+ Ns Np Cin + Cr Ns Co - Vo - FIG 1 A conventional SAC engine in a simplified format. ILoad VP LV and HV ports of the SAC. The next step involves applying voltage to the controller that is referenced to the SAC's primary side. A small current passes through the diode from the HV source to the SAC controller, which is energized through bias circuits internally connected to the HV port. As soon as the controller switches on and carries out its prestart sequence of checks, it enables the SAC power train. In this case, with the HVDC source preset to 255 V, a voltage step develops at the HV port that shifts from the initial level of 255 V directly to 333 V. The BCM is observed porting current in reverse mode from the SAC's HV port into the E-LOAD resistor, once the E-LOAD switch is closed. The bucking and boosting actions associated with voltage and current are interchanged between forward and reverse modes. Irrespective of the mode of operation (1) is universally applicable to SAC action as V I K = Vout = I in . in out IP (1) VOUT Modified Bidirectional Converter IS Time FIG 2 Oscillograms associated with zero voltage switching in the SAC associated with a load step. VP is the voltage across the primary winding induced by the switching of diagonal pairs of MOSFETs in the primary section of the SAC. Switching frequency is precisely set to the resonant frequency of the low-Q tank circuit formed by C r and the power magnetic's leakage inductance. Note that the primary and secondary currents I P and I S are sinusoidal in nature. A slight drop in voltage with the onset of the load is afforded by finite interconnect impedance in the SAC. 55.5 * 6 = 333 V 255 V HVDC + - Source E-Load +In +Out 1/K = 6 K = 1/6 -In -Out 55.5 V 255/6 = 42.5 V + LVDC - Source A particular need in switched mode power systems (SMPS) is to provide boost conversion factors in excess of five. The SAC inherently operates very efficiently in reverse mode at boost factors above five. From this point, it is only a question of scaling up power levels, which has been achieved in a modified architecture. This arrangement is suited for use in 384-V dc distribution schemes that have exhibited good conversion efficiency at limited power throughputs. The reverse SAC changes the design space on the basis of this reverse processing capability. The primary side consists of a stacked half-bridge arrangement. The input stack is realized with lowcost MOSFETs that have moderate figures of merit. This facilitates a front end that has very good standoff voltage capability [5]. Table 1 shows a number of performance parameters aligned to compare the old and new SAC topologies for bidirectional conversion. Power Flow Classifications FIG 3 An example of the SAC reverse start-up. 68 IEEE PowEr ElECtronICS MagazInE z June 2018 We can broadly classify the applications in accordance with the basic configurations shown in Figure 4. The unique aspect of the