DAEWOO DVK882SYAN
Manual de Servicio

When the voltage of C813 reaches Vbe(0.7V) of Q802, Q802 is on and Q801 is off. The on-time of Q801 depends on the time constant R810, DZ801 and C813. When Q801 is off, the energy stored on LP of primary is output through a rectifying diode of secondary part. Then, when the core energy is used up, the diode is switched off but some energy is left in Ns winding. The left energy which cause the voltage in Nb winding, repeats the switching operation to turn on Q801.
Np Ns + Q801

+

C807

Nb

4. SOFT START A soft start circuit, as the name implies, starts to the power supply in a smooth fashion, reducing the inrush courrent exhibited by all switching power supplies. When the power supply is start up, Ic current is flowing in Q801 and C813 is charged from 0V. 5. CONTROL OF RATED VOLTAGE The SMPS applies VCR controlled by detecting voltage fluctuation of 5.8V output. Generally, IC of MICOM uses 5V Vc which has a low fluctuation. Rated voltage is adjusted by detecting the voltage fluctuation of 5.8V, changing charge time of C813 and controlling the switching time of Q801. IC802 consists of an OP-AMP and a zener diode which has a reference voltage(Vref). If the output voltage is lower than 5.8V, the charge time of C813 becomes longer. On the core of the voltage output being over 5.8V because(-) is higher than Vref Output of OP-AMP. These changes are fed back to the primary side via the photo coupler to the switching control circuit. As a result of it, Q801 ON/OFF frequency is changed, changing charge time of C813 to contain the constant out put Voltage. 6. FOLD BACK CHARACTERISTICS The most rudimentary form of fold back is like the shape shown below figure. As the load gets bigger, the on-time of Q801 becomes longer so as to supply energy; however, the on-time of Q801 in determined by the serial equivalent resistor time constant of C813,R810,DZ801. The on time cannot pass this time constant. So if the load requires more on-time, It does not get and output voltage of Np winding, so the off time of Q801 becomes longer and the next on time a Q801 becomes shorter. Ic(collector current) is reduced and output voltage goes down.
Vo

Io Vo: Secondary output voltage Io: Secondary output current

7. SNUBBER CIRCUIT The circuit protects Q801 from surge voltage generated when the switching transistor turns off it is also required to prevent noise. Snubber circuit operates when Q801 is in off. the surge voltage is rectified on D802, smoothed on C808 and then the energy is dissipated as heat on R806.

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