System and method for safe switching in an AC-to-AC converter

A load control device to control current flow to an AC load, the load control device comprising:

• a circuit including:
• an input connectable to a line terminal of an AC source;
• an output connectable to a load terminal of an AC load;
• one or more supply lines connecting the input and output to transmit power from the AC source to the AC load, each supply line corresponding to a phase in the AC load;
• a plurality of line-side switches connected between the line terminals and the load terminals, such that each supply line includes at least one line-side switch connected thereto;
• and
• a plurality of floating-neutral side switches connected to the load terminal at one end and together at a common connection at another end, such that each supply line includes at least one floating-neutral side switch connected thereto;
• and
• a controller connected to the circuit and programmed to:
• implement a switching pattern for each of the plurality of line-side switches and each of the plurality of floating-neutral side switches based on a determined direction of current flow on each of the supply lines;
• and
• transition between switching patterns responsive to a changing direction of current flow on a supply line, wherein in transitioning between switching patterns the controller is further programmed to:
• establish a transition window;
• and
• implement a transition switching pattern for each of the plurality of line-side switches and each of the plurality of floating-neutral side switches for a duration of the transition window, to prevent a voltage spike from occurring.

A method for controlling current flow to an AC load comprising:

• providing a circuit in series between an AC power source and the AC load, the circuit comprising a plurality of switches forming a group of line-side switches connected to one or more supply lines between the line terminals of the AC power source and load terminals of the AC load and a group of floating-neutral side switches connected to the one or more supply lines at one end and together at a common connection at another end;
• selectively operating the circuit in an active mode and a free-wheeling mode so as to selectively provide a full phase voltage to the load terminals during the active mode and a zero voltage to the load terminals during the free-wheeling mode of operation, wherein operating the circuit comprises:
• implementing a first switching pattern for each of the line-side switches and each of the floating-neutral side switches based on a current sign on each of the one or more supply lines, the current sign comprising a positive current or negative current;
• and
• implementing a second switching pattern for each of the line-side switches and each of the floating-neutral side switches upon current on one supply lines changing its current sign upon traversing a zero crossing;
• wherein, in transitioning from the first switching pattern to the second switching pattern, a transition switching pattern is implemented during a transition window established about the zero crossing of the current changing its sign.