BBK DV725SI, Руководство по обслуживанию

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3.5.12 function introduction to FDSH321
Pin 6.7.8 of 5L0380 is controlled by feedback pin 3 to make it on for a while and off for another while to
form pulse DC to control coupling quantity of transformer. When secondary output voltage of transformer is on
the high side, under the control of pin 3, disconnection time of pin 6¡¢7¡¢8 gets longer and coupling quantity of
transformer decreases to make output voltage get lower gradually; when secondary output voltage of
transformer is on the low side, disconnection time of pin 6¡¢7¡¢8gets shorter, connection time gets longer
(connect inside fsdh321), coupling quantity of transformer decreases and secondary output voltage gets
larger gradually. Pin function is shown as the following table:
Pin Symbol Description
1 GND Sense FET source terminal on primary side and internal control ground.
2 VCC Positive supply voltage input.
3 Vfd The feedback voltage pin is the non-inverting input to the PWM comparator.
4 Ipk Pin to adjust the current limit of the Sense FET.
5 Vstr This pin connects directly to the rectified AC line voltage source.
6 DRAIN
7 DRAIN
8 DRAIN
The Drain pin is designed to connect directly to the primary lead of the transformer and is capable of
switching a maximum of 650V. Minimizing the length of the trace connecting this pin to the transformer will
decrease leakage inductance.
3.5.13 function introduction to LM431A
LM431A is a 2.5V comparator, shown as the figure 3.5.13.1. Compared the inputted voltage of R end with
2.5V, when voltage of R end is more than 2.5V, KA end is on and photoelectric coupler starts to send out
photocurrent; when voltage of R end is less than 2.5V, KA end is cutoff and photoelectric coupler does not
send out photocurrent. CPU+3.3V in power board circuit must be kept in 3.3V, for the function of comparator.
No matter more than or less than 3.3V, through on and off status of comparator, it will control the on state of
the output end of photoelectric coupler LM431A to adjust the output space occupation ratio of switch module
to control the output voltage of transformer and masthead the power supply of the whole system.
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Figure 3.5.13.1 LM431A outside drawing
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