USB Chargers Conduction and Radiation
At present, the electromagnetic compatibility of electronic products has been paid more and more attention, especially in the developed countries of the world, has formed a complete electromagnetic compatibility system, and our country is also establishing electromagnetic compatibility system, therefore, to achieve the product of electromagnetic compatibility is Access to international market. For the USB Chargers, due to the switch, rectifier work in high current, high voltage conditions, the outside will have a strong electromagnetic interference, so the USB Chargers conduction and electromagnetic radiation launch relative to other products More difficult to achieve electromagnetic compatibility, but if we understand the principle of electromagnetic interference on the switching power supply is clear, it is not difficult to find a suitable strategy to transmit the emission level and radiation emission level down to the appropriate level, to achieve electromagnetic compatibility design.
2 USB Chargers Conducted harassment
2.1 Generation of conducted emissions
The conduction of the USB Chargers is electromagnetic interference that propagates out of the power supply line of the power supply. In the switching power supply input power line out of the harassment, both differential mode harassment, and common mode harassment, common mode disturbance than the differential mode harassment produces stronger radiation harassment.
In the frequency range of 0.15MHz ~ 1MHz, harassment mainly exists in the form of common mode. In the frequency range of 1MHz ~ 10MHz, the form of harassment is the coexistence of differential mode and common mode. In the form of 10MHz or more, Mainly. The transmission of the differential mode is mainly due to the switch in the switch state, when the switch is turned on, the current flowing through the power line linear rise, the switch turns off when the current mutation to 0, so the current flowing through the power line Is a high-frequency triangular pulsating current, which is rich in high-frequency harmonic components. As the frequency increases, the amplitude of the harmonic component becomes smaller and smaller, so the differential mode disturbance decreases with the increase of frequency. As shown in Fig. 1, due to the presence of the capacitor C5, it forms a low-pass filter with the inductor L3, so that the differential mode conducted disturbances mainly exist in the low frequency section.
The main reason for the generation of common mode disturbances is that there is a distributed capacitance between the power supply and the earth (protection ground). The high frequency harmonic components of the square wave voltage in the circuit are transmitted to the earth through the distributed capacitance and form a loop with the power line to produce common mode disturbance The
As shown in Figure 1, L, N for the power input, C1, C2, C3, C4, C5, L1, L2 composition of the input EMI filter, DB1 rectifier bridge, L1, VD1, C6 and VT2 for the power factor correction main circuit , VT2 for the switch, switch D pole connected with the tube radiator, switch installed in the radiator, and the radiator to form a coupling capacitor, as shown in Figure 1 C7, switch VT2 work In the switch state, the D pole voltage is high frequency square wave, square wave frequency is the switching frequency of the switch, the square wave in the harmonics will be through the coupling capacitor, L, N power lines constitute a loop, resulting in a total Mode harassment. Power supply and the earth's distributed capacitance is scattered, it is difficult to estimate, but from the above Figure 1, the switch tube VT2 D pole and the heat sink between the role of coupling capacitor in the above Figure 1, from the rectifier bridge to the inductance L3 voltage between the 100Hz frequency waveform, and from the inductor L3 to the diode VD1 and switch VT2D pole between the voltage of the connection are square wave voltage, containing a large number of higher harmonics. Second, the impact of inductance L3 is relatively large, but L3 and the chassis of the distance is far, the distribution of capacitance than the switch tube and the heat sink between the coupling capacitance is much smaller, so we mainly consider the switch tube and the heat sink between the coupling capacitor.