Electronics Guide

Understanding ESD Threats

ESD events arise from various sources, each with characteristic waveforms:

• Human body model (HBM): Simulates discharge from a person, characterized by 100 pF capacitance and 1500 ohm series resistance, producing relatively slow rise times around 5-10 nanoseconds
• Machine model (MM): Represents discharge from manufacturing equipment with 200 pF and near-zero resistance, creating faster, more severe transients
• Charged device model (CDM): Occurs when a charged component itself discharges, producing extremely fast sub-nanosecond events
• IEC 61000-4-2: Defines standard test levels from 2 kV to 15 kV for system-level testing

Understanding which ESD models apply to a particular application guides the selection and design of protection circuits.

TVS Diode Protection

Transient voltage suppressor diodes are the primary component for ESD protection at circuit inputs:

• Operating principle: TVS diodes clamp voltage through avalanche breakdown, diverting transient current away from protected circuits
• Response time: Sub-nanosecond response captures even the fastest ESD events
• Clamping voltage: The voltage during clamping must remain below the damage threshold of protected devices
• Capacitance considerations: Standard TVS diodes may add tens to hundreds of picofarads; low-capacitance types available for high-speed signals
• Bidirectional types: Protect against transients of either polarity, essential for AC-coupled and differential signals

ESD Protection Diode Arrays

Multi-channel protection arrays offer advantages for protecting parallel data buses and multi-pin interfaces:

• Matched characteristics: All channels exhibit similar capacitance and clamping for consistent signal timing
• Integrated rail clamps: Common-mode transients are clamped to supply rails
• Compact footprint: Single package replaces multiple discrete devices
• Application-specific designs: Arrays optimized for USB, HDMI, Ethernet, and other standard interfaces

Spark Gaps and Gas Discharge Tubes

For very high-energy ESD or surge events, spark gaps and gas discharge tubes provide robust protection:

• High energy capability: Can handle hundreds of amperes for microseconds
• Higher trigger voltage: Typically 75-600 V breakdown, suitable for telecom and industrial interfaces
• Arc voltage: Once triggered, voltage drops to 10-20 V, effectively shorting the transient
• Response time: Slower than TVS diodes, typically 0.5-5 microseconds
• Coordination: Often used as first stage with TVS diodes as secondary protection

ESD Layout Considerations

Physical layout significantly affects ESD protection effectiveness:

• Placement: ESD protection devices must be located directly at the connector, before signal traces route into the PCB
• Ground path: Low-inductance connection to chassis or circuit ground is essential
• Guard rings: Prevent discharge from jumping to adjacent traces
• Trace routing: Avoid running protected and unprotected signals in parallel
• Via placement: Multiple vias reduce ground path inductance

Zener Diode Clamping

Zener diodes provide simple, effective voltage clamping for many applications:

• Operating principle: Reverse breakdown at a predictable voltage allows clamping with minimal components
• Voltage selection: Choose Zener voltage above maximum signal level but below protected device ratings
• Dynamic impedance: Voltage rises somewhat with current; must be factored into protection margin
• Power dissipation: Zener must handle I2R heating during overvoltage; select appropriate power rating
• Temperature coefficient: Breakdown voltage varies with temperature; specify over operating range

Schottky Diode Clamping

Schottky diodes clamp voltage to the power supply rails with minimal forward voltage drop:

• Rail-to-rail protection: Diodes to both positive and negative rails limit voltage swing
• Low forward voltage: 0.2-0.4 V forward drop minimizes clamping voltage
• Fast switching: No minority carrier storage for fast response
• Leakage current: Higher than silicon diodes; increases with temperature
• Reverse breakdown: Lower than silicon; avoid exceeding ratings

This technique is particularly effective for op-amp and comparator inputs where overvoltage beyond the supply rails can damage input structures.

Active Clamp Circuits

Active clamping uses amplifiers to achieve precise voltage limiting:

• Precision threshold: Op-amp feedback eliminates diode voltage drop variation
• Adjustable limits: Threshold set by resistor divider or reference voltage
• Current limiting: Active clamps can simultaneously limit current
• Speed limitations: Amplifier bandwidth limits response to fast transients

Active clamps are best suited for controlled environments where fast transients are unlikely, complementing passive protection for ESD and surges.

