The first active-OR controller was introduced last year. Now there are two, and the price has been squeezed down so a controller and MOSFET cost about the same as passive ORing with Schottkys.
It has been customary to use multiple ac-dc converters for redundancy in 48-V dc systems for telco and server applications, isolating them from each other with Schottky diodes. Essentially, the diodes perform a logical-OR function on the supply voltages.
This is simple and expedient, but each diode's forward voltage drop accounts for a few extra watts of power dissipation at heavy loads. These conduction losses become significant in a large switch room or server array. Therefore, the same arguments for using MOSFETS in synchronous rectification can be made here. All that has been needed to replace passive ORing with active ORing is a control IC to drive an N-channel MOSFET.
The IR5001 active ORing controller from International Rectifier handles the wide range of input voltages specified for telco systems. It activates an external MOSFET when the associated supply is functional and turns it off when the supply fails by sensing the direction of current flow through the MOSFET.
Passive ORing with Schottkys creates another problem. It's hard to tell if they are still good after the system has been up and running for a while. For self-test, the IR5001 accepts an external signal to momentarily turn off the MOSFET and verify the voltage across it. An external pin reports the results.
The IR5001S costs $0.67 in 10,000-unit quantities.
International Rectifier
www.irf.com