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The penetration of semiconductors inside an automobile is forecasted to increase rapidly in the coming years. Today, a high-end car is estimated to contain 70 to 100 modules containing semiconductors worth more than $1000. In fact, electronics is responsible for almost 25% of the vehicle cost.
Automotive electronic modules are being populated with an increasing number of semiconductor components that implement a wide range of applications. In addition, semiconductors are replacing numerous mechanical components. Due to this evolution, higher and higher current loads need to be driven with more and more ‘intelligence’ and diagnostics capabilities to help identify and debug problems. The ability to implement smaller, smarter, higher current-density devices, while simultaneously improving vehicle quality and reliability results in a significant enhancement for automotive applications.
According to industry analysts, the body segment is one of the fastest growing areas in automotive electronics (CAGR in excess of 12% from 2004 to 2010). The body segment encompasses most of the vehicle applications directly affecting driver comfort and convenience. Thus, the modern car is becoming a sophisticated electronic device with many onboard features such as dome and door zone controls, HVAC systems, wiper and seat controls, and lighting controls, just to name a few. These features are present on even low-end vehicles.
Many of these modules drive a myriad of loads. For example, the light center or junction box (Figure 1) can drive more than 80 different loads from small lamps for the license plate and park lamp to the high/low beams. This module also includes drivers for motors such as those of the washer pump and cooling fan. Historically, many of these loads were driven with standard electromechanical relays. Today, all of them can be driven with semiconductors also known as intelligent power switches (IPS). These power components need to operate reliably in a harsh environment, protecting the load, the board, and itself with the highest level of quality. Only a few semiconductor companies possess the know-how to design and manufacture these devices reliably.
SMART JUNCTION BOX
The junction box, which is a power distribution center with fuses and relays, was traditionally filled solely with electromechanical components without any intelligence. Today, semiconductors are at the heart of these applications, whether you need to drive an LED, 5 W or 65 W bulb; a 1 A or 20 A motor; high-side/low-side switches (from 0.5 Ohms down to less than 2 mOhms); multiple outputs per package; multiple configurations; or full- and/or half-bridge drivers.
The advantages of using semiconductors are manifold:
- They are substantially more reliable and durable than mechanical relays.
- They are smaller and lighter.
- They incorporate self-diagnostic capabilities.
- They offer self-protection and external protection capabilities.
- You can find practically any functionality you need.
Electromechanical relays are constantly being replaced by high-side drivers, which are also known as intelligent power switches or IPS. These devices are commonly made in a monolithic technology in which a vertical Power MOSFET (power output) is integrated with a low-voltage control circuitry based on NMOS components (Figure 2).
The availability of the low-voltage control circuitry allows the implementation of standard protections required for driving automotive loads. These protections include current limiting, voltage clamp, thermal protection, open-load, current sense, and electrostatic discharge (Figure 3). These protections allow the device to operate in harsh automotive environments and to survive abnormal (but not so rare) conditions such as reverse battery and load dump.
In addition, using these switches allows the implementation of new features that are typically available on today's vehicles, such as automatic head-lamp leveling or bending, low-frequency PWM techniques to stabilize bulb voltages and extend its life time, lamp outage detection and failsafe.
NOVEL CONTROL STRATEGY FOR HSD
Today, the reliability and quality expectations for electronics in general, and IPS switches in particular, are much more demanding. The greater expectations come despite operation in extended temperature environments at higher junction temperatures, higher thermal loads producing increased current densities and the elevated number of drivers present in a vehicle electrical system.
Engineers need to analyze potentially harmful conditions to eliminate degradation mechanisms of solid-state devices, such as thermally induced stress due to power surges, which may cause device degradation.
An accurate prediction of the instantaneous junction temperature of power devices, depending on the application and the load, is imperative to reach optimized designs and estimate reliability levels.
Recent developments in next-generation IPS families are generating innovative solutions that allow the user to rely on the IPS to protect itself and the surrounding environment without the need to perform elaborate thermal calculations and/or implement complicated software algorithms.
