IEEE Power Electronics Magazine - June 2021 - 35

Short-Circuit in a Motor Drive
Inverter and SiC MOSFETs
SC is an abnormal, and infrequent condition, encountered
especially in electric motor drive inverters, as illustrated in
Figure 1(a). Figure 1(b) shows the SC test circuit, and typical
gate voltage, drain voltage, drain current Id
and temperature
rise waveforms during the SC event are shown in Figure
1(c). In a SC event the transistor current rapidly rises to
5-7 times the nominal current condition, and simultaneously
the applied drain-source voltage equals the dc link
supply voltage VIN of several hundred volts. This situation
results in several tens of kW of instantaneous power that
the transistor must dissipate until the protection circuitry
turns off the transistor.
Actually measured SC test waveforms are shown in Figure
2. The graph shows (a) the gate voltage V ,gs (b) the drain
current I ,d and (c) the gate terminal current I .g The drain
current is rising to a peak value of approximately 300 A in
the first microsecond, and the device goes to thermal runaway
and failing (destroyed) at around ..
tt .1
is considered to cause the degradation
on-resistance (),RON
55 sn The gate current
Ig is observed to abnormally and steeply rising starting
from the instant = The large increase in the gate current
Ig from time t1
of the gate oxide film [5], [6], and it is expected to degrade
the electrical characteristics (,Vth
of the transistor. These phenomena can be observed on
almost all SiC MOSFETs (with some differences) and need
to be evaluated for each technology.
Additional SC test data is presented in Figure 3 along
with modeling and simulation results of the SC drain current
etc.)
and the transistor internal temperature Tj [2]. A Cauer
electro-thermal impedance model was used for simulation.
Using this model, and the measured SC current waveform,
the temperature rise is simulated and the critical temperature
can be extracted. In this case, the Tj
destruction ()T ,j crit
is estimated to be + 1100 Cc
t .,, n when the gate current starts to flow, the transistor
temperature is T 800 C .
1 27 s
j , c
In the next sections, the SC robustness of advanced
SiC MOSFETs is considered. Table 1 summarizes the main
characteristics of the four evaluated 1200 V SiC MOSFETs,
namely the nominal ON-state resistance
R ,ON dc-rated drain
RxAON
at the time of
. At the time
current, die area and the specific ON resistance () as
a Figure-of-Merit. All the evaluated devices were introduced
in production in the 2016-2018 timeframe. Table 1 contains
the die photographs of the tested devices, and the results of
the SC strength evaluation (rows 9 to 15).
Short-Circuit Robustness: Modeling and
Evaluation Framework
For all the evaluated devices, the failure is found to be a
thermally-driven phenomenon [7]-[9]. To understand and
compare data from different manufacturers, the heating
mechanisms need to be considered.
Basically, the transistor's temperature rises due to heat
generation in the JFET region between the Pwell diffusions
in the transistor [7]. Heating power is PV Idd d
two heating mechanisms in the semiconductor are,
= # (the product
of the drain voltage and the drain current). The transistor
fails when the critical temperature T ,j crit
is reached. The
dc Link
Vac
M
Id
GND
(a)
LStray Vdd
DUT
Buffer
Id
CBank
∆Tj
∆Tj
Time
Avg Pd = Vdd × Id
tp
Time
Inverter
Vin
Vdd
Vgs
Avg Pd
VG
LS
Id Sense
(b)
(c)
FIG 1 (a) Typical 3-phase motor-inverter drive circuit and example of SC events, along with (b) the simplified short-circuit test circuit,
and (c) typical voltage and current switching waveforms, and the device temperature rise TTj during a short-circuit event.
June 2021 z IEEE POWER ELECTRONICS MAGAZINE 35
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