Crowbar Overvoltage Protection

For severe overvoltage conditions, crowbar circuits provide ultimate protection by shorting the input:

• SCR-based crowbar: Silicon controlled rectifier fires when voltage exceeds threshold, shorting input and blowing a fuse
• Thyristor surge protectors: Self-triggering devices that fire at a specified voltage
• Destructive nature: Crowbar operation requires fuse or breaker replacement
• Application: Used where clamping alone cannot dissipate fault energy

Series Resistor Limiting

The simplest current limiting approach uses a series resistance:

• Ohm's law protection: Resistor limits current to V/R regardless of load conditions
• Combined with clamp: Resistor limits current through clamping diode, protecting both diode and source
• Voltage drop: Resistor causes signal attenuation; must be acceptable for application
• Bandwidth effects: Forms RC filter with input capacitance, limiting bandwidth
• Thermal considerations: Resistor must dissipate I2R power during fault

For high-impedance inputs such as op-amp inputs, series resistors of 1-100 kohm provide excellent protection with minimal impact on circuit performance.

PTC Thermistor Protection

Positive temperature coefficient thermistors provide resettable current limiting:

• Normal operation: Low resistance allows normal current flow
• Fault response: Overcurrent causes self-heating, dramatically increasing resistance
• Latching behavior: Remains high-resistance until power is removed and device cools
• Current rating: Specified as hold current (stays low) and trip current (transitions high)
• Response time: Relatively slow, typically hundreds of milliseconds to seconds

Active Current Limiting

Active circuits provide precise, fast current limiting with adjustable thresholds:

• Current sense feedback: Amplifier monitors current through sense resistor and regulates drive
• Foldback limiting: Current limit decreases as voltage increases, reducing power dissipation
• Current mirror limiters: Transistor current mirrors provide simple current sources
• Integrated solutions: Many interface ICs include built-in current limiting

Fuse Protection

Fuses provide one-time current limiting for catastrophic faults:

• Fast-acting fuses: Clear quickly to protect semiconductors
• Slow-blow fuses: Tolerate brief inrush currents while protecting against sustained faults
• I2t coordination: Fuse must clear before protected device is damaged
• Surface mount fuses: Available for PCB mounting in space-constrained designs

RF Filtering

Low-pass filtering removes RF energy while passing desired signals:

• Capacitive filtering: Shunt capacitors bypass RF to ground; effective for high-impedance inputs
• Ferrite beads: Lossy inductors absorb RF energy, converting it to heat
• LC filters: Combined inductors and capacitors provide steeper rolloff
• Feedthrough capacitors: Integrated into connector hardware for effective shielding penetration

Shielding and Grounding

Preventing RF from reaching inputs is often more effective than filtering:

• Shielded cables: Braided or foil shields block electromagnetic fields
• Shield grounding: Proper connection of shield to chassis ground at appropriate points
• Enclosure design: Conductive enclosures with proper seam treatment and filtered penetrations
• Differential signaling: Common-mode RF rejection through balanced transmission

RF Rectification Prevention

Non-linear semiconductor junctions can rectify RF, creating DC offsets or low-frequency interference:

• Input filtering: Remove RF before it reaches semiconductor junctions
• Matched input impedance: Prevents reflection and standing waves that concentrate RF energy
• Anti-parallel diodes: Balanced diode pairs cancel rectification effects
• Bandwidth limiting: Restrict amplifier bandwidth to below RF frequencies where possible

RF-Resistant Design Practices

Design choices that minimize RF susceptibility:

• Short input traces: Minimize antenna effect of input wiring
• Ground plane: Continuous ground plane under sensitive inputs
• Guard traces: Grounded traces between sensitive and noisy signals
• Component selection: Choose RF-rated components for critical positions

Common-Mode Voltage Sources

Understanding common-mode sources helps in designing appropriate protection:

• Ground potential differences: Different ground reference points at signal source and receiver
• Inductive coupling: Magnetic fields from power wiring induce common-mode voltages
• Capacitive coupling: Electric fields from high-voltage sources couple to signal wiring
• Lightning events: Ground current flow creates potential differences across large distances

Common-Mode Clamping

Clamping limits common-mode excursion relative to local ground:

