How does a coupling CAP react to ESD?

[Saimoun]

Hi

Just wondering, say I have an audio output circuit with a coupling cap:

opamp -> 10R res -> 47uF coupling cap (electrolytic) -> connector

How would the cap react to an ESD event? Because ESD is very high frequency I guess it would go straight through?
Then could a (say) 16V rated cap be damaged by letting through an ESD event of 2kV? (the obvious answer is YES - just hoping to get a bit more explanation here )

Thanks!
Simon

[TheUnnamedNewbie]

to me it actually seems unlikely that a 16V big cap will be damaged. Most ESD models assume that you have a resistance in series from the contact/air/whatever, and then a 100pF or so charged to a high voltage. Charge redistribution from a 2kV 100pF to a 16V 47uF cap will probably show that your 47uF cap will never see more than a few volts, if that.

but citation needed, ESD is a mess.

[srb1954]

to me it actually seems unlikely that a 16V big cap will be damaged. Most ESD models assume that you have a resistance in series from the contact/air/whatever, and then a 100pF or so charged to a high voltage. Charge redistribution from a 2kV 100pF to a 16V 47uF cap will probably show that your 47uF cap will never see more than a few volts, if that.

but citation needed, ESD is a mess.

It is more than just the charge redistribution across the capacitor. Because ESD pulses have such short rise times and the pulse currents are moderately high the inductance of the cap becomes very important. Even a few 10's of nH of inductance will drop an appreciable voltage. The ESR of the capacitor can also be important when considering the peak voltages developed.

It is instructive to perform SPICE simulations of such situations but to get useful results you need to ensure that your component models incorporate all necessary stray parameters to accurately model real world behaviour under the extreme conditions of an ESD pulse.

[Saimoun]

Oh ok I did not expect that actually - I did not think of it like that, but yes it makes sense the charges just go from one cap to the other.

The cap is electrolytic so I guess it has a few ohms ESR. Inductance I do not know, and the datasheet does not mention it.

[Manul]

It is hard to model accurately because of details like like internal inductance, so it is hard to say is overvoltage condition possible or not. But even assuming that overvoltage is possible (across whole or part of active surface) it is fair to say, that damage is very unlikely if energy is limited.

Unipolar aluminum electrolytic capacitors act similar to a zener diode. One way you get breakdown at a low voltage (like a forward bias of a zener) and the other way you get a breakdown somewhere above its rated voltage. The oxide layer is not permamently damaged like film or ceramic isolators. If the breakdown is brief and low energy, the oxide should repair itself no problem. Bipolar aluminum electrolytic capacitor will act similar as two back to back zeners.

[T3sl4co1l]

The basic RLC model won't exactly be valid at such frequencies (i.e. including low 100s MHz), but we can assume some kind of transmission line behavior above the usual range (simple RLC might be within say 20% from some kHz to ~10MHz?), maybe with a distributed capacitance characteristic (waves diffusing into the capacitor construction?) or losses or whatever.  At the highest frequencies, body capacitance and lead length take over, in a somewhat more straightforward way (in that, they're chunks of metal, so we can draw a fairly representative TL network or RLC model of them).  In any case, the impedance won't be crazy; and where that impedance occurs, physically, will be nothing more than bulk materials -- no hazard of, say, chemical reaction (the pulse is quite brief and delivers little charge), overheating, etc.

Unipolar aluminum electrolytic capacitors act similar to a zener diode. One way you get breakdown at a low voltage (like a forward bias of a zener) and the other way you get a breakdown somewhere above its rated voltage. The oxide layer is not permamently damaged like film or ceramic isolators. If the breakdown is brief and low energy, the oxide should repair itself no problem. Bipolar aluminum electrolytic capacitor will act similar as two back to back zeners.

It's not obvious that rectification or breakdown effects will be relevant here, or if capacitance will dominate; but interesting to think about.  If so, just chalk that up as another contributor towards lower [average or incremental] impedance!

Tim

[Saimoun]

Thanks for the replies guys. I understand from your replies that the coupling cap will be fine (in most cases at least - which is good enough for me).

I know it's a bit out of the topic, but what about the opamp output, will the ESD pulse be stopped by the capacitor or will it reach the opamp?

See attached file - I added a capacitor in parallel which should block some of the pulse (100n, with 10 ohm the cut-off frequency is still way above the audio range). The question is whether the TVS is overkill or not.

Thanks

[T3sl4co1l]

Low series impedance (at best 100s ohms) vs. kohms of source -- essentially no effect.

