Electronics Guide

Transient Voltage Suppressors

TVS diodes provide fast clamping of overvoltage transients on power supply lines:

• Operating principle: Avalanche breakdown clamps voltage to a predictable level while shunting transient energy
• Response time: Sub-nanosecond response captures even the fastest transients
• Standby power: Negligible leakage current during normal operation below breakdown voltage
• Clamping voltage: Select to exceed maximum normal supply voltage but remain below component ratings
• Peak power rating: Must handle maximum expected transient energy without failure
• Unidirectional vs bidirectional: Unidirectional types are typically used for DC supply protection

For automotive load dump protection, high-power TVS diodes rated for 1500 W or more absorb the energy released when alternator load is suddenly disconnected.

Zener Diode Clamping

Zener diodes provide simple, cost-effective overvoltage clamping for moderate power applications:

• Voltage selection: Choose Zener voltage above maximum normal supply but below component ratings
• Dynamic impedance: Zener voltage rises with current; factor this into protection margin
• Power dissipation: Must handle worst-case overvoltage power indefinitely if condition persists
• Temperature coefficient: Zener voltage varies with temperature; specify over operating range
• Series resistance: May require series resistor to limit current through Zener

Zener diodes work well for low-power circuits where the source impedance naturally limits fault current. For higher power applications, TVS diodes offer better peak power handling.

Crowbar Circuits

Crowbar circuits provide ultimate overvoltage protection by shorting the supply when voltage exceeds a threshold:

• SCR-based crowbar: Silicon controlled rectifier fires when triggered, creating a near-short that blows a fuse
• Trigger circuit: Zener diode or voltage comparator triggers SCR at overvoltage threshold
• Holding current: Once triggered, SCR latches on until current falls below holding current
• Fuse coordination: Fuse must clear before SCR is damaged by fault current
• Response time: Slower than TVS diodes; typically microseconds to trigger

Crowbar protection is appropriate where a brief overvoltage could cause catastrophic damage to expensive downstream components. The tradeoff is that triggering requires fuse replacement.

Active Overvoltage Protection

Active circuits provide sophisticated overvoltage protection with programmable thresholds and controlled response:

• Overvoltage protection ICs: Integrated controllers monitor voltage and disconnect load when threshold is exceeded
• Series pass element: MOSFET or transistor in series with supply is turned off during overvoltage
• Programmable threshold: Set by resistor divider or external reference
• Hysteresis: Prevents oscillation at threshold; requires voltage to drop before reconnecting
• Status indication: Fault flag output enables system-level response to overvoltage conditions
• Auto-retry: Some controllers periodically retry after overvoltage to enable recovery

Linear Regulator Protection

Linear regulators can provide inherent overvoltage protection when properly applied:

• Voltage clamping: Regulator maintains constant output regardless of input variations within its range
• Input voltage limits: Regulator must have sufficient input voltage rating for worst-case overvoltage
• Power dissipation: Dropout voltage times current determines regulator heating during overvoltage
• Thermal shutdown: Many regulators include thermal protection that limits dissipation
• Pre-regulator protection: TVS or crowbar may still be needed to protect regulator input

Undervoltage Lockout

Undervoltage lockout (UVLO) circuits prevent operation below a minimum voltage threshold:

• Threshold voltage: Set above minimum operating voltage of protected circuit
• Hysteresis: Turn-on threshold higher than turn-off prevents oscillation during slow voltage ramps
• Enable output: UVLO provides enable signal that gates power or operation of downstream circuits
• Power consumption: UVLO circuit itself must operate at voltages below its threshold
• Startup behavior: Consider timing of UVLO release relative to circuit initialization requirements

Voltage Supervisor ICs

Dedicated voltage supervisor ICs provide precise undervoltage detection with integrated features:

• Fixed threshold: Factory-trimmed threshold voltage for common supply values
• Adjustable threshold: External resistors set custom threshold voltage
• Reset output: Active-low or active-high reset signal for microcontrollers
• Reset delay: Programmable delay ensures supply is stable before releasing reset
• Watchdog function: Some supervisors include watchdog timer for processor monitoring
• Multiple channels: Monitor multiple supply rails with single IC

Voltage supervisors are essential for microcontroller systems where low supply voltage can cause code execution errors or memory corruption.

