2
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This image was taken from UCC28881 (page 16)

enter image description here

The desciption in the body says:

A half-wave rectifier is chosen and implemented by diode D2 (1N4937). It is a general purpose 1-A, 600-V rated diode. It has a fast reverse recovery time (200 ns) for improved differential-mode-conducted EMI noise performance. Diode D3 (1N4007) is a general purpose 1-A, 1-kV rated diode with standard reverse recovery time (>500 ns), and is added for improved common-mode-conducted EMI noise performance. D3 can be removed and replaced by a short if not needed.

My question is how does D3 improve the EMI noise performance ?

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    \$\begingroup\$ common mode noise flows in the same direction down both wires, the diodes ensure that such a thing can't happen, thus limiting you to differential noise only (which can sometimes be easier to filter) \$\endgroup\$
    – Sam
    Nov 22, 2016 at 22:01
  • \$\begingroup\$ @Sam ah. That makes sense. \$\endgroup\$
    – efox29
    Nov 22, 2016 at 22:17

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