LEDs drive brighter car headlights

In many high-brightness LED (HB-LED) applications, one of the main issues to be addressed is the powering of the devices from a wide input voltage source. This is especially true in automotive applications where the +12V battery power source may fluctuate between 8V and 18V.

HB-LEDs need to be driven by a current source rather than a voltage source since the forward voltage rating varies from part to part and over temperature. To obtain predictable and matched luminosity and chromacity it is also desirable to drive the LEDs with a constant current.

To decide the best cost/performance trade-off for driving HB-LEDs, it is useful to segment applications and their corresponding solutions according to their approximate LED drive current, number of LEDs per string and system complexity.

I identify three groups:

? Low-power and low-complexity applications such as most of the rear lighting functions, which are typically addressed with linear regulators.
? Medium-power applications with low system complexity, such as vehicle interior lighting. In this category, the most popular device is a standard switching regulator.
? Medium to high-power front lighting applications with high-end system requirements to deal with the safety regulations and standards.

It is possible to use standard buck or boost switching regulators plus the supporting circuitry for the control of higher power front lighting applications.

However, application-specific devices can provide a more elegant approach that incorporates a wide range of important additional functionality to improve reliability and reduce overall cost.

Able to drive and control more than one HB-LED string and incorporating a host of diagnostics monitoring and protection, application ­specific devices allow the implementation of a single-chip self-contained lighting module that also negates the need for a microcontroller.

They are able to generate pulse width modulation (PWM) dimming above 500Hz, whereby the duty-cycle of the LED current is varied to achieve a dimming effect. This approach overcomes the problems associated with analogue dimming, where LED light colour (colour temperature) varies visibly with reduced applied current.

It is anticipated that future HB-LED-based headlamps will emit more lumens than their predecessors. It is then likely that replacement LED lighting modules fitted to a vehicle damaged in an accident may have better performance and light output than those fitted when the vehicle was manufactured.

This could cause a visible difference in brightness between the old and new lighting modules. The PWM dimming feature embedded in an integrated device is able to compensate for this, as well as for the natural degradation in light over the lifetime of the vehicle.

Temperature monitoring and diagnostics are two important functions that integrated drivers can include.

Temperature monitoring is achieved via an interface to the appropriate sensors. With the life and reliability of HB-LEDs closely tied to their junction temperature, this can prove vital in helping achieve acceptable longevity for the LEDs.

It can also protect the LED module in case of a system failure caused by, for example, an inoperative cooling fan or a short circuit in the wiring to the LEDs.

Diagnostics support compliance with safety and regulatory requirements. Typical problems that integrated diagnostics are able to detect include shorted and open circuit LED strings and the detection of a single shorted LED that may cause the light beam to be non-compliant with regulations.

Application-specific devices can also incorporate EMC filtering to ensure stringent automotive requirements and standards are satisfied.

Input current filtering is particularly important for HB-LEDs during PWM dimming when they are continuously switched on and off. External filtering for standard switching regulators can be expensive and difficult to implement.

A typical boost application will generate high ripple currents from the battery. During dimming, PWM switching generates fast current changes in the LED strings which causes them to act like an antenna. A standard switching regulator approach requires expensive filtering to overcome these effects.

A design approach that uses an integrated device generates much lower input ripple, and in most cases can filter out PWM dimming effects without the need for external components.

Paul Decloedt is business development manager at ON Semiconductor

Tags: automotive, cars, LED lights, LEDs

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