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I want to understand how my switching power supply work and how to modify value for CC and CV mode. The SMPSU I got is almost identical to this reverse engineer schematic. (link here) I alo inserte the schematic image to confirm accessibillity. The question is.

  1. How this circuit start up? It needed voltage supply to controller ICs where it came from?
  2. How the current control work in this design? It seem very different and more complicare than the circuit from datasheet. enter image description here

This circuit provided by tl494 datasheet enter image description here for datasheet circuit I can find CC mode set point by make both side of error amp equals. From inverting input it got 1V from voltage divider and non-negative side, it got 1V from 0.1R resistor which mean current must flow 10A at CC mode.

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  • \$\begingroup\$ In what way is the constant current control more complicated? Both have a resistor for current sense, a reference and a built in op-amp for the purpose. \$\endgroup\$
    – winny
    Commented Nov 30, 2020 at 18:01
  • \$\begingroup\$ The circuit initially gets a little power from the secondary - enough for it to start up properly. \$\endgroup\$
    – Andy aka
    Commented Nov 30, 2020 at 18:04
  • \$\begingroup\$ @winny I edit my question to answer your comment. \$\endgroup\$
    – M lab
    Commented Nov 30, 2020 at 18:13
  • \$\begingroup\$ @Andyaka so the little current must flow through primary side right? please tell me which path the current flow. \$\endgroup\$
    – M lab
    Commented Nov 30, 2020 at 18:14
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    \$\begingroup\$ Notice the start-up resistors 2x150k/1W at the half-bridge transistor bases \$\endgroup\$
    – G36
    Commented Nov 30, 2020 at 18:45

1 Answer 1

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At start up the TL494 is dead and primary of the drive transformer has a high impedance across it .Main switch halfbridge transistors Q1 Q2 are biased slightly into class A due to the 150K bias resistors.They now have gain and form an oscillater due to positive feedback provided by the current overwind on the secondary of the drive transformer .The frequency of this prospective oscillation is on this circuit determined by main transformer saturation .The duty cycle of this start up oscillation is close to 50% ,in other words there is only a small amount of dead time .After a few cycles the TL494 activates because it now has Vcc .The TL494 now controls the power supply via the drive transformer primary .Output current is sensed along with output voltage and fed into the TL494 control loop in an orthodox manner .Under current limiting the average DC output voltage falls but Vcc which is peak rectified should stay relatively constant so the TL494 still runs .This old circuit was really clever in its day.It could be tarted up for greater reliability ,lower switching losses and outperform a lot of modern stuff .

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  • \$\begingroup\$ For the love of god, can you please stop putting space before comma and period? \$\endgroup\$
    – winny
    Commented Nov 30, 2020 at 22:48
  • \$\begingroup\$ Can you explain current control loop? Snd setpoint calculation? \$\endgroup\$
    – M lab
    Commented Dec 1, 2020 at 19:17

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