EMC basics: I/O

EMC Basics #5: I/O as critical circuits article gives some useful tips on the EMC issues related to inputs and outputs.

Digital inputs/outputs — The key concern for digital interfaces is ESD. A secondary concern is radiated emissions. Radiated susceptibility is rare with digital I/O, although possible at very high RF levels. The solutions for both radiated problems include filtering at the interface and/or or shielding of external cables.

Analog inputs/outputs — The key concern for analog interfaces is RF. High RF levels can cause rectification in the I/O circuits causing errors and/or noise. Typical solutions include high frequency filters and/or shielding of the external cables.

Relay outputs — Since relay drivers are usually digital, the regular digital concerns apply. In addition, inductive transients from the relay coils may pose a self-compatibility problem. Snubber circuits may be needed at either the relay (best) or at the driving circuit on the boards.

Contact inputs — Since the receiving circuits are usually digital, the regular digital concerns apply.

When designing or reviewing circuit boards for EMI, ALL of the I/O circuits deserve EMI attention!

I have some additions to those suggestions:

Opto-isolators (also known as optocouplers) work to protect the receiving system at the expense of the sending system needing to drive the cables/interconnects. They are a great way to isolate digital from power circuits but have limited bandwidths. Fairchild Application Note AN-3001 Optocoupler Input Drive Circuits gives some implementation tips for optocoupler based input circuits.

optocoupler

Using a balanced line interface for sensitive and/or fast signal is a very good idea. Using balanced interface reduces EMI pickup and radiated EMI considerably compared to single-ended signals. Applications like telephone lines, analogue instrumentation, professional audio signals, fast serial bus standards and Ethernet all use balanced interfaces to get good noise performance.

Be careful on the grounding of cable shield when they enter the cabinet. The cable shields should be grounded at the point where they enter the metal cabinet. This will stop the RFI from entering inside the device. This advice applies especially to sensitive analogue circuits like audio interfaces. Proper grounding is essential in keeping RFI and ground loop noise away.

In many power controlling applications you can’t beat a relay for isolation or low on-resistance, as well as low cost. For relay outputs you need to carefully consider the need for snubber circuits. When talking about snubber circuits there are two kind of applications for them: Snubber cuircuit in parallel with the relay coil and snubber circuits in parallel with the relay output.

For the relay coil driven with DC voltage at known polarity an inexpensive diode in parallel with the coil works well. If the relay is switched with AC, the DC polarity is not known or you need very fast operation (parallel diode can slow down relay release time).

You need to consider snubber circuit also at the relay contact side especially if you are switching anything that is even slightly inductive. Relay contacts can arch. The end result of Contact Arc Phenomenon is shortened contact life. In addition to that arching causes lots of electromagnetic interference.

Relay Contact Life article tells that perhaps the most popular method of quenching an arc between separating contacts is with an R-C network placed directly across the contacts. Contact Protection and Arc Suppression Methods for Mechanical Relays gives information how to design a suitable R-C network for quenching an arc.

mechrela

Some relay users connect a diode across the inductive load to prevent counter-voltage from reaching the contacts. In some application zener diodes are used. The MOV performs in a manner similar to back-to-back zener diodes, and can be used in both AC and DC circuits.

An added benefit of arc suppression is the minimization of EMI. An unsuppressed arc between contacts is an excellent noise generator. Arc may radiate energy across a wide spectrum of frequencies. By suppressing the arc, electromagnetic interference is held to a minimum. By quenching the arc quickly, this action is held to a minimum. The result often is a considerably lessened amount of electromagnetic and radio frequency interference. Contact arc noise can be troublesome to sensitive components in a circuit. In worst-case conditions, EMI can cause unwanted turn-on of IC logic gates, SCRs, and triacs, and can cause damage to other semiconductor devices.

206 Comments

  1. Tomi Engdahl says:

    Is This the End for the Optocoupler in High-Voltage Isolation?
    Sept. 19, 2023
    Learn more about TI’s new family of opto-emulators and how they differ from traditional optocouplers.
    https://www.electronicdesign.com/technologies/power/article/21273990/electronic-design-is-this-the-end-for-the-optocoupler-in-highvoltage-isolation?utm_source=EG+ED+Auto+Electronics&utm_medium=email&utm_campaign=CPS231006018&o_eid=7211D2691390C9R&rdx.identpull=omeda|7211D2691390C9R&oly_enc_id=7211D2691390C9R

    What you’ll learn

    The importance of signal isolation in high-voltage design
    The difference between optocouplers and opto-emulators
    Details on TI’s silicon dioxide (SiO2) isolation technology
    Isolation is invaluable in everything from industrial motor drives on factory floors to electric-vehicle (EV) traction inverters, where it’s necessary to deter dangerous high voltages from entering the more fragile parts of the system.

    Optocouplers have been the “go-to” for isolating analog and high-speed digital signals traveling through power supplies and systems for decades, enhancing reliability and safety in the process, said Tsedeniya Abraham, VP and GM of interface products at TI. She added that while they can facilitate the safe transfer of signals and data from the input stage in a power supply and its output terminals or load, the optocoupler is no longer cutting it as power levels rise in the automotive and industrial sectors.

    TI is trying to bring the optocoupler into the future with a new family of what it calls “opto-emulators.” Based on its silicon-dioxide (SiO2) isolation technology, the parts are pin-to-pin alternatives to optocouplers for high-voltage signal isolation. And the company claims they bring better power efficiency, signal integrity, and reliability to the table while extending the lifespan of high-voltage industrial and automotive systems.

    Reply
  2. Tomi Engdahl says:

    Opto-emulators explained: Why you should upgrade your optocoupler technology
    https://e2e.ti.com/blogs_/b/analogwire/posts/opto_2d00_emulators-explained-why-you-should-upgrade-your-optocoupler-technology?HQS=asc-mdbu-catan-anahub_isom8x-agg-ta-electronicdesign_ana-wwe_int&DCM=yes&dclid=CJPqxpLVi4IDFZOMmwodzuAPyw

    Optocouplers, also known as photocouplers, opto-isolators and optical isolators, have long been an option for designers seeking galvanic isolation for their system signals. Around since the 1970s, these semiconductor devices have played an important role in providing safety isolation for industrial and automotive end equipment. However, despite significant advancements, there appears to be a limitation to their progress in electrical characteristics, high-voltage reliability and integration capabilities, prompting designers to explore alternatives.

    Technologies such as capacitive and magnetic isolation have emerged as alternatives, offering better overall performance compared to optocouplers. Texas Instruments (TI) has invested in silicon dioxide (SiO2)-based digital isolation technology since the early 2000s, providing digital isolator products that offer the same functionality as optocouplers with some distinct advantages.

    Reply
  3. Tomi Engdahl says:

    Submitting a new device for electromagnetic compatibility (EMC) testing seems a little like showing up for the final exam after skipping all the lectures. You might get lucky and pass, but it really would have been smarter to take a few of the quizzes to see how things were going during the semester. Similarly, it would be nice to know you’re not making any boneheaded mistakes early in the design process, which is what……

    HOMEBREW TEM CELL LETS YOU EMC TEST YOUR OWN DEVICES
    https://hackaday.com/2023/11/02/homebrew-tem-cell-lets-you-emc-test-your-own-devices/?fbclid=IwAR17xB0vOKsk-MbNtwckAkHmGF4w7Z4E8AfDqtwor7GeN9gEyIUOWkCP3As

    Reply

Leave a Comment

Your email address will not be published. Required fields are marked *

*

*