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A Samsung switching power supply AD-3014STN delivers 14V 2.14A. The rectifier consists of 3 SB3150 (150V 3A) diodes connected in parallel. One of the diodes was short-circuited, see picture, the defective diode is removed.

Photo of PCB component side

What I don't understand is, why are 3 diodes connected in parallel that can deliver a total of 9A, but the power supply only delivers 2.14A. One diode should do it, right?

Furthermore the high blocking voltage of 150V is mysterious to me. Does anyone know the answer to these questions?


EDIT: Here is the solder side photo:

Photo of PCB solder side

A power supply for an LCD monitor is an extremely price-sensitive product, so I wonder about the 3 diodes in parallel and also the high reverse voltage. In this diode family there are products with a lower reverse voltage which are also cheaper. Using only one diode with a lower reverse voltage may save approx. 0.10 USD per unit, but with 1,000,000 power supplies that's 100,000 USD savings, so why does a manufacturer install 3 diodes?

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    \$\begingroup\$ Can you show us a photo of the underside of the board? Parallel diodes are generally useless as the one with the lowest forward voltage, hogs more than its fair share of the current, gets hotter which reduces the forward voltage further so that it ends up hogging most of the current. The others may as well not be there. \$\endgroup\$
    – Transistor
    Dec 1, 2019 at 16:31
  • \$\begingroup\$ You can probably ignore the blocking voltage; diodes commonly have high blocking voltages and these are probably just the ones they found the best deal on. \$\endgroup\$
    – Hearth
    Dec 1, 2019 at 17:33
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    \$\begingroup\$ Transistor is correct. We need to see the other side. At a minimum, I'd guess that the diodes are not, in fact, in parallel. Most likely the transformer is center-tapped, and the two diodes form a half-bridge. \$\endgroup\$ Dec 1, 2019 at 17:37
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    \$\begingroup\$ Looks excessive, but assuming flyback topology and wide range input, the peak current may be considerable despite average isn’t. \$\endgroup\$
    – winny
    Dec 3, 2019 at 16:51
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    \$\begingroup\$ OK, so I was wrong. From the other side of the board, it's clear that the diodes are in parallel. \$\endgroup\$ Dec 3, 2019 at 18:05

1 Answer 1

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Because of the lower junction temperature compared to one single Schottky. Look at this link:

Nexperia Application note AN11358 - Parallel Schottkys as secondary rectifiers in flyback adapter: power losses and junction temperatures

From the conclusion in that application note:

There are two main causes for a reduction of junction temperatures of two parallel rectifiers compared to a single one:

a. Electrical power losses are reduced due the nonlinear VF versus IF characteristic of Schottky rectifiers

b. Thermal resistance junction to solder point Rth(j-sp) is reduced because of the doubled amount of silicon, leadframes, leads and die bonds, resulting in a smaller total thermal resistance from junction to ambient Rth(j-a).

On dual layer PCB with 0.5 cm2 mounting pads, thermal simulation results show between 73.5 °C and 79 °C junction temperature for two parallel PMEG4050ETP Schottky rectifiers, depending on peak current conditions. This is a temperature decrease of about 20 % compared to Tj of a single rectifier.

Both the reduction of forward power losses and the lowered Rth(j-sp) add to more or less 10 % decrease of junction temperatures in a 2.5 A total forward current application, independent from the PCB layout.

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  • \$\begingroup\$ Attached AN assumes thermal coupling (which is required to prevent thermal run-away of one of the diodes), but I fail to see the thermal coupling of both diodes in the board OP shows here. BTW the AN only shows SMT where therm coupling is way more realistic. Could you or one of the three upvoters enlighten me? \$\endgroup\$
    – Huisman
    Jan 9, 2020 at 19:22

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