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Why must the Schottky diode be fully rated in a synchronous buck converter?

In a sync buck converter, to reduce reverse recovery losses it is desirable to have a fully rated Schottky diode in parallel with the synchronous FET (sync FET).    To ensure that that the body PN diode does not turn on, the forward voltage drop of the Schottky diode must be significantly less than the voltage drop of the body PN diode.  When a Schottky diode is capable of handling all the sync FET current at a forward voltage drop small enough to keep the body diode from conducting, it is said to be "fully rated."  Most device manufacturers scale down the size of the internal Schottky to maximize active area of the switch in products that have a MOSFET and Schottky in the same package.  The reduced size internal Schottky diodes are rated to carry only a fraction of fully rated MOSFET current.  At current levels above the Schottky diode rating, the forward voltage drop of the Schottky diode becomes larger than the body PN diode and the current in the sync buck circuit starts to conduct through the body diode leading to reverse recovery losses.  Therefore, a partially rated Schottky diode is not effective in clamping the body diode off, and becomes ineffective at higher load currents.  The preferred solution is to use a fully rated integrated Schottky diode.

The Junction Barrier Schottky FET (JBSFET^TM) has an integrated Schottky diode that is capable of conducting the maximum sync FET current at a low forward voltage—ensuring that the body diode will not turn on.  The innovative design of the JBSFET^TM is such that the Schottky diode has the same current conduction area as the switch, and is capable of handling the same peak currents as the FET.  Schottky diode leakage is minimized by the junction barrier shielding of the FET PN junction.  The design of the JBSFET^TM is optimized for sync FET applications with the JBSFET^TM realizing a significant efficiency improvement over competing trench FET devices.