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Analysis of SiC MOSFET dI/dt and its temperature dependence
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DOI: 10.1049/iet-pel.2017.0203
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Research Article
Analysis of SiC MOSFET dI/dt and its
temperature dependence
ISSN 1755-4535
Received on 17th March 2017
Revised 18th September 2017
Accepted on 30th October 2017
doi: 10.1049/iet-pel.2017.0203
www.ietdl.org
Hui Li1, Xinglin Liao1 , Yaogang Hu1, Zheng Zeng1, Erbing Song1, Hongwei Xiao1
1State
Key Laboratory of Power Transmission Equipment & System Security and New Technology, School of Electrical Engineering, Chongqing
University, Chongqing 400044, People's Republic of China
E-mail: lxl108381@126.com
Abstract: A change regulation of variation in drain current (dID/dt) of silicon carbide (SiC) metal–oxide–semiconductor fieldeffect transistors (MOSFETs) and their temperature dependencies are examined. Experimental results show that the magnitude
of turn-off dID/dt decreases with temperature and turn-on dID/dt increases with increasing temperature. Further analysis shows
that turn-on dID/dt is better than turn-off dID/dt in terms of temperature dependency and exhibits good linearity. This behaviour
results from the positive temperature coefficient of the intrinsic carrier concentration and the negative temperature coefficient of
the effective mobility of the electrons in the SiC MOSFET. Other factors that affect the temperature dependency of dID/dt, such
as supply voltage, load current, and gate resistance, are also discussed. A temperature-based analytical model of dID/dt for the
SiC MOSFET is derived using fundamental device physics equations. The calculations generally fit the measurements well.
These results are beneficial since they provide a potential approach for junction temperature estimation in the SiC MOSFET. In
SiC MOSFET-based practical applications, if turn-on dID/dt is sensed, then the junction temperature can be derived from the
relationship curve of turn-on dID/dt versus temperature drawn experimentally in advance.
1
Introduction
The superiority of a silicon carbide (SiC) metal–oxide–
semiconductor field-effect transistor (MOSFET) in static and
switching performance to Si devices has been demonstrated [1, 2].
To promote and expand its applications, many efforts have been
devoted [3, 4]. In the future, the SiC MOSFET may replace Si
insulated-gate bipolar transistors (IGBTs) in the voltage range of
1200 V and above due to its similar on-state performance and a
faster switching speed.
However, SiC MOSFETs are prone to oscillations during
switching because of the coupling effects between the variation of
drain current (dID/dt) and the parasitic inductances from the
package design of the power module, which results in voltage
overshoot [5, 6], false turn-on and unreliability [7, 8], and even
damage to the devices [9]. With the development of the packaging
technology, parasitic inductances are becoming increasingly small,
but it cannot be avoided. Since the SiC MOSFET possesses a
larger dID/dt compared with a Si MOSFET, even small parasitic
inductances also result in significant oscillations. In practical
applications, dID/dt requires close attention. In addition, their
dynamic performance is also influenced by temperature [10, 11].
However, due to the limited knowledge of the SiC MOSFET, the
theoretical analysis and the change regulation in terms of the
effects of temperature on its switching characteristics have not
been fully characterised and understood. Previous work by Zhu et
al. [12] analysed the temperature dependency of on-resistance. In
[13, 14], the threshold voltage, which was unstable under different
temperatures because of electron tunnelling into and out of oxide
traps, was investigated. The switching characteristics of the SiC
MOSFET were investigated in [15, 16], whereas the impact of
temperature was ignored. The relationship between dID/dt and
temperature for the SiC MOSFET can be observed in a few
published reports [17–20]. In [17], the characterisation and
comparison of three types of 1.2 kV SiC MOSFETs from different
manufacturers were presented at 25 and 175°C. Similar
measurements were also carried out in [18, 19]. In [20], a SiC
implantation and epitaxial MOSFET were evaluated at
temperatures of 25 and 125°C. In [21], the effects of temperature
on the turn-on dID/dt of the SiC MOSFET were investigated. It is
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suggested that turn-on dID/dt can be used as an approach for
junction temperature measurement in the SiC MOSFET.
Unfortunately, many issues still remain unclear. For the SiC
MOSFET-based practical applications, the effects of temperature
on its switching characteristic, which is useful for safely using the
device, system reliability, and actual system design, should be
investigated. Alternatively, in the future SiC MOSFET-based
practical applications, junction temperature measurement will
become an important topic. For the SiC MOSFET, the temperature
sensitivity for the same electrical parameters in Si IGBT is low
owing to its unipolar nature. Some conventional indicators of
junction temperature estimation for Si devices will fail for the SiC
MOSFET because of its wide bandgap, low intrinsic carrier
concentration, and fast switching speed [22, 23]. Hence, it is also
important to find a suitable temperature-sensitive electrical
parameter for junction temperature measurement of the SiC
MOSFET.
