Texas Instruments | Understanding the Short Circuit Protection for Silicon Carbide MOSFETs (Rev. A) | Application notes | Texas Instruments Understanding the Short Circuit Protection for Silicon Carbide MOSFETs (Rev. A) Application notes

Texas Instruments Understanding the Short Circuit Protection for Silicon Carbide MOSFETs (Rev. A) Application notes
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Understanding the short circuit protection for silicon
carbide MOSFETs
Silicon Carbide (SiC) MOSFET has become the potential substitute of Silicon (Si) IGBT for various
applications such as solar inverter, on board and off board battery charger, traction inverter, etc.
Comparing with Si IGBT, SiC MOSFET has more stringent short circuit protection requirements. To make
the most use of SiC MOSFET and ensures a robust system operation, a fast and reliable short circuit
protection circuit is needed. Different characteristics of SiC MOSFET and Si IGBT will be discussed. Three
short circuit protection methods will be illustrated and compared. The requirements of short circuit
protection for SiC MOSFETs will be summarized. TI’s UCC217xx family, a single channel isolated gate
driver for IGBT and SiC with advanced protection feature, can be used in various system designs to
protect the switch from all types of overcurrent and short circuit faults. The best in-class fast protection
and high noise immunity improves the versatility of system design and robustness of the system.
higher VDS. The drain current keeps increasing along
with the increasing Vds. The device will be destroyed
1
Characteristics of SiC MOSFET
before reaching the transition point. These
and Si IGBT
characteristics make the short circuit protection for SiC
The key to make the best utilize of the SiC MOSFET is
MOSFETs very different than IGBT.
to fully understand the device characteristics. The
different characteristics of SiC MOSFET and Si IGBT
2
Short Circuit Protection Methods
have impact on their short circuit protection schemes.
Comparison
Comparing with IGBT which has similar blocking
voltage and current rating, SiC MOSFET has smaller
chip area, which makes the parasitic capacitance
smaller than IGBT and increases the intrinsic switching
speed. However, the smaller chip area means the SiC
MOSFET die has lower thermal dissipation capability.
During short circuit conditions, the surge current
generates a significant amount of joule heating and the
die can be destroyed in a short period of time without
enough capability to dissipate the heat. With smaller
die size, the surge current capability of SiC MOSFET
is lower than that of IGBT.
The output characteristics of SiC MOSFET and IGBT
are different too. IGBT typically works in the saturation
region during the normal ON state. When a short
circuit happens, the collector current IC increases and
goes through a sharp transition from the saturation
region to the active region. The collector current gets
self-limited and becomes independent of VCE.
Consequently, the increase in IGBT current and hence
power dissipation gets self-limited.
On the other hand, SiC MOSFET works in the linear
region during normal ON operation. During a short
circuit event, the SiC MOSFET enters the saturation
region. Different than that of an IGBT, SiC MOSFET
has a larger linear region. The transition from linear
region to saturation region happens at significantly
SLUA863A – January 2018 – Revised March 2019
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The short circuit protection is important to
ensure a robust system and best utilize the
device. A qualified short circuit protection circuit
should realize a fast detection and shut down
the device without false trigger. Three short
circuit protection schemes which are commonly
used today will be analyzed and compared,
including desaturation detection, shunt resistor
sensing scheme and senseFET current sensing
scheme. The desaturation detection circuit is
shown in Figure 1.
VDD
DESAT
Fault
RBLK
DHV
+
CBLK
+
VDESAT
t
Figure 1. Desaturation Detection Circuit
The circuit is consisted by a resistor, a blanking
capacitor and a diode. When the device turns
on, a current source charges the blanking
capacitor and the diode is conducted. During
normal operation, the capacitor voltage is
Understanding the short circuit protection for silicon carbide MOSFETs
Copyright © 2018–2019, Texas Instruments Incorporated
1
Summary
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clamped at the forward voltage of the device.
When short circuit happens, the capacitor
voltage is quickly charged to the threshold
voltage which triggers the device shutdown. The
capacitor charging time is called the blanking
time, which is calculated as:
C
t BCAP
BLK
The senseFET current sensing scheme is
shown in Figure 3. The senseFET is normally
integrated in the power module, connecting in
parallel with the main device to scale down the
device current. The scaled down current is then
measured by an accurate shunt resistor. The
detection time is short as the sensed current is
synchronous with the device current. As the
senseFET is integrated in the power module,
low noise will be generated due to the small
parasitic inductance. Although this scheme has
many advantages, a power module with
senseFET is required, which increase the
system cost.
u V DESAT
I CHG
(1)
For IGBT, the desaturation threshold voltage is
normally set around the transition voltage, as
the current can be virtually limited afterward for
IGBT to withstand longer period of time. It
needs more attention to design desaturation
circuit for SiC MOSFET. The blanking time
designed for IGBT is too long to protect SiC
MOSFET. On one hand, the transition voltage
of SiC MOSFET is normally very high so the
current cannot be limited. With the preferred
short circuit shutdown time less than 2 µs, the
desaturation threshold voltage needs to be set
lower. On the other hand, fast switching speed
of SiC MOSFET generates noise during turn on
transition. The short circuit detection time
should be designed long enough to avoid the
false trigger, which makes the desaturation
circuit design challenging for SiC MOSFET.
The shunt resistor sensing scheme is shown in
Figure 2. A small resistor is connected in series
in the power loop to sense the current. This
scheme is straight forward and can be flexibly
adopted in any system. High precision resistor
and fast ADC are needed to guarantee the
accuracy of the signal and detection time. The
drawback of this method lies in the power loss.
In high power system, high current generates
large power loss on the shunt resistor. In low
power system, larger resistance is needed to
guarantee the accuracy of the sensing signal,
which also generates loss and reduces
efficiency in low power applications.
+
OC Fault
+
t
VOCTH
3
+
+
t
CFLT
RS
Figure 3. SenseFET Current Sensing Scheme
OC Fault
VOCTH
CFLT
RS
Summary
SiC MOSFET is a promising substitute for IGBT
to achieve a more compact and efficient
system. A short circuit scheme for SiC
MOSFET should be evaluated from the
following aspects: fast response time, low
power loss, high accuracy, high noise immunity
and low cost. Efforts should be made from the
protection circuits, gate driver and PCB layout
to improve the overall performance. UCC217XX
family has the best in-class overcurrent and
short circuit protection feature. With the short
detection time and fault reporting time, the gate
driver can shut down the IGBT and SiC
MOSFET module promptly after fault happens,
and report the fault to the isolated input side.
The UCC217XX family supports all three
detection schemes above, which makes the
driver versatile to various system designs. The
driver provides a reliable protection for
overcurrent and short circuit fault, and increases
the robustness of the system.
Figure 2. Shunt Resistor Current Sensing Scheme
2
Understanding the short circuit protection for silicon carbide MOSFETs
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Revision History
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Revision History
NOTE: Page numbers for previous revisions may differ from page numbers in the current version.
Changes from Original (January 2019) to A Revision .................................................................................................... Page
•
Added additional detail to the abstract and summary regarding the UCC217xx family. ......................................... 1
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