Application Note
Power switching device

Precautions during
measurementgate-source

voltagePower devices such as MOSFETs and IGBTs are used as switching devices in various power supply applications. Due to the steep voltage andcurrent slopes present during the switching transients of a SiC MOSFET device, the measurement of the gate-source voltage should be performed
carefully. As described in “Gate-source voltage behaviour in a bridge configuration” [1] application note, it is necessary to determine with accuracy
the voltage peaks between gate and source, so that the limits specified in the data sheet are not violated. This application note will explain the
precautions that need to be taken during the gate - source voltage measurement.

General measurement method

We conducted a double pulse test with the half bridge
configuration shown in Figure 2 using this measurement

In many cases, the device terminals are not accessible tomethod. ROHM SCT3040KR SiC MOSFETs were selected forthe probes due to layout and space constrains of the circuit

the high-side (HS) and low-side (LS) switches. The LS switch

itself. Because of that, it might be necessary to solder additional

was kept permanently OFF (0V) and the HS side was switched.

cables to the terminals to connect the probes.

The cable extension shown in Figure 1 is soldered to the gate

Figure 1 shows an example of a voltage probe connected toand source terminals.

a cable extension in our evaluation board (P02SCT3040KR-EVK-001). The extension cable (12cm long) for the voltage
probe is soldered to the terminals of the device under test
(DUT), and the cables are twisted, in order to minimize picking

VG

up radiated noise.

(HS)

RG_EXT

E

High
Side

L

VSW
(LS)

RG_EXT

Low
Side

Figure 2. Double pulse test circuit

Measured gate-source voltage waveforms are shown in
Figure 3. The effect of using extension cables with RG_EXT = 10
Ohm is negligible. However, if RG_EXT is reduced to 3.3 Ohm,

Figure 1. Gate- source voltage measurement using cablethe measured waveforms clearly show some ringing due to theextension© 2020 ROHM Co., Ltd.

noise from the steeper voltage and current slopes and/or

1/11

No. 62AN085E Rev.002
Apr.2020

Precautions during gate-source voltage measurement
induced by high-frequency operation.

Application Noteconnected between the probe and the DUT will cause a

The extension cable in the system will cause changes in theresonance between the stray inductance LEXT and the input

frequency band due to its additional impedance. This will result

capacitance C of the voltage probe. This resonance willin a distortion of the original waveform if a probe is connected tosuperimpose high frequency voltage ringing to the originalthem.

voltage waveform. Thus, the measured surge voltage will behigher than the one at the device terminals.
VDS=800V, ID=50A

25

20

35

10Ω

15

30

10

25

5

Differential
Probe

20

HS

0

15

-5

10

-10

5

LS

-15

0

-20

-5

-25

-10

-30
18.8

-15
18.9

19.0
Time [μs]

19.1

DUT
VGS_LS [V]

VGS_HS [V]

40

3.3Ω

Extended
cable

LEXT

19.2

FG
VDS=800V, ID=50A

HS

3.3Ω
10Ω

15

Figure 4. Equivalent circuit of differential probe
40

35

Connecting probes

30

10

25

As previously explained, the measured waveform will be520

significantly affected by the way the voltage probe is connected0

15

to the device. To verify the differences of the measurement-5

10

results depending on how it was connected, following cases

5

were studied:

-10

LS

-15

0

-20

-5

-25

-10

-30
21.40

21.50

21.60
Time [μs]21.70

VGS_LS [V]

VGS_HS [V]

20

Probe
Body

LEXT

(a)
25

Probe
Tip

-15
21.80

(a)

Directly connect probe tip to the device terminal(b)

Use twisted extension wire to connect probe tip:
A 12-cm long twisted extension cable was soldered
to the DUT terminals. The voltage probe was(b)connected to the other end.

Figure 3. Gate-source voltage waveform(c)

Use long twisted extension wire with 100 Ω:
The 100 Ω resistor was placed in the middle of the(a)Turn-on, (b)Turn-off

twisted cable used in (b).
(d)

Figure 4 describes the equivalent circuit of a differential

The cable used in (c) was shortened to 4 cm andvoltage probe connected to the DUT through an extension

connected between DUT and voltage probe.

cable [2, 6]. The added impedance of the extension cable© 2020 ROHM Co., Ltd.

