Applications Information
The Cree SiC MOSFET has removed the upper voltage limit of silicon MOSFETs.
However, there are some differences in characteristics when compared to what is
usually expected with high voltage silicon MOSFETs. These differences need to be
carefully addressed to get maximum benefit from the SiC MOSFET. In general,
although the SiC MOSFET is a superior switch compared to its silicon counterparts,
it should not be considered as a direct drop-in replacement in existing applications.
There are two key characteristics that need to be kept in mind when applying the
SiC MOSFETs: modest transconductance requires that V
GS
needs to be 20 V to
optimize performance. This can be see in the Output and Transfer Characteristics
shown in Figures 1-3. The modest transconductance also affects the transition
where the device behaves as a voltage controlled resistance to where it behaves as
a voltage controlled current source as a funtion of V
DS
.
The result is that the
transition occurs over higher values of V
DS
than are usually experienced with Si
MOSFETs and IGBTs. This might affect the operation anti-desaturation circuits,
especially if the circuit takes advantage of the device entering the constant current
region at low values of forward voltage.
The modest transconductance needs to be carefully considered in the design of the
gate drive circuit. The first obvious requirement is that the gate be capable
of a >22 V (+20 V to -2V) swing. The recommended on state V
GS
is +20 V and the
recommended off state V
GS
is between -2 V to -5 V. Please carefully note that
although the gate voltage swing is higher than the typical silicon MOSFETs and
IGBTs, the total gate charge of the SiC MOSFET is considerably lower. In fact, the
product of gate voltage swing and gate charge for the SiC MOSFET is lower than
comparable silicon devices. The gate voltage must have a fast dV/dt to achieve
fast switching times which indicates that a very low impedance driver is necessary.
Lastly, the fidelity of the gate drive pulse must be carefully controlled. The nominal
2.5V
threshold voltage is 2.5V and the device is not fully on (dV
DS
/dt≈0)
until the V
GS
is
above 16V. This is a noticeably wider range than what is typically experienced with
silicon MOSFETs and IGBTs. The net result of this is that the SiC MOSFET has a
somewhat lower ‘noise margin’. Any excessive ringing that is present on the gate
drive signal could cause unintentional turn-on or partial turn-off of the device. The
gate resistance should be carefully selected to ensure that the gate drive pulse is
adequately dampened. To first order, the gate circuit can be approximated as a
simple series RLC circuit driven by a voltage pulse as shown below.
4
CMF20120D Rev. -
CREE [ CREE, INC ]
