[Technical Talk] Fuse Selection Guide (Final Chapter)
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Technical Talk
Techtalk
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Let's continue with the last topic Let's go !
4. Overcurrent protection of fuse (i.e. time-current curve)
The overcurrent protection capability of the fuse is mainly based on the following time-current curve, namely TC curve
The X-axis represents the current; the Y-axis represents the fusing time
Assuming the fault current is 100A, find 100A on the X-axis and draw a line perpendicular to the Y-axis. There will be three intersections with the three TC curves. Moving the intersections horizontally to the Y-axis, we can get three times. These three times are the corresponding melting times of the three fuses under the same 100A condition.
(1) 606040 (40A fuse) -> Fuse time: 4.5s
(2) 606050 (50A fuse) -> Fuse time: 25s
(3) 606063 (63A fuse) -> Fuse time: 150s
It can be seen that the larger the rated current of the fuse, the longer the blowing time under the condition of the same current. During the design stage, engineers can use the TC curve to roughly estimate the blowing time of the fuse under different fault currents; they can also compare the blowing time of two different fuses under the same fault current. These can help them to make better designs and selections. Of course, one thing to note is that the TC curve provided by the manufacturer is generally the average data at room temperature (25°C), so in actual use, some derating needs to be considered according to different environments. (In addition, there is a blue 40A line in the above figure, which has no intersection with the three TC curves, which means that under the condition of 40A, these three fuses will not blow within 10000s)
Finally, let's talk a little more about the fast-breaking and slow-breaking issues of fuses.
First of all, only the IEC 60127 standard has clear definitions of fuses: very fast breaking, fast breaking, slow breaking, and very slow breaking. See the following table:
That is to say, the fuse needs to be tested according to this standard to distinguish between fast breaking and slow breaking. But this standard does not apply to all fuses , or many fuses are outside the scope of this standard, so it is impossible to clearly define whether the fuse is fast or slow breaking. Therefore, now you will see that more and more fuses will not define fast breaking or slow breaking in the datasheet , but instead give you the I^2t parameter and TC curve , allowing you to evaluate the fuse time yourself to see if it can meet the design requirements. This approach is actually closer to practical applications and more reasonable.
5. The maximum allowable fault current of the fuse (i.e. the breaking capacity of the fuse)
Interrupting Rating = IR
Breaking Capacity = BC
Both of the above phrases may appear in the fuse datasheet and have the same meaning, indicating the breaking capacity of the fuse.
The selection criteria is that the maximum fault current (or short-circuit current) in the circuit must be less than the breaking capacity of the fuse. This is a safety test used to indicate the maximum current that the fuse can safely break at a certain voltage. Safety means no fire, no smoke, and no explosion.
A few points to note:
(1) The breaking capacity will change with the change of voltage. The breaking capacity given in the datasheet is generally at the maximum operating voltage of the fuse. If the breaking capacity at other voltages is required, it needs to be re-evaluated. Generally speaking, for the same fuse, the voltage and breaking capacity are inversely proportional, that is, the higher the voltage, the smaller the breaking capacity; the lower the voltage, the greater the breaking capacity.
(2) There is a big difference in breaking capacity between AC and DC voltages. Generally speaking, the breaking capacity of AC is much greater than that of DC. The reason is that AC voltage has a zero point, which helps to extinguish the arc when the fuse blows, while DC voltage does not. Refer to the figure below:
IR =50kA@500VAC vs IR = 20kA@500VDC
( 3 ) Generally speaking, the size of a fuse is proportional to its breaking capacity, i.e. the larger the size, the greater the breaking capacity; the smaller the size, the smaller the breaking capacity. (See the figure below)
6. Impact of pulse current on fuse selection
First of all, not all applications will have pulses, so if not, then don’t consider this. Usually, the start-up of the switching power supply, lightning surge testing, motor start and stop, hot plug, etc. will have obvious pulses, so this must be considered when selecting the fuse.
