Recovery time of 'television grade' EHT rectifiers?

Discussion in 'General Electronics Chat' started by Hypatia's Protege, Mar 3, 2016.

  1. Hypatia's Protege

    Thread Starter Distinguished Member

    Mar 1, 2015
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    Kind friends

    As per the title, this is to inquire as to the reverse recovery time (Trr) of discrete EHT rectifiers marketed as semiconductor retrofits for high vacuum diodes applied to development of accelerating potential in CRT display systems...

    Said data is annoyingly absent in the technical literature for said devices --- Although I had assumed the 'standard' 100ns typically applicable to low current (<100 mA) EHT rectifiers 'Speced' to VRRMs ≤ 50kV -- The drastically improved 30kHz+ performance witnessed with 'mainstream' "high frequency" EHT rectifiers (e.g. certain 'members' of the 100ns '2CLG' series) leaves me in doubt...

    Granted! I could merely characterize them myself (and may yet need to do):rolleyes: -- "Six-Figure Sad Experience", however, has led me to the philosophy that EHT and VLSI based instrumentation are straight up incompatible (HV Probes, etc... notwithstanding)!:eek::eek::eek:

    Many advanced thanks for any info and/or insight!:):):)

    Best regards
    HP

    PS
    FWIW here are the 'sketchy' specifications and an image of a device typical of that in question:

    Vrrm=45kV
    If (Average) = 5 mA
    If (Surge) = 1A

    EHT_Retrofit.JPG
     
    Last edited: Mar 3, 2016
  2. RichardO

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    May 4, 2013
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    Interesting question. I would assume that the reverse recovery time is fairly low or they would not work well at 15750 Hz.

    Seeing your picture reminds me that I have a thick GE diode catalog that is many years old. Not near the bookshelves right now... I will have a look later and get back to you.
     
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  3. ian field

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    Oct 27, 2012
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    In VGA monitor work; 35nS diodes were pretty much the norm for scan derived supplies, but resolutions evolved with faster scan rate than TVs.

    You wouldn't want to go any slower than about 60 - 70nS.
     
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  4. JohnInTX

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    Jun 26, 2012
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    Datasheet for the diode - looks like its a scan of the original VARO one. 45KV 175ns. The -4 suffix may mean a better/faster one than this. GE may have a better datasheet.
     
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  5. ian field

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    Apparently selenium sticks are faster than standard Trr silicon - about 100kHz IIRC.

    They'd probably have kept on using them if they didn't release toxic hydrogen selenide when they burn out.
     
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  6. RichardO

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    Wow. This is surprising to me.

    Not immediatley toxic. :) I have lasted decades after smelling selenium rectifiers in my youth. :D
     
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  7. ian field

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    When something smells extremely strongly of rotting fish - you tend not to breathe any more of it than you have to.
     
  8. Aleph(0)

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    Mar 14, 2015
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    HP I say is sweet of you to be planing to use 3 series 2clg30kv/20ma diodes for each rectifier to save tutorial builders expense:)! But now I'm saying you're not thinking completely through:(! I know that three 30kv diodes are ok for steady state cuz 50kv total across rectifier (which is Vwinding + V(doubler_capacitor) allows three 30kv piv diodes to divide voltage (which is from reverse leakage current) well enough that all the diodes are stressed less than 30kv:)! But now you are forgetting about voltage distribution during switching transient cuz diode that switches first will have to block all 50kv until another diode switches:eek:! So are you saying junction capacitance is enough to handle it for those frequencies? I say that can work for equalization only if recovery charge and so Δ(recovery charge) is small enough! HP I say you should be sure cuz burned out cheapo diodes are more galling to builders than expensive ones that just work:D! I am saying they can make equalizing capacitors from ptfe and foil if they have to cuz a few pf is probably going to be enough for 60khz:cool:!
    HP If everything I said is wrong just tell me straight out so I can get some Henna asap:D!
     
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  9. Hypatia's Protege

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    Oddly enough, TV EHT rectifiers' performance markedly declines above 40kHz whereas devices speced at 100ns preform acceptably well beyond 100kHz?:confused:

    Best regards
    HP
     
    Last edited: Mar 4, 2016
  10. Hypatia's Protege

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    Congratulations on finding a datasheet!!!:):):) --- I have searched for weeks!:rolleyes: -- In my experience search results pointing to 'datasheet sites' are generally false 'hits' leading to a rigmarole of ads and come-ons for products and services having no bearing upon the search!:mad:

    So... 175ns --- That'd explain it!:):):)

    I likewise find it interesting disappointing that, quite despite its 'advertised ratings' (i.e. 45kV @ 5mA), the device is limited to continuous operation at EMFs of no more than 38kV (and then to only 1.5mA) -- and to 'best case' continuous currents of 2.2mA..:(:(:rolleyes:.

