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August 17, 1998

Automotive power semis rev up to replace mechanical devices

Solid-state switches and ICs are meeting cost and performance goals in winning the race against traditional electromechanical devices.

Bill Travis, Senior Technical Editor

The value of electronic devices in a car's sticker price is steadily rising. Discrete semiconductors and ICs are well on their way to totally replacing electromechanical devices in automobiles. Solid-state switches—insulated-gate bipolar transistors (IGBTs), bipolar transistors, and MOSFETS—are, more and more, assuming the character of ICs, incorporating "smart" control and diagnostic circuitry that can communicate with J1850 and CAN protocols, for example. This report describes the special attributes of several automotive power devices. ("Power" here does not denote computing power, which abounds in engine-control µCs, for example.)

Table 1 gives some, but not all, of the applications for power semiconductors in cars. It's easy to think of many more—cruise control, power-seat controllers, Global Positioning System (GPS) components, and entertainment-system power devices, for example. In many of the applications in Table 1, the solid-state devices replace electromechanical switches and controllers used in the past. The advantages of solid-state devices are clear: theoretically infinite lifetime as opposed to the inevitable wearout of mechanical contacts, lower power consumption, less weight and volume, and more "smart" features. However, as the sidebar, "Want to deal with Detroit? Be prepared," points out, the solid-state devices need special attributes to satisfy car makers. Most notably, these devices must be cost-competitive with the electromechanical devices they strive to replace.

It's a harsh, harsh world

The electrical systems in a car teem with undesirable transients and failure-provoking mechanisms. Table 2, derived from Harris Semiconductor's home page, www.semi.harris.com/auto/autotvs/solution.htm, shows some typical transients that can crop up in an automobile. You can see that some of the transients—for example, accessory noise and coupled ignition noise—are benign. However, others are potentially lethal to electronic devices. Designers of automotive systems must either specify and use switches, controllers, and power ICs that can survive these conditions or else install protective devices that prevent the transients from attacking the switches and ICs.

To protect sensitive electronic devices from lethal voltage transients, Harris Semiconductor offers a line of transient-suppression devices using two technologies: MOV (metal-oxide varistor) and multilayer ceramic. The MOVs are through-hole, radial-leaded units; the multilayer suppressors are surface-mount chips. The surface-mount chips come in sizes from 0603 to 2220. The MOVs can absorb transient energy levels from 2 to 160J; the multilayer suppressors, from 0.1 to 25J. For overcurrent protection, auto makers use traditional one-time fuses and, more recently, resettable (more accurately, self-resetting) polymer fuses, developed by Raychem Corp (www.raychem.com) and now available from several manufacturers.

To cope with the demanding voltage, current, and protection requirements of ICs used in automobiles, semiconductor manufacturers have developed several robust processes. STMicroelectronics (ST, formerly SGS-Thomson), for example, fulfills Detroit's needs using four process families: BCD (bipolar-CMOS-DMOS), VI Power (vertical intelligent power), bipolar, and ASD (application-specific discrete). Figure 1 shows a typical BCD circuit. The L9822E is an octal, low-side driver suitable for driving lamps, relays, and solenoids. It has eight 1A DMOS outputs with 36V transient clamping. The driver accepts 8-bit serial data using the serial-peripheral-interface (SPI) protocol. The driver offers a serial diagnostic output that signals overload and open-circuit conditions.

The L9822E is a low-side switch; it switches a load that's connected to the 12V battery line. Many auto applications, however, mandate high-side switching; in other words, switching a load that's connected to ground (chassis). ST's VI Power family provides high-side switching using n-channel power MOSFETs as output devices. Figure 2 shows a typical VI Power device. The VN 610 is a single-channel, high-side solid-state relay designed to switch grounded resistive or inductive loads. It provides 45A with 12-mOhm on-resistance and handles 36V on the battery line. The IC is typical of circuits used in cars. Its protection features include an overvoltage clamp, thermal shutdown, undervoltage and overvoltage shutdown, and current limiting. The switch provides an analog current-sense output that furnishes a current proportional to the load current.

