Read Sharp 28JW-74H (serv.man2) Service Manual online
50Hz Widescreen Colour Television – GA20 Chassis
Sharp Electronics (UK) Ltd
Technical Support
Issued June 02
Page 3
Control (Pin 3)
Burst Standby mode
The CTRL pin (pin 3) is also used to implement the burst mode standby. In burst mode standby, the
power supply enters a special low dissipation state, where it typically consumes less than 1W of input
power, but is still able to supply a microprocessor. The system enters burst mode standby when the
microprocessor turns on Q754 in the isolated side of the power supply which will result in the opto-
coupler, IC702 turning on, taking the control pin of the TEA1507 high. In response to this signal, the
IC stops switching and enters a ‘hiccup’ mode. This burst activation signal should be present for a
period, longer than 30 ms.
power supply enters a special low dissipation state, where it typically consumes less than 1W of input
power, but is still able to supply a microprocessor. The system enters burst mode standby when the
microprocessor turns on Q754 in the isolated side of the power supply which will result in the opto-
coupler, IC702 turning on, taking the control pin of the TEA1507 high. In response to this signal, the
IC stops switching and enters a ‘hiccup’ mode. This burst activation signal should be present for a
period, longer than 30 ms.
Hiccup Mode
The hiccup mode during burst mode standby operation does not differ from the hiccup mode in safe-
restart mode during a system fault condition (e.g. OVP or output short circuit).
restart mode during a system fault condition (e.g. OVP or output short circuit).
Demagnetization (Pin 4)
The system will be in discontinuous conduction mode all the time. The internal oscillator will not start
a new primary stroke until the secondary stroke has ended. Demagnetization features a cycle-by-
cycle output short-circuit protection by immediately lowering the frequency (longer off-time), thereby
reducing the power level.
Demagnetization recognition is suppressed during the first tsuppr time. This suppression may be
necessary in applications where the transformer has a large leakage inductance and at low output
voltages/start-up.
a new primary stroke until the secondary stroke has ended. Demagnetization features a cycle-by-
cycle output short-circuit protection by immediately lowering the frequency (longer off-time), thereby
reducing the power level.
Demagnetization recognition is suppressed during the first tsuppr time. This suppression may be
necessary in applications where the transformer has a large leakage inductance and at low output
voltages/start-up.
Overvoltage
An overvoltage (OVP) mode is implemented by sensing the voltage via the current flowing into pin 4
DEM during the secondary stroke. The secondary winding voltage is a well-defined replica of the
output voltage. Any voltage spikes are averaged by an internal filter. If the output voltage exceeds the
OVP trip level, the OVP circuit switches Q702. Next, the controller waits until the UVLO level is
reached on pin 1, VCC. This is followed by a safe restart cycle, after which switching starts again.
This process is repeated as long as the OVP condition exists. The output voltage at which the OVP
function trips, Vo(OVP) is set by the series/parallel network comprising of R706, R709, R710, R714
and D701
DEM during the secondary stroke. The secondary winding voltage is a well-defined replica of the
output voltage. Any voltage spikes are averaged by an internal filter. If the output voltage exceeds the
OVP trip level, the OVP circuit switches Q702. Next, the controller waits until the UVLO level is
reached on pin 1, VCC. This is followed by a safe restart cycle, after which switching starts again.
This process is repeated as long as the OVP condition exists. The output voltage at which the OVP
function trips, Vo(OVP) is set by the series/parallel network comprising of R706, R709, R710, R714
and D701
Overpower
This pin is also used to monitor the power that is being drawn 'Overpower Protection' (OPP) During
the primary stroke, the rectified mains input voltage is measured by sensing the current drawn from
pin 4 (DEM).
The current information is used to adjust the peak drain current, which is measured via pin 5 (I
the primary stroke, the rectified mains input voltage is measured by sensing the current drawn from
pin 4 (DEM).
The current information is used to adjust the peak drain current, which is measured via pin 5 (I
sense
).
The internal compensation is such that an almost mains independent maximum output power can be
realised.
realised.
Current Sense (Pin 5)
Short Winding Protection
After the leading edge blanking time, the short winding protection circuit is activated. If the ‘sense’
voltage exceeds the short winding protection voltage Vswp, the converter will stop switching. Once
V
voltage exceeds the short winding protection voltage Vswp, the converter will stop switching. Once
V
CC
drops below the UVLO level, capacitor C705 will be recharged and the supply will restart again.
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