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Design Talk: Mechatronics

Thu, 02/19/2009 - 8:04am

Mechatronics and Assisted Living

By Louise Merriman and Charles Watts, Fairchild Semiconductor, www.fairchildsemi.com

ec93dt100aec93dt100bMore than 50 million people in the United States have disabilities, a number that is growing rapidly as the population ages. This growth has sparked renewed interest in developing more sophisticated assistive technologies to enable an oftentimes disabling environment. This growth presents semiconductor suppliers with an opportunity to enable this field with innovative, highly efficient solutions, which means being able to travel further, workout longer, and use lighter and more powerful aids. For example, power assisted wheelchairs at one time were limited on how long they could go on one charge. Due to recent technological improvements, battery life has been extended and this has opened up new possibilities for people with disabilities.

All battery technologies require specific charge and discharge rates to achieve long life cycles. So a battery management system (BMS) is needed. A BMS must provide full cell protection to cover almost any eventuality. Operating a battery outside of its specified design limits will inevitably lead to failure of the battery. This is particularly true in high power applications where batteries must operate in hostile environments and which at the same time are subject to abuse by the user. So the BMS manages the rechargeable battery (cell or battery pack) by monitoring its state, calculating secondary data, reporting that data to an external source, protecting it, controlling its environment, and/or balancing it. Semiconductors play an important role in meeting these requirements.

ec93dt100cLeveraging the tremendous advancements in silicon and in packaging, semiconductor solutions can bring assistive technologies a level of sophistication that they never had before. More efficient Buck regulators and MOSFETs not only make products more efficient, but simplify the actual thermal design. This innovation can lead to a significant reduction in the mechanical constructs required for cooling the application and ultimately decreasing the overall system cost. This, in turn, leads to ergonomic improvements in assistive technology designs, making the device more appealing from a user standpoint. This simplified design and elimination of extra components leads to cost reductions in the end applications and hence, makes these devices more accessible to the disability community.

Assistive devices are moving from standard DC motors to Brushless DC motors (BLDC). BLDC motors are the new high efficiency standard in motors. As the name implies, brushless DC motors do not use brushes for commutation; instead, they are electronically commutated. They offer compelling advantages including higher speed ranges, noiseless operation, long operating, high efficiency, high dynamic response and better speed versus torque characteristics.

To facilitate design in these applications, semiconductor suppliers are providing assistive device manufacturers with discrete or integrated and modular solutions to drive these motors. Modular solutions offer the value of integrating functionality into one package or one die to drive these highly efficient motors. Semiconductor suppliers that have both advanced packaging and MOSFET technologies can provide true advantages in extending battery life in assistive device applications. A prime example of this are N-Channel, 60V to 100V MOSFETs fashioned with PowerTrench® technology that offer both low gate charge and low RDS(ON). The required MOSFET voltage rating depends on the battery voltage used, which ranges from 12V to 48V.

With assistive device application frequencies at 20 kHz, a low RDS(ON) power MOSFET is used to reduce power consumption, extending the battery life. In fact, MOSFETs are switched in parallel in these applications to further reduce conduction losses. Switching low gate charge MOSFETs in parallel is more difficult than switching higher gate charge MOSFETs. As the switching frequency is only 20kHz, problems with extra switching losses are less severe to address than the problems with parallel switching and with the higher EMI generated by faster switching.

In BLDC motors, the MOSFETs are configured in a half-bridge configuration, often with a number of MOSFETs in parallel on both the high and the low sides. So a high and low side driver is needed to drive switches. The FAN7390 highly integrated drivers have high gate drive (4.5A sink/5A source) needed for switching high and low side MOSFETs in parallel. The drivers are robust, improving system reliability in assistive drive applications. First, they are highly immune to dv/dt-induced disturbance caused by the fast MOSFET switching. Second, they can tolerate a high level of negative voltage on the pin connected to the mid-point of the half bridge, which often presents difficulties when driving half bridges.

For lower output loads, TinyBuck Buck regulators offer a completely integrated solution as they integrate the PWM controller IC and power FETs in a single multi-chip module, from 3 to 8A in a 5mm x 6mm MLP package, hence offering highest efficiency for the smallest form factor. The small internal dimensions of the power path combined with the low package and interconnection inductance result in reduced EMI, further improving system reliability for assistive drives.

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