Electronic Component and Design Guide

Circuit Protection and ESD
Component Selection Guide

From the moment a product reaches a user's hands, its circuits face electrical stresses that its designers may not have anticipated — static discharge from a finger, surges from an AC power line, overcurrent from a fault, or a reversed battery connection. Getting protection design wrong increases failure rates, causes certification failures, and in severe cases triggers recalls. This guide covers the threats, the protection components, and the design principles that keep circuits safe.

Circuit Protection · ESD · Surge · EMC 7-min read TVS · MOV · GDT · PTC · IEC 61000-4-2

Five electrical threat types that circuits face; eight protection component types (TVS diode, low-capacitance ESD diode, MOV varistor, GDT gas discharge tube, PTC resettable fuse, standard fuse, circuit breaker, EMC filter) with characteristics and key manufacturers; four fundamental protection design rules; interface-specific protection for USB, Ethernet/PoE, HDMI, power input, and battery charging; and IEC 61000-4-2 ESD test level requirements.

POINT 01

Five Electrical Threats That Circuits Face

Protection design starts with understanding which threats your product will actually face. Different threats have different energy levels, time profiles, and entry paths — and require different protection strategies. Designing for ESD-only when your product also faces surge is insufficient; designing maximum surge protection on every signal line when only ESD is likely over-complicates the design unnecessarily.

ESD

Electrostatic Discharge

Human or object static charge discharges into the product on contact. Voltage: several thousand to 30,000V. Energy: very small. Duration: nanoseconds. Entry: connectors, buttons, switches, and any exposed metallic surface accessible to users. Primary tool: TVS diodes or dedicated ESD protection ICs at the entry point.

EFT / BURST

Electrical Fast Transient

Rapid repetitive high-voltage pulses conducted through power or signal lines. Caused by relay switching, motor commutation, contactor operation. Lower energy than surge but high repetition rate. Simulated by IEC 61000-4-4. Primary tool: TVS diodes plus common-mode chokes and EMC filters.

SURGE

Surge / Lightning Transient

High-energy overvoltage from indirect lightning (electromagnetic induction) or utility switching operations. Energy is far greater than ESD — it can destroy unprotected components irreversibly. Simulated by IEC 61000-4-5. Requires high-energy-capable protection: MOV varistor + GDT in multi-stage configurations for AC mains applications.

OVERCURRENT

Overcurrent / Short Circuit

Current exceeding design rating due to short circuit, abnormal load, or component failure. Causes thermal damage — wires burn, PCB traces arc, components fail from heat. Primary tools: fuses (one-shot) and PTC resettable fuses. The protection must clear fast enough to prevent thermal damage to the wiring and PCB before the fuse opens.

OVERVOLTAGE

Overvoltage

Supply voltage exceeds the circuit's rated operating voltage due to power supply fault, load dump (automotive), or input miswiring. Sustained (not transient) overvoltage destroys ICs that survive transient ESD. Protection: TVS diode (for transient), crowbar circuit or linear regulator with overvoltage shutdown for sustained conditions.

REVERSE POL.

Reverse Polarity Connection

Power supply or battery connected with reversed polarity — destroys any unprotected circuit in milliseconds. Extremely common in field-serviceable equipment. Protection: series Schottky diode (simple, low-cost, adds forward voltage drop), or P-channel MOSFET reverse polarity switch (near-zero voltage drop, preferred for high-efficiency designs).

Identify your threats before selecting components: A product with an exposed USB-C connector faces ESD on every plug/unplug cycle. A product connected to AC mains needs surge protection. A portable device with a replaceable battery needs reverse polarity protection. A motor-driven industrial product needs EFT suppression. The right protection design starts with a realistic threat assessment for your specific product, not a generic checklist.

POINT 02

Eight Protection Component Types: Characteristics and Manufacturers

Each protection component type occupies a specific role in the protection hierarchy. Understanding which threats each type handles — and its limitations — is the prerequisite for correct selection and multi-stage design.