Comparator-Based Detection

Simple comparator circuits can implement undervoltage detection:

• Reference voltage: Stable reference establishes comparison threshold
• Resistor divider: Divides supply voltage to match reference for desired threshold
• Hysteresis resistor: Positive feedback provides switching hysteresis
• Open-drain output: Enables wire-OR connection of multiple supervisors
• Rail-to-rail operation: Comparator must function down to minimum supply voltage

Brown-Out Detection

Brown-out detection responds to temporary undervoltage conditions:

• Brown-out vs power-fail: Distinguish between recoverable dips and complete power loss
• Detection delay: Brief glitches should not trigger protection; sustained drops should
• Recovery timing: Circuit must remain disabled until supply fully recovers
• Data protection: Brown-out detection triggers save of critical data before shutdown
• Graceful degradation: Some systems reduce functionality during brown-out rather than shutting down

Battery Discharge Protection

Battery-powered systems require protection against over-discharge that can damage batteries:

• Cutoff voltage: Disconnect load before battery voltage drops to damaging levels
• Cell chemistry: Different chemistries have different minimum voltage requirements
• Load disconnect: MOSFET switch isolates load when voltage is too low
• Leakage current: Protection circuit itself must have minimal quiescent current
• Battery management ICs: Integrated solutions include cell balancing and charge control

Series Diode Protection

A diode in series with the positive supply blocks reverse current:

• Simple implementation: Single component provides complete protection
• Forward voltage drop: 0.3-0.7 V depending on diode type reduces available supply voltage
• Power dissipation: Continuous loss of Vf times load current reduces efficiency
• Schottky diodes: Lower forward voltage minimizes losses but higher leakage
• Current rating: Diode must handle maximum load current with margin for transients
• Reverse voltage rating: Must exceed maximum possible reverse voltage

Series diode protection is simple and reliable but the power loss may be unacceptable for low-voltage or high-current applications.

Shunt Diode with Fuse

A reverse-connected diode conducts if polarity is reversed, blowing a protective fuse:

• Normal operation: Diode is reverse-biased and draws negligible current
• Reverse polarity: Diode forward conducts, creating near-short that clears fuse
• No voltage drop: Zero insertion loss during normal operation
• Surge capability: Diode must survive fuse clearing current
• I2t coordination: Fuse must clear before diode is damaged
• Service required: Fuse replacement needed after reverse polarity event

P-Channel MOSFET Protection

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

• Circuit configuration: MOSFET placed in positive rail with source toward supply, drain toward load
• Normal operation: Negative gate voltage (relative to source) turns MOSFET fully on
• Reverse polarity: Gate-source voltage becomes positive, MOSFET turns off
• Body diode: Intrinsic body diode briefly conducts during initial turn-on
• Low on-resistance: Milliohm Rds(on) creates minimal voltage drop at full current
• Gate voltage limit: Zener clamp may be needed to protect gate for high input voltages

P-channel MOSFET protection offers excellent efficiency but requires careful selection to ensure adequate voltage and current ratings.

N-Channel MOSFET Protection

N-channel MOSFETs offer lower on-resistance for a given die size:

• Circuit configuration: MOSFET placed in negative rail with source toward ground
• Gate drive: Requires gate voltage above source; gate driven from positive supply through resistor
• Lower Rds(on): N-channel devices have better conductivity than equivalent P-channel
• Ground shift: Circuit ground is elevated by Rds(on) drop; may affect sensitive circuits
• Charge pump drive: For high-side N-channel, charge pump generates gate drive above supply

Ideal Diode Controllers

Ideal diode controllers actively manage a MOSFET to emulate an ideal diode:

• Low forward drop: Controller keeps MOSFET on for minimal forward voltage drop
• Fast reverse blocking: Detects reverse current and turns off MOSFET before body diode conducts significantly
• Reverse current threshold: Adjustable threshold determines when to block reverse current
• Load sharing: Multiple ideal diodes can share current from parallel supplies
• Fault indication: Status outputs signal reverse voltage or other fault conditions

Ideal diode controllers are particularly useful in power OR-ing applications where multiple supplies feed a common load.

