Inductive power transfer system and method for operating an inductive power transfer system

1. An inductive power transfer system for supplying electrical power from a transmitter-side subsystem connectable to a mains to a receiver-side subsystem connectable to a load, the system comprising:

a transmitter coil and a receiver coil;

a transmitter-side power converter including a mains rectifier stage to power a transmitter-side dc-bus and arranged to control a transmitter-side dc-bus voltage U 1,dc according to a corresponding reference value U 1,dc *;

a transmitter-side inverter stage with a controllable turn-off time of switches of the transmitter-side inverter stage to supply the transmitter coil with an alternating current;

a receiver-side power converter having a receiver-side rectifier stage to rectify a voltage induced in the receiver coil and to power a receiver-side dc-bus and a receiver-side charging converter arranged to control a receiver-side dc-bus voltage U 2,dc according to a corresponding reference value U 2,dc * ;

one or more power controllers arranged to determine a value that is indicative of a power transfer efficiency of the power transfer and to calculate, from this value indicative of the power transfer efficiency, the reference values U 1,dc *, U 2,dc * for the transmitter-side dc-bus voltage U 1,dc and the receiver-side dc-bus voltage U 2,dc ,

an inverter stage switching controller arranged to control the turn-off time of the switches of the transmitter-side inverter stage to at least minimize switching losses in the transmitter-side inverter stage; and

a communication channel for transmitting the reference value U 1,dc * for the transmitter-side dc-bus voltage U 1,dc from the receiver-side subsystem to the transmitter-side subsystem.

2. The inductive power transfer system of claim 1 , wherein the value that is indicative of the power transfer efficiency is a magnetic coupling k between the transmitter coil and receiver coil.

3. The inductive power transfer system of claim 1 , wherein the value indicative of the power transfer efficiency is computed from a measurement of an input power P in input into the transmitter-side inverter stage or the transmitter coil and a measurement of a received power P out that is output by the receiver coil or the receiver-side rectifier stage.

4. The inductive power transfer system of claim 1 , wherein at least one of the reference values U 1,dc *, U 2,dc * for the transmitter-side dc-bus voltage U 1,dc and the receiver-side dc-bus voltage U 2,dc is determined from a look-up table wherein reference values obtained from an off-line calculation are stored as a function of transmitter and receiver coil currents and a switching frequency f sw of the transmitter-side inverter stage.

5. The inductive power transfer system of claim 1 , wherein the one or more power controllers are configured to compute the reference value U 2,dc * for the receiver-side dc-bus voltage U 2,dc from a measured difference between a load current I batt and a reference value I batt * for the load current; and to compute the reference value U 1,dc * for the transmitter-side dc-bus voltage U 1,dc from the reference value I batt * for the load current and a measured value of the load voltage U batt .

6. The inductive power transfer system of claim 1 , wherein the value that is indicative of the power transfer efficiency is a magnetic coupling k between the transmitter coil and receiver coil.

7. The inductive power transfer system of claim 1 , wherein the value indicative of the power transfer efficiency is computed from a measurement of an input power P in input into the transmitter-side inverter stage or the transmitter coil and a measurement of a received power P out that is output by the receiver coil or the receiver-side rectifier stage.

8. The inductive power transfer system of claim 1 , wherein at least one of the reference values U 1,dc *, U 2,dc * for the transmitter-side dc-bus voltage U 1,dc and the receiver-side dc-bus voltage U 2,dc is determined from a look-up table wherein reference values obtained from an off-line calculation are stored as a function of transmitter and receiver coil currents and a switching frequency f sw of the transmitter-side inverter stage.

9. The inductive power transfer system of claim 1 , wherein the one or more power controllers are configured to compute the reference value U 2,dc * for the receiver-side dc-bus voltage U 2,dc from a measured difference between a load current I batt and a reference value I batt * for the load current; and to compute the reference value U 1,dc * for the transmitter-side dc-bus voltage U 1,dc from the reference value I batt * for the load current and a measured value of the load voltage U batt .

10. The inductive power transfer system of claim 1 , wherein the inverter stage switching controller is arranged to control the transmitter-side inverter stage in an inductive region close to the resonant frequency f 0 of the transmitter for a switch-off current i 1,off , corresponding to a value of the transmitter coil current i 1 (t) at the time of commutation, that is at a value that minimizes a switching loss of the transmitter-side inverter stage.

