Control For Voltage-Sharing Of Series-Connected IGBTs

The direct series connection of multiple IGBT is one of the effective approaches to realize high voltage power conversion,and the dynamic voltage sharing between series-connected IGBT is the key to its safe and reliable operation.An analytical model is proposed to describe the FS-IGBT turn-off transients,which is simplified from the IGBT physical model and extensive experiments observations.The turn-off transient process of IGBT can generally be divided into five stages,of which stage 3 and 4 are most complicated and time consuming in calculation,due to its intrinsically complicated physics process during transients.The equivalent circuit is presented for each stage,and the key features of the model are:in stage 3 where the C-E voltage building up fast,the gate voltage is equivalent to a linearly decreasing voltage.In stage 4 where the drain current decreasing fast,the device is equivalent to a sub-circuit of a current source,which representing the total base charge and base carrier lifetime,and a variable capacitor,which representing the displacement current and the redistribution current,in parallel.As a result,the calculation burden on the proposed model is significantly reduced by using the explicit closed-form expressions of IGBT terminal voltage and current in each turn-off stage instead of solving the complicated differential equations in the physical model.The comparisons between simulation and experimental results verify that the proposed analytical model matches the experimental one pretty well.The proposed Simplified Analytical Model offers an alternative solution for incorporating the IGBT model into the circuitry level simulation with less computation time and good trade-off accuracy,in the view point of power circuit applications.A novel parameter extraction method,combining the turning-on and-off transient test of the terminal voltage and current of the device and the physical analysis on the semi-conductive process,is proposed,with the consideration of the simplification and the accuracy.The method is based upon the inherent relationship between the terminal data of the device and the semiconductor characteristics of the model,built from the operational principle analysis of the IGBT.A control for dynamic voltage sharing of series-connected IGBTs during turning-off is proposed,which is responsive to series IGBT characteristics and adjusts the delay time(or lead time)of the IGBT switches in real-time.The time adjustment is calculated by the two-section type regulation algorithm,which consists of the square proportion algorithm to realize regulation rapidity with larger voltage un-balance degree and the proportional algorithm to achieve voltage stability with smaller voltage un-balance degree.The time adjustment of each IGBT is introduced into its next IGBT time adjustment to further optimize the regulation speed.It is revealed that the existing turn-off process voltage sharing methods are not suitable for the turn-off process of series-connected IGBTs.An active clamp circuit for the turn-on process voltage sharing is proposed,which detects the C-E voltage of the leading IGBT,generates gate pumping current,slows down the turn-on speed,and thus reduces the voltage difference between the IGBTs.A turn-on edge adjustment algorithm of the driving signal is proposed,which further reduces the voltage spike and the turn-on loss,optimize the regulation speed.A time detection circuit is used to detect the pulse signal indicating the working time of the active clamp circuit.The adjustment direction and adjustment amount of each IGBT s turn-on edge is determined according to the feature that the rising edge of the pulse signal coincides with the corresponding IGBT turn-on edge delay.Finally,experimental results verify the feasibility and effectiveness of the modeling and voltage shareing methods proposed.