Passive Balancing of Switching Transients between Paralleled SiC MOSFETs

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2018-02-19
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Virginia Tech
Abstract

The SiC MOSFET has attracted interest due to its superior characteristics compared to its Si counterpart. Several SiC MOSFETs are usually paralleled to increase current capability, considering cost effectiveness and manufacturability. Current unbalance among the MOSFETs is a concern as it affects reliability. The two main causes are asymmetrical layout and parameter mismatch. The variation in parameters, unlike circuit or module layout, is unavoidable during production. Among all the parameters of MOSFET, the spreads in on-state resistance (Rds(on)) and threshold voltage (Vth) are the major concerns during paralleling. The disparity in Rds(on) causes static current unbalance which is self-limited due to the positive temperature coefficient of Rds(on). Its influence is not investigated here. The threshold voltage Vth has a negative temperature coefficient, forcing the MOSFET with lower Vth to carry more current during switching transient. Paralleled MOSFETs are usually de-rated to guarantee safe operation. Balancing of peak currents during switching transient isthe goal of this work.

Integration of current/voltage sensors into paralleled structure is difficult in real application. Complicated feedback loop design and separate gate drivers also need to be avoided in perspective of cost and volume. Passive balancing solutions are investigated in this dissertation. The inductors and resistors most effective in improving current sharing are identified by parametric analysis. Their current balancing mechanisms are analyzed in circuit point of view. The design guidelines involving the magnitude of Vth mismatch, current rise time, and unbalance percentage are derived for the selection of passive components. The theory upholds well when substantial parasitics from device package and layout exist.

Several passive balancing structures are analyzed and compared in terms of current balancing capability, voltage stress, total switching loss, and switching loss difference. All of them can provide much better current and power balancing without increasing switching loss. Some of the them may increase the stress-inducing inductance, which can be reduced by negative magnetic coupling. Perfect coupling between power-source inductors would enable current matching without penalty on voltage stress.

Common-source inductance (Lcm) is effective in dynamic balancing, but at the expense of higher switching loss. It is not considered in power module application because Kelvin connection is normally applied. However, wire bond inside the package of discrete MOSFETs and part of the external leads are inevitable and add to Lcm. Peak-current and switching energy mismatches vary with operating conditions (including input voltage, input current, and switching speed). Design guidelines and procedures that are valid for wide operating range are provided for cases with and without Lcm.

This dissertation also models the switching energy and switching energy mismatch of paralleled MOSFETs. The influence of operating conditions, passive balancing components, layout and package parasitic inductances, nonlinear channel performance, and voltage dependent parasitic capacitors are included in the modeling process. The resulting high order system is simplified by reducing the number of passive components and number of devices without losing accuracy. The influence of current balancing components and magnitude of threshold voltage mismatch on sharing are discussed based on modeling results.

In conclusion, this dissertation balances the transient currents between paralleled SiC MOSFETs automatically by inductance, resistance and magnetic coupling. This procedure is done utilizing one gate driver without current/voltage sensors and feedback loop. Those solutions work for both polarities of Vth mismatch and force balancing from the first current peak. Design guidelines involving the magnitude of Vth mismatch, current rise time, and maximum peak-current difference are derived to guide the choice of passive components. The detail design procedures are recommended to force currents to share over wide operating range. The aforementioned benefits are demonstrated by two paralleled SiC MOSFETs (C2M0160120D) tested at variant operating conditions. The difference of peak currents can be reduced below 5% of steady-state current in every switching transient. Switching energy mismatch percentage can be reduced by 6 times without increasing total switching energy.

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Keywords
paralleled SiC MOSFETs, dynamic balancing, passive compensation, magnetic coupling, Modeling
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