Electronic Design

Air Conditioner Chip Set Is Way Cool

Whte-goods designers must maximize efficiency to meet the demands set by new regulations and markets.

If you think appliances built for the North American market lag in innovation, you're probably right. For decades, white goods built for Europe and Asia have been quieter and more efficient. But as the implications of rising energy costs sink in, this is changing. Some companies in the U.S. and Canada are updating their product emphasis to take advantage of overseas markets' latest goal—efficiency without a price premium.

For instance, take International Rectifier's latest iMotion ICs, which were unveiled at PCIM China in Shanghai earlier this month. They target the first appliance that research shows Chinese citizens look to acquire when they get some disposable income—an air conditioner.

Rather than trying merely to squeeze incremental improvements out of its classic power discretes, IR chose to engineer an efficient motor-control subsystem that would enable air-conditioner OEMs to cut their total materials and assembly costs by accommodating the latest trends in motors and compressors.

GOODBYE INDUCTION MOTORS
The biggest new trend is the total eclipse of induction motors by permanent-magnet synchronous motors (PMSMs) (Fig. 1). Once considerably less expensive, induction motor prices have risen, largely due to a worldwide increase in steel and copper costs. Meanwhile, PMSM prices have risen less steeply. As a result, IR optimized its chip set for the more efficient motor.

PMSMs were always attractive because of their lower conduction losses, good low-speed torque, simple speed control, and compact size. Where induction motor currents flow in both stator and rotor windings, PMSMs have less copper and lower conduction losses. Where iron cores in induction motors saturate, requiring voltage to be varied along with frequency to achieve speed control, PMSM speed depends solely on frequency.

From a system standpoint, IR's iMotion products remove a potential reliability problem. For motor control, air conditioner OEMs have been accustomed to using motors with built-in Hall-effect sensors to monitor phase currents. The IR chips achieve the same effect by using on-chip sense resistors to externally sample the dc current to the pulse-width modulation (PWM) driver at precisely selected times.

ADAPTING TO TORQUE VARIATIONS
While it doesn't cost the OEM any more to use a PMSM, compressors are a different story. Also, IR had to understand cultural differences in the market to engineer the iMotion products.

Rotary compressors comprise an eccentric circular cam like rotor inside a larger circular cavity. Pressure cycling and the eccentric weight distribution of the rotor create mechanical vibration and a cyclically varying motor load. Scroll compressors comprise an eccentric helical rotor that orbits within a stationary helix. They're more efficient, and they tend to represent a more constant load on the motor.

Unfortunately, the precision machining required to make them increases their cost. While generally affluent Japanese consumers will absorb the premium in return for higher efficiency and appliances that produce less noise, more price-sensitive Chinese buyers will not. So, OEMs use rotary compressors for the Chinese market.

IR's controllers deal with the potential for vibration and varying loads presented by rotary compressors by creating a more complex driving function than three phases of simple sinusoids.

The design challenges don't stop with the compressor, though. Both compressor and fan motors require speed control, but their controller requirements are different.

Speed control is necessary in both cases to allow the air conditioner to adjust its output to match conditions and maintain a steady temperature for the best operational efficiency, rather than merely cycling on and off under the control of a simple on/off thermostat. Studies show that up to 40% efficiency can be gained in systems that allow the fan and compressor to run at variable speeds.

DON'T FORGET PFC
Just like Japan, Europe, and parts of India, China requires appliances to meet power-factor-correction (PFC) standards.

Therefore, the PWMs that control the motors must provide PFC, even though it adds losses.

To meet that challenge, IR's platform incorporates a low-loss PFC algorithm that minimizes losses at the low switching frequencies used to drive the motors. IR's iMotion "integrated design platform" comprises a digital control IC and a driver IC that integrates the latest of the company's IGBTs and FETs (Fig. 2).

The IRMCF3xx iMotion digital controller is a multicore design. One side is built around an 8051 core that handles the user interface. The other is built around a patented Motion Control Engine (MCE), which implements a complex, sensorless, PMSM-control algorithm in hardware.

The two cores communicate via a RAM register block that provides set-points for the MCE and diagnostic feedback to the MCU. While the MCU is programmed conventionally, the MCE algorithm is implemented via a graphical drag-and-drop approach using a library of analog functions. In addition to the MCE, the chip integrates a mixed-signal block called the embedded analog signal engine (ASE), which integrates all signal-conditioning and conversion circuits for sensorless motor control using built-in current shunts mentioned previously.

Implementing the MCE in hardware provides fast loop response without a super-fast DSP or CPU core. In fact, the MCE executes the sensorless field-oriented-control (FOC) algorithm in about 11µs, enabling simultaneous control of the fan and compressor motors as well as power-factor correction. And since the OEM can tweak the outer loop by graphically manipulating a library of analog functional blocks, this approach eliminates software coding from the development process.

The analog block in IR's platform consists of the IRS2136D family of three-phase analog driver and protection ICs. These ICs incorporate three independent 600-V half-bridge inverter gate drivers with built-in bootstrap diodes, along with the IGBTs and FETs.

Typically in these air conditioners, the chips' target fan motor capacity is about 100 W, while the compressor motor can reach up to 1 kW, depending on the air conditioner's cooling capacity. FETs perform the fan switching, while IR's latest depletion-stop trench IGBTs accomplish the compressor motor's switching. These IGBTs provide lower collector-to-emitter saturation voltage (on the order of 1.7 V) and total switching loss (slightly more than 300 J) than punch-through and non-punch-through IGBTs.

MAKING IT WORK
Developing the user interface for an air-conditioner application takes place on a RAM-based evaluation system. Application programs that run on the 8051 controller are developed in C or assembly language using whatever third-party tools are the most familiar to the engineer. A JTAG emulator enables debugging. When using the reference design, developers can change set-points and read out diagnostic information on their PCs. Isolation lets designers run the reference design live during development.

At this time, IR mask-programs the apps code in the final product. Some customers who anticipate the need to update application features and don't want to be stuck with a large inventory of mask-programmed chips containing the old code are asking for flash memory. IR says it's working to accommodate those customers.

There are additional tools for adapting the generic motor-control engine that IR supplies with the reference design to any particular motor. These consist of a library of analog blocks that can be graphically configured and simulated using the Mathworks' MatLab and a compiler that converts the results to a netlist.

By staying in the graphical mode, engineers can configure the control loop quickly, with less chance of coding error than if they were writing code. In the development environment, the motor-control algorithm can be updated as required.

For more, see "Bringing It Home: AC Efficiency" online at Drill Deeper 12212, www.electronicdesign.com

International Rectifier
www.irf.com

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