Sub Power Supply - Toshiba 1997 G Series Troubleshooting Manual

10. SUB POWER SUPPLY.

The Sub Power Supply is a switching supply. It uses a
Free-Running Oscillator made up of Q802, T888 and
C855. R852 supplies the start-up voltage, and C855 sup-
plies a positive feedback to maintain switching (Fig.14).
The Sup Power Supply supplies a 16V, -16V and 29V to
the Convergence Outputs and the Convergence Digital
Control. This supply also operates as a Main Power Sup-
ply in some of the 19" TVs and is similar in operation to
the supplies used in Toshiba's VCRs and DVD players.
Basic operation:
Refer to Figure 12. RS supplies a start-up voltage to the
base of Q1 turning Q1 on. With Q1 on, current slowly
increases through the primary winding of the transformer.
An electromagnetic field builds while the current in-
creases. This field will induce an electromagnetic field
Rs
V
I
Q1
Base winding
Feedback.
For stabilization, a negative feedback is supplied to the
Base of Q1. The feedback signal is developed form the
Detection winding, rectified by D856 and controls the
bias of Q1. If the output is too high, the frequency of
Primary winding
- +
+
-
+
Secondary winding
-
C1
+
Primary winding
Rs
V
I
and a positive voltage across the Base winding of the trans-
former increasing the current through the Base of Q1.
Eventually, the current through the Collector of Q1 satu-
rates, stops increasing and levels off. At that point, the
electromagnetic field across the Base Winding collapses,
supplies a negative voltage to the Base of Q1, and charges
C1.
The negative voltage on the Base of Q1 turns Q1 off. The
electromagnetic field on the primary collapses, induces
current in the secondary winding, and induces a potential
on the detection winding which continues to hold Base of
Q1 low.
After the electomagnetic field is fully collapsed, C1 dis-
charges truning Q1 on. and this starts the process over again.
The start up voltage in no longer used once the supply starts
switching.
V
CE
On
Time
0
V
B
V
O
0
Ic
0
I
D
0
Fig. 12
Q1 increases resulting in decreased on-time of Q1. This
will then decrease the output voltage. If the output is
too low, the frequency of Q1 decreases. In return, it
increases the on-time of Q1
Secondary winding
Vo
Base winding
D z
Detection winding
D856
Fig. 13
14
Off
Time
Slant
V
I
L
P
Slant
V
O
L
S