The maximum primary-to-secondary turns ratio can be determined by the target maximum switching frequency at full load, the minimum input capacitor bulk voltage, and the estimated DCM quasi-resonant time.

Initially determine the maximum available total duty cycle of the on time and secondary conduction time based on target switching frequency and DCM resonant time. For DCM resonant time, assume 500 kHz if you do not have an estimate from previous designs. For the transition mode operation limit, the period required from the end of secondary current conduction to the first valley of the V_DS voltage is ½ of the DCM resonant period, or 1 µs assuming 500-kHz resonant frequency. D_MAX can be determined using the equation below.

Equation 21.

Once D_MAX is known, the maximum turns ratio of the primary to secondary can be determined with the equation below. D_MAGCC is defined as the secondary diode conduction duty cycle during constant-current, CC, operation. It is set internally by the UCC28700-Q1 at 0.425. The total voltage on the secondary winding needs to be determined; which is the sum of V_OCV, the secondary rectifier V_F, and the cable compensation voltage (V_OCBC). For the 5-V USB charger applications, a turns ratio range of 13 to 15 is typically used.

Equation 22.

Once an optimum turns ratio is determined from a detailed transformer design, use this ratio for the following parameters.

The UCC28700-Q1 controller constant-current regulation is achieved by maintaining a maximum D_MAG duty cycle of 0.425 at the maximum primary current setting. The transformer turns ratio and constant-current regulating voltage determine the current sense resistor for a target constant current.

Since not all of the energy stored in the transformer is transferred to the secondary, a transformer efficiency term is included. This efficiency number includes the core and winding losses, leakage inductance ratio, and bias power ratio to rated output power. For a 5-V, 1-A charger example, bias power of 1.5% is a good estimate. An overall transformer efficiency of 0.9 is a good estimate to include 3.5% leakage inductance, 5% core and winding loss, and 1.5% bias power.

Equation 23.

The primary transformer inductance can be calculated using the standard energy storage equation for flyback transformers. Primary current, maximum switching frequency and output and transformer power losses are included in the equation below. Initially determine transformer primary current.

Primary current is simply the maximum current sense threshold divided by the current sense resistance.

Equation 24.

Equation 25.

The secondary winding to auxiliary winding transformer turns ratio (N_AS) is determined by the lowest target operating output voltage in constant-current regulation and the VDD UVLO of the UCC28700-Q1. There is additional energy supplied to VDD from the transformer leakage inductance energy which allows a lower turns ratio to be used in many designs.

Equation 26.