Comparison of commutation transients of inverters with silicon carbide JFETs with and without

body diodes

Björn Ållebrand and Hans-Peter NeeElectrical Machines and Power Electronics

Department of Electrical EngineeringKTH

Background I

• SiC has approximately ten times higher critical electric field compared to Si.

• This makes SiC attractive for majority carrier (uni-polar) devices.

• For bi-polar devices the large band gap will lead to high losses.

• This means that a SiC IGBT will only be interesting for the highest voltage levels.

Background II

• There is problems with manufacturing SiC MOSFETs.

• Therefore the interest has shifted to SiC JFETs.

• There are different ways of designing SiC JFETs.

• Buried gate JFETs and Vertical JFETs.

Different SiC JFET designs

Bg-JFET VJFET

Inverter topology

Commutation Procedure I

Commutation Procedure II

Simulations

• Simulations show that it is not much difference between inverters using the different SiC JFETs.

• The switching losses will be slightly larger for inverters with SiC JFETs without body diodes.

Short-circuit current

Large gate-drain capacitances

Reducing short-circuit currents

• Reducing the gate-drain capacitance (redesign of the component).

• Increasing the gate voltage to a higher value (may lead to that the component must be redesigned).

New Simulations

• In simulations with the gate-drain capacitance lowered by a factor of two, the switching losses were reduced.

Conclusions I• Using an inverter with only SiC JFETs is possible. • Using different SiC JFETs will not affect

performance that much.• A drawback is that short-circuit currents will

occur and this increases the switching losses.• The short-circuit currents can be reduced by

different means.

Conclusions II

• The gate-drain capacitance has to be reduced.

• Or a higher gate voltage needs to be used.

Future Work

• Investigate how the short-circuit current will be for larger devices.

• How will the stray inductance affect this short-circuit current.