18.104.22.168.5 Turns of Auxiliary Winding (NA)
Turns of the auxiliary winding (NA) is an integer value usually chosen to provide a nominal VVDD that satisfies all devices powered from VVDD, such as the gate driver, UCC28780, etc. NA is determined by the following design considerations:
- VVDD must be lower than the maximum rating voltage of VDD pin (VVDD(MAX)) at maximum output voltage (VO(MAX)). VVDD(MAX) is limited by the lowest voltage rating of the devices connected to VDD pin.
- The nominal VVDD should consider the impact on the standby power. Higher VVDD results in the static-loss increase with the total bias current of the devices connected to VDD pin.
- VVDD should be higher than the 11-V threshold voltage of survival mode, which is the sum of VVDD(OFF) and VVDD(PCT), at the minimum sustained output voltage (VO(MIN)). ΔV represents the voltage difference between the nominal VVDD and the survival-mode threshold. A minimum of 3 V is a recommended design margin of ΔV.