Operation - Toshiba TP43H60 Service Training

Operation

The main power supply is a current resonant switch-
ing power supply. Figure 2 is a basic block dia-
gram for this supply. The primary winding of T862
and capacitor C870 create an LC series resonant
circuit. An oscillator (OSC), drive circuit, and two
MOSFETs are located internally to Q801 (STR-
Z4117). The OSC determines the power supply's
switching frequency. The drive circuit alternately
switches the MOSFETs ON and OFF. The two
power MOSFETs, in a push-pull configuration,
alternate the current flow through the LC circuit
during normal operation. The alternating current
continually builds and collapses an electromagnetic
field around T862's primary windings. The col-
lapsing of the electromagnetic field induces cur-
rent into the secondary windings of T862. A full-
wave bridge rectifier converts the induced current
into 125Vdc.
To regulate the 125Vdc, an error amplifier moni-
D
O
r
S i
v
C
e
Q801
Figure 2
Operational Block Diagram
tors the voltage and supplies a negative feedback
to the oscillator through photo-coupler Q862.
Q862 isolates the primary side of the power sup-
ply from the secondary side.
Refer to Figure 3. The power supply's switching
frequency operates above the LC resonant fre-
quency. When the load on the secondary side of
the power supply increases and requires more cur-
125V +B
T862
C870
Error
Amp
Photo
Coupler
Z801
Q862
rent, the oscillator frequency decreases and oper-
ates closer to the LC resonant frequency. The closer
the switching frequency is to resonance, the higher
the current flow through the primary windings of
T862 and the larger the electromagnetic field. The
larger the electromagnetic field is when it collapses,
the higher the induced current is in the secondary
winding. When the load decreases and requires
less current, the switching frequency increases and
moves away from resonance. As a result, less cur-
rent is induced in the secondary windings.
Load
Current
Normal Operating Frequency is 70-80kHz
Load Current Characteristics
5
Increased
Load
Decreased
Load
Switching
Freq.
Figure 3