Research On Gate Driver With Protection Of IGBT And The Series Connection

With the development of power electronic devices,voltage-source converters based on insulated gate bipolar transistors(IGBTs)have been widely adopted in flexible DC power transmission systems.As the vital power semiconductor device,IGBT must be in safe and reliable operation,which is highly relevant to the normal operation of the system,therefore,the gate driver design of IGBT is very important.In addition,the voltage rating of a single IGBT can not meet the requirement of high voltage,which makes the series connection of IGBTs very valuable.Firstly,the switching characteristics of IGBT is analyzed in this paper,and the current overshoot during the turning-on process and voltage overshoot during the turning-off process are focused on.Compared with the traditional gate driver,the active gate driver can effectively suppress the current and voltage overshoots,but its fixed overshoot suppression ability will cause extra switching loss unnecessarily when the load current or DC bus voltage is low.To further optimize IGBT switching performance,a self-adaptive active gate driver(SAGD)is proposed,which is particularly suitable for applications where the load current varies over time or the DC bus voltage is changed.With neither A/D nor D/A converters applied,the proposed driver indirectly detects the load current and DC bus voltage as time quantities,based on status monitoring,then self-adaptively chooses the optimal gate resistance increment online during the specific voltage or current rising phase.In this way,the current or voltage with overshoot is suppressed to be inside the safe operation area of IGBT while the switching loss is minimized,thus a better tradeoff between overshoots and switching losses is achieved.Then its feasibility is verified by experiments.Furthermore,the voltage unbalancing of IGBTs in series is analyzed,and a digital active voltage balancing circuit is proposed.By comparing the gate-emitter voltage of IGBT with the preset threshold voltages,the statuses of discrete digital signals are obtained as the feedbacks,and then the gate loop is compensated properly and accurately by directly controlling the pulse width of the current source with the digital signal.Therefore,the purpose of voltage balancing is achieved.Since it adopts the plug-in structure,it can work with commercial IGBT gate driver directly.The additional current source inside only provides a small power to fine-tune gate loop,and the functions provided by the commercial gate driver are not affected;moreover,the control scheme is easy to realize in field programmable gate array(FPGA),and good robustness to the bus voltage variation is possessed.With the active voltage balancing circuit,the voltage unbalancing in the current switching cycle can be greatly reduced,and the voltage unbalancing can be basically eliminated after several switching cycles with the cycle-by-cycle control.Finally,the experimental results verify the effectiveness of the circuit.Thanks to the development of wide band gap devices,the new silicon carbide-based(Si C)IGBT has higher voltage rating and faster switching speed than the traditional silicon-based(Si)IGBT.Therefore,some research on silicon carbide IGBT is also made in this paper.Aiming at the series connection,a single voltage balancing gate driver combined with limiting snubber circuits is proposed in this paper.Since Si C IGBT and Si C metal-oxide-semiconductor field-effect transistor(MOSFET)have some features in common like switching speed,the design and verification are based on Si C MOSFETs.This gate driver only requires one standard driver circuit to drive two series connected Si C MOSFETs by adding simple coupling circuits,and limiting snubber circuits are applied for voltage balancing,and thus,low cost,simple structure and high reliability are acquired.In LTSpice simulation,the proposed gate driver shows good voltage balancing performance as power loop current increasing.Besides,the branch of series connected Si C MOSFETs is in reliable on or off state during steady process.Finally,the experimental results further verify the good performance of the proposed single voltage balancing gate driver.