Use short twisted extension wire with 100 Ω:

2/11

No. 62AN085E Rev.002
Apr.2020

Precautions during gate-source voltage measurement

Application Note

Figure 5 shows actual extension cables used in themeasurement and Figure 6 shows how the probe tips wereconnected.

(b)

(c)

Figure 5. Extension Cables(d)Figure 6. Voltage probe connection(b)Long extension cable,(a)(c)Long extension cable +100 Ω,

Figure 6. Voltage probe connection(d)Short extension cable +100 Ω(a)Directly

Figure 7 shows the gate-source voltage waveforms forturn-on and turn-off using the connections from Figure 6 (a) to(d).
The LS waveforms show major differences depending on
the measurement conditions. With the measurement condition© 2020 ROHM Co., Ltd.

3/11

No. 62AN085E Rev.002
Apr.2020

Precautions during gate-source voltage measurement

Application Note(a) there is a loop formed by the probe tips, probe cables and(c) and (d) do not show a negative peak, which is closer to thethe device, see Figure 8. There will be a magnetic flux change

actual behaviour of the device [3].

in this loop during the switching transient when HS MOSFET

With condition (b), on the other hand, a significant ringing

turns on.

can be observed. This comes from the ringing induced by the

The waveforms show that for method (a) there is a negative

added impedance of the extension cable.

voltage peak first. This peak is the result of the current inducedby the magnetic flux change, which charges the capacitance of
the LS probe body, see Figure 4(c). Measurement methods

VDS=800V, ID=50A, RG_EXT=3.3ohm

50

25

45

20

40

15

35

10
5

30

HS

0

(a)Directly

25(b)Long extension cable20(c)Long extension cable+100Ω

-5(d)Short extension cable+100Ω

-10
-15

15

10

LS

5

Figure 8. Electromotive force induced during commutation
VGS_LS [V]

VGS_HS [V]30

of the power device

-20

0

-25

-5

Therefore, it is necessary to minimize the loop formed by the

-30

-10

probe tips and gate-source terminal connections to reduce the

-35
18.8

-15
18.9

19.0
Time [μs]19.1

influence of the magnetic flux variation due to the commutation

19.2

of the device current in the measurement. One way of achieving(a)VDS=800V, ID=50A, RG_EXT=3.3ohm

3020(a)DirectlyHS50

directly to the device terminals.

45(b)Log extension cable

15

minimized by soldering the tightly twisted extension cables(c)Long extension cable+100Ω

40(d)Short extension cable+100Ω

35

Selecting the measurement point

10

30

5

25

0

20

-5

15

connection to the device terminals is not always possible due to

-10

10

layout or space restrictions. As a result, the selected

-15

LS

5

-20

0

-25

-5

-30

-10

-35
21.40

21.50

21.60
Time [μs]21.70

When measuring power devices usually the probes areconnected to an easily accessible point of the circuit. A direct

VGS_LS [V]

VGS_HS [V]25

this can be seen in Figure 6 (b) to (d), where the loops aremeasurement point might not be the optimal one. To verify this,four different measurement methods have been selected:(a) Probe tip adjacent to the DUT mold package

-15
21.80

(b) Connection to the PCB soldering pad of the DUT(b)Figure 7.

(c) Connection to a test pin soldered to the PCB(d) Like (c) + twisted wire + 100 ΩComparison of waveforms with different
voltage probe connection methods(a)Turn-on, (b)Turn-off© 2020 ROHM Co., Ltd.

4/11

No. 62AN085E Rev.002
Apr.2020

Precautions during gate-source voltage measurement

Application Note

Test
Pin

PCB

PCB

(a)

(c)

Solder side

DUT

VDS
Probe
Head

VGS
Probe
Head

VGS
Probe
Head

Switching
Current

DUT

Inductor

PCB

PCB
(b)
(d)
Figure 9. Measurement point(a)Direct to lead, (b)Tip of the lead, (c)On PCB, (d)On PCB＋twisted cable＋100 Ω© 2020 ROHM Co., Ltd.

