Choosing ESD Suppression Diodes

[bamix]

I've tried to use various sources online to answer the following questions, but I haven't found any that matches exactly my concerns. This is my first time designing a circuit, so please bear with me.

I'm designing a circuit with the following components needing ESD protection:

RS-485 cable connection to 5x2 header with 5 signals included: +5V power, Rx+, Rx-, Tx+, Tx-; SN75176BP is transceiver used; signals are 0 to 5 VDC; transceiver has absolute max voltage rating of +15V

ESE-24SV3 detector switches connected to AT89S8253 MCU; MCU ports read in logic High-Low; Vcc for MCU is +5V; MCU has absolute max voltage rating of +7V

I have found TVS diodes with standoff voltages greater than the normal operating voltages of the signals and clamping voltages less than the MCU and transceiver absolute max ratings. However, the clamping voltages increase as more current is applied, and at ESD discharge of 8kV the clamping voltages are way higher than the absolute maximum rating, while lower discharge voltages stay under the clamping voltage requirement.

Questions:
1. I'm wondering how I should go about choosing the diodes, since it seems like there will always be a point where the diodes will not be able to clamp voltage low enough.

2. For example, if I want my standoff voltage to be >5.5V and clamping <6 V, and I can only manage to find diodes that have clamping voltage < 6V while standoff voltage >20 V, would it be okay to choose such diodes? My main concern is that the standoff voltage is so much higher than the normal operating range, which means that the diode will not start conducting until the device is basically exposed to a dangerous voltage.

 

[djs]

Instead of a single component, use three. The current state of the art for surge suppressors is a three stage device: starting from the un-protected side a MOV or glass discharge tube which can handle large currents but might be slow and have high let through current. A second stage consisting of a small inductor or resister and finally a quick acting semi-conducter diode with low let-through voltage. There are commerically made units available in this configuration.

 

[IRstuff]

Sorry, but you generate all these words, but hardly anything about why you have 8kV ESD, and why it's high current.

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[bamix]

Well I'm just going off of the some of the datasheet plots and values, which I suppose could be due to my lack of understanding on how to read the diode's datasheets.

For instance, the part DF2S14P2CTC, on page 4 with the table of characteristics, for different IPP and ITLP different VCLs exist. For IPP of 50 A, for example, the VCL is well above the absolute max voltage the IC can withstand, which I feel like would be a problem.

As for why I have 8kV ESD, isn't 8 kV contact discharge one of the levels of IEC 61000-4-2? And why it's high current... well I suppose that's dependent on the device.

If it makes my questions less complicated, disregard anything I said about 8kV ESD and high current, since the actual questions listed are my questions.

 

[IRstuff]

Agree with djs; but note that any external ESD and TVS will add capacitive loading to your transceiver circuit, which could affect its SNR and frequency performance. The more integrated the solution, the less parasitics intrusion into the normal operation of the circuit.

You might want to start by looking at the links below.

https://www.analog.com/media/en/technical-documentation/application-notes/AN-1436.pdf

https://www.analog.com/media/en/technical-documentation/data-sheets/ADG5412F-EP.pdf

TTFN (ta ta for now)
I can do absolutely anything. I'm an expert!

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[djs]

I guess a question needs to be asked what baud rate is being used? Commercially available three stage SS are commonly available up to 100K or so. Some specialized ones got to 12 Mhz.

 

[DRELECTRON]

If you know the bandwidth required and capacitance you can use primary and secondary protection with a resistor in series between the protections. The resistor could be they 10-500 ohms depending upon the requirements. The primary protection connected to the connector should be IEC/cdm capable.
Look at CM6302 from onsemi.