Innovative and patented control strategies reduce the effect of stress on the most vulnerable circuit elements through active power limitation. While the die size (and hence the package size that directly effects the system cost) required to implement a given power-handling specification has been reduced by an average of about 40%, the device robustness has been increased by orders of magnitude.
In Figure 4, the behavior of a 16 mOhm device, housed in a PowerSS012 package (same body size of a standard SO8), while driving a 55 W lamp is shown. The integrated control logic is transparent during the normal warm-up phase of the lamp filament.
Figure 5 illustrates the behavior of the same 16 mOhm device, subject to an overload condition (2 W × 55 W lamps). After turn-on, the integrated control logic automatically modulates the dissipated power. After a few power cycles, the lamp filaments warm-up, its impedance changes, and the current decreases. The device is now able to turn the bulbs on. The device is automatically limiting any fast thermal transients that may affect the device's long-term reliability.
In Figure 6, the behavior of the same the device, in heavy overload conditions (3 W × 55 W lamps), is shown. The modulated power that is delivered to the load is not sufficient to warm up the bulb filaments. When an anomalous overload condition lasts longer than a certain time, the internal control logic takes specific precautions to protect the device.
This innovative control strategy is a key characteristic of next-generation high-side drivers: enhanced durability with simplified software control that does not impact the system cost.
MORE COMFORT WITH MOTORS
The number of motors inside a car has been steadily increasing in the past several years. Motors are used in front and rear wipers, headlamp wipers, seat-belt tensioners, the sunroof, door-lock, window lift, throttle-control, HVAC, and anti-lock brakes, among others. In the recent past only high-end cars offered some of these options and they were traditionally driven by electromechanical relays. The trend for having more comfort and convenience onboard means that the next generation of cars will require most of these features as standard offerings on all platforms. Thanks to the advances in silicon technology most of these loads are now driven electronically.
New fully integrated H-bridge devices (Figure 7) are being designed to drive medium/high-current dc motors (up to 30 A stall currents). Housed in small and thin power plastic packages, the motors are protected from the most common faults, such as open or shorted load, excessive internal junction temperature, under/overvoltage condition, diagnostic feedback, and motor current feedback. In addition, the device integrates control logic that protects and monitors the motor under normal, such as shoot-through protection, or abnormal conditions, such as that caused by a short to battery or ground. The input signals interface directly with an external microprocessor, which offers the possibility of selecting the motor direction and brake condition while monitoring the motor current.
The main features of these H-bridge devices are:
- very low RDS(on) : starting from 19 m per leg;
- PWM: operation up to at 20 kHz;
- Overcurrent protection;
- current sense feedback;
- short to battery or ground detection and protection;
- overvoltage and undervoltage protection; and
- cross conduction or shoot-through protection greatly simplifying the system software.
For these fully integrated H-bridge devices, the packaging technology is evolving toward a surface-mount (SMD) approach even for higher-power applications. System miniaturization requires thinner packages with an improved ratio between silicon die area and package footprint. In a hybrid approach (Figure 8), different chips can be placed on electrically isolated islands within the same package for an efficient and cost-effective solution.
Wire-bond technology is also evolving. Packages are moving toward mixed Au, Al, and Cu wiring. The adoption of mixed-wiring technology removes limitations related to allowable pulsed and direct-current capability in higher-power applications, increasing the overall system reliability and performance. The reduction of interconnections among chips, while simplifying the design, also decreases the assembly cost, increases system density, and improves reliability of the complete system.
An additional benefit of this approach is a considerable reduction in the number of signal and power interconnections on the PCB. In particular, the reduction in the number of power interconnections significantly reduces the power losses caused by the PCB traces, GND bounce/shifts, and EMI problems due to the parasitic effects of these traces.
The advantages are numerous compared to standard packages:
- Top-side or bottom-side slug is soldered to the silicon providing a low thermal resistance path from the chip to the external slug and a large thermal capacitor to absorb power peaks during switching.
