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difference between a normal diode and a schottky diode

Discussion in 'General Electronics Chat' started by jin29_neci, Sep 11, 2005.

  1. jin29_neci

    jin29_neci New Member

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    :lol:
    hi guys Im experiencing between the two diodes. Im confuse about them how do they differ in physical appeareances? and how to check a schottky diode using an analog meter?

    And how they are used in a circuit? thanks.

    POST PLS. GOD bless you all
     
  2. Someone Electro

    Someone Electro New Member

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    Schottkey didodes have an much lower forward breakdown voltage.Thats all.They act as an nomal diode.And you check it the same way too.
     
  3. Sebi

    Sebi New Member

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    And the scottky is fast recovery, so useable in switchmode PSU as rectifier up to about 1MHz. (depending from type)
     
  4. dave

    Dave New Member

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  5. Glyph

    Glyph New Member

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    Schottky vs "standard" silicon diode

    Schottky generally has lower reverse voltage max. You can easily find silicon diodes with 200-1400v ranges, while commonly schottky is 20v-60v.

    Schottky is extremely fast recovery, even faster than so-called "fast recovery" silicon diodes. So you can use them in high frequency SMPS.

    Forward voltage drop is lower. In silicon you get 0.6v-0.7v at rated current. In schottky you can get 0.3v which is great for SMPS.

    Most schottky diodes look exactly the same is a regular diode.

    if you want to test a schottky with an multimeter, set it to measure forward drop and measure the drop of the diode if its 0.3-0.4 its a working schottky. (or a burned silicon....)
     
  6. Styx

    Styx Active Member

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    Silicon diode: Use of P-doped silicon and N-doped silicon to make a P-N junction that causes the blocking effect

    Schokky diode: Use of (normally) P-doped silicon bonded to metal. The junction effect is at the metal connection and it is because only one doped-tpye of silicon is used you end up with the lower on-state

    seems everyone is champion Schokky's her, there is a slight disadvantage with them with nice high-power electronics in that their reverse recovery is alot higher then a standard diode when operated as a free-wheel diode.

    This comes from their ohmic connection

    Saying that though Schokky (SiC devices) are comming in, although the reverse recovery is worse the lower on-state losses is very attractive
     
  7. Someone Electro

    Someone Electro New Member

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    I think i have an scottkey rectefier someware.Its two powerful diodes in an TO-220.There is much less heating than an normal diode.I cod run 10A trough it and it only got slitly warm.
     
  8. Roff

    Roff Well-Known Member

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    Styx, I think I have heard this same statement from you in the past. Can you cite evidence for this claim? My understanding is the exact opposite (and a Google search confirms this).
     
  9. Styx

    Styx Active Member

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    Sure will do.
    I am not allowed to use Schokky diodes anywhere in my control cct at work for one thing.

    But the power electron side of thing's is in my power-electronic book on my desk at work.


    The only exception to the rule of "no schokky diodes in free-wheel" is with SiC since there are some extra benefits from SiC
     
  10. Roff

    Roff Well-Known Member

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    Here's a quote from International Rectifier:
    "This explains the Schottky diode's lack of reverse recovery, making it ideal for high frequency applications."
    A Google search for "schottky reverse recovery" yields several similar hits. Make sure you spell "Schottky" right, or you won't find anything.
    I don't think a decree from your boss is a "cite". How about one from your power-electronic book on your desk at work?
     
  11. Oznog

    Oznog Active Member

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    Schottky diodes typically have a higher reverse leakage current.

    Typically Schottky diodes have a forward voltage which rises more in response to current than a std rectifier. But it's a bit of an apples-to-oranges comparison, it depends on the junction area.
     
  12. Styx

    Styx Active Member

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    Power Electronics: Converters, Applications and Design. 2nd Edition
    Mohan/Underland/Robbins
    ISBN: 0-471-58408-8

    Ch20: Power diodes – Schottky diodes.

    Not quoted but summarised

    Advantage:
    Speed (thus lower switching losses)
    Low forward volt drop (thus lower on-state losses)
    Low voltage overshoot at turn-off

    Disadvantage:
    Silicon based Schottky’s limited to 200V reverse blocking (a P-N can block 1200V for a doping of 10^4 cm-3)
    The reverse recovery is comparable to the reverse-recovery of a P-N diode (extreamly unexceptable)
    Ther reverse leakage is worse for a comparable P-N diode



    SiC have solved the reverse blocking (can now get 1200V) but that is only due to the SiC being able to support kV’s of voltages

    --EDIT--
    corrected a mistake
     
  13. Roff

    Roff Well-Known Member

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    Styx, reiterating: this is the statement I am questioning. You seem to be sidestepping it. I totally agree about all the other properties of Schottky diodes that you listed. Can you support the above quote with something from the book on your desk, or some other source?
    What does this mean? Leakage is measured in amperes, and reverse recovery is measured in seconds.
    Styx, I'm not picking on you - I just don't think we should be disseminating misinformation here. If I am wrong, I'll be happy (but not too happy) to admit it.
     
  14. Styx

    Styx Active Member

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    Errr post above yrs my me full book refernce ISBN provided an all.
    Well reverse recovery is actually measureed in AMp-seconds ;)

    That aside tho.
    The full quote is

    Thus I stand by the fact that Schottky's have a reverse-recovery problem just like a standard diode. The only difference is the source of this reverse -recovery charge.

    in a PN it is the stored charge (ie hte diode acting like a capacitor) and in a Schottky it is behaving like a short-cct while the volts across the device a building up[/quote]


    This has been taken from the 2nd edition of the "power electronics bible" and I have also confirmed it is in the 3rd edition (which is less then a year old)


    So The fact that they do exibit reverse-recovery (that is comparable to the reverse recovery of a standard PN) means no advantage or dissadvantage.

