ISO7741: SPI isolation to cross continuous 4-5kVDC

John Ranson

Part Number: ISO7741
Other Parts Discussed in Thread: ISO7841, ISO6441, ISO6741,

Tool/software:

Is there a proven solution for using TI digital isolators to reliably operate across a 4-5kVDC continuous working voltage? The particular application is SPI.

The best parts that I found, like the ISO774x and ISO774xTA, have a VIOWM of 2121VDC. In this portion of the design, I believe we need only functional insulation, but with very good long term reliability. Based on the datasheet graph, the 1ppm TDDB for these parts at 4kVDC is on the order of a year.

An additional concern is power, both for the isolator(s), and the low power sensing circuit beyond it.

Thanks for any help you can provide.

Best regards!

22 days ago

0 Aaditya V. 21 days ago

TI__Genius 17345 points

Hi John,

Thanks for reaching out.

I understand you are looking for a digital isolator that offers VIOWM of 4-5kV DC. Generally, the VIOWM is based off of the dielectric breakdown time. This means that the working voltage is based on a lifetime of the isolation barrier. Considering this, operating digital isolators at a higher working voltage significantly reduces overall lifetime of the device. More information on the VIOWM specification is in this FAQ: [FAQ] What is Maximum Working Isolation Voltage (VIOWM) and Maximum Repetitive Peak Isolation Voltage (VIORM)? - Isolation forum - Isolation - TI E2E support forums

Considering the above, the highest VIOWM specification device that we currently offer is the ISO7741-Q1 and ISO7841 in DWW package, with VIOWM = 2828 VDC.

TI digital isolator datasheets also include the VISO/VIOTM specifications that define the withstand capability of the isolation barrier when exposed to transients to breakdown of the dielectric. Since you had mentioned the 4-5kV specifically, I would recommend checking this FAQ for clarification on if this specification is something you are considering in relation to choosing a digital isolator for your system: [FAQ] What is Withstand Isolation Voltage (VISO) and Maximum Transient Isolation Voltage (VIOTM)? - Isolation forum - Isolation - TI E2E support forums

If the above specification is of interest or is originally what you meant, the ISO6441 (newest generation of digital isolator), ISO7741, ISO6741, etc. are all suitable options.

Regards,
Aaditya V

0 John Ranson 21 days ago in reply to Aaditya V.

Prodigy 235 points

I'm looking for circuits to use your digital isolators across a 4-5kVDC continuous working voltage. In other E2E threads, Manuel Chavez from TI notes that isolators can be used in series, and that TI has tested such circuits.

Stacking multiple digital isolators

ADC to uController Isolation

Stacking Multiple Isolation

I would like to work from an existing TI tested solution. If I design my own, I'm concerned that I won't end up with the matched impedances that allow decent voltage division across the isolators.

Thank you for pointing me to the DWW package parts. I now have a separate question about them which I will ask in another thread.

Best regards,

John

0 Aaditya V. 21 days ago in reply to John Ranson

TI__Genius 17345 points

Hi John,

Thanks for your reply.

John Ranson said:
If I design my own, I'm concerned that I won't end up with the matched impedances that allow decent voltage division across the isolators.

Although series connecting was considered many years ago, your concern about the voltage splitting across the isolators is exactly why we do not guarantee the performance of the isolators in regards to the working voltage and the lifetime when connecting in series.

When isolators are connected in series, the working voltage may split across the two device unevenly, causing one isolator to be exposed to more working voltage that the ratings defined on the datasheet compared to the second isolator. This can cause the lifetime of either isolator to be greatly reduced since the VIOWM specification would be exceeded. Therefore, it would not be a stable solution in terms of the lifetime of the isolators in the system.

Regards,
Aaditya V