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Why doesn’t an external Schottky diode work in a synchronous buck converter?

To limit the reverse recovery losses due to the body diode of the synchronous FET (sync FET), designers often place a Schottky diode in parallel with the sync FET.  Body diode turn-on and reverse recovery losses can occur despite the use of an external Schottky diode.  This is because of the presence of parasitic inductances between the external Schottky and the sync FET as shown in Fig. 1.  These parasitic inductances could be due to the Schottky package bond wires or circuit board traces.  The transfer of current from MOSFET to Schottky diode is impeded by these parasitics, and in most cases the benefits of the Schottky are fully negated.

Some device manufacturers have made attempts to reduce reverse recovery effects by placing a Schottky diode inside the device package.  This co-packaged Schottky reduces the size of the FET and the result is, at best, a compromise: higher RDSON and a partially rated Schottky diode.  The forward voltage drop of the Schottky diode must be significantly less than the voltage drop of the body diode.  Bond wires between the Schottky diode and the FET body diode are a significant source of parasitic inductance.

Figure 1.  Sync buck circuit with external Schottky diode.  Parasitic inductors Lp1 and Lp2 decrease effectiveness of Schottky diode.

An ideal solution for the reverse recovery problem is to use a Junction Barrier Schottky FET (JBSFET^TM) for the sync socket such as the SSCJ3003S.  The JBSFET^TM has an integrated Schottky diode that clamps the body diode reverse recovery using a low leakage, low forward voltage drop Schottky structure interdigitated with the FET cell structure.  The interdigitated design virtually eliminates parasitic inductance between the Schottky and body diode.  A JBSFET^TM is more than just a MOSFET plus Schottky diode.  Within the structure of the JBSFET^TM, the Schottky has the same current conduction area as the switch FET without compromising the size of the FET or its RDSON value.  In addition, the PN junction of the MOSFET provides junction barrier shielding to the Schottky diode which helps keep leakage to a minimum.