ZHCSBB1D July 2013 – March 2018 UCC28740
PRODUCTION DATA.
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, f_{MAX}, and DCM resonant time. For DCM resonant frequency, assume 500 kHz if an estimate from previous designs is not available. At the transition-mode operation limit of DCM, the interval required from the end of secondary current conduction to the first valley of the V_{DS} voltage is ½ of the DCM resonant period (t_{R}), or 1 µs assuming 500 kHz resonant frequency. The maximum allowable MOSFET on-time D_{MAX} is determined using Equation 12.
When D_{MAX} is known, the maximum primary-to-secondary turns-ratio is determined with Equation 13. D_{MAGCC} is defined as the secondary-diode conduction duty-cycle during CC operation and is fixed internally by the UCC28740 at 0.425. The total voltage on the secondary winding must be determined, which is the sum of V_{OCV}, V_{F}, and V_{OCBC}. For the 5-V USB-charger applications, a turns ratio range of 13 to 15 is typically used.
A higher turns-ratio generally improves efficiency, but may limit operation at low input voltage. Transformer design iterations are generally necessary to evaluate system-level performance trade-offs. When the optimum turns-ratio N_{PS} is determined from a detailed transformer design, use this ratio for the following parameters.
The UCC28740 constant-current regulation is achieved by maintaining D_{MAGCC} at the maximum primary peak current setting. The product of D_{MAGCC} and V_{CST(max)} defines a CC-regulating voltage factor V_{CCR} which is used with N_{PS} to determine the current-sense resistor value necessary to achieve the regulated CC target, I_{OCC} (see Equation 14).
Because a small portion of the energy stored in the transformer does not transfer to the output, a transformer-efficiency term is included in the R_{CS} equation. This efficiency number includes the core and winding losses, the leakage-inductance ratio, and a bias-power to maximum-output-power ratio. An overall-transformer efficiency of 0.91 is a good estimate based on 3.5% leakage inductance, 5% core & winding loss, and 0.5% bias power, for example. Adjust these estimates as appropriate based on each specific application.
The primary transformer inductance is calculated using the standard energy storage equation for flyback transformers. Primary current, maximum switching frequency, output voltage and current targets, and transformer power losses are included in Equation 16.
First, determine the transformer primary peak current using Equation 15. Peak primary current is the maximum current-sense threshold divided by the current-sense resistance.
N_{AS} is determined by the lowest target operating output voltage while in constant-current regulation and by the VDD UVLO turnoff threshold of the UCC28740. Additional energy is supplied to VDD from the transformer leakage-inductance which allows a lower turns ratio to be used in many designs.