    Best regards and many thanks!
    HP:)
     
    Last edited: Mar 4, 2016
  11. Hypatia's Protege

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    Oh! Those! Uggg! -- My 'first move' in restoring vintage gear following 're-capping' is retrofiring all Se rectifiers with Si diodes! -- A procedure rendered irksome via the required addition of 'dropping resistors' to 'emulate' the Se units' comparatively large forward drop!:mad: --- Why were they adopted!? They offer no advantages over electron tube rectifiers in the ways of reliability or compactness!!!

    I would have said "putrefying onions":eek::eek::eek::D -- Seems fetor is in the olfactory receptors of the... um... huffer?:eek:;):D

    Best regards
    HP:)
     
    Last edited: Mar 5, 2016
  12. Hypatia's Protege

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    My blushes:oops::oops::oops::D

    I might have known the 'accolade train' was approaching the buffers!:(:D

    Correct! -- Application of resistive equalization networks to modest current EHT rectification is a veritable nightmare! -- Whereas implementation of multiple series connected diodes is an easy and reliable 'work-around'!:cool:

    Not quite...;)

    Your observation is correct -- as far as it goes....:cool:

    Correct -- in consideration of the frequency of operation, the waveform (sinusoid), the Qrr and the Trr figures of the diodes in question, no external shunt caps are required...

    There's a lesson in this @Aleph(0) -- to wit: Listen very, very closely to the lady in the mirror! -- She's ever so clever! - You see? - You Ahem -- she - 'answered' all of your questions prior to their asking!:rolleyes::rolleyes::rolleyes::)

    Or, indeed, petroleum wax dielectric inasmuch as the assembly is to be potted in same --- A moot point at best...

    What? And lose your pet scapegoat? -- I don't think so!:p:p:p

    All the best
    HP:)
     
    Last edited: Mar 5, 2016
  13. Aleph(0)

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    HP I'm thinking you're right cuz if driver is running at 100khz (which is way higher than it should be) 100ns is like 3.6° which is 3.14kv when Qrr is for sure gone cuz Trr is 100ns. So I say plenty of leeway for 30kv piv diodes! And for 60khz driver it would only be 2.16° so 1.885kv:cool: I am hoping this is making sense:)?

    PS HP I hope you know that my ideas in this post are for sine wave driver which I think is the plan atm? You know that amplitude should be set all the way to minimum during power up and, cuz of bounce, during power down too! Otherwise fast non sine transitions will gank diodes sooner or later:(!
     
    Last edited: Mar 6, 2016
  14. Aleph(0)

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    HP In case you read my last post already plz read new post script:)!
     
  15. #12

    Expert

    Nov 30, 2010
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    Always a vocabulary lesson with you two!:D
     
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  16. Hypatia's Protege

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    Keep the faith! -- It may just happen one fine day!;)

    Hey @Aleph(0) What's the deal?!:confused: -- First it was 'collectivist buzzwords' on the EHT thread and now ---UGGG!!!--- Degrees!?:eek::rolleyes:

    Ok then -- for starters, let's make this a bit more 'reader friendly':

    (360°*100kHz)*100ns=3.6°
    Sin (3.6°)*50kV ≈ 3.14kV

    AND

    (360°*60kHz)*100ns=2.16°
    Sin (2.16°)*50kV ≈ 1.885kV

    Correct and correct!:)

    So, apart from demonstration of proficiency at 7'th grade math, what's your point?:confused::p:D

    Best regards
    HP:)
     
  17. Aleph(0)

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    HP is just like I said! I was thinking out loud to understand that shunt capacitors aren't necessary! But now I'm saying you have to be careful about noise on lv power supply cuz it can cause non sine transient on osc output! So saying fr xfmr and ups but I say best to filter cascade input and that's not so simple for 50kv:(
     
  18. Hypatia's Protege

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    Hey @Aleph(0) --- Short answer -- No worries!!!:) --- Let me get back to you on this (Mon Late/Tue) with some reassuring 'evidence'....K?:)

    All the best
    HP
     
  19. Aleph(0)

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    Well? (Taping fingers):p
     
  20. Hypatia's Protege

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    Patience!:rolleyes: Rome wasn't burned um.. built in a day!:D

    All the best
    HP
     
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