LED-based vehicle headlights are the hot exterior feature in luxury models. LED headlights are cutting-edge technology that gives car designers unprecedented freedom in headlight design. Their distinctive look and environmentally green technologies give luxury carmakers bragging rights in the marketplace and differentiation in a competitive environment.
For instance, witness the Audi R8 LED daytime running lights introduced in a 2008 Superbowl commercial. The headlight was the star. Despite their “star power,” LEDs are not power hungry, expensive or fragile. They run cool and efficiently, saving valuable battery and generator bandwidth for the countless other electronic features in today's high-end vehicles and they are far more durable than other lighting technologies.
Currently, there are three types of bulb-based headlights on the market that can be easily recognized by the color of the light output: incandescent (yellowish light), halogen (yellow-white light) and HID (high-intensity discharge — blue light). Incandescent and halogen bulbs are filament-based and destined to eventually burn themselves out. Their dim yellow color appearance is equated in the current market with old and, for lack of a better word, cheap.
HID bulbs are filament free and last longer, using a high-voltage electric charge to light up the gas to produce a very bright light, but the high-voltage electronics and bulbs are expensive and fragile, and the high temperature color results in a blue tint. Even with the wow factor of bright blue (or purple) HID lamps, their expense is a significant deterrent to widespread use. Plus, they cannot be dimmed.
White high-brightness (HB) LED strings allow headlight designers to give their new models complete lighting makeovers. The strings can be spread out or reshaped into interesting and distinctive, never-before-seen shapes. The Audi R8 LED daytime running lights are a good example, featuring an approximately 10-W LED string. Imagine a complete headlight using 50 W of LEDs. The Cadillac Escalade Platinum released the first 50-W LED headlight this summer and Mercedes, Lexus, and Audi have pledged to follow suit. The race is on.
LED Advantages
LED headlights offer several technical advantages over other headlamp types. Efficiency is a key benefit. The growth of vehicle electronics and the ballooning cost of fuel create a need for high-efficiency electronics to minimize the load on the generator and the drain on the battery. LED drivers, with their high-efficiency (>93%), combine with high lumens-per-watt white LEDs to keep the required battery current low.
The ability to tailor lighting color is another advantage. LED color can be either true white light (as close to daylight as possible) or any warm white color in order to resemble “old-style” headlights for the nuevo-retro look, where the lights look old-school, but bright.
A further advantage is long operating life. The lifetime of the LED strings is expected to exceed that of the vehicle, giving the owner peace of mind that expensive headlamp failures and replacements will be minimized, if not completely eliminated.
And not to be overlooked is the relatively low cost and ease-of-use of the electronics that drive 50-W LED headlamps. The DC-DC converter LED driver attaches directly to the battery and maintains a well-regulated output over the battery's wide voltage range. In contrast, other high-power headlight technologies such as HID require very high voltage (difficult-to-work-with and dangerous) electronics and the expense that comes with them. The LEDs themselves are much cheaper than the alternative, and a broken LED is not expensive to replace.
LEDs also make possible instant turn on and turn off. Becasue they are powered by a controlled constant-current drive, they can also be PWM (pulse width modulation) dimmed and quickly adjusted over a wide range of brightness settings without changing the color.
The DC-DC converter that drives the LED is a crucial piece of the headlight design. The converter must be able to efficiently power a 50-W LED headlamp with constant and controlled LED current while drawing on a battery voltage that can wander over a wide range. Trying to do this with discrete components or even a linear regulator is difficult to impossible.
To make things even more complicated, add some other basic requirements, such as high dimming ratios at constant color (PWM dimming), low current consumption, high efficiency (to preserve the battery system), relatively low cost, low EMI, operability in high temperature, short-circuit protection and open LED protection, and the problem looks intractable.
However, an LED driver design based on a switching regulator can satisfy these requirements. For example, Linear Technology's LT3755 is a high-efficiency LED driver boost converter IC that is specifically designed to drive automotive headlights.
LED strings can be driven from any DC-DC converter that can produce the proper voltage across the LED string and regulate the current through the string. In most cases, the most difficult problem is choosing the best regulator IC for the job. Depending on the device that's selected, the features offered by the driver chip can simplify the task of driving a 50-W LED string from an automobile battery while maintaining high LED current accuracy.
An automotive battery operates in the range of 8 V to 16 V with transients upward of 36 V for short durations. The input voltage range of the 50-W headlamp driver shown in Figure 1 is 8 V to 36 V for a 50-V string of 1-A LEDs. Given that the output voltage is greater than the input voltage, a boost topology is the natural choice for the most efficient and simplest DC-DC converter solution.
The high efficiency of this circuit shown in Figure 2 minimizes the strain that the headlamps place on the extremely demanding electronic loading in today's luxury vehicles. The efficiency of this circuit is greater than 93 percent at 12-V input and ranges from about 92 percent at VIN=10 V to about 97 percent at VIN=40 V. For the circuit in Figure 1, when the lights are off, the 12-V shutdown current is less than 50 µA.
The ability to vary the brightness of headlamps opens up a host of new feature possibilities. The brightness of a string of LEDs is decreased by simply reducing the current through the LEDs. This can be done by lowering the constant current (analog dimming) or by pulsing the LED current on and off at full current (PWM dimming).
PWM dimming is obviously more involved than analog dimming, but it has a major advantage: PWM dimming maintains the same LED color regardless of brightness, whereas LEDs change color with analog dimming. This is because the color of any typical LED changes with operating current.
