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This is again from an HP power supply (PSU). The incoming 230 V is rectified into 340 VDC and is fed to a switched-mode supply. The odd thing here is that the manual for that unit, HP6023A, has different diodes for CR1, CR3, and CR2, CR4.

CR1 and CR3 are labeled "pwr rect 600V 3A 200ns" (MR856) whereas the other two are just "pwr rect 600V 3A" (1N5406). In reality the unit did have four identical MUR 460s which is indeed a 200 ns part. If we look at a more recent schematic for a similar PSU (HP6038) then they are labelled "1901-1199 diode-power rectifier 600V 3A":

Schematic

Now 1901-1199 is indeed MUR460 in real life. Is there a reason to use fast recovery diodes in a bridge for 50 or 60 Hz? Is it because it drives a switcher?

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As well as possibly rationalizing their BOM (though I would expect Schottky diodes to be used on the low voltage side rather than fast recovery conventional diodes), they would possibly have had to put a high voltage capacitor across each of the 1N540x diodes to reduce EMI (from the glacial reverse recovery times- unspecified generally, but in the microseconds- and subsequent fast snap-off), and the ultra-fast diodes don't require that.

Here is a paper from Power Integrations which covers the comparison, from which the below graph was drawn:

enter image description here

Interestingly, the author says that it is sufficient that two of the diodes are fast-recovery.

This is unrelated to the switching regulator- you'll see this topology (bridge with four capacitors) used in transformer linear supplies used in audio equipment to prevent annoying 100/120Hz buzz.

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    \$\begingroup\$ +1 on BOM rationalizing. \$\endgroup\$ – winny Sep 25 at 17:38
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    \$\begingroup\$ "Interestingly, the author says that it is sufficient that two of the diodes are fast-recovery." Interesting indeed. If one of the diodes shuts off then it doesn't matter if the other is slow. It's just a matter of making sure that one is covering positive half-cycles and the other the negative half-cycles. \$\endgroup\$ – Transistor Sep 25 at 18:22
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    \$\begingroup\$ Great thanks! I will print out and read tomorrow! \$\endgroup\$ – AndersG Sep 25 at 19:08
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    \$\begingroup\$ +1 Extremely interesting! Thank you very much for this insight. Although my answer is not made wrong by yours, I think yours is the one to be accepted. \$\endgroup\$ – Lorenzo Donati supports Monica Sep 26 at 9:45
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The fact that it drives a switcher doesn't mean too much, since even quick current pulses are smoothed out by the output filter (called "input filter" in the schematic), especially the series inductor L1, which will hold the current drawn from the bridge constant (ideally).

Most probably they already had such diodes in their bill of materials for other parts of the circuit. Perhaps the rectifier circuits after the switching transformer, where the AC power is high frequency and a fast diode is needed to minimize losses.

So it makes sense to optimize the BOM and reusing a part that must be already in stock.

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  • \$\begingroup\$ Thanks. That is what I thought as well. \$\endgroup\$ – AndersG Sep 25 at 16:48
  • \$\begingroup\$ This explains the actual fitment, but not so much the design schematic. (unless the BOM quantisation was factored into the original design). \$\endgroup\$ – Sean Houlihane Sep 27 at 8:55

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