Ruiling Semiconductor (Shenzhen) Co., Ltd.

PPTC Derating Guide

PPTC devices, also known as polymeric positive temperature coefficient resettable fuses, are widely used for overcurrent protection in electronic and electrical systems. Unlike a conventional one-time fuse, a PPTC fuse can return toward its normal low-resistance state after the fault is removed and the device cools.

However, one important characteristic must be considered during PPTC fuse selection:the electrical performance of a PPTC changes with temperature.

A PPTC fuse rated for a specific hold current at 25°C cannot necessarily carry the same current continuously at 60°C, 80°C, or 100°C. As temperature increases, the available hold current generally decreases. This is why engineers need to use a PPTC derating curve rather than selecting a device only from its room-temperature current rating.

This PPTC Derating Guide explains how temperature affects PPTC performance, how to read a thermal derating curve, how to calculate a practical current requirement, and how to select a PPTC resettable fuse for real-world applications.

What Is PPTC Derating?

PPTC derating refers to the reduction in the allowable operating current of a PPTC device as its operating temperature increases.

A PPTC is a thermally sensitive overcurrent protection component. Its resistance increases significantly when the device is heated by excessive current. Therefore, the temperature surrounding the component directly influences how much current it can carry without entering its high-resistance state.

In general:

At lower temperatures, a PPTC can carry more current before tripping.

At the rated reference temperature, the specified Ihold applies.

At higher temperatures, the allowable hold current decreases.

At sufficiently high temperature, a relatively small increase in current can cause the PPTC to trip.

Manufacturer thermal derating curves are therefore essential when designing PPTC protection for equipment operating above normal room temperature. Eaton similarly recommends selecting a resettable PTC based on the maximum ambient temperature and steady-state current, then using the thermal derating graph during selection.

Why Is PPTC Derating Important?

A common mistake in PPTC selection is to compare the circuit operating current directly with the Ihold value shown in a datasheet.

For example, suppose a circuit continuously draws:

Iload = 1.0 A

An engineer may select a PPTC with:

Ihold = 1.1 A @ 25°C

At 25°C, this may appear acceptable.

However, if the actual operating temperature around the PPTC reaches 60°C, the device may have a substantially lower allowable current than its nominal 25°C rating.

The actual design condition is therefore not:

1.0 A < 1.1 A

It should instead be evaluated as:

Actual operating current < temperature-adjusted allowable hold current

This distinction becomes particularly important in:

Automotive electronics

Battery protection

Power supplies

Industrial controls

USB and Type-C interfaces

Consumer electronics

Telecommunications equipment

Smart home equipment

IoT devices

Medical electronics

High-density PCB designs

How Temperature Affects PPTC Performance

The most important parameters affected by temperature include:

Hold current (Ihold)

Trip current (Itrip)

Resistance

Time-to-trip

Power dissipation

Reset behavior

1. Hold Current

Ihold is the maximum current that a PPTC can carry continuously without tripping under specified test conditions.

The important point is that Ihold is specified at a particular temperature and test condition.

For many PPTC products, the reference condition is around room temperature, but engineers must always verify the exact test temperature in the manufacturer's datasheet.

As temperature increases, Ihold generally decreases.

Therefore:

Higher temperature → lower allowable continuous current

This is the primary reason thermal derating must be considered during PPTC selection.

2. Trip Current

Itrip represents the minimum current specified to cause the PPTC to transition toward its high-resistance state under defined test conditions.

Temperature also affects trip behavior.

At elevated temperatures, the PPTC requires less additional heating to reach its switching condition. Consequently, a PPTC may trip at a lower current than an engineer might expect from its room-temperature characteristics.

The area between Ihold and Itrip should also be treated carefully. The exact behavior in this region is not simply an on/off threshold and can depend on initial resistance, temperature, mounting conditions and other factors.

For this reason, engineers should not design normal continuous operation close to the Itrip region.

Understanding the PPTC Thermal Derating Curve

A PPTC thermal derating curve normally shows the relationship between:

Ambient or operating temperature

Allowable current

Percentage of nominal Ihold

The horizontal axis normally represents temperature, while the vertical axis represents the percentage of rated current.

A typical conceptual curve may look like this:

These values are only illustrative.The actual derating values must always be taken from the specific PPTC manufacturer's datasheet.

For example, one published PPTC series shows an Ihold derating of 100% at 25°C, decreasing to approximately 85% at 40°C, 76% at 50°C, 67% at 60°C and 47% at 85°C. This demonstrates why the same nominal PPTC rating cannot be treated as constant across the entire operating-temperature range.

