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.

154 Comments

  1. Tomi Engdahl says:

    Reliable and Affordable Isolation for High-Voltage Designs
    Galvanic Isolation: The Key to Reliability and Safety
    https://storydesign.electronicdesign.com/galvanic-isolation/landing-page-438DY-2084UE.html

    Galvanic isolation prevents direct currents from flowing from one subcircuit to another. Functional-level isolation facilitates the proper operation of equipment when subcircuits use different voltage domains and operate at different ground potentials. Two additional levels of isolation, basic and reinforced, enhance reliability and safety.

    Reply
  2. Tomi Engdahl says:

    EEVblog 1409 – The DANGERS of Inductor Back EMF
    https://www.youtube.com/watch?v=hReCPMIcLHg

    A practical demonstration of Lenz’s law and back EMF in an inductive relay coil and how to solve it using a Freewheeling/Flywheel/Flyback/Snubber/Clamp diode. Also the downsides of clamping diodes, and switch arcing supression.
    Also a look at an AMAZING potential phenomenon you probably haven’t seen before!
    Actually, two rather cool things you probably haven’t seen before.
    Along with transistor ratings, transistor storage current, and Collector-Emitter breakdown voltage, there is a lot to unpack in this video.

    00:00 – Recap of Relays, Inductors, Faraday & Lenz’s Laws
    02:30 – Relay Back EMF Explained
    07:09 – The Flywheel analogy of Inductors
    08:30 – Relay circuit demonstration
    12:35 – 700V Back EMF!
    14:43 – BJT Transistor Storage Time
    17:03 – Back EMF Diode clamp demonstrated
    19:06 – An AMAZING demonstration!
    24:43 – Trap for young players
    25:23 – DOWNSIDES of Back EMF Diodes
    28:38 – BONUS cool effect of Back EMF diode DEMONSTRATED

    Reply
  3. Tomi Engdahl says:

    Galvanic isolation prevents direct currents from flowing from one subcircuit to another. Two types of galvanic isolation find use in signal chain and power supply designs: capacitive and magnetic isolation. Capacitive isolation exhibits low propagation delay and supports high data rates, but it requires separate bias supply voltages on each side of the isolation barrier.

    Reliable and Affordable Isolation for High-Voltage Designs
    Galvanic Isolation: The Key to Reliability and Safety
    https://storydesign.electronicdesign.com/galvanic-isolation?pk=TISD2-09232022&utm_source=EG+ED++Sponsor+Paid+Promos&utm_medium=email&utm_campaign=CPS220916111&o_eid=7211D2691390C9R&rdx.ident%5Bpull%5D=omeda%7C7211D2691390C9R&oly_enc_id=7211D2691390C9R

    Galvanic isolation prevents direct currents from flowing from one subcircuit to another. Functional-level isolation facilitates the proper operation of equipment when subcircuits use different voltage domains and operate at different ground potentials. Two additional levels of isolation, basic and reinforced, enhance reliability and safety.

    Two types of galvanic isolation find use in signal chain and power supply designs: capacitive and magnetic isolation.

    Capacitive isolation exhibits low propagation delay and supports high data rates, but it requires separate bias supply voltages on each side of the isolation barrier.

    Magnetic isolation handles power in excess of hundreds of milliwatts, but it is difficult to increase isolation through winding separation within the confines of an IC.

    Reply
  4. Tomi Engdahl says:

    How Does EMI Harm—and Help—in the Robotics World?
    Sept. 22, 2022
    Design engineers must pay attention to potential EMI issues early in the design cycle and determine how proper motor selection could manage these threats. Sometimes, though, EMI can be intentionally used for security reasons.
    https://www.electronicdesign.com/power-management/whitepaper/21251247/electronic-design-how-does-emi-harmand-helpin-the-robotics-world?utm_source=EG+ED+Analog+%26+Power+Source&utm_medium=email&utm_campaign=CPS220915021&o_eid=7211D2691390C9R&rdx.identpull=omeda|7211D2691390C9R&oly_enc_id=7211D2691390C9R

    What you’ll learn:

    How do conductive and radiative emissions affect BLDCs?
    Impact of EMI on drones and UAVs.
    How EMI is used to disable illegal drones and UAVs.

    High-power IEMI targets electronic circuitry via an antenna deploying a high-power EMI wave, which will destroy or degrade the offending drone device:

    Targeting an unprotected electronic system via a Cassegrain Antenna with 37- to 40-dB gain using a pulse method with a few kV/m peak field that has a pulse repetition frequency (PRF) of 300 Hz to 1 kHz.
    Targeting a commercial drone, such as DJI Phantom 3, with an ultra-wideband (UWB) electromagnetic pulse (EMP).
    Targeting a commercial quadcopter drone with a horn antenna using a narrowband pulse from 100 MHz to 3.4 GHz that has a PRF of 1 kHz.
    Targeting a minimal sensor network (MULLE) using a horn antenna with a continuous wave (CW) at 2 to 3 GHz with a peak field of 0.24 to 0.36 kV/m.
    Targeting a commercial off-the-shelf (COTS) quadcopter with an antenna that has a CW at 100 MHz to 2 GHz and a field from 75 to 95 V/m.

    Low-power IEMI targets the following:

    An analog sensor target can be disrupted via an antenna coil using resonant frequency for efficient coupling.
    A digital sensor target using Bulk Current Injection (BCI) or Direct Power Injection (DPI).
    Targeting the communication module using an antenna with in-band jamming

    Non-RF methods:

    An acoustic MEMS sensor using mechanical resonance.
    Optical flow using a laser that will degrade the received image of the optical flow sensor, leading to malfunction.

    Reply

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