Control method of automobile LED light

1. Current control

A fundamental problem with LEDs is that LEDs are current-controlled devices with relatively low voltage drops. The easiest way is to use a resistor to limit the current of the LED, but this method is not suitable for systems with 12V or 24V battery ratings, as the actual voltage of the battery is from 6V to 18V or 12V to 36V. Therefore, constant current control is necessary if brightness needs to be maintained.

2. Linear control of current

Linear control refers to keeping the current through the LED constant through a linear regulator. Linear control is inefficient in some cases, for example, a single 1A (3W) LED with a forward voltage of 3.5V requires the regulator to reduce the rated 12V supply to 8.5V while maintaining 1A current, so using a 3W LED will waste 8.5W. Linear current control is the least noisy technique and is the safest from an EMC perspective.

3. Switching regulator

Inductive switching constant current technology produces more electronic noise, but it is more efficient. Depending on the number of LEDs used, a step-down or drop/boost regulator can be used.

4. EMC issues

Radiated and conducted noise must be minimized to keep noise within allowable limits. Although the PWM method has a fixed frequency and is relatively easy to filter, due to the stable LED load, hysteresis controllers and PFM are suitable choices if appropriate measures are taken. The trend in switching regulators is that the frequency will be higher to reduce the volume of the inductor/capacitor. This is always the best solution for automotive applications. Keeping the frequency low helps avoid interference problems.
Fundamental "jitter" or "spread" techniques do help meet similar peak EMC test requirements, but the best approach is to produce no radiation, which is difficult for any switching regulator to achieve.

Radiant Heat, Conductive Heat and Thermal Management One of the key issues and biggest challenges facing users of high-brightness LEDs (especially in the automotive industry) is the self-heating problem of LEDs. The lumens per watt of LEDs have been greatly improved, but in fact most of the electrical energy of LEDs is converted into conducted heat. LEDs produce less radiant heat suitable for cabin lighting, but in cold climates, the radiant heat of headlights can effectively melt snow on the lens. Therefore, thermal management is the key to reliable control of LEDs.

Thermal management mainly refers to reducing the current when the temperature increases. The advantage of using high-brightness LEDs is that when the current changes greatly, the eye cannot detect the brightness change. Generally speaking, the current drops by 25%, and the brightness change of a single LED is not obvious.

However, LEDs change color with changes in temperature and current, and whether this will affect automotive lighting applications remains to be discussed. Whether the spectrum of LEDs is suitable for lighting and whether it will affect the driver's sense of distance under normal night vision effects may be more important.

Using the PWM method to reduce the brightness ratio, rather than DC control, can obtain a larger light-to-dark ratio, and the color temperature will not change, so using the PWM method to reduce the brightness is a better method. However, the choice of frequency is also important. It is generally considered that a frequency of 200Hz is better, because the human eye does not feel the flicker of 200Hz light, and a lower frequency ensures that it is lower than the switching frequency of the switching regulator. However, the potential problem of stroboscopic effect in headlights must be foreseen. A more appropriate method is to use a higher frequency to adjust the brightness of the LED, thereby avoiding the "yaw" effect. In addition, inductors must be carefully selected to avoid audible noise in the car.

The temperature sensing of LEDs is also an issue that needs to be addressed. Thermistors are a widely used method, but great care must be taken when using thermistors, and the temperature control response should be set to the upper temperature limit corresponding to the current that the LED needs to reduce. When the ambient temperature decreases, simple temperature control can cause the current of the LED to increase. Figure 2 shows typical response requirements for LEDs to ambient temperature.