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© 2020 ROHM Co., Ltd.
No. 62AN134E Rev.002
JUNE 2022
Application Note
Power Device
Calculating Power Loss from Measured Waveforms
This application note describes how to calculate the power loss of a SiC MOSFET from measured switching waveforms in a
switching circuit with a SiC MOSFET.
Measurement of switching waveforms
Figure 1 shows a switching circuit and probes for monitoring
waveforms. The voltage between the drain and source of the
MOSFET is measured using a differential voltage probe. In
addition, a current probe is used for the drain current.
Figure 2 shows the waveform of each component and the
power loss is shown (shaded area). t
on
and t
off
represent the
turn on and turn off times, respectively. The switching loss
occurs where V
DS
and I
D
overlap in these sections. During the
turn ON, since this circuit has an inductance load, I
D
starts
changing first and then V
DS
starts changing after the current
change is completed. During the turn OFF, on the other hand,
V
DS
starts changing first and then I
D
starts changing after the
voltage change is completed. Next, T
ON
is the section where
the MOSFET is in an ON state, and conduction loss occurs
due to I
D
and the on-resistance of the MOSFET.
The latest oscilloscopes may automatically compute the
shaded area and display the power loss. However, you need
to calculate the power loss from the measured waveforms
when using oscilloscopes without this function.
G
L
1
V
DD
Q
1
Q
2
I
D
V
DS
V
GS
差動電圧プローブ
電流プローブ
Figure 1. Switching circuit and probes for monitoring
waveforms
The following are cautions for the measurement. The first point
is the sampling number of the oscilloscope. The detail of the
waveforms cannot be captured with an insufficient sampling
number, causing an error in the measurement result. It is
necessary to display the sampling points and check whether
the waveforms are traced accurately. The second point is that,
since the characteristics of the delay time are different
between the voltage and current probes, the measured
waveforms include an error due to this difference in delay.
Without correction, deviation is produced along the direction
of the time axis between voltage and current, resulting in an
incorrect shaded area in Figure 2. This leads to a larger or
smaller loss than the actual value, giving erroneous results. To
eliminate the difference in delay from the measurement
system, it is necessary to perform a skew correction (de-skew).
For the method, refer to the instruction manuals of the
measuring instruments or the technical materials of the
manufacturers of the measuring instruments.
V
GS
I
D
t
t
V
GS(ON)
V
GS(OFF)
V
DS
t
t
on
t
off
T
ON
T
OFF
W
t
Figure 2. Waveform of each component and power loss
(shaded area)
Differential
voltage probe
Current probe
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© 2020 ROHM Co., Ltd.
No. 62AN134E Rev.002
JUNE 2022
Application Note
Calculating Power Loss from Measured Waveforms
Approximate calculation for waveforms
From the measured waveforms, calculate the power loss by
dividing them into sections where a linear approximation can
be performed. First, calculate power losses P
ton
and P
toff
to be
consumed during the turn on and turn off times, respectively.
The power loss is calculated with the approximate equations
in Table 1. Since the calculation formula depends on waveform
shapes, select one that is close to the shape of the measured
waveform.
In the example of the waveforms shown in Figure 3, divide the
waveforms during the turn ON into two sections, and use case
2 in Table 1 for the former section (t
on1
). In addition, use the
equation under I
D1
0 as the condition. For the latter section
(t
on2
), use the equation under V
DS2
0 in case 3. Although
voltage is produced by the on-resistance of the MOSFET and
I
D
in Figure 3, V
DS2(on)
is treated as 0 if it is sufficiently small
relative to the High voltage of V
DS
. As a result, the power loss
during the turn ON can be approximated with the following
equation.
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Similarly, divide the waveform during the turn OFF into two
sections, and use the equation under V
DS1
0 in case 1 for
the former section (t
off1
). For the latter section (t
off2
), use the
equation under I
D2
0 in case 8. Although voltage is produced
in Figure 3 for the same reason as mentioned above, V
DS1(off)
is treated as 0 if it is sufficiently small relative to the High
voltage of V
DS
. As a result, the power loss during the turn OFF
can be approximated with the following equation.
Next, calculate the power loss to be consumed during
conduction. Figure 4 shows an example of the waveforms to
determine the conduction loss. Since the MOSFET is
energized in the T
ON
section, V
DS
is the product of the on-
resistance of the MOSFET and I
D
. Refer to the data sheet for
the value of the on-resistance. To calculate the power loss,
select one of the cases in Table 2 where the shape is close to
the waveform and use the approximate equation.
For this example, case 1 in Table 2 is used. The conduction
loss can be calculated with the following equation.
I
D
t
t
V
GS(ON)
V
GS(OFF)
V
DS
t
t
on
V
DS1(on)
t
off
V
DS2(off)
I
D3(on)
I
D1(off)
T
ON
T
OFF
W
t
I
D1(on)
I
D3(off)
V
DS2(on)
V
DS1(off)
T=
f
1
I
D2(on)
I
D2(off)
t
on1
t
on2
t
off1
t
off2
Figure 3. Example of switching loss waveforms
I
D
t
t
V
GS(ON)
V
GS(OFF)
V
DS
t
t
on
t
off
I
D1(ON)
I
D2(ON)
T
ON
T
OFF
W
t
T=
f
1
Figure 4. Example of conduction loss waveforms