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I'm trying to build a AC/DC buck converter and I've found an application note from ST. Two diodes are connected series at input to rectify the AC, but one is enough to have a half wave rectification.

https://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=&ved=2ahUKEwj1sN3e483uAhUDnVwKHegCBboQFjAAegQIBBAC&url=https%3A%2F%2Fwww.st.com%2Fresource%2Fen%2Fapplication_note%2Fdm00286922-stevalisa178v1-5-v1-w-30-khz-buck-demo-with-viper01-stmicroelectronics.pdf&usg=AOvVaw0-4Brdx97P7k1oa_wdEtXu

What is the purpose of second diode here? enter image description here

Das D.
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  • Please link to the document where the picture came from. Put the link in your question. – Andy aka Feb 03 '21 at 12:49
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    *What is the purpose of second diode here?* Consider that it might not actually be a "good" design or have any purpose. An answer below suggests an increased tolerance against (negative) voltage spikes. If that was needed, **I** would use **one** diode with a higher reverse voltage or (better) a fuse, capacitor and/or MOV to make a **proper** protection circuit. My point: there might not be a good reason to use two diodes in series. The designer might have made a mistake. – Bimpelrekkie Feb 03 '21 at 13:27
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    They have used a 22 ohm resistor at the input and realistically this should be a fusible resistor but, the data sheet for that part is not totally conclusive that it is a fusible resistor. I am suspicious about this design. – Andy aka Feb 03 '21 at 13:34
  • I’ll bite and go ahead by calling it a bad design. Don’t connect semiconductors in series and expect any increased voltage rating save for some extreme edge cases where you know what you are doing. – winny Feb 03 '21 at 14:01

2 Answers2

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This will improve how the circuit handles reverse voltage spikes as it may be seen by the designer (a bad design is not excluded).

It will not, however, simply double the maximum reverse voltage - it's a lot more complicated than that.

It will also allow the circuit to continue working in case one of the diodes fails as a short - this might be a common enough failure mode. (as OP I don't count on the probability of such event but I respect the editor)

Edited by @FrancoVS

Paul Ghobril
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    No it won't be effective because you cannot rely on the two diodes sharing that voltage equally. – Andy aka Feb 03 '21 at 13:06
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    @Andyaka I didn't say it will double the reverse voltage tolerance. I said it will increase it. – Paul Ghobril Feb 03 '21 at 13:07
  • OK fair point - it might be relied upon to increase it from 1000 volts to 1100 volts reliably if you dug deep into the unwritten performance of the MRA4007. – Andy aka Feb 03 '21 at 13:09
  • One Diode handles up to 1000V already.. Using two to increase the reverse whitstand seems not the main reason to me. – stowoda Feb 03 '21 at 13:11
  • This is the same argument as putting TVS or varistors in series. Check datasheets and application notes and then downvote. And by the way find the answer to this question. – Paul Ghobril Feb 03 '21 at 13:26
  • @PaulGhobril, could you explain plz? "the same" = ? – stowoda Feb 03 '21 at 13:37
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    @FrancoVS I will. I am explaining the reason but I am not responsible if the design is bad or not. – Paul Ghobril Feb 03 '21 at 14:04
  • @stowoda When putting varistors or TVS in series we have the same problem of voltage distribution discussed in many datasheets and application notes however used in some cases. I repeat: I am not responsible for the design if it is bad or not. I am suggesting the reason. – Paul Ghobril Feb 03 '21 at 14:05
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    Who cares if the diodes share the voltage equally? If one starts to break down earlier the other better one gets more voltage, that's all. Unlike insulation breakdown, reverse breakdown is reversible and not harmful to a rectifier unless there is a lot of current flowing. I think this is the correct answer. You get a 2kV rating with two cheap parts. High voltage rectifiers are made this way too, stacking dice with lower ratings, as you can tell from the forward voltage, and no they don't have anything else in the package. – Spehro Pefhany Feb 03 '21 at 14:23
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It would appear that, indeed as the first answer suggests, increasing the voltage tolerance for reverse breakdown is the main purpose. As this article here suggests, it may not be enough and it will be further mitigated by the addition of high value resistors to further increase the load balancing between the diodes due to reverse leakage differences...

citizen
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