Like most IC-based and discrete switches using n-channel MOSFETs, the VN 610 relies on charge-pump circuitry to generate the gate-source voltage needed to turn on the FET. The use of p-channel MOSFETs would eliminate the need for such circuit tricks, but an unlucky law of nature dictates that, for equivalent voltage, current, and on-resistance ratings, ap-channel MOSFET requires approximately three times the silicon area of its n-channel counterpart. So, it's much more economical to use a small-area charge pump to control an n-channel FET. The VN 610 is a single-channel VI Power device. It has several siblings of increased complexity, including the dual-channel VN 600, the dual-output, bridge-configuration VN 670, and the quad-output, full-H-bridge VN 770 and VN 771.

Bipolar ICs continue tradition

MOSFETs make ideal switches. For linear applications, however, bipolar ICs offer many attractive features. In car radios, for example, a couple of npn transistors connected in a push-pull bridge configuration can provide low-distortion drive to a 4 Ohm speaker. In a bridge connection, each npn sees a 2 Ohm equivalent load. Two car-radio amplifiers from ST typify bipolar automotive ICs. The TDA7396 provides power as high as 60W to a 4 Ohm speaker. The TDA7385 offers four channels of 22W each. The ICs provide diagnostic feedback for clipping and output short circuit (to ground or battery) and include thermal shutdown. The amplifiers are designed to survive highly inductive loads and load-dump conditions. Another appropriate area for bipolar technology is in voltage regulators, in which a pnp pass transistor provides low-dropout performance. ST's L4938, for example, is a dual, multifunction regulator that supplies 5V µC systems in automobiles.

Another appropriate application for bipolar devices is in switching the ignition coil. An IGBT, which is a cross between MOSFET and bipolar technology, also provides efficient ignition-coil switching. ST uses a Darlington output structure in its VI Power ignition switches (Figure 3). The VB 027 and VB 130 coil drivers use logic-level inputs and offer programmable coil-current limiting and collector-voltage clamping. A discrete IGBT used for coil switching uses less drive power and offers lower conduction losses than does the bipolar Darlington configuration, but the IGBT is not amenable to the levels of integration in VB circuits, which provide several smart features, such as current and voltage flags.

The remaining member of ST's automotive family is a line of ASDs. This family combines various semiconductors with integrated resistors and capacitors to provide complete functions. The process uses lateral and vertical current flow and can handle voltages to 1300V and current densities as high as 100A/mm (in 1-msec pulses). Semiconductor cells in the ASD library include diodes, pnp and npn transistors, and plain and gate-turnoff thyristors. Protection devices with either fixed or programmable overvoltage thresholds are also available. The software tools available for ASDs include a cell and macrocell library, a stimuli and waveform generator, design rules, and component-layout libraries.

Packaging is a crucial consideration in the design of automotive power devices. ST, the inventor of the Multiwatt (the "double TO-220'') power package in the late 1970s, has introduced a new surface-mount power package, the PowerSO-20. The package incorporates a massive internal slug, which provides the same thermal impedance as that of traditional insertion packages in a small surface-mount format. The family, whose JEDEC-registration number is MO-166, includes packages with lead counts of 20 to 36. Lead pitches of 1.27, 1.0, 0.8, and 0.65 mm are available. An application note from ST goes into considerable detail about the thermal characteristics of the new package and gives mounting and heat-sinking advice for circuits with various power levels (Reference 1).

Harris Semiconductor also has a large portfolio of automotive ICs using a variety of technologies (bipolar, BiCMOS, DMOS, dielectric isolation, MOSFET). As an example, Figure 4 shows an IGBT-based coil-switching circuit. The circuit uses the CA3274 ignition predriver, a member of Harris' engine-control/ignition family of ICs. Other members include fuel-injection controllers, MOSFET-output drivers, voltage regulators, protection circuits, and an accelerate-by-wire chip set. Harris also produces IC families for air-bag deployment, cruise-control management, antilock-brake systems, electric-seat and -window control, the instrument cluster, transmission control, security functions, and radio receivers.

