Digital Active Common Mode Electro Magnetic Interference Suppression Technique For Power Converters

With the application of high-speed high-power switch devices, the dynamic performance of power electronic products has been improved and the volumes and weight have become small. However, severe electromagnetic interference (EMI) is produced due to high dv/dt and high di/dt caused by high speed switching of power devices. There are basically two types of EMI in power electronics, namely common mode EMI and differential mode EMI. Common mode EMI usually constitutes a major part in total EMI. Recently, many efficient CM EMI suppression methods have been presented. They can be categorized into three classes: modified PWM strategies, passive filters and active/passive compensatory filters. To address the problem existed in the reported common mode EMI suppression technique, a novel active common mode EMI suppression method with digital closed-loop control is proposed in this dissertation. The six main researches are listed as following:Firstly, common mode conducted EMI in single-phase full-bridge converter is analyzed in this dissertation, it discuss the noise source and the flowing path of common mode current. Appoint the shortcoming of common mode current equivalent circuit based on noise source in series, and propose a novel equivalent cirvuit of common mode EMI based on noise source in parallel. Conducted EMI in single-phase full-bridge is analysize and identified in time domain and frequency domain. Simulation and experimental results prove the validity of proposed common mode EMI equivalent circuit. It is conclusion the proposed equivalent model can be the same with superposition principle, can make clear the radiate mechanism of common mode current in multi-bridge converter. Moreover, the equivalent parasitic capacitance and the magnitude of noise source voltage in proposed equivalent circuit can be different. The poposed equivalent circuit based on noise source in parallel is the basis of the active common mode EMI suppression method and the design of compensating circuit.Secondly, compared the conducted common mode EMI in full-bridge circuit with bipolar SPWM and unipolar SPWM modulate technique. It is conclusion that the conducted common mode current with bipolar SPWM modulation technique is theoretically zero because the two legs provide perfect CM current compensation for each other. However, different transmission delays of two drive signals in bipolar SPWM modulation technique always produce large CM current in real case. Experimental results validate fine-tunning driving signals before they are transmitted can suppression the common mode EMI in full-bridge converter efficiently, then an idea of fine-tunning driving signals technique with close-loop control system is proposed.Aim at the conditions without two complementary common mode noise sources, an active compensatory common mode EMI suppression technique with an external compensating circuit is proposed. In active compensatory common mode EMI suppression, when the compensating current and noise source common mode current are the same maginidute and anti-phase, whether driving signals transimmission delay of compensating circuit and main bridge are the same or not will impact the effect of common mode current suppression. It is conclusion the problem of anti-phase alignment and different transmission delays of two drive signals in bipolar SPWM modulation technique are similar, whose resolving method is the same. The two key techniques in fine-tuning system of the drive signals are common mode noise evaluation index and close-loop control stratery.Thirdly, Due to the change direction of common mode current level is needed in digitalized close-loop control system, and not the detailed current spectrum. A common mode noise evaluation index of the'energy'of the common mode current is adopted here. The relationship between the common mode current energy and the delay time of the drive signals on the cross of two legs is studied. The result indicates that common mode current energy has several minimum values in the range of available time delay and the curve is symmetrical damp surge in'V'figure.Aim at the detecting technique of common mode current, two schemes are put forward, they are as follows: one is common mode current detecting system based on FPGA. FPGA collect the common mode current and figure out the energy specially. The main controller DSP performs the converter control and picks up the common mode current energy values from FPGA, which considered as the feedback of digitalized close-loop control system. The other detecting technique is common mode current is treated by full wave rectifier and filter circuit firstly, then figure out the energy. This scheme is proved feasibility and validity in theory. At the time, DSP not only perform the converter control but also collect common mode current and calculate its energy.Fourthly, there are more than one local minimums of the common mode current energy in the relationship between the energy of common mode current the delay time of drive signals. In most cases the delay time of driving signals is in the range of±T/2, in which T the oscillating period of common mode current. It is proposed a local searching algorithm to search the minimum energy of CM current in the range of±T/2 when the curve is symmetrical damp surge in'V'figure. At the same time it is explained the local searching algorithm is easy to get trapped at local minimums when there are more than one minimums energy of CM current in the curve. Considering of the fact that searching algorithm is preformed in DSP, this dissertation adopt simulated annealing algorithm to avoid getting trapped at local minimums. Application of simulated annealing algorithm (SAA) in find the global optimal energy value of common mode current is studied, and an improved SAA is proposed. In improved SAA, the ending criterion of annealing process and sampling process is modified in order to reserve the state of'Best So Far'and improve the searching efficient. Simulation results show that the improved SAA has been increased in searching speed and searching precision.Fifthly, the basic configuration of digitalized close-loop control common mode EMI suppression technique is illustrated. Two kind of push-pull compensating circuit where the DC biased voltage source is provided by DC bus voltage of converter or external electric supply is discussed. The calculating process of output capacitor and coupling capacitor in active compensating circuit by using the common mode current equivalent mode based on noise source in parrellel. Experimental results verified the dv/dt of output voltage and output capacitance in active compensating circuit affects the peal value and the osicillation frequency of compensating current.At last, an experimental bench of single-phase half-bridge converter consists of digitalized active EMI suppression system based on F2812. High frequency common mode current is treated by high precision full-wave rectifier and filter circuit. Driving signals of compensating circuit are pre-adjusted by close-loop control system based on improved SAA. Experimental results show that Closed-loop control system with improved SAA can regulate the delay time of compensating current and suppress the common mode current noise level effectively.