AC to AC converter

1. An apparatus comprising:

an input capacitor connected between a first terminal and a second terminal;

an output capacitor connected between the second terminal and a third terminal;

a plurality of switches comprising:

a first switch connected between the first terminal and a first node;

a second switch connected between the first node and a second node;

a third switch connected between the second node and a third node; and

a fourth switch connected between the third node and the third terminal;

an inductor connected between the second node and the second terminal; and

a controller configured to:

cause the apparatus to output a single-phase alternating current (AC) voltage of a first frequency between the second terminal and the third terminal by switching, at a second frequency different from the first frequency, the first, the second, the third, and the fourth switches, wherein the switching includes:

during each first half period of the second frequency, turn on the first switch and the third switch, and turn off the second switch and the fourth switch; and

during each second half period of the second frequency, turn on the second switch and the fourth switch, and turn off the first switch and the third switch.

2. The apparatus of claim 1 , wherein the apparatus is configured to output the single-phase AC voltage by:

receiving a second single-phase AC voltage across the first terminal and the second terminal; and

converting, based on the switching, the second single-phase AC voltage to the single-phase AC voltage.

3. The apparatus of claim 2 , wherein the single-phase AC voltage and the second single-phase AC voltage have opposite phases.

4. The apparatus of claim 1 , further comprising a capacitor connected between the first node and the third node.

5. The apparatus of claim 1 , wherein the controller is further configured to, during each first half period of the second frequency, turn on the first switch and the third switch at substantially a same time.

6. The apparatus of claim 5 , wherein the controller is configured to, during each first half period of the second frequency, turn off the third switch prior to turning off the first switch.

7. The apparatus of claim 5 , wherein the controller is configured to switch the first switch under zero-current conditions.

8. The apparatus of claim 1 , wherein the controller is further configured to:

determine a resonant frequency of a current flowing through the inductor; and

adjust, based on the resonant frequency, the second frequency.

9. The apparatus of claim 1 , wherein each of the first switch, the second switch, the third switch, and the fourth switch comprises a body diode.

10. The apparatus of claim 1 , wherein the plurality of switches further comprises:

a fifth switch connected between the first terminal and the first node;

a sixth switch connected between the first node and the second node;

a seventh switch connected between the second node and the third node; and

an eight switch connected between the third node and the third terminal, and wherein the controller is further configured to:

sense a second single-phase AC voltage between the first terminal and the second terminal; and

during a positive half-cycle or a negative half-cycle of the second single-phase AC voltage, turn off the fifth switch, the sixth switch, the seventh switch, and the eighth switch.

11. An apparatus comprising:

an input capacitor connected between a first terminal and a second terminal;

an output capacitor connected between the second terminal and a third terminal;

a plurality of parallel-connected alternating current to alternating current (AC-to-AC) converters, wherein each of the plurality of parallel-connected AC-to-AC converters comprises:

a node;

a first plurality of series-connected switches connected between the first terminal and the node;

a second plurality of series-connected switches connected between the node and the third terminal; and

an inductor connected between the node and the second terminal; and

a controller configured to:

receive an input single-phase alternating current (AC) voltage between the first terminal and the second terminal; and

convert the input single-phase AC voltage to an output single-phase alternating current (AC) voltage of a first frequency between the second terminal and the third terminal by switching, at a second frequency different from the first frequency, a first switch and a second switch of the first plurality of series-connected switches, and a third switch and a fourth switch of the second plurality of series-connected switches, wherein the switching includes, during each first half period of the second frequency and for each of the plurality of parallel-connected AC-to-AC converters:

turn on the first switch, and turn off the second switch, and

turn on the third switch, and turn off the fourth switch.

12. The apparatus of claim 11 , wherein the output AC voltage and the input AC voltage have opposite phases.

13. The apparatus of claim 11 , wherein, in each of the plurality of parallel-connected AC-to-AC converters, the controller is further configured to:

determine a resonant frequency of a current flowing through the inductor; and

adjust, based on the resonant frequency, the second frequency.

14. The apparatus of claim 11 , wherein each of the plurality of parallel-connected AC-to-AC converters further comprises a capacitor connected between the first switch of the first plurality of series-connected switches and the fourth switch of the second plurality of series-connected switches.

15. The apparatus of claim 11 , wherein the controller is further configured to, during each first half period of the second frequency, turn on the first switch and the third switch at substantially a same time.

16. The apparatus of claim 11 , wherein the controller is configured to switch the first switch under zero-current conditions.

17. The apparatus of claim 11 , wherein each of the first plurality of series-connected switches and the second plurality of series-connected switches comprises a body diode.

18. A method comprising:

receiving, by an alternating current to alternating current (AC-to-AC) converter and between an input terminal of the AC-to-AC converter and a neutral terminal of the AC-to-AC converter, an input alternating current (AC) voltage provided by a single-phase AC voltage source;

producing, between an output terminal of the AC-to-AC converter and the neutral terminal, an output AC voltage of a first frequency by switching, at a second frequency different from the first frequency, a plurality of series-connected switches connected between the input terminal and the output terminal, wherein the switching includes:

during each first half period of the second frequency, turning on, by a controller, a first switch and a second switch, of the plurality of series-connected switches and turning off, by the controller, a third switch and a fourth switch, of the plurality of series-connected switches; and

during each second half period of the second frequency, turning off, by the controller, the first switch and the second switch, and turning on, by the controller, the third switch and the fourth switch; and

causing a resonance, at a resonant frequency, between an inductor, connected to a midpoint of the plurality of series-connected switches and the neutral terminal, and a capacitor connected across two switches of the plurality of series-connected switches.

19. The method of claim 18 , further comprising:

turning off the first switch of the plurality of series-connected switches;

sensing a current flowing through a body diode of the first switch; and

turning off, based on a determination that the current is zero, the second switch of the plurality of series-connected switches.

20. The method of claim 18 , further comprising:

determining the resonant frequency; and

adjusting the second frequency based on the resonant frequency.

21. The method of claim 18 , wherein the output AC voltage and the input AC voltage have opposite phases.