are two basic types of current regulators, respectively.

A linear buck constant current regulator is an example of an energy consuming constant current regulator. For a given current, the energy consumption represented by the voltage drop across the current regulator is consumed. The other case is an energy conversion current regulator that attempts to store the energy difference between different levels.

The equation used to describe this energy conversion is one of the basic laws of thermodynamics:

Input power = output power

Given with W=V·I (given), replace W in the formula:

Vin·Iin=Vout·Iout+(100-X% efficiency) W fever (fever)

If the forward voltage of the LED is taken as Vout and the required current is taken as Iout, a general equation describing the LED driver will be obtained.

Edit: Leo


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