Diodes and zener diodes

Discussion in 'General Electronics Chat' started by Shocker33, Jun 11, 2015.

  1. Shocker33

    Thread Starter New Member

    Oct 28, 2013
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    Is there such a thing as fast acting diodes and zener diodes? Or is this just an advertising buzz word to attempt you to purchase it instead of from a competitor?
    If there are such things, what in the datasheet can i look at to distinguish this? And even better, its performance?
     
  2. dl324

    Distinguished Member

    Mar 30, 2015
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    Can you give some examples? Fast shouldn't be relevant in zener diodes due to how they're typically used. Reverse recovery time for diodes that alternate between forward and reverse bias can be important. More specifics will give you a more specific answer...
     
  3. Shocker33

    Thread Starter New Member

    Oct 28, 2013
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    Hmmm.....maybe i've got a little mixed up. I've had a search for the for fast acting zeners and i've been unsuccessful. Although i do want to say that i've seen at least one datasheet that stated that it was fast acting. I definitely know that datasheets for some normal diodes can have fast acting. For example: On Semi - MUR105
    I just wonder if they are actually faster than other diodes.
     
  4. dl324

    Distinguished Member

    Mar 30, 2015
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    Pls elaborate on what you mean by fast or faster.

    Faster than parts that are marketed as substitutes for MUR105?

    Faster than rectifier diodes that don't say they're fast?

    Faster than other fast diodes?

    What is your application? What frequency? For rectifiers used in line voltage rectification, it's probably a don't care...
     
  5. ian field

    Distinguished Member

    Oct 27, 2012
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    Shottky barrier diodes are among the fastest, but they have significant reverse leakage and notoriously low reverse breakdown voltage. The new kid on the block is silicon carbide rectifiers, which are claimed to be bindingly fast - but they have massive Vf and I haven't heard them mentioned at all in recent times.

    Zeners are most often put to work passing a small but steady reverse current to stabilise a voltage - I've never seen any published data on fast transient response. The latest thing in transient protection is the sidac, I haven't had anything to do with them yet but as far as I can gather they're a sort of diac-zilla - a breakdown diode like a diac but rated to clamp a lot more energy.
     
  6. crutschow

    Expert

    Mar 14, 2008
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    Typically the turn-on time of all diodes is relatively fast.
    It's the turn-off (recovery) time that is usually being referred to when talking about diodes and fast-recovery diodes are faster than standard diodes.
    Schottky diodes have no significant reverse-recover time so all of those types are "fast".
    Do you need the Zener diode to be fast during turn-on?
    Transorb transient-suppression type Zeners are designed to be very fast to protect against high frequency transients.
     
  7. ian field

    Distinguished Member

    Oct 27, 2012
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    Its always the Trr spec we look for in the datasheet, but when I serviced CRT PCs for a living I had opportunities to observe the risetime of the flyback pulse (most of them used flyback converters) and the delay before anything happened at the other end of the rectifier. Probably worth noting that some monitors had a sync winding round the core of the horizontal output transformer, so the PSU was going as fast as the scan rate of whatever VGA mode it was displaying.

    In the dim and distant past, some genius at Philips circulated a memo stating that modern stable PSUs meant it was no longer necessary to run CRT heaters at 6.3V, and it should be reduced to 6.15V. Apparently quite a few manufacturers fell for this April fools prank, because I was getting a LOT of monitors with poisoned CRT cathodes, and the heater voltage had been set to around 6.15 give or take.

    Some engineers cured this by cranking up the PSU till the heater voltage came right - at the risk of increasing X-ray emission.

    In most such monitors I was able to fix the problem by replacing the fast-soft recovery heater rectifier with Shottky barrier. The heater reservoir electrolytic usually needed replacing anyway, but I beefed it up and padded it with a low ESR non-electrolytic. It wasn't just the lower VF of the SB that got me the required heater voltage - a few extra uS of each flyback pulse got through the rectifier.

    The next step was to display a scanned in blank sheet of paper and leave the monitor on overnight with the brightness and contrast up full, about 99% recovered enough to get me paid.
     
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  8. Bordodynov

    Active Member

    May 20, 2015
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    Few will tell about the zener. My colleague used a zener diode for general use. Its working voltage was 15 volts. He used a zener diode as a surge voltage. In the observation process on the oscilloscope was seen zener inertia. Delay (breakdown) was ~ 300 ns. The initial moment of tension was about more than one volt operating voltage of the zener diode. As a result, my colleague began to use special limiting diodes (eg Transil). For these diodes start delay the breakdown of a picosecond.
    For normal operation of the zener diode (for stabilization), no need for high speed.
    There is a little known fact to the general public for semiconductor diodes. With the rapid inclusion of the forward voltage of the diodes may be greater than the nominal 1-5 volts. This pseudo inductance diode. At the initial moment in the base there is no carrier, and base resistance (weakly ligated region) large. When the 1N4148.png base appear carriers, the conductivity modulation occurs. The base resistance drops sharply. Especially this effect is that when a single pulse. Therefore, the use of silicon diodes as pulse limiters at the inputs of the high-frequency transistors may not be effective. Modulation effects are not Schottky diodes.
     
  9. ian field

    Distinguished Member

    Oct 27, 2012
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    Just an observation - when testing diodes on the DMM diode check, zeners come in closest to the theoretical 0.7V. Mains frequency types tend to read somewhere around 0.65 and fast diodes can be as low as 0.55 or so, Shottky come in at the lowest, but are very dependant on size, the little signal ones are close to the oft quoted 0.4, but the stud variety can read as low as 0.1.

    These readings are generally lower than the Vf under real current - some datasheets state 0.8 - 1.1V at full rated current for standard rectifiers.
     
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