Toshiba CF35E50, Technical Training Manual

Page: 102 / 139

102
6. ACTUAL CIRCUIT
Two MOS FET’s, driver which drives FET’s and frequency control IC are combined inside HIC (Q801 ).
Converter transformer T862 is designed to have loose coupling between the primary and the secondary, and to have
some extent of leakage inductance. This is the reason why L and C (leakage inductance and resonant capacitor) are
resonated during period that rectifying circuit (diode) connected to the secondary winding conducts. Rectifying
circuit of the secondary winding uses double wave rectifier considering current balance of switching device, because
converter transformer is driven in push-pull. The function of STR-Z3201 is explained below. Fig. 14-6 shows
block diagram and figure 14-7 shows waveforms at main terminals.
<<FUNCTION OF HIC>>
(1) Output switching element
Uses two power MOS FET’s, and operates in push-pull. Voltage across the switching element does not increase more than
power line voltage basically, and therefore, element of low rating voltage (enduring 200V) is used.
(2) Driving circuit
Drives output switching element. MOS FET is specially used for driving element of high side, and to drive this, bootstrap
circuit is equipped.
(3) Dead time
To avoid that two switching element turns ON at the same time in push-pull operation, dead time is arranged.
(4) CT terminal (Pin 5) About basic oscillation
Variable frequency oscillator is equipped inside frequency control IC, charge and discharge of the capacitor C862
connected to this terminal decide oscillation frequency and dead time. This oscillator generates triangle wave signal with
low level of 2.5V(TYP) and with high level of 4V(TYP). Charging time of oscillator becomes output-on period, and
discharging time becomes dead time.
(5) RT terminal (Pin 5) About lowest oscillation frequency
Lowest oscillation frequency is decided by capacitor C862 connected to pin 4 and resistor R867 connected to this terminal.
(6) CONT terminal (Pin 6) About frequency control
Current flowing out of this terminal varies charging current of oscillating capacitor C862. Therefore, flowing of CONT
terminal current corresponding to feedback quantity from photo coupler (Q862) varies charging time of C862 and controls
oscillation frequency. Maximum oscillation frequency is decided by R864 connected to CONT terminal.
(7) Css terminal (Pin 8) About soft start
Capacitor (C866) and resistor (R863) for soft start are connected to make TV start at high frequency in the time of power
on and gradually make frequency lower. This function suppresses rush current in POWER MOS FET output and provides
stable starting of TV.
(8) CD terminal (Pin 9)
Latch circuit detects abnormal operation to hold the status of operation seizing, and if following condition as a result of
detecting abnormal operation comes, the latch circuit begins to start.
*In operation of over voltage protection (OVP) circuit
*In operation of thermal shock detection (TSD) circuit
*In operation of over current protection (OCP) circuit
*In going down and no recovery of Main +B output voltage
Until latch function begins to operate, the charging time of capacitor C869 connected to CD terminal (Pin 9) is utilized
to produce delay time. To release the latch function after operating once, turn off power and turn on again.
(9) OC terminal (Pin 12) About over current protection (OCP) function
This is to detect current in LC series resonant circuit, and to suppress over current to stop operation.
(10) Over voltage protection (OVP) circuit
This is to make latch circuit operate when voltage at Vcc terminal (Pin 10) exceeds 22V (TYP).
(11) Thermal shock detection (TSD) circuit
This is to make latch circuit operate when temperature inside IC exceeds 150°C.