The transformer of the design consists of two major values, turns ratio and primary side inductance. There is no minimum limit to the turns ratio of the transformer, just a maximum limit. The equation below will give the turns ratio as a function of duty cycle which if the maximum duty cycle of the converter is used will give you a maximum turns ratio. The UC1825B-SP design targeted a duty cycle of 30%. Since this design is for a dual output device the duty cycle must stay below 50%. If both outputs were running above 50% duty cycle they would have to overlap which is not possible for the topology. The equation of the turns ratio of the transformer is Equation 4.

Often the turns ratio will slightly change in design due to how the transformer is manufactured. For the UC1825B-SP design a turns ratio of 2.2 was used. Another turns ratio that is important is the turns ratio of the auxiliary winding. The auxiliary winding is found by figuring out what positive voltage is needed from the auxiliary winding. Selecting this voltage lets one pick the turns ratio from the secondary to the auxiliary winding, which in turn allows for the turns ratio from primary to auxiliary to be found. The equation for the turns ratio is Equation 6.

An auxiliary winding of 1.5 was used for the UC1825B-SP design. The primary inductance of the transformer is found from picking an appropriate magnetizing current. The magnetizing current of the transformer is the amount of current drawn through the windings of the transformer when the output is open circuited. Decreasing the magnetizing current will increase the inductance of the transformer, perhaps to unreasonable values. Increasing the magnetizing current will cause efficiency to decrease. It is desirable to keep the magnetizing current low, thus 6% was picked for the design value. The equation for the auxiliary winding turns ratio is Equation 8.

There are quite a few physical limitations when making transformers that will affect the inductance value. For the UC1825B-SP design a primary inductance of 120 µH was used. The output inductor was then picked based on the output inductor ripple current with Equation 10.

In the final design, a 2.2-μH inductor was used. The peak and primary currents of the transformer are also generally useful for figuring out the physical structure of the transformer, so equations are listed below. Note these equations are only true for continuous conduction mode. Peak currents are higher at the maximum input voltage while the RMS current is highest at the minimum input voltage. These are also idea values and don't take into account efficiency. Final designs needs to be optimized depending on the specific application requirements. Equations that show how to calculate these for this design are below: