Active-ORing Controller Features High Speed and Versatility

May 11, 2005 2:52 PM
By David Morrison, Editor, PETech

From International Rectifier (IR), the IR5001S is a universal high-speed controller/n-channel power MOSFET driver for active-ORing circuits. Housed in an SO-8 package, the active-ORing IC is used with an external MOSFET, replacing traditional diode-ORing to increase efficiency and reduce power dissipation. The IR5001S is distinguished from other active-ORing controllers by its ability to turn off the power MOSFET very quickly, by its low pin count and by its wide input-voltage range.

“Compared with typical Schottky diode-ORing circuits, a system using the IR5001S active-ORing IC and an optimized external MOSFET will reduce on-board power dissipation by up to 85% and can be made at least 50% smaller than diode solutions. ORing circuits made with Schottky diodes run hotter for the same power level, and in many cases, this means that through-hole devices with heatsinks are needed, increasing bulk and manufacturing complexity,” said Carl Smith, marketing manager for Networking and Telecommunication Products at IR. The IR5001S also provides additional diagnostic features that increase system up-time.

Active-ORing combines two or more power sources to create a redundant power source, preserving the input power supply when one of the sources fails. The IR5001S is suitable for a range of applications such as -48-V or-24-V input active-ORing for carrier-class communication equipment and 24-V or 48-V output active-ORing for redundant ac-dc rectifiers. Other uses include 12-V output active-ORing for multiple-output supplies and for low-voltage-output redundant VRM power. In 12-V output systems, ORing circuits capable of handling currents of 100 A can be made using four IRF6609 DirectFET MOSFETs in parallel.

Another use for the IR5001S is in reverse-polarity applications for 48-V or 24-V systems, replacing diodes in D² Paks and an expensive relay. When using the IR5001S with the 100-V IRF6655 DirectFET MOSFET, 30-W to 60-W board power levels can be addressed. With the addition of a simple charge pump circuit, the IR5001S also can be implemented in ground-ORing in -48-V systems, as required by the ATCA specification.

The input terminals (INN and INP) are used to determine the output voltage by measuring the differential voltage and polarity across the drain and source terminals of the external MOSFET. The output voltage (V_OUT) of the IC drives the gate of the external MOSFET.

If the current reverses polarity, the IC will quickly switch the active-ORing MOSFET off by pulling the V_OUT pin low. The turn-off delay for IR5001S is 130 ns typ., with a typ. 20-ns FET turn-off time, which is twice as fast as existing active-ORing controllers. Unlike other designs where the reverse polarity protection circuitry must be mounted on the rack or panel in the system, an IR5001S-based design is small enough to be mounted on the pc board.

The IR5001S IC includes diagnostic pins (FET Check “FETch” and FET Status “FETst”) to determine the status of the active-ORing circuit. When a logic signal is sent to the FET Check pin, the output of the IR5001S (V_OUT), or the gate drive to the MOSFET, will pull low, turning the MOSFET off. This momentary off period should cause the voltage across the drain-to-source terminals of the MOSFET to increase, as the current is diverted from the MOSFET channel to the body drain diode. If the voltage rises from approximately ‹100 mV to 700 mV (as expected for a body drain diode), the MOSFET is functioning normally. If the voltage does not rise above a 300-mV threshold, the MOSFET is shorted. The FET status pin provides an output signal to the system, depending on the status of the external MOSFET. This feature enables system-level monitoring of the active-ORing circuitry, allowing scheduled maintenance rather than the inconvenience of random system downtime.

Asymmetrical offset voltage prevents output oscillations under light load, increasing reliability. The IR5001S can be powered from either a 36-V to 75-V telecom bus voltage (100 V max.) or from an external bias supply and a bias resistor. The IR5001S also can withstand continuous gate-short conditions and up to 100-V continuous on the input pins.

Other specifications include a V_CC max of 15 V, a UVLO V_CC (on) of 8.3 V to 10.0 V and a turn-off gate drive of 3-A peak.

Priced at $0.67 each in 10,000-unit quantities, the IR5001S is available now. Data sheets are posted at www.irf.com.

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