In general, blocking ESD is almost impossible.  Even strong insulators (like, ~10 mil polyester sheet will block direct strikes) may exhibit partial discharge (that static-y sound you hear when rubbing plastic and other surfaces, is charge getting stuck into the surface in various places, redistributing with a small weblike discharge), which is just the same problem over again, merely smaller.

Having a series 10 ohm gives you the opportunity to clamp the direct output with an ESD diode.  This will have some voltage drop still (usually in the ballpark of 30V), and quite a low impedance this time, which is where the series 10 ohm comes in.  Now to the IC, it looks like a mere, oh, 2.5kV or so ESD?  Which it's probably quite capable of handling.

Remember to put a nice big cap between +V/GND near the ESD diode, assuming it's a clamping type (diodes from GND to output to +V).  Over 0.1uF is needed.  Can be the same one used to bypass the chip, or somewhat further away on the power plane if applicable.

Zener type TVS, can be unidirectional on the amp side of the coupling cap, or bidirectional on the outside.

I wouldn't bother with the cap, and what you have for TVS is fine.

Tim

[MrAl]

Hi

Just wondering, say I have an audio output circuit with a coupling cap:

opamp -> 10R res -> 47uF coupling cap (electrolytic) -> connector

How would the cap react to an ESD event? Because ESD is very high frequency I guess it would go straight through?
Then could a (say) 16V rated cap be damaged by letting through an ESD event of 2kV? (the obvious answer is YES - just hoping to get a bit more explanation here )

Thanks!
Simon

One thing to keep in mind is that the capacitor has to charge in order to see an overvoltage.  If it does not have time to charge, it will never see much voltage.  In this case it would have to have time to charge up to 16v and above in order to do damage to it due to overvoltage.

[Saimoun]

Low series impedance (at best 100s ohms) vs. kohms of source -- essentially no effect.

In general, blocking ESD is almost impossible.  Even strong insulators (like, ~10 mil polyester sheet will block direct strikes) may exhibit partial discharge (that static-y sound you hear when rubbing plastic and other surfaces, is charge getting stuck into the surface in various places, redistributing with a small weblike discharge), which is just the same problem over again, merely smaller.

Having a series 10 ohm gives you the opportunity to clamp the direct output with an ESD diode.  This will have some voltage drop still (usually in the ballpark of 30V), and quite a low impedance this time, which is where the series 10 ohm comes in.  Now to the IC, it looks like a mere, oh, 2.5kV or so ESD?  Which it's probably quite capable of handling.

Remember to put a nice big cap between +V/GND near the ESD diode, assuming it's a clamping type (diodes from GND to output to +V).  Over 0.1uF is needed.  Can be the same one used to bypass the chip, or somewhat further away on the power plane if applicable.

Zener type TVS, can be unidirectional on the amp side of the coupling cap, or bidirectional on the outside.

I wouldn't bother with the cap, and what you have for TVS is fine.

Tim

Thank you Tim for the long and detailed explanation
Though I am not sure if you are saying the opamp internal diodes should be able to handle the rest of the ESD event (after going through the TVS/Zener), or if I should add 2 diodes (one between GND and output and one between output and +V) ?
I removed the extra cap and change the TVS based on your comment.

[T3sl4co1l]

To clarify, the bidirectional TVS you had, is fine.  More or less equivalent.  Consider it a parts choice option (maybe you're using one or the other elsewhere in the design already, etc.).

The amp's internal diodes might appreciate a little more help, I don't know, but I definitely wouldn't worry if it's rated 4kV+ HBM.  2kV is probably fine; lower, I might add clamp diodes (BAV99?).

Tim

[Saimoun]

Awesome. And yes I'm using unidirectional TVS somewhere else so since you said it made no difference this is better
Opamp is rated 8kV according to the datasheet 

So all in all everything is good - I was mainly worried about that cap. Thank you all for the help!

[Infraviolet]

Am I right in thinking the choice of bidirectional vs unidirectional TVS diode is dependent on the signal type you actually expect on the conductor rather than which way (always either way) you fear ESD may occur. So if that op amp always outputs above Gnd you can use either type, if it outputs above and below Gnd at different times than you need a bidriectional? The TVS can clamp an ESD spike in either direction, but a unidirectional would also do unwanted clamping of any signals going more than about 1V below Gnd, whereas a bidirectional doesn't start conducting until the selected voltage either above or below gnd?

