Control modes for resonant DC-to-DC-converter

1. An electrical power converter, comprising:

an inverter with semiconductor switches;

a resonant circuit coupled with the inverter; and

a controller for switching the semiconductor switches of the inverter to switching states,

wherein the controller is adapted for periodically switching the inverter between switching states, such that a periodic resonant current is generated in the resonant circuit, the switching states comprising a free-wheeling state for short-circuiting the resonant circuit and at least one of:

a power forward state, in which a DC voltage is applied to the resonant circuit, which has the same polarity as the resonant current, and

a power backward state, in which a DC voltage is applied to the resonant circuit that has the opposite polarity as the resonant current,

wherein the controller is adapted for determining a zero crossing of the resonant current as a specific periodic point, and wherein the controller is adapted for switching the inverter to the power forward state and to the power backward state at a predefined first time period before the zero crossing,

wherein the controller is adapted for switching the inverter to the free-wheeling switching state at a predefined second time period after the zero crossing and the predefined second time period is a different time period than the predefined first time period,

wherein the controller is adapted for synchronizing switching events of the switching states with the periodic resonant current, such that a switching state is applied to the inverter at a time point associated with the specific periodic point of the periodic resonant current, and

wherein the controller is adapted during a low-power mode for applying a plurality of the switching states such that an overall power feed backward from the resonant circuit to an input of the inverter is balanced with an overall power feed forward from the input of the inverter to the resonant circuit.

2. The electrical power converter of claim 1 ,

wherein at least two switching events occur during one half-cycle of the period of the resonant current.

3. The electrical power converter of claim 1 ,

wherein the controller is adapted for measuring the resonant current in the resonant circuit.

4. The electrical power converter of claim 1 ,

wherein the inverter is a full-bridge inverter, which comprises two half-bridges connected in parallel to a DC input, and

wherein each half-bridge comprises two branches, each branch having a semiconductor switch, a free-wheeling diode and a snubber capacitor connected in parallel across the semiconductor switch and the free-wheeling diode.

5. The electrical power converter of claim 1 ,

wherein the resonant circuit comprises an inductor and a capacitor connected in series.

6. The electrical power converter of claim 1 ,

wherein a primary winding of a transformer is part of an inductivity of the resonant circuit.

7. A DC-to-DC-converter, comprising:

a power converter according to claim 1 ;

a transformer for generating a high AC voltage from an AC voltage in the resonant circuit; and

a rectifier for generating a high DC voltage from the high AC voltage from the transformer.

8. An X-ray device, comprising:

a DC-to-DC converter according to claim 7 ; and

an X-ray tube supplied with the high DC voltage.

9. The electrical power converter of claim 1 ,

wherein the predefined second time period is larger than the predefined first time period.

10. A method for controlling an inverter, the method comprising acts of:

periodically switching the inverter between switching states, such that a periodic resonant current is generated in a resonant circuit coupled to the inverter, the switching states comprising a free-wheeling state for short-circuiting the resonant circuit;

synchronizing switching events of the switching states with the periodic resonant current, such that a switching state is applied to the inverter at a time point associated with a specific periodic point of the periodic resonant current;

determining a zero crossing of the resonant current as the specific periodic point;

switching the inverter to a power forward state and to a power backward state at a predefined first time period before the zero crossing;

switching the inverter to the free-wheeling switching state at a predefined second time period after the zero crossing, wherein the predefined second time period is a different time period than the predefined first time period; and

applying a plurality of the switching states during a low-power mode such that an overall power feed backward from the resonant circuit to an input of the inverter is balanced with an overall power feed forward from the input of the inverter to the resonant circuit.