UCC25800-Q1: Turns ratio calculation for the LLC converter transformer.

Part Number: UCC25800-Q1

Tool/software:

On page no. 30 of the datasheet, the turns ratio calculation shown for the design process of the LLC converter's transformer, the formula includes the voltage drop of both the diodes of the output rectifier instead of just one. The LLC topology that they have considered for this design is the "Secondary side resonant LLC converter with voltage doubler rectifier" shown as Fig 9.5 on page no.28 of the datasheet. As per my understanding only one of these two diodes will conduct during any instant of time, so only one diode's voltage drop should be considered instead of two for the turns ratio calculation. In the design process they have mentioned that they are going for the voltage doubler rectifier topology instead of the full bridge one. If it was a full bridge topology design then including the voltage drops of two diodes makes sense but I am unable to figure out why they did this for the voltage doubler configuration. 

   

  • DC voltage gain relates the output voltage to the input voltage over an entire switching cycle. You are correct that only one output rectifier diode is switching at any instant in time which means that one diode is conducting during the positive half-cycle and the second diode is conducting during the negative half-cycle. Added together over a full switching cycle and this is where we get 2*VF.

    Steve