Toshiba ESTUDIO170F, Service Manual

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7
January 2005 © TOSHIBA TEC e-STUDIO170F Circuit Description
7 - 71
7.6.1 Power Supply Voltage Generation Circuit
Fig.7-52
The power supply voltage generation circuit converts the AC voltage to DC voltages of +24V, ±12V and
+5V and delivers them to the entire system.
When the power switch of this machine is turned on, an AC voltage is input from the inlet. The input AC
voltage is passed through the noise filter, rectified to a DC voltage by the diode bridge (DB1), and then
smoothed by C10.
When the DC voltage is applied, voltage is applied to the gate of Q2 through R7, R8, R10, and R14 to
turn on Q3 and current flows through winding Np in the primary circuit of the transformer (T1). Then
voltage is produced in winding Nb and voltage is applied to the gate of Q3 through R20 and R21 to
maintain the on state of Q3. At the same time, current begins to flow in the base of Q2 through R19 and
ZD1.
As Q3 continues to maintain its on state, the voltage produced from Nb rises. Then, the base current of
Q2 also increases to turn on Q2. This causes the gate voltage of Q3 to drop to turn off Q3. When Q3
turns off, the energy accumulated in the transformer allows current to flow from each winding in the sec-
ondary circuit.
After that, the current ends flowing as the accumulated energy runs out. However, residual energy is
present in each winding in the secondary circuit. This energy causes voltage to be produced in winding
Nb in the primary circuit to turn on Q3 again.
In this way, the switching operation performed by Q3 allows a high-frequency voltage to be produced in
each winding in the secondary circuit. The produced voltage is rectified by the diodes (D9, D10, D12)
and smoothed by the capacitors (C17, C18, C26, C29). Then, it is converted to stable DC voltages
(+24V, ±12V, +5V) by the regulators (IC2 to IC4) before being delivered to the system.
As the output load of the LVPS becomes larger, the time for the on state of Q3 becomes longer to
obtain the energy required. However, being determined according to the time constant of the equivalent
series resistance of R19 and ZD1, the maximum on time of Q3 does not extend beyond a certain value
and the output voltage drops for protection against overcurrent.
Q3
+
DB1
-
~
~
R7,R8,
R10,R14
Np
Nb
R19
LIVE
NEUTRAL
R20 R21
ZD1
T1
+5VPS
+5V
-12V
+12V
C/O-IN
IC4
1
2
12,13,15
4
5
2
CN1
+24VILS 10,11
+24V
14
3
6
7
D9
D12
C26,
C29
C18
C17
Q10
Q8
D10
IC3
Front cover
open switch
Main PBA
IC2
CN3
AC120V/
AC220-240V
LVPS
Power switch
Q2
PG
AG
SG
Inlet
Noise
filter
circuit
C10
CN4
CN5