Axpert MEX-1500/ MEX-3000 Service manual
7
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.
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 positive with respect to their no-dot ends. Output rectifier diodes D70, D67,
D54 and D57 are reverse-biased and all the output load currents are supplied from storage filter
capacitors C116, C79, C78 and C75.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 C116,
C79, C78 and C75.
As shown in figure 3.1, this circuit may generate several output voltage, such as +12V_RELAY,-12V,
+VCC, +5V, HFPW+.