PPTC Derating Calculation Example

Consider a circuit with the following requirements:

Normal operating current:1.0 A

Maximum ambient temperature:60°C

Required operating margin: application dependent

PPTC reference Ihold: specified at 25°C

Assume, for illustration only, that the selected PPTC's thermal derating curve indicates that its allowable current at 60°C is approximately 67% of the nominal 25°C Ihold.

The required nominal Ihold can then be estimated as:

Required Ihold ≥ Iload ÷ Derating Factor

Therefore:

Required Ihold ≥ 1.0 A ÷ 0.67

Required Ihold ≥ 1.49 A

In this example, selecting a device with an Ihold of only 1.1 A or 1.2 A at 25°C would not provide sufficient thermal margin at 60°C.

A device with a nominal Ihold around 1.5 A or higher may be a starting point for evaluation, subject to the device's Itrip, Vmax, Imax, resistance, time-to-trip and actual application testing.

Important:The 67% factor in this example is illustrative. Engineers must use the actual derating curve for the specific Ruilin PPTC part number.

Ambient Temperature Is Not the Only Temperature to Consider

One of the most important points in PPTC thermal design is that ambient temperature is not necessarily the same as the temperature experienced by the PPTC itself.

For example, a system may have:

Ambient temperature = 50°C

But the PPTC may be located next to:

A power MOSFET

DC/DC converter

Power resistor

Battery cell

Processor

Motor driver

High-current PCB trace

Transformer

Local heat from these components can raise the PPTC's actual operating temperature.

Therefore, engineers should consider:

T_PPTC ≈ Ambient Temperature + Local Temperature Rise

The actual relationship depends on PCB construction, airflow, copper area, enclosure design, component placement and thermal coupling.

This is particularly important for compact SMD PPTCs.

PCB Layout Can Affect PPTC Derating

Two identical PPTC components can show different thermal behavior when mounted on different PCBs.

Factors include:

Copper Area

A larger copper area can improve heat spreading and influence the thermal equilibrium of the PPTC.

PCB Thickness

PCB thickness and copper structure affect heat dissipation.

Airflow

Forced airflow can change the thermal environment around the component.

Component Placement

Placing a PPTC close to a hot component can increase its actual temperature.

Enclosure

A sealed enclosure may have significantly less heat dissipation than an open-air test environment.

Soldering and Mounting Conditions

The actual mounting configuration can affect thermal performance compared with laboratory test conditions.

For these reasons, the PPTC datasheet should be treated as the starting point for design rather than a substitute for application-level validation.

PPTC Derating vs. Ihold Selection

PPTC selection should not be based on Ihold alone.

A practical selection process should consider at least:

Eaton's recommended selection process similarly includes determining circuit current, voltage, interrupt current and maximum ambient temperature, followed by thermal derating, rating comparison, time-to-trip evaluation and operating-temperature verification.

How Much Safety Margin Should a PPTC Have?

There is no universal percentage that should be applied to every PPTC design.

A suitable margin depends on:

Normal current tolerance

Input voltage variation

Startup current

Inrush current

Ambient temperature

PCB temperature rise

Component tolerance

Aging

Fault current

Required protection level

Time-to-trip requirements

For example, simply choosing a PPTC with a very large Ihold may reduce nuisance tripping, but it can also move the protection point too far away from the normal operating current.

Therefore, the design objective is not:

"Choose the largest PPTC possible."

Instead, the objective is:

"Choose a PPTC that remains stable during all normal operating conditions while providing appropriate protection during abnormal current conditions."

PPTC Derating at High Temperature

High-temperature environments deserve special attention.

Typical applications include:

Automotive Electronics

Automotive systems can experience elevated ambient temperatures and significant local PCB heating.

A PPTC designed for a 25°C laboratory environment may not provide the same current capability inside an engine-compartment or other high-temperature environment.

Battery Protection

Battery packs can experience temperature changes during charging, discharging and fault conditions.

The PPTC must be evaluated at the actual operating temperature rather than only at room temperature.

Industrial Equipment

Industrial control systems may operate inside cabinets where ambient temperatures are substantially higher than the surrounding room.

Power Supplies

Power converters and regulators generate heat that can raise the local PCB temperature around the PPTC.

Consumer Electronics

Compact enclosures have limited thermal dissipation, making local temperature rise important.

PPTC Derating at Low Temperature

Thermal derating is not only a high-temperature issue.

At temperatures below the reference temperature, a PPTC may have a higher current-carrying capability than its nominal room-temperature value.

However, designers should still use the manufacturer's specified thermal derating curve.

Low-temperature operation may also affect:

Resistance

Heating behavior

Time-to-trip

Reset characteristics

Mechanical and environmental reliability

Therefore, a wide-temperature application should be evaluated across the complete specified temperature range.

PPTC Derating and Time-to-Trip

Thermal derating should also be considered together with the time-to-trip curve.

