Electronics Guide

Diode Clipping Circuits

Diode clippers limit signal amplitude by conducting when voltage exceeds a threshold determined by the diode forward voltage and any series bias voltage:

• Series clippers: The diode is placed in series with the signal path, blocking signals of one polarity while passing the other
• Shunt clippers: The diode is placed in parallel with the load, conducting and shorting excess voltage when the threshold is exceeded
• Biased clippers: Adding DC bias allows the clipping threshold to be set at any desired level
• Dual clippers: Back-to-back diodes or antiparallel configurations clip both positive and negative excursions

For transient protection, shunt clippers are most common. The diode must conduct fast enough to catch the transient and must handle the peak current and energy without damage. Silicon switching diodes work for moderate transients, while specialized avalanche diodes handle higher energies.

Zener Diode Clipping

Zener diodes provide voltage-referenced clipping independent of current magnitude over a wide range:

• Operating principle: Zener breakdown at a specified voltage allows the diode to clamp while conducting significant current
• Bidirectional protection: Back-to-back Zeners protect against transients of either polarity
• Voltage selection: The Zener voltage is chosen above normal signal peaks but below the damage threshold of protected components
• Dynamic resistance: The finite dynamic resistance of Zeners causes some voltage rise with increasing current

Zener clippers are effective for protecting inputs against moderate overvoltage but have limited energy handling capability compared to specialized transient suppressors.

Clamping Circuits

Unlike clippers that remove portions of a waveform, clamping circuits shift the entire waveform to a new DC reference while preserving its shape:

• Capacitor coupling: A series capacitor blocks DC, allowing the diode to establish a new reference level
• Positive clamping: Establishes the negative peak at a reference voltage
• Negative clamping: Establishes the positive peak at a reference voltage
• Biased clamping: Combines diode clamping with a voltage source for arbitrary reference levels

While primarily used in signal processing, clamping techniques also find application in protecting AC-coupled stages against DC shifts that could bias transistors into damaging operating regions.

Active Clipping Circuits

Operational amplifiers and comparators enable active clipping with precisely defined thresholds and fast response:

• Precision clippers: Op-amp circuits eliminate diode forward voltage drop, providing accurate threshold voltage
• Programmable limiters: Digitally controlled references allow threshold adjustment
• Current-limited clamping: Active circuits can limit current as well as voltage
• Speed limitations: Op-amp slew rate and bandwidth limit response to fast transients

Active clippers are best suited for signal conditioning rather than high-energy transient protection, where passive devices offer faster response and greater ruggedness.

SCR Crowbar Circuits

Silicon controlled rectifiers form the most common crowbar implementation:

• Trigger mechanism: A voltage sensing circuit fires the SCR gate when the threshold is exceeded
• Latching action: Once triggered, the SCR remains conducting until current falls below holding current
• Current handling: SCRs can handle surge currents of hundreds or thousands of amperes for short durations
• Response time: Turn-on time is typically 1-5 microseconds, fast enough for many applications

The SCR crowbar is typically used in power supply protection, where an overvoltage condition from regulator failure could damage downstream circuitry. The crowbar fires, blowing the input fuse and disconnecting power.

Crowbar Design Considerations

Effective crowbar design requires attention to several factors:

• Threshold accuracy: The trigger circuit must respond reliably at the set voltage while avoiding false triggering from noise
• Trigger delay: Some designs incorporate time delay to ignore brief transients that would not damage protected circuits
• Fuse coordination: The fuse must clear before the SCR's I2t rating is exceeded
• Gate drive: Adequate gate current ensures fast, reliable turn-on under all conditions
• Reset mechanism: Some applications require manual reset; others use resettable fuses or automatic recovery

Thyristor Surge Protectors

Specialized thyristor devices designed specifically for surge protection offer advantages over conventional SCRs:

• Breakover voltage triggering: Self-triggering at a specified voltage eliminates need for external sensing circuit
• Fast turn-on: Optimized for sub-microsecond response
• High surge capability: Designed to handle multiple surge events without degradation
• Bidirectional types: Triacs and specialized devices protect against surges of either polarity

Crowbar versus Clamping

The choice between crowbar and clamping protection depends on the application:

• Crowbar advantages: Provides near-zero voltage during protection, handles very high energies, definitive shutdown
• Crowbar disadvantages: Destructive (requires fuse replacement), cannot discriminate between minor and major events
• Clamping advantages: Non-destructive, automatically recovers, allows continued operation
• Clamping disadvantages: Limited energy handling, some let-through voltage during clamping

Many robust designs combine both approaches: clamping devices handle routine transients, while the crowbar provides backup for catastrophic events.

