I'm studying the schematic for an electronic load available as a kit from Jameco: http://www.jameco.com/Jameco/Products/ProdDS/2161107%20Schematic.pdf

There is a Schottky diode shunting the source-to-drain nodes of the MOSFET, D1 in this simplified schematic:

enter image description here

The kit instructions describe its purpose as: "used to protect the MOSFET from voltage transients".

I've searched and searched and can't find an explanation of how it would do that. I've found circuits that have a Schottky placed this way, but they do that for improved performance in switching power supplies and similar applications.

Would this diode do anything useful in this circuit or should I just leave it out?

  • 1
    \$\begingroup\$ Yes, it's used as an overvoltage protection and reverse polarity. Due the the day acting characteristics of this diode the mosfet is protected against peaks, overvoltage and reverse polarity. The only note is that the power supply is equal to the reverse voltage of the diode I would suggest to take one with a higher reverse voltage. E.g. the 1N5818. \$\endgroup\$ – rfkortekaas Aug 2 '15 at 9:02
  • \$\begingroup\$ Across IN+ to IN- (reverse biased) is of some use BUT I rating should be simila to I out \$\endgroup\$ – Russell McMahon Aug 2 '15 at 12:26

I don't see any useful functionality for that part. In the Jameco schematic the IRF46 MOSFET used will handle very large transient currents through the body diode, which is directly in parallel. It also has a reasonably healthy 28A/11mJ avalanche rating.

A reverse connection of the load will put all the source voltage (minus a diode drop) across the sense resistor. Chances are that will damage the resistors before anything else.

A series Schottky might be more useful, as might a TVS or other overvoltage protection (the low voltage Schottky will break down but that's hardly reliable).

Speculation as to how this ended up here- copying of some bridge switching configuration or perhaps they had problems with MOSFETs dying and upgraded the MOSFET and added the diode blindly. Some MOSFETs don't do well in linear mode, datasheet SOA notwithstanding.


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