EMI simulation design of switching power supply
With the increase of switching frequency and power density, the electromagnetic environment inside the switching power supply is becoming more and more complicated, and its electromagnetic compatibility has become a major focus and a major difficulty in power supply design. In the conventional design method, the EMC problem is dealt with by empirical design, and the EMC problem can only be considered finally after the prototype is established. The traditional EMC remedy can only add additional components, which may affect the original control loop bandwidth, resulting in the worst case of redesigning the whole system and increasing the design cost. In order to avoid this situation, it is necessary to consider EMC's problems in the design process, analyze and predict the EMI of switching power supply with certain accuracy, and improve the design according to the mechanism of interference and its distribution in each frequency band to reduce the EMI level, thus reducing the design cost.
2 switching power supply EMI characteristics and classification
In order to predict the conducted electromagnetic interference of switching power supply, it is necessary to make clear its generation mechanism and the characteristics of noise sources. Because of the high-speed switching action of the power switch tube, its voltage and current change rate are very high, and the rising edge and falling edge contain rich higher harmonics, so the electromagnetic interference intensity is large; The electromagnetic interference of switching power supply is mainly concentrated in the vicinity of diodes, power switching devices, radiators and high-frequency transformers connected with them; Because the switching frequency of the switching tube ranges from tens of kHz to several MHz, the interference forms of the switching power supply are mainly conducted interference and near-field interference. Among them, conducted interference will be injected into the power grid through the noise propagation path and interfere with other devices connected to the power grid.
Conducted interference of switching power supply can be divided into two categories.
1) Differential Mode (DM) interference. DM noise is mainly caused by di/dt. Through parasitic inductance and resistance, it propagates in the loop between live wire and neutral wire, generating current Idm between the two wires, which does not form a loop with the ground wire.
2) common mode (CM) interference. CM noise is mainly caused by dv/dt. The stray capacitance of PCB propagates in the loop between two power lines and ground, and the interference intrudes between the line and ground. The interference current flows in half on each of the two lines, with ground as the common loop. In the actual circuit, due to the unbalanced line impedance, the common-mode signal interference will be transformed into crosstalk interference that is not easy to eliminate.
Simulation analysis of EMI in switching power supply
Theoretically speaking, whether it is time domain simulation or frequency domain simulation, as long as a reasonable analysis model is established, the simulation results can correctly reflect the EMI quantization degree of the system.
Time-domain simulation method needs to establish a circuit model including all component parameters in the converter, use PSPICE or Saber software for simulation analysis, and use the fast Fourier analysis tool to get the spectrum waveform of EMI. This method has been verified in the analysis of DM noise. However, the nonlinear characteristics and stray parameters of semiconductor devices such as MOSFET and IGBT in switching power supply make the model very complicated, and the circuit topology of switching power supply changes constantly when it works, which leads to the problem of non-convergence in simulation. When studying CM noise, all parasitic element parameters must be included. Due to the influence of parasitic parameters, the FFT results are difficult to match the experimental results. Switching power converters usually work in a large range of time constants, mainly including three groups of time constants: time constants related to the basic frequency of the output terminal (tens of ms); Time constant (tens of μ s) related to the switching frequency of switching elements; Time constant (several ns) related to the rising time and falling time when the switching element is turned on or off.
For this reason, in time domain simulation, a very small calculation step must be used, and it takes a long time to complete the calculation; In addition, the results obtained by time domain method often can not clearly analyze the influence of various variables in the circuit on interference, can not deeply explain the EMI behavior of switching power supply, and lack the judgment of EMI mechanism, and can not give a clear solution to reduce EMI.
