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issue: January 2004 APPLIANCE Magazine

Electronics Report
Integrated Power Switch

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A new power switch from Fairchild Semiconductor is said to offer improved reliability in d.c./d.c. power supply designs.

The FS6X1220RT Fairchild Power Switch (FPS), from Fairchild Semiconductor International (San Jose, CA, U.S.), is a highly integrated off-line power switch designed for appliances that use an internal medium-voltage d.c. bus for distribution of power (48 V to 72 V d.c.). Possible applications include digital feature phones, VOIP (Voice Over Internet Protocol) phones, and telecom central office power supplies.


Fairchild Semiconductor's new FS6X1220RT features an internal current sensing, high-voltage SenseFET, which is said to offer a 200-V minimum breakdown rating.

According to the company, the new power switch integrates a fully avalanche-rated SenseFET (200-V minimum breakdown rating) with a current mode PWM (pulse width modulation) IC––a unique combination that minimizes external components, simplifies designs, and lowers cost in target applications.

Madhu Rayabhari, Fairchild's strategic marketing director of Integrated Circuits Group, explains, "SenseFET is a special power MOSFET that has a single gate and drain but two sources - one for the main power path and another that provides a proportional current output. This proportional current output feeds our PWM IC directly on chip. Hence, designing for an external current sense resistor is eliminated, which, in turn, minimizes external components and simplifies designs. As this approach eliminates the use of a precision, high-power resistor, it also reduces the cost."

Designs based on the integrated FS6X1220RT are also said to achieve a higher performance than offered by discrete MOSFET/controller or RCC switching converter solutions while benefiting from reduced board space and improved reliability.

"VOIP phones, for example, require extremely compact power supplies that feature high reliability and efficiency," Mr. Rayabhari tells APPLIANCE. "The FS6X1220RT enhances system reliability by minimizing the number of components needed and providing a variety of protection features such as overload and overcurrent protection. High performance is provided by the current mode PWM function. It also enhances system efficiency through optimized switching characteristics."

An integrated 300-kHz oscillator helps to further increase system efficiency and allows for the use of smaller transformers. "This device has optimized gate drivers and internal SenseFET that minimize the switching losses for the 300-KHz frequency. This combination of frequency choice and internal component performance enhances efficiency," Mr. Rayabhari explains. "By employing a higher switching frequency than conventional PWM ICs, the size of the transformer - and also, associated capacitors - is reduced."

The FS6X1220RT also features line-under voltage detection, remote sleep on/off, under-voltage lock out, and an optimized blanking and gate turn-on/turn-off driver. In addition, the device has temperature-compensated, precision current sources for loop compensation and fault protection circuitry.


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