5/11

No. 62AN085E Rev.002
Apr.2020

Precautions during gate-source voltage measurement
Figure 9 describes the different measurement methods.

Application Note

Figure 10 shows the measurement results. The difference

Measurement method (a) has the shortest distance from thebetween the different methods is clear.

probe tip to the DUT chip. In method (b), the probes are

Method (a), with the probe tip close to the DUT, shows aconnected to the bottom side of the PCB, where the DUT pins

stable waveform. Method (b) catches the EMF from theare soldered. For measurement method (c) a test pinmeasurement network through the package leads. Methods (c)located close to the DUT was used. Method (d) uses theand (d) collect the noise through the closed loop formed by thesame point as (c) but the probe is connected through a

DUT and the test pin.

twisted wire with a damping resistor to the DUT.
VDS=800V, ID=50A, RG_EXT=3.3ohm

select the measurement point as near to the DUT as possible.
50

25

45

20

40

15

35

10

30

5

25

0

HS

-15

(c)on PCB

15

power switching devices, attention is needed when placing the(d)On PCB+Twisted cable+100Ω

10probe. The influence of the magnetic flux change will be

5

LS

-20

0

-25

-5

-30

-10

-35
18.8

-15
18.9

the probe tip can be critical.
Due to the noisy environment generated by high-speed(b)Tip of the lead

-10

Besides the measurement point itself, also the placement of

20(a)Direct to lead

-5

Probe tip placement

19.0
Time [μs]19.1

VGS_LS [V]

VGS_HS [V]30

It is clear that, according to these results, it is desirable tosuperimposed to the measured waveforms if the tip is notcarefully placed.
To understand this better, four different placement locations
and connection methods have been tested:

19.2(a) Probe placed inside of the main circuit loop(a)VDS=800V, ID=50A, RG_EXT=3.3ohm

3020(a)Direct to leadHS50(b)Tip of the lead

45(c)On PCB

40(d)On PCB+Twisted cable+100Ω

15(c) Probe out of the main circuit loop connected through 12
cm twisted wire to the PCB(d) Same as (c), but with an additional 100 Ω damping
resistor

35

10

30

5

25

0

20

corresponding current path of the main circuit loop. Placement-515(a) exposes the probe to the maximum dφ/dt. In (b) the probe is

-10

10

placed at the edge of the main loop, where d φ/dt is still

-15

5

LS

-20

0

-25

-5

-30

-10

-35
21.40

21.50

21.60
Time [μs]21.70

Figure 11 shows each of these placement methods and the

VGS_LS [V]

VGS_HS [V]25(b) Probe placed outside of the main circuit loop

relatively large. In the case of (c), the probe is placed away fromthe main loop to minimize dφ/dt influence. Method (d) adds 100
Ω damping resistance to the cable connection.

-15
21.80

(b)
Figure 10. Waveform comparison for different measurement
points(a)Turn-on, (b)Turn-off© 2020 ROHM Co., Ltd.

6/11

No. 62AN085E Rev.002
Apr.2020

Precautions during gate-source voltage measurement

Application Note

PCB

PCB

(c)

(a)

damping
resistor

PCB

PCB

(d)

(b)
Figure 11. Probe placement(a)Inside main circuit loop, (b)outside main circuit loop,(c)outside using extension cable, (d)apart using extension cable +100 Ω© 2020 ROHM Co., Ltd.

7/11

No. 62AN085E Rev.002
Apr.2020

Precautions during gate-source voltage measurement
Figure

12

shows

the

different

gate-source

Application Note

voltage

The test was conducted by switching the HS device andwaveforms for the aforementioned placement methods. It cankeeping the LV device off (0 V). Since the LS switch has nobe appreciated that the voltage surge at the LS device is thedriving current flowing into the gate it is more susceptible to thelargest for method (a), where the flux change rate is at its

flux change than the HS device, which is currently being turned

maximum. In the case of (c), this shows a larger fluctuation thanon. This results in more stable waveforms for the HS device.