Cherry Semiconductor's BiCMOS process, PowerSense, combines the robustness of bipolar with the dense logic capability of CMOS and the power capabilities of DMOS. Cherry produces an extensive line of automotive ICs using both pure-bipolar and BiCMOS processes. The CS-1107 (single), 1108 (single), and 1109 (dual) drivers, for example, use bipolar Darlington outputs to provide 350-mA drive for relays and lamps. The drivers include overtemperature shutdown and fault-reporting diagnostic pins. The devices go into PWM mode during overcurrent conditions, thus limiting power by restricting the output duty cycle.

Cherry uses its bipolar expertise to good advantage in a range of linear regulators for automotive applications. A series of low-dropout (600-mV) regulators, for example, uses pnp pass transistors to provide 5 or 12V outputs at currents of 50 to 750 mA. The regulators incorporate overtemperature shutdown and protection against reverse-battery and load-dump conditions. High-current regulators come in a five-lead TO-220 package; devices with lower current ratings use narrow- or wide-body SOIC packages.

One interesting application for automotive ICs is air-bag deployment. Cherry's CS-2082, for example, contains two independent firing-squib drivers: one low side and one high side. As are most automotive ICs, the 2082 is full of smart features, such as a charge pump for its MOS outputs, squib-resistance measurements, continuous short-to-ground and -battery measurements, power-on reset, overtemperature protection, 60V load-dump immunity, a monitor to ensure firing potential, and an analog multiplexer. The CS-2092 is a dual version of the 2082, integrating four squib drivers as well as all the other amenities of the 2082.

Temic Semiconductors also offers a range of air-bag-interface ICs. Like the Cherry devices, Temic's circuits partition the squib-firing stages in high- and low-side sections. Thus, a malfunction or short-circuit condition in one switch is without effect. Temic's air-bag controllers include monitoring and diagnostics and come in versions that handle two front air bags (bipolar ICs) and two front plus two side air bags (a two-chip BCDMOS set).

Silicon replaces heavy metal

The advantages of solid-state switches over their electromechanical counterparts are indisputable. The coil-and-metal-contact parts are subject to inevitable wearout, they're subject to environmental contamination, they're heavy and bulky, and they take considerable drive power. Their two possible advantages over solid-state devices are low cost and high transient and overload immunity. Solid-state switches, however, are becoming more cost-competitive, and various built-in and external protective measures ensure their survival in harsh environments.

International Rectifier (IR) offers an array of power MOSFETs and IGBTs for the automotive market. Traditionally, the MOSFETs (dubbed HEXFETs by IR) needed 55V ratings to survive load-dump conditions. Car makers are now incorporating a zener diode in the alternator to reduce load-dump transients. Now, the use of 40V MOSFETs in many automotive subsystems allows increased power efficiency (because of lower on-resistance) or a reduction in device cost (by using less silicon). IR offers a range of 40V MOSFETs with on-resistance of 6, 8, or 9 mOhm and with standard or logic-level input drive. The 6-mOhm IRL1004 handles currents as high as 110A; the 9-mOhm IRF1104 is rated at 100A. Both devices can operate to 175ºC junction temperature.

IR has customized a Web site for design engineers who need detailed information on automotive-system design requirements. The Web site, www.irf.com/auto, focuses on design approaches for various automotive systems, such as high-intensity-discharge lighting, electronic power steering, and fractional-horsepower motor drive. You'll find schematic diagrams; suggestions for cost and size reduction; and IR's entire library of data sheets, application notes, and design tips. The Web site contains detailed designs for the following automotive applications:

• High-intensity-discharge lighting
• Air-bag deployment
• Controller-area-network or J1850 interface
• Fractional-horsepower motor drive
• Climate-control system
• Electronic engine control
• Transmission control
• Alternator duty-cycle control
• Diesel-engine control
• Electronic power steering
• Antilock-brake systems
• Traction-control systems
• Cruise control
• Active suspension

Recent MOSFETs from Vishay-Siliconix feature the 60V rating needed for unprotected circuits in an automotive environment. The single p-channel Si9407AEY, the dual n-channel Si9945AEY, and the dual p-channel Si9948AEY come in SO-8 packages (dubbed Little Foot). On-resistance ranges from 80 to 170 mOhm, and 4.5V logic-level signals drive the gates. Two new D²PAK-encased devices also use 4.5V logic-level drive. The n-channel SUD15N06-90L specs 90-mOhm on-resistance; the p-channel SUD10P6-280L specs 280 mOhm.