Component	Response	Best Threat	Key Parameters	Leading Manufacturers
TVS Diode Transient Voltage Suppressor	Picoseconds	ESD, EFT, small-to-medium transients	Clamping voltage (VCL), peak pulse power (PPP), breakdown voltage (VBR), uni/bidirectional	Bourns · Littelfuse · Nexperia · Vishay · STMicro · Infineon · onsemi
ESD Protection Diode Low-capacitance TVS variant	Picoseconds	ESD on high-speed signal lines (USB, HDMI, Ethernet, CAN, MIPI)	Line capacitance (Cd, must be very low for high-speed signals), working voltage, clamping voltage, channel count per package	Nexperia · onsemi · Bourns · Littelfuse · TI · Diodes Inc.
MOV (Varistor) Metal Oxide Varistor	Nanoseconds	High-energy surge (AC mains, lightning-induced), power line protection	Clamping voltage, energy absorption (joules), peak current (A), life (number of surges)	Littelfuse · Bourns · TDK Epcos · Panasonic · KEMET (Yageo)
GDT Gas Discharge Tube	Microseconds	Extreme energy surge — telecom lines, outdoor equipment, direct lightning path	DC sparkover voltage, impulse sparkover voltage, follow current capability, arc voltage	Bourns · Littelfuse · TDK Epcos · Phoenix Contact
PTC Resettable Fuse Polyfuse / PolySwitch	Seconds (thermal)	Overcurrent — self-recovering after fault clears	Hold current (Ih), trip current (It), max voltage, recovery time, resistance in tripped state	Littelfuse · Bourns · TDK · TE Connectivity
Fuse One-shot overcurrent protection	Milliseconds to seconds	Overcurrent — positive indication of fault (requires replacement)	Rated current, voltage rating, interrupting capacity (breaking capacity), time-current characteristic (fast/slow/time-lag)	Littelfuse · Bel Fuse · Schurter · Eaton (Bussmann)
Circuit Breaker Resettable, mechanical	Milliseconds	Overcurrent in panels, systems where manual reset is acceptable	Rated current, breaking capacity, trip curve, operating temperature range	Carling Technologies · TE Connectivity · Eaton · Siemens
EMC Filter Common-mode choke, ferrite bead, LC filter	Frequency-dependent	High-frequency conducted noise — EMC certification, noise reduction	Impedance vs. frequency, rated current, insertion loss (dB), common/differential mode attenuation	Murata · TDK · Würth Elektronik · Coilcraft · Vishay

MOV wear-out is a lifetime management issue: Unlike TVS diodes, MOV varistors degrade with each surge event — each absorbed pulse permanently reduces the clamping energy capacity. MOVs near the end of their wear life can fail short-circuit, creating a fire risk from continuous current flow. For AC mains applications, MOVs should either be in series with a thermal fuse (most common) or the equipment should have a defined service life expectation for the protection assembly.

POINT 03

Four Fundamental Protection Design Rules

Protection components only work when the design around them allows them to. The most common reason protection circuits fail in the field is not wrong component selection — it's correct components placed incorrectly or connected to an inadequate ground. These four design rules address the most common failure modes.

Multi-Stage Defense: Stack Devices to Match Threat Energy and Speed

Single-device protection cannot simultaneously handle extreme energy (GDT/MOV territory) and picosecond response time (TVS territory) — the physics don't permit it. The correct approach for high-energy inputs is staged: GDT at the port entry to divert extreme surge energy to chassis ground; MOV to absorb residual surge voltage; TVS on the downstream signal line to clamp fast residual transients and ESD. Each stage handles what it's designed for. The decoupling impedance between stages (typically a series resistor or inductor) allows each stage to function without defeating the next.

Place Protection Components at the Threat Entry Point — Not Downstream

A TVS diode placed far from the connector it's protecting is largely useless. Before the TVS sees the transient, the fast voltage edge has already propagated along the trace to the IC it was meant to protect. The rule: protection components must be physically between the threat entry point (connector, external terminal, cable pad) and the rest of the circuit. Place TVS diodes on connector footprints or immediately adjacent — within 1–2 mm where possible. Do not route the signal through vias and copper to an inner layer before reaching the protection device.

Ground Path: Short, Wide, Low-Impedance — Directly to Chassis

The suppressed transient energy goes somewhere — it must flow through the protection device to ground. If the ground path from the protection device is long, narrow, or high-inductance, the protection device's advantage is partially negated: the trace inductance momentarily maintains a high voltage even as the TVS conducts. Requirements: use the shortest possible ground return from the protection component; connect directly to the chassis ground pour (not the signal/circuit ground, which can be raised in potential); use multiple ground vias; and keep the protection device's ground return separate from the functional circuit ground to prevent ground bounce propagation.

Physically Separate the Protection Zone from the Functional Circuit Zone

When surge or ESD energy flows through a protection device, it causes a brief but significant ground bounce in the local ground area. If the protection device's ground is shared directly with sensitive analog or digital circuits, this bounce can propagate into the functional circuit and cause malfunction or damage. The solution: define a physical boundary between the "dirty" external-facing area (where protection devices are located) and the "clean" internal circuit area. Use a single ground connection point (guard ring, stitching via row) between the two zones so that transient energy flows to chassis without contaminating the internal ground plane.

Layout matters more than component selection: In protection design, a correctly selected TVS diode in the wrong position with a long ground return is less effective than a slightly lower-spec TVS in the ideal position. If your protection design fails in ESD testing, look at placement and ground path before changing the component specification. Most ESD test failures in production designs are layout problems, not specification problems.

POINT 04

Interface-Specific Protection and IEC 61000-4-2 Test Requirements

Each interface type has specific protection requirements driven by its operating voltage, signal speed, hot-plug behavior, and the standards it must pass. One-size-fits-all protection is never correct.

Protection by Interface Type

🔌

USB 2.0 / 3.x / USB-C

Hot-plugging creates ESD on every connection. Data lines (D+/D−, SS TX/RX) require low-capacitance ESD protection — even small capacitance degradates USB signal integrity. Power lines (VBUS) need overvoltage and overcurrent protection. USB-C introduces CC lines (protocol negotiation) that also need protection. USB-PD (up to 20V/5A) adds additional overvoltage protection requirements on VBUS.

