Start-Up And Over Voltage Protect - Toshiba TP43H60 Service Training

Start-up and Over Voltage Protect

The positive cycle of the AC line input supplies a
16V start-up pulse to pin 8 of Q801 via resistor
R861. Figure 5 is the block diagram for this cir-
cuit. After start-up, a drive circuit consisting of a
secondary winding of T862, diode D864, and ca-
pacitor C868 supply 16-20Vdc to pin 8 of Q801 to
maintain its operation. The voltage developed by
the drive circuit fluctuates with the switching fre-
quency of the power supply. Therefore, the volt-
age on pin 8 is also applied to an over voltage pro-
tect (OVP) block internal to Q801. If the voltage
on pin 8 increases to 25V, the OVP triggers the
latch and switching stops. Refer to the Latch sec-
tion for further information. D876 is a 27V zener
diode that protects Q801 by preventing excess volt-
age increases on pin 8.
R861 D864 R871
From
T862
D876
D801
C868
16-20 Vdc
8
Vcc
Q801
Start
OVP
Figure 5
Start-up and Over Current Protect
7