In this study, an analysis of the variation in drain current for the
SiC MOSFET during turn-on and turn-off is completed by
considering the effects of temperature. It is shown experimentally
that the magnitude of turn-off dID/dt decreases with temperature
and turn-on dID/dt increases with increasing temperature. Further
analysis shows that turn-on dID/dt is better than turn-off dID/dt in
terms of temperature dependency and exhibits good linearity. The
result is of great interest since it provides a potential approach to
the junction temperature of the SiC MOSFET. From the
relationship curve of turn-on dID/dt versus temperature, the
junction temperature of the SiC MOSFET can be derived. In
addition, a temperature-based analytical model of variation in drain
current is presented, and the effects of the supply voltage, load
current, and gate resistance on the temperature dependency of
variation in drain current are analysed.
2 Temperature dependency characterisation of
variation in drain current
2.1 Overview of the turn-on and turn-off process
The typical structure of a 1200 V SiC MOSFET that consists of
electrodes, gate oxide, junction field-effect transistor (JFET)
1
2.2 Temperature-based model of dID/dt
Drain current rises during turn-on because VGS reaches the
threshold voltage VTH. An N-type conducting channel connecting
the source to the JFET region is created, while an accumulation
layer is formed under the gate oxide at the top of the JFET region,
providing current spreading for the electron current flowing from
the channel into the JFET region, as shown in Fig. 1a. During turnoff, the condition is the opposite. Before the MOSFET is turned
off, it is effectively in the normal operation condition, of which its
drain-source voltage is approximately equal to VGS-VTH. Assuming
that the drain current is approximately equal to the MOSFET
channel current, the drain current can be written as
ID =
W μ0COX
V GS − V TH 2 1 + λ(V GS − V TH) ,
2L
(1)
where W is the channel width, L is the channel length, VTH is the
threshold voltage, λ is the channel length modulation parameter, μ0
is the effective mobility of the carriers in the channel of the SiC
MOSFET, and COX is the gate oxide capacitor. The time derivative
of the drain current is given by
dID W μ0COX
dV GS
=
V GS − V TH 2 + 3λ(V GS − V TH)
.
dt
2L
dt
(2)
The gate–source voltages (VGS) for turn-off and turn-on are given
below
Fig. 1 The structure and typical switching process
(a) Structure of the SiC MOSFET, (b) Simplified equivalent circuit of the SiC
MOSFET, (c) Typical switching process under an inductive load
V GS_off = V GGe−(t / RgCISS),
(3a)
V GS_on = V GG(1 − e−(t / RgCISS)),
(3b)
where VGG is the gate driver voltage, Rg is the gate resistance that
is composed of gate driver resistance and internal gate resistance,
CISS = CGS + CGD is the input capacitance, where CGS and CGD are
the gate–source capacitance and gate–drain capacitance,
respectively. Taking the derivation of (3) with respect to time and
substituting it into (2), the turn-off and turn-on dID/dt are shown as
Fig. 2 Experimental switching process of the SiC MOSFET: VGS 20 V/div
(orange line), VDS 400 V/div (blue line), ID 10 A/div (green line), t 100
ns/div
region, and N-drift is shown in Fig. 1a, which is a vertical device
with a planar gate. Fig. 1b shows its equivalent circuit, which
contains three internal capacitances between each pair of nodes:
gate–drain (CGD), gate–source (CGS), and drain–source (CDS).
The switching process of power MOSFET based on an
inductive load circuit is illustrated in Fig. 1c, which shows the four
phases during turn-on and turn-off. Owing to the existence of
switching loop stray inductances, the voltage drop across stray
inductances reshapes the waveforms of VDS at the drain current rise
phase in turn-on and fall phase in turn-off. As a result, a notch
during turn-on and a peak during turn-off is provided because of
the induced positive voltages and negative voltages, respectively.
During turn-on, the drain current over-shoot is also indicated,
which results from the reverse recovery current of the freewheeling
diode (FWD). Meanwhile, during turn-off, drain current drops
before the metal–oxide–semiconductor (MOS) channel starts to
cut-off. This condition is due to the existence of the FWD junction
capacitance discharged by the drawing part of the drain current. In
addition, the turn-on gate–source voltage VGS presents a higher
plateau voltage level than that of the turn-off because of the effect
of CDS. Fig. 2 shows the turn-on and turn-off experimental
measurements at a supply voltage of 450 V, a load current of 12 A,
and a gate resistor of 10 Ω.