(b) due to the added inductance of the extension cable. The 100
Ω damping resistance helps in diminishing the amplitude of the

Precaution with bridge configuration

oscillations, as it can be seen in (d).
VDS=800V, ID=50A, RG_EXT=3.3ohm

45

voltage measurements of power devices in half-bridge

20

40

configuration.

15

35

10

30

0

(a)Inside main circuit loop

25(b)Outside main circuit loop20(c)Apart using extension cable-5(d)Apart using extension cable+100Ω

-15

5

LS

35

10

30
Switching
side

25

0

HS switching

20-5

LS switching

15

-20

0

-25

-5

-30

-10

-20

0

-15

-25

-5

-30

-10

-35
18.8

18.9

19.0
Time [μs]

19.1

-10
-15

19.2

HS

(a)Inside main circuit loop(c)Apart using extension cable15(d)Apart using extension cable+100Ω

10

25

0

20

-15

15
10

5

LS

-20

0

-25

-5

-30

-10

-35
21.40

20

35

5

-10

21.50

21.60
Time [μs]

21.70

19.0
Time [μs]

19.1

19.2

VDS=800V, ID=50A, RG_EXT=3.3ohm

25

30

-5

-15
18.9

(a)
40

(b)Outside main circuit loop

545

VGS [V]20

VDS=800V, ID=50A, RG_EXT=3.3ohm

VGS_LS [V]25

10Commutation
side

-35
18.8

(a)

VGS_HS [V]

40

15

5

10
VGS_LS [V]

-10

15

20

VGS [V]HS

45

Switching side1540

LS switching

35

10

30

5

25

0

20

-5

15

-10

10

-15

5

-20
-25
-30

-15
21.80

0
Commutation
side

-35
21.40

(b)

45

HS switching

21.50

VGS [V]5

VDS=800V, ID=50A, RG_EXT=3.3ohm

25

VGS [V]25
VGS_HS [V]High voltage differential probes are commonly used for

-5
-10
21.60
Time [μs]

21.70

-15
21.80

(b)
Figure 13. VGS waveform when HS/LS switching

Figure 12. Waveform comparison for different probe placement(a)Turn-on, (b)Turn-off

locations(a)Turn-on, (b)Turn-off© 2020 ROHM Co., Ltd.

8/11

No. 62AN085E Rev.002
Apr.2020

Precautions during gate-source voltage measurement

Application Note

However, it is necessary to be aware of the distortion

VDS [V]

VGS measurement is particularly critical, as the distortion

15

500

12.5

10

HS
0

the

waveforms

for a

7.5

-250

half-bridge-500

configuration with either the HS or the LS device switching. The

5
2.5

HS

-750

instrument corresponds to a YOKOGAWA 701297 active0LS

differential probe (150MHz, 1400V). It can be appreciated thatthe ringing of the commutation side device is higher when the

-1000

-2.5

-1250

-5

-1500
18.8

LS device is switched. This is caused by the limited CMRR

-7.5

18.9

19.0

19.1

19.2

19.3

19.4

Time [μs]when the device is switching at a dV/dt in the range of 20 to 50
V/ns.

(a)

Figure 14 shows CMRR performance of the differentialofProbes”

[2]

15

500

12.5

250

are connected together to the source terminal. Please refer to
“ABCs

750

0

for

10

LS

7.5

-250

measurement method detail.

5

-500

Drain-source voltage VDS switches at about 200 kHz (Duty

2.5

LS
-750

50%) on 800V DC and HS and LS VDRIVER SOURCE is
-1000

0

HS

-2.5

synchronized to VDS change. The LS one can keep zero (0)
-1250

-5

VDRIVER SOURCE [V]

