Designers face challenges in providing secure and reliable control devices for automotive lighting.
In the old days, life was much simpler: there were very few switches in your car and if you got lucky, the directional light switched on at the exact spot you wanted it to. If a bulb didn’t work, the horns of nearby drivers announced you were about to be blindsided. Nowadays, defective light bulbs are almost history. LEDs with their extended lifetime have become more and more common. It is a success story and power semiconductors have quite their share in it.
Failing head- or tail-lights are real safety issues. In the 1980’s, monitoring their function and extending their lifetime was a developer’s goal. These were the days of the notorious central relay box, which held the relays to switch on a single lamp or a group of lamps. Finding solutions that could monitor these relays took quite a lot of effort: the bulb current had to be measured and analyzed accordingly. But still, a relay‘s lifetime was as limited as an incandescent bulb’s lifetime.
As soon as they were able to replace the technology of the old relays with new intelligent semiconductor switches in terms of cost, semiconductor manufacturers had their great moment. It became possible to extend a relay’s lifetime, implement diagnosis, and increase reliability.
The launch of the LED was the second crucial step. but it also complicated things. They needed completely different control, made possible only by advanced electronics.
Beyond incandescent light bulbs and the increasing amount of LEDs, there was and still is a phase of using XENON gas discharge bulbs for headlights.
The number of light functions on a car’s exterior didn’t change much over the years (other than their form) in contrast to lighting on the interior. Dashboards glitter like the Las Vegas Strip at night. With the introduction of RGB-LEDs, the inside of your car can light up according to your mood, your clothes, or your nail polish.
Using smart power switches: Not just a relay replacement
Technology and circuit development
In the early ’90s, the first smart power drivers came on the market, and replaced relays in terms of power, voltage, and cost. BMW was one of the first car manufacturers to have introduced light modules for the control of lamps, first in the luxury class models.
At that time a broad range of power MOSFETs (metal oxide semiconductor field effect transistors) became available and increasingly replaced the classic bipolar transistor. With the introduction of additional layers in that standard power transistor process, additional functions could be integrated on the same chip. The transistor became ‘smart.’ The invention of VIPower (vertical intelligent power ) from STMicroelectronics was the start of many innovations to control high power in a car.
There are two possible configurations for switching a load: high side and low side. In a high side configuration, the load is connected between the switch and ground (GND). In a low side configuration, the load is connected between battery voltage and the switch (Figure 1). For safety reasons, the switch (especially for bulbs) is always connected between GND and the source of the high side driver.
Figure 1 Two ways to control a load
This prevents the load from being switched on unintentionally in the event of the most common short circuit (against ground). This would have fatal consequences for lamps: accidental activation of fog lights, high beams, and brake lights are a great safety hazard, as you can imagine.
This requires so-called high-side switches that require a voltage at the internal gate for control that must be higher than the supply voltage (which is on the drain). The device needs an internal charge pump, which must be efficient and should not have electromagnetic radiation.
Bulbs have a special current characteristic. When turned on, the current is more than 10× higher than the continuous current for a short time. And this characteristic is strongly dependent on temperature. This means the driver is further challenged to realize a special current profile of the current limit to guarantee a sufficient lifetime of the bulb.
A further decisive advantage switches have over relays is the possibility of a series of diagnosis and protection elements. Lamps can be completely monitored and protected with the internal measurement and evaluation of the load current. A basic block diagram is shown in Figure 2.
A smart high-side driver should have the following:
Application options and examples
Today, smart power drivers are used in one- to multi-channel versions and in different configurations for all lighting controls in the motor vehicle. Here, relays are almost completely absent. Either all light functions are concentrated in a central control unit (formerly so-called light modules) or are a component of a central body domain controller. The split in a front and rear module is also common today.
Figure 3 shows the printed circuit board of a body control module on which one can detect the large number of smart power devices. When selecting the drivers that are available in multi-channel versions and with different ON resistors, the user can select drivers from 3mΩ to 140mΩ. As an example, a selection of available high-side drivers of the current technology generation M07 from STMicroelectronics are compiled. This family offers further low impedance variants and smart power devices that remain safely switched on even when the supply voltage is very low (as when cranking the engine).
Depending on the loads to be controlled (current, ambient temperature, type of bulb), the corresponding drivers are selected. H4 (two filament bulbs) and H7 bulbs for the headlights are still mainstream for low and medium cost cars. These lamps are directly controlled by the smart power drivers.
Figure 3 In a body control module, single- and multi-channel devices control bulbs and other loads.
In partitioning, appropriate security considerations must be performed in order to have certain redundancies in the system in case of failure of individual bulbs. One hot topic was and still is the limited lifetime of lamps with filaments. Constructive improvements have been achieved to increase their lifetime, and it also has helped to have the ability to drive the smart switches by pulse width modulation (PWM).
[Continue reading on EDN US: Xenon gas discharge bulbs vs LEDs]
Jochen Barthel is Senior Technical Marketing Manager for Smart Power (VIPower), Motor Control, and Lighting Products at STMicroelectronics.
Gert Rudolph is Technical Marketing Manager of Automotive at STMicroelectronics.