Sensitivity of Schottky Diodes to ESD, effectiveness vs TVS diodes

Question

I'm trying to protect a component against ESD strikes - this component is particularly sensitive, so I am also trying to protect against sub-ns strikes. Forward voltage (max allowable) is 8V, reverse is 2V. Normally operated at 7V.

The normal approach would be to specify a ESD specific unidirectional TVS connected in parallel as close to this component as possible. One with vary fast transient response.

Another might be to use a high speed varistor (such as TDK Ceradiode, claimed <0.5ns response), however this is bi-directional and the clamping voltage may be a little high.

The addition of a ceramic capacitor in parallel may also help absorb some of the excess energy.

After chatting with someone on this topic, they insisted that schottky diode connected anti parallel is the best solution - their claim 'nothing is faster' might be true but I have three concerns:

1. Schottky diodes may actually be damaged by these transients themselves.
2. It will only protect against ESD strikes in the one direction.
3. The forward voltage of a Schottky may be lower, but will it actually be quicker or perform (responsiveness / clamping below 2V) any better than a TVS diode built for the job such as this? (in the one direction).

The forward voltage of unidirectional TVS's don't seem to be specified either.

I'm interested to hear what people think about each of the points above - especially if there would be any benefit at all to using a Schottky in combination with the above.

Answer 1

The part you link to shows the inherent problem with all TVS devices; the clamping voltage has a huge range:

Looking at the 7.5V part, it has a reverse standoff voltage of 7.5V (quite reasonably). At this voltage there will be no more than 50μA of leakage current, but the maximum clamping voltage (at maximum clamp current) is 12.9V.

The Littelfuse devices are very fast acting (because the internal parasitics are minimised) but I have used some parts that do not properly clamp for several hundred nanoseconds even though the manufacturers model showed instantaneous clamping (and lightning testing destroyed several parts before that was tamed with added circuitry).

To guarantee clamping voltages, I would normally use a pair of schottky diodes with a series resistor (to limit the clamping current) and a small capacitor (not always possible - high speed data lines are the very devil to properly protect).

^{simulate this circuit – Schematic created using CircuitLab}

With schottkys, be careful of reverse leakage; a properly chosen part will be ok, but the leakage is exponential with temperature and can cause thermal runaway in certain circumstances.