Datasheet

SB SB _ CONV

P P 2.5 mW= +

UCC28710, UCC28711

UCC28712, UCC28713

UCC28714, UCC28715

SLUSB86 –NOVEMBER 2012

www.ti.com

Stand-by Power Estimate

Assuming no-load stand-by power is a critical design parameter, determine estimated no-load power based on

target converter maximum switching frequency and output power rating.

The following equation estimates the stand-by power of the converter.

(7)

For a typical USB charger application, the bias power during no-load is approximately 2.5 mW. This is based on

25-V VDD and 100-µA bias current. The output preload resistor can be estimated by V

OCV

and the difference in

the converter stand-by power and the bias power. The equation for output preload resistance accounts for bias

power estimated at 2.5 mW.

(8)

The capacitor bulk voltage for the loss estimation is the highest voltage for the stand-by power measurement,

typically 325 V

For the total stand-by power estimation add an estimated 2.5 mW for snubber loss to the converter stand-by

power loss.

(9)

Input Bulk Capacitance and Minimum Bulk Voltage

Determine the minimum voltage on the input capacitance, C

and C

total, in order to determine the maximum

Np to Ns turns ratio of the transformer. The input power of the converter based on target full-load efficiency,

minimum input RMS voltage, and minimum AC input frequency are used to determine the input capacitance

requirement.

Maximum input power is determined based on V

OCV

, I

OCC

, and the full-load efficiency target.

(10)

The below equation provides an accurate solution for input capacitance based on a target minimum bulk

capacitor voltage. To target a given input capacitance value, iterate the minimum capacitor voltage to achieve the

target capacitance.

(11)

Product Folder Links: UCC28710, UCC28711 UCC28712, UCC28713 UCC28714, UCC28715