source:Industry News Popular:adapter release time:2022-03-24 10:16:52 Article author:sznbone
Conducted interference mainly evaluates the interference noise flowing on the input and output lines. The test method is shown in Figure 1. The EUT of the equipment to be tested is connected to a clean AC power source through an impedance matching network LISN.
1) The role of LISN is as follows:
1. Isolate the EUT of the device under test and the AC input power supply, and filter out the noise and interference introduced by the input power line.
2. The interference noise generated by the EUT passes through the high-pass filter inside the LISN and the 50 Ω resistor in turn, and the corresponding signal value is obtained on the 50 Ω resistor and sent to the receiver for analysis.
(2) Analysis of test principle:
Conducted interference comes from differential mode current noise and common mode current noise, these two types of noise interference
1. The differential mode current flows in the opposite direction between the two input power lines, and the two form a current loop with each other, that is, one is the source line of the differential mode current, and the other is the return line of the differential mode current.
2. The common mode current flows in the same direction on the two input power lines, and they respectively form a current loop with the ground, that is, they simultaneously serve as the source line or return line of the common mode current.
What is differential mode current:
Differential mode current noise is mainly generated by high frequency switching currents of power switching devices.
1. Cause 1: The power device is turned on
There is a current spike at the moment when the power device is turned on
The turn-on current spike consists of three components:
(1) The charging current of the interlayer capacitance of the primary winding of the transformer.
(2) The discharge current of the MOSFET drain-source capacitance.
(3) The reverse recovery current of the output diode working in CCM mode.
The turn-on current spike cannot be bypassed by the input filtered DC electrolytic capacitor, because the input filtered DC electrolytic capacitor has an equivalent series inductance ESL and resistor ESR, which produces a differential mode current that flows between the two input lines of the power supply.
The differential mode current formed when the power device is turned on is IDM:
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