Intelligent Power Switches for 24 V Vehicular Systems

Automotive body electronics modules routinely use intelligent power switches to control loads such as lamps, LEDs, solenoids and motors. The trend to replace relays with solid state switches began more than a decade ago when system requirements mandated using solid state switches. The need to PWM the lamps to extend lamp lifetimes, requirements for dimming such as daytime running lamps, the desire to reduce mechanical noise, demands to shrink module size while increasing functionality, the need for accurate diagnostics, etc., all worked against using relays.

Since those early days, the market has exploded with specialized devices designed to meet the many demanding and sometimes unique requirements of this application. Many years of development have produced today’s low cost devices that are efficient, safe, flexible, reliable, robust and fault tolerant. Now, those same advances are being extended to intelligent power switches designed for the more demanding requirements of 24 V systems. Requirements of a solid state switch for 24 V truck and bus systems must consider what we have already learned from the use of solid state switches in 12 V systems. Many of the requirements of 12 and 24 V systems are similar.

Perhaps the primary requirement is low cost. Here, the entire system cost as well as the device cost is of interest. This includes the cost of thermal management, MCU overhead and pin count, PCB area for mounting and routing, additional circuitry needed for diagnostics and fault management, protection components such as capacitors needed to suppress voltage transients, etc. To minimize system costs associated with managing power, the latest devices have very low on-resistances to reduce power dissipation. Additionally, their SPI interface makes many control and diagnostic features possible and reduces MCU overhead and pin count. The SPI interface also greatly reduces routing complexity and saves PCB area.

Next, the system must be safe. Exterior lighting is a critical safety feature, so the intelligent driver must be compatible with the module’s overall fault management and failsafe strategy. The power switches must respond to any failsafe condition and autonomously activate safety critical lamps or other loads.

Flexibility Required

Third, the system must be flexible to accommodate variations in the number, size and types of loads. For example, lighting loads are now often a varying mix of incandescent lamps and LEDs. Lamp and LED power levels, start-up behavior and fault thresholds differ considerably, but the power switch might be required to drive either type. PWMing of incandescent lamps has become quite common, because it allows limiting lamp power at elevated battery voltage, which can greatly increase lamp lifetime. It also allows dimming of lamps for daytime running lights or “theater dimming,” a slow increase or decrease in light output.

Fourth, the power devices must be fault tolerant. The automotive electrical environment is notorious for its number of possible faults and the need to tolerate them. Shorts can be “soft” or “hard.” They can be intermittent. And, the output can be shorted to battery as well as to ground. The battery may be reversed. Many of these faults must be detected and reported, even though the current magnitude of the load might vary with load type and the state of the load.

Fifth, the power switches must be efficient. Better efficiency, of course, is highly desired to improve overall vehicle efficiency. A second and very important benefit of higher device efficiency is that more efficient devices generate less power in their module. Most modules have plastic housings and often the printed circuit board provides the only heatsinking for the power components. Since a control module may require many outputs, it is paramount that each driver minimizes its contribution to the module’s thermal load.

Finally, the system must meet many immutable requirements of automotive body electronics. For example, the vast majority of the loads are ground referenced and must be controlled by a “high side switch”. Some loads must be PWMed at a specific frequency. Ambient temperatures and power density are high while the thermal environment is often poor. Quiescent current must be low to minimize battery drain when the vehicle is parked. Thermal cycling, radiated and conducted emissions and susceptibility, ESD and mechanical stresses are other requirements that have to be considered during device as well as module and system development.

Besides the obvious requirement to manage a higher system voltage and its new set of transients, a 24 V system is likely to have a longer and more severe service life, a greater variety of load types and sizes, more numerous loads and longer wiring harnesses. Each of these is discussed briefly below.

Higher Operating and Transient Voltages

Vehicular voltage transient specifications are more demanding in 24 V systems. Table 1 compares the various ISO7637-2 transients for 12 and 24 V systems. Operating voltage ranges are also given.

