SLLSFM0A March 2022 – June 2022 SN6507
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 time this voltage is applied to the primary for half the period of the lowest frequency at the specified input voltage. For designs using duty cycle control, the maximum V-t applied by the device can be calculated by the typical voltage applied for one quarter of the period of the lowest switching frequency. For systems using a clock frequency set by RCLK, fmin can be estimated as 15% below the typical or approximate switching frequency value, fSW, for the corresponding RCLK from Section 8.3.4. For systems where the CLK pin is connected to GND, the minimum specified FSW from Section 6 should be used. Therefore, the transformer’s minimum V-t product is determined through Equation 9 for fixed inputs and Equation 10 for wide-ranging inputs using duty cycle control:
Example of Fixed Input:
For a fixed input system with fSW(min) of 780 kHz and a VIN = 24 V supply with ±10 % tolerance, Equation 9 yields the minimum V-t product of:
Example of Wide-Ranging Input:
Taking the assumption of fSW(min) as 780 kHz with a VIN(typ) 24 V supply, Equation 10 yields the minimum V-t product of:
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.