VDS [V]and LS of respectively. The plus and minus tips of that probe

note

17.5

HS

and turn-off (b). It is measured voltage of source terminal of HS

application

RG_EXT=10ohm, ID=50A

1000

probe by VDRIVER SOURCE measurement result during turn-on (a)Tektronix

VDRIVER SOURCE [V]the measured signal.
shows

750

250

through CM voltages can be of the same order of magnitude of

13

17.5

LS

mode rejection ratio (CMRR) in the high frequency band. The

FigureRG_EXT=10ohm, ID=50A

1000

introduced to the voltage waveforms due to the limited common

voltage level after switching, see Figure 14(a). On the other
-1500
21.4

hand, the HS switch shows an offset of a few volts on

-7.5

21.5

21.6

21.7

VDRIVER_SOUCE, which is positive before the LS turn-off andTime [μs]negative after it. This is just a CMRR error. The LS(b)

VDRIVER_SOURCE voltage shows a positive surge when LS VDS

21.9

22.0

Figure 14. Isolated voltage probe CMRR characteristics

falls and a negative surge when it rises. However, that may(a)Turn-on, (b)Turn-offchange depending on the differential probe characteristics.

© 2020 ROHM Co., Ltd.

21.8

9/11

No. 62AN085E Rev.002
Apr.2020

Precautions during gate-source voltage measurement

Application Note

Optically isolated differential probes offer an improved

VDS=800V, ID=50A, RG_EXT=10ohm

30

VGS [V]measurements. Figure 16 shows a comparison of gate-source
voltage measurements performed with an optical probe(Tektronix IsoVu®) and a traditional differential probe.

35

15

30

10

25

20

HS

0

connector

40

Optical Probe

20

5

MMCX

45

Isolation Probe

25

15

CMRR Error

HS VGS-5

10

Probing

-10

5

-15
-20

0

LS

-5

-25

LS VGS

-10

-30
18.8

Probing

VGS [V]CMRR rating, which can lead to distortion-free high-side VGS

-15
18.9

19.0
Time [μs]

19.1

19.2

(a)
30

cable

25
VGS [V]

OpticalProbe

Figure 15. Optically-isolated probe (bottom) and traditional
isolated differential probes (top) connection

HS

Optical Probe

CMRR Error

The traditional differential probe was connected with a short

35
30
25

5

20

0

15

-5

10

-10

5
0

LS

-30
21.40

points as the traditional probe, see Figure 15.

40

10

-5

-25

the optical probe was connected to the same measurement

45

15

-20

the measurement lacked of proper MMCX connection interface,

Isolation Probe

20

-15

Since the PCB board (P02SCT3040KR-EVK-001) used for

VDS=800V, ID=50A, RG_EXT=10ohm

VGS [V]

coaxial

-10
21.50

21.60
Time [μs]

21.70

-15
21.80

(b)

extension cable and 100 Ω damping resistor to the PCB, as
this minimizes the flux change influence on the measurement.

Figure 16. VGS waveform when HS switching(a)Turn-on, (b)Turn-off

The results show that the measurement with the nonoptically isolated probe has some distortion, exceeding 18 V
when turning on, and falling below 0 V when turning off. The
optical probe, on the other hand, shows accurate 0 V and 18 V
levels without ringing, which match the gate driver setup.

© 2020 ROHM Co., Ltd.

10/11

No. 62AN085E Rev.002
Apr.2020

Precautions during gate-source voltage measurement
Optically Isolated Probe
Traditional Differential Probe

180

Reference:*1 「Gate-source voltage behaviour in a bridge configuration」

160

Application Note (No. 60AN135ERev.001）

CMRR [dB]140

RHOM Co., Ltd., May, 2018

120

*2 「ABCs of Probes」

100

Application Note (No. EA 60W-6053-14）

80

Tektronix, January, 2016

60

*3 「Improvement of switching loss by driver Source」

40

Application Note (No. 62AN040ERev.001）

20

ROHM Co., Ltd., October, 2019

00.0001

Application Note

0.001

0.01

0.1

1

10

100

*4 「Complete ISOLATION Extreme COMMON MODE

1000

ｆrequency [MHz]REJECTION」
White Paper (0/16 51W-60485-1)

Figure 17. CMRR frequency response characteristics

Tektronix, 2016

Tektronix ; MMCX50 (IsoVu®)*5 「User’s Manual Model 701927 PBDH0150 Differential

YOKOGAWA ; 701927

Probe」
User’s Manual (IM701927-01EN, 6th Edition)

The improved results obtained with the optical probe can beeasily explained by the higher value of the CMRR at highfrequencies, as shown in Figure 17 [4, 5].

