Hello guys,

I have a question regarding ESD protection.

I have seen many designs that use ESD capacitor and TVS diode on the input side.
The most TVS diodes have very high ESD voltage more than you could achieve with MLCC 100nF.

Let say if you want to protect15K V human body air discharge.

Then why people keep both TVS and a cap instead of just single TVS to prevent ESD?

Thanks in advance.

Justin.

 

To stop the harmonic noise that may be generated by the TVS..

 

To stop the harmonic noise that may be generated by the TVS..

Thanks for the comment.

Just rephrase your words, a capacitor is not there for ESD purpose it is more like a filter for harmonic noise. Then how would you choose the capacitance?

 

Hello guys,

I have a question regarding ESD protection.

I have seen many designs that use ESD capacitor and TVS diode on the input side.
The most TVS diodes have very high ESD voltage more than you could achieve with MLCC 100nF.

Let say if you want to protect15K V human body air discharge.

Then why people keep both TVS and a cap instead of just single TVS to prevent ESD?

Thanks in advance.

Justin.

This one knows what he is doing.

Most of us pick the capacitors from practice, we do not calculate them. We just test them by putting 1 value, if its not enough we increase it.

 

Without a sample circuit or two it is hard to say what is intended. You may be looking at examples that have been carefully designed by someone who understands all of the details or examples where someone has just slapped in some components without any real knowledge.

Where circumstances are difficult, such as where protection against lightning is required, it is not unusual to find at least two types of protective device with added impedances to help assure that each can perform its task effectively. For example, in devices connected to telephone lines there will be protective devices at the entry of the line to the premises, then TVS diodes and possibly a gas-discharge tube in the connected equipment.

A capacitor to ground at an input with a series resistor to the vulnerable point (connector, etc.) can go a long way to providing some degree of ESD protection. In some circuits, such as switch debouncers, the R and C are necessary anyway. In some circuits, neither the R nor the C can be tolerated. Larger TVS diodes have a lot of capacitance that is likewise intolerable in some applications, such as protecting high-speed digital signals like USB. For this sort of application specially designed TVS diodes with ultra low capacitance are necessary. IC manufacturers go to considerable trouble to "harden" I/Os against ESD especially where ESD is to be expected as a regular occurrence and the signal characteristics place constraints on what can be done externally.

In order for ESD protective devices to work effectively it is essential to manage parasitic inductance carefully. Inductance in the right place is beneficial, but in the wrong place can be seriously detrimental.

You should be able to find lots of good applications info on the web from manufacturers of ESD protection devices. Littelfuse has a fairly broad line of products. NXP (Nexperia) produces some low-capacitance TVS arrays suitable for fast signals. I believe Vishay and ON Semi both produce TVS diodes. You should also be able to find info from manufacturers of ICs that need extra protection.