The selection criteria is to calculate the pulse I^2t and then compare it with the I^2t in the fuse datasheet. Pulse waveforms are of various shapes and sizes, but they can basically be classified into the six waveforms A to F in the figure below. For the convenience of calculation, the figure below also gives the formula for calculating I^2t for different waveforms. The parameters you need to know are the peak current Ip and the pulse duration t.
Look at the actual example below, the pulse waveform is very close to "E", Ip=8.0A, t=0.004s
pulse I^2t =1/5 x 8^2 x 0.004 = 0.0512A^2s
Finished? Don't worry! It's not over yet! Every time the fuse is subjected to a pulse, it will produce loss. This is determined by physical properties and there is no way to change it. So we also need to consider the number of pulses. The following figure shows the relationship between the number of pulses and pulse I^2t/fuse I^2t.
Here we consider the most stringent number of 100,000 pulses,
fuse fuseI^2t=pulseI^2t/0.22=0.0512/0.22=0.2327A^2s
Therefore, we need to select a fuse with I^2t > 0.2327A^2s to avoid it being blown by the energy generated by the pulse. Of course, considering factors such as product tolerance and ambient temperature, more margins are often considered in actual selection. For example, in this example, if an additional 50% margin is considered, then I^2t > 0.2327/0.5 = 0.4654A^2s
Let's compare the datasheet parameters of the following two fuses: fuse A and fuse B.
FuseA:32VDC/2.5A/IR=50A, I^2t=0.1560A^2s< 0.4654A^2s
FuseB:32VDC/2.5A/IR=50A, I^2t=0.65A^2s>0.4654A^2s
As you can see, except for I^2t, all other parameters of the two are the same. In this example, fuse B is obviously a better choice because its I^2t is greater than the 0.4654A^2s we just calculated.
7. Physical dimensions of the fuse
If fuses are classified according to physical size, there are many types. Here are two common examples to start your discussion!
(1) Cartridge fuse
Named by diameter x length (mm), 2AG, 3AG, 5AG are the more common sizes.
(2) SMT fuse
0402, 0603, 1206, 2410 These are the same packages as chip resistors and capacitors. Generally, the current is small (<10A) as shown below:
10.1x3.12x3.12(mm),12.1x4.5x4.5(mm),10x10x6(mm) These are special large-size SMT fuses , generally with larger current ( >10A ), as shown below:
8. Safety certification of fuse
The main fuse safety certifications in the world are UL 248 and IEC/EN 60127. UL 248 is the standard for North America, mainly used in the United States and Canada; IEC/EN 60127 is the standard of the International Electrotechnical Commission, mainly used in Europe. Most of the standards of other countries and regions will continue to use or refer to these two standards.
UL sets its own certification standards and also issues its own certificates.
IEC/EN only formulates certification standards and does not issue certificates itself. Instead, certification bodies in various European countries issue their corresponding certificates, such as the more common TUV and VDE.
The following two pictures show the standards, safety regulations and symbols of different regions and countries around the world for your reference. (Pictures from Littelfuse)
9. Fuse installation method
Fuses of different sizes and shapes have different installation methods. Here are a few typical examples!
Cartridge tube: Need to be used with base, clip, box and other accessories
Axial lead:
The lead needs to be shaped before use
THT through-hole soldering: PCB through-hole soldering
Bolt down: Need to use screws and base for installation and fixing
SMT patch: PCB surface mount welding
10. Fuse accessories (base, rack, box, etc.)
The main purpose of fuse accessories is to facilitate the installation and replacement of fuses, and they can also fix and protect fuses. The following figure gives three common examples:
Holder: The fuse is installed inside the holder, which is a fully enclosed form. It isolates the influence of the external environment on the fuse and can be dustproof and waterproof. It is generally used in some large equipment and cabinets.
Block: Blocks can be combined together, so they are more suitable for applications that require current distribution.
Clip: Small and cheap, just solder the through-hole on the PCB and put in the fuse, which is also easy to replace.
OK, I have finally finished writing it. Thank you for reading to the end. Thank you !
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