Yokogawa Test & Measurement Corporation, March, 2018
*6 「WaveLink Medium Bandwidth(8-13GHz) Differential Probe」
Operator’s Manual（924243-00）
TELEDYNE LECROY, May 2014

Summary
In order to improve the measurement accuracy with

IsoVu® is registered trademark of Tektronix, Inc.

differential probes, following measures can be taken:
 Minimize the loop created by the probe tip and cables.
 Minimize the loop created between the device
terminals and the measurement point.
 Minimize the influence of the flux change from themain circuit when placing the probe.

The results shown in this paper indicate that the solution
with the most accurate output is the use of an extension cableplus damping resistor to connect the probe leads. The cableshould be connected directly at the device terminals. Thisshouldbeadaptedforeach

individual

measurement

configuration.
It is expected that with the help of this document, the user ofhigh-speed power devices is able to judge in a more critical way
the measured voltage waveforms and avoid possible error
sources.
© 2020 ROHM Co., Ltd.

11/11

No. 62AN085E Rev.002
Apr.2020

NoticeNotes1) The information contained herein is subject to change without notice.
2) Before you use our Products, please contact our sales representative and verify the latest specifications :3) Although ROHM is continuously working to improve product reliability and quality, semiconductors can break down and malfunction due to various factors.
Therefore, in order to prevent personal injury or fire arising from failure, please take safetymeasures such as complying with the derating characteristics, implementing redundant andfire prevention designs, and utilizing backups and fail-safe procedures. ROHM shall have no
responsibility for any damages arising out of the use of our Poducts beyond the rating specified by
ROHM.
4) Examples of application circuits, circuit constants and any other information contained herein areprovided only to illustrate the standard usage and operations of the Products. The peripheral
conditions must be taken into account when designing circuits for mass production.
5) The technical information specified herein is intended only to show the typical functions of andexamples of application circuits for the Products. ROHM does not grant you, explicitly or implicitly,
any license to use or exercise intellectual property or other rights held by ROHM or any otherparties. ROHM shall have no responsibility whatsoever for any dispute arising out of the use ofsuch technical information.
6) The Products specified in this document are not designed to be radiation tolerant.
7) For use of our Products in applications requiring a high degree of reliability (as exemplified
below), please contact and consult with a ROHM representative : transportation equipment (i.e.
cars, ships, trains), primary communication equipment, traffic lights, fire/crime prevention, safety
equipment, medical systems, servers, solar cells, and power transmission systems.
8) Do not use our Products in applications requiring extremely high reliability, such as aerospaceequipment, nuclear power control systems, and submarine repeaters.
9) ROHM shall have no responsibility for any damages or injury arising from non-compliance withthe recommended usage conditions and specifications contained herein.
10) ROHM has used reasonable care to ensurH the accuracy of the information contained in thisdocument. However, ROHM does not warrants that such information is error-free, and ROHM
shall have no responsibility for any damages arising from any inaccuracy or misprint of suchinformation.
11) Please use the Products in accordance with any applicable environmental laws and regulations,such as the RoHS Directive. For more details, including RoHS compatibility, please contact a
ROHM sales office. ROHM shall have no responsibility for any damages or losses resulting
non-compliance with any applicable laws or regulations.
12) When providing our Products and technologies contained in this document to other countries,
you must abide by the procedures and provisions stipulated in all applicable export laws andregulations, including without limitation the US Export Administration Regulations and the Foreign
Exchange and Foreign Trade Act.
13) This document, in part or in whole, may not be reprinted or reproduced without prior consent of
ROHM.

Thank you for your accessing to ROHM product informations.
More detail product informations and catalogs are available, please contact us.

ROHM Customer Support System
http://www.rohm.com/contact/

ZZZURKPFRP652+0&R/WG$OOULJKWVUHVHUYHG

5%