Study Of EMC And Stability For AC-DC Switch Power Supply System

With high efficiency, small volume, low power consumption, reliable performance, high output voltage range and many other advantages, AC-DC switch power supply system has been widely applied in electronic equipments for household and industry. However, because of its work in the switch state with high voltage and large current, its Electromagnetic Compatibility (EMC) problems is obviously serious. Today the traditional design concept can not follow the development trend of electronic equipments, such as higher conversion efficiency, lower standby consumption, and lower Electromagnetic Interference (EMI). As a result, the EMC problem and related series of circuit stability problems have become more and more important in the design of AC-DC switch power supply systems.In this background, this paper focuses on typical AC-DC switch power supply system and the mechanism of its electromagnetic interference, then the bottom-up solutions for the EMC problems and stability problems in AC-DC switch power supply system design are presented. The main research contents include the following parts:1) To improve the frequency characteristics of the control IC in AC-DC converter, the improvements of two critical circuits—error amplifier and power mosfet driver are discussed. And a rail-to-rail Operational Transconductance Amplifier (OTA) with a source cross coupling input stage is proposed, based on a premise that output ability is improved, the static current and loop bandwidth can be effectively reduced. Thus high frequency interference signals can be filtered out effectively; Then the importance of balanced output for the power mosfet driver is researched, to achieve the noise elimination effectively, a very simple driver circuit with good balance on drive ability, drive delay and dead time is presented. According to the improvements of these two key circuits in control loop, the performance on electromagnetic compatibility of the internal control circuit will be better.2) The improvement methods of the control strategy for better stability are put forward. A compensation method for error correction of primary inductance is given, the output can be more stable, thus high frequency noise and the risk loop oscillation caused by inductance error can be avoided. Further, a simpler, more efficient Pulse Skipping Modulation (PSM) strategy, by replacing the error amplifier with an error comparator, and reducing output sampling time, it can weaken the probability of the coupled noise in the control loop. These methods can raise the entire loop EMC reliability based on the principle level of loop control strategy.3) For suppression of the substrate coupling noise, four methods such as physical isolation, layout optimization, routing optimization and decoupling capacitance are discussed. As an example, a layout distribution of a power factor correction IC is explained. This paper provides the basis of rational use of these methods in IC layout, and the chips can achieve expected design with good noise suppression.4) For noise suppression of board level in AC-DC switch power supply system, Electromagnetic-bandgap (EBG) structure is utilized to realize the suppression of EMI and high frequency switch noise. Existing EBG structure still have some shortcomings, so a kind of three-dimensional EBG structure with external L bridge and a two-dimensional EBG structure with a snake-like long bridge are proposed respectively. The two structures can effectively prevent the transmission of EMI and high frequency switch circuit noise in the Printed Circuit Board (PCB), key signaling pathways can be secured, and the circuit stability and EMC problem can be improved.The EMC solution for the AC-DC switch power supply system at all levels is systematically studied in this dissertation, the results of these work are concluded by strict simulations and experiments. Therefore, this work has valuable contribution for the actual AC-DC switch power supply system design.