PMP20878: PMP20878: How to calculate the turns ratio and inductance of the transformer (primary: auxiliary)? Can you provide related calculation formulas and parameters?

SLUP262 (attached) has good information on active clamp forward design.  There is some basic converter information in the first few pages and the active clamp forward information starts on page 15.  Looking at figure 21, you can see that the drain voltage of the primary FET increases significantly at low input voltages/high duty cycle.  Because of this, you will want to clamp the maximum duty cycle at about 75% for a standard input of 36-60V.  To account for transients the actual maximum working duty cycle should be 65-70%.  Using 65% and the duty cycle equation for a forward converter, you will get a turns ratio of approximately 1:1 for a 24V output.  The bias winding is proportional to the main output.  For a 12V bias with a 24V output, the 24V winding to bias winding ratio will be 2:1 (which in this case is also the primary to bias turns ratio).  If using sync rectifiers on the output and diodes on the bias, the bias voltage will be slightly lower due to the diode drop.

The primary (magnetizing) inductance determines the peak magnetizing current.  This helps drive the resonant (zero voltage) transition of the primary FETs, which reduces EMI and improves efficiency.  Since this current flows though the transformer it also results in copper loss.  For converters operating in the 200-300kHz range at 48V nominal and 25-100W, 100uH is a good value to use to provide sufficient energy for the zero voltage transition while not resulting in too much copper loss.

Thanks, David

slup262.pdf