Alternators are used to power the automotive electrical system and charge the battery during normal run time of the vehicle. Rectified alternator output contains residual AC ripple voltage superimposed on the DC battery voltage due to various reasons. These reasons include engine speed variation, regulator duty cycle with field switching ON and OFF, and electrical load variations. On a 12-V battery supply, alternator output voltage is regulated by a voltage regulator between 14.5 V to 12.5 V by controlling the field current of alternator rotor. All electronic modules are tested for proper operation with superimposed AC ripple on the DC battery voltage. AC super imposed test specified in ISO 16750-2 and LV124 E-06 requires AC ripple of 2-V peak-peak on a 13.5-V DC battery voltage, swept from 15 Hz to 30 kHz. LM74722-Q1 rectifies the AC superimposed voltage by turning the MOSFET Q1 OFF quickly to cut off reverse current and turning the MOSFET Q1 ON quickly during forward conduction. Figure 9-3 shows active rectification of 6-V peak-peak 100-kHz AC input by LM74722-Q1. Fast turn-off and quick turn-on of the MOSFET reduces power dissipation in the MOSFET Q1, and active rectification reduces power dissipation in the output hold-up capacitor ESR by half. Figure 9-4 shows active rectification of 2-V peak-peak 200-kHz AC input.

Figure 9-3 AC Super Imposed Test - 6-V Peak-Peak 100 kHz

Figure 9-4 AC Super Imposed Test - 2-V Peak-Peak 200 kHz