DC Power Supply EMC Filter for Surge / EFT & ESD

[eengr]

Hi There
I am trying to design an input filter and protection circuitry for my PCB.
The circuit contains,

• Over current protection
• Reverse polarity protection
• Over Voltage protection
• EMC protection against ESD, EFT, Surge and RF Common Mode conducted immunity + radiation

I have two circuit options as shown below as Circuit-1 & Circuit-2



The only difference between the two circuits is the type of reverse polarity protection.

• Circuit-1 uses PMOSFET (hence low series voltage drop)
• Circuit-2 used Diode only

I have tested the filter around Common mode choke (i.e., Common mode choke with three capacitors on each side) at a test house and it has passed RF Common Mode conducted Immunity + Radiation test.
There were some other bugs in the circuit (in rest of electronics – nothing to do with this section of the circuit that require re-spinning the board). But, I did not have enough time left to test the circuit for ESD, EFT and Surge. Before next board manufacturing & EMC testing, I would like to make some changes to this circuit.
The question I have is specific to the protection against these tests i.e.,
ESD, EFT and Surge
The circuit is DC Power operated. (Input Vol_Pos is connected to voltage in the range of 12-30V DC)
The maximum current that circuit takes is 400mA at 12V
The levels that I would be testing against are:

• Surge: +/-1kV – L-L & +/-2kV L-E ------- (1,2/50 -----8/20------Tr/Th us)
• ESD: +/- 6kV contact
• EFT: +/-2kV -------------- (5/50 ------Tr/Th – ns) (5kHz repetition)

I understand that MOV and TVS are recommended at Input of DC Supply to protect the circuitry
From the application note of TVS selection guide:
https://www.microsemi.com/document-portal/doc_view/14650-how-to-select-a-transient-voltage-suppressor



there are three different levels of protections that could be achieved using TVS as:

• board level
• secondary level
• primary level

I would like to protect my reverse polarity protection components as well as the rest of electronics (that comes after Common mode choke). By putting a TVS after the Common choke, I could satisfy the ‘secondary level’ of protection as the series resistance of Common Mode choke and the fuse will help in limiting the current impulses. It will only protect the downstream electronics in that case. But in order to protect the Reverse polarity circuit (PMOSFET for Circuit-1 or Diode for Circuit-2), I need to have this TVS before the protection circuitry as shown in the figures above. The type of TVS I am using is SM15T39A ---- 1500W)
https://www.farnell.com/datasheets/1670952.pdf?_ga=2.260830862.201980731.1494499452-1067632968.1490966146

The questions:
would it be safe to have this Power rating TVS installed at the location shown in the figures and Not get damaged by Surge/EFT & ESD test?

What component rating I need to look into when selecting the reverse protection MOSFET or Diode (in addition to their current handling capacity) so that they are still safe during the Surge/EFT & ESD test?

What headroom is considered safe above the clamping / breakdown voltage of MOV & TVS for the MOSFET / Diode?

Do I need bidirectional OR unidirectional TVS? (The input is DC voltage). Surge/EFT & ESD are +/- voltages.

BOM:
Fuse: 500mA
TVS: SM15T39A (link above)
MOV: Wurth Elect: 820 513 001
https://katalog.we-online.de/en/pbs/WE-VD/820513001?m=n&sp=http%3A%2F%2Fwww.we-online.de%2Fweb%2Fen%2Felectronic_components%2Fsearchpage_PBS.php%3Fsearch%3D820%2B513%2B001#vs_ct:3
MOSFET I am planning to use: FDN5618P
https://www.mouser.com/ds/2/149/FDN5618P-888996.pdf
Rated for 60V max & 1.25A ---SOT23 package
Diode – Not sure
Zener is there for addition protection against overvoltage to blow the fuse in case of continuous overvoltage applied.
Filter caps on each side of the CM coke are: 100uF / 100nF & 10nF

Kind Regards

 

[westom]

Your circuit (for 120 volt operation) must withstand up to 600 volts without damage. Since that was an international design standard even before PCs existed.

You have assumed normal mode transients. Numerous and completely different parameters exist for each anomaly (ie esd, surge, overvoltage, RFI, reverse polarity, floating ground, noise, etc). Also relevant are constant current source anomalies verses a voltage source. And, of course, some transients can be normal mode. Most destructive types are longitudinal mode.

Many mistakenly assume transients in terms of a neutral wire to hot wire voltage. That is not a typically destructive transient.

Protection also means this question is answered. Where do hundreds of thousands of joules harmlessly dissipate? Protection from anomalies that cause damage? Or protection that only cause data problems or intermittents?