Behind The Mystery of Electromagnetic Compatibility Design 22
Otherwise, this might cause uncontrolled turn-on/off of the MOSFET
and thermal issues. Some SiC MOSFETs are designed with an
additional low-impedance Kelvin source connection (Figure 4) for
a gate current return path. This connection does not carry the high
switching current and has a lower interference potential than the
source connection, which significantly improves gate driving (such as
Infineon IMZ120R045M1 1200V/52A),
Regarding the auxiliary supply, it should be compact with its output
capacitors (with minimal ESL and ESR) placed very close to the gate
driver and SiC/GaN device to minimize the gate current loop and
associated parasitic effects.
Isolated Gate Driver Power-Supply Requirement
A large selection of compact, isolated 1 – 2W DC/DC converters
are available on the market. For a SiC-MOSFET such as the Infineon
IMZ120R045M1 1200V/52A, up to 1W power requirement per
device can be estimated (see example calculation (1)). However, an
application with over 5kW load power would require the use of either
a SiC-MOSFET module (ROHM BSM600D12P3G001 1200V/600A)
or alternatively several discrete SiC-MOSFETs in parallel (current
sharing). In a module solution, several semiconductor dies are
paralleled to form the final SiC-MOSFET. This technique reduces
the effective R
DS(ON)
but results in a very high Total Gate Charge,
which places a higher power requirement on the gate-driver system
power supply (example calculation (2)). Above 2W of power, a limited
number of off-the-shelf isolated DC-DC converter modules are
available. Despite their convenience, the modules are often large,
weighty, expensive, and with efficiencies lining under 79 percent.
P
Gate
Total power required to drive the SiC device gate
P
Driver
Power loss in the gate driver section (approx. 0.3W)
Q
Gate
Total Gate Charge value (from datasheet)
F
SW
Maximum switching frequency
∆V
Gate
Maximum voltage swing at the gate from –V
ee
to +V
dd
(e.g. -4V to +15V = 19V)
Example calculation (1) with Infineon IMZ120R045M1 1200V / 52A:
Example calculation (2) with ROHM BSM600D12P3G001 1200V /
600A:
Figure 4: Kelvin connections and critical parasitic inductances in a half-bridge
configuration. (Source: Würth Elektronik Group)
