Research On Switching Oscillation Suppression For SiC MOSFET By Inductively Coupled Damping

The wide bandgap semiconductor devices represented by silicon carbide（SiC）MOSFET has been revolutionizing the power electronics technology,bringing about high-efficiency and high-power-density power converters.Compared with conventional Si devices,SiC MOSFET can achieve higher operating voltage and lower on-state resistance,faster switching speed and higher operating junction temperature as alternatives to Si IGBT,thus having been widely applied to electric traction,photovoltaic power,smart grid and so on.However,a big challenge of severe switching oscillation exists for their efficient and reliable applications,which is mainly excited by ultrafast switching and caused by low on-state loss,inevitably leading to the severe electromagnetic interference,additional oscillation losses,and system breakdown by wrongly triggered switching.Thus,it presents a serious challenge to adopting SiC MOSFET into the mainstream power electronic applications.To take full advantage of the extremely fast switching and low loss of SiC MOSFET,the mechanism of switching oscillations should be well investigated and advanced techniques are expected to suppress the troublesome switching oscillations.In this paper,a novel inductively coupled damping is proposed to suppress switching oscillation for SiC MOSFET.Existing analytic design methods of inductively coupled damping have the following deficiencies:Generally speaking,prior-art analytical design techniques mainly rely on scrutinizing the peak characteristic impedance and analyse the relationship between the peak characteristic impedance and the secondary resistance R₂.It is proposed that the larger the peak characteristic impedance,the better the suppression effect.Thus,an empirical range for the damping circuit components R₂ is provided.However,it cannot quantitatively analyse the influence of the damping circuit components on the the oscillation suppression performance.Furthermore,they are discussed under a specific coupling by only tuning a single snubber parameter,i.e.the secondary resistance R₂.The design of the coupling inductors and the secondary capacitors that have a great impact on the oscillation suppression performance is not involved and therefore,only limited suppression effect is obtained.In view of the above problems,based on different theoretical analysis,this paper proposes more accurate and comprehensive inductive coupling damping optimization design methods,which can not only reflect the evolution trend of switching oscillation frequency and attenuation with the loss elements,but also quantify the effect of inductive coupling damping on switching oscillation,thereby determining the snubber region with excellent damping performance.This paper is organized as follows:（1）Employing SiC MOSFET,we firstly demonstrate the transient parity-time（PT）symmetry in electronics,which can be used for switching oscillation suppression in switch-mode electronics.First,a switch-mode electronic converter is constructed by taking advantage of the low loss characteristics of SiC devices.According to the switching states of electronic devices,a pair of different equivalent RLC oscillators at both turn-on and turn-off oscillations can be derived after a series of circuit simplification and transformation.Then,with the phasor method and Laplace transformation,we derive the hidden PT-symmetric Hamiltonian in the switching oscillation,which are characterized by free oscillation modes.Next,the experimental circuit diagram of the switch-mode circuit at the primary side with a wirelessly coupled PCB circuit at the secondary side is constructed.And the design scheme of PCB coils is proposed with the help of ANASYS Q3D finite element software.Both spectral and dynamic properties of the PT electronic structure demonstrate frequency splitting,loss rate splitting and the phase transition with eigenmode orthogonality.Importantly,the observed transient PT symmetry enables exceptional-point-induced optimal switching oscillation suppression,which shows the significance of PT symmetry in electronic systems with temporary responses.（2）To effectively suppress the switching oscillation,the effect of inductive coupling damping on the switching ringing and switching loss of SiC MOSFET under strong coupling regime is studied in detail.By viewing from the terminals of both upper and lower SiC MOSFETs in a half-leg circuit,two-port networks are linearized from switching oscillation and developed to deduce the exact characteristic equations.Since the damping ratio of all poles reflects the damping characteristic,an optimized design method of inductively coupled damping circuit is developed by the way of actively assigning poles.The root locus method is used to discuss the influence of snubber parameters on the trajectory of the poles.Then,the optimal snubber configuration along with two key parameters,i.e.,the inductance factor A_Land the secondary resistance R₂,are directly and precisely determined via the root locus method.It is revealed that the damping effect of RL in the strong coupling region is even better than that of the secondary RLC oscillator.Additionally,ferrite cores of nickel-zinc material are adopted to enhance the inductive coupling with a smaller-volume and lighter-weight coil.The experiments under different operating conditions reveal that applying inductively coupled RL damping circuit can not only remarkably dampen the switching oscillations and relieve EMI,but also greatly lower switching losses of SiC MOSFET.（3）Taking the inductively-coupled RLC damping circuit as the analysis object,the effect of all key snubber elements of RLC damping on the switching oscillation is thoroughly discussed to completely eliminate switching oscillation.After equivalent circuit transformations and simplifications,two-port networks including RLC damping is established,and the fourth-order characteristic equation is deduced.Then,the root locus method is used to quantitatively analyze the influence of the inductance factor A_L,the secondary capacitance C₂,and the secondary resistance R₂ on the motion trajectories of poles.Taking the damping ratio of 1 as the design standard for selecting the snubber parameters,A_L-C₂-R₂ regions of the RLC snubber where oscillations can be completely eliminated are determined.Finally,standard half-bridge double-pulse experiments are conducted to verify the the effectiveness and correctness of the design method,which show the designed RLC damping can completely eliminate the switching oscillation and have better damping performance than the conventional RLC resonator.