8.2.2.2 Standby Power Estimate and No-Load Switching Frequency

Assuming minimal no-load standby power is a critical design requirement, determine the estimated no-load power loss based on an accounting of all no-load operating and leakage currents at their respective voltages. Close attention to detail is necessary to account for all of the sources of leakage, however, in many cases, prototype measurement is the only means to obtain a realistic estimation of total primary and secondary leakage currents. At present, converter standby power is certified by compliance-agency authorities based on steady-state room-temperature operation at the highest nominal input voltage rating (typically 230 Vrms).

Equation 6 estimates the standby power loss from the sum of all leakage currents of the primary-side components of the converter. These leakage currents are measured in aggregate by disconnecting the HV input of the controller from the bulk-voltage rail to prevent operating currents from interfering with the leakage measurement.

Equation 6.

Equation 7 estimates the standby power loss from the sum of all leakage and operating currents of the secondary-side components on the output of the converter. Leakage currents result from reverse voltage applied across the output rectifier and capacitors, while the operating current includes currents required by the shunt-regulator, optocoupler, and associated components.

Equation 7.

Equation 8 estimates the standby power loss from the sum of all leakage and operating currents of the auxiliary-side components on the controller of the converter. Leakage currents of the auxiliary diode and capacitor are usually negligible. The operating current includes the wait-state current, I_WAIT, of the UCC28740 controller, plus the optocoupler-output current for the FB network in the steady-state no-load condition. The VDD voltage in the no-load condition V_VDDNL are the lowest practicable value to minimize loss.

Equation 8.

Note that P_{PRI_SB} is the only loss that is not dependent on transformer conversion efficiency. P_{SEC_SB} and P_{AUX_SB} are processed through the transformer and incur additional losses as a consequence. Typically, the transformer no-load conversion efficiency η_SWNL lies in the range of 0.50 to 0.70. Total standby input power (no-load condition) is estimated by Equation 9.

Equation 9.

Although the UCC28740 is capable of operating at the minimum switching frequency of 170 Hz, a typical converter is likely to require a higher frequency to sustain operation at no-load. An accurate estimate of the no-load switching frequency f_SWNL entails a thorough accounting of all switching-related energy losses within the converter including parasitic elements of the power-train components. In general, f_SWNL is likely to lie within the range of 400 Hz to 800 Hz. A more detailed treatment of standby power and no-load frequency is beyond the scope of this data sheet.