122 Automotive Power Systems

FIGURE 7.12 Various steps in operation.

and the light shifts from the blue to the white range. Finally, for a continuous action,
the lamp is operated at a stabilized power rating for a still arc, showing no flicker.
Figure 7.12 sketches all the steps in the operation, while Figure 7.13 illustrates the
subsequent usage of various control algorithms on the same hardware platform. The
control software switches between current control and voltage control, depending on
the operation stage.

• During the turn-on phase, a voltage feedback control is used to adjust the
dc bus voltage (Figure 7.14) in order to delivered the dc bus voltage with no
power to the lamp.
• During the warm-up phase and at the beginning of the run-up phase, a
current-feedback control mode is required (Figure 7.15) in order to keep a
constant current while settling the power level within the lamp.
• During the steady-state (continuous) phase, a power-feedback control
mode is required in order to maintain a constant power level within the arc
(Figure 7.16).

An example of the complete schematics for the power electronic circuits used within
the controller is shown in Figure 7.17. The battery protection, reverse protection,
EMI filter, sensors, digital (DSP) controller are not shown.

7.4 LED LIGHTS AND THEIR ELECTRONIC CONTROL

Rubin Braunstein of the Radio Corporation of America and Robert Biard and
Gary Pittman of Texas Instruments contributed to the development of the infrared
LED. In 1962, a GE scientist, Nick Holonyak, developed the first visible [red] light
LED, which was followed by George Craford with the yellow light LED. In 1994,
Lighting 123

FIGURE 7.13 Various control methods adopted on the same power stage.

FIGURE 7.14 Voltage feedback control.

Hewlett-Packard increased the efficiency of the LED to ten times the efficiency of a
red filtered light bulb.
White LED is used today for lighting applications. This was first demonstrated by
Zhao et al. in 2002, by locating red, green, and blue LEDs adjacent to one another,
and properly mixing the amount of their output. A poor color rendering was achieved
124 Automotive Power Systems

FIGURE 7.15 Current-feedback control mode.

FIGURE 7.16 Power-feedback control mode.

since only three narrow bands of wavelengths of light were emitted. Later on, bet­
ter results were achieved with phosphor: a Y3Al5O12:Ce (known as “YAG”) cerium
doped phosphor coating produces yellow light through fluorescence. The combina­
tion of yellow with the remaining blue light appears white to the eye.
Using different phosphors produces green and red light through fluorescence. The
mixture of red, green, and blue is perceived as white light, with improved color ren­
dering compared to the blue LED/YAG phosphor combination.
The principle for the operation of any LED device is rather simple (Figure 7.18).
When a voltage is applied across the electrodes, the current flows from anode (P) to
the cathode (N), not unlike a conventional diode. When an electron meets a hole at
Lighting 125

FIGURE 7.17 Power electronic circuit.

FIGURE 7.18 Principle of a LED device.

the P–N junction, it falls into a lower energy state. The difference in energy of the
two states is called the “band gap,” which is a characteristic of the material com­
prising the P–N junction. The excess energy of the electron is emitted as a photon.
The more the “band gap,” the higher is the energy difference, and the shorter the
wavelength of the light emitted.
Control means producing a quasi-dc current through the LED, Figure 7.19.
LED color is determined by the energy required for electrons to cross the band
gap of the semiconductor and this depends on the semiconductor used:

• Indium gallium nitride (InGaN): blue, green, and ultraviolet high-bright­

ness LEDs;
• Aluminum gallium indium phosphide (AlGaInP): yellow, orange, and red
high-brightness LEDs;
126 Automotive Power Systems

FIGURE 7.19 LED bias.

• Aluminum gallium arsenide (AlGaAs): red and infrared LEDs;

• Gallium phosphide (GaP): yellow and green LEDs.

