description:
Figure 1 shows a simplified circuit of a typical electronics power supply section. C1 is a filter capacitor in parallel with the load. At the moment of power-on, the capacitor voltage cannot be abruptly changed, thus generating a large charging current. According to the first-order linear non-homogeneous equation established by the first-order circuit zero-state response model, the current value obtained by short-circuiting the filter capacitor can be obtained. This current is what we often call the input surge current. It is generated when the filter capacitor is initially charged. The size depends on the amplitude of the input voltage when the power is turned on and the bridge rectifier and electrolytic capacitor. The total resistance of the loop.
Assume that the input voltage V1 is 220Vac, and the internal resistance of the entire grid (including rectifier bridge and filter capacitor) Rs=1Ω. If the power input waveform is turned on when the input waveform reaches 90 degrees, the peak value of the surge current will reach I=220. ×1.414/1=311(A). Although this surge current is very short, if it is not suppressed, it will shorten the life of the input capacitor and rectifier bridge, and may also cause the input power supply voltage to decrease, so that other power equipment using the same input power supply will be powered down instantaneously. The normal operation of the device causes interference.
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