2
dID_off
W μ0COXV GG
= −
a(2 + 3λa) e−(t / RgCISS),
dt
2LRgCISS
(4a)
dID_on W μ0COXV GG
=
b(2 + 3λb) e−(t / RgCISS),
dt
2LRgCISS
(4b)
with
a = V GG e−(t / RgCISS) − V TH,
b = V GG(1 − e−(t / RgCISS)) − V TH .
Equation (4) is useful for understanding how the variation in drain
current is determined by the gate resistance, load current, system
voltage, and temperature. The value of dID/dt is also dependent on
input capacitance Ciss, which dominates the switching time
constant of the SiC MOSFET. Indeed, since the gate–drain
capacitor is formed by a combination of constant gate-oxide
capacitor and bias-dependent depletion capacitance under the gate,
the input capacitance Ciss is non-linear and depends largely on the
terminal voltage of the drain–source that has been revealed in
numerous presented literature and datasheets. For a fixed drain–
source voltage, the value of Ciss can be obtained according to the
curve of Ciss versus drain–source voltage. The curve is normally
provided by the manufacturer or experimental measurements. The
switching process in Fig. 1c shows that during the drain current
rise stage in turn-on and the fall stage in turn-off, the terminal
voltage of the drain–source is equal to system voltage VDC. In
practical application, system voltage VDC is typically fixed in the
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existing assembled converters. Therefore, Ciss can be considered a
fixed value in the presented model. As reported by Chen et al. [15],
experimentally, the C–V characteristics of the SiC MOSFET almost
overlap under different temperatures. Hence, the temperature
dependency of CISS is neglected. However, threshold voltage VTH
and carrier effective mobility μ0 are dependent on temperature. The
temperature dependency of dID/dt can be evaluated by taking the
derivative of (4) with respect to temperature. For turn-off and turnon, the results are shown as (5a) and (5b), respectively
d2ID_off
WCOX V GG
dμ
= −
(2a + 3λa2) 0
dtdT
2L RgCISS
dT
dV TH −(t / RgCISS)
−(2 + 6λa)μ0
e
,
dT
d2ID_on WCOX V GG
dμ
=
(2b + 3λb2) 0
dtdT
2L RgCISS
dT
−μ0(2 + 6λb)
dV TH −(t / RgCISS)
e
.
dT
(5b)
(6)
By replacing a with b and multiplying by −1 in (6), the expression
of the temperature dependency of turn-on dID/dt can be obtained.
Ultimately, the temperature dependency of the variation in drain
current is dominated by the temperature dependency of the
threshold voltage.
The threshold voltage is the value of the gate voltage at which
the surface potential of the channel in the MOSFET is exactly two
times the bulk potential. In other words, the gate potential has
induced sufficient band bending for the intrinsic Fermi level in the
p-type body of the device to be below the Fermi level. Hence, the
electron concentration in the channel is exactly equal to the p-body
doping and the channel is properly inverted, that is, the minimum
gate voltage required to switch on the device. Hence, the threshold
voltage can be calculated as shown below in (7), where ψB is the
Fermi potential, ɛSiC is the dielectric constant of carbide silicon, NA
is the p-body doping, Qf is the fixed oxide charges, COX is the
oxide capacitance, and φms is the metal–semiconductor work
function difference [25]
4εSiCqNAψ B
Qf
V TH = ϕms −
+ 2ψ B +
,
COX
COX
(7)
where
kT NA
ψB =
ln
.
q
ni
Given that the work function difference (φms) and the oxide
charges (Qf) are essentially independent of temperature, the
threshold voltage temperature dependency can be written
approximately as
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(8)
Nevertheless, the analytical expressions of the threshold voltage
and its temperature dependency are difficult to use for common
electrical engineers due to several parameters based on the device
fabrication that is not provided usually by manufacturers. A simple
and feasible temperature-sensitive parameter of the threshold
voltage can be generally obtained by experimental measurements.
