Why the voltage drop is so high in my Half-Bridge SMPS?
Is your transformer secondary 12 turns total or 12 turns on each side of the center tap? If it's the former, that's why you can only get 30 V under load. I calculate it this way:

Input voltage is 220 VRMS full-wave rectified to about 310 VDC.

This means that your half-bridge is driving the transformer with a voltage whose peak is half of this, or 155 V.

The 33:12 transformer is going to turn this into a peak voltage of about 56 V.

If the secondary is center-tapped, then you're only hitting the rectifiers with a peak of 28 V.



As for the excessive rise at low loads &mdash; well, that's why lots of SMPS specify a minimum load. It's actually quite difficult to design an efficient one that also has a huge dynamic range. One problem might be excessive leakage inductance (i.e., less than perfect coupling) in your transformer.



EDIT: Since I can't put this drawing in the comments, I'll add it here. Your transformer drive waveform always needs to be symmetric. At 50% duty cycle, it should look like a square wave, with a small amount of "crossover distortion" created by the dead time:

  --------            --------
 |        |          |        |
 |        |          |        |
 |        |          |        |
 |        |          |        |
           |        |          |        |
           |        |          |        |
           |        |          |        |
           |        |          |        |
            --------            --------


But at lower duty cycles, it still needs to be symmetric, with longer "off" periods between the alternating pulses. It should look like this:

  --                  --
 |  |                |  |
 |  |                |  |
 |  |                |  |
 |  |                |  |
-    ------    ------    ------    --
           |  |                |  |
           |  |                |  |
           |  |                |  |
           |  |                |  |
            --                  --


This is the sort of waveform that the drivers on the SG3525 are designed to produce.