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Posted on 17 July 2019

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Next-Generation Enhancements to Automotive MOSFETs

There is a steadily increasing quantity of electric motors in modern vehicles as electric actuation becomes the norm for numerous features ranging from air-conditioning controls and seat and mirror adjusters to headlamp positioning and electric power steering.

By Dr Georges Tchouangue, Principal Engineer, Power Semiconductors, Toshiba Electronics Europe GmbH

 

Although various motor-control architectures and algorithms are employed, depending on the application, final delivery of the calculated PWM signal to the motor is the responsibility of the trusty power bridge circuit comprising four or six power MOSFETs. In automotive motor control applications, the MOSFETs employed typically must offer a number of attributes including small size, high current-handling capability, high reliability, and the ability to withstand many thousands of power cycles. Since MOSFET reliability is related to operating temperature, low-loss device design is imperative to minimise the heating effects of continuous and pulse currents to which the device may be exposed. Enhancements to device design are required at both the package level and the silicon level, to satisfy these demands.

At the package level optimising the characteristics of the device leads and internal ohmic connections help to minimise I2R heating by reducing electrical resistance. Low thermal resistance throughout the leads, connections and overmold is also necessary, to help the device dissipate generated heat as efficiently as possible. As far as the silicon is concerned, lower on-resistance is a perennial goal for device designers, to minimise I2R heating within the die. At the same time, a low input capacitance, Ciss, is highly desirable for automotive MOSFETs. This reduces turn-on energy and allows fast response to control signals. For H-bridge use, the turn off behaviour is not an issue. For three-phase BLDC, the dead time has to be controlled, which means the turn off time has to be fast enough to prevent highand low- side MOSFETs from short circuit conditions.

Next-generation MOSFET silicon

Trench architecture is generally preferred among automotive MOSFETs, to achieve the desired characteristics of low RDS(ON), low input capacitance, low gate charge and high current-handling capability. Compared to trench devices, planar MOSFET technology has historically delivered desirable characteristics such as high avalancheenergy handling and latch-up immunity. More recently, trench technology has been able to approach the ruggedness of planar devices with the advantages of lower on-resistance per unit area, delivering clear advantages for automotive designers.

Trench MOSFETs for automotive applications continue to evolve, as device developers target improvements such as smaller feature sizes to further reduce RDS(ON), gate charge (Qg) and Ciss. In addition, optimising trench width and depth allows higher channel density leading to higher current-carrying capability.

Toshiba U-MOS is a trench technology delivering low-loss and high current-handling performance for automotive applications. The latest U-MOS IV generation achieves a reduction in cell size that simultaneously enables lower RDS(ON) as well as lower Ciss. This has enabled a significant improvement in the RDS(ON) x Ciss figure of merit, translating into overall improvements in reliability, efficiency and switching performance.

Package-Level Innovations

Historically, automotive MOSFETs have used conventional package architectures and materials. To further improve reliability, which is important given the high number of power MOSFETs built into modern vehicles, automotive MOSFET design is incorporating new and higher-performing package features to help minimise the total device on-resistance. Attention is focusing on optimal material selection and dimensioning of leads, and on implementing low-loss interconnections between the leads and the die.

Improvements to the bondwires between the package leads and the MOSFET die enable designers to improve reliability and deliver higher current-handling capability within a given package size. Some technologies, for example, have implemented multiple bondwires per terminal, thereby effectively increasing the cross-sectional area of the interconnection. This has the effect of reducing the overall resistivity of the interconnect, leading to reduced I2R heating.

Later developments at the lead-to-die interconnection have resulted in new packages that feature a copper clamp in place of conventional aluminium bondwires. The clamping mechanism maintains a reliable mechanical connection capable of withstanding repeated power cycling as well as exposure to shock and vibration. With a larger cross-sectional area than a multi-bondwire interconnect, combined with the higher electrical conductivity of the copper material, this design minimises I2R heating due to package losses. Replacing the conventional bondwires with the copper clamp also delivers a reduction in package inductance, which makes a further contribution to reducing heat generation as well as improving noise performance and enabling faster device operation.

To take full advantage of this copper clamp technology, an enlarged source terminal (Figure 1) creates a low-resistance pathway for current entering the device, which translates into a lower source temperature during operation. The enhanced channel structure also improves the package power-dissipation capabilities. Figure 1 illustrates the improvement in package thermal resistance achieved through combining the direct copper clamping structure and wide source lead, highlighting around 20% reduction in channel-to-case thermal resistance.

Package innovation

Combined Strength

Toshiba has combined the latest developments in U-MOS silicon with the enhanced source termination and copper-based lead-to-die clamping to develop its latest family of MOSFETs, which have been optimised for automotive applications. These devices have high current- handling capability, up to 150A, as well as maximum voltage of 75V(VDSS). The trench technology contributes to typical RDS(ON) as low as 1.7m? and typical Ciss down to 4500pF. The robust package design featuring copper connections and the enlarged source terminal has resulted in a predicted lifetime of high power cycles for these devices.

In addition, the package thickness of 3.7mm is 21% thinner than existing TO-220SM (also known as D2PAK) package technology. This improves power dissipation by reducing the die-to-case thermal resistance, and also provides extra opportunities for designers to build smaller control modules that can be mounted nearby the motor being driven. The package is qualified to AEC-Q101 at a channel temperature of 175ºC, and TS16949 approval has also been secured.

The improvements in performance throughout the package and the die have enabled a valuable reduction in electrical losses combined with improved heat dissipation. As a result, the average MOSFET operating temperature is appreciably lower, as the comparison in figure 2 illustrates.

New technology delivers thermal resistance improvement

The graph shown compares the operating temperature as measured at the drain, package surface and source lead of automotive trench MOSFETs in the standard TO-220SM/D2PAK package and the TO- 220SM(W) WARP package. The wider terminal of the WARP package results in a significantly lower source temperature, and also influences the temperature measured at the MOSFET body. The temperature curves demonstrate how the latest package and process technologies achieve almost a two-fold increase in current rating within the industry-standard TO-220 footprint.

Reducing MOSFET source temperature

Conclusion

Environmental concerns are changing buyers’ expectations of cars and the automotive industry. One constant, however, will be the demand for continued improvement in performance, economy, comfort and value. The steadily increasing number of electrical systems built into modern vehicles has delivered clear progress toward all of these goals. Their success is partly due to improved motor types and new control techniques, but continuous improvement in power-electronic technology is critical to meeting all of the demands placed on modern vehicle electrical systems.

The latest generations of trench MOSFETs, incorporating improvements to silicon and package construction, allow designers to deliver extra functions, higher performance and increased reliability while also achieving valuable size and cost savings.

 

 

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