ESD and surge Protection

I have been going through the reference designs from microcontroller vendors and schematics of several development boards and everyone seem to include ESD and surge protection only for USB and CAN connectors but not for Audio (Line-in, Mic, Line-out), SD/MMC or RS232 ports. Is there any specific reason for this? Also, is there any board level solution for ESD and surge protection instead of trying to protect every port on the board?



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Bhavani enquires

When choosing a surge protection device, you don't want it to interfere with the signal it guards. The obvious parameter to watch is its breakdown voltage, but there are a couple of others:

- High speed signals, and this includes some types of high speed comms lines, don't like lots of capacitance on them. "Lots" can be, say, 20pF

- basically if the time constant of the transmission line and this C is more than the risetime of the signals, you have problems. Imagine a

10MHz signal trying to get through a 1MHz low pass filter. Complicating the issue is that the capacitance of the protection devices varies with the voltage, so you need to check the details in the curves of the device datasheet.

This also affects many analogue signals. I'm not sure where you begin to hear audio effects, but let's say you can hear a loss of treble if you lose harmonics above 15kHz. Audio signals are often on 600 ohm lines, so more than 17nF of capacitance on the line will be audible. (That might be a bad example because it would be tricky to find a transient suppressor more than about 100pF. But you get the idea.)

- The breakdown voltage of transient suppressors is not very sharp. A 5V "holdoff" means it has maybe 1uA leakage at 5V, and is fully turned on around 9V. See device datasheet for details. And this will vary with temperature. While a digital signal won't much care about 10 or 100uA leakage, a high quality analogue signal will - especially as this is a non linear leakage (increasing as you get nearer the clamping voltage).

You certainly can get ESD protection devices for RS232 voltage levels. People used to use varistors a lot, but personally I went off those in favour of bidirectional transient suppressors when they became cheap, because varistors seemed to sometimes fail in a medium-impedance leaky mode. Maybe people don't use them much now because the ESD protection

*inside* the RS232 IC's is greatly improved now. But I would always add tranzorbs to those lines, unless it was REALLY cheap equipment. I have also seen people deliberately add series R, and C to earth on slow signals like RS232 thus:

IC---100 ohms---+------ input / ESD spark | == 1nF | 0V

the idea is that incoming ESD sparks at say 15kV from a standard "human body model" ESD generator (which is discharged from a 150pF capacitor) are divided down to about one tenth that voltage by the 1nF capacitor, then the resistor takes some more of the strain of dealing with the ESD off the IC's internal catch diodes. The resistors need a good voltage rating. This is rare these days as signal frequencies have increased, the RC time constant is a problem.

Another thing to consider: if your connector has no metal showing, there is nothing for ESD sparks to hit. So less need to protect.

For a magic board-level component you can use for multiple lines, I suggest you search for "transient suppressor arrays" in your favourite online catalogue.

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The USER needs to be ESD aware. That means family members... everyone. It is just common sense to know that ESD and your gadgets do not go together.

Note how most such connectors as those you mention have the ground shell making contact before any other elements of the connector. One should be constantly aware of ESD hazards whenever one is working around open circuits.

In other words, you should always make it a point to ground yourself often, releasing any incidental charges you may and usually do accumulate. Then, if your device was recently plugged in, you can be reasonably sure that it was/is at close to ground level, so you are not imbalanced with it either, which if you did not constantly ground yourself, you WILL build a charge, and it only take a mere few volts to breach a modern chip element as they are very tiny.

My Atom processed PC (7" square) is inside a can inside a plastic box. ALL of the connectors at ALL locations around this board have their ground shroud connected to this can, as is the motherboard itself.

This is ideal. Try to reduce your charge levels whenever you get a chance, and refrain from doing it only when you are in or near a lightning storm.

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