Protecting electronics hardware against damage from issues like shorts, transients or electromagnetic interference poses myriad reliability challenges. A variety of defensive design approaches combat threats which can emerge across product lifecycles – from manufacturing through field operation.
This guide covers common protection methods available to fortify circuit integrity as well as tradeoffs weighing dependencies unique to application contexts from cost to performance needs.
Key Failure Modes
To assess protective requirements, understanding damage risks proves essential:
Transient Voltages
- Electrostatic discharge
- Lightning strikes
- Power grid fluctuations
Fault Conditions
- Output shorts
- Thermal overloads
- Component failures
Interference Sources
- AC power line noise
- Magnetic flux coupling
- Radio emissions
Judiciously applying safeguards against such eventually likely occurrences reduces probability of field failures disrupting customer experiences. But balancing protection costs against risks warrants thoughtful analysis.
Fusing Against Faults
One historically common component protecting dangerous fault conditions manifesting from shorts or excessive currents remains the venerable fuse – basically a controlled weak spot intentionally designed to fail open in surpassing specifications. This protects downstream elements from succumbing to thermal heat caused byALLOWABLE signal levels vastly exceeded under irregular conditions.
Fuse characteristics require matching to application needs:
Voltage Rating
Must withstand normal bias supply levels
Current Rating
Tolerance band adequate for load flows anticipate under standard use above triggering levels
Blowing Time
Faster or time-delayed modes balance temporary spikes against disruptions
Type
Resettable, single blow fuse sizes, specialty form factors, compatibility with automated assembly processes
Here is an example circuit using a polyswitch current limiting resettable fuse device:
And a comparison table of some various fuse types available:
| Fuse Type | Size Range | Description | Key Characteristics |
|---|---|---|---|
| PTC Resettable | 0402-1210 SMD / Through Hole | Polymer heats up increasing resistance when overcurrent flows | Reusable protection, lower cost for short term minor faults |
| Ceramic Tube | 5x20mm / 10x38mm | Cylindrical tube physically cracks during overcurrent fault | Very fast reaction, surface mount sizes available |
| Blade Terminal | ATO / ATC Various sizes | Widely used terminal block through hole fuse | Easy to replace physical fusing allows circuit restoration after major fault |
| Chip SMD | 0402-1206 sizes | Thin profile surface mount fuse in common SMT sizes | Integrates protection without significant space overhead |
Clamping Transients
Lightning strikes or electrostatic buildup intermittently introduce extremely brief high voltage spikes across electronics vulnerable to exceeded ratings. Transorbs and avalanche diodes actively clamp such abruptly seen potential surges harmlessly bleeding excesses before damage ensues inside integrated circuits or semiconductor junctions.
Voltage Dependent Resistors like varistors dynamically decrease impedance when applied potential increases thus limiting transient amplitudes passed further through the circuitry.
Gas Discharge Tubes literally arc ionized pathways steering current flow away from sensitive nodes for nanosecond scale immense inrush events.
Here a transorb array protects a cable from nearby lightning air discharge entering through the physical interconnect:
Snubbing Spikes
Less severe but still problematic voltage spikes arise from inductive loads suddenly collapsing magnetic fields after cycling off during routine operation - relays, solenoids, motors. The generated inverse EMF can disturb analog processing or memory retention. Snubbers present alternate routing to safely divert turn-off spikes away from sensitive nodes.
RC Snubbers dissipate energy recombining charges across resistive elements without interrupting intended connectivity. Values match impedance properties of loads.
DIODE Snubbers literally circulate spawned losses using reverse currents through additional paths back to source origins neutralizing effects.
Strategic snubber placement targets specific inductive loads pronouncing troublesome back EMF when switching analog sensor signals or power drivers.
Isolating Noise - Opto-Couplers
Electrically noisy elements like motors, power supplies and actuators wreak havoc on signal integrity when direct wire couplings infiltrate ground planes spreading interference widely across boards.
Opto-isolators provide electrical insulation using internal LED/phototransistors transforming signals into light pulses conveying data across dielectric barriers. This breaks ground continuity allowing very noisy or high voltage sub-systems separation from vulnerable analog signal chains sensitive to EM disturbances.
Shielding Enclosure Design
PHYSICAL constructs play important roles barricading environmental noise infiltration sources ranging from metal chassis shields to carefully engineered partitioned coplanar cutouts and ground walls built right into the PCB layer stackup configuration. Such tactics enclose circuitry blocking external interference coupling through open airpaths.
Common shielding methods include:
- Copper chassis, cans, covers, boxes
- Gaskets/fasteners for light sealing around doors
- Grounding schemes tying barriers to common nodes
- Electrostatic & electromagnetic solutions
Building protection directly into cases rather than just circuits counteracts unintended leakage points across design gaps challenging to completely close through components alone.
Thermal Protections
Excessive ambient heat degrades performance parameters altering precision and reliability capabilities through temperature dependencies built right into integrated circuits. Once thresholds exceed absolute maximum ratings stated on datasheets, permanent damage can manifest in transistor leakage or dielectric breakdown leading towards eventual failure over time.
Thermal cutoffs proactively open circuits once reaching sets limits through thermally sensitive components like:
- Resettable thermistors with resistance reacting to temp
- Single use thermal fuses permanently opening on rise
- Temperature diodes providing monitoring telemetry
- Failsafe timers disabling dangerous faults
Provisions preventing thermal runaway faults helps ensure graceful performance degradations into recovering safe modes.
Redundancy - Backup & Failover
No single absolute prevention mechanism proves universally robust against all potential environmental risks electronics routinely face across mission critical reliability requirements over extended lifetimes. However layers of strategically coordinated redundant backup protections compensate weaknesses fortifying overall hardening throughDefense-in-Depth approaches:
- Parallel redundancies – doubling devices
- Standby backup activations on primary failure signals
- Watchdog timers resetting unresponsive latches
- Alternate ground paths circumventing susceptibility
- Replaceable modular components enabling field services
With risks emerging linearly but protections increasing exponentially with layers, measured allocations towards failsafe investments pay reliability dividends over total cost of ownership. Survivability sees no single solution - both circuits and full systems engineering integrate protections balancing importance against economics through redundancy commitments commensurate with demands.
In Summary
Electronics face myriad risks from transient faults, shorts failures, interference coupling, thermal stresses and uncontrolled usages across product lifetimes. Hardening systems against eventual likelihoods proves vital towards sustaining experiences customers anticipate - but requires holistic thinking weighing capabilities against complex tradeoffs only bounded by physics and budgets.
PCB designers build foundations supporting reliable operation through application tailored component selections and layout considerations realizing protections
- either off-the-shelf or custom where needed. Measured resilience promises performance sustainability.
Frequently Asked Questions
Here are some common questions around providing circuit protection:
Q: Where should resettable polymeric fuses get placed on a board?
A: Polymer positive temp coefficient devices best mount physically close to components needing protection placed along the supply provision trace feeding the associated loads downstream. This helps sensing heat nearest actual faults.
Q: What considerations help select appropriate surface mount fuse sizes?
A: Calculate maximum sustained current draw through protected path at nominal supply voltage along with applicable deratings. Then source fuses allowing some margin above this - for example 15-20%. Size also based on fault clearing capacity needed.
Q: How much spacing should separate ground planes when isolation becomes necessary?
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