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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
11.1 Types of protection circuits
This board has a built-in protection circuit that absorbs surge voltage generated at the gate-source pin of the MOSFET. The
protection circuit has three functions (clamp circuit) and four countermeasure circuits as shown in Table 7.
Since the gate-
source voltage is greatly affected by the change in V
DS
and I
D
during switching operation, it is necessary to suppress the surge
voltage appropriately with these protective circuits. For more information on gate-source voltage behavior and surge
suppression methods, please refer to the application notes (*1) and (*2) that have been published separately.
Figure 23 shows the circuit diagram.
Table 7 Protection circuit and operation details
Item
Function (clamp
circuit)
Target
component
Operation details
Default
setting
(I)
Positive surge
suppression
D53, C51
D153, C151
Positive surge that occurs when the V
DS
change of the
MOSFET is completed.
During turn-on, it may exceed the maximum V
DS
rating, so it
is clamped to Vcc2 by D53 (D153). C51 (C151) is a bypass
capacitor and is laid out in the vicinity of D53 (D153).
Mounting
(II)
Negative surge
suppression
D54, C64
D154, C164
During turn-off operation of the MOSFET in bridge
configuration, a negative surge occurs in V
GS
of the non-
switching side MOSFET, which may exceed the maximum
rating. Therefore, it is clamped to the turn-off drive voltage
(VEE2). C64 (C164) is a bypass capacitor and is laid out in
the vicinity of D54 (D154).
Mounting
(III)
Self-turn-on prevention
Self-turn-on surge
suppression
R80
R180
During turn-on operation of the MOSFET in bridge
configuration, V
GS
of the non-switching side MOSFET may
rise (linked to the rise of V
DS
) and turn on when the gate
threshold V
GS(th)
is exceeded (so-called self-turn-on).
Therefore, this rise of V
GS
is suppressed by clamping it to
the turn-off drive voltage using the MC signal of the driver
IC (BM61M41RFV-C).
Mounting
(IV)
C52
C152
As the Crss/Ciss ratio of the MOSFET becomes larger, V
GS
tends to rise. (Because the charging current is proportional
to Crss)
By adding capacitance in parallel with Ciss and reducing
the Crss/Ciss ratio, VGS can be prevented from rising
above V
GS(th)
. The larger the capacitance value to be added,
the smaller the amount of rise becomes, but the additional
capacitance must be determined while considering heat
generation, because it not only requires more drive
capability but also involves an increase in switching loss.
Not
mounted
Figure 23 Protection circuit for gate-source signal (PCB004P)
(I)
(III)
(II)
(IV)
