Datasheet

UCC28700, UCC28701

UCC28702, UCC28703

www.ti.com

SLUSB41 – JULY 2012

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)

Typical start-up resistance values for R

STR

range from 13 MΩ to 20 MΩ to achieve 1-s start-up time. The

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

typically 325 V

(9)

For the total stand-by power estimation add an estimated 2.5 mW for snubber loss to the start-up resistance and

converter stand-by power loss.

(10)

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.

(11)

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.

(12)

Product Folder Link(s): UCC28700, UCC28701 UCC28702, UCC28703