Inrush Current Limiting

Large capacitors at power supply inputs draw enormous currents when first connected to a voltage source:

• Problem magnitude: Inrush current can exceed steady-state current by factors of 10-100 or more
• Damage mechanisms: Contact welding in relays and switches, fuse fatigue, capacitor stress, connector damage
• Resistor limiting: A series resistor limits peak current but wastes power in steady state
• NTC thermistor: High cold resistance limits inrush; self-heating reduces resistance during operation
• Active circuits: FET or relay bypasses the limiting element after initial charge period

Voltage Ramping

Gradually increasing the applied voltage reduces both electrical and thermal stress:

• Linear ramp: Voltage increases at a constant rate to the final value
• Exponential approach: Natural RC charging provides soft approach to final voltage
• Stepped ramp: Discrete voltage steps, common in digital control systems
• Controlled rise time: Specified in milliseconds to seconds depending on application

Many integrated power supply controllers incorporate soft-start functionality, ramping the output voltage over a programmable interval after enable.

Sequential Power-Up

In systems with multiple supply rails, the order and timing of power application can be critical:

• Core before I/O: Many modern processors require lower core voltage to be established before higher I/O voltage
• Analog before digital: Prevents digital noise from affecting analog circuits during initialization
• Power good signaling: Each supply signals when stable, triggering the next in sequence
• Timing controllers: Dedicated ICs manage complex sequencing requirements

Motor Soft Starters

Electric motors present extreme inrush current and mechanical stress challenges:

• Starting current: Induction motors draw 6-8 times rated current at standstill
• Mechanical stress: Sudden torque application stresses shafts, couplings, and driven equipment
• Phase-controlled soft start: SCRs or triacs control voltage by phase angle, gradually increasing to full voltage
• Variable frequency drives: Starting at low frequency provides soft start with full torque capability

Inductive di/dt Limiting

Inductors inherently limit the rate of current change:

• Fundamental relationship: V = L(di/dt), so di/dt = V/L
• Series inductors: Placed in the power path to limit current rise rate
• Saturation considerations: The inductor must not saturate at peak current, or limiting is lost
• Energy storage: The inductor stores energy that must be dissipated or returned

Di/dt limiting is critical for thyristor protection, as excessive di/dt at turn-on concentrates current in a small area near the gate, causing localized overheating and device failure.

Capacitive dv/dt Limiting

Capacitors limit voltage rate of change by absorbing current:

• Fundamental relationship: I = C(dv/dt), so dv/dt = I/C
• Bypass capacitors: Shunt high-frequency transients to ground
• Snubber function: Capacitors slow voltage rise across switching devices
• Size tradeoff: Larger capacitance provides better limiting but increases stored energy

Excessive dv/dt can cause spurious turn-on of thyristors through displacement current in the junction capacitance, leading to loss of control or device failure.

Combined Rate Limiting

Many applications require limiting both di/dt and dv/dt:

• LC networks: The combination provides both di/dt and dv/dt limiting with oscillatory response
• RLC damping: Adding resistance damps oscillation at the cost of power dissipation
• Critical damping: Optimal resistance eliminates overshoot while maintaining fast response
• Snubber networks: RC and RCD circuits specifically designed for rate limiting across switching devices

Device Ratings

Semiconductor devices specify rate-of-change limits that must be respected:

• Thyristor di/dt: Typically 50-500 A/microsecond depending on device size and design
• Thyristor dv/dt: Typically 50-1000 V/microsecond for static conditions
• MOSFET dv/dt: Body diode recovery can cause issues at high dv/dt
• IGBT constraints: Excessive di/dt or dv/dt can cause latch-up or voltage overshoot

RC Snubbers

The simplest snubber consists of a resistor and capacitor in series across the switching device:

• Function: The capacitor limits dv/dt during turn-off; the resistor damps ringing and limits discharge current during turn-on
• Capacitor selection: Must be large enough to limit dv/dt below device rating
• Resistor selection: Chosen to critically damp the resonance with circuit inductance
• Power dissipation: The resistor dissipates 0.5CV2f watts, where f is switching frequency

RC snubbers are effective for moderate-speed switching and provide turn-off protection but load the switch with discharge current at turn-on.

