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2. Working principle: this power circuit is composed of two parts, which use the common
electromagnetic interference filtering circuit and bridge rectification circuit and filtering circuit. The first
part circuit produces P+28V DC used to supply power for power amplifier circuit; the other part is
responsible for the power supply of other module circuit of the player. The working principle of each
composed part is shown as follows:
(1) Electromagnetic interference filtering circuit: various electromagnetic radiation exists in the
surrounding environment, so it will affect the inputted AC, and the function of electromagnetic
interference filtering circuit is to filter these interference to make those that enter bridge rectification
circuit is pure 220V AC.
(2) Bridge rectification and filtering circuit: the function of this circuit is to produce a 310V DC used
for rear stage.
(3)Start-up circuit: when power on, transformer does not begin to work, now the start-up circuit
provides switch module with a power supply voltage to make it work, after transformer begins to work
normally, the voltage provided for switch module by power supply circuit maintains the working of switch
module.
(4)Absorption loop:the switch module performs on/off action in a high frequency, so a strong self-
inductance voltage will produce in transformer primary coil and switch module will probably be damaged.
The function of absorption loop is to form a loop for this self inductance to ensure the normal working of
circuit.
(5) Switch module: that inputted from transformer is 310V DC. To make transformer work, AC shape
voltage must be presented. The function of switch module is to control this 310V DC to make it on for a
while and then off for a while to produce a high/low voltage change in the primary stage of transformer,
thus the transformer can work.
(6) Power supply circuit: provides a power supply voltage for switch module.
(7) Rectification diode: the voltage that outputs from transformer is pulse DC, the function of
rectification diode is to change pulse DC to DC together with the filtering circuit behind.
(8) Feedback loop: the time of “on” and “Off” in the same cycle inside switch module 5L0380R is
decided by feedback loop. Feedback loop perform sampling to +3.3V output voltage, when output
voltage is too high, through feedback loop, the space occupation ratio of pin 4 signal of 5L0380R is
changed, the “on” time decreases, and output voltage begins to decrease. When output voltage is too
low, the voltage sampled is on low side, through feedback loop, space occupation ratio of 5L0380R
increases, output voltage increases to make power board output stable voltage through the function of
feedback loop. LM431 used in this power is a 2.5V comparator, compare sampling voltage with this 2.5V
voltage, when sampling voltage is more than 2.5V (means output voltage is on high side), LM431 is on,
light emission diode in photoelectric coupler begins to emit light to make the other end of photoelectric
coupler begin to be on, the light emission is stronger, the “on” degree is large, the “on” time of switch
module 5L0380R decreases, output voltage begins to decrease. When sampling voltage is less than
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