TIDUF04 December 2022
The target maximum switching frequency at full load, the minimum input-capacitor bulk voltage, and the estimated DCM resonant time determine the maximum primary-to-secondary turns-ratio of the transformer. Initially, determine the maximum-available total duty-cycle of the on-time and secondary conduction time based on the target switching frequency (fMAX) and DCM resonant time (tR).
At the transition-mode operation limit of DCM, the interval required from the end of the secondary current conduction to the first valley of the VDS voltage is ½ of the DCM resonant period (tR), or 1 µs assuming 500-kHz DCM resonant frequency. The maximum allowable MOSFET on-time DMAX is determined using Equation 4:
where
When DMAX is known, the maximum primary-to-secondary turns ratio is determined with Equation 5. The total voltage on the secondary winding must be determined, which is the sum of VOCV and VF.
Assuming VF = 0.8 V:
A higher turns-ratio generally improves efficiency, but can limit operation at a low input voltage. Transformer design iterations are generally necessary to evaluate system-level performance trade-offs.
The primary transformer inductance is calculated using the standard energy storage equation for flyback transformers. The primary current, maximum switching frequency, output voltage and current targets, and transformer power losses are included in Equation 8:
The UCC28742 CC regulation is achieved by maintaining DMAGCC at the maximum primary peak current setting. The product of DMAGCC and VCST(max) defines a CC-regulating voltage factor VCCR which is used with NPS to determine the current-sense resistor value necessary to achieve the regulated CC target, IOCC (see Equation 10).
where
NAS is determined by the lowest target operating output voltage while in CC regulation and by the VDD UVLO turnoff threshold of the UCC28742 device. Additional energy is supplied to VDD from the transformer leakage-inductance which allows a lower turns ratio to be used in many designs.
where
This implies:
Since the ±14-V rails are unregulated, the turn ratio determines their output voltage: