Application Note
SiC Power Device Series

Method for Monitoring Switching Waveform
To monitor a location where switching is performed by a power device element, for example, in a switching power supply or a motordrive circuit, an oscilloscope and a voltage probe are commonly used. The switching waveform is a rectangular wave, and contains
high frequencies with high power in the case of power circuits. The correct waveform cannot be monitored unless the probe, which isthe entrance of the waveforms, is appropriately used. This application note explains how to correctly monitor the switching waveforms.

Switching location
For example, even if a switching power supply circuit is createdand produces a correct value of the output voltage, it is stillnecessary to confirm that the switching operation does not
malfunction and the switching waveform does not exceed therated voltage of the element. Since the switching waveform is arectangular wave and contains high order harmonic components,
the frequency may reach several GHz. In addition, since thepower processed by power circuits is large, high voltage isgenerated with minimal influence from the parasitic elementsFigure 1. Monitoring of switching location with voltage probe; the(the inductance component, especially).

ground side connected with the ground lead with attachedalligator clip.

Waveform monitoring with voltage probe
Ideally, a differential probe should be used to monitor switching
Switching
20V/div

locations. However, since differential probes are expensive, apassive voltage probe is frequently used for low voltage circuitswith a voltage of 300 V or less in general. The photograph in
Figure 1 shows monitoring of a switching location with a voltage

VOUT
0.2V/div(AC)probe. The end of the probe is connected to the switching
location, and the ground side is connected with the attachedground lead with alligator clip. Figure 2 shows the waveforms
monitored. The top shows the switching waveform and the4 μs/div

bottom shows the output waveform of the DC/DC converter.

Figure 2. Waveforms monitored using the probe connected withRinging is observed when the switching signal changes abruptly.

© 2019 ROHM Co., Ltd.

the ground lead with alligator clip

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No. 62AN095E Rev.001
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Method for Monitoring Switching Waveform

Application NoteSince the ground lead of the probe is added as inductance, thisringing is caused by a series resonance with the input
capacitance of the probe. Therefore, the correct waveforms
cannot be monitored with this method.
Figure 3 shows an example of an equivalent circuit to the probe.
In addition, the resonance equation is expressed as Equation (1).
When the ground lead is 160 mm in length and the inductanceper 1 mm is estimated at 1 nH, the inductance is 160 nH. From
Equation (1), the resonance frequency is approximately 200MHz.

Figure 5. Ground spring(Tektronix)

入力端子Input terminal
CPROBE
4pF

Groundlead
グラウンドリードRPROBE
10MΩ

The state where the switching location is monitored using theLGROUNDprobe tip adapter is shown in Figure 6. The measurement resultis shown in Figure 7. Compared with the above waveforms thatFigure 3. Example of an equivalent circuit to the probe

are monitored using the probe connected with the ground leadwith alligator clip (Figure 2), the ringing caused by the probe

𝑓𝑟 =12𝜋√𝐿𝐺𝑅𝑂𝑈𝑁𝐷 × 𝐶𝑃𝑅𝑂𝐵𝐸[𝐻𝑧]

almost disappears and the obtained waveform is faithful to the(1)original waveform.
The above method is effective if the operating voltage of theTo prevent the resonance and obtain the correct waveforms, thecircuit is low. However, correct waveforms cannot be obtained ifinductance added by the ground lead should be reduced. By

the operating voltage is high. In such cases, use a high voltage

shortening the ground lead and reducing the inductance, thedifferential probe or an optically isolated differential probe.

resonance frequency can be increased beyond the frequency
band of the measuring equipment.
Figures 4 and 5 show examples of a short ground lead. Figure 4shows a type to be mounted on the PCB, and Figure 5 shows atype to be contacted with the PCB. Especially probe tip adaptershave a ground lead of a few mm in length, reducing theinductance to a few nH. These accessories are sold together
with voltage probes.

Figure 6. State where the switching location is monitored using
Figure 4. Probe tip adapter

the probe tip adapter(Tektronix)© 2019 ROHM Co., Ltd.

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Method for Monitoring Switching Waveform

Application NoteSwitching
20V/div

VOUT
0.2V/div(AC)

4 μs/divFigure 7. Waveforms monitored with the probe using the probe
tip adapter

Figure 9. Creating the tip (end) support part of the adapter

Simplified probe tip adapter
As mentioned above, the probe tip adapters are highly effective.
However, when they are not available at hand, a simplified
device may be used as a substitute. The procedure to self-build
the simplified device is described below.
1. Wind a leaded resistor or 0.5 mm tinned wire around theground part of a probe to create the ground support part ofthe adapter (Figure 8).
2. Wind a leaded resistor or 0.5 mm tinned wire around the tip ofa tweezer to create the tip (end) support part of the adapter(Figure 9).
3. Figure 10 shows the created adapter and the probe to be used.
Figure 10. Created simplified probe tip adapter
4. Mount the adapter on the board with soldering before using(Figure 11).

Figure 11. State where the adapter is mounted on the board
Figure 8. Creating the ground support part of the adapter© 2019 ROHM Co., Ltd.

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Method for Monitoring Switching Waveform

Application Note150

There may be cases where the simulation results do not match

100Voltage (V)Influence of probe in simulations
the waveforms that are actually measured. One of the causes isdifference in the measurement system. Since the measurement
probe in simulations is an ideal probe, its input impedance is500

infinite. On the other hand, since the measuring equipment isconnected to the monitoring point in actual measurements,

-50
6.5

various parasitic impedances are added. Since the ground leadof the probe mentioned above is one of such parasitic

7.0

7.5
Time (μs)

8.08.5Figure 13. Waveform resulting from the simulation in Figure 11components, if a simulated waveform does not match thewaveform that is actually measured, it is also necessary to add
the equivalent circuit of the measuring equipment to theV1
100V

simulation circuit and perform verification.

M1
SCT3120AL

L1500μH

Probe(+)

Figure 12 shows a circuit diagram to simulate the switching by
M2

MOSFETs and Figure 13 shows the result. In this result, aSCT3120AL

waveform with almost no ringing is monitored.

GV2

C1100pF

CPROBE
4pF

LGROUND160nH

RPROBE
10MΩ

Probe(-)

Next, on the assumption that a probe is used for monitoring,
Figure 14 shows a circuit diagram where the simulation isPULSE(0 18 0 1n 1n 1u 2u 0)

performed with the addition of the equivalent circuit in Figure 3.

Figure 14. Simulation circuit with the addition of an equivalent

The result is shown in Figure 15. The ground lead is assumed tocircuit to the probebe 160 mm in length, having 160 nH of inductance. A large
ringing occurs in the waveform, showing a major difference fromthe original waveform.

150Although the above is one example, it can be seen that the100

Voltage (V)equivalent circuit added by the measurement system affects themonitored waveform.

500V1100V

M1SCT3120AL

L1500μH-50Probe(+)

6.5

M2SCT3120AL

GV2

C1100pF

7.0

7.5
Time (μs)

8.0

8.5

Figure 15. Waveform resulting from the simulation in Figure 13
Probe(-)

PULSE(0 18 0 1n 1n 1u 2u 0)

Figure 12. Circuit to simulate the switching by MOSFETs© 2019 ROHM Co., Ltd.

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NoticeNotes1) The information contained herein is subject to change without notice.
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