Positive temperature coefficient thermal materials have been widely used in many fields such as overheat protection, overcurrent protection, temperature detection, temperature compensation, degaussing circuits, motor starters, and constant temperature heaters. The positive temperature coefficient thermistor (FTCR) is the main branch of PTC material application. According to materials, PTCR can be divided into ceramic PTCR and organic PTCR. Organic PTCR has the advantages of low price, low resistance at room temperature, and ceramic PTCR.
PTCR has excellent over-current protection characteristics and has been widely used in the communications industry. Since the communication signal is a high-frequency signal, to understand the influence of PTCR on a communication line, it is necessary to analyze its high-frequency characteristics. At present, no relevant reports have been found.
This article analyzes and compares these two types of thermistors from the perspective of frequency characteristics, and correlates the high frequency characteristics of the two. In order to reduce the adverse effect of other factors on the test results, the wire uses a microwave transmission line with a characteristic impedance of 50n and a 50n matching resistor.
With the resistance reflection coefficient Ir = 0, where the standing wave ratio P = reflection coefficient 1r = Sf, Zl is the load impedance, and the measurement point; Z0 is the characteristic impedance of the transmission line, and the characteristics of the microwave transmission line.
If PTCR is suitable for over-current protection of high-frequency circuits, it should meet: at room temperature, standing wave ratio P =1, reflection coefficient KZ) = 0. Exceptional current When the temperature of PTCR rises, VSWR p = Reflex The coefficient r(z)=1. Network analyzer test line Microwave transmission line The common state is as follows: When the micro-wavelength line terminal is short-circuited, open circuit or pure reactance load, the incident wave of the terminal is totally reflected, and the standing wave ratio is p = m, reflection coefficient Ir 2) traveling wave state When the micro wavelength line is semi-infinite or when the load impedance is equal to the characteristic impedance of the long line, the power of the incident wave is fully absorbed by the load, that is, the load matches the long line, and the standing wave ratio p = On the left side of the two curves (portion of the standing wave ratio P>50) is a very steep curve (not shown due to the limit of the frame). A number of tests were performed and the results were very reproducible.
From the test results, it can be found that the organic PTCR has a larger VS ratio (P > 4) over a wide frequency range (0 to 1.5 GHz) when the abnormal current passes, while the ceramic PTCR is in a narrow frequency range ( There is a large standing wave ratio (P > 4) within 0~1.5MHz. It can be found that the organic PTCR may be more suitable for over-current protection at high frequencies, while the applicable frequency range of ceramics is narrower. The end result of the ceramic PTCR has a good agreement with the conclusion that the ceramic PTC element no longer has a positive temperature coefficient at 1 MHz.
3 Application example Communication protection circuit is generally composed of over-current protection device (organic PTCR) and overvoltage protection device (as shown) In the field of communications, the useful signal frequency can reach 1.8GHz, and even higher frequency. From the foregoing analysis, it can be concluded that organic PTCR still has a good PTC effect on high-frequency signals, especially for high-frequency impact energy below 1 GHz.
Typical Protection Circuit Diagram When the protected circuit is subjected to large voltage and large current, the overvoltage protection device can suppress the large voltage, and the generated large current is grounded through the overvoltage protection device to absorb the impact energy. At the same time, the organic PTCR action is triggered so that the current passing through is reduced and the partial pressure on the organic PTCR is increased, so that the energy is introduced as far as the external release of the high voltage side, thereby effectively protecting the overvoltage protection device and the internal device.
4 Conclusion The PTC thermistor has been widely used in overcurrent protection, overheat protection, constant temperature heaters, etc. Especially in the rapid development of the communications industry today, its overcurrent protection performance has been adopted by many designers to study its high frequency The characteristics are more realistic.
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