During turn-off, the time it takes for drain current to fall to zero
from the load current can be modelled as
(5a)
Equations (5a) and (5b) show two temperature-sensitive
parameters: the threshold voltage and the effective mobility of the
carriers. Effective mobility in the SiC MOSFET, as reported in
[24], initially shows an increasing trend with temperature due to
the early movement of the Fermi level towards the band gap,
whereas the mobility shows a negative temperature-sensitive
parameter at high temperature because lattice scattering dominates
and begins to release the interface trap charges. Therefore, in the
practical working temperature range of 300–600 K, the mobility
can be considered approximately constant for the SiC MOSFET,
and dμ0/dT can be ignored for simplicity. Hence, (5a) can be
simplified as
d2ID_off W μ0COX V GG −(t / RgCISS)
dV TH
=
e
(2 + 6λa)
.
dtdT
2L RgCISS
dT
dV TH dψ B
1 εSiCqNA
=
2+
.
dT
dT
COX
ψB
tif = RgCISSln
V GP
,
V TH
(9)
where VGP is the plateau voltage of VGS in turn-off defined as
(IL/gm + VTH) and IL is the load current. Equation (9) shows that
time tif is a function of load current (IL). Equation (9) is substituted
into (4a) and the derivative with respect to IL is expressed as
follows:
d2ID_off W μ0COXV GGV TH
=
(2 + 3λa(3a + 2V TH)) e−(t / RgCISS) .(10)
dtdIL
2LRgCISSV GPgm
In (10), time t is equal to the drain current fall time tif. Similarly,
for turn-on, the load current dependency of dID/dt is written as
d2ID_on W μ0COXV GG(2 + 3λb(3b + 2V TH)) −(t / RgCISS)
=
e
, (11)
dtdIL
2LRgCISSgm(V GG − V GP)
where the value of time t is the drain current rise time given as tir =
RgCISSln((VGG−VTH)/(VGG−VGP)). Regardless of the situation, (10)
and (11) are positive, which means that dID/dt increases with the
load current during turn-off and turn-on.
During turn-off, the excess carriers in the N-drift region are
swept out to form a depletion layer in order to block the supply
voltage. The depletion layer capacitance (Cdep) is defined as
Cdep =
εSiC A
.
2εSiCV /qNA
(12)
The input capacitance (CISS) consists of gate–source capacitance
CGS and gate–drain capacitance CGD. The former is constant and
independent of voltage. The latter includes two components,
namely, fixed gate oxide capacitor COX and varied depletion layer
capacitance Cdep, which are in series. Hence, the derivative of CISS
with respect to voltage can be obtained as
2
dCISS
COX
dCdep
=
Cdep + COX dV
dV
= −
2 2
COX
εSiC A 2εSiCV
Cdep + COX qNA qNA
−(3/2)
(13)
.
The voltage dependency of dID/dt can be evaluated by taking the
derivative of (4) with respect to voltage and substituting (13) into
the derivative. For turn-off, the dependency relation is exhibited in
(14a), and for turn-on, the relation is in (14b).
d2ID_of f
W μ0COXV GG εSiCqNA
= −
2
2A
dtdV
4LRgCISS
1/2
2
COX
×
a(2 + 3λa)e−(t / RgCISS),
Cdep + COX
(14a)
3
d2ID_on W μ0COXV GG εSiCqNA
=
2
dtdV
2A
4LRgCISS
×
1/2
2
COX
b(2 + 3λb)(1 − e−(t / RgCISS)) .
Cdep + COX
(14b)
It is shown in (14) that dID/dt decreases with the increase in
voltage during turn-off because its second-order derivative is
negative. Also, for turn-on, dID/dt increases with voltage because
of the positive second-order derivative. The relationships between
temperature, load current, supply voltage, and variation in drain
current during turn-off and turn-on are summarised in Table 1.
When temperature increases, the variation of drain current
decreases monotonically in turn-off. In the case of a turn-on, the
trend increases monotonically. At a fixed temperature, the variation
in drain current decreases with the increase in load current during
turn-off and increases during turn-on. In addition, the variations in
drain current during turn-off and turn-on show an increasing shift
in supply voltage at the same temperature and load current. These
results are interesting because the variation in drain current may be
used as a potential indicator of junction temperature in the SiC
MOSFET due to the monotonicity in temperature variation, load
current, and supply voltage.
3
Experimental detail
The static characteristics of the SiC MOSFET are measured using
the Agilent B1505A curve tracer with the device placed in an
environment chamber to control the temperature. The switching
characteristics are determined by using the clamped inductive
double-pulse test circuit shown in Fig. 3a.
