Fixed-frequency high-voltage air conditioning (AC) systems typically operate at around 60-percent efficiency. Using a systems approach, International Rectifier (IR) engineers have designed digital and analog ICs and the digital control algorithms for variable-speed motion control achieving more than 90-percent efficiency.
Variable-speed motion control is replacing fixed-frequency control in appliances for several reasons. "You are looking for some performance advantage, adding some features, or looking for energy efficiency," explains Aengus Murray, director of marketing for IR's Digital IC Center. "And, energy efficiency is becoming a more and more important topic." With the increased cost of steel that has brought permanent-magnet motors into cost parity with induction motors, a low-cost, high-efficiency, variable-frequency motor design needs only low-cost, digital-controlled power electronics.
Digital Dynamics
A lower-cost digital design that controls both the AC fan and the compressor motors meets the constraints of higher efficiency. Using its patented Motion Control Engine (MCE) as a starting point, IR engineers added system enhancements consisting of a standard 8-bit MCU based on the popular 8051 core and an analog signal engine (ASE) to create the IRMCF3xx digital-control IC. For a complete systems approach, the unit directly interfaces to an analog IC in the power stage.
The digital IC executes a proprietary algorithm for controlling interior permanent-magnet synchronous motors (IPMSMs) in the compressor and fan. By executing the sensorless Field Oriented Control (FOC) algorithm in about 11 µsec, the MCE enables simultaneous control of the fan and compressor motors, as well as the power-factor-correction (PFC) circuits. Eliminating the Hall-Effect position sensors in the motor, and using an 8-bit MCU with enhancements for the control instead of a 32-bit MCU or DSP, meets the cost and reliability goals in the AC application.
Variable-speed sinusoidal-current control eliminates the need for position sensors to achieve smooth torque output and lower acoustic noise in the motors. The embedded ASE integrates the amplification, signal conditioning and ADC circuitry for a single-current-shunt, sensorless control. The digital controller uses the single current-sense output from the compressor and fan-motor power stage and interfaces directly to a separate high-voltage IC, the IRS2136D, in each power module.
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 The IRMCF3xx provides the digital control for the IRS2136 D analog interface in the Power Module (PM) for the AC fan motor and Integrated Power Module (IPM) for the condenser motor. PFC control from the IRMCF3xx completes the system design. Click here for a larger view.
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Analog Aspects
Designed to work with IR's highly efficient depletion-stop trench insulated gate bipoloar transistors (IGBTs), the IRS2136D three-phase analog drivers incorporate three independent 600V half-bridge drivers with built-in bootstrap diodes and provide other system functions. To use a single dc link on the low-side IGBTs for current sensing, the IC has separate power- and signal-ground connections.
The single sensor resistor demonstrates the overall attention to system design details that both the digital and the analog portions need to design a cost-effective, high-efficiency AC control. As Murray notes, "If you time your sampling correctly, you can measure the motors' currents just from what's going through that sense resistor."
For more information about International Rectifier's iMOTION integrated power-design platform, go to: http://rbi.ims.ca/4920-552.
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