Galvanic isolated output

Adding an additional secondary winding will provide you with an isolated output.

Z. Cochrane

Strangely, that would be obvious, even to me.
But how will such an additional 'floating' winding affect the transformer performance ?
How add the additional winding to the transformer calculation ?

For a flyback topology, the outputs are cross regulated by the secondary turns ratios. (Nfloat)/(Nmon)=(Vfloat + Vdiode)/(Vmon + Vdiode)
where:
Nfloat is the floating winding secondary turns
Nmon is the number of turns of the monitored (feedback connected)secondary
Vfloat is the desired output voltage of the floating output
Vdiode is the forward drop of the output rectifier (use 0.5 for schottky and 0.7 for all others silicon diode types)
Vmon is the regulated output (where the feedback is connected)

For example, if Vmon = 5V, Nmon = 8 turns, desired Vfloat = 12V
lets say Vmon used a schottky diode and Vfloat used a ultra fast diode.

Nfloat= (12+0.7)/(5+0.5)*8 turns = 18.5 turns

since turns can only be integer numbers then if you used 18 turns, Vfloat = 11.7 Volts; if you use 19 turns then Vfloat = 12.4 Volts.

You should get good cross regulation (about 15%)as long as the load currents for each output are within 25% to 100% of full load. If for example the monitored output is at full load and the floating output is at no load, then the cross regulation falls apart due to leakage inductance and secondary resistance. A preload resistor on the floating output would help in this case.

If you download our free software PI Expert, it will design the transformer for you. To get a floating output click on "stacking" in the Design Tree View window on the left once the design is complete. Then select floating in the pull down box in the "type" column. You must select floating for both outputs. This will change the output from AC stacking to floating.

Z. Cochrane