Low-C ESD diode · VBUS TVS · PTC fuse

🌐

Ethernet (including PoE)

Transformer isolation provides inherent galvanic separation but doesn't fully suppress surge energy. Surge (IEC 61000-4-5) and ESD protection is required — typically TVS diodes on each wire pair before the transformer, plus a GDT for outdoor/industrial installations. PoE (Power over Ethernet, up to 90W at 802.3bt) adds power handling requirements: the protection components must handle PoE current without excessive voltage drop.

Wire-pair TVS · transformer isolation · GDT (outdoor)

🖥️

HDMI / DisplayPort

Multi-Gbps differential signals (HDMI 2.1 up to 48 Gbps) are extremely sensitive to capacitance — use ESD protection ICs with line capacitance below 0.2 pF per line. TMDS and HBR3 signals are differential; the ESD device must have matched capacitance on both lines of each pair to avoid differential-to-common-mode conversion that degrades the eye diagram. Dedicated HDMI/DP protection ICs from Nexperia, onsemi, and TI handle this correctly.

Ultra-low-C ESD IC · dedicated HDMI/DP protection IC

⚡

AC Power Input

The highest threat environment. Multiple hazards: lightning-induced surge, utility switching transients (IEC 61000-4-5), EFT/burst (IEC 61000-4-4), conducted emissions filtering requirements. Standard protection architecture: EMC input filter (common-mode choke + Y-capacitors) → MOV for surge → fuse or circuit breaker for overcurrent. Metal-enclosure designs require chassis bonding for effective surge diversion.

MOV · line fuse · EMC filter · GDT (telecom/outdoor)

🔋

DC Power Input / Battery

Reverse polarity: P-channel MOSFET ideal switch (near-zero drop) or series Schottky (simpler, adds ~0.3V drop). Overvoltage: TVS diode sized to clamp load dump transients. Overcurrent: PTC fuse for user-accessible ports; standard fuse for fixed supply circuits. Battery protection requires a dedicated BMS IC integrating overcharge (cell voltage), overdischarge, overcurrent, and overtemperature protection.

Reverse polarity MOSFET · TVS · PTC or fuse · BMS IC

🏭

Industrial / RS-485 / CAN

Industrial buses face severe ESD and EFT from motor-driven environments. RS-485 and CAN transceivers have rated ESD immunity, but bus pins exposed to industrial cabling face much higher stress in practice. Use bidirectional TVS arrays (4- or 8-channel) optimized for the bus voltage and speed. CAN FD at higher data rates requires lower-capacitance protection than classic CAN — verify that the TVS capacitance doesn't degrade signal integrity at the intended data rate.

Bus TVS array · bidirectional TVS · common-mode choke

IEC 61000-4-2 ESD Immunity Test Levels

IEC 61000-4-2 is the international standard for ESD immunity testing of electronic equipment. Understanding the test levels helps set correct design targets — and identifies the minimum protection specification required to pass certification testing.

Level	Contact Discharge	Air Discharge	Typical Application
Level 1	±2 kV	±2 kV	Protected environments (controlled access, indoor)
Level 2	±4 kV	±4 kV	Consumer electronics, general commercial products
Level 3	±6 kV	±8 kV	Industrial products, outdoor exposure, high-traffic public use
Level 4	±8 kV	±15 kV	Demanding industrial, medical, automotive environments
X (Special)	Specified in product standard	Specified in product standard	Application-specific levels beyond Level 4

Performance criteria define acceptable behavior during and after the test: Criterion A — normal operation during test (most stringent); Criterion B — temporary degradation, self-recovery; Criterion C — loss of function requiring operator reset; Criterion D — damage (not acceptable for any commercial product).

Design to the test level plus margin: If your product must pass Level 3 contact discharge (±6 kV), design and verify your protection circuit to handle ±8 kV or higher in the lab. ESD test variance, real-world conditions exceeding lab parameters, and production component variation all consume margin. A design that barely passes in the lab will fail in the field or in a third-party certification test. The difference in protection component cost between a design targeting 6 kV and one targeting 8 kV is negligible.

Key Takeaways

Effective circuit protection starts with the threat landscape for your specific product, then applies the right protection components in the right configuration. TVS diodes for fast transients (ESD/EFT) — place them at the connector, with the shortest possible ground return; MOV varistors for high-energy surge on power lines; GDT for extreme energy on telecom or outdoor connections (always in combination with faster downstream devices); PTC resettable fuses for user-accessible overcurrent paths; standard fuses for power circuits requiring positive fault indication; EMC filters for conducted noise. Multi-stage defense, correct placement at the port boundary, low-impedance chassis ground connection, and physical separation of the protection zone from the functional circuit zone are the four design rules that determine whether your protection components actually work. Design to exceed your target IEC 61000-4-2 test level by at least one level to have reliable certification margin.

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Circuit Protection and ESDComponent Selection Guide