Samsung HPS5073X/XAA, Service Manual

Page: 122 / 144

Circuit Description
13-4 Samsung Electronics
ㆍ
Over Heat Protection
If the temperature on the chip parts is higher than about 135
℃
, this power board operates in thermal protection mode and
makes Latch on, finally this power board is shut down.
⑤
PFC Circuit
【
Basic Operation
】
After the sub-dc voltage source is on, Vcc of PFC controller IC(UCC2818A) induces the PSON port to be
"
L
"
level and this
makes the oscillation with a frequency (RT:R8239, CT:C8235) in it. The PFC block consists of two inside feedback
loop to control FET duty ratio. The first is the voltage loop that keeps the output voltage to be constant by sensing the output
voltage in 11 pin. And the second loop controls the power factor by comparing the input line voltage (6pin) and reference in
it. By this current loop, the input current waveform can be sinusoidal and now we can get the higher power factor (almost
1.0) in this power board.
【
Protection Function
】
ㆍ
Over Voltage Protection
Because the PFC controller IC(UCC2818A) has the function of over voltage protection about the high output voltage limit,
even if the overshoot and not-controlled state occur with the deep condition in ac input source or output step load, this
controller protects the abnormal output voltage state by decreasing the FET ON ratio. The high output limit level is set to
110% value of voltage across the dividing resistors ; R8247, R8248, R8249, R8250, R8251
(this power board has also the high limit O.V.P level about 440V)
ㆍ
Over Current Protection
PFC controller has the function of over current protection by sensing the voltage (2 pin) coming from the current-sensing
resistor R8201. If the over current flows across the sensingresistor, controller decreases the FET duty ratio for protection of
part's damage.
< Vds Voltage Waveform on Sub-power Circuit FET >
< FET Vds Waveform (100Vac input) > < FET Vds Waveform (230Vac input) >
0.6
RT x CT