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F1206A0R50FWTR Datasheet(PDF) 9 Page - AVX Corporation |
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F1206A0R50FWTR Datasheet(HTML) 9 Page - AVX Corporation |
9 / 12 page Accu-Guard® SMD Thin-Film Fuse Correct choice of an Accu-Guard® fuse for a given applica- tion is fairly straightforward. The factor of pre-arc I2t, howev- er, requires clarification. The proper design for pre-arc I2t is presented by way of example. DESIGN PARAMETERS 1. Operating Temperature The Accu-Guard® is specified for operation in the tempera- ture range of -55°C to +125°C. Note, however, that fusing current is sensitive to temperature. This means that the fuse must be derated or uprated at circuit temperatures other than 25°C: 2. Circuit Voltage Maximum Voltage: Accu-Guard® is specified for circuits of up to rated voltage. Accu-Guard® will successfully break currents at higher voltages as well, but over voltage may crack the fuse body. Minimum Voltage: Accu-Guard® cannot be used in circuits with voltage of about 0.5V and less. The internal resistance of the fuse will limit the fault current to a value which will pre- vent reliable actuation of the fuse (<2 x rated current). 3. Maximum Fault Current Accu-Guard® is fully tested and specified for fault currents up to 50A. Accu-Guard® will successfully break currents above 50A, but such over current may crack the fuse body or damage the fuse terminations. 4. Steady-State Current The Accu-Guard® current rating is based on IEC Specifica- tion 127-3. In accordance with this international standard, Accu-Guard® is specified to operate at least 4 hours at rated current without fusing (25°C). Engineering tests have shown that F0805B and F1206A/B Accu-Guard® will in fact operate at least 20,000 hours at rated current without fusing (25°C). 5. Switch-on and Other Pulse Current Many circuits generate a large current pulse when initially connected to power. There are also circuits which are sub- ject to momentary current pulses due to external sources; telephone line cards which are subject to lightning-induced pulses are one example. These current pulses must be passed by the fuse without causing actuation. These puls- es may be so large that they are the determining factor for choosing the Accu-Guard® current rating; not necessarily steady state current. In order to design for current pulses, the concept of fuse pre-arc Joule integral, I2t, must be understood. Fuse current rating is defined by the requirement that 2 x I R will cause actuation in <5 seconds. This rating does not indicate how the fuse will react to very high currents of very short duration. Rather, the fusing characteristic at very high currents is specified by I2t-t curves (or I2t-I). I2t expresses the amount of energy required to actuate the fuse. Total I2t expresses the total energy which will be passed by the fuse until total cessation of current flow. Pre-arc I2t expresses that energy required to cause large irreversible damage to the fuse element (Total I2t = pre-arc I2t + arc I2t). If the Joule integral of the switch-on pulse is larger than the fuse pre-arc I2t, nuisance actuation will occur. In order to choose the proper Accu-Guard® current rating for a given application, it is necessary to calculate the I2t Joule integral of the circuit switch-on and other current pulses and compare them to the Accu-Guard® I2t-t curves. An Accu- Guard® fuse must be chosen such that the pulse I2t is no more than 50% of the pre-arc I2t of the prospective fuse. Pre-arc I2t of the Accu-Guard® fuses is well characterized; I2t-t and I2t-I graphs are in this catalog. The problem is cal- culating the I2t of the circuit current pulses. This concept is not familiar to most engineers. Correct calculation of pulse Joule integral and subsequent choice of Accu-Guard® current rating is illustrated by way of the attached examples. HOW TO CHOOSE THE CORRECT ACCU-GUARD® FUSE FOR CIRCUIT PROTECTION Environmental Accu-Guard® Temperature Current Carrying Capacity* F0402E, F0805B, F1206A, F0805B 2.50A F0603C F0612D F0603E F1206B & 3.00A -55°C to -11°C 1.07 x I R 1.07 x I R 1.07 x I R 1.07 x I R 1.07 x I R -10°C to 60°C I R I R I R I R I R 61°C to 100°C 0.85 x I R 0.93 x I R 0.90 x I R 0.90 x I R 0.80 x I R 101°C to 125°C 0.80 x I R 0.90 x I R 0.90 x I R 0.75 x I R 0.75 x I R *As a function of nominal rated current, I R. |
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