Power Demands Drive Advances In Automotive Applications

Bill Grant, Steve Clemente, Jim Tompkins, and Anthony Murray analyse the growing importance of power MOSFETs for car manufacturers.

Increased demand for electrical power in new and emerging automotive applications are forcing today's car makers to challenge established power control and conversion techniques. This is driving rapid advances in power semiconductor and packaging technologies for automotive applications, not least in the area of the power MOSFET.

Compared to general-purpose power semiconductors, those destined for automotive applications typically face more stringent demands on voltage and current handling capability, switching frequency, power loss, dynamic characteristics and protection. There are also numerous requirements for DC-DC conversions, especially as the industry migrates from 14V to 42V operation. X-by-wire applications such as brake-by-wire and steer-by-wire are also driving demand for innovations in automotive power electronics.

VEHICLE REVOLUTION
At the hub of the electric vehicle revolution, the Integrated Starter Alternator (ISA) represents a fully reversible electrical machine that enables a host of efficient electronic functions with high peak-power requirements. Firstly, it allows the engine to be cut off — while the car is sitting at traffic lights, for example — as the alternator is capable of making the vehicle jump when the driver presses the accelerator. This reduces emissions when they are at their most noxious and improves fuel economy.

The ISA is also crucial in implementing electronic power steering (EPS) and the X-by-wire functions, as well as active suspension, electronic turbo assist, electronic valve control and variable-speed air conditioning, among others.

The technological challenges of creating an ISA centre on the design of the power electronic control section. This is typically a 3-phase inverter/rectifier responsible for supplying power to the 42V loads, and charging the 36V battery in rectifier mode. Inverter action is required for supplying power to the starter motor while starting the engine. To provide a suitable power section, a number of specific requirements for appropriate power electronic devices can be identified. Figure 1 shows a sample schematic.

First, current-carrying capability must be high enough to allow the ISA to satisfy the highest loads that will be imposed. Typically, this is cold starting of the combustion engine at low ambient temperatures. Most carmakers believe a 10kW ISA will be sufficient. For a 10kW system working with a 42V power scheme, this implies that the peak current-carrying capability of each switch in the inverter must be in the region of 400A.

It is also worth noting that although restarting a hot engine draws less current, the current rating of a power MOSFET also decreases with increasing temperature, making adequate cooling for the ISA power section essential. Also, to maximise the current rating, a MOSFET technology with low RDS(ON) is necessary.

Next, scalability to meet the needs of larger or smaller vehicles is also a desirable characteristic of a suitable ISA power section.

A low forward voltage drop boosts efficiency of generator operation. Low parasitic inductances are needed to minimise over-voltage peaks caused by switching. Since significant inductance is associated with device packaging, many consider a module solution to be optimal. However, as the module package can limit current-carrying capability, some developers favour a discrete implementation while others consider advanced packaging solutions to maximise current capability and minimise parasitic inductances.

Other requirements include switching behaviour optimised for the target switching frequency, typically several kilohertz. The ISA inverter/rectifier must also be capable of delivering full power at very high efficiency, at ambient temperatures up to 150°C.

Finally, overall cost is arguably of greatest importance. Although continuous performance improvement has characterised the auto industry since its inception, cost continues to be the dominant influence. Improvements will only be accepted if they come at little or no price premium.

MODULAR SOLUTION
The module shown in Figure 2 was designed for a 42V mild hybrid vehicle, where the combustion engine provides the major share of motive power. It is a half-bridge built with two large power MOSFET dice (150mm2 each) capable of switching 600A on a 42V bus. To reduce thermal stresses, the FETs are mounted on a ceramic substrate with a temperature coefficient matching that of silicon perfectly.

Advanced wirebonding techniques ensure that the module can withstand temperature-cycling and power-cycling stresses normally associated with automotive environments. The power leads are designed to mate perfectly with a laminated buss bar to minimise stray inductance, which is less than 8nH.

To minimise EMI susceptibility, the gate drive, sensing and protection circuitry is mounted on a small PCB that covers the module. The gate drive circuitry is capable of driving these FETs at 20kHz. Sensing and translation of these parameters is also performed on this small board.

POWER MOSFETS FOR EPS
Although debate continues over the ideal layout and execution of a suitable power inverter/rectifier, it is clear that the 42V ISA is a key enabler of future electrical automotive systems such as EPS. Here, however, the current carrying capability of discrete plastic MOSFET packages again limits performance.

For example, the standard 3-lead D2Pak will carry between 75A and 100A, depending on the lead cross section and the supplier's specification method. For a mid-market EPS system, this limitation will force designers to use multiple discrete devices in parallel, or take a module approach.

Increasing the current-carrying capability of the discrete package will enable system costs for medium and lower power EPS systems to be reduced. Increasing the cross-sectional area of the source lead relative to the die size is one solution. On the other hand, increasing the lead count in the standard D2Pak footprint distributes the current across five leads in parallel rather than just one.

This approach has the advantage of reducing the lead temperature at the board level, resulting in greater reliability of solder joints and effectively doubling the current-handling capability of a standard 3-pin package. Furthermore, the extra leads allow a larger leadframe T-post inside the package, resulting in additional wirebond area capability and the added benefit of reducing the die free package resistance by more than 50% compared to the standard package. This in turn leads to a reduction of RDS(ON).

DIE-ON-LEADFRAME
The silicon power MOSFET devices in an EPS power inverter are typically the most expensive components, closely followed by the other primary components of the package, including substrate, leadframe and housing. Package improvements, particularly those that reduce thermal and electrical resistance, allow smaller MOSFETs, with higher RDS(ON), to reduce the overall cost of the module. Conversely, for the same silicon, lower losses and greater efficiency can be achieved.

Most automotive modules today use Insulated Metal Substrate (IMS), Direct Bonded Copper to a ceramic substrate (DBC), thick film substrates or PCB-based modules in lower current applications. A new concept in power inverter packaging in this medium-to-low power range is die-on-leadframe (DOL).

In this case, there is no separate substrate: the MOSFETs are mounted directly onto the same insert moulded leadframe used to make the terminals for external connection, which are continued into the housing of the module. Figure 3 compares a DOL approach (a) with a more conventional approach using die on DBC (b).

DOL eliminates the substrate without adding an equivalent cost to the housing or leadframe. Typically, a PCB containing the primary system control and gate drive circuitry is located in close proximity, resulting in an extremely cost-effective approach to the system design.

To gain the maximum performance from DOL modules, which are not electrically insulated and therefore do not have the disadvantage of higher thermal and electrical resistance, a highly filled silicone adhesive is used to secure the bottom surface of the module to the heatsink or baseplate. These materials provide both good thermal transfer and electrical insulation, but require additional measures to minimise and control the thickness of the material deposited.

CONCLUSION
MOSFETs are the device of choice for power control duties in next-generation 42V-based automotive electrical systems. However, these devices must adapt to the automotive environment to support the functionality, efficiency and reliability the market will expect. Power semiconductor manufacturers are meeting these challenges with new device and package technologies.

These will allow major vehicle components, such as valve control and steering systems, to be replaced with next-generation electrical equivalents, saving cost, weight and fuel consumption, and providing car makers with the flexibility they need to investigate new concepts in personal transport.

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