I am trying to design an input power supply filter circuit inorder to protect from ISO 7637 pulses.

I am referring this Little fuse app note

There seem to be different pulses. Pulse 1, 2a, 2b and so on.

In page two of the app note, they have given the description of the pulses.

My questions:

  1. Why are some pulses negative and some pulses positive? I just want to understand how some pulses can be positive and can be negative

  2. Can someone explain in simple terms regarding the pulse 1 and pulse 2 description in page 2? Like please explain "disconnection of the power supply from an inductive load " and "Interruption of series inductive load"? Any block diagram or explanation with the concept and direction of current and voltage would be very helpful to me.

Quotes phrases are verbatim.

(P.S. I understand when an Inductive load is disconnected, we need to protect DUT with the help of a flyback diode to prevent from heavy voltage build-up. Is this the reason why some voltage pulses are positive and some are negative.)


Here are some answers but, because you are not making quotes verbatim there are some answers that I cannot give. May I suggest you copy and paste the exact section you refer to.

  • Positive and negative pulses can come from the alternator when the battery is disconnected or connected (worse when disconnected)
  • Negative pulses can arise if there is an inductive load and the battery is disconnected - the inductance will try and take its former positive lead to a large negative value in order to keep current circulating (as per the normal inductor formula)

"disconnecting the inductive load from a power supply"

\$\color{red}{\text{That sentence doesn't appear in the document}}\$

"series interruption of inductive load"

\$\color{red}{\text{That sentence doesn't appear in the document}}\$

I tried partial sentence searches but I'm wasting my time realistically.

EDIT section with verbatim quotes

disconnection of the power supply from an inductive load

I believe I explained that under the 2nd bullet point above - a negative pulse appears on the previous connection to battery positive.

Interruption of series inductive load

This is the more regular flyback situation we see when we use a flyback diode to protect a transistor that might be controlling said inductive load.

  • \$\begingroup\$ Sorry, I have edited the question now. Now the phrases within quotes are verbatim. I have taken those phrases from Page 2 of the Application note document \$\endgroup\$ – Newbie May 4 '20 at 4:29
  • \$\begingroup\$ @Newbie added bits as per verbatim quotes. \$\endgroup\$ – Andy aka May 4 '20 at 8:44
  • \$\begingroup\$ Yes accepted the answer \$\endgroup\$ – Newbie May 8 '20 at 9:54

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