The switching waveforms are captured using a Lecroy 610Zi
digital oscilloscope, which possesses a bandwidth of 1 GHz and a
Table 1 Variation trend of drain current over operation
conditions
Turn-on
Turn-off
dID/dt
dID/dt
operation
condition
temperature ↑
load current ↑
supply voltage ↑
gate resistance ↑
↓
↓
↑
↓
↑
↑
↓
↓
sample rate of 20 GS/s. Current is measured using a Pearson
current sensor that is connected to the oscilloscope. During the
experiment, different ambient temperatures are simulated using a
heater, and the SiC MOSFET is mounted at the bottom of the test
circuit board connected to the heater through an aluminium plate
with some thermal grease for reliable heat transfer, as shown in
Fig. 3b. The heater can vary the temperature from room
temperature to 450°C. To guarantee measurement accuracy, a fan is
used to avoid the effects of temperature on the other components.
Fig. 3c shows the test rig.
4
Experimental results
4.1 Threshold voltage and transconductance temperature
sensitivity
Figs. 4a and b show the SiC MOSFET transfer and output
characteristics at varying temperatures, respectively. The
temperature dependencies of the threshold voltage can be obtained,
as shown in Fig. 4c. The square represents the tested values under
different temperatures, while the solid line represents the fitted
values. It can be seen that the threshold voltage significantly
decreases with the increase in temperature, which is typical for 4HSiC MOSFET and has been observed in previous studies [17, 18].
The effect is caused by the increase in intrinsic carrier
concentration at high temperatures, as expressed by (7), due to the
increased thermal generation of carriers across the band gap, which
forms the channel easily.
Fig. 4c shows that the threshold voltage temperature
dependency is approximately linear. The temperature sensitivity
coefficient (kVT) is ∼−6.37 mV/°C, which causes the derivative of
dID/dt with respect to the temperature for the turn-off to be
negative according to (6). As predicted, dID/dt decreases with the
increase in temperature during turn-off. In the case of a turn-on,
dID/dt increases with temperature because of the positive
temperature derivative of dID/dt. If the nominal threshold voltage
(VTH0) at room temperature is known, then a simple expression can
be used to describe the threshold voltage at any measured
temperature T
V TH(T) = V TH0 − kVT(T − T 0) .
(15)
The temperature dependency of transconductance gm is shown in
Fig. 4d. The stars represent the tested values under different
Fig. 3 Experiment details
(a) Schematics of test, (b) Simulation of different ambient temperatures, (c) Experiment rig components
4
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Fig. 4 Static characteristics under different temperatures for the SiC MOSFET
(a) Temperature-dependent transfer characteristics, (b) Temperature-dependent output characteristics, (c) Temperature-dependent threshold voltage, (d) Temperature-dependent
transconductance
temperatures, while the solid line represents the fitted values.
Fig. 4d shows that transconductance increases with temperature
and the temperature sensitivity coefficient kGT is ∼16.19 mS/°C.
The positive temperature coefficient of the transconductance arises
from the increased MOS channel inversion electron density and
mobility at high temperatures due to the increased detrapping of
electrons at the high density of interface traps at the SiC/SiO2
interface. A simple expression similar to (15) can also be used to
describe the transconductance at any measured temperature (T),
where gm0 is the nominal transconductance at room temperature
gm(T) = gm0 + kGT(T − T 0) .
(16)
4.2 Impact of temperature on dID/dt
The switching waveforms of the SiC MOSFET are recorded at
different temperatures, load currents, gate resistors, and supply
voltages to study the effects of these conditions on dID/dt. Figs. 5a
and b show the turn-off waveforms of the drain current under
different gate resistors at a supply voltage of 200 V and a load
current of 15 A. The temperature varies with a range of room
temperature to 175°C at 25°C intervals. The turn-on waveforms for
drain current are also exhibited in Figs. 5c and d. It can be seen that
the temperature dependency of dID/dt varies with gate resistors
during turn-off and turn-on. During turn-off, the time at which the
drain current starts to fall increases with temperature at small
values of the gate resistor and is approximately the same for large
values of the gate resistor. During turn-on, the time, at which the
drain current starts to increase, decreases with temperature at large
values of the gate resistor and is nearly the same at small values of
the gate resistor. The phenomenon is a result of the negative
temperature coefficient of the threshold voltage shown in Fig. 4c,
which causes the device to switch later or sooner at high
temperatures during turn-off and turn-on, respectively. In addition,
the variation in drain current varies with temperature during turnoff and turn-on, especially for a large gate resistor during turn-on.
