TIDA-020031: Max duty cycle calculation

Part Number: TIDA-020031

Customer is referring to both TIDA-020031 and SLUA560D

They have some questions about how max Duty cycle is selected. 

SLUA560D use 70% as the max duty cycle in equation (4), 

In design, use a center-tap transformer, which leads to a 21:1:1 transformer. 

In TIDA-020031, use 'secondary duty max' as 90%. This design uses a current doubler on the secondary side. 

1. Eq (4) in SLUA560D and Eq(4) in TIDA-020031 similar other than "Dmax" and "Dsec_max".  
   1. Can you explain the differences in "Dmax" and "Dsec_max"?
   2. How should we define Duty_max for our design?
2. Eq(5) in TIDA-020031, How to define Vsec_min ?