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 a gate driver, or the UCC28781-Q1. 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 and rectifier forward drop
(VO(MAX) + VF). VVDD(MAX) is also limited
by the lowest maximum voltage rating of any other devices connected to the VDD
pin. For designs with a fixed output voltage or a narrow output range, the
maximum Auxiliary winding turns (NA(max)) is given by the following
equation.
Equation 34. ![GUID-FD33122D-4F68-446B-BEE3-D04F5ECA0E2B-low.gif](/ods/images/JAJSLC6/GUID-FD33122D-4F68-446B-BEE3-D04F5ECA0E2B-low.gif)
- The nominal VVDD calculation must
consider the impact on the stand-by power. Higher VVDD results in a
static loss increase with the total bias current of all devices connected to the
VDD pin.
- VVDD should be higher than the 13-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 here represents the
voltage difference between the nominal VVDD and the survival-mode
threshold. A minimum of 3 V is a recommended design margin for ΔV.
Equation 35. ![GUID-4C7D9CC8-36F3-48DA-B9A3-CDD3AC21BA98-low.gif](/ods/images/JAJSLC6/GUID-4C7D9CC8-36F3-48DA-B9A3-CDD3AC21BA98-low.gif)
NA(min) must also accommodate the
highest VVDD(off) threshold of other devices powered by VDD, if any.
Select an integer value for NA between the lowest NA(MAX) and
the highest NA(MIN) with consideration of #2. For best performance,
design the DC resistance of the auxiliary winding to be < 0.1 Ω.