[T3sl4co1l]

ESD is tested both directions so it doesn't much matter there, and it's down to what the nominal voltage range of the node is.

You can use a clamp diode when the voltage range is above or below a supply rail.

You can use unidirectional TVS when the voltage range would permit one clamp, but another rail is unavailable for clamping positive and negative (or various other limitations).

You can use bidirectional TVS when the voltage range is symmetrical around one rail.

If the node's range is fully constrained positive and negative, then you're done.  So, two clamp diodes between supply rails, one bidirectional to ground, etc.

When ranges are not symmetrical around one rail, you can use dissimilar unidirectionals in anti-series.  To, uh save cost maybe I suppose, you can use diodes to clamp to rails when applicable, along with either type of TVS as suitable.  Which can be regular diodes (BAT54S, BAV99, etc.), say when you have them elsewhere in the design, or they might be cheaper in some cases.

Mind that, as you make series combinations, of course the ESL increases, which is to say the peak clamping voltages add.  But adds up faster because you're also adding trace length to connect them up.

This works fine for things other than ESD, of course.  Sometimes you might use a TVS in series with a diode to get well-defined flyback voltage on a relay or solenoid driver (and occasionally SMPS, like monolithic offline regulators often suggest this), and the MOSFET (if it's big enough, anyway -- a few amps say) body diode handles the negative direction.

To clarify, MOSFETs handle ESD fine so long as it either causes avalanche within energy and current ratings, or body diode forward bias.  That is, through the channel specifically.  It's the gate that's vulnerable to voltage alone.  Note that ESD through the D-S or S-D path could still cause lift of the source, reversing the gate and causing destruction.  So keep source inductance low, or allow gate to rise together with it (e.g. differential drive with Kelvin connection), if this is a necessary feature.

Note that schottky aren't actually very good with ESD; they have higher ESR than PN diodes, and in fact generally integrate PN diodes (guard rings), which activate at higher voltage drop (which is mainly for avalanche capability AFAIK, but helps in forward bias as well).  I don't know what BAT54S for example is actually rated at (no one provides these data).  (I can say they've passed 8/15kV 10 pulses +/- plenty of times.)  But they're convenient, and maybe you need the lower signal-clamping voltage, like for using logic gates (GND/VCC clamping, or GND unidirectional type) with certain circuits that produce excess voltage (high to low level conversion, RC timer, etc.).

Tim

[Saimoun]

Am I right in thinking the choice of bidirectional vs unidirectional TVS diode is dependent on the signal type you actually expect on the conductor rather than which way (always either way) you fear ESD may occur.

Well I am not an expert like Tim - but yea that's how I do it at least Unidirectional when signal is within 0V <-> +V, and bidrectional when signal is -V <-> +V.

And I think you're fine if you use bidirectional instead of unidirectional, yes the negative clamping will be a few volts higher but like it has been mentioned, on an ESD event there will still be 30V left anyways (to be taken care of by the IC's diode, or your own BAT54 like Tim mentioned). So whether it is 33V or 30V I do not think it matters much

[MathWizard]

What about the gate of some FET's, is the insulating layer so thin that ESD burns a hole or something ?

[T3sl4co1l]

The gate oxide is just any other capacitor, with some leakage below breakdown, and, uh... breakdown above.

Generally it fails catastrophically.  Over a short term it might handle 20V, 30V, have heard even 70V -- but when it goes, it literally blows a hole out of the chip.  Typically making a three-way short of modest resistance (10s to k's of ohms).  Which is to say, a point failure, the resistance is from the entire surrounding area down to a tiny cross section.

Which, if the resistance isn't too low, you can still demonstrate "transistance" of the undamaged rest of the chip... it's just not very useful with the gate "leaking" so hard (and usually drain too)!

Compare to BJTs, which have diode and avalanche characteristics.  They can still fail suddenly; avalanche can manifest as a rapid discharge, with hot-spotting and subsequent pinhole failure if current isn't limited.  But that's kind of a special case, and other than that, they handle things alright.  At least, big enough ones do; you can always pop a microscopic RF BJT or whatever.

Power MOSFETs tend to be less vulnerable (and often aren't equipped with internal ESD diodes), due to their sheer capacitance (~nF).  But it's still just down to gate voltage, and there not being any sink current (or, more than nA~uA), until it explodes.

Small MOSFETs often provide internal ESD diodes, but they're still very small -- only good to cover nominal like 1-2kV ESD.  Presumably so as not to add too much gate leakage in the process.

Tim