Permanent-magnet synchronous motors are beginning to replace split-phase induction motors in appliances as a means of boosting efficiency while reducing part costs. PM synchronous motors work at variable speeds and may become part of the appliance structure itself, perhaps eliminating the need for separate gearboxes and belt drives used to harness power from induction motors. |
New International Rectifier chips are designed specifically to control PMS motors in split-system-type air-conditioning units. |
IC maker International Rectifier in El Segundo, Calif., says it has devised special interface chips for appliances and switching power supplies that will facilitate use of advanced motors and controls.
One new chip targets permanent-magnet synchronous (PMS) motors now being designed into major appliances. These motors are starting to be specified in place of ordinary split-phase induction motors, which have served in washer/dryers and refrigerators for decades. Besides being more energy efficient, PMS motors are smaller than comparable induction motors and less expensive as well, partly because they use less steel.
PMS motors have also begun showing up in condensing units of split-system residential air conditioners. Panasonic, for example, has devised a single-rotary compressor incorporating a PMS motor that is said to be as efficient as more-expensive scroll compressors.
The chipmaker is targeting motors in condensing units specifically with the recent development of what it calls an iMotion integrated power-design platform. It lets appliance makers control PMS motors with variable-speed sinusoidal current control. This control technique eliminates the need for Hall-effect sensors that would ordinarily be necessary to gauge rotor position.
The IR platform, dubbed the IRMCF3xx family, is billed as an appliance-control system on one IC, complete with power-factor correction. The only feedback it needs for operation comes from a dc link-current sensor. It runs embedded motor-control algorithms with an independent application layer processor that handles tasks specific to different classes of appliances. The chip can be keyed to different kinds of appliances through the use of appliance-specific high-voltage IC sections and intelligent power modules already in place.
For example, the airconditioning platform incorporates a proprietary algorithm for interior PMS motors driving both the AC compressor and fan. Combined with application-specific HVICs and discrete trench IGBTs to handle the output power, the controller is said to hit 95% converter efficiency. In this regard, IR says it works with individual PMS motor manufacturers to tune converter parameters for best efficiency.
Digital controls on the chips consist of a Motion Control Engine (MCE) that implements the sensorless algorithm in hardware, plus an embedded analog signal engine that handles all signal-conditioning and conversion circuits for single-current shunt sensorless control. The MCE executes the sensorless field-oriented control algorithm in about 11 sec. This speedy execution is key to handling fan and compressor motors simultaneously, says IR.
An application layer processor on the chip defines the operation of the air-conditioning system independently from the MCE that controls the fan and compressor motors. The intent of this separate processor is to let appliance makers focus on application issues rather than on motor control, says IR.
The analog block consists of IRS2136D threephase analog driver and protection ICs. They have three independent 60-V half-bridge inverter-gate drivers with built-in bootstrap diodes. Protection functions include inverter overcurrent trip and undervoltage lockout with an automatic fault-clear function.
Outputs go to IGBTs that the company also offers.
In a separate development, the company devised a special chip for ac/dc power converters used in laptops, LCD and PDP TVs, game systems, and so forth. Called the SmartRectifier IC, it is said to improve the efficiency of secondary synchronous rectification in high-power flyback and resonant half-bridge converter circuits. It eliminates the need for the current transformer now necessary in these circuits to detect the polarity of load current. Instead it senses voltage thresholds across synchronous rectification Mosfets as a way to minimize power losses. The chip, bearing the part number IR1167, is said to boost overall system efficiency by 1% and reduce the temperature of Mosfets in the power conversion circuit by 10°C. It also cuts the number of parts used for synchronous rectification by 75%.
The new IC works independently from the primary control, so it can serve in transformer switching modes and applications with capacitive output filters. Independence from the primary side means the IR1167 operates in low-power burst modes to enable a 1-W standby and comply with California Energy Commission (CEC) 80Plus directives.
The SmartRectifier technique halves the losses dissipated in the synchronous rectifier Mosfet, says IR. This reduces the number of Mosfets needed in the power supply and allows a change from large TO-220 packages to small, surface-mount SO-8 parts.
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