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B2B Zener vs TVS for AC clipping

Discussion in 'Electronic Projects Design/Ideas/Reviews' started by ACharnley, Sep 15, 2017.

  1. ACharnley

    ACharnley Member

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    Hi,

    I've yet to use TVS in an application due to typically using two large back-to-back zeners. Investigating TVS diodes as a cleaner replacement it appears they are not merely drop-in. My questions are;

    1. TVS appear to have a breakdown voltage much lower than the clamp point. For the 35V AC I want to clip at many TVS's begin at 18V. Granted this may be at minor current, however zener's don't have such a wide range (typically 5% variance).

    2. It's not clear if TVS's shunt the entire voltage once they breakdown. I assume this isn't the case as it's not logical, so a 30V TVS supplied 40V will shunt 10V?

    3. TVS's have a much higher power rating than zeners (which is why I ask 2.) but is this value for an ad-hoc spike rather than a SINE? In which case, could zener's actually be more tolerant than TVS's over time?

    I should mention that I'm not expecting the input signal to be > 35V AC often or for long periods of time, say a minute tops, and even then it'll not be over by much, say 10V. Current is fixed at 500mA, so 5W dissipation.

    Cheers,

    Andrew
     
  2. dknguyen

    dknguyen Well-Known Member

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    What is the application? Need more info. Too many unknowns.

    Do you need the clipped signal to maintain it's current or power? Or is it for clipping it so it can be read in as a signal?

    Are you wanting to maintain the analog integrity of the signal while unclipped (like measuring an analog signal and clipping to prevent overvoltage) or do you not care about integrity (i.e. just measuring the zero-cross and you don't care what the rest of the waveform looks like).

    There are also other alternatives ways to clip that remove the need for worrying about power handling of the diode or accuracy of the breakdown voltage. For example, using a series resistor before the diodes to drop the excess voltage across so the diode need not handle all the current, or clipping to some supply rails which removes the need for a carefully selected breakdown voltage and will be more accurate than any diode..


    TVS breakdown is not very well defined or precises and can vary quite a bit depending on how much current you expect it to be carrying.
     
    Last edited: Sep 15, 2017
  3. ACharnley

    ACharnley Member

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    It's to prevent over-voltage to mosfets and any measuring will be zero cross only. Damage will occur at 40V therefore clipping the AC at 35V. It's a power waveform.

    At first I assumed a TVS would behave like the Zeners and so dump the excess as heat however reading into it I'm no longer sure.
     
  4. dave

    Dave New Member

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  5. dknguyen

    dknguyen Well-Known Member

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    In other words, you don't care about waveform integrity and you don't need to maintain the waveform's power capability after it's been clipped.

    If that's what you want, just put a resistor after the input to the clipping circuit followed by two schotkeys in anti-parallel to the ground rail. They'll clamp the entire signal down to +/-0.5V (you just use a single one with anode on the line and cathode on the rail if it's has zero chance of going negative). Just size the resistor appropriately and it will dissipate most of the heat while limiting the current through the diode and the diode will dissipate practically no power so you don't have to worry about it's size anymore.

    That 0.5V margin should give your electronics enough room to detect the zero cross. If it doesn't you can use more diodes in series to increase the clamping voltage a bit or just put zeners back in since the resistor makes it so power handling capability of the zener a minor issue. It's literally the same concept of what the resistor is meant to do in a zener regulator:
    http://www.reuk.co.uk/wordpress/electric-circuit/zener-diode-voltage-regulator/

    The point I'm trying to make is, use a series resistor. Also, with the resistor the clipped waveform is isolated from the primary waveform so it can continue doing whatever you want (like supplying high currents and power to something bigger).

    TVS diodes are much faster than zeners, but like I mentioned in my first post, they aren't built for super-accurate and stable breakdown unlike zeners.

    TVS power ratings are based on a waveform, not continuous operation, due to their intended application. That waveform usually is described by 10/1000 or something similarwhich represents timing points in the transient or ESD spike waveform. That's part of why their power ratings appear to be so much higher.
     
    Last edited: Sep 15, 2017
  6. ACharnley

    ACharnley Member

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    I may be confused here as I need the 35V (it'll be rectified). Anything above that is throwaway.
     
  7. dknguyen

    dknguyen Well-Known Member

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    Well, we're talking about two different clamps in your circuit here: One for zero-crossing and one to protect the MOSFETs.

    1. For the zero-cross clamping, you don't need to preserve the power capability or integrity of the waveform. Remember, that with the series resistor only the signal AFTER the resistor will be clipped to detect the zero-cross. It also lets low voltage electronics process the zero-crossing. The waveform before the resistor will continue to go up to 40V and with it's full power capability.

    2. As for clamping for the purposes of protecting the MOSFETs (in which case you do need to preserve the power capability of this particular signal, that's tougher. You can't really protect against a continuous overvoltage since the currents will blow the clamping devices and you can't use a series resistor to limit diode current the same way as the zero-cross circuit since that works by limiting the power.

    I assume you're just trying to protect against brief, irregular transients and such in which case I would use TVSs diodes for their speed. It won't be zener-accurate but it will kick in fast enough to help absorb those fast spikes which is typically more important than power handling since power handling means nothing if it's too slow to save the MOSFET. What type of overvoltage are you trying to protect against? Not against a user mishap right?
     
    Last edited: Sep 15, 2017
  8. ACharnley

    ACharnley Member

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    I don't, forget the crossing zero completely it's not relevant, this is a power AC wave for which all power under 35AC will be utilised.
     
  9. dknguyen

    dknguyen Well-Known Member

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    Then what kind of overvoltages are you trying to protect against? You might not even need anything depending on where this AC wave is coming from.
     
  10. ACharnley

    ACharnley Member

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    0 - 20V. More is possible but unlikely.
     
  11. dknguyen

    dknguyen Well-Known Member

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    Not the level, th cause. What would be the cause of such overvoltages?
     

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