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My objective is to protect the pins of a device.

The pins are connected to an external connector which might be exposed to an ESD event.

Hence, I need to place an ESD protection device on that line.

So, my understanding is that, suppose, let me assume that the pin I am trying to protect is a USB signal from a USB hub which is going to a USB connector.

What I do is, search for a diode. The only parameter that I look for is the clamping voltage of the diode. Since the USB signal voltages during normal operation don't go beyond 4V, I select a diode that has a clamping voltage of 5V (max) @ 1A. And I make sure this clamping voltage of 5V doesn't exceed the absolute maximum ratings of the device's pin in the Hub, which is 5.5V.

Is my approach correct? If not please tell me what is wrong.

Also, please let me know if I have to consider additional paramters while selecting an ESD diode to protect the pin of a device in general.

Thanks.

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  • \$\begingroup\$ What ESD specification are you working with and, what discharge voltage category are you considering? \$\endgroup\$
    – Andy aka
    Commented May 19, 2023 at 11:01
  • \$\begingroup\$ "Hence" doesn't quite follow, actually: USB connectors are shielded, and it takes quite a clever fool* to get ESD into the signal pins! (*As the saying goes: "nature will always make a better fool.") That said: a charged-cable event could be possible, which has very similar consequences to ESD. \$\endgroup\$ Commented May 19, 2023 at 11:14
  • \$\begingroup\$ @Andyaka, at this moment, I do not have any specification nor I do know of it. Just wanted to get my understanding right? Whether this is the right approach? \$\endgroup\$
    – Freshman
    Commented May 19, 2023 at 11:18
  • \$\begingroup\$ @TimWilliams , ok, Whether my understanding is correct? \$\endgroup\$
    – Freshman
    Commented May 19, 2023 at 11:18

2 Answers 2

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Fortunately a solution exists: the clamping voltage is not so strict as you imagine.

Ideally, we would have clamping voltage precisely no more than the maximum rating of the device (for a USB transceiver, that might be -0.3V to VCC + 0.3V), for any input current.

Alas, that is infeasible: the only clamping devices that precise (i.e. within fractions of a volt) are shunt regulators, and those contain an internal control loop which means they cannot respond immediately, but must ramp up their response -- so as to avoid overshoot or oscillation, and so as to land precisely on the target voltage -- precisely, but eventually. (The response time might be some microseconds, say for a TL431 with current-boosting transistor.)

There are other reasons, too: to sink say the 10A pulse of an ESD strike, quite a large transistor would be needed. So not only would the clamp take some microseconds to respond, but it would load the signal with some nF of junction capacitance -- pretty much making a USB link impossible, certainly at high speed.

In contrast, devices with low enough capacitance to be suitable, fast enough response speed, and high enough voltage rating not to load normal signals, have peak clamping voltages of say 10-20V.

So we must fall back on a less clear-cut clamping strategy.

The next best approach is to (in part) utilize the device's own clamping diodes. Say it's rated for 2kV HBM. If the application requires 8/15kV (contact/air discharge), it must be attenuated by about 8x, while presenting the same source impedance as seen by the IO pin. Or if the impedance is lower, the attenuation must be proportionally higher.

This is an important insight, because when we clamp ESD with a diode, the diode's internal resistance determines the circuit impedance. It might be a few ohms -- which provides effective clamping of the ~10A peak pulse -- but the remaining 10-20V then has that as the source impedance. Which means, if we wire it directly to the IO pin, the "10V ESD" pulse, at 1.5Ω, looks an awful lot like 10kV at 1.5kΩ -- if the IO pin is capable of sinking the same amount of current that is.

In practice, the IO pin will have some internal resistance too, and likely more than the external TVS diode -- so the two protective devices act together as a current divider, with more likely going to the external diode, and so ESD rating is already improved. But we're still putting quite a bit of energy into the IO pin, and it would be nice to avoid that.

The simple solution is to add resistance between them.

schematic

simulate this circuit – Schematic created using CircuitLab

Since the source impedance is quite low (ohms if that?), not much resistance is needed; 10Ω is enough to make a significant difference, and the more the merrier -- limited by signal bandwidth, and current requirements (when the IO is outputting).

