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Part Number: ISO7241C
ISO7241C datasheet states:
1. "This coupler is suitable for basic electrical insulation only within the maximum operating ratings."
Viorm is 560 Vpk.
If permanent votage across I/O isolation barrier is much less than 560 Vpk in application, e.g. 50 Vpk, can ISO7241C be declared as reinforced isolation component?
2. "Compliance with the safety ratings shall be ensured by means of suitable protective circuits."
Is there a reference design of such protctive circuit, e.g. to achieve safety limiting values?
Hi Marin,Welcome to the E2E forums! Thank you for posting your questions.The ISO7241C can withstand 50Vpk over its lifetime since this is, as you mentioned, within the Viorm rating of 560Vpk.Basic and reinforced isolation however are physical capabilities of each device and their certified isolation ratings ( >1kV = operational, >2.5kV = basic, >5kV = reinforced). Since the ISO7241C is certified for 4kV peak, it will only provide basic isolation to the systems it is in.I've attached a design guide which can be found in the "Technical Documents" tab of the ISO7241C product folder. Where did you find the quotation "Compliance with the safety ratings shall be ensured by means of suitable protective circuits."? Methods to protect the IC include zener clamps on the supply pins and using a y-capacitor between ground planes, but they are not always necessary.TI has compatible isolators which can achieve higher isolation performance, like the ISO7741 and ISO7841. Is reinforced isolation a requirement for your system? Feel free to respond with your desired design specs and we can optimize the isolation subcircuit.I hope this response is helpful! Please let me know which additional expertise I can offer.Respectfully,Manuel ChavezDigital Isolator Design Guide - SLLA284b.pdf
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In reply to Manuel Chavez:
Thank you for response.
Manuel ChavezWhere did you find the quotation "Compliance with the safety ratings shall be ensured by means of suitable protective circuits."?
is in datasheet rev. T, on page 11, note (2).
2. Is it applicable to use miniature fuse on power supply and ground pins to ensure "safety input, output, or supply current" from datasheet point 7.8?
Is current limiting resistor on signal input/output pins needed?
3. In case both sides of ISO7421C are floating (no connection to earth potential or other defined potential):
Y-capacitor between ground planes is to reduce AC and transient stress on the isolation barrier?
Is resistor (e.g.1 MOhm) between ground planes needed to reduce DC voltage stress on the isolation barrier?
4. ISO7741 is also a suitable alternative.
There is a requirement in our application for reinforced isolation for functional safety (not electrical hazad). On both sides of ISO7421C are microcontroller I/O pins.
Concern is that in a case of single fault of isolation barrier, some other catastrophic single fault on one side of the barrier (e.g. short to higher power supply) will not destroy circuits on the other side of the barrier too.
Under which condition is low resistance path between ISO7421C I/O pins possible?
In ISO7241C datasheet there is "isolation capacitor lifetime projection". What is end-of-life criterion for capacitor?
There are many questions above, answer to at least some would be beneficiary.
In reply to Marin Babic:
Hi Mr. Babic,
Thank you as well for the information provided. Below are my answers to your points:
Conserving the isolation barrier relies on protecting the device from most absolute maximum ratings violations. Within these conditions, the device's isolation barrier life depends on the working voltage (Viorm) it is continuously exposed to on a day-to-day basis. As Viorm decreases, the insulation capacitor's lifetime increases logarithmically.
I hope to have been helpful with this response! Please let me know if elaboration on any points is desired or there are additional questions.
Thank you for your time,
Understanding Failure Modes in Isolators - slyy081a.pdf
Thank you for the replies.
we will certainly consider reinforced digital isolators in future designs.
Just for completeness, in question 3 "floating" means that local ground potentials are isolated from earth potential, or any other common potential. ISO device pins are directly connected to local ground.
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