The afternoon sun on Saturday passed through the window of the room, the light was reflected in the window of the snow, the warm golden yellow sun was comfortably sprinkled on the face, and spent an afternoon on the soft sofa. For a long time, it was not so comfortable, and suddenly the cell phone sounded in the distance. Still thinking about whether or not to pick up, I never thought that the caller ID is a foreign technical service manager. It is a matter of urgency. It is really bad to answer the call. Our products start up and cause other devices of the system to restart.
Zuo Siyou must think that the capacity of the fuseless switch in the system is too small. It is recommended to increase the capacity of the fuseless switch in the safety specification. The problem temporarily solves the inrush current of the product after entering the office the next day. Sure enough, as shown in Figure 1, the instantaneous inrush current is 44 amps. Although there is no hard international specification for inrush current, it is necessary to study the design and site considerations and restrictions of some specific industry applications. To all readers.
Figure 1: Displaying an instantaneous inrush current of 44 amps
What is inrush current?
The inrush current is a spike of current generated when the power is turned on. Figure 2 is a typical DC to DC power system.
The EMI filter input line contains some capacitors. Similarly, the DC-DC converter also contains capacitors at the input and output terminals. The load terminals may contain other additional capacitors and stray capacitance. The instantaneous to steady-state voltage that the current charges its capacitor when the power is turned on is called the inrush current. When the front end of the capacitor is not connected to any damping, the AC or DC grid instantaneously causes the capacitor to resemble a short circuit. Of course, the instantaneous surge current also causes the life of the capacitor to be degraded.
Figure 2: Typical DC to DC power system
The general power supply is the first to protect its protection device. The current protection component, commonly known as the fuse, can be subdivided into one or more fuses (One TIme or Resettable fuse). Its working principle is to use the Joule law of current:
Figure 3: Joule's law
The current is converted into heat, which provides the energy of the metal layer in the fuse to open the circuit, thus protecting the safety of the back-end components. The bottleneck of the low-current fuse is how to overcome the inrush current. To avoid permanent damage to the fuse caused by the surge current during startup, it is recommended to use appropriate circuitry to reduce the inrush current.
For example, the power supply of a PC is assumed to have a power of 300 watts and an efficiency of 90% if there is no component design that limits the inrush current. Then the path of the input current is the power supply 110V recharged to the capacitor through the rectifier diode, according to the power supply 110V when the input voltage is at the peak for a short time.
Simply use Om's law for I=V/R.
V peak=110*root number 2=110*1.414=155.5V.
The R value is the total network impedance, including the dynamic equivalent impedance of the rectifier diode and the equivalent impedance (ESR) of the capacitor.
Assuming that the dynamic equivalent impedance of the rectifier diode is approximately 0.1 ohms, the equivalent impedance (ESR) of the capacitor is approximately 2 ohms.
Then I=V/R=155.5/(2+0.1)=74A whose instantaneous inrush current is 74A, assuming t=0.05sec Inrush current =74A. The fuse you use must meet the following Joule formula:
The answer obtained by substituting the Joule formula is equivalent to 0.037 degrees, and the instantaneous power consumption at startup is about 0.037 degrees, which is equivalent to the continuous use of 12 seconds of power, which naturally reduces the life of the product.
The calculated value of Joule can be found in the table as shown in Table 1. In order to meet the inrush current, the value is much higher than the nominal current, which may cause abnormal power supply. Because the selected fuse capacity exceeds the input mean current average and cannot be cut-off in real time, causing permanent damage to the back-end components.
Figure 4: General fuse capacity? Selection table (Note 1), Note 1: This table is taken from the littlefuse product website (http://)
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