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TO-247-4L Half-Bridge Evaluation Board
User’s Guide
©
2019 ROHM Co., Ltd.
No. 61UG047E Rev.001
August.2019
15
．
Precautions when measuring the Gate-Source signal
Drain current di/dt of the MOSFETs with driver source pin is in several A/ns level. Thus, the generation of radiation noise is quite
big. Therefore, pay attention to the connection of the voltage probes when measuring comparatively low voltage signals. Figure 36
shows the influence of the voltage probe connection on the measured waveforms.
The comparison is done between two types of connections
when connecting an isolated probe, one is when there is a “Big Loop
S
ensing” where there is space created between the (+) and (-) of the clips, and the other one is when the sensing wire is directly
soldered to the device’s package root with a 100Ω damping resistor. This 100Ω damping resistor removes the voltage oscillation
caused by the sensing wire inductance and does not affect the actual measurement waveforms.
The commutation side Vgs (Vgs_LS) surge polarity is different depending on measurement method. Originally, at turn on, the
commutation side current decreases, Vf of the body diode decreases and Coss and Crss are charged. Since this charged current is
flowing from drain pin to gate pin, positive surge should occur in Vgs. However, in
”Big Loop Sensing”, a large surge is generated on
the negative side. This happens as the di/dt of the current flowing on the PCB creates the radiation noise, and the error in
measurement occurs when there is space between the probe connections.
Therefore, consider that the commutation side of Vgs positive surge can be correctly measured by soldering the sensing wire
directly to the
device’s package root in order to minimize the connection space. However, connection space also exists between
PCB and the device leads, that creates the radiation noise. Therefore, even a measurement by using a
50Ω coaxial cable with BNC
connector directly mounted on the PCB can still cause a measurement error.
Figure 36. Probe connection and measured waveforms
（
SCT3040KR with Vgs protection circuits
）
ON
OFF
V
gs_H
S
[V]
V
gs_LS
[V
]
V
ds_H
S
[V]
Id_H
S
[A
]
V
ds_
L
S
[V
]
Big loop sensing
100ohm damping
Big Loop Sensing
100ohm Damping
Damping
resistor
Damping
resistor
sensing of 100ohm damping
minimize
loop areas
Negative surge
is disappeared.
DUT
DUT
DUT
DUT
DUT
Time [s]
HS
LS
