Essential Terminology for PPTC Selection
Key Electrical Parameters (Ih, Itrip, Vmax, Imax)
IH: Holding Current - The maximum current at which a PPTC will not operate in a static air environment at 25°C.
IT: Trigger Current - The minimum current required for a PPTC to operate in a static air environment at 25°C.
TTRIP: Trip Time - The maximum trip time at 5 times IH in a static air environment at 25°C.
Vmax: Maximum Operating Voltage - The maximum operating voltage of a PPTC.
Imax: Maximum Withstand Current of a PPTC.
Rmin: Minimum Resistance - The minimum zero-power resistance of a PPTC at an ambient temperature of 25°C.
Rmax: Maximum Resistance - The maximum zero-power resistance of a PPTC at an ambient temperature of 25°C.
The 7-Step PPTC Self-Resetting Fuse Selection Process
Step 1: Determine Circuit Electrical Parameters
Maximum normal operating voltage.
Maximum operating temperature and environmental conditions.
Maximum normal operating current.
Maximum circuit protection current.
Step 2: Determine Maximum Working Voltage（Vmax）
Determine the product mounting method: through-hole or surface mount.
Then find the corresponding voltage rating (Vmax) of the selected component. The theoretical voltage rating of the selected component should be greater than the operating voltage multiplied by 1.2.
For example: for a 12V circuit, the component's Vmax should be greater than or equal to 12V * 1.2. A voltage rating of 1.0 is also acceptable for circuit stability.
Determine the component series: Through-hole components: 6V, 16V, 30V, 60V, 72V, 250V, 600V; Surface mount components: SMD0603, SMD0805, SMD1206, SMD1812, SMD2012, SMD2920.
Step 3: Determine Operating Current based on Environment
Referring to the current and temperature reduction ratio comparison table in the technical data, select the operating current device that best matches the environmental conditions.
The current reduction under the environmental temperature conditions should be greater than or equal to the operating current value.
Step 4: Determine Maximum Interrupt Current（Imax）
The maximum operating voltage determined above, and the short-circuit current caused by a circuit short circuit or fault, should be less than the maximum current that the device can withstand; that is, IMAX is greater than the maximum short-circuit current.
Note: Theoretically, the short-circuit current is infinite, but this is not actually the case.
Step 5: Determine Trip Time（TTRIP）
Ensure the trip time is not too slow (to prevent circuit damage) or too fast (to prevent nuisance tripping).
Step 6: Verify Environmental Temperature Feasibility
To ensure that the device you choose can achieve the best performance, ensure that the minimum and maximum ambient temperatures of the application are within the operating temperature range of the resettable fuse. The operating temperature of the device is -40℃~85℃.
Step 7: Confirm Installation Size
Refer to the product series technical data to select the appropriate product for the PCBA and the space range of the product mounting enclosure.
PPTC Application Examples and Typical Circuits
Case Study: Automotive USB 2.0 Interface Protection
Power supply voltage requirement: 4.75-5.25V
Operating environment: -40°C-60°C
Typical current draw: 350mA. Mounted on the vehicle's cab console.
Because the car battery supplies 12V or 24V, and the regulated current provides a USB voltage of around 5V, considering a 1.2x multiplier, select components with a Vmax greater than 6V, namely: SMD0805 and SMD1206.
Based on the product derating ratio, a component with a derating current of 350mA at 60℃ should have an Ih greater than or equal to 500mA at 25℃.
If a derating current of 500mA is required at 60℃, then an Ih greater than or equal to 750mA at 25℃ is needed.
Considering the PCBA space provided by the automotive electronics products, select SMD1812-050 or SMD1206-050. To provide a 500mA load current, select SMD1812-075 or a component with a voltage of 6V or higher and an Ih greater than 750mA at 25℃.
Typical circuit USB peripheral
| USB power regulation (host, hub, remote charger)
|
Top 10 Frequently Asked Questions about Self-Resetting Fuses
Question 1: What is the working principle of a PPTC fuse?
Polymer resettable fuses consist of a polymer matrix and carbon black particles that make them conductive. Since polymer resettable fuses are conductors, current flows through them. When an overcurrent flows through the polymer resettable fuse, the generated heat (I² multiplied by R) causes it to expand. This causes the carbon black particles to separate, and the resistance of the polymer resettable fuse increases. This causes the polymer resettable fuse to heat up more quickly and expand even further, further increasing the resistance. When the temperature reaches 125°C, the resistance change is significant, resulting in a noticeable decrease in current. At this point, a small current flowing through the polymer resettable fuse is sufficient to maintain it at this temperature and in a high-resistance state. When the fault is cleared, the polymer resettable fuse shrinks back to its original shape, reconnecting the carbon black particles, thereby reducing the resistance to the level required for the specified holding current. This process can be repeated multiple times.