From the curves above, the measured values of dID/dt, as a
function of temperature under different measurement conditions,
are obtained. The results are shown in Fig. 6 for turn-off and Fig. 7
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for turn-off, respectively. In Fig. 6, the magnitude of dID/dt is
adopted for plotting. The calculated values are also shown in these
figures. In the calculations, the values of CGS and COX are 2.192
and 3.387 nF, respectively, which are extracted by a constant gate
current circuit during turn-on. Indeed, for the SiC MOSFET, the
relative variation of channel length is very small compared with the
long channel, and λ can be treated as zero. The values of W and L
are calculated by choosing points from the saturation region of the
device output characteristic curves in the datasheet. The drain–
source overlap area ADS is 5.3676 mm2. The temperature
dependency of the effective mobility of the carriers is taken from
[26].
For a fixed voltage of 400 V and a gate resistance of 47 Ω, the
relationship between dID/dt and temperature under different load
currents is presented in Fig. 6a. The load current values are 10, 15,
and 20 A. Fig. 6b shows the results at supply voltages of 200, 400,
and 600 V with a fixed load current of 15 A and a gate resistance
of 47 Ω. The calculated values can be obtained by (4a) combined
with (15). It can be seen that turn-off dID/dt decreases as
temperature increases with fixed voltage, load current, and gate
resistance, which is consistent with (4a) and (6). At a fixed
temperature, turn-off dID/dt increases with load current as
predicted by (10). The increase of the gate voltage plateau with
load current will increase dID/dt, while the increase in time it takes
for gate voltage to fall to threshold voltage from plateau with
voltage plateau will result in a small dID/dt. Hence, assuming a
fixed supply voltage and device temperature, the more dominant
parameter will determine how dID/dt varies with temperature.
Indeed, the load current dependency of dID/dt is dominated mainly
by the gate voltage plateau. At large load current, dID/dt is large.
At fixed load current, turn-off dID/dt varies with supply voltage. It
can be seen in Fig. 6b that turn-off dID/dt decreases with the
increase in supply voltage as predicted by (14a). This characteristic
is a result of the change in Miller capacitance under different
supply voltages. Further analysis of the measurements in Figs. 6a
and b reveals that regardless of the measurement conditions, the
temperature sensitivity of dID/dt is approximately common.
5
Fig. 5 Turn-off and -on waveforms for ID at a supply voltage of 200 V and a load current 15 A under different temperatures
(a) Turn-off waveforms for ID at Rg = 10 Ω, (b) Turn-off waveforms for ID at Rg = 47 Ω, (c) Turn-on waveforms for ID at Rg = 10 Ω, (d) Turn-on waveforms for ID at Rg = 47 Ω
Fig. 6 Turn-off dID/dt as a function of temperature
(a) Turn-off dID/dt at different load currents, (b) Turn-off dID/dt at different supply voltages, (c) Turn-off dID/dt at different gate resistances
Moreover, the dependency of dID/dt on the gate resistor and
temperature is investigated by experimental measurements.
The results are exhibited in Fig. 6c, where the temperature
dependencies of dID/dt are shown for different gate resistors with a
supply voltage of 400 V and a load current of 15 A. It can be seen
from Fig. 6c that the temperature sensitivity of dID/dt varies with
6
different gate resistors, which is small at small and large values of
the gate resistor and large at intermediate values of the gate
resistor, which is due to the fact that d2ID/dtdT as the varying gate
resistor shows an upside-down bell-shaped characteristic, as
presented in Fig. 8.
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In the case of a turn-on, the temperature dependency of dID/dt
at different load currents is shown in Fig. 7a, while the results
under different supply voltages are exhibited in Fig. 7b. It is shown
that turn-on dID/dt increases with temperature as expected. The
variation trend is different from that for turn-off. For the load
current, its impacts on dID/dt are positive as predicted by (11),
which is similar to that for turn-off. For supply voltage, its effect is
also positive as predicted by (14b), but is contrary to that for turnoff. This means that turn-on dID/dt increases with increasing
temperature and supply voltage. The temperature sensitivity of
dID/dt is also approximately the same in any test condition. Fig. 7c
presents the temperature dependency of dID/dt for different gate
resistors. It is noted that the impacts of the gate resistor are similar
to those for turn-off. At the intermediate gate resistor value, the
temperature sensitivity of dID/dt is large. In addition, the
comparisons of calculated and measured dID/dt in the figures above
show that the calculations generally fit the measurements well. The
results obtained are interesting and useful because they provide a
potential approach for junction temperature measurement in the
SiC MOSFET.
