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Posted on 01 October 2019

Automotive Lighting Solutions

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The inrush current creates a thermal stress on the driver

Despite the development of new lighting technologies such as the Xenon lamp (HID) or LED lights, the incandescent bulb will remain the most commonly used light source for the next generations of cars.

By Jerome Pillet, Senior Engineer Intelligent Power Devices,Power Semiconductors Product Unit, Automotive Business Group, NEC Electronics (Europe) GmbH

 

Consisting of a tungsten filament inside an evacuated glass bulb, these lamps, as used in today’s automotive market, are available in a wide variety of types, but strictly specified to international standards according to their mechanical, electrical and luminous characteristics for exterior lighting applications. The following table shows the most commonly used lamps in the European automotive market, as classified per ECE R37.

Lamps used in European car applications

Light control in modern cars

In the past, lamps were directly controlled by mechanical switches at the steering wheel or on the dashboard. Nowadays, light control is via a dedicated electronic module. Usually called Body Control Unit or Module (BCU / BCM), this device receives information through the CAN network from various command modules, e.g. the top head column, dashboard panels, light sensors, etc. The BCU switches the lamps on or off, but also contains diagnostic features. Indeed, for obvious safety reasons, both legislators and car makers nowadays require increasingly more diagnostic functions for light systems. Thus defective lamps for key functions such as brake lights or position lights must be reported to the driver via the dashboard.

In most cases, the output of the BCU must be able to drive either LEDs or bulbs, or Xenon light or incandescent bulbs, depending on the lamp function. The selection is usually made by software configuration, so the lamp driver must normally have extended performance to support various types of loads.

An advantage of the BCU microcontroller is that bulbs can be driven in Pulse Width Modulation (PWM) mode to regulate the RMS voltage, allowing light intensity adjustment, or simply avoiding light intensity variation over battery voltage. This feature is especially useful when driving LED lights, since a single LED matrix can be used for position or brake lights, simply by varying the duty cycle of the PWM signal to the lamp driver.

For all these reasons, silicon drivers, rather than relays, are today the preferred solution for lighting applications. Naturally, silicon drivers also offer advantages in system integration and the mounting process.

Bulb properties

In filament lamps, the luminous energy is produced by a tungsten filament heated within a few ms from ambient temperature up to about 2,800°C. Electrically speaking, the filament impedance increases strongly at lamp switchon, which corresponds to an inrush current phase. The peak current can be much more than 10 times the nominal current. The graph below shows the inrush current of an H9 bulb at 40°C, driven by a car battery at 15V with the NEC driver μPD166007. The peak current reaches 80A for a nominal current of 6.2A.

The driver should not limit the inrush current, otherwise the lamp might not switch on or could switch on with a significant delay, which is clearly inacceptable from a safety point of view.

The inrush current creates a thermal stress on the driver, which must therefore be designed to handle repeated inrush currents of this magnitude without any loss of reliability and performance over the expected lifetime. As an example, the graph below shows the power dissipation and junction temperature increase of the µPD166007 when switching on the H9 bulb. The channel temperature rises by about 50°C within 300µs!!!

Bulb Properties

Drivers for lights

A silicon driver for automotive applications is an integrated device, usually directly driven from a microcontroller port. The figure below shows a block diagram of the NEC Intelligent Power Device (IPD) µPD166007.

Power losses and channel temperature of the µPD166007 driving an H9 bulb at 40°C

The power part is made of an N-Channel Power MOSFET, into which a current sensor and a temperature sensor have been integrated. The size of the power part, and consequently the Rdson, will depend on the target load.

The control part realizes the gate drive along with protection features and diagnostics.

- Gate drive: the Power MOSFET is usually connected in high side position (i.e. to the car battery driving a load connected to ground). The drive circuitry includes a charge pump to supply the appropriate gate voltage.
- Protection features: the protection functions are mainly overcurrent and overtemperature detection.

This driver consists of two main parts (1) a power switch and (2) a control part

Overcurrent protection

The overcurrent protection has a short reaction time and continuously monitors the output current. It reacts if the output of the device is shorted to ground in a very low impedance path. The overcurrent protection must be designed to allow the inrush current of the lamp flow. When an overcurrent is detected, the device reacts to prevent power switch destruction, either by limiting the current or switching the device off. Usually, diagnostic information will be output at one dedicated pin of the device.

Overtemperature protection

Excessive output current due to a temporary or permanent overload at the driver output may cause the channel temperature to rise above the maximum value. In this case, the overtemperature protection prevents the destruction of the device by switching off. Typically, the overtemperature protection will react to a “short circuit” occurring in a light module at the cable termination.

Power, Channels and Packages

Silicon drivers and short circuit

A load can be connected to the BCU by a cable several meters in length. A short between any point of this cable and ground would create a “short-circuit” ranging from a few mOhm to several 100mOhm impedance. Ideally, such a “short-circuit” will trigger one of the protection mechanisms, but it could also create an excessive power dissipation situation below the levels at which protection mechanisms are activated. This situation would drastically reduce the driver lifetime and could lead to early failure of the driver.

In practice, a feature that can read back the load current is central to ensuring the reliability of a system. Silicon drivers can provide the microcontroller with a proportional image of the current into the load. This information is usually handled by the microcontroller’s A/D converter. Such a setup also enables an open load situation to be detected. In fact, this feature is set to become a mandatory requirement of car makers. Naturally, NEC drivers will support this feature.

NEC Electronics is developing a comprehensive family of Intelligent Power Drivers (IPD) targeting the control of automotive lights.

 

 

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