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issue: November 2006 APPLIANCE Magazine

Motor Technology
Driving Smart

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Three new power modules are designed to reduce component count and board space while increasing system reliability and efficiency in air-conditioner motor drives.

Adding to the Smart Power Module (SPM™) product line, Fairchild Semiconductor has designed three new modules for full-switching power factor correction (PFC) in motor drive applications in the 3- to 6-kW power range. Each PFC-SPM device integrates two fast-recovery diodes, two freewheeling diodes, two IGBTs, a gate-driver IC, a shunt resistor, and a thermistor into a 44- by 26.8-mm package.
According to the company, the devices enhance reliability in end systems such as air-conditioner motor drives by integrating a thermistor for temperature monitoring and a shunt resistor for current sensing. The built-in resistor also reduces part count by eliminating the need for the external components required by discrete solutions. Further reliability features include under-voltage and over-current protection provided by the integrated gate-driving IC.
The modules are said to save 50 percent board space compared to discrete solutions and provide 99-percent power factor. “Fairchild’s PFC-SPM devices serve as one component in an input PFC supply that meets 99-percent power factor,” says Ron Randall, staff applications engineer, Fairchild Semiconductor (South Portland, Maine, U.S.) “It is the interaction of this module with an external PFC controller IC and good engineering design practice that results in near ideal power factor.”
Randall says that this feature is important to air-conditioner engineers for several reasons. First, it increases the power available to drive HVAC systems for a given AC mains current rating. “As an example, in the U.S., a standard 15 A service is UL-recognized for
12 A @ nominal 115 V AC input,” he tells APPLIANCE. “This equates to approximately 1,350 W @ 99-percent power factor. If the input power factor is 75 percent, which is typical of conventional input bulk rectification, the available power would be limited to 1,000 W.”
In addition, the power factor is said to help engineers meet international low-order harmonic limits such as the mandatory PFC standard, IEC61000-3-2. It also provides a regulated DC bus for powering variable-speed motor drives over a broad AC input voltage range. “The PFC circuit can provide constant 400 V DC over the universal AC input range (90 V AC to 256 V AC),” Randall says.
He adds that the device’s regulated voltage allows for a more efficient motor drive system by increasing the available drive output voltage and decreasing the motor current. “If the motor is optimized for this additional voltage, a more efficient drive can be realized,” he says.
To optimize thermal efficiency, the modules utilize a direct bonded copper (DBC)-based transfer-molded packaging technology. According to Randall, the DBC construction technique offers excellent thermal interface between the SPM power semiconductors and the module’s external thermal interface, while providing electrical isolation. “Essentially, it is a sandwiched construction of two copper planes with a good thermal interface such as Al2O3 or ALN,” he explains. “The copper on the semiconductor side of this sandwich is etched to form copper traces for internal power connections between the module semiconductors. The transfer mold process is similar to the process used in today’s ICs and can be automated to reduce manufacturing cost.”
In order to use the new modules—FPDB20PH60 (600 V/20 A), FPDB30PH60 (600 V/30 A) and FPDB50PH60 (600 V/50 A)—Randall says air-conditioning engineers should have working knowledge of active PFC circuit techniques. “They will need to assess the overall application requirements such as maximum power out, minimum AC input and the tradeoffs between switching frequency, efficiency and physical size,” he says. “The engineer should consider the overall system to take full advantage of the regulated DC bus voltage supplied by the PFC.”

Suppliers mentioned in this article:
Fairchild Semiconductor

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