Question 2: What is the difference between Rmin, Rmax, and R1max?
Rmin is the minimum resistance specified for the polymer resettable fuse supplied by yint, which determines the minimum operating current of the polymer resettable fuse. Rmax is the maximum resistance specified for the polymer resettable fuse supplied by yint. R1max is the maximum resistance that the polymer resettable fuse should reach after tripping, and its value determines the maximum holding current of the polymer resettable fuse. After the polymer resettable fuse trips, the resistance of the resistor supplied by yint (greater than or equal to Rmin and less than or equal to Rmax) will rise to less than or equal to R1max.
Question 3: What is the voltage drop Vdop across a polymer resettable fuse?
This varies depending on the circuit. Generally, if the resistance and the equilibrium current are known, the voltage drop can be calculated. The maximum voltage drop of a polymer resettable fuse is calculated using the resistance value R1max; a typical voltage drop can be calculated using the resistance value Rmax or, if Rmax is not provided, the average of Rmin and R1max. If Ih is the normal operating current, and R is the resistance of the polymer resettable fuse (R1max, Rmax, or (Rmin + R1max)/2), then the voltage drop across the polymer resettable fuse in the circuit is: Vdrop = Ih x R
Question 4: How many times can a polymer resettable fuse operate under maximum voltage and inrush current?
Each polymer resettable fuse has a specific operating voltage and can withstand a specific inrush current. Safety regulations stipulate that polymer resettable fuses must still exhibit the PTC effect after 6,000 operations. For polymer resettable fuses used in communication equipment, specifications stipulate that after a dozen to hundreds of trips at maximum voltage, their various performance parameters should remain within their original range.
Hardware designers should recognize that polymer resettable fuses are for protection, not for treating their continuous tripping as normal operation.
Question 5: How long does it take for a polymer resettable fuse to recover after tripping?
The time it takes for a polymer resettable fuse to return to its low-resistance state after tripping is affected by the following factors: the type of polymer resettable fuse; how it is mounted or fixed; ambient temperature; the internal cause of the trip and its duration.
Generally speaking, although many reset within seconds, most polymer resettable fuses will reset within minutes. Different operating currents also have a significant impact; the higher the operating current, the longer the recovery time. Ambient temperature is also an important factor.
Question 6: How long can a PPTC remain in the tripped state?
Safety standards require the device to stay in the tripped state for 1000 hours at maximum voltage without losing its PTC characteristic.
Question 7: What is the maximum ambient temperature for operation?
For polymer resettable fuses in the active state, the range depends on the product type. For most of our products, this range is up to 85°C, and some can reach up to 125°C. Some polymer resettable fuses in the non-active state can withstand shorter reflow soldering temperatures, and both lead-free and leaded soldering are permitted.
Question 8: Can a PPTC switch between states?
Unless device protection is eliminated, the polymer resettable fuse will not transition between its normal and activated states. When a polymer resettable fuse operates, its resistance changes from low to high. In the high-resistance state, a trace amount of fault current still exists (from a microscopic or differential perspective, this occurs during the switching process, but this state is not detectable during current flow). This small fault current is sufficient to maintain the high-resistance state. Only when the fault is cleared can the polymer resettable fuse cool down and return to its low-resistance state.
Question 9: What is the difference between IH (Holding Current) and IT (Trip Current)?
IH is the highest current that a non-triggered resistor can pass through a circuit in still air (ranging from 20°C to 25°C depending on the product), i.e., the highest operating current at room temperature. IT is the minimum current that a polymer resettable fuse will trip in still air (ranging from 20°C to 25°C depending on the product), i.e., the minimum fault current at room temperature.
For most of our products, the ratio of IT to IH is 2:1, but for some products it may be as low as 1.7:1, and for others it may be as high as 3:1. Differences in materials and manufacturing methods, as well as changes in resistance after tripping, will determine this ratio.
Question 10: How does pressure affect a PPTC fuse?
Atmospheric pressure affects the electrical performance of polymer resettable fuses. If the pressure is too high during operation and restricts the expansion of the polymer resettable fuse, it will not activate as intended.
