Machine Design

Power Control

Power control is provided by high-power devices and their drive circuits. A variety of power devices are used, including bipolar and MOSFET transistors, insulated-gate bipolar transistors (IGBTs), and MOS-controlled thyristors. MOS-gated devices increasingly are favored. The ability to switch both high voltage and high current is the trademark of smart-power technology.

Drive circuits, on the other hand, are typically made using a high-voltage CMOS process. Operating at up to 30 V, they provide sufficient gate voltage to control the power devices. To accommodate totem-pole configurations, where two or more devices are connected in series, drive circuits must also be capable of level shifting to higher voltages.

Sensing and protection is also important. In addition to detecting excessive current, voltage, and temperature, many power ICs also sense when a load has been disconnected and when load voltage falls below a safe level. Undervoltage warnings can prevent extreme power dissipation during load start-up by properly biasing the power devices. Current sensing is done with minimum power loss by partitioning a few cells from the power device and feeding the small current to the control circuit.

Feedback loops typically contain high-frequency bipolar transistors. Fast response is critical for a benign shutdown because system current increases rapidly during a fault. To maintain speed and accuracy, this portion of the chip uses high-performance analog circuits.

Interface functions are typically provided by high-density CMOS logic circuits. These circuits perform encode and decode operations to communicate with microprocessors and other logic components. Not only do smart-power ICs respond to microcontrollers, they also send messages regarding operating status, load monitoring, and diagnostics. Messages may include overtemperature shutdown requests and no-load or short-circuit warnings.

Intelligent power is expected to have the greatest impact on applications where solid-state switches are already used. Here, loads range from 10 to 10,000 V and current from a few milliamps to several hundred amperes. On one end of the spectrum are display drives that generate less than 0.1 A at 100 V or so. Because displays are typically linked to microprocessors, these applications are ideal for power ICs.

At the high end are vehicle traction and high-voltage dc (HVDC) transmission systems, where voltage requirements exceed 1,000 V and switching currents are several hundred amperes. Novel MOS-bipolar devices under development could enable smart power to penetrate even these applications. Meanwhile, intense efforts are being directed to motor control, factory automation and robotics, computer power supplies, and automotive electronics.

Another area where smart power is thriving is in disk drives. Spindle motors that rotate the hard disk must maintain constant rpm, while voice-coil motors that position read/write heads must be accurate and fast.

TAGS: Technologies
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