Download Minimizing oscillations in the Diode

Comparison of commutation
transients of inverters with silicon
carbide JFETs with and without
body diodes
Björn Ållebrand and Hans-Peter Nee
Electrical Machines and Power Electronics
Department of Electrical Engineering
KTH
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.