- Package symmetry: non-parallel solder joint, typical of asymmetrical packages, is eliminated.
- Exposed pads offer improved thermal performance. The thermal resistance Rthj-a is equal to 26 °C/W when the slug is placed on a standard FR4 of 7 cm2. This outstanding characteristic eliminates the need for large heat sinks that are required by most of the existing solutions and allows designers to achieve significant space and cost savings.
Along with smart junction boxes, door modules are among the body electronics systems that semiconductors have revolutionized. A typical door module is responsible for controlling power windows; exterior mirrors, including heating elements, motors for mirror adjustment and fold, electrochromic, and automatic dimming mirrors; memory seat selection; door locks; puddle lamps; and side-turn indicators. Figure 9 shows the typical system architecture of a standard front-door module found on various vehicles on the road today.
The door area is tight and constraints on space are demanding. The door compartment is subject to strong vibrations and is narrow and not easily accessible by assembly plant workers. Second, the wire-harness bundle that can be routed to the door needs to be limited since the door is separate from the vehicle body. In spite of these limitations, the number of features is constantly increasing. These constraints are even more demanding if the mirror is taken into consideration.
To reduce costs, implementations of specific functions for door modules need to be flexible and tailored to specific customer specifications. For example, the customer may specify single or double motor lock, with or without folding mirrors. Additional variants can depend on the vehicle partitioning and distribution of loads between the front and rear doors. (see Figures 9 and 10 for some examples).
Another key characteristic for door modules is the need to communicate with the rest of the vehicle via a CAN or LIN bus for the identification of the key from the ignition lock, vehicle speed, stored memory seat settings, window position, and diagnosis requests or messages.
Advances in silicon technology combined with packaging developments allow the implementation of advanced functions, such as full control of the exterior mirror, in a single chip. A state-of-the-art actuator designed in a smart power process, which ST calls BCDTM because it delivers Bipolar + CMOS + DMOS, is capable of satisfying all the system requirements — quality, cost and performance — of a medium/high-end vehicle and is shown in Figure 11.
The main features of the actuator are:
- Standby mode for very low power consumption;
- SPI controlled;
- protection and diagnostic feedback: over/open load, power supply over/undervoltage, temperature warning and protection, load current monitoring;
- PWM input: each driver has a corresponding PWM enable bit that can be programmed by the SPI interface;
- cross-conduction protection: avoids faulty bridge operation while simplifying the system software; and
- programmable soft start function to drive loads with higher inrush currents.
Development trends in the area of exterior mirrors are approaching the direction of mechatronic solutions, without giving up the concept of a central control module. As the mirror poses a major constraint on the number of wires it can host due to the limited space available, single-chip components are the most effective solutions.
A single-chip integrated circuit offering a complete mirror actuator including a LIN transceiver, eight-bit (ST7) core, memory (EPROM and RAM), timers, analog-to-digital converter (ADC), regulators, and drivers is available on the market and currently being installed on various vehicle platforms. This device uses STMicroelectronics' BCDTM smart-power technology. The system-on-chip approach allows the device to be mounted directly in the mirror housing. Only three wires are sufficient to control all of the required functions: battery, ground, and a dedicated wire for the serial LIN communication.
The car is becoming a sophisticated electronic device. Several factors are influencing the way system complexity has increased:
- system performance;
- quality, reliability and cost; and
- consumer convenience and comfort.
Semiconductors are the enabling technology supporting this revolution in body electronics. Both high power and high-density logic devices are required to replace older, and often less reliable, technologies, such as electromechanical relays. In the end, the semiconductor content in a vehicle is increasing and, at the same time, systems are performing better and becoming more cost effective.
ABOUT THE AUTHORS
Nicola Liporace is technical marketing & business development manager for STMicroelectronics' Car Body Division, Automotive Products Group.
Joseph Notaro (to come).
Giovanni Torrisi is technical marketing & business development senior engineer for STMIcroelectronics' Car Body Division, Automotive Products Group.