Enhancing TVS diode circuit

Question

I was reading into Electromagnetic Compatibility Engineering by Henry W. Ott about ESD protections and I was interested by the following part.

First, a series resistor (or ferrite) added in front of the diodes to limit the magnitude of the ESD current, and second, additional bulk capacitance (5 to 50 mF) added across the power supply rails. Both of these approaches are shown in Fig. 15-18B

This picture reminded me of the schematics seen inside a TVS Diode. The TVS diodes I am using up till now are the TVS DIODE (würth 8240116)

• Can the shown figure actually be compared to a TVS diode?

• If so, can the functionality of TVS diodes be improved by adding the current limiting series resistance and a bulk capacitance?

• Why aren't any of these improvements suggested in the datasheet just like decoupling or bypass capacitors are recommended in every IC datasheet?

Answer 1

Can the shown figure actually be compared to a TVS diode?

A TVS has a similar function as two regular diodes, but the ESD current can be shunted to ground instead of one of the rails. The diagram above is mainly for overcurrent\overvoltage protection. A TVS has a higher breakdown voltage which is mainly useful for very high voltages such as those from ESD. You probably wouldn't want the ESD spike ending up on your power rails, even with a limiting resistor, so a port to the outside world should have TVS diodes close to the entrance of the input to the board to shunt the currents directly to ground.

Source: https://www.slideshare.net/KevinDuke/aodesignprotection2015nonda

If so, can the functionality of TVS diodes be improved by adding the current limiting series resistance and a bulk capacitance?

See this answer TVS diode before or behind resistor :

You have three components there that are all there for protecting the AVR, but all are doing a different job.

The resistor is there to stop steady state high voltages.

The capacitor is to remove ripple/RF/slow transients.

The TVS is to suppress fast transients.

In order to get the best out of your protection, you need to have the shortest (lowest inductance) path back for the fast transient impulses (such as ESD). To do this, you fit the TVS (the fastest responding device) as close to the input to the board as possible. The capacitor would then be a bit further in (depending on the layout and design) and the resistor (which only deals with very slow, or steady state situations) can pretty much be on the pin of the AVR

Why aren't any of these improvements suggested in the datasheet just like decoupling or bypass capacitors are recommended in every IC datasheet?

Because the IC datasheets don't know what the designer is going to connect the ports to. Most IC pins are connected internally on the board, and don't require extra protection. They do specify what not to do to the IC in the absolute maximum ratings section and in the ratings section for different pins. To provide this information on every datasheet would provide too much information. This information is provided in the form of app notes as it applies to many different IC's

Analog: Solving IEC System Protection for Analog Inputs and Using ESD Diodes as Voltage Clamps

TI: http://www.ti.com/interface/circuit-protection/esd-protection-and-tvs-surge-diodes/technical-documents.html

ST micro: https://www.st.com/content/st_com/en/support/resources/resource-selector.html?querycriteria=productId=CL1137$resourceCategory=technical_literature$resourceType=application_note

Answer 2

Can the shown figure actually be compared to a TVS diode?

It can be compared to the TVS diode array you referenced. Both work by having diodes to prevent the signal level from exceeding the power rails by more than a diode drop.

If so, can the functionality of TVS diodes be improved by adding the current limiting series resistance and a bulk capacitance?

Series resistance, if practical with your circuit requirements, will reduce the stress on any ESD protection device. The bulk capacitance above, from the ESD protection standpoint, fulfills the same function as the zener in the 8240116, keeping the power rail from moving much.

Why aren't any of these improvements suggested in the datasheet just like decoupling or bypass capacitors are recommended in every IC datasheet?

Because they don't know your exact design requirements. Static protection on a cell phone is different from static protection for guys jumping out of helicopters.

Answer 3

The drawing in your post does not show a TVS diode, but does have a diode array clamping to the power-rail (positive polarity) or GND. You can look at an app note (here is an OnSemi one). Usually TVS datasheets are pretty bare bones, and app notes have more information.

Having a diode array is a way to remove the capacitance of the TVS diode or capacitors that the ESD (or lightning) sourced current is being shunted into. It also allows small diode arrays to steer multiple inputs into a single power rail or protective device.

Including bulk capacitance is probably not relevant for situations that include a TVS, although it does impact the amount of over-voltage excursion between the TVS hitting threshold and actually clamping.

Using resistors is common in front of most clamping methods. If the parts are already designed to handle transients, it might not be necessary. And series resistors can also have a dramatic input on the possible signal rate you can get through a net.

As an aside on clamping to a power rail as shown in your initial post, that setup can easily cause system upsets / resets if any device you are using needs consistent power rails. Having more capacitance can limit the amount of movement, but you can also use a resistor to isolate your power rail from the capacitor that clamping current is being dumped into.

Answer 4

TVS diodes are multi- and single-purpose depending line impedance and energy absorption levels.

The part you showed has a differential line with a full-wave bridge and a power Zener.

vs.

The block diagram shows a single line with a half-bridge to a power Zener to a bulk storage cap.

Current limiting is normally externally dependant on signal impedance requirements . For example all CMOS being low input current , high speed, use a series R of 10k for inputs with 2 high speed diodes in half bridge to each rail rated for 5mA DC followed by a 2nd similar diode pair to rails for each input. Pulse current drops voltage in the series R then clamps further to <=0.2 outside each rail with Schottky diodes. This is adequate for 100pF 1k~2kV ESD discharges only.

Ferrite would be used for low impedance circuits near transients << 1us.