SETfuse T115 Technical Analysis: The Definitive Guide to Industrial Resettable Fuses

Experiencing frequent control board failures caused by motor short circuits? Or are your power modules repeatedly damaged due to overcurrent conditions?

The SETfuse T115, a high-reliability PPTC (Polymeric Positive Temperature Coefficient) resettable fuse, is purpose-built for overcurrent protection in demanding industrial environments. Unlike traditional fuses, it automatically resets once the fault condition is cleared—minimizing downtime and maintenance effort.

Core Specifications & Selection Criteria (Engineer’s Must-Read)

Design Advantages

• Millisecond-Level Overcurrent Response: Trips within ≤ 0.15 s at 5×I_hold, 25% faster than MF-RX110.
• Anti-Aging Protective Coating: Ceramic base epoxy layer withstands over 1,000 reset cycles—exceeding industrial endurance standards.
• Low Internal Resistance: Typical R_max ≤ 0.08 Ω, minimizing power loss and heat buildup (15% lower than MF-RX110 based on field tests).

Top 3 Industrial Applications & Circuit Design Examples

1. DC Motor Stall Protection

Problem: When the motor stalls, inrush current can damage the driver IC.

Solution:

• Install a SETfuse T115 (I_hold = 1.15 A) in series with the motor’s power supply line.
• Add a TVS diode across the motor terminals to suppress back-EMF.
• Circuit topology: Power → T115 → Motor → TVS → GND

Test Result: Auto-reset time ≤ 60 s after trip, reducing unplanned downtime significantly.

2. USB-C Port Overcurrent Protection

Challenge: Hot-plugging causes occasional port shorts, damaging power stages.

Design Implementation:

• Place T115 inline with the VBUS rail; I_hold = 1.15 A is suitable for USB PD 3.0 specification.
• Ensure layout distance from USB connector is <10 mm to minimize parasitic inductance and ensure fast trip response.

Case Study: A charger manufacturer reduced warranty failure rate by 72% after T115 adoption.

3. Photovoltaic (PV) Inverter DC-Side Protection

Risk: Reversed polarity or short circuit on solar panel strings can lead to fire hazards.

Solution:

• Connect T115 in parallel across each PV string input (voltage range: 25–60 V DC).
• Thermal Management: Allocate a 3 × 3 mm copper thermal pad on the PCB under the fuse to reduce hotspot temperature by 40%.

Common Faults & Diagnostic Workflow

Fault 1: Nuisance Tripping (Frequent Interruptions)

Causes:

• Ambient temperature above 70°C reduces I_hold by ~30%.
• High current ripple > 40%, typically from low-quality switching power supplies.

Solutions:

• Use a derated fuse with higher I_hold (e.g., 1.5 A model for high-temp environments).
• Add input LC filtering: 10 μH inductor 100 μF capacitor recommended.

Fault 2: Permanent Open (Non-reset Condition)

Diagnosis Steps:

• Measure resistance across terminals: Normal < 0.1 Ω; Failed > 1 kΩ.
• Check whether max fault current (40 A for T115) was exceeded.

Root Cause: Prolonged overload permanently damages the polymer matrix, making the device non-recoverable.

Fault 3: Delayed Response Leading to Downstream Damage

Recommended Improvements:

• Install a fast-blow fuse (e.g., 5A / 250V) in parallel as secondary protection.
• Minimize PCB trace length to < 15 mm to reduce parasitic inductance and enhance trip speed.

Diagnostic Workflow Summary:

Symptom Evaluation → Resistance Measurement → Environmental Conditions Review → Current Waveform Analysis → Component Replacement Verification

Alternative Models & Emergency Repair Solutions

Emergency Repair Tips

• Temporary Substitution: Use two T110 fuses in parallel to achieve ~2.2 A total I_hold. Ensure proper current balancing between devices.
• Permanent Replacement: Thoroughly clean oxidized solder pads before installation. Keep soldering time under 3 seconds to prevent thermal damage to the fuse element.

PCB Layout & Reliability Design Best Practices

Current Path Optimization

• Trace Width: Ensure PCB traces before and after the fuse are ≥ 2 mm wide to safely handle up to 5 A of continuous current.
• Avoid 90° Angles: Use 45° bends or curved traces to minimize impedance discontinuities and reduce potential hotspots.

Thermal Management

• Keep-Out Zone: Reserve at least a 3 mm clearance zone around the fuse to isolate it from temperature-sensitive components.
• Thermal Vias: Deploy heat-dissipating via arrays beneath the fuse area (hole diameter: 0.3 mm; pitch: 1.0 mm) to improve thermal conduction to inner or bottom copper layers.

Frequently Asked Questions (FAQ)

Q1: Can the T115 be used in 220V AC systems?

No. The T115 is rated for 60 V DC only and is not suitable for AC applications. For 220V AC circuits, use the TR600 series, which is rated up to 250 V AC.

Q2: What factors affect the reset time after tripping?

Key influences:

• Ambient temperature: At 25°C, typical reset time is ~60 seconds; at 85°C, it can extend to ~120 seconds.
• Fault current level: Lower currents (e.g., 10 A) result in quicker resets compared to higher currents (e.g., 40 A).

Q3: How can I detect if the fuse has aged or degraded?

Recommended diagnostic methods:

• Measure the fuse’s cold resistance. If it exceeds 0.15 Ω, the part is considered aged and should be replaced.
• Apply rated current continuously for 1 hour. If the temperature rise exceeds 40°C, aging is suspected.

Author & Reference

Technical Review: This article has been reviewed by a Master of Electrical Engineering (specializing in Power Protection Systems).

Reference Document: For more detailed specifications and application notes, please refer to the official Littelfuse SETfuse datasheet:

📄 Littelfuse SETfuse Technical Datasheet (PDF)