Temic and Toshiba are using multimillion-cell technology to produce MOSFETs with unprecedented low on-resistance. Temic's SUP75N03-04 (TO-220) and SUB75N03-04 (D²PAK) spec 4-mOhm on-resistance and handle 75A currents. Note that these devices target 30V automotive applications that have transient-clamping protection. Toshiba's trench-technology S2J80 operates at 60V and offers 4.4-mOhm on-resistance at 45A and 10V gate-source drive. At 4V gate-source drive, the on-resistance increases marginally to 5.6 mOhm. Powerex also uses trench-gate technology to attain low on-resistance values (for example, 7.5 mOhm in a 60V device).

Philips Semiconductors serves the automotive-electronics market with thyristors, triacs, rectifiers, and power MOSFETs. More than 100 low-on-resistance devices are available in the company's TrenchMOS family. The recently developed TOPFET2 (temperature-overload-protected field-effect transistor) family combines TrenchMOS processing with TOPFET protection features to provide high current handling with complete overload protection.

Thanks to progress in electronic devices, it's not your grandfather's car anymore. No more replacing distributor points (and the "condenser") with annoying frequency; no more tinkering with the carburetor to adjust the idling or fuel/air mixture. Automotive power devices have made cars more reliable, more fuel-efficient, and more pleasurable to drive. Now, if only car prices would follow computer curves.

Reference

1. Casati, P, and C Cognetti, "A New High Power IC Surface Mount Package: Power SO-20 Power IC Packaging from Insertion to Surface Mounting," STMicroelectronics, application note.

Want to deal with Detroit? Be prepared

Designing and producing devices for auto makers' electronic systems pose special challenges. You have to deal with issues of specifications, quality, delivery, and—especially—cost that, taken as an ensemble, prevail in no other industry sector. The difficulties notwithstanding, the allure of vast quantities is irresistible for many suppliers who manage to successfully fulfill the industry's rigorous demands. Most suppliers to the automotive industry have dedicated account managers who juggle the technical, scheduling, and myriad other aspects of dealing with the auto makers.

Most of the auto makers have skilled system and circuit designers who create the specs and, sometimes, complete designs, for vendors to bid on. Often, for complex ICs, the designers create complete schematic- and chip-level designs and treat vendors simply as foundries. Other times, the auto makers gain the rights to a circuit design, for which they pay nonrecurring engineering (NRE) charges to a vendor and then put the schematic and chip design out for bid. Vendors that apply no NRE charge for projects can negotiate to keep the rights to their intellectual property in some cases. For simpler designs, the auto makers often request quotes on a black-box or "gray-box" basis, by specifying I/O specs, by issuing a block diagram, or by doing both.

Meeting the auto makers' specs and quality requirements is not easy. In many ways, both aspects are more rigorous than military requirements but at a subcommercial price. Specs are often brutal, entailing several kinds of protection against overvoltage, overcurrent, and reversed polarity. Temperature specs often far surpass the relatively wimpy 125ºC military limit, reaching 150 or 175ºC, for example. American car makers almost universally require QS9000 quality-control certification, a process that's more rigorous than that of the ubiquitous ISO9000 certification. Acquiring QS9000 certification is an expensive proposition.

The auto makers also often cut costs by dispensing with incoming inspection and putting received parts directly into production. They instead impose tight, "parts-per-million" quality specs on their vendors. Just-in-time shipping joins the other special challenges of dealing with the car makers. Because of fluctuating production schedules, the auto producers require vendors to maintain a buffer inventory at the vendors' expense. The penalty for failing to meet technical, quality, and delivery requirements is simple: no more business. This carrot-and-stick arrangement is an effective one; no vendor wants to lose out on million-quantity orders.

Bill Travis, Senior Technical Editor

You can reach Senior Technical Editor Bill Travis at 1-617-558-4471, fax 1-617-558-4470, b.travis@cahners.com.

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