BBK DK2810HD, Service Manual

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2.Working principle:
Power supply is divided into two channels to supply power. One channel supplies power for
decode board and other small power IC with small power and low voltage; the other channel supply
power for power amplifier with high voltage and large power. But the design method is the same and the
structure is as above:
(1) Electric wire netting filtering circuit as there are various of magnetic radiation, input
alternating circuit must be disturbed. The function of electric wire netting filtering circuit is to filter these
disturbance, so the voltage entering bridge rectifying circuit is pure 220V alternative power.
2 Bridge rectifier and filter circuit: the function of this circuit is to change AC into DC. Voltage
after rectified and filtered is 1.414 times of input power. So DC voltages of TC501 two sides are about
300V.
3 Absorb circuit: as power supply always works in on/off state, during which it produces high
peak voltage. In order to protect switch IC better, a peak absorb circuit is added.
4 Filter circuit: its function is to produce a more stable and less ripple DC voltage. Filter circuit
mostly adopts shaped filter. Feature of capacitance is high impedance resistor. Filter is evident
when current is small. While for indutance filter, filter function is evident when current is large. Make
capacitance to be shaped to gain a better filter effect.
5 Feedback loop: In switch module NCP1200D, time length of "on" and "off" depend on
feedback circuit. Feedback samples for voltage of
NCP1200D is changed through feedback loop to
decrease conductivity time and output voltage begins to drop. When output voltage is low, sample
voltage is low. Duty-cycle of
D+5V output stage. When output stage voltage is
high, sample voltage is high. Signal duty-cycle of
NCP1200D is increased through feedback loop and output voltage begins
to rise. Power board outputs stable voltage through feedback loop. TL431, which is used in this power
supply is a 2.5V comparator. Compare sample voltage with the 2.5V voltage, when sample voltage is
more than 2.5V (indicate output voltage is higher), TL431 is conducted. Light emitting diode of opto-
coupler begins to flick and other side of opto-coupler begins to conduct. The stronger light emitting diode
flicks, the larger extent it conducts. When conductivity time of switch module NCP1200D decreases,
output voltage begins to drop. When sample voltage is less than 2.5V indicates output stage voltage is
lower TL431 is off. When conductivity time of NCP1200D in creases, output voltage increases. Thus
power board outputs stable voltage through automatic controlling function of feedback loop.
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