EMC IEC 61508


Part 2 Section 7.2.3.3 Item f


Part 2 Section 7.3.2.2 Item f


Part 2 Annex A Table A.16 and A.18


Part 2 E.1 Items m and n


Description


Design requirements shall contain details about the required electromagnetic immunity levels


Validation planning shall consider electromagnetic immunity performance criteria to be applied


Measures against electromagnetic disturbances are mandatory regardless of SIL, with corresponding levels of effectiveness


Test requirements for safety functions, including integrated circuits with or without additional safety margins


Table 2. Examples of electromagnetic immunity related requirements based on IEC 61508-2:20103,4


Improving electromagnetic immunity 


There are different ways to improve immunity from electromagnetic disturbances. Table 3 shows examples of such external discrete solutions alongside their purpose. One of the most used discrete surge protection devices is the transient voltage suppressor (TVS) diode. A TVS diode – typically connected in parallel with the protected load – clamps overvoltage spikes and dissipates transient energy (Figure 1a). In normal operation, it remains nonconductive, but when an instantaneous high energy transient causes the voltage across its terminals to exceed its breakdown voltage (VBR), the device avalanches and rapidly shunts surge current to ground, limiting the voltage to its specified clamping level (VC) and protecting downstream circuitry (Figure 1b).8 Once the surge subsides and the line voltage falls back below the breakdown threshold, the TVS diode quickly returns to its high impedance state, restoring normal circuit operation. This reversible behaviour makes TVS diodes highly effective for protecting sensitive electronics against electrostatic discharge (ESD), lightning-induced transients, and switching surges.9


There are downsides to this approach. Solution Pros


Ferrite Beads Attenuate high frequency and fast voltage spikes when used below the self-resonant frequency.


RC Snubber Low-cost damping elements that reduce ringing by slowing edge rates and dissipating energy.


Voltage Suppressors


Clamp fast low voltage transients very quickly and precisely, making them excellent for protecting sensitive electronics.


Voltage Dependent Resistor


Absorb high energy surges effectively and offer broad clamping capability across a wide voltage range.


www.cieonline.co.uk Cons


Provide limited energy absorption and can create unwanted resonances if operated near or above the self-resonant frequency or not paired with capacitors.


Introduce signal loss and power dissipation, making them unsuitable for high speed or high current paths.


Have limited surge-energy capability due to form factor constraints and significant junction capacitance that can impact fast systems.


Exhibit high leakage near breakdown and add substantial capacitance, requiring decoupling or additional circuitry in high-speed designs.


Table 3. Examples of discrete solutions for improving electromagnetic immunity8


rating makes it challenging to select a suitable TVS diode, especially when residual surge voltage at the system level remains high, a surge stopper can be added as a secondary series clamp to regulate the output to a safer voltage level (Figure 2). This approach improves system robustness by preventing excessive stress on the TVS diode and downstream components during high energy surge events. Using a two-stage protection scheme separates surge energy absorption from precise voltage limiting, resulting in a more robust and reliable solution that is well-suited for meeting IEC surge immunity requirements.


Optimising electromagnetic immunity performance with integrated solutions


As EMC and functional safety requirements become more stringent, discrete protection components alone are often insufficient


The clamping can convert an AC signal to have a large DC component that then easily passes through any subsequent AC filtering. In cases where the load’s maximum voltage


Figure 2. Simplified schematic of a surge stopper implementation using the LT4356.


Figure 3. ADI integrated solutions for electromagnetic immunity.


to guarantee predictable behaviour under severe electromagnetic stress. High-energy surge, tight voltage margins, and increasing system complexity can expose the limitations of passive solutions particularly with higher residual voltages, uncontrolled energy dissipation, and a growing component count. These challenges are best addressed with protection that goes beyond passive clamping. Analog Devices offers a portfolio of integrated power-protection solutions that actively manage voltage, current, and power during abnormal operating conditions, significantly improving electromagnetic immunity robustness while simplifying system design as shown in Figure 3.


Ideal diode controller


Standard Schottky diode ORing introduces significant forward voltage drops and heat, Continues on page 40


Components in Electronics June 2026 39


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