In recent years, high-brightness LED applications have developed rapidly, especially in terms of signs and traffic lights. For automotive applications, LEDs also have great appeal. Long-life, shock-resistant, efficient, and good control of the light source are its advantages. Of course, compared to incandescent lamps, LEDs need drive circuits, and automotive electronics are powered by acid-lead batteries. They are mechanically driven alternators. These batteries are suitable for incandescent lamps and are not suitable for LEDs. Therefore, design is stable. A drive circuit with good voltage performance and low noise is very necessary.
In theory, the LED light output is related to the drive current, independent of the supply voltage. For the most demanding applications, a resistor can limit the current if the supply voltage is stable. It is worth noting that for this simplest application circuit, LEDs exhibit self-stabilization characteristics to some extent. That is, if the temperature rises, the light output of the LED decreases, but at the same time its forward voltage drop also decreases, causing the drive current to increase, thereby compensating for the reduction in light output at higher temperatures.
Unfortunately, the range of automotive power supply is very large, between 8V and 18V, the peak voltage can reach tens of volts. In addition, the high-brightness LED drive current is large, which generates a large amount of heat in the resistor, complicating the heat dissipation design.
A relatively simple solution is to use a linear buck regulator (Figure 1). D1 is a Zener diode and the current through the LED is set to VD1/RSET. D2 performs humidity compensation on the base diode. This circuit still has energy loss problems and resistance heat dissipation problems. This circuit is a cost-effective solution for low current LEDs, especially where the forward voltage drop of the LEDs in series is slightly lower than the supply voltage.
Figure 1 simple steady current circuit
In most cases, switching power supplies provide a better electrical solution. As the name suggests, the switching power supply operates as a switch that charges the RLC circuit in one cycle; in the next cycle, the stored energy is used to drive the load. These circuits are extremely efficient, typically up to 90%. Switching regulators can boost voltage, lower voltage, and generate voltages of opposite polarity, which are not available with linear regulators.
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