RCD Snubbers

Adding a diode to the RC snubber separates the charge and discharge paths:

• Polarized design: The diode allows fast capacitor charging during turn-off while the resistor provides slow discharge
• Reduced turn-on stress: The capacitor discharges through the resistor, not through the switch
• Energy recovery versions: Modified topologies can return snubber energy to the supply
• Diode requirements: Fast recovery diodes are needed to avoid reverse recovery effects

Turn-On Snubbers

While most snubbers focus on turn-off, turn-on protection may also be required:

• Series inductor: Limits di/dt during turn-on
• Saturable inductor: Provides high impedance during initial turn-on, then saturates for low on-state loss
• Combined snubber: Addresses both turn-on and turn-off requirements
• Active snubbers: Use auxiliary switches to control snubber operation

Snubber Design Procedure

Systematic snubber design follows these steps:

1. Identify requirements: Determine whether dv/dt, di/dt, or both need limiting
2. Characterize circuit: Measure or calculate stray inductance and capacitance
3. Calculate snubber capacitor: Based on dv/dt limit and turn-off current
4. Calculate snubber resistor: Based on damping requirements and acceptable discharge current
5. Verify power dissipation: Ensure resistor can handle the switching losses
6. Test and iterate: Observe actual waveforms and adjust as needed

Snubber Component Selection

Component selection is critical for snubber reliability:

• Capacitors: Film types (polypropylene, polyester) handle high dv/dt and current; avoid electrolytics
• Resistors: Non-inductive types required; wire-wound resistors add problematic inductance
• Power rating: Derate significantly due to pulse stress and heating
• Voltage rating: Capacitor must withstand peak transient voltage

TVS Diode Characteristics

TVS diodes are optimized avalanche diodes with key characteristics:

• Breakdown voltage (VBR): The voltage at which the device begins to conduct, specified at a small test current
• Clamping voltage (VC): The voltage across the device at peak pulse current; higher than VBR due to dynamic resistance
• Peak pulse current (IPP): Maximum current the device can handle for a specified pulse duration
• Peak pulse power (PPP): Maximum power dissipation, typically specified for 10/1000 microsecond pulse
• Standoff voltage (VRWM): Maximum continuous voltage that can be applied without significant leakage

TVS Selection Criteria

Selecting the appropriate TVS requires balancing several factors:

• Standoff voltage: Must exceed the maximum normal operating voltage including tolerances
• Clamping voltage: Must be below the damage threshold of protected components
• Power rating: Must exceed the maximum expected transient energy
• Response time: Sub-nanosecond for most TVS diodes, but lead inductance may limit effective speed
• Capacitance: Important for high-speed data lines; low-capacitance TVS types available

Unidirectional versus Bidirectional TVS

TVS devices are available in two configurations:

• Unidirectional: Clamps in one polarity; acts as a forward diode in the other direction. Use for DC lines where polarity is known
• Bidirectional: Clamps symmetrically in both directions. Use for AC signals and differential data lines

Bidirectional devices have slightly higher capacitance and clamping voltage than unidirectional types of the same power rating.

TVS Arrays

Multi-channel TVS arrays protect multiple lines in a single package:

• Data line protection: Arrays for USB, HDMI, Ethernet, and other multi-line interfaces
• Matched characteristics: All channels have similar capacitance and clamping for signal integrity
• Common-mode protection: Some arrays include rail-to-rail clamping for common-mode transients
• Space efficiency: Single package replaces multiple discrete devices

Metal Oxide Varistors

Metal oxide varistors (MOVs) offer an alternative to silicon TVS devices:

• Operating principle: Zinc oxide grains with intergranular barriers provide nonlinear voltage-current characteristic
• High energy capability: MOVs handle very large surge energies, suitable for AC line protection
• Degradation: Repeated surges cause gradual increase in leakage current and decrease in clamping voltage
• Response time: Slower than TVS diodes, typically tens of nanoseconds
• Voltage range: Available for voltages from tens of volts to thousands of volts

MOVs are the standard protection device for AC power line transients but degrade with use and should be replaced periodically in critical applications.

Multi-Stage Protection

A coordinated protection system typically includes three stages:

• Primary protection: Spark gaps or large MOVs at the service entrance handle the bulk of lightning surge energy
• Secondary protection: Smaller MOVs or gas discharge tubes at panel or equipment level reduce residual transient
• Tertiary protection: TVS diodes at the circuit board level provide final clamping

Each stage must be coordinated so that the upstream device operates first, absorbing most of the energy before the downstream device responds.

Coordination Requirements

Proper coordination depends on several factors:

• Clamping voltage differential: Upstream devices must have higher clamping voltage to conduct first
• Series impedance: Inductance or resistance between stages allows voltage to build upstream
• Response time matching: Faster devices should be downstream to catch any let-through
• Energy rating progression: Upstream devices handle more energy; downstream devices see only residual

Decoupling Impedance

The impedance between protection stages is critical for coordination:

• Wire inductance: Approximately 1 microhenry per meter of wire provides natural decoupling
• Added inductors: Ferrite beads or small inductors enhance decoupling
• Minimum separation: At least 10 meters of wire or equivalent inductance recommended between stages
• Voltage drop calculation: V = L(di/dt); with 10 microsecond rise time and 10 microhenry, get 1 volt per ampere

Let-Through Voltage

The voltage that appears at downstream equipment despite protection measures:

• Device clamping voltage: The voltage across the suppressor during conduction
• Lead inductance: Voltage drop in the leads during fast current changes
• Device response time: Voltage may rise before the device fully responds
• Source impedance: Lower source impedance means higher peak current and higher clamping voltage

The total let-through must remain below the susceptibility threshold of protected equipment. Testing under realistic surge conditions validates the protection design.