11. The inductive power transfer system of claim 1 , wherein the inverter stage switching controller is arranged to control the turn-off time of the switches of the transmitter-side inverter stage for a switch-off current i 1,off to follow a switch-off current reference value.

12. The inductive power transfer system of claim 1 , wherein the inverter stage switching controller is arranged to estimate a power loss in the semiconductor switches of the transmitter-side inverter stage and to perform an iterative search procedure, varying the turn-off time of the switches of the transmitter-side inverter stage to minimize the power loss.

13. The inductive power transfer system of claim 1 , wherein the inverter stage switching controller is arranged to control the turn-off time of the switches of the transmitter-side inverter stage for the transmitter coil current i 1 (t) to have a constant phase shift relative to the output voltage u 1 (t) of the transmitter-side inverter stage.

14. A transmitter side subsystem of an inductive power transfer system, connectable to a mains and being able to supply electrical power to a receiver-side subsystem, the transmitter side subsystem comprising:

a transmitter coil;

a transmitter-side power converter including a mains rectifier stage to power a transmitter-side dc-bus and arranged to control a transmitter-side dc-bus voltage U 1,dc according to a corresponding reference value U 1,dc *, and

a transmitter-side inverter stage with a controllable turn-off time of switches of the transmitter-side inverter stage to supply the transmitter coil with an alternating current;

transmitter-side communication means for receiving the reference value U 1,dc * for the transmitter-side dc-bus voltage U 1,dc from the receiver-side subsystem; and

an inverter stage switching controller arranged to control the turn-off time of the switches of the transmitter-side inverter stage to at least minimize switching losses in the transmitter-side inverter stage transmitter-side inverter stag.

15. A receiver side subsystem of an inductive power transfer system, the receiver-side subsystem being connectable to a load and being able to receive electrical power from a transmitter-side subsystem, the receiver-side subsystem comprising:

a receiver coil;

a receiver-side power converter including a receiver-side rectifier stage to rectify a voltage induced in the receiver coil and to power a receiver-side dc-bus and a receiver-side charging converter arranged to control a receiver-side dc-bus voltage U 2,dc according to a corresponding reference value U 2,dc *;

a receiver-side controller arranged to determine a value that is indicative of a power transfer efficiency of the power transfer and to calculate, from this value indicative of the power transfer efficiency, reference values U 1,dc *, U 2,dc * for a transmitter-side dc-bus voltage U 1,dc and the receiver-side dc-bus voltage U 2,dc , and

receiver-side communication means for transmitting the reference value U 1,dc * for the transmitter-side dc-bus voltage U 1,dc from the receiver-side subsystem to the transmitter-side subsystem.

16. A method for controlling an inductive power transfer system for supplying electrical power from a transmitter-side subsystem connectable to a mains to a receiver-side subsystem that is connectable to a load, the inductive power transfer system that includes:

a transmitter coil and a receiver coil;

a transmitter-side power converter having a mains rectifier stage to power a transmitter-side dc-bus;

a transmitter-side inverter stage with a controllable turn-off time of switches of the transmitter-side inverter stage to supply the transmitter coil with an alternating current; and

a receiver-side power converter including a receiver-side rectifier stage to rectify a voltage induced in the receiver coil and to power a receiver-side dc-bus and a receiver-side charging converter arranged to control a receiver-side dc-bus voltage U 2,dc according to a corresponding reference value U 2,dc *;

the method comprising the steps of:

determining a value that is indicative of a power transfer efficiency of the power transfer and calculating, from this value indicative of the power transfer efficiency, reference values U 1,dc *, U 2,dc * for a transmitter-side dc-bus voltage U 1,dc and a receiver-side dc-bus voltage U 2,dc ,

transmitting the reference value U 1,dc * for the transmitter-side dc-bus voltage U 1,dc from the receiver-side subsystem to the transmitter-side subsystem;

controlling the mains rectifier stage for the transmitter-side dc-bus voltage U 1,dc of the transmitter-side dc-bus to follow the corresponding reference value U 1,dc *;

controlling the receiver-side charging converter for the receiver-side dc-bus voltage U 2,dc of the receiver-side dc-bus to follow the corresponding reference value U 2,dc *; and

controlling the turn-off time of the switches of the transmitter-side inverter stage to at least minimize switching losses in the transmitter-side inverter stage transmitter-side inverter stage.