[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]

RE: Disabling the freewheling diode



Original poster: "Mccauley, Daniel H" <daniel.h.mccauley-at-lmco-dot-com> 


The reason for this is that the freewheeling current is much less than
the current that the MOSFET conducts when it
is ON.

The Captain



 > All,
 >
 > In a SSTC H-bridge, the internal freewheeling diode of the MOSFET is
 > disabled by a clever arrangement of external diodes.
 > A schottkey diode above the drain keeps the internal diode
 > from getting
 > forward biased, so that an ultrafast recovery external diode
 > can provide
 > the path of any reverse EMF from the load.
 > Any current passing trough the MOSFET must pass trough the schottkey
 > diode.
 >
 > In a typical design, I see MOSFETS capable of handling 20A continous,
 > with a 3 A schottkey diode on top.
 >
 > Why is the schottkey diode not dimentioned for a similar 20A?
 >
 > Cheers, Finn Hammer
 >
 >                                      |
 >                                      |
 >                                      |
 >                                      |
 >                                     -+--------------+
 >                                    __|__            |
 >                                     \ /             |
 >                                    --+--            |
 >                                      |              |
 >                         oooooooooo   |   D          |
 >                     oooo          oooo              |
 >                  ooo    |            |ooo           |
 >                oo   |   |------------|   oo         |
 >               o     |   |            |     o        |
 >              o      |                |      o       |
 >             o       |             /--+---/   o    --+---
 >             o       |   |           / \      o     / \
 >            o        |   |<----+    --+--      o   --+--
 >            o        |   |     |      |        o     |
 >             o       |         |      |       o      |
 >             o       |         |      |       o      |
 >              o      |   |     |      |      o       |
 >    G          o     |   |------------|     o        |
 > --------------oo---+   |            |   oo         |
 >                  ooo                 |ooo           |
 >                     oooo          oooo              |
 >                         oooooooooo   |              |
 >                                      |   S          |
 >                                     -+--------------+
 >                                      |
 >                                      |
 >                                      |
 >
 >
 >