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Application Note
© 2023 ROHM Co., Ltd.
No. 65AN133E Rev.001
APR.2023
Power Device
Estimation of switching losses in IGBTs
operating with resistive load
In most of the applications where IGBTs are used the device
switches an inductive load. Datasheets and simulation models
are made with this kind of operation in mind. However, there’s
a growing interest in using IGBTs with resistive loads, mainly
driven by high-voltage (HV) heaters used in electric vehicles.
They rely on IGBTs for regulating the heating power. For the
IGBTs, this translates into switching a predominantly resistive
load on and off. A proper estimation of the losses during turn-
on and turn-off is needed to avoid device failure, but common
loss estimation methods cannot be applied to this case. In this
application note, a reliable method to estimate the switching
losses and instantaneous junction temperature considering
these operating conditions is presented.
Switching behavior of an ideal switch –
Inductive and resistive loads
For a better understanding of the differences between both
types of loads, the switching behavior here only considers
ideal switches and diodes. Taking this into account, the volt-
age and current waveforms for an ideal switch during an
inductive turn-on switching transient can be seen in Figure 1a.
The main characteristic of this operating mode is that the volt-
age can only start to decrease once the load current I
L
has
been reached. Conversely, during turn-off the voltage must
reach the total dc-link voltage
for the current to decrease.
The maximum instantaneous power during this mode of oper-
ation is
.
Resistive-switching waveforms look significantly different, as
shown in Figure 1b. In this mode of operation, the factor
must remain constant and equal to the load resistance . The
maximum stress for the device will happen at the middle of the
switching transient,
. The total switching energy
can be expressed analytically as
Switching waveforms of an IGBT
operating with resistive load
As we know, IGBTs do not behave as ideal switches. In the
case of inductive switching, the “tail current” during IGBT turn-
off is a good example of this. When the IGBTs are operated
with a resistive load, its losses will be higher than in the ideal
case:
��
where the term
accounts for the losses due to the con-
ductivity modulation process in the IGBT. In an IGBT and other
bipolar devices, carrier injection from collector into the drift re-
gion of the device improves the conductivity of the device
when it is turned on. This is a dynamic process that happens
each time the device changes its state from OFF to ON . The
device behaves as a time-dependent variable resistance. For
a detailed explanation, please refer to [1].
Exemplary waveforms of an IGBT during turn-on have been
Figure 1. Ideal turn-on waveforms for (a) inductive and (b)
resistive switching
I V
t
V
dc
I
L
(a)
I V
t
V
dc
I
L
(b)
t
sw
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Application Note
Switching losses in IGBTs operating with resistive load
© 2023 ROHM Co., Ltd.
No. 65AN133E Rev.001
APR.2023
sketched in Figure 3. The main difference between ideal and
real devices can be found during phase III, where the conduc-
tivity modulation takes place. This phase depends on the IGBT
technology, and its length is inversely proportional to the gate
current.
Depending on the device selected and its operating conditions,
the additional losses
can be significant. For the case
shown in Figure 2, the ideal losses
calculated according
to (1) amount to 83 mJ, but the total losses
equal to 142
mJ. Hence,
is equal to 59 mJ, 20% less than
. Note
that the example shown in Figure 2 represents an extreme
case, in which a 1200-V IGBT is operated at a voltage of 470
V.
SPICE models: Are they good enough?
After realizing that ideal waveforms are not sufficient for accu-
rate estimation of switching losses, it is important to evaluate
the accuracy of SPICE-based IGBT models, such as the one
provided by ROHM. However, when comparing experimental
waveforms with simulated ones, it is necessary to consider
that the IGBT in simulation will switch faster than in reality for
the same
value. In the case of the example shown in Fig-
ure 2, the simulation predicts a switching time of 38 µs, while
the measured value is 45 µs. This discrepancy can lead to un-
derestimation of losses if not compensated for. Specifically,
using the original
value in the simulation results in losses
of 82 mJ, while the measured losses are 142 mJ.
To match the experimental switching time, the value of
in
the simulation model was increased from 23 kΩ to 34 kΩ, as
shown in Figure 2. With this modification, the simulation repro-
duced a switching time of 45 µs, which matched the experi-
mental value. However, the SPICE model still did not accu-
rately reproduce the conductivity modulation phase, resulting
in 19% lower
losses compared to the experimental values.
While more accurate than ideal waveforms, standard SPICE-
based IGBT models remain limited in its ability to provide a
proper estimation of switching losses. The limitations of these
models have been well-documented in previous studies [1].
Proposed estimation method
Given that both ideal and simulated waveforms do not accu-
rately reflect the behavior of the IGBT for estimating switching
losses, a hybrid method is proposed here. This method in-
volves extracting device losses based on measurements and
reproducing the temperature swing based on worst-case op-
erating conditions using an equivalent RC thermal network.
Part 1: Experimental waveforms
To capture the
and
waveforms during turn-on and
turn-off, a single pulse test of the IGBT driving a resistive load
is carried out, using the circuit shown in Figure 4. To reflect the
temperature at which the device will be operating, the IGBT
should be heated up externally to a selected value between
100 and 150 °C. However, if the
value is high, setting a
Figure 3. Resistive turn-on waveforms (simplified) in an
IGBT.
I
C
V
CE
t
I
L
V
dc
V
GE
V
GE,on
V
Miller
V
th
Fast MOSFET phase
Conductivity
modulation
P = V
CE
I
C
E
id
E
cm
II III IVI
t
t
Figure 2. Turn-on of IGBT RGS80TSX2DHRC11 at 470 V,
23.5 A, and and t
sw
= 45 µs, based on measurements (exp.)
with R
G
= 23 kΩ and simulated (sim.) with R
G
= 34 kΩ.
Losses E
on
= 142 mJ (exp.) vs. E
on
= 115 mJ (sim.)
0 50 100 150 200 250
0
100
200
300
400
500
Volt. (V), curr. (A)
CE
exp.
10
C
exp.
CE
sim.
10
C
sim.
0 50 100 150 200 250
0
2
4
6
8
10
12
14
Voltage (V)
GE
exp.
GE
sim.
0 50 100 150 200 250
-0.5
0
0.5
1
1.5
2
2.5
3
Time ( s)
Power (kW), Energy (J)
exp.
10 exp.
sim.
10 sim.