1. Current Control
A fundamental issue with LEDs is that they are current-controlled devices with relatively low voltage drop. The simplest method is to limit the LED current using a resistor, but this is unsuitable for systems using 12V or 24V batteries, as the actual battery voltage ranges from 6V to 18V or 12V to 36V. Therefore, constant current control is necessary to maintain brightness.
2. Linear Current Control
Linear control refers to maintaining a constant current through the LED using a linear regulator. Linear control is inefficient in some situations; for example, a single 1A (3W) LED with a forward voltage of 3.5V requires the regulator to drop the rated 12V supply to 8.5V while maintaining the 1A current, resulting in a wasted 8.5W of power when using a 3W LED. Linear current control generates the least noise and is the safest from an EMC perspective.
3. Switching Regulator
While inductive switching constant current technology generates more electronic noise, it is more efficient. Depending on the number of LEDs used, buck or buck/boost regulators can be used.
4. EMC Issues Radiated and conducted noise must be minimized and kept within acceptable limits. Although the PWM method has a fixed frequency and is relatively easy to filter, due to the more stable nature of the LED load, a hysteresis controller and PFM are suitable choices if appropriate measures are taken.
