9.2.2.2 Transformer Design

The transformer design starts with selecting a suitable switching frequency. Generally the switching frequency selection is based on a tradeoff between the converter size and efficiency, based on the simple Flyback topology. Normally, higher switching frequency results in smaller transformer size. However, the switching loss is increased and hurts the efficiency. Sometimes, the switching frequency is selected to avoid certain communication bands to prevent noise interference with the communication. The frequency selection is beyond the scope of this data sheet.

The switching frequency is targeted for 110 kHz, to minimize the transformer size. At the same time, because EMI regulations start to limit conducted noise at 150 kHz, choosing 110-kHz switching frequency can help to reduce the EMI filter size.

The transformer turns ratio can be selected based on the desired MOSFET voltage rating and diode voltage rating. Because maximum input voltage is 265 V AC, the peak bulk voltage can be calculated by Equation 6.

Equation 6.

To minimize the cost of the system, a popular 650-V MOSFET is selected. Considering the design margin and extra voltage ringing on the MOSFET drain, the reflected output voltage must be less than 120 V. The transformer turns ratio can be selected by Equation 7.

Equation 7.

The transformer inductance selection is based on the continuous conduction mode (CCM) condition. Higher inductance would allow the converter to stay in CCM longer. However, it tends to increase the transformer size. Normally, the transformer magnetizing inductance is selected so that the converter enters CCM operation at about 50% load at minimum line voltage. This would be a tradeoff between the transformer size and the efficiency. In this particular design, due to the higher output current, it is desired to keep the converter deeper in CCM and minimize the conduction loss and output ripple. The converter enters CCM operation at about 10% load at minimum bulk voltage.

The inductor can be calculated as Equation 8.

Equation 8.

In this equation, the switching frequency is 110 kHz. Therefore, the transformer inductance must be about 1.7 mH. 1.5 mH is chosen as the magnetizing inductance value.

The auxiliary winding provides the bias power for UCC2813-0 normal operation. The auxiliary winding voltage is the output voltage reflected to the primary side. It is desired to have higher reflected voltage so that the IC can quickly get energy from the transformer and make start-up under heavy load easier. However, higher reflected voltage makes the IC consume more power. Therefore, a tradeoff is required.

In this design, the auxiliary winding voltage is selected to be the same as the output voltage so that it is above the UVLO level but keeps the IC and driving loss low. Therefore, the auxiliary winding to the output winding turns ratio is selected by Equation 9.

Equation 9.

Based on calculated primary inductance value and the switching frequency, the current stress of the MOSFET and diode can be calculated.