Fig. 7 Calculated d2ID/dtdT for varying gate resistor at 25°C
5
Discussion
The aforementioned analysis indicates that the threshold voltage
will vary at different temperatures, thereby causing the device to
switch sooner or later. Hence, delay time also varies with
temperature during turn-on and turn-off. The results for two
temperatures, namely, 25 and 175°C, are presented in Fig. 9, where
the delay time during turn-off and turn-on is defined according to
the report in [27]. The two gate resistances of 10 and 47 Ω are also
used in the measurement. Fig. 9a shows the turn-on delay time of
the SiC MOSFET at 25 and 175°C with a gate resistance of 10 Ω.
The supply voltage and load current are 200 V and 15 A,
respectively. At the same conditions, the turn-off delay time of the
SiC MOSFET is shown in Fig. 9b. Delay time versus temperature
is repeated at a gate resistance of 47 Ω as shown in Figs. 9c and d
for turn-on and turn-off, respectively. It is noted from Fig. 9 that
delay time varies with temperature and that gate resistance exerts a
significant influence on the temperature dependency of delay time.
For a fixed gate resistance of 10 Ω, the turn-on delay time varies
from ∼50.44 ns at 25°C to 61.11 ns at 175°C, thereby exhibiting a
small variation trend as temperature increases. However, during
turn-off, delay time ranges from 132.71 to 199.88 ns when
temperature changes from 25 to 175°C. With a large gate resistance
of 47 Ω, the turn-on delay time shows a significant decrease from
109.59 ns at 25°C to 57.76 ns at 175°C, while the turn-off delay
time is approximately invariant with temperature. The values are
∼564.88 ns for two temperature conditions. For a certain gate
resistance, either turn-on delay time is strongly correlated with
temperature or turn-off delay time varies significantly with
temperature. Hence, the delay time can also be used as a thermosensitive electrical parameter for junction temperature
measurement in the SiC MOSFET. However, since the time is
relatively small, the delicate measurement of delay time in
practical applications is a challenge.
The variation trend between dID/dt and temperature for the SiC
MOSFET can also be observed in other published reports on the
performance of the SiC MOSFET at different temperatures. OrtizGanzalez et al. [21] investigated the temperature dependency of
turn-on dID/dt. Measurements were performed for three different
Fig. 8 Turn-on dID/dt as a function of temperature
(a) Turn-on dID/dt at different load currents, (b) Turn-on dID/dt at different supply voltages, (c) Turn-on dID/dt at different gate resistances
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7
Fig. 9 Delay times at 25 and 175°C at a supply voltage of 200 V and a load current 15 A with different gate resistances
(a) Turn-on delay time at Rg = 10 Ω, (b) Turn-off delay time at Rg = 10 Ω, (c) Turn-on delay time at Rg = 47 Ω, (d) Turn-off delay time at Rg = 47 Ω
rating current devices (10, 24, and 42 A) at 25, 75, 105, and 150°C.
Their results showed that turn-on dID/dt linearly increases with
temperature and that the temperature sensitivity of dID/dt increases
with gate resistance and current rating, which are consistent with
the results. Compared with other published literature in terms of
the temperature dependency of turn-off dID/dt, an interesting
phenomenon can be found. The turn-off dID/dt is either
temperature invariant or decreases with temperature at a small gate
resistance. In DiMarino et al. [17] and Chen et al. [18], the
measurements were performed at a supply voltage of 600 V and a
load current of 10 A with 10 Ω gate resistance. The magnitude of
turn-off dID/dt decreases with temperature. The values are ∼2.22,
1.9, 1.79, and 1.64 A/ns at 25, 125, 175, and 200°C, respectively.
Othman et al. [19] experimentally showed that the temperature
sensitivity of turn-off dID/dt is very low and approximately
constant under 400 V, 15 A, and a gate resistance of 28 Ω. When
the temperature increases from 25 to 175°C, the value of dID/dt is
∼0.96 A/ns. As reported by Takao et al. [20], the same conclusions
were drawn from the switching waveforms of the SiC MOSFET at
25 and 125°C at 600 V, 10 A, and a gate resistance of 11.36 Ω.
Thus, it can be concluded that the temperature sensitivity of turnoff dID/dt differs for different devices coming from different
manufacturers even at the same gate resistance. In addition, the
temperature dependency of the gate current varies for different SiC
MOSFETs, as presented in Fig. 10, where the SiC MOSFETs are
SCT2080KE and C2M0080120 and the gate resistance is 220 Ω. In
both measurements, the temperatures are 25, 75 and 150°C.
