For the switching power supply, if the large capacitor of the energy storage filter after the rectifier bridge is an ideal capacitor, that is, the equivalent series resistance is zero (ignoring all parasitic parameters of the capacitor), all possible differential mode noise sources input to the power supply will be affected by this Capacitors are completely bypassed or decoupled, but the equivalent series resistance of bulk capacitors is not zero. Therefore, the equivalent series resistance of the input capacitor is the major part of the impedance Zdm seen from the differential mode noise generator. In addition to bearing the working current flowing in from the power line, the input capacitor also provides the high-frequency pulse current required by the switching tube, but in any case, the current flowing through the resistor will inevitably produce a voltage drop, such as the equivalent series resistance of the capacitor, so the input filter There will be high frequency voltage ripple at both ends of the capacitor, and the high frequency and high voltage ripple will come from the differential mode current. It is basically a voltage source (due to the equivalent series resistance). Theoretically, when the rectifier bridge is turned on, the high frequency ripple noise should only appear on the input side of the rectifier bridge. In fact, when the rectifier bridge is turned off, the noise will leak through the parasitic capacitance of the rectifier bridge diode.
There are many accidental paths for high frequency current to flow into the enclosure. When the drain of the main switch tube in the switching power supply jumps high and low, the current flows through the parasitic capacitance between the switch tube and the radiator (the radiator is connected to the casing or the radiator is the casing). When the AC grid current keeps the rectifier bridge on, the noise injected into the chassis encounters almost equal impedance and therefore flows equally into the neutral and live wires. So this is pure common mode noise.
There are many accidental paths for high frequency current to flow into the enclosure. When the drain of the main switch tube in the switching power supply jumps high and low, the current flows through the parasitic capacitance between the switch tube and the radiator (the radiator is connected to the casing or the radiator is the casing). When the AC grid current keeps the rectifier bridge on, the noise injected into the chassis encounters almost equal impedance and therefore flows equally into the neutral and live wires. So this is pure common mode noise.