Automotive Reverse Polarity Requirements

Automotive applications have specific reverse polarity test requirements:

• ISO 16750-2: Defines -14 V reverse voltage test for 12 V systems
• Duration: Test typically applied for 60 seconds minimum
• Jump-start conditions: Must withstand reverse connection from another vehicle
• Battery reversal: Full battery voltage applied in reverse during service
• Double-battery: 24 V from truck battery connected to 12 V system

Hot-Swap Controllers

Hot-swap controllers manage all aspects of connecting a circuit to a live power bus:

• Inrush limiting: Controls capacitor charging current during insertion
• Overvoltage protection: Blocks connection if voltage exceeds safe limits
• Undervoltage lockout: Prevents connection until voltage is adequate
• Overcurrent protection: Limits and monitors current during and after connection
• Short-circuit protection: Fast response to short circuits on load side
• Power good indication: Signals when power is stable and within limits

Integrated Protection ICs

Single-chip solutions combine multiple protection functions:

• Power management ICs: Include protection as part of voltage regulation
• Automotive supply ICs: Address load dump, cold crank, and reverse polarity
• Industrial interface protectors: Handle overvoltage on 24 V industrial supplies
• USB power controllers: Manage VBUS with overvoltage and overcurrent protection

Multi-Rail Protection

Systems with multiple supply rails require coordinated protection:

• Sequencing: Rails must power up and down in correct order
• Tracking: Some rails must maintain specific relationships during transitions
• Fault propagation: Fault on one rail should not damage circuits on other rails
• Cross-rail protection: Interface circuits spanning rails need protection on both sides
• Supervisory ICs: Multi-channel supervisors monitor all rails and coordinate response

Redundant Supply Systems

Critical systems use redundant supplies with protection for seamless failover:

• Diode OR-ing: Simple diodes combine supplies but have forward drop losses
• Ideal diode OR-ing: MOSFET-based OR-ing minimizes losses
• Active load sharing: Parallel supplies share load current equally
• Fault isolation: Failed supply is isolated without affecting system operation
• Hold-up capacitance: Capacitors maintain supply during switchover

TVS Diode Selection

Key parameters for TVS diode selection:

• Working voltage: Maximum continuous voltage; select above maximum supply
• Breakdown voltage: Voltage at which clamping begins; typically 10% above working voltage
• Clamping voltage: Voltage at peak current; must be below protected device ratings
• Peak pulse current: Maximum current during transient; match to expected surge
• Peak pulse power: Energy handling capability for specified pulse duration
• Capacitance: Affects high-frequency supply impedance

MOSFET Selection for Reverse Protection

Critical MOSFET parameters for reverse polarity protection:

• Drain-source voltage rating: Must exceed maximum reverse voltage applied
• Continuous drain current: Must handle maximum load current
• On-resistance: Determines forward voltage drop and power dissipation
• Gate-source voltage rating: May need protection Zener if supply exceeds rating
• Body diode rating: Must handle inrush current during initial turn-on
• Thermal capability: Package must dissipate I2R heating

Fuse Selection

Fuse selection for supply protection circuits:

• Current rating: Must handle normal operating current without degradation
• Voltage rating: Must safely interrupt at maximum supply voltage
• I2t rating: Must clear before protected devices are damaged
• Response time: Fast-acting for semiconductor protection; slow-blow for inrush tolerance
• Breaking capacity: Must interrupt maximum available fault current
• Form factor: Surface mount, through-hole, or panel mount as appropriate

Voltage Supervisor Selection

Parameters for voltage supervisor selection:

• Threshold voltage: Match to supply voltage and protection requirements
• Threshold accuracy: Tolerance determines protection margin needed
• Hysteresis: Sufficient to prevent oscillation during slow voltage changes
• Reset delay: Match to circuit initialization requirements
• Output type: Push-pull or open-drain; active-high or active-low
• Quiescent current: Critical for battery-powered applications

Protection Device Placement

Optimal placement of supply protection components:

• Input location: Place protection at power entry point before supply branches
• Short connections: Minimize trace length between protection device and connector
• Low inductance: Wide traces and multiple vias reduce parasitic inductance
• Thermal relief: Provide adequate copper for heat dissipation
• Current path: Fault current path should not pass through sensitive circuits

Ground and Return Paths

Ground layout affects protection performance:

• Low impedance ground: Protection devices need low-impedance return path
• Star grounding: Prevent protection fault currents from flowing through signal grounds
• Ground plane: Solid ground plane provides lowest impedance
• Via placement: Multiple vias reduce ground path inductance for protection devices

Thermal Management

Protection devices may dissipate significant power during fault conditions:

• Copper area: Use thermal relief pads to spread heat
• Component spacing: Prevent heating of temperature-sensitive components
• Airflow: Consider ventilation for continuous fault power dissipation
• Derating: Apply thermal derating based on ambient temperature

Decoupling Strategy

Decoupling capacitors interact with supply protection:

• Placement: Decoupling after protection isolates capacitor charging from protection response
• Inrush consideration: Large capacitance after series protection devices draws inrush current
• Voltage rating: Decoupling capacitors must withstand clamped voltage, not just nominal supply
• Reverse voltage: Consider capacitor behavior during reverse polarity events

Overvoltage Testing

Methods for verifying overvoltage protection:

• DC overvoltage: Apply steady-state overvoltage and verify clamping or shutdown
• Transient testing: Use surge generators per IEC 61000-4-5 for transient testing
• Load dump simulation: Apply automotive load dump waveform per ISO 7637-2
• Clamping voltage measurement: Capture actual clamping voltage during transients
• Repeated stress: Verify protection survives multiple events without degradation

Undervoltage Testing

Verifying undervoltage protection behavior:

• Threshold accuracy: Measure actual trip and release voltages
• Hysteresis verification: Confirm adequate hysteresis prevents oscillation
• Response time: Measure time from undervoltage to circuit response
• Brown-out immunity: Test with brief voltage dips to verify proper behavior
• Temperature variation: Verify threshold over operating temperature range

Reverse Polarity Testing

Testing reverse polarity protection:

• Full reverse voltage: Apply maximum expected reverse voltage
• Leakage measurement: Verify no significant reverse current flows to circuit
• Sustained exposure: Apply reverse voltage for extended period per specifications
• Recovery verification: Confirm normal operation after reverse polarity event
• Current-limited source: Use current-limited supply to prevent damage during testing

Integration Testing

System-level testing of supply protection:

• Startup sequencing: Verify proper power-up with protection enabled
• Fault injection: Inject faults and verify protected circuits survive
• Marginal testing: Test at limits of protection thresholds
• Environmental testing: Verify protection over temperature and humidity ranges
• EMC interaction: Ensure protection does not compromise EMC performance

Automotive Electronics

Automotive applications face harsh supply conditions:

• Load dump: Up to 40 V transients when alternator load disconnects
• Cold crank: Supply voltage drops to 6 V or below during engine starting
• Jump start: Reverse polarity and 24 V double-battery conditions
• Solution: TVS for load dump, UVLO for cold crank, P-FET for reverse protection

Industrial Control Systems

Industrial environments have specific protection needs:

• 24 V DC supplies: Wide tolerance from 18-32 V typical
• Surge exposure: Lightning and switching transients on long cable runs
• Reverse protection: Field wiring errors during installation
• Solution: TVS with sufficient headroom, UVLO, and reverse protection

Battery-Powered Devices

Battery applications balance protection with efficiency:

• Efficiency priority: Minimize losses to extend battery life
• Reverse insertion: Users may insert batteries incorrectly
• Over-discharge: Protect battery from damage due to deep discharge
• Solution: Low Rds(on) P-FET for reverse protection, integrated battery management

Server and Telecom

High-availability systems require robust protection:

• Hot-swap capability: Cards inserted into live backplanes
• Redundant supplies: Multiple supplies with seamless failover
• Inrush control: Limit current during insertion to prevent supply disturbance
• Solution: Hot-swap controllers with integrated OR-ing and protection