A PPTC does not necessarily switch instantaneously when current exceeds Ihold.

The time required to reach the high-resistance state depends on factors including:

Overcurrent magnitude

Ambient temperature

Initial device resistance

Device construction

PCB mounting conditions

Heat dissipation

At higher ambient temperatures, less additional heating may be required to reach the PPTC's switching condition.

Therefore:

Temperature + Current + Thermal Environment → PPTC Trip Behavior

This is why an engineer should review both the thermal derating curve and time-to-trip curve during final selection.

Common PPTC Derating Mistakes

Mistake 1: Using the 25°C Ihold Directly

The most common error is treating the nominal Ihold as a temperature-independent rating.

Better approach:Use the thermal derating curve at the maximum actual operating temperature.

Mistake 2: Considering Only Ambient Temperature

An enclosure may have an ambient temperature of 50°C while the PPTC itself operates at a higher temperature due to nearby heat sources.

Better approach:Evaluate the actual PPTC operating environment.

Mistake 3: Choosing a PPTC Only by Ihold

A higher Ihold does not automatically mean better protection.

Better approach:Evaluate Ihold, Itrip, Vmax, Imax, resistance and time-to-trip together.

Mistake 4: Ignoring Startup Current

Some circuits have a short-duration inrush current that is substantially higher than the normal operating current.

Better approach:Check the startup waveform against the PPTC time-to-trip characteristics.

Mistake 5: Ignoring PCB Layout

The same component can behave differently depending on its PCB environment.

Better approach:Validate the selected PPTC on the actual PCB.

Practical PPTC Derating Design Procedure

Ruilin Semiconductor recommends using the following engineering process when evaluating PPTC protection:

Step 1: Determine Normal Operating Current

Identify the maximum continuous load current under normal operating conditions.

Step 2: Determine Maximum Operating Temperature

Consider:

Ambient temperature

Enclosure temperature

PCB temperature

Nearby heat sources

Self-heating

Step 3: Check the PPTC Thermal Derating Curve

Determine the allowable current at the actual operating temperature.

Step 4: Select the Nominal Ihold

Calculate the required nominal Ihold based on the derating factor.

Step 5: Check Itrip

Confirm that the trip current provides appropriate protection without causing unwanted nuisance trips.

Step 6: Check Vmax

Ensure that the maximum working voltage of the PPTC is suitable for the protected circuit.

Step 7: Check Imax

Verify that the device can withstand the available fault current.

Step 8: Check Time-to-Trip

Compare the PPTC time-to-trip characteristics with the fault tolerance of the protected circuit.

Step 9: Check Resistance

Evaluate normal-state resistance and resulting voltage drop and power dissipation.

Step 10: Validate on the Actual PCB

Perform testing under the application's maximum temperature, maximum load and representative fault conditions.

PPTC Derating Formula

A simplified approach to thermal derating can be expressed as:

Required Nominal Ihold ≥ Maximum Operating Current ÷ Derating Factor

For example:

Maximum Operating Current = 1.2 A

Derating Factor at Maximum Temperature = 0.75

Then:

Required Nominal Ihold ≥ 1.2 ÷ 0.75 = 1.60 A

The engineer would then select a suitable PPTC with a nominal Ihold above the calculated requirement and verify the complete datasheet specifications.

This formula is useful as an initial screening method, but it should not replace the manufacturer's actual thermal derating curve and application testing.

PPTC Derating Guide for Engineers

When selecting a PPTC resettable fuse, the following checklist can be used:

Circuit requirements

Maximum normal current

Minimum and maximum operating voltage

Maximum fault current

Startup and inrush current

Required protection response

Thermal requirements

Minimum ambient temperature

Maximum ambient temperature

PPTC local temperature

PCB temperature rise

Nearby heat sources

Airflow and enclosure conditions

PPTC parameters

Ihold

Itrip

Vmax

Imax

Time-to-trip

Initial resistance

Operating temperature range

Thermal derating curve

Validation

Normal operation test

High-temperature test

Low-temperature test

Startup/inrush test

Overcurrent test

Fault test

Reset test

Repeated trip-cycle test

PPTC Derating vs. Traditional Fuse Selection

A conventional fuse and a PPTC resettable fuse both provide overcurrent protection, but their thermal behavior and reset mechanisms are different.

A traditional fuse normally opens the circuit when its fusible element melts. A PPTC instead increases its resistance dramatically as it heats and enters a high-resistance state.

This thermal operating principle makes temperature an especially important factor in PPTC selection.

For a PPTC:

Current generates heat → Temperature rises → Resistance increases → Current is limited

Consequently, the surrounding thermal environment is an integral part of the protection design.

