PolySwitch component protection features

I. Introduction:

The PolySwitch self-recovery fuse, developed by Raychem in the USA, is a modern component made from polymer-doped conductive materials. These materials exhibit a positive temperature coefficient (PTC) effect, meaning their resistance increases with temperature, especially near the melting point of the polymer. This thermal switching behavior allows the device to act as a protective element in electrical circuits.

When an overcurrent occurs, the PolySwitch rapidly increases its resistance, effectively limiting the current flow and acting like an open circuit to protect the system. Once the overcurrent condition is removed, the device cools down and returns to its normal state, allowing the circuit to function again. Due to this automatic recovery feature, PolySwitch components are widely used in both civil and industrial applications, offering advantages over traditional fuses, such as no need for replacement after each fault.

However, selecting the right PolySwitch component can be challenging. Conventional methods, such as using lookup tables, are often time-consuming and lack precision. This paper presents a more accurate approach by analyzing the action protection characteristic curves provided in the PolySwitch manual and establishing a mathematical model based on these characteristics. Experimental verification shows that the simulation results align well with the actual performance data, validating the model’s accuracy.

II. Main Content of the Program:

1. Analysis of PolySwitch action protection characteristics

2. Development of a mathematical model for protection behavior

3. Determination of critical points in the protection curve

4. Examination of various PolySwitch models and their application scenarios

5. Example: RXE series components at 20°C ambient temperature

Table 1 lists the electrical characteristics of the RXE series components at 20°C, including parameters such as maximum operating current (IH), minimum trip current (IT), maximum voltage (Vmax), and others. It shows that the maximum current these components can withstand is 40A, while the minimum current triggering protection is twice the maximum operating current.

Figure 1 illustrates the action protection characteristic curve of the RXE series at 20°C, with the x-axis representing fault current (in amps) and the y-axis showing the action time (in seconds). Both axes use logarithmic scales. Each curve corresponds to a specific model, forming a cluster of protection curves for the series. Table 2 provides a comparison of curve labels and corresponding component models.

III. Mathematical Model Development:

In the logarithmic coordinate system shown in Figure 1, the PolySwitch action protection curve consists of two parts: a curved section and a straight line. A separate mathematical model can be established for each part. Taking the RXE160 curve as an example, the process involves identifying the critical point where the curve transitions from a curved shape to a linear one.

IV. Critical Point Determination:

By measuring the curve in Figure 1, the critical point between the curved and straight sections can be identified. For the RXE160 model, this critical point is approximately at 4.3A. The straight-line portion extends up to the maximum current of 40A, which is listed in Table 1.

Assuming a logarithmic relationship between the action time (t) and the fault current (I), the following equation can be derived:

log(t) = a * log(I) + b

Using computer processing, the constants were determined as a = 813 and b = 22.54, resulting in the formula:

t = 813 * I^22.54

For the curved section, the PTC effect of the PolySwitch suggests using a power or logarithmic function. The general form of the equation is:

log(t) = t0 + b * log(I - I0)

Based on measurements, the constants were found to be t0 = -38.93, I0 = 4.42, and b = -1.83, leading to the final equation:

t = -38.93 + (-1.83) * log(I - 4.42)

V. Conclusion:

To validate the mathematical model, the RXE160 component was tested in a DC power supply protection circuit. The experimental data, shown in Table 3, was plotted alongside the simulation curve generated from the mathematical model. As illustrated in Figure 2, the simulated curve closely matches the shape of the original protection characteristic curve from the PolySwitch manual, confirming the model's accuracy.

This study demonstrates that the mathematical modeling approach significantly improves the selection process of PolySwitch components, offering greater precision compared to traditional lookup tables. The results confirm that the simulation curve aligns well with the measured data, making this method a valuable tool for engineers and designers.