byExiMAdmin/September 16, 2024/inEigensicherheit - Berechnungen, English
Method for Calculating the Pulse Load of Zener Diodes in Intrinsic Safety for Zener Barriers Using the Melting Integral and Cold Resistance and Their Issues
Introduction
In the development of intrinsically safe devices, the precise calculation of the pulse load on Zener diodes is essential, particularly for devices operating with mains voltage. The ability to correctly assess such pulse loads ensures that the Zener diodes operate within their safe operating limits, thus maintaining the integrity of the entire device. This blog post introduces a method for calculating the pulse load that takes into account the melting integral and cold resistance. Although this method doesn’t fully align with physical realities, it is a known practice applied by developers and manufacturers of intrinsically safe devices, as well as by certification bodies for ATEX or IECEx approval. There are several points of criticism regarding this method, which will be discussed in this article. Many thanks for the comments and suggestions on this topic. In case of doubt, it is advisable to conduct practical testing or seek further information from the manufacturer of the diodes and fuses.
The Importance of Pulse Load
Considering pulse load is of crucial importance, especially for devices powered by mains voltage. The Zener diodes in such barriers are often protected by a fuse. However, since fuses are relatively slow in response to transient events, the diodes can be exposed to pulse currents. If these loads are not accurately calculated and considered, it could lead to diode failure and, in the worst case, a safety hazard. Therefore, a precise and reliable method for calculating the pulse load is necessary to ensure the safety and reliability of the devices.
The Melting Integral
The melting integral, also known as the i²t value or limit load integral, is a measure of the short-term overload capability of electrical or electronic components under pulsed loads. It describes how a fuse reacts to such a load: Once the i²t value is reached, the fuse will melt and interrupt the circuit.
The i²t value specified in the datasheet serves for selecting the fuse and only indicates the energy at which the fuse wire starts to melt (melting i²t). After the wire has melted, there is a period during which an arc is sustained (arcing i²t). Only after this period is over will the fuse open. The total time, also known as the clearing i²t, is not provided in the datasheet.
Further information can be found in the “Fuseology Design Guide” by Littelfuse.
Cold Resistance
The cold resistance of a fuse is the ohmic resistance, usually specified at room temperature. Since the fuse wire is typically made of a metal that behaves as a positive temperature coefficient conductor, its resistance increases as it heats up. The cold resistance is the value when the fuse is not carrying current. In normal operation, the value is higher as the fuse heats up due to the current flow.
In the case of a short-duration voltage pulse, the ohmic resistance acts as a current limiter and can reduce the pulse current.