• Rail-to-rail clamping: Diodes to both supply rails limit common-mode range
• Symmetrical clamping: Equal clamping on all conductors maintains signal integrity
• Gas discharge tubes: High-voltage clamping for industrial and telecom applications
• TVS arrays: Integrated common-mode and differential protection

Galvanic Isolation

Complete isolation eliminates ground-related common-mode issues:

• Transformer coupling: Magnetic coupling transfers signal without galvanic connection
• Optocoupler isolation: Optical path provides complete electrical isolation
• Capacitive isolators: High-voltage capacitors pass signal while blocking DC and low-frequency common-mode
• Magnetic isolators: Integrated magnetic coupling for digital signals
• Isolation voltage rating: Specify continuous and transient isolation voltage requirements

Common-Mode Chokes

Common-mode chokes attenuate common-mode signals while passing differential signals:

• Operating principle: Coupled windings present high impedance to common-mode current but cancel for differential current
• Frequency range: Select core material and winding for the frequency range of concern
• Current rating: Must handle signal current without saturation
• Leakage inductance: Affects differential mode signals at high frequencies

Balanced Protection Networks

Protection components must be matched between differential conductors:

• Matched TVS diodes: Equal clamping voltage and capacitance on both lines
• Series resistor matching: Equal resistance in both signal paths
• Symmetrical layout: Equal trace lengths and similar routing for both conductors
• TVS arrays: Integrated protection with factory-matched characteristics

Differential Clamping

In addition to clamping each line to ground, differential clamping limits the voltage between the two conductors:

• Line-to-line TVS: Clamps differential voltage directly
• Back-to-back Zeners: Provides bidirectional differential clamping
• Integrated protection: Many differential interface ICs include internal protection
• Clamping level: Must exceed maximum differential signal but protect receiver inputs

Differential Interface Protection

Standard differential interfaces have specific protection requirements:

• RS-485/RS-422: Designed for +/-7 V common-mode; may need protection for industrial environments
• CAN bus: Built-in protection to +/-16 V; additional protection for automotive transients
• LVDS: Low voltage, high speed requires low-capacitance protection
• Ethernet: Transformer isolation plus TVS protection for PoE and ESD

Hot-Plug Challenges

Connecting circuits under power creates several stress conditions:

• Capacitor charging: Bulk capacitance draws large inrush current when first connected
• Contact bounce: Multiple make-break cycles create repeated transients
• Sequence uncertainty: Ground, power, and signal pins may connect in any order
• Voltage transients: Stored energy in cables and circuits creates voltage spikes

Inrush Current Limiting

Limiting current during hot-plug prevents contact damage and supply disturbance:

• Series FET limiting: FET gate ramp controls current during connection
• Hot-swap controllers: Integrated circuits manage inrush, fault protection, and sequencing
• Current sense feedback: Active current limiting until load stabilizes
• Soft-start timing: Controlled ramp time allows capacitors to charge gradually

Pre-charge Circuits

Pre-charging bulk capacitance before full connection reduces inrush:

• Long-pin connectors: Staggered pin lengths ensure ground connects first, then pre-charge, then power
• Pre-charge resistor: Initial connection through current-limiting resistor
• Relay sequencing: Pre-charge through resistor, then bypass after voltage rises
• Active pre-charge: Controlled current source charges capacitance before main connection

Contact Protection

Protecting connector contacts from hot-plug damage:

• Arc suppression: RC networks across power contacts prevent arcing
• Contact rating: Select connectors rated for hot-plug operation
• Making current: Contacts must handle inrush without welding
• Breaking current: Disconnection under load creates arcing if not controlled

Signal Integrity During Hot-Plug

Maintaining signal integrity during connection and disconnection:

• Defined states: Pull-up or pull-down resistors ensure defined logic levels during partial connection
• Debounce circuits: Filter contact bounce before processing connection status
• Sequenced enables: Enable signal drivers and receivers only after stable connection
• Fail-safe receivers: Open or shorted inputs produce defined output states

Series Diode Protection

The simplest protection places a diode in series with the power input:

• Forward drop penalty: Silicon diodes drop 0.6-0.7 V; Schottky diodes reduce this to 0.2-0.4 V
• Power loss: Continuous I*Vf power dissipation reduces efficiency
• Current rating: Diode must handle maximum load current with margin
• Reverse voltage: Diode must block maximum possible reverse voltage
• Simplicity: No control circuitry required; always protected

