Talking about the selection of fuse from a practical application case
July 30, 2021
As an overcurrent protection device, fuses have been widely used in various electrical products. However, many domestic design and development personnel lack sufficient knowledge of fuse devices, and their selection is relatively random. Usually, they simply consider the operating voltage/operating current parameters of the device. Because the development cycle of domestic projects is usually tight, engineers rarely pay attention to fuse devices, and their protection functions and effects have not been fully verified, causing customer complaints after the end products are put on the market.
Common fuse selection has the following types of problems:
If the current specification of the fuse is too high, the fuse cannot be blown in time under the over-current situation, and the fuse will not be able to protect it and cause danger;
If the current specification of the fuse is too low, it will fuse under normal working conditions, causing the product to fail to work normally;
The wrong fuse type is selected, and its impact resistance is not enough. Under the impact conditions of normal application, the fuse will blow out and the product will not work;
Choose low-end or counterfeit brand fuses, the performance of the device does not meet its own specified parameters, and cannot achieve the due protection function.
Here is an actual case, combined with the characteristics of TE/FUSE parameters to talk about how to choose a fuse that meets the requirements of the actual application:
Collect the fuse requirements of the end product design/application:
Product working temperature range: -40°C to +85°C
Normal working voltage range: 6V to 18V
Normal working stable working current = 0.36 A / Normal working peak maximum current = 1.68A
Impulse resistance test requirements:
Pulse impact test requirements for normal switching:
Imax = 12.94A/duration time 200us/switching times greater than 120,000 times;
Start pulse shock test under 24V condition:
Imax = 27.56A/continuous 200us/impact at least 5 times;
Pulse 1 pulse shock Imax=7A/continuous 10ms/500 shocks;
Pulse 2a pulse shock Imax=23.38A/continuous 30us/500 shocks
Packaging requirements: 1206/SMD
Step 1: Determine the steady-state rated current of the fuse Ifuse
Consider the most extreme use cases:
Ifuse=Max Isys / (standard steady-state reduction application temperature reduction) = 1.68A/(0.750.85)=2.64A
According to TE/1206/Fuse current specification table/Figure 1, the closest specification greater than 2.64A is 3A / specification 1206SFS300F/32 (32Vdc meets application voltage requirements).
Preliminarily select the I²t value of Fuse through the conversion of the actual application pulse I²t value comparison step 1. It is necessary to meet the former and the latter for the appropriate selection.
1) The impulse impact test requirements of normal switching:
Imax=12.94A/duration 200us/switching times greater than 120,000 times;
Based on the above data, it can be estimated that I²t=0.5x (12.94)²x 200us = 0.0167A²s
Consider the application of pulse cycle (see Figure 3) / temperature (see Figure 2) / circuit difference reduction
(According to experience, take the original value x 0.7), after conversion I²t=0.0167/(0.390.850.7)=0.072A²s
Comparing and querying TE/1206/SFS fuse I²t curve Figure 4, it can be seen that the 1.25A specification corresponding to I²t=0.15 is greater than the application I²t requirement, so the higher current specification 3A can meet the application requirements.