ESD Protection for low power

Thread Starter

Gorden

Joined Oct 30, 2019
25
Hi,

I want to understand the use (need) of ESD protection diodes.

I have a small sensor board located a few metres from a main board, it operates at 5V. So i have a +V, 0V and a signal. The signal carries simple serial data between two microcontrollers. (just every now and again)

I have already used a diode array, but my question is:-

1, Why do these devices exist in unidirectional mode - if transients could be both positive and negative on any line relative to any other line, is it not better to use a bi directional all the time?

2, What effect does choosing asymmetrical or symmetrical have on the actual funtion - i can see that in asymmetrical direction you have the breakdown voltage, in the other you have the forward voltage, and in symmetrical both are the same, based on breakdown.

3, I have used the device GSOT-05C, this is confusing becuase the datasheet describes using all three pins for a two line protection (bi directional asymmetrical mode) or using two pins for a one line protection (bi directional symmetyrical mode) - can anyone help with this?

Is this device really going to help much with potential pickup on the cable from things turning on and off....

Any guidence would be appreciated.

Thanks
 

dl324

Joined Mar 30, 2015
14,462
There's a reason why your post has 47 views and no responses.

Show us a schematic or block diagram for the circuit in question so we can see what you're trying to protect.

Save us some bother and post the datasheet you referenced.
 

Thread Starter

Gorden

Joined Oct 30, 2019
25
Hi dl324,

The datasheet is GSOT-05C page 5, 8.

The question relates to the general use of TVS diodes, not a specific circuit.

As explained, i was trying to understand the need for Uni-directional devices, when Bi-directional would appear to work in both cases.

The datasheet confused me because it appears to show a Uni-directional diagram (Page 5) but calls it a Bi-directional device.

Thanks.





 

MisterBill2

Joined Jan 23, 2018
11,880
OK, here is an answer for a general input circuit working with a 5 volt system. For starters, the protection system is intended to prevent input voltages from reaching the protected device, which device specification must not have input voltages above five volts or below zero volts. That represents the safe input voltage range, although usually it is negative 1 volt and positive 6 volts for the limits.
Now for protection we assume that the 5 volt supply is a low resistance source, so that the 5 volts will not vary with current drawn.
So from the input line two diodes are connected, one diode so that it is biased into forward conduction when the input source exceeds 5 volts, the other diode is connected so that it is biased when the input drops below zero volts. There must always be a resistor in series with the input source to limit the current when the input biases a diode into conduction.
There you have the functional description of the diode resistor input protection scheme, and how it works. No schematic needed, but if any are not able to visualize such a simple arrangement that is too bad.
 

dl324

Joined Mar 30, 2015
14,462
As explained, i was trying to understand the need for Uni-directional devices, when Bi-directional would appear to work in both cases.

The datasheet confused me because it appears to show a Uni-directional diagram (Page 5) but calls it a Bi-directional device.
All TVS diodes are bi-directional; they're just not symmetrical. Typical ESD protection doesn't use TVS diodes. They use diodes that clamp voltages from ESD events to the supply rails. Protection is typically only tested to a few thousand volts.

From RCA:
1653664508470.png

Whether you even need protection depends on the environmental conditions.

If you have signals going over long wires that are susceptible to things like lightning strikes, then you'd want something that could handle high transient voltages and their associated currents. Operating in electrically noisy environments, e.g. cars, also requires protection, but it's not considered ESD and protection is typically only used on power rails.
 
Last edited:

DrBearEE

Joined Feb 3, 2020
8
I think perhaps the confusion is that a "unidirectional" protection diode such as this is called that because its intended to protect a *unidirectional signal* (where the voltage is normally always at or above the ground reference). The *device* conducts in *both * directions, as required to protect against transient voltages above the signal maximum, or below ground.

Vishay actually did a good job explaining the operation in the referenced datasheet.

"As soon as any positive transient voltage signal exceeds the breakdown voltage level of the protection diode, the diode becomes conductive and shorts the transient current to ground. Now the protection device behaves like a closed switch. The Clamping Voltage (VC) is defined by the breakdown voltage (VBR) level plus the voltage drop at the series impedance (resistance and inductance) of the protection diode...

...Any negative transient signal will be clamped accordingly. The negative transient current is flowing in the forward direction through the protection diode. The low Forward Voltage (VF) clamps the negative transient close to the ground level...

...Due to the different clamping levels in forward and reverse direction the GSOTxxC clamping behavior is Bidirectional and Asymmetrical (BiAs)."
 

Thread Starter

Gorden

Joined Oct 30, 2019
25
Thankyou MisterBill2 & dl324,

I think perhaps i did not ask the question correctly. I should really have referenced the datasheet more, because thats the bit that confused me.

When you say all TVS diodes are bi-directional, i think that was my point, why do some manufacturers sell specifically
uni-directional.

Why would you not always want a bi-directional symmetrical function?

Thanks.
 

dl324

Joined Mar 30, 2015
14,462
When you say all TVS diodes are bi-directional, i think that was my point, why do some manufacturers sell specifically
uni-directional.
Sounds like it's just marketing to me. Like buying ham that's gluten free. Everyone should know ham doesn't have gluten, but maybe some don't.
 

Thread Starter

Gorden

Joined Oct 30, 2019
25
Hello DrBearEE,

Just crossed over in replies.

Thank you for the help.

But this is still the problem for me - they are calling this a bi-directional, asymmetrical in one configuration and symmetrical in the other - is that not the case with all TVS diodes, so my question why do manufacturers sell uni-directional.

As dl324 said, all TVS diodes are bi-directional in that they will conduct both ways, just not equally.
 

dl324

Joined Mar 30, 2015
14,462
As dl324 said, all TVS diodes are bi-directional in that they will conduct both ways, just not equally.
It could be that bi-directional is implied to be symmetrical unless it's specifically noted that it's asymmetrical.

If you put two "unidirectional" TVS diodes in series, you can make one that's symmetrical when operated bi-directionally.
 

MisterBill2

Joined Jan 23, 2018
11,880
Fig 2 in post #5 shows what I was describing. And most non-linear devices are not really exactly symmetrical. And besides all of that, there is the fact that there are two points where protection must start, and an interval between those points where protection should have no effect. In the example, from zero to +5 volts is OK, and so there are two end points of the safe zone, so a bidirectional protection device will interfere.
 
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