White light is obtained by using multiple semiconductors or a layer of light-emitting

phosphor on the semiconductor device. The exact choice of the semiconductor mate­
rial used will determine the overall wavelength of the photon light emissions and
therefore the resulting color of the light emitted.
Until recently, legislation has prevented the use of LEDs for exterior lighting.
An LED can produce approximately 100 lm/W. The current state-of-the-art
(SOTA), phosphor-coated LEDs regularly achieve 140 lm/W for “warm white”
LEDs and 160 lm/W for “cool white” lighting, and this technology is expected to be
limited to an efficacy of 255 lm/W. The greatest advantage comes from reliability
since LEDs have a typical rated life of over 50,000 hours, compared with just a few
thousand for incandescent lamps.
Another advantage for automotive applications: the turn-on times are about 130
msec for the LEDs (due to minority carrier), and 200 msec for bulbs. This advantage
is especially beneficial for brake lights.
Finally, there is a better thermal response than with any other technology. This
presents a major advantage since all former technologies needed more space for
natural cooling. Modern LED lights allow different light receptacle shapes, with
smaller, slender curves (Figure 7.20).
The LED lighting technology evolution can be followed with examples from a
world leader in vehicle lighting technologies:

• 2004: Audi A8 W12 with LED daytime running lights

• 2008: Audi R8 with all‑LED headlights
• 2010: Audi A8 in which the headlights are networked with the navigation
data
• 2012: Audi R8 with dynamic turn signal lights
• 2013: Audi A8 with Matrix LED headlights
Lighting 127

FIGURE 7.20 New shapes for the headlights equipped with LED lights.

At higher power levels, a power converter replaces the voltage source from Figure
7.19, because high-intensity LEDs are designed to work with a higher current. This
is possible with the pulse-width modulation (PWM) operation of a power converter
(Figure 7.21) so that a high-intensity producing current is delivered to the LED and
the average current is low. This secures a good energy efficiency. If the pulse fre­
quency is high enough, this “on–off” flashing does not affect what is seen by the
human eye as it “fills” in the gaps between the “on” and “off” light pulses, making
it appear as a continuous light output. Interestingly enough, pulses at a frequency
of 100 Hz or more actually appear brighter to the eye than a continuous light of the
same average intensity.
The brightness of an LED is approximately proportional to its average current.
The brightness is adjusted through a process called dimming. Both amplitude mod­
ulation (AM) and pulse-width modulation are used for dimming that changes the
average value of the current based on a reference command (Figure 7.21).
For a simple control, the duty cycle of the train of pulses is adjusted to change the
average value of current through the LED. A higher performance control avoids a
color temperature shift. This problem relates to the fact that current variation in an
LED may cause a color temperature shift. To prevent the LED lighting from color
shift, the LED lamp can be dimmed by low-frequency PWM control and usage of a
single current level through an LED. This process is illustrated in Figure 7.22.
Numerous integrated modules have been developed for automotive LED con­
trol (Figure 7.23). For example, MPM6010. The MPM6010 is a high-frequency,

FIGURE 7.21 PWM control of the LED current.

128 Automotive Power Systems

FIGURE 7.22 Advanced PWM control to avoid color temperature shift.

FIGURE 7.23 Integrated circuit for LED lighting control.

synchronous, rectified, step-down, switch-mode, white LED driver with built-in

power MOSFETs, an integrated inductor, and two capacitors, which Also includes
a control with an internal compensation network along with multiple protection fea­
tures (Figure 7.23).

7.5 LASER LIGHTS

Laser lighting is very similar to LED devices and their control. The laser diode is
a solid-state device, which emits light through photons. Instead of the P–N junc­
tion, the laser diode uses either a P–I–N structure or a N–p–P double heterostruc­
ture, where a narrower band gap semiconductor (for example, GaAs) is sandwiched
between two semiconductors of a wider band gap (such as AlGaAs). This solution
is used in most cases. The surface of the area around the junction is prepared to a
mirror-like finish. Hence, the photons are bouncing around the polished resonant
cavity, creating the laser effect.