EEVblog Electronics Community Forum
Electronics => Beginners => Topic started by: king.oslo on October 21, 2012, 12:54:13 am
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Hello there,
I was thinking, if I find a 12V light bulb, and I connect it to only the positive terminal of a 12V battery, without a connection to the negative terminal, it doesn't light up.
So I would think that if I am electrostatically charged up to 8kV from walking across my carpeted floors, and touch the pins my IC, no current would flow, and no damage would be done to the IC. Is this the case? Does the IC have the be grounded to the same point as I am, to cause ESD-damage to it?
I tried to simulate it using LTspice. I got some results which I thought were weird: LTspice says current will flow be moved "through" C1, even without GND! What is more confusing to me: it seems like my input protection is helpless in making the 8kV propagate to the IC.
Can somebody please explain what is going on in the real world?
Thanks.
Kind regards,
Marius
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Can somebody please explain what is going on in the real world?
In a word, capacitance. Just as water always tries to find a level, so does voltage. If you are charged up to 8 kV and a poor unsuspecting chip is sitting at 0 V, then when you touch the pins of the chip the voltage on your hands will try to bring the chip up to 8 kV to equalize the voltages. For voltage to change, current must flow. Therefore, current will flow from your fingers into the pins of the chip, and will try to jump across the delicate gates of the transistors inside the chip, punching holes in them (they are able to handle 3.3 V or 5 V, not 100 V, and certainly not 8 kV).
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also the point that all voltage is absolute, just commonly measured as a relative voltage between 2 points,
your carpet is generally at or close to your houses ground potential due to leakages, where that ground sits in the scheme of things may well be a few Million volt absolute but we call it ground and assume its 0, the battery has also been sitting on materials with leakages so it on its own over time will approach becoming centered around ground, but AC noise and other EMI may shift it one way or another by a bit,
then comes you the user you walk along the carpet and charge up a few KV faster than the carpet can dissipate it, when you go and touch either side of your battery powered / isolated system, its unlikely you will be at the exact same potential, so depending on how great the difference, there will be a spike where the 2 potentials try to equalise, as 2 small capacitances, with current flowing from the small resistance value of a few K ohms to whatever is on the other side of the circuit, or when the voltage is higher, you get an arc, or static zap, with is down in the low hundreds of ohms,
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Can somebody please explain what is going on in the real world?
In a word, capacitance. Just as water always tries to find a level, so does voltage. If you are charged up to 8 kV and a poor unsuspecting chip is sitting at 0 V, then when you touch the pins of the chip the voltage on your hands will try to bring the chip up to 8 kV to equalize the voltages. For voltage to change, current must flow. Therefore, current will flow from your fingers into the pins of the chip, and will try to jump across the delicate gates of the transistors inside the chip, punching holes in them (they are able to handle 3.3 V or 5 V, not 100 V, and certainly not 8 kV).
A similar situation occurs when the chip was charged and you ground it, and that's why ESD straps and mats are resistive and not just short circuits; to limit the current that can flow as the charges equalise.
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This is not what I wanted to ask.
If you look at my schematic, the load and input protection circuitry are missing earth GND. If they were grounded to the same point as the ESD source, they would be highly effective against the ESD, but it seems to me, that the 8kV will propagate throughout the circuit if the other circuitry isnt grounded.
So is my imput protection useless if it is, say battery powered, or disconnected from earth ground?
Does battery powered devices with a high impedance to ground not require ESD input protection?
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The IC is hopefully connected to the lower end of the transient clamp. This then does provide protection, but any other inputs or outputs, including the other supply rails, still need the same sype of ESD clamp on them as well, otherwise you have still got the fast flow of charge from the transient event, but now on those other pins via their capacitance to the ground plane. The ESD protection needs to be on all pins of the device that extend out of the ground plane of the board.
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This is not what I wanted to ask.
If you look at my schematic, the load and input protection circuitry are missing earth GND. If they were grounded to the same point as the ESD source, they would be highly effective against the ESD, but it seems to me, that the 8kV will propagate throughout the circuit if the other circuitry isnt grounded.
So is my imput protection useless if it is, say battery powered, or disconnected from earth ground?
Does battery powered devices with a high impedance to ground not require ESD input protection?
It's not really a matter of earth ground, or whether anything is connected to earth ground or not.
To protect against ESD damage, what matters is to avoid large voltage differentials between different pins on the device. Keeping all pins at the same potential is one way to do this, which is why static sensitive chips are inserted in conductive foam pads for shipping. When installed in a circuit, the surrounding circuitry will usually provide some measure of protection by joining the pins up and preventing them floating to wild potentials.
But really, the only sure way to protect against ESD damage is to enclose the circuit and prevent any high voltages getting near it. You can damage a memory card while holding it in your hand, but after it has been plugged into the memory socket and the case put back on the computer it is reasonably safe.