Toshiba TLP-X2500C Service Manual page 53

OPERATION OF POWER SUPPLY
1. Transition from AC ON to Standby mode
When the AC source is supplied to this power circuit, the voltage rectified by D101 and smoothed by
C105 is supplied to VH pin of IC102 through R125.
VCC pin voltage of IC102 rises by current supplied to VCC pin through VH pin.
And then, IC102 starts up and the DC/DC converter begins to operate. (6V, 14V and 17V start up.)
* This DC/DC converter is a PWM switching circuit, which is composed of IC102, T201 and Q103 mainly.
2. Transition from Standby mode to Typical Operation mode
When the level of ST/OP pin in the connector CN201 is Low or open, only the DC/DC converter is in the
switching operation. Each output does not have the on/off control which works solo, except primary 16V
line. The voltage level of primary 370V line becomes voltage after smoothing through bridge rectifier
circuit. (e.g. Input: 100Vac, Output: approx. 140Vdc)
This status is standby mode.
When the level of ST/OP pin in the connector CN201 is High, Q105 is turned on through a photo-coupler
PC103, and then the primary 16V source is supplied to the Ballast and IC101 of PFC circuit.
Because VCC voltage of IC101 reaches threshold level of starting voltage, IC101 starts up activity and the
PFC circuit begins to operate. The voltage level of primary 370V line becomes approximately 370V which is
stable against variation of AC input voltage.
* ST/OP pin is a pin to control for switching mode between standby and typical operation.
3. Protection against abnormal condition
If Overvoltage or overheat or overload happens, the DC/DC converter stops the switching operation by
putting IC102 into the latch mode. Every output (They are primary 16V, secondary 6V, 14V and 17V.) of
the DC/DC converter is turned off, and this is maintained. PFC circuit stops the switching operation
because +16V line which is power source for the control IC of PFC is turned off. The voltage of 370V line
varies to voltage that rectified the AC source. (i.e. The boost stops.)
About the latch mode of IC102
In normal operation, CS pin voltage of IC102 is clamped by an inside 4V zener diode.
Externally forcing CS pin voltage to increase to the threshold voltage, 8.2V, for the latch mode allows the
IC102 to stop its operation for protection.
When CS pin voltage reaches to 8.2V, the inside source in IC102 is turned off, and OUT pin (for gate
drive of FET in the DC/DC converter) is set to the Low level.
Then, the start-up circuit is activated again, and VCC voltage is held at approximately 22V. This status is
the latch mode of IC102. The latch mode is maintained as long as supply to VCC pin continues.
It takes time for the latch mode to be rest because charged C105 voltage is applied to the VH pin even if
IC102 have changed to the latch mode.
Cutting off the input voltage decreases VH pin voltage, supplying no current to the VCC pin.
Thereafter, the latch mode is reset when VCC drops below the OFF threshold level, 8V min.
6–10
Chapter 6