NEC LCD1550X Service Manual page 42

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MB3778PFV-G-BND-EF (IC608) Study of DC/DC power MOS FET drive circuit
Power MOS FET : Vishay Siliconix product P-Channel MOS FET : Si3457DV (CP260P470A11) (IC607,
IC609)
[Rating]
V
= -30V, I
(DC)= -3.4A (Ta=70˚C), I
DS
D
[Specifications]
on resistor R
= 0.1Ω (V
DS(on)
t
(on) = 15nsec (Max.)
d
t
(off) = 50nsec (Max.)
d
C
= 11000nsec
iss
The turn On + turn Off time is 65nsec, and is sufficiently small compared to the switching cycle
5000nsec (oscillation frequency 200kHz). (There is little switching loss.)
1/10W 3.3k-J
MB3778
(IC608)
Open corrector output
10
(
7
Output drive terminal
V
CE (sat)
(Io
MAX
Max.:1.4V
Typ.:1.1V
When switching is On, the power MOS-FET gate charge is pulled by the open collector transis-
tor (base Hi) built-in the DC/DC IC (MB3778) to set the gate voltage V
(The DC/DC open collector transistor operates actively, so the power MOS-FET turns ON at a high speed.)
On the other hand, the turn OFF time slows during OFF due to the open collector's pull-up
resistor R1 (base L
). To charge the power MOS-FET gate, Tr1 in this drive circuit turns ON
o
when the base voltage has been increased by 0.6V in respect to the emitter by the pull-up
resistor R1. It then turns OFF at a high speed as the power MOS-FET gate is charged by Tr1's
active operation.
(Tr1 can be turned OFF at a high speed because the pull-up resistor is bypassed.)
The drive circuit consumes power only when the consumption power is turned ON and OFF by
the Tr1 and R2, and thus can be ignored. When the power consumed by the pull-up resistor
R1 during ON is considered, the following is attained.
Current consumption (during ON) = (12-V
Power consumption = 3.3mA (at ON) x (12-V
The DC/DC IC (MB3778) used with this circuit has a two-channel output, so there are two
driver circuits. The I
CC
(=2.4mA x 12V), and the total power consumption including the driver circuit is 64.8mW
(=18mW x 2Ch+28.8mW).
(PULSE)= -10A, P
D
= -4.5V, I
= -3.4A)
GS
D
Power MOS-FET (IC607,IC609)
+12V
At Off
R1
Tr1:2SC2412K-R (Q601,Q602)
)
On time
= 50mA)
1/10W 10-J
D614, D615
V
= 0.3V (Typ.)
F
of the DC/DC IC is 2.4mA, so the power consumption is 28.8mW
= 1.3W (Ta=70˚C)
D
Voltage between Gate and Source (5V/Div)
Drain current (0.5A/Div)
Tr1:2SC2412K-R (Q601)
R2
)V / 3.3kΩ = 3.3mA
CE(sat)
)V x 0.5 (On Duty) = 18mW
CE(sat)
1 - 36
Voltage between Collector
and Emitter
V
(5VDiv)
CE
Corrector current Ic (1A/Div)
Collector loss
Pd
to 12V- (V
+V
).
GS
CE(sat)
F

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