Switch mode power supply electromagnetic interference analysis
1 Internal sources of interference
● Switching circuit
The switching circuit is mainly composed of a switching tube and a high-frequency transformer. The switching tube and its heat sink have distributed capacitance with the lead wires inside the case and power supply, and the du/dt it generates has a large amplitude pulse with wide bandwidth and rich in harmonics. Switching tube load for high-frequency transformer primary coil, is an inductive load. When the original conduction of the switching tube off, the leakage inductance of the high-frequency transformer generates a counterpotential E = -Ldi/dt, the value of which is proportional to the rate of change of the collector current, proportional to the leakage inductance, iterated on the shutdown voltage, the formation of the shutdown voltage spike, thus forming a conduction interference.
● Rectifier diode of rectifier circuit
The output rectifier diode cuts off with a reverse current whose recovery time to zero is related to factors such as junction capacitance. It produces a large current variation di/dt under the influence of transformer leakage inductance and other distribution parameters, generating strong high-frequency interference with frequencies up to tens of megahertz.
● Spurious parameters
As a result of operating at higher frequencies, the characteristics of the low-frequency components in the switching power supply change, resulting in noise. At high frequencies, the spurious parameters have a great influence on the characteristics of the coupling channel, and the distribution capacitance becomes the channel for electromagnetic interference.
2 External sources of interference
External sources of interference can be divided into power supply interference and lightning interference, and power supply interference to "common mode" and "differential mode" way. At the same time, due to the AC power grid directly connected to the rectifier bridge and filter circuit, in half a cycle, only the peak time of the input voltage input current, resulting in a very low input power factor of the power supply (about 0.6). Moreover, this current contains a large number of current harmonic components, which can "pollute" the grid with harmonics.