Figs. 10a and b show the measured turn-on and turn-off gate
current waveforms as a function of time for SCT2080KE, whereas
Figs. 10c and d show the results for C2M0080120 during turn-on
and turn-off. It can be seen that the turn-on gate current plateau
increases with temperature, which is similar to those reported in
Ortiz-Ganzalez paper [21]. However, for C2M0080120, the turn-on
gate current plateau is approximately invariant with temperature. It
indicates this parameter is not utilised for junction temperature
measurement in C2M0080120. In the case of turn-off gate current,
its temperature dependency in SCT2080KE is not significant
8
shown in Fig. 9b, while it exhibits a strong temperature
dependency in C2M0080120 shown in Fig. 9d, which may be used
as a potential approach in junction temperature measurement for
C2M0080120. Hence, it is concluded from the above analysis that
some parameters in the dynamic properties of the SiC MOSFET
during turn-off at special operation conditions may be suitable
temperature-sensitive electrical parameters for junction
temperature measurement in SiC MOSFET.
In the case of the SiC MOSFET used in this study, the
relationship curve between turn-off dID/dt and temperature is nearlinear under large gate resistor conditions, which may be as a
temperature-sensitive electrical parameter for junction temperature
measurement. However, further analysis of the curves of dID/dt
versus temperature during turn-off and turn-on shows that the turnon dID/dt exhibits better temperature sensitivity compared with that
of turn-off dID/dt at the same gate resistor. The linearity with
temperature for turn-on dID/dt is higher than turn-off dID/dt. The
higher linearity brings benefits to the calibration step since it does
not need a lot of measurement points. Hence, turn-on dID/dt is
more suitable to be used as a temperature-sensitive electrical
parameter. Indeed, dID/dt can be easily measured by the induced
voltage across the parasitic inductance between the Kelvin and the
power emitter terminal of a power module [28]. For a discrete SiC
MOSFET with TO-247 package, the Kelvin connection is realised
by adding an auxiliary source pin at the root of the packaging
power source lead. In addition, dID/dt can also be measured
directly by the Rogowski coil or the magnetoresistive current
sensors [29, 30]. For SiC MOSFET-based practical applications,
the type of device and system parameters, such as voltage, load
current, and gate resistance, are usually fixed. Once turn-on dID/dt
is sensed, the junction temperature of the SiC MOSFET can be
derived from the relationship curve of turn-on dID/dt versus
temperature drawn experimentally in advance.
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Fig. 10 Temperature dependency of gate current for different SiC MOSFETs during turn-on and turn-off
(a) Turn-on gate current for SCT2080KE, (b) Turn-off gate current for SCT2080KE, (c) Turn-on gate current for C2M0080120, (d) Turn-off gate current for C2M0080120
6
Conclusion
The temperature dependency of dID/dt is presented in this study. It
is shown that dID/dt is a function of temperature. Compared with
turn-off dID/dt, the temperature dependency of turn-on dID/dt is
better and exhibits a near linear dependency on temperature. This
behaviour is a result of the coupling of the positive temperature
effect of the intrinsic carrier concentration and the negative
temperature effect of the effective mobility of the electrons in the
SiC MOSFET. The effects of supply voltages, load currents, and
gate resistances are also discussed. During turn-off, dID/dt
increases with load current but decreases with supply voltage. In
the case of a turn-on, dID/dt increases with the increase in load
current and supply voltage. However, for different supply voltages
and load currents, the temperature dependency of dID/dt is
approximately the same. With fixed supply voltage and load
current, the temperature dependency of dID/dt varies for different
gate resistors, which is small at small and large values of the gate
resistor and large at intermediate values of the gate resistor. In
addition, delay time also varies with temperature and gate resistors
under fixed supply voltage and load current. A temperature-based
analytical model of dID/dt for the SiC MOSFET is proposed and is
shown to account for the experimental measurements. Due to good
linearity, it can be concluded that turn-on dID/dt may be considered
a practical temperature-sensitive electrical parameter for the
junction temperature measurement of the SiC MOSFET. With a
database of turn-on dID/dt with temperature, supply voltage, load
current, and gate resistance, the junction temperature of the SiC
MOSFET can be derived from the relationship curve of turn-on
dID/dt versus temperature.
(CYB16020), the National Natural Science Foundation of China
(51377184, 51607016), and the Chongqing Scientific and
Technological Talents Program (KJXX2017009).
8
[1]
[2]
[3]
[4]
[5]
[6]
[7]
[8]
[9]
[10]
[11]
[12]
7
Acknowledgments
This paper is supported by the Fundamental Research Funds for the
Central Universities (106112016CDJZR158802), the Graduate
Scientific Research and Innovation Foundation of Chongqing
IET Power Electron.
© The Institution of Engineering and Technology 2017
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