FAQs

What is PPTC derating?

PPTC derating is the reduction in allowable continuous current as the operating temperature changes, particularly as temperature rises. Engineers use the manufacturer's thermal derating curve to determine the appropriate current capability at the actual operating temperature.

Why does PPTC Ihold decrease with temperature?

A PPTC is a thermally sensitive polymeric device. As its temperature increases, less additional heating is required to reach the high-resistance switching condition. Therefore, the allowable continuous current generally decreases at higher temperatures.

Does PPTC Ihold change with temperature?

Yes. Ihold is specified under defined test conditions and is not a temperature-independent value. The actual allowable current must be determined from the PPTC's thermal derating curve.

Can I use the 25°C Ihold value for a 70°C application?

Not without checking the thermal derating curve. The 25°C Ihold should not automatically be treated as the allowable continuous current at 70°C.

What is the difference between PPTC derating and PPTC Ihold?

Ihold is the specified hold-current rating under defined conditions. Derating describes how the usable current capability changes with temperature.

Does PPTC Itrip also change with temperature?

Yes. PPTC trip behavior is temperature dependent. Higher temperature can cause the device to enter its high-resistance state at a lower current than under cooler conditions.

How do I calculate the required PPTC Ihold?

A simplified calculation is:

Required Ihold ≥ Maximum Operating Current ÷ Thermal Derating Factor

The final component selection should then be checked against Itrip, Vmax, Imax, resistance, time-to-trip and the manufacturer's datasheet.

Does PCB layout affect PPTC derating?

Yes. PCB copper area, thermal spreading, airflow, component placement and nearby heat sources can all affect the actual temperature of the PPTC and therefore its current-carrying capability.

Should I choose a higher Ihold to avoid nuisance tripping?

A higher Ihold can provide additional operating margin, but selecting an unnecessarily high rating may reduce the effectiveness of overcurrent protection. The correct selection must balance normal operating current, temperature, Itrip, fault current and time-to-trip.

Is PPTC derating important for automotive applications?

Yes. Automotive electronics can experience wide temperature ranges and significant local heat sources. Automotive PPTC selection should therefore consider the actual operating temperature and the appropriate thermal derating characteristics. High-temperature PPTC series are available for applications requiring extended temperature capability; for example, some commercial automotive PPTC families are specified for operation up to +125°C.

What information should I provide to a PPTC manufacturer for selection?

For a technical recommendation, provide:

Normal operating current

Maximum operating current

Operating voltage

Maximum ambient temperature

Minimum ambient temperature

Startup/inrush current

Available fault current

PCB mounting information

Application type

Required protection behavior

This information allows the manufacturer to evaluate the PPTC more accurately than using nominal current alone.

About Ruilin Semiconductor

Ruilin Semiconductor (Shenzhen) Co., Ltd.is a manufacturer focused on the research, development, production and supply of PPTC resettable fuse and polymeric positive temperature coefficient protection devices.

Ruilin Semiconductor provides PPTC solutions for electronic circuit overcurrent protection, with product options covering different package sizes, current ratings, voltage ratings and application requirements.

The company's PPTC product portfolio includes solutions for applications such as:

Consumer electronics

Battery protection

USB and USB Type-C interfaces

Automotive electronics

Industrial equipment

Power supplies

Telecommunications

Smart devices

Electronic control systems

As a PPTC manufacturer, Ruilin Semiconductor focuses not only on component specifications but also on practical application requirements such as Ihold, Itrip, thermal derating, resistance, time-to-trip and operating temperature.

For high-temperature or temperature-variable applications, Ruilin Semiconductor can assist engineers in evaluating PPTC selection based on actual circuit current, voltage, temperature and protection requirements.

When selecting a PPTC fuse, engineers should always refer to the applicable Ruilin Semiconductor datasheet and thermal derating curve for the specific part number rather than applying a generic derating factor across different PPTC series.

Conclusion

PPTC derating is an essential part of reliable resettable fuse selection.

The most important principle is:

Do not select a PPTC based only on its nominal 25°C Ihold rating.

Instead, evaluate:

Maximum operating current + actual PPTC temperature + thermal derating + Itrip + Vmax + Imax + time-to-trip + resistance

A well-designed PPTC protection circuit should remain stable during the entire normal operating range while still providing effective protection during abnormal current conditions.

For engineers designing products for high-temperature, automotive, battery, industrial or compact electronic applications, the manufacturer's PPTC thermal derating curve should be considered an essential part of the component-selection process.

Ruilin Semiconductor can provide PPTC resettable fuse solutions based on specific application requirements, including operating current, voltage, temperature range, package size and protection characteristics.

For PPTC selection support, contact Ruilin Semiconductor with your circuit operating conditions and application requirements.