16. PPTC Resettable Fuse for PoE Ethernet Port Protection

A Polyfuse (also called a Resettable Fuse or PPTC device) is a polymeric positive temperature coefficient device used for resettable overcurrent protection.

• At normal current → very low resistance → circuit runs normally.
• At fault current/short → device heats up → resistance increases sharply → limits current flow (like an open fuse).
• When the fault is cleared, and power is removed → it cools and resets, ready for reuse.

Key difference from a normal fuse: A glass fuse must be replaced once blown, while a Polyfuse resets automatically.

Schematic Symbols of Polyfuse:

• Prevent damage from overcurrent and short circuits.
• Save cost and downtime → no need for manual replacement.
• Provide user-friendly protection in consumer and embedded products.
• Essential for meeting safety and USB/IEC standards in many devices.

Real-world example:

• In a USB port, if a user connects a faulty cable drawing >1 A, the Polyfuse trips → saves the motherboard without blowing permanently.

• USB ports – resettable protection against shorts.
• Battery packs (Li-ion) – protect cells from overcurrent or runaway.
• Consumer devices – toys, cameras, chargers, IoT nodes.
• Telecom & networking – line cards, Ethernet ports.
• Automotive & Industrial – ECUs, infotainment, instrument clusters.

• Constructed from a polymer matrix with conductive carbon particles.
• Normal condition: Carbon paths conduct → device acts like a low-resistance resistor.
• Overcurrent condition: Current heats polymer → expansion separates carbon particles → resistance rises rapidly (hundreds of ohms) → limits current.
• Cooling/reset: When power is removed and device cools, the carbon paths reconnect → low resistance restored.

1. Hold Current (Ihold):
   • The maximum continuous current the device can carry without tripping.
2. Trip Current (Itrip):
   • Current at which the device “trips” into high resistance.
3. Time-to-Trip:
   • The time it takes to respond once Itrip is reached.
   • Can be ms to seconds (not instant like TVS diodes).
4. Rated Voltage (Vmax):
   • The maximum circuit voltage it can withstand.
5. Initial Resistance (Rmin, Rmax):
   • Device resistance before tripping.
   • Important for low-voltage circuits (USB, battery packs).
6. Post-Trip Resistance (R1):
   • Resistance after tripping (very high, limits current).
7. Power Dissipation (Pd):
   • Heat is generated in normal operation.
8. Reset Cycles:
   • Can reset hundreds–thousands of times, but resistance increases slightly with age.

Example Littelfuse PolySwitch RHEF series Ployfuse Specifications:

Example Polyfuse Temperature Rerating Curve:

• Define normal operating current (Ihold): Choose a PPTC with hold current higher than your circuit’s maximum steady current so it doesn’t trip during normal operation.
• Check trip current (Itrip): Ensure the trip current is below the fault current level, so the fuse reliably activates during overload or short-circuit conditions.
• Match voltage rating: Select a device with a maximum voltage rating equal to or higher than your circuit voltage.
• Consider ambient temperature: PPTC performance changes with temperature, so apply derating if the device operates in high-temperature environments.
• Evaluate resistance (Rinitial): Lower resistance reduces voltage drop and power loss during normal operation.
• Verify response time and reset behavior: Ensure the trip time suits your protection needs and that the reset characteristics match how quickly the circuit should recover.

Advantages:

• Resettable → no replacement
• Low cost, small size
• Perfect for user-accessible ports (USB, chargers).

Limitations:

• Slower response (ms) → cannot catch fast spikes (use TVS diodes instead).
• Resistance increases after every trip (aging).
• Degradation Over Cycles: PPTCs have a limited "trip cycle" life. Repeated tripping causes thermal stress on the polymer matrix, which can lead to permanent changes in trip characteristics or eventual failure.

• Polyfuses protect against overcurrent — not voltage surges.
• Always combine with TVS diodes or MOVs for complete protection.
• Perfect for resettable user-accessible protection (USB ports, batteries).
• Always check Ihold vs system current and Itrip vs fault current