Standards and Testing

Industry standards define surge environments and test methods:

• IEEE C62.41: Defines surge environment categories for AC power systems
• IEC 61000-4-5: Surge immunity test standard with defined waveforms
• Combination wave: 1.2/50 microsecond voltage with 8/20 microsecond current is standard test
• Category ratings: Categories A, B, and C define expected surge levels at different locations

Arc Formation Mechanism

Understanding arc formation helps in designing effective suppression:

• Contact separation: As contacts begin to separate, current constricts to a small area
• Metal vapor: Intense heating vaporizes contact material, creating a conductive path
• Ionization: The arc ionizes surrounding gas, sustaining conduction
• Minimum arc voltage: DC arcs sustain at approximately 10-15 volts; AC arcs extinguish at zero crossing
• Minimum arc current: Below approximately 300-500 milliamperes, arcs typically do not sustain

RC Arc Suppression

An RC network across the contacts reduces voltage rate of rise:

• Capacitor function: Limits dv/dt across opening contacts, preventing arc initiation
• Resistor function: Limits discharge current when contacts close, preventing contact welding
• Typical values: 0.01-0.1 microfarad with 10-100 ohms, adjusted for contact current and voltage
• Limitations: Less effective for inductive loads with high stored energy

Diode Suppression

For DC circuits with inductive loads, freewheeling diodes are highly effective:

• Principle: The diode provides an alternative path for inductive current when contacts open
• Placement: Connected across the load with polarity to conduct when the switch opens
• Energy dissipation: Inductive energy dissipates in the load resistance rather than the arc
• Delay effect: Current decay is slower because there is no arc voltage to speed de-energization

Adding a resistor in series with the diode speeds current decay at the cost of some arc energy.

Varistor Suppression

Varistors offer advantages for arc suppression in certain applications:

• Bidirectional: Works for AC or DC circuits of either polarity
• Voltage clamping: Limits voltage to a defined level, reducing arc energy
• Faster decay: Higher clamping voltage than diode results in faster inductive current decay
• Degradation concern: MOVs degrade with repeated operation; may need replacement

Magnetic Blowout

Magnetic fields can be used to extinguish arcs in high-power applications:

• Principle: Magnetic field exerts force on the arc, stretching and cooling it
• Permanent magnets: Simple implementation for DC contactors
• Series coils: Load current creates the extinguishing field automatically
• Arc chutes: Divide the arc into multiple segments, increasing total arc voltage

Zero-Cross Switching

For AC circuits, timing contact operation to coincide with current zero crossings minimizes arcing:

• Opening at zero: Interrupting at current zero means no arc can form
• Closing at zero: Avoids inrush from charging capacitive loads at peak voltage
• Solid-state implementation: Triacs with zero-cross optoisolator drivers
• Hybrid switches: Mechanical contacts with parallel solid-state switch for zero-cross operation

Layout and Wiring

Physical implementation significantly affects protection effectiveness:

• Lead length: Keep leads short; 20 nH per inch of wire causes significant voltage drop at high di/dt
• Suppressor placement: Mount directly at the protected device, not remotely connected by long wires
• Ground connections: Use low-inductance paths to ground; wide traces or planes preferred
• Separation: Route protected lines away from potential noise sources

Failure Modes

Understanding how protection devices fail aids in designing robust systems:

• TVS diodes: Typically fail short, providing continued protection but potentially affecting circuit operation
• MOVs: Can fail short (safe) or open (no protection); some include thermal disconnects
• Gas discharge tubes: May fail short or develop increased firing voltage
• Fusing: Consider adding series fusing to prevent fire hazard from shorted protection device

Thermal Management

Transient suppression devices must dissipate absorbed energy:

• Single pulse rating: Maximum energy for one event with cold device
• Repetitive rating: Reduced capability for repeated events due to heating
• Steady-state dissipation: Leakage current causes continuous power dissipation
• Heat sinking: Large TVS devices may require heat sinking for full rating

Testing and Verification

Protection designs should be verified under realistic conditions:

• Surge generators: Standard waveforms per IEC or IEEE specifications
• ESD testing: IEC 61000-4-2 defines human body and machine models
• Waveform monitoring: Observe clamping voltage and current during test events
• Margin verification: Test at levels exceeding expected worst case