Now, this is a solution, but mind what it's doing: it's still relying on internal clamp diodes. If current flow through these should cause internal latchup (typically CMOS devices exhibit this behavior for more than 100 or 200 mA of injected current), you have to power cycle the unit to restore normal operation. Which is termed class C compliance (device malfunctions until user intervention i.e. turn it off and back on, but otherwise survives without permanent damage.)

If you require class A or B compliance (no effect on performance or functionality; adverse effect, but recovers automatically), then a lower level of current injection is required. Perhaps a second stage clamp can be employed (diode, resistor, diode, resistor, IO pin). If the device gives a maximum current injection spec while maintaining normal operation (e.g., many MCUs indicate 5 or 10mA is acceptable on general-purpose pins), increase the resistance to meet that rating.

These are very good reasons why high-speed communications cables, like USB, Firewire, HDMI, etc. use shielded connectors with small pins hidden inside a shroud -- it is very difficult to subject such pins to errant ESD strikes without hitting the shroud instead, greatly increasing reliability without having to add any ESD devices at all.

It's still possible to strike such pins, of course -- a highly determined fool might hold a wire in their hand while probing into the housing, thus exposing individual pins to hazardous discharge. But that's a pretty contrived situation. And the more contrived these situations get, the less likely they are to occur in normal use. So connector and cable design is a big part of these standards being as reliable as they are.

The other possible situation is static charge accumulating on a cable. This is also somewhat contrived, as cables don't normally accumulate charge between their conductors. (On the whole thing, if unplugged at both ends -- sure, that's easy; but between wires in the cable itself?) But there are circumstances that can lead to that, and so a charged-cable event may be a similar concern.

Charged-cable transients are more powerful than ESD -- the impedance is lower, the characteristic impedance of the cable (say 50 to 120Ω) rather than an ESD network (300Ω or more?). But the voltage may also not be much, either, and so the effect can be quite similar to ESD -- in any case, it's handled the same way, too.

Mind that this all assumes that voltages and currents are where they are assumed to be. Typically this will be the case with a ground-plane PCB design, with the USB (or etc.) shield/shell grounded solidly to the plane. (It doesn't have to be a galvanic connection, the AC grounding is what's important; but if you're allowing some DC isolation, several bypass capacitors in parallel must be used, spread evenly around the connector / ground pins.) It will not, in general, be the case for a loose layout, like free wires in space (prototyping board), or a naive point-to-point layout without ground pour/plane. ESD contains frequencies that "do not like to stay in wires", and ground planes and shields are required to direct it into the relevant structures.

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  • \$\begingroup\$ Thank you for the answer. I am going through the answer and trying to understand it. I do not understand this sentence of yours, " So not only would the clamp take some microseconds to respond, but it would load the signal with some nF of junction capacitance". How does the capacitance gets added to the signal when ESD clamping occurs? \$\endgroup\$
    – Freshman
    Commented May 19, 2023 at 12:08
  • \$\begingroup\$ Could you also clarify, "which provides effective clamping of the ~10A peak pulse -- but the remaining 10-20V then has that as the source impedance. " From where is the 10V-20V coming from? \$\endgroup\$
    – Freshman
    Commented May 19, 2023 at 12:11
  • \$\begingroup\$ The capacitance would be always present, a consequence of the large (but most of the time, inactive) transistor required by the regulator. For example, suppose a TL431 shunt regulator were used with a 2SCR582D3TL1 in a current-boosted shunt regulator circuit. As for the "10-20V", consider the voltage drop on "TVS" in the diagram when exposed to say a 10A ESD pulse. \$\endgroup\$ Commented May 19, 2023 at 12:14
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What you'll need is a TVS diode array, which is connected in parallel to those pins. The TVS diode is shunted to the ground. So that will take care of any ESD events.

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  • \$\begingroup\$ This answer provides zero quantitative guidance. Randomly picking "TVS diodes" will get them nowhere. \$\endgroup\$
    – tobalt
    Commented May 19, 2023 at 12:51

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