An apparatus comprising: first (C1; B1), second (C2; B2), third (C3; B3) and fourth (C4; B4) serially-connected energy storage devices; first, second, third and fourth serially-connected windings; a first switch configured to connect a first tap of the first winding to a first terminal of the first energy storage device (C1; B1); a second switch configured to connect a second tap of the first winding and a first tap of the second winding to a second terminal of the first energy storage device (C1; B1) and a first terminal of the second energy storage device (C2; B2), wherein a second tap of the second winding and a first tap of the third winding are connected to a second terminal of the second energy storage device (C2; B2) and a first terminal of the third energy storage device (C3; B3), wherein the second switch is a bidirectional switch; a third switch configured to connect a second tap of the third winding and a first tap of the fourth winding to a second terminal of the third energy storage device (C3; B3) and a first terminal of the fourth energy storage device (C4; B4), wherein the third switch is a bidirectional switch; a fourth switch configured to connect a second tap of the fourth winding to a second terminal of the fourth energy storage device (C4; B4); a first diode connected between the first terminal of the first energy storage device (C1; B1) and the second terminal of the fourth winding wherein the anode of the first diode is connected to the second terminal of the fourth winding and the cathode of the first diode is connected to the first terminal of the first energy storage device (C1; B1); and a second diode connected between second terminal of the fourth energy storage device (C4; B4) and the first terminal of the first winding and wherein the anode of the second diode is connected to the second terminal of the fourth energy storage device (C4; B4) and the cathode of the second diode is connected to the first terminal of the first winding; and a control circuit configured to operate the first, second, third and fourth switches at a same duty cycle; wherein the first and second windings are magnetically coupled to one another and wherein the third and fourth windings are magnetically coupled to one another, wherein the first and second windings are wound on a first core and wherein the third and fourth windings are wound on a second core. first (C1; B1), second (C2; B2), third (C3; B3) and fourth (C4; B4) serially-connected energy storage devices; first, second, third and fourth serially-connected windings; a first switch configured to connect a first tap of the first winding to a first terminal of the first energy storage device (C1; B1); a second switch configured to connect a second tap of the first winding and a first tap of the second winding to a second terminal of the first energy storage device (C1; B1) and a first terminal of the second energy storage device (C2; B2), wherein a second tap of the second winding and a first tap of the third winding are connected to a second terminal of the second energy storage device (C2; B2) and a first terminal of the third energy storage device (C3; B3), wherein the second switch is a bidirectional switch; a third switch configured to connect a second tap of the third winding and a first tap of the fourth winding to a second terminal of the third energy storage device (C3; B3) and a first terminal of the fourth energy storage device (C4; B4), wherein the third switch is a bidirectional switch; a fourth switch configured to connect a second tap of the fourth winding to a second terminal of the fourth energy storage device (C4; B4); a first diode connected between the first terminal of the first energy storage device (C1; B1) and the second terminal of the fourth winding wherein the anode of the first diode is connected to the second terminal of the fourth winding and the cathode of the first diode is connected to the first terminal of the first energy storage device (C1; B1); and a second diode connected between second terminal of the fourth energy storage device (C4; B4) and the first terminal of the first winding and wherein the anode of the second diode is connected to the second terminal of the fourth energy storage device (C4; B4) and the cathode of the second diode is connected to the first terminal of the first winding; and a control circuit configured to operate the first, second, third and fourth switches at a same duty cycle; wherein the first and second windings are magnetically coupled to one another and wherein the third and fourth windings are magnetically coupled to one another, wherein the first and second windings are wound on a first core and wherein the third and fourth windings are wound on a second core.