Axpert MKS-4000/KS-5000 Service manual
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3. Working principle of the major functional block
3.1 Switch Power Supply
The switch power supply (SPS) supplies DC power for Inverter/Charger operation. The input voltage of
the SPS is the battery or AC Charger output voltage.
Figure 3.1 basic circuit of power supply
This is the fly-back DC-DC converter, fly-back operation can be easily recognized from the position of the
dots on the transformer primary and secondary(these dots show starts of the winds). When Q36 is ON,
the dot ends of all winds are negative with respect to their no-dot ends. Output rectifier diodes D54, D57,
D67 and D70 are reverse-biased and all the output load currents are supplied from storage filter
capacitors C78, C75, C79 and C116.The primary coil of the transformer acts as an inductor and stored
energy.
When Q36 is OFF, the stored energy in the primary coil is delivered to secondary filter capacitors C78,
C75, C79 and C116.
As shown in figure 3.1, this circuit may generate several output voltage, such as +12V,-12V, +15V, +5V,
HFPW+.
