Confusing Power Supply Schematic

The above is the schematic for the power supply portion of a board. As a electronics noob, I find it highly confusing and hence would appreciate some help in unraveling this schematic.

The EE13 transformer has 2 secondaries, one for 5v and another for 220v. I would appreciate some insight on the following:

1) What are the diodes on the primary input for?

2) How is the 5v secondary rectified to DC?

3) How is the 220v secondary rectified to DC using the TOP264EG component?


closed as unclear what you're asking by winny, Dwayne Reid, Finbarr, Sparky256, Lior Bilia Dec 24 '18 at 3:09

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  • \$\begingroup\$ rough description: the 220VAC is rectified by the bridge rectifier D9-D12 ..... the voltage across C23 is a DC voltage ...... U6 produces a pulse width modulated signal on pin 2 of T1 ..... that signal is varied in a way as to produce the desired voltage on the T1 seconary \$\endgroup\$ – jsotola Dec 17 '18 at 6:05
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    \$\begingroup\$ Please edit your question to add "a" and "b". Please clarify what output you mean, the output of TOP264 or of the whole circuit? \$\endgroup\$ – try-catch-finally Dec 17 '18 at 8:52
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    \$\begingroup\$ If i don't see a 5v on the output, there has to be a problem with the Transformer windings I can list at least 10 more things that could prevent this circuit from working properly even with a fully functional transformer. I suggest that you do some serious studying on switched converters because "guessing" will not get you anywhere to solve the problem. \$\endgroup\$ – Bimpelrekkie Dec 17 '18 at 9:28
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    \$\begingroup\$ The transformer most certainly does not have 220V on the second winding. The first (top) winding is used to generate the 5V output. The second (lower) winding is used to generate the voltage for the feedback signal to the controller. The capacitor that smooths the generated DC is only rated for 50V, so that winding had better not generate anything that can be rectified to more than 50VDC. 220V out of the lower winding would kill C28 in short order. \$\endgroup\$ – JRE Dec 17 '18 at 10:41
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    \$\begingroup\$ ”The EE13 transformer has 2 secondaries, one for 5v and another for 220v.” Please show where you got that information. \$\endgroup\$ – winny Dec 17 '18 at 11:28

This is a secondary side regulated flyback architecture. The diodes are used to rectify the 220V to have a sine wave between 0 - 220V. The capacitor is to get a somehow DC like voltage.

The U6 TOP264EG switches the current through the coil of the primary side. This induce a voltage on the two secondary side winding's.

The TOP264EG regulates the switching according to its input pin 3. The optocuppler ISO5 (and the feadback network R29, C33, U7, R31, R32) basically "tells" the U6 if the secondary 5V output voltage is above or below the limit. Based on this the duty cycle of the switching is changed according to the needs of the secondary 5V side.

The third winding is to give the ISO5 a fitting supply voltage around 5V based on the primary side ground.

The 5v secondary side is rectified to DC by C31, L1 and C32

  • \$\begingroup\$ Thank you for the detailed response. I need to read-up on the "Flyback" architecture. So if i don't see 5v on the output would i be correct in assuming that the Transformer windings are the source of the problem? Or could the TOP264EG itself be the source of the problem? \$\endgroup\$ – RamanathanR Dec 17 '18 at 17:31
  • \$\begingroup\$ If the transformer’s dots are in the right place, this is not a flyback converter. Secondary conduction occurs when the primary switch is closed. \$\endgroup\$ – Edgar Brown Dec 18 '18 at 4:55

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