In PWM dimming, brightness is controlled by turning the LEDs on and off at a constant frequency. It is important to use a frequency that is high enough to be undetectable as flicker to the human eye (≧100 Hz). Because the “on” part of the cycle is always the same current, the color of the LEDs is not affected. Dimming is achieved by varying the duty cycle. Although the actual current through the LEDs at any given time is either 0 or “full-on,” changing the duty cycle effectively changes the average current through the LEDs.
In the LT3755 headlight circuit, brightness can be dimmed fairly accurately via analog dimming to 1/50 of its full current level through the CTRL pin voltage. The LT3755 also has the ability to provide much higher and very accurate dimming ratios with a microcontroller input to the PWM pin. The PWM dimming waveforms in Figure 3 shows a 250:1 PWM dimming ratio for up to 0.4 percent brightness with a 100-Hz PWM dimming frequency.
The frequency of PWM dimming should be 100 Hz or greater so that it cannot be detected by the normal human eye under low-light conditions when the lights will most likely be running. Human peripheral vision and low-light vision can typically detect up to 80 Hz and higher in some individuals. Increasing the PWM dimming frequency is possible, but there is a proportional decrease in the maximum PWM dimming ratio. Higher DC-DC converter switching frequencies also allow higher PWM dimming ratios, but 400 kHz is chosen here to keep the main EMI content of the switcher outside of the AM broadcast band.
The filter on the front end of the DC-DC converter shown in Figure 1 limits conducted EMI. A few simple L and C components reduce the conducted spectrum looking back to the battery and other electronics. Setting 400 kHz as the switching frequency keeps the main content out of the AM band from 500 kHz to 2MHz. The high power of the headlamp circuit means that EMI content would be significant in the switcher if left unfiltered, but Figure 4 shows how the switching content and the EMI spectrum is below the required CISPR 25 Class 5 limits for the 50-W boost circuit.
If one or all of the LEDs are suddenly removed from the output, the converter output voltage climbs to the overvoltage protect level of 56 V and stops. When set properly, overvoltage protection allows the output to be opened and closed without any damage to the controller circuit or LEDs. The OPENLED output flag of the LT3755 gives feedback to the diagnostic microcontroller that there is an open output condition because the output voltage has climbed too high.
Short-circuit protection is a concern for most headlamp manufacturers. The failure mode of each LED might be either short or open, and an outside factor such as removal of the LEDs while the lights are on or improper connection of the headlamp may lead to an open or short circuit at the output of the LED+ and LED- (or GND/chassis) terminals.
The SEPIC topology uses a second inductor and a coupling capacitor to provide a DC block for short-circuit protection from LED+ to LED- or LED+ to GND as an improvement to the boost topology in Figure 1. Although the boost topology is simpler, the LT3755 SEPIC shown in Figure 5 has similar efficiency as the boost and has the addition of short-circuit protection. The efficiency for the SEPIC ranges from about 88 percent to 91 percent over an input voltage range of 10 V to 40 V.
The short-circuit waveform shows how the converter maintains control of the inductor current, and thus switch current, during a shorted output. The switch current is the sum of the inductor currents during switch on time and the catch diode current is the sum of the inductor currents during switch off time. The ability to survive the harsh short-circuit condition makes the SEPIC topology particularly robust. The unique short-circuit detect circuitry inside the controller IC is able to distinguish between collapsed output voltage due to short circuit and that due to startup.
Similar to the boost, conducted EMI of the SEPIC is well controlled with a simple filter on the front end in Figure 6. The conducted EMI measurements are similar to the boost measurements and they also meet the CISPR 25 Class 5 standard.
Although the switching frequency of the controller IC is adjustable from 100 kHz to 1 MHz, 300 kHz to 400 kHz is the frequency range of choice for automotive applications, set to be as high as possible while remaining outside of the AM band. The main spike in the conducted EMI spectrum is understandably at the switching frequency of 350 kHz in this application.
Switching frequency also affects solution size, efficiency, ripple current, and thermals. Higher switching frequency results in smaller components and a lower-cost solution, but increases AC switching losses in the switch (M1) and catch diode (D1). Lower switching frequency returns more ripple on the inductor current and can increase the heat rise of the inductor if a larger inductor is not chosen to reduce ripple.
In this application, 350 kHz provides a balance of thermal management, efficiency, and small solution size. The choice of MOSFET is optimized for the 350-kHz application with a combination of low RDS(ON), a 100-V drain-to-source rating, high rise and fall times at 7-V, 1-A gate drive and low gate charge.
In a 350-kHz, high-voltage and high current (8-A+ peak switch current limit) switcher, the rise and fall time of the main power switch is just as important as the low RDS(ON) rating. The high power gate driver of the LT3755 and the low gate charge of the Si7454DP 100-V MOSFET (Si7850DP 60-V MOSFET for the boost) are a nice match for automotive headlight applications.
The boost and SEPIC 50-V/1-A LED drivers described here are high performance and robust solutions for powering automotive headlights. Both are small, easy to apply and include many built-in features required in headlamp applications. These two ap proaches give designers alternatives for optimizing their LED driver designs. While the boost circuit offers the smallest solution size, the SEPIC provides short-circuit protection.
Keith Szolusha is a senior applications engineer at Linear Technology in Milpitas, CA, with a BSEE and MSEE from MIT.