SBVS343A March 2019 – September 2019 TPS7A78
The AC input current cannot be directly calculated because of the active bridge control; see the Active Bridge Control section. The AC input current through the AC+ and AC– pins is a combination of two current components, as shown in Figure 24: ISHUNT and IPEAK. The ISHUNT current component is identified by its wave profile because this component is the AC charging current supplied by the cap-drop capacitor CS. The IPEAK current component is identified by its instantaneous peak current profile.
The frequency of the shunt activity is uncorrelated to the AC input frequency. Therefore, the standby power must be measured with a power analyzer. Fortunately, using a power analyzer is relatively simple and the measurement setup shown in Figure 25 and Figure 26 can be used to measure the standby power and the output efficiency.
If the application has an upstream current-limit circuit that limits any high-transient input currents, such as surge or hot-plug currents, the requirement for the surge resistor RS can be relaxed. The input transient current-limit circuit allows the RS resistor to be removed, thus significantly improving the standby power and output efficiency because no power loss is dissipated in RS.
|VAC = 120 VRMS at 60 Hz, FB, VLDO_OUT = 5.0 V,
IOUT = 0 mA
|VAC = 120 VRMS at 60 Hz, HB, CS = 150 nF, VLDO_OUT = 5.0 V|
|VAC = 120 VRMS at 60 Hz, FB, CS = 150 nF, VLDO_OUT = 5.0 V|