To prevent a transformer from saturation its V-t product must be greater than the maximum V-t product applied by the device. The maximum voltage delivered by the device is the nominal converter input plus 10%. The maximum time this voltage is applied to the primary is half the period of the lowest frequency at the specified input voltage. Therefore, the transformer’s minimum V-t product is determined through:

Equation 3.

Taking an example of f_min as 138 kHz for SN6505A and 363 kHZ for SN6505B with a 5 V supply, Equation 3 yields the minimum V-t products of:

Equation 4.

Common V-t values for low-power center-tapped transformers range from 22 Vμs to 150 Vμs with typical footprints of 10 mm x 12 mm. However, transformers specifically designed for PCMCIA applications provide as little as 11 Vμs and come with a significantly reduced footprint of 6 mm x 6 mm only.

While Vt-wise all of these transformers can be driven by the device, other important factors such as isolation voltage, transformer wattage, and turns ratio must be considered before making the final decision.