    The on-state losses gives a fantastic advantage, but the increase reverse leakage will increase the losses during the portions where the Schottky is blocking

    Yes in the last few post mixing terminology a bit (soz) but that is the definitive statement that backs my 1st post in this thread abt the reverse-recovery of Schottky diodes
     
  15. Dr.EM

    Dr.EM New Member

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    I experimented using some in a distortion effect, I thought that they sounded better than silicon as clipping diodes, though i'm sure they can do more in other situations :lol:

    Mine appear quite differently actually, they are blue with a white stripe on them, but i'm sure a lot of them look like ordinary diodes.
     
  16. Glyph

    Glyph New Member

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    Just checking:

    So by styx's assertion Schottky diodes should not be used as freewheeling diodes because their internal parasitic capacitance is actually GREATER than the equivalent reverse recovery loss of a standard silicon.

    i just want to make sure of this point for myself. I'll start ripping them out of my SMPS if its true.
     
  17. audioguru

    audioguru Well-Known Member Most Helpful Member

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    He, he. Then they aren't Schottky diodes. They are insects that are from the Amazon. :lol: :lol:
     
  18. Styx

    Styx Active Member

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    not saying that at all. I am just pointing out the bad points abt Schottky's
    WhenI first learnt abt them at Uni they sounded great (low volt drop, fast switching) and I wander why they were not used that much.

    The fact is they are great and have their place, when you need to have as much volts as possible in a situation where the diode-effect is needed - the 0.3V is too attractive.

    So you have a switching cct one with a diode and one with a Schottky

    The diode one will have a high on-state losses since it has a higher on-state voltage, so the Schottky ends up winnning since its conduction losses are 50% better

    So the diode is blocking it has a leakage current, the Schottky also has one that is worse then a standard diode, but I am talking abt 100s of uA so might not be significant

    So you come to turn-OFF of the diode and turn-ON of your switch. As stated the diode will "dump" its stored charge to provide the reverse-recovery current. This extra blat of current will increase the switching losses of both devices (the diode has to turn-off with that extra current and the FET/IGBT must switch into an extra current of that amount)

    diodes tent to have switching losses in the order of single mJ so an extra few hunder As wont do much. The problem exists for the switching device who (as seen in my piccy) at its turn-on has to turn-on into extra current, be that extra current for a finite time, the time it take to fully turn-on

    All that means is the controlled devices switching losses will go up. ie cool it better
     

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  19. Roff

    Roff Well-Known Member

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    Styx, I see your points, and I appreciate being enlightened. I guess that with the relatively recent availability of really fast recovery PN diodes, the Schottky advantages tend to pale. I was only dimly aware of these advances in diode technology until I started researching this issue.
    Nevertheless, I still saw a lot of Schottkys being used in low-voltage freewheeling and flyback applications when I was researching this. Your company apparently believes that PN diodes are superior for freewheeling applications. Do you think that they are a clearly superior choice, or is it still pretty close?

    I have a more detailed question. If the forward current in the diode has gone to zero (inductor has fully discharged) before the next turn-on of the controlled device, then it seems that the PN diode would be a clear choice, because stored charge would be essentially zero, and the junction capacitance is much less than that of a comparable Schottky. Is that a fair statement?
     
  20. Styx

    Styx Active Member

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    no problem, we are all here to learn.
    Schottky's are still great and as I have said they have their place. I was just putting forward a couple of points abt Schottky's

    As to hte nevative points of a Schottky (leakage, REverse-recovery,blocking voltage)
    The only one of major concern to where I work is the blocking voltage
    leakage might be higher then a PN, but we are still talking abt 100's of uA so not that significant (if used in power ccts)
    The reverse-recovery as has been shown exist for a Schottky as well as a PN and if the benefits of a Schottky are needed then cope with the slight higher (then to a PN) with better heatsinking.
    The only major problem is the blocking voltage - 200V is far too low

    Now for SMPS from 12V, 28V Schottky's are by far the superior option, but for higher voltage stuff then they cannot even be touched. this is changing with Silicon Carbide - We have just been offered a 1200V 3ph H-bridge IGBT module with SiC for the diodes !!!! looking very good

    that is a fair statement, leaving the only deciding decision between PN and Schottky being down to the volt-drop (be it for the conduction losses or for trying to recover as many volts as possible)
     
  21. Claude Abraham

    Claude Abraham Member

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    Schottky's have a capacitance which must be discharged when switching between on & off states. The reverse recovery of a Schottky is negligible. Although the junction capacitance of a Schottky is higher than that of a p-n junction, the p-n exhibits higher switching losses due to stored minority carrier charge.

    It's been my experience that in the same application, the Schottky exhibits lower forward conduction loss, lower switching loss, but higher reverse leakage loss.

    This is very well known. During switching, the Schottky will incur greater loss due to capacitance than a p-n, as the Schottky has greater capacitance vs. a p-n. But the losses incurred in a p-n via minority charge is even greater yet. So Schottky diodes are preferred for switching at low voltages where the reverse leakage isn't too great.

    At high voltages, Schottky diodes are less desirable. Their availability is limited to SiC parts. But an SiC Schottky has high forward voltage drop. At high voltages this may not be a problem. I use ultrafast p-n diodes for anything over 100 volts.
     

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