Description Of Each Block Circuits - LG MW-60SZ12 Service Manual

1. Power Block
Circuit is to use the FLY BACK CONVERTER way for insula-
tion basically and composed of HYBRID IC STR F6658B for
switching power, which has a POWER MOSFET within, and
STR G6153 as the circuit for stand-by. And it is operated by
the ERROR AMP (IC802, SE105) which transfers electric sig-
nal in output part to the primary part with insulating primary
and secondary part.
1.1 Block Diagram (STR F6668B)
Vcc
5
START OVP
Internal Bias
Reg1 (9.4V)
Internal Bias
Reg2 (7.2V)
Internal Bias
Reg3 (7.2V)
TSD
Counter
1.2 Pin Description
No. Symbol Name
Over-current
/Feed-Back
1 O.C.P/F.B
terminal
Source
2 S
terminal
3 D
terminal
Voltage input
4 Vin
terminal
Ground
5 GND
terminal
1.3 Others
Symbol
O.V.P Internal Protect circuit against over-voltage
T.S.D Internal Protect circuit against overheat

Description of each Block Circuits

Latch
OSC
S
R
Q
FB OCP
Mirror
FB Current
Detector
4
FB
Function
Input signal of detecting
over-current & to input
signal for controlling con-
stant voltage.
MOSFET source
Drain MOSFET drain
Input control circuit power
Ground
Function
1.4 Function & Description of each terminals
(1) Vin terminal (4 PIN), Start circuit
The Start circuit performs the operation start and stop of con-
trol IC by voltage of Vin terminal (4 PIN).
In power start, C2 is charged through the start resistor Rs.
If the Vin terminal voltage reaches 16V (TYP), control IC
starts operating and current increases by the operation of
Start circuit. If this voltage decreases and reaches lower than
10V (TYP), the control IC stops and returns initial states by
the operation of UVLO (operation stop circuit).
The control circuit gains power by rectifying the trans wire
wound D after operation.
As the voltage of trans wire wound doesn't go up to the set
voltage just after operation, the Vin terminal voltage begins to
decrease. But as the voltage of operation stop is set to 11V
(Max), the voltage of trans wire wound reaches the set value
1
and control IC keeps operating while the Vin terminal voltage
D
Drive
decreases to the voltage of operation stop.
The Vin Start voltage (Terminal 4) is 16V (TYP), the voltage
of operation stop is 10V (TYP) and the Vovp voltage is 22.5V
Buffer
2
Amp.
(TYP).
OCP
3
GND
Vcc
(5pin)
Start Circuit
(2) OCP / FB terminal (Terminal 1), Oscillator
and Constant voltage control circuit
The oscillator is to use charge or discharge of internal con-
densor C1 and to select PRC (Pulse Ratio Control) which
fixes off time of MOSFET and changes on time of it.
The voltage regulator Vr4, which is generated when the FB
current through Photo Coupler flows in R4, is approved by
Vr5. Because of this, Comp. 1 reverses and needed voltage
value of Vr5 (peak value of Id) is supplied from Vr4(FB cur-
rent). Like this Output control is current mode control.
(3) OCP / FB terminal (Terminal 1) and OCP circuit
OCP circuit is the Protection circuit against over-current
which selects the pulse by pulse way. This way is to reverse
oscillator output by detecting peak value of MOSFET drain
current every pulse.
Detecting MOSFET drain current is done by connecting R5
with MOSFET source terminal (Terminal 2) and GND terminal
(Terminal 3) and by inputting regulated voltage to OCP/FB
terminal. The threshold voltage Vth (1) of OCP terminal is
about 0.73V compared with GND. External components R4
and R5 are Filter circuit to protect malfunction by surge cur-
rent which is generated in MOSFET On.
- 17 -
Icc
15mA(TYP)
13mA(TYP)
Internal Bias
Reg.
70µA(MAX)
Vcc(OFF) 15V
Vin terminal voltage waveform
Vcc
Vcc(ON) 20V