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TO-247N(-4L) Half-Bridge Evaluation Board
User’s Guide
© 2022 ROHM Co., Ltd.
No. 63UG0
60E
Rev.001
2022.2
The non-discharge snubber circuit is ideal for high-frequency switching circuits because only the surge exceeding the high-
voltage input HVdc is consumed by the resistors in the snubber circuit. However, since the pattern layout is complicated, it
should be used on boards with four or more layers.
The power
P
SNB
consumed by the resistor in the snubber circuit is entirely consumed by the resistor
R
SNB
in the snubber
circuit. The following equation applies:
Here,
L
MAIN
is the wiring inductance of the main circuit up to the bulk capacitor,
I
MAIN
is the drain current of the MOSFET at turn-
off, and
f
SW
is the switching frequency of the MOSFET.
On the other hand, the capacitance
C
SNB
of the snubber capacitor is calculated from the energy stored in the inductance by
the following equation
Where V
HVdc
is the high-voltage power supply and V
SURGE
is the maximum surge voltage.
Furthermore, the resistance of
R
SNB
can be obtained by the following equation.
This maximizes the surge absorption effect by discharging all the energy absorbed by
C
SNB
during one cycle of the MOSFET.
Figure 22 shows the verification results of the snubber circuit of the SCT4018KR from double-pulse testing. During turn-on,
there is no difference between with and without snubber circuit, but during turn-off, the V
DS
turn-off surge is reduced from 1207 V
to 1070 V. On the other hand, there is little effect on the switching
speed.
11. Protection Circuit for Gate-Source Signal
𝑃
SNB
=
𝐿
MAIN
× 𝐼
MAIN
2
× 𝑓
SW
2
𝐶
SNB
=
𝐿
MAIN
× 𝐼
MAIN
2
𝑉
SURGE
2
− 𝑉
HVdc
2
𝑅
SNB
<
−1
𝐶
SNB
× ln[(𝑉
SURGE
− 𝑉
SNB
)/(𝑉
SURGE
)]
×
1
𝑓
sw
Figure 22 Effect of snubber circuit
(SCT4018KR)