Very High Quality and Reliability

Vehicles using 24 V systems are often commercial or heavy duty vehicles. In terms of reliability, mission profile, ambient conditions, etc., the requirements of 24 V systems exceed those of their 12 V counterparts. Table 2 shows that heavy duty vehicles typically have a shorter service life but a much more demanding operating profile.For example, typical requirements for 24 V systems are:

• Lifetime expectancy of over 30,000 hrs or 1.5 million km
• Operating profile for trucks up to 300,000km/year
• Ambient operating temperature range of -40°C to 125°C
• Very low failure rate to minimize service disruption and vehicle down time
• System must be very robust with respect to vehicle transients and load faults
• Components must be robust with respect to wear out mechanisms

More Loads and More Types of Loads

Lighting loads predominate in most automotive body control modules. While 24 V body control modules may control even more lighting loads, they also tend to control many solenoids and motors as well. They also provide power to other modules. In broad terms, here are some of the features of their loads and how they differ from those of 12 V systems:

• Higher nominal power to service a larger vehicle
• Higher inductance of inductive loads
• Systems have more loads than 12 V systems and more of those loads are motors and solenoids
• Motors are often PWMed @ ~1 kHz from 5% to 100% whereas lighting is PWMed at 100 to 200 Hz

Wiring Harness Length

Wire harnesses for a 24 V system are often much longer than in that of a typical passenger vehicle. The load can be up to 20 meters from a module within the vehicle, and up.

24 V Family of Dual, High Side Intelligent Switches

Semiconductor manufacturers are developing intelligent high side switches for the 24 V vehicle market. Table 3 lists the first components in Freescale Semiconductor’s offering.

Each of these devices is housed in the 23 lead, 12 mm by 12mm, PQFN package (Fig. 1). This package was developed for intelligent high side switches and features excellent thermal conductivity, a small footprint and high reliability. Its high pin count allows SPI control, sophisticated fault diagnostics and fault management and failsafe features. These devices have the same pinout and feature set except for those features related to the on-resistance and its associated current and energy capacity. Fig. 2 shows the internal functional block description of this family of intelligent power switches.

Numerous Features

Each device has a vast array of features, which include:

• Dual, low on-resistance, self protected, high side MOSFETs
• 16-bit SPI interface for device configuration, control and diagnostics; 3.3V and 5V compatible
• Configurable to drive incandescent lamps, LEDs, DC motors or solenoids
• Independent control of each channel by any of the following:
  • Internally generated, PWM clock autonomous operating mode
  • External clock-signal, modulated or not
  • Two dedicated direct inputs
  • Fail-safe operation mode, allowing continued basic control if communication with the master device is lost
  • Programmable over current profile to provide protection without avoid false over current faults during lamp inrush
  • Flexible current sensing allows monitoring either output separately in synchronous & asynchronous mode
  • Controlled output-voltage slew-rates (individually programmable) for EMC compliance.
  • Parallel bit to control a higher current load

An example of the programmability is the ability to tailor the overcurrent profile for the expected load. Fig. 3 shows the possible over current profiles that can be chosen for a particular load. Through a hardware pin configuration, the device offers either a static profile suited for loads having an inrush current (like lamps) or a dynamic profile adapted for inductive loads (like DC motors). Any load current excursion beyond the over current profile creates an over current fault and immediately turns off the output thereby reducing stress in the switch, the harness, the supply and the load. An automatic retry is embedded in the device to allow a proper activation of the load even in case of an intermittent short circuit event. This allows minimizing the device’s aging under repetitive overstress conditions to reach 10x better robustness than for standard solutions.

Fewer Components and I/Os

The ability to program the intelligent switch through an SPI communications interface not only gives the systems designer an exhaustive load protection and diagnostics, but it also reduces the number of MCU I/Os and discrete components. This is particularly important for many 24 V vehicle applications, for which more than 20 power outputs are common to control an increasingly vast repetoire of functions as vehicles continue to add safety and convenience features.

Listing the benefits of using an SPI controlled device can be eye opening. As an example, consider a control module that has twenty six 1 to 2A loads. The user can control these loads using 13 dual switches such as Freescale’s MC10XS4200 intelligent switch, or 26 single output drivers in a TO-252 style package. Freescale’s switch is a clear choice.

The MC10XS4200 is a highly versatile 24 V intelligent switch. Fig. 4 shows a typical application circuit for the MC10XS4200 power switch, which can drive a 6 A load per channel. The dual channel, high-side intelligent switch has a complete range of I/O, clock, sync, and other functions to control large lamps and small motors typically encountered in vehicles.

Given that LEDs are increasingly part of the load mix in today’s vehicles, the current sense accuracy requirements are much more demanding. This in turn requires the addition of op amps and sense resistors. Using an SPI enabled device such as the MC10XS4200 power switch brings out most of the functionality within the device. For the system designer, this versatility results in far fewer external components, greatly reduced and simplified routing of control signals, less PCB area, and a more flexible system.

References:

[1] ISO7637-2 Road vehicles — Electrical disturbance by conduction and coupling — Part 2: Vehicles with nominal 12 V or 24 V supply voltage.Electrical transient conduction is along supply lines.

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