I have two options for input filter having ferrite bead and reverse polarity protection diode as shown below


simulate this circuit – Schematic created using CircuitLab

The input supply is from 9 - 16 VDC. This is meant to protect the solution from EM immunity requirements as per ISO 7637.

I am also unsure about the values of the capacitor to be used in option 2.

The input current is around 1.5A.

Which of the above two options fits the best?

Also, how should I place these components on the PCB.

  • \$\begingroup\$ D1 has significant capacitance because its a power diode or better still a schottky.This means low impedence at most relevent EMC frequencies.Hence the slightly more complex option 2 wont outperform option 1 so option 1. \$\endgroup\$
    – Autistic
    Nov 21, 2015 at 9:57

1 Answer 1


The ISO 7637 pulse test requirement is this: -

enter image description here

And, with those sort of slow transients a ferrite bead will be absolutely useless. Both circuits will be ineffective against pulses. These are the sorts of voltages you need to be protecting from: -

enter image description here

You should look to protect your circuit from load dumps like this: -

enter image description here

But be aware the currents can be high. Maybe you look at a series regulation method of protection like this: -

enter image description here

Picture taken from this useful article on load dumps (Maxim app note 4081)

  • \$\begingroup\$ The scale on the pulse graph seems slightly unusual when 87V is higher than 100V! \$\endgroup\$
    – David
    Nov 21, 2015 at 23:38
  • \$\begingroup\$ Sorry, I think I mentioned the conducted immunity standard. What about the contribution of the ferrite bead and the capacitor filter in protecting against the radiated immunity requirements of ISO 11452 standards? In case of BCI and ALSE methods, would option 1 fits the best or option 2 fits best? \$\endgroup\$
    – Shaun
    Nov 22, 2015 at 7:07
  • \$\begingroup\$ I read the literature and believed the pulse test to be part of that standard. It's the only one I could find online. If you know where to find the spec you mean then please leave a link. Ditto ISO 11452 although this wasn't part of your original question. \$\endgroup\$
    – Andy aka
    Nov 22, 2015 at 12:00
  • \$\begingroup\$ Here you can download the standard: emfine.cn/FileUPLoad%5CarticleFile%5C635392015568460876.PDF \$\endgroup\$
    – Shaun
    Nov 23, 2015 at 3:52
  • \$\begingroup\$ It appears that tests invoked range from 10 kHz to 18 GHz so ferrite beads are going to be useful starting around 30 MHz to low GHz but this leaves big gaps where they will likely not offer any help whatsoever. \$\endgroup\$
    – Andy aka
    Nov 23, 2015 at 8:55

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