BBK DV718SI, Руководство по обслуживанию

Страница: 33 / 120

USBP
XS601
USB
JK602
USBM
USB 5V
A5V
(2) Bridge rectification and filtering circuit: the function of this circuit is to converse electric supply
into DC power, the voltage after being rectified and filtered is 1.414 times of input power, so the DC
voltage at the two ends of TC501 is about equal to 300V
(3) Absorption loop: for power is always working in on/off state and will produce very high peak
voltage, in order to well protect switch IC, a peak absorption loop is added.
(4) Filtering circuit: the function is to produce a stable and slamm-wave DC voltage. In filtering
circuit, “ " type filter is mostly adopted. The feature of capacitor filtering is high load resistance, when
current is small, filtering is obvious, but inductor filtering is small load resistance, when current is large,
filtering is obvious. To compose capacitor into “ " type filter can make better filtering effect.
(5) Feedback loop: the time length of “on” and “off” within the same cycle inside switch module
VEPR22 is decided by feedback loop. Feedback loop performs sampling to +3.3V output stage voltage,
when output stage voltage is too high, the sampled voltage is on high side, through feedback loop, to
change the duty ratio of pin 3 signal of VEPR22 and reduce on time, and output voltage begins to reduce.
When output voltage is too low, the sampled voltage is on low side, through feedback loop, to make duty
radio of VEPR22 increase, output voltage increases, through the function of feedback loop, power board
is made to output stable voltage. The used LM431 in this power is a 2.5V comparator, sampling voltage
is compare with this 2.5V voltage, when sampling voltage is more than 2.5V (means that output voltage
is on high side), LM431 is on, light emitting diode in photoelectric diode begins to emit light to make the
other end of photoelectric coupler begin to be on, light emitting diode is stronger, the on degree bigger,
the on time of switch module VEPR22 decreases, output voltage begins to decrease. When sampling
voltage is less than 2.5V (means output stage voltage is on low side), Lm431 is cut off, on time of
VEPR22 increases, output voltage increases. Thus through auto control function of feedback loop,
power board is made to output stable voltage.
3.2.13 USB jack circuit
1. USB jack circuit block diagram is shown in the figure 3.2.13.1:
Figure 3.2.13.1 USB jack circuit block diagram
2. Working principle: USB jack is composed of two data lines of USBP and USBM, one power supply
line USB5V and grounding line (outside casing). Pin 48,49 of MT1389 is USB data jack, which outputs to
USB jack through XS601 and can read data inside USB device.
- 29 -