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 — 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:
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But at lower duty cycles, it still needs to be symmetric, with longer "off" periods between the alternating pulses. It should look like this:
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This is the sort of waveform that the drivers on the SG3525 are designed to produce.
The 60Vdc input from wains transformer is Valid .This means a 60 volt switching waveform and not a 360Vdc switching waveform.Linear post reg is good ,if there are concerns about HF rejection then extra LC filter before the reg is good.The coils and caops for this are not a drama at an average current of 5A .Synchronous will as orthodoxely implemented will be much more noisey at HF .If you still want synch then run the bottom switch in "FIODE " mode .This means that the switching regime makes the fet behave like a diode but with a lower volt drop that is dictated by RDS on .I think that a schottky diode rated at say 100V will be more sensible.Now deal with your turn on by whatever means because turnon into a conducting diode makes lots of HF and VHF noise which is more difficult to filter because capacitors look inductive and inductors look capacitive.I have used a S trap buck for Variable volts off inputs up to 820VDC so 60 wont be a problem . The S trap is a second cousin of a valley switch. Use lots of MLC caps because they are better at HF ripple suppression than ALIMINIUM ELECTROLYTIC CAPS.Sure you need thousands of microfarad on buck input due to 50/60Hz power but your output wont need much electro .Most importantly the ceremics deal with the switchmode ripple .Your design should be doable in SMD.
Best Answer
Most low-power-conscious power supplies have power-saving features included, similar to what you have described. There are some considerations to keep in mind.
All switching power supplies have a finite start-up time. The power rails need to be steady, reference voltages need to establish and stabilize, and the switching converter needs to ramp (or soft-start) in order to avoid large stresses on the powertrain elements. A total turn-off usually means a recovery time of hundreds of milliseconds, which could be bad if there's a sudden surge in the load. (In your case, not so much, but power IC designers need to consider all possibilities).
Another method is to keep the controller alive, but simply stop operating the powertrain. The controller will still consume power but there is a net savings as the powertrain devices are not switching all the time.
All basic PWM controllers will pulse-skip by nature of the control algorithm (PWM width proportional to the error voltage: if error voltage > reference voltage, PWM = 0%). So this isn't so much a power-saving feature, rather how the controller inherently behaves at light loads. Hysteretic regulation is closer to the spirit of your idea - let the output sag a little to the lower threshold, then send some pulses to recover to the slightly higher threshold.
In both of these cases, because the controller remains active, there is a faster recovery if there is some input disturbance or load step.
Keeping the pulses more regular results in a smoother output, which your load would likely appreciate. Also, when there is a lot of capacitance on the output, keeping the pulses regular also avoids a large inrush of current being drawn when the capacitor is depleted and needs to be recharged, which is good for component stress and for EMI.