Over the last 15 to 20 years, Automotive Power MOSFETs have evolved from a rather plain topic into a dynamic business. Power MOSFETs were selected based on their ability to tolerate the high-voltage transients such as load dump and field decay experienced in vehicle electrical systems. Packaging was straightforward and utilized mostly TO220 and TO247 leaded packages. Applications such as electric windows, fuel injection, intermittent wipers and cruise control were becoming standard on most automobiles and required similar power devices in their design. During this period, Automotive Power MOSFETs proliferated with the growing use of electric motors, solenoids and fuel injectors.
Today's automotive systems have ushered in a new age for power devices. This article explores several of the new applications driving a revolution in power devices within the automobile. It also looks at some of the developments in power MOSFETs enabling today's automotive systems. These developments will have an accelerating impact on the progress of automotive electronics, particularly in emerging markets such as China.
There are four forces in the new automotive applications driving this evolution in Automotive Power devices, including:
Required stand-off voltage (BVDSS)
System power requirements
Product and system cost
Historically, power devices of approximately 60V BVDSS were used in automotive applications. However, new systems use both higher and lower voltages to more cost-effectively deliver system performance not previously available.
Two applications gaining popularity are High Pressure Piezo and Magnetic Electric Injection Systems, and High Intensity Lighting. These applications require Power MOSFETs with breakdown capability as high as 150 to 300V. The higher potential piezo and magnetic electric fuel-injection systems can deliver better performance by providing more accurate injection of the fuel and a finer air/fuel mixture, allowing improved combustion with fewer emissions.
The high-intensity lighting generates a brighter light with less energy than standard incandescent lights. This improves visibility at night or in poor weather conditions. It also enhances the ability of other drivers to see the vehicle.
In addition to the higher voltage automotive power applications, the explosion of consumer applications moving into the vehicle have pushed the low end of the voltage spectrum. CD/DVD players, satellite radio, cell phones, GPS navigation and MP3 player interfaces are several of the newer applications requiring power MOSFET devices. The required power used in these applications is typically lower, using smaller discrete, or possibly integrated power devices. They have the same characteristics as the consumer products they are based upon. Size is very important in these applications, and newer power devices utilizing smaller PC board package footprints and surface mount assembly are standard. Packages such as the Power 56 and the Super SOT SSOT 6 are commonly used for these applications offering considerable power handing capability in very small packages.
Many automotive systems, which were traditionally mechanical or hydraulic, are converting to electrical or electrical/ hydraulic systems. One of the first to change over was the radiator fan. Through the use of an electric motor, the fan belt can be eliminated and the control of the fan can be tied to the actual engine or coolant temperature more accurately. Other similar applications are electric power steering (EPS), integrated starter alternator (IAS) and active suspension systems. Electric power steering and active suspension systems give the automotive system designer the flexibility of using a single hardware system on multiple vehicle platforms, while modifying the feel of the vehicle from sporty to luxury through software modifications.
One characteristic of these electromechanical systems is the use of extremely high power levels. These high-energy systems require high-current power switches. In order to provide the highest current switching at lower losses, these applications typically use high-performance trench MOSFETs with 30 to 40V ratings.
Power MOSFET Solutions
The use of trench MOSFETs has become the standard for most of today's automotive applications. Historically, planar MOSFETs were built on the surface of the silicon wafer. Trench MOSFETs etch vertical trenches into the silicon allowing higher cell density and lower on resistance for the power switches. Since most of these electromechanical systems utilize MOSFET H bridge motor drive configurations, two devices are always in series, thus allowing the use of lower voltage MOSFETs, while still withstanding the traditional high-voltage automotive transients. These lower voltage breakdown devices can offer up to a 50 percent reduction in the switch on resistance as compared to 60V MOSFETs. This translates directly into a 50 percent reduction in system power dissipation, reducing system heat and minimizing heat sinking requirements.
As automotive system designers gain more experience in working with these lower voltage power MOSFETs and begin to realize the performance and cost advantages to lower voltage power MOSFETs, their use is expanding into other lower power systems such as braking and control of displays.
Today's power trench MOSFETs have switch on resistance (RDS(ON)) down to 1 or 2 milliohms. This reduces system power dissipation dramatically, but adds another complexity for the automotive system designer. It is now probable that the interconnect resistance, including the board interconnection, system wiring and even the bond wires in the package, add more resistance to the system than the actual MOSFET. One way to drive continual reduction in on resistance and get improved power density is the use of hybrid modules. Many applications are moving away from the traditional power package solutions to use bare die mounted on insulating substrates made from Insulated Metal Substrate (IMS) or Direct Bond Copper (DBC) material. These modules offer higher energy and higher current capability, as compared to discrete power packages even when using the same power silicon die. Modules offer higher density bonding or larger wire bonding of the die, lowering the connection resistance, while also minimizing the distance between power components. This higher-density energy capability comes at an increased component cost over discrete package alternatives. However, system size and performance improvements in high-energy systems more than compensate for this increased device cost.
Another direction automotive power MOSFETs is taking is the increased ability to sense, control and protect the power switch. Power devices are being integrated with the intelligence of the automotive system. At the lowest level, MOSFETs are now available with sense elements included on the power device. These sense elements can measure current or temperature and be connected to electronics to monitor the system performance and protect the power device in the event of an over current or temperature condition.
Low power devices in the 30 to 60V range are being integrated into monolithic ICs, including serial interfaces and microcontrollers. This single, application-specific IC can control a small motor, or possibly an entire door node, with motors and locks. For higher energy applications, where the cost or technology of a monolithic IC is not practical, integration can occur through the use of innovative packaging solutions. Through the combination of high-power MOSFETs with control integrated circuits contained within a single package, very high power intelligent systems are being built.
These intelligent devices offer improved monitoring of system performance and increase the reliability of the power system through integrated protection. Features like over current, over voltage and over temperature protection are standard on these types of products. When the device senses the presence of one of these potentially catastrophic conditions, it can put the power MOSFET into a condition to self protect the entire system. Additional diagnostic features that monitor for open or shorted loads and help direct auto mechanics to isolate and correct issues in the vehicle can be integrated.
The last, and perhaps most important driving force behind all of these applications, and why many of these technologies are available, is product and system cost. In the automotive business, there is a constant push to reduce product and system cost. This is not only a push on the component cost, but the entire cost of ownership of a vehicle. For this article, reliability is also included as a cost driver. A low-cost power device that leads to line or field failures is not a low-cost device. When making decisions on components to be utilized in an automobile, the system designer must take cost and reliability as one major driving force. The products discussed in this article are being designed specifically for automotive applications and systems, and are characterized and qualified for automotive end use. The automotive market has established several product qualification standards for power and intelligent power devices. These are known as the AECQ100 and AECQ101 standards. Products developed and offered to the automotive market must be designed and characterized to meet these tough standards to assure the end system performance will meet the designer's and, more importantly, the vehicle owner's expectations for product value.
The breadth of power devices and design considerations in today's automobile has come a long way over the years from the 60 or 55V alternative that used to be one of the only design questions. With the proliferation of electronic systems for entertainment, instrumentation, power train control, safety, chassis and stability controls, and body and convenience controls, the number of power devices in the typical automobile is in the hundreds and growing. Selecting the right device is now a complex challenge with many technical options available to meet the desired performance and cost objectives.