Shunt Diode Protection

A reverse-connected diode across the input conducts and blows a fuse if polarity is reversed:

• Low forward drop: No voltage drop during normal operation
• Fuse requirement: Must include fuse to clear the shorted condition
• Diode surge rating: Must survive until fuse clears
• Reset needed: Fuse replacement required after reverse polarity event

MOSFET Reverse Protection

A P-channel MOSFET provides low-loss reverse polarity protection:

• Operating principle: MOSFET body diode conducts briefly on power-up; gate voltage then turns on channel for low on-resistance
• Low voltage drop: Milliohm on-resistance creates minimal drop at full current
• Reverse blocking: With reversed polarity, gate has no drive and MOSFET blocks
• N-channel variation: Requires gate drive circuit but offers lower on-resistance for given size

Active Reverse Protection

Integrated controllers provide advanced reverse protection features:

• Fast turn-off: Detects reverse condition and turns off MOSFET before body diode conducts
• Load dump protection: Combined reverse and overvoltage protection for automotive applications
• Ideal diode controllers: Actively controls MOSFET to emulate ideal diode characteristics
• Power path management: Manages multiple power sources with reverse blocking

Automotive Considerations

Automotive applications present specific reverse polarity challenges:

• Jump-start scenarios: Reversed jumper cable connection applies high-current reverse voltage
• Battery reversal: Full battery voltage applied in reverse during service
• Double-battery jump: 24 V from truck battery to 12 V system
• Standards compliance: ISO 16750 defines test conditions for reverse polarity

Protection ICs

Integrated circuits designed specifically for input protection:

• Interface protection ICs: ESD, overvoltage, and fault protection for USB, HDMI, and other interfaces
• Industrial input protectors: High-voltage tolerant interfaces for field wiring
• Automotive protection ICs: Meet automotive transient requirements including load dump
• Telecom protectors: GR-1089 compliant protection for central office equipment

Protected Interface ICs

Many interface ICs include built-in protection:

• RS-485/RS-422 transceivers: Integrated ESD protection to 15 kV or higher
• CAN transceivers: Bus fault protection and high-voltage tolerance
• Analog input multiplexers: Input protection allows overvoltage on unpowered devices
• Operational amplifiers: Input protection diodes and output current limiting

Protection Module Assemblies

Pre-assembled protection modules simplify design:

• Signal line protectors: Complete multi-stage protection in single package
• Power entry modules: Combined filtering, surge protection, and EMI suppression
• Connector-integrated protection: Protection components built into connector assembly
• Modular terminal blocks: Protection modules plug into standard terminal systems

Threat Analysis

Begin by identifying potential threats to the input:

• ESD environment: Level of human contact, handling procedures, packaging
• Overvoltage sources: Connected equipment, power supplies, field wiring exposure
• Current fault sources: Short-circuit paths, load faults, cable damage
• Environmental factors: Lightning exposure, industrial machinery, RF sources
• User behavior: Hot-plug likelihood, polarity reversal risk, abuse potential

Protection Level Selection

Match protection levels to identified threats and circuit sensitivity:

• ESD ratings: IEC 61000-4-2 levels from 2 kV contact to 15 kV air discharge
• Surge ratings: IEC 61000-4-5 levels define test waveforms and magnitudes
• Industry standards: Automotive (ISO 16750), telecom (GR-1089), medical (IEC 60601)
• Design margin: Protect to levels exceeding minimum requirements

Protection Coordination

Multiple protection stages must work together effectively:

• Voltage coordination: Upstream devices trigger first with higher clamping voltage
• Energy sharing: Impedance between stages allows voltage buildup upstream
• Response time: Faster devices downstream catch let-through from slower devices
• Failure mode analysis: Consider protection behavior if one stage fails

Verification and Testing

Validate protection design through testing:

• ESD testing: Per IEC 61000-4-2 using calibrated ESD simulator
• Surge testing: Per IEC 61000-4-5 with appropriate combination wave generator
• Waveform capture: Monitor clamping voltage and current during tests
• Repeated stress: Verify protection survives multiple events without degradation
• Marginal testing: Test at levels above specification to verify design margin