Power supply circuit

1. A power supply circuit comprising:

an alternating-current signal synthesis unit that synthesizes a plurality of input alternating-current signals to output the synthesized signal as an output alternating-current signal;

an AC/DC conversion circuit that converts the output alternating-current signal into a direct-current voltage signal; and

a control circuit that generates an input selection signal, the input selection signal specifying a combination of the input alternating-current signals to be synthesized so that the output alternating-current signal or the direct-current voltage signal is maximized,

wherein the alternating-current signal synthesis unit synthesizes the input alternating-current signals to be synthesized specified by the input selection signal to output the output alternating-current signal, and

wherein the alternating-current signal synthesis unit comprises:

a plurality of alternating-current coupling elements each having primary-side input terminals to which the respective input alternating-current signals are input and secondary-side terminals connected in series with each other; and

a plurality of input selection switches that short-circuit or open at least one of the two primary-side terminals and the two secondary-side terminals of the respective alternating-current coupling elements according to the input selection signal.

2. The power supply circuit according to claim 1 , wherein the plurality of input alternating-current signals are reception signals generated through an antenna which receives a radio signal.

3. The power supply circuit according to claim 1 , further comprising a voltage detection circuit that detects a voltage of the direct-current voltage signal, wherein:

the control circuit switches, by the input selection signal, the combination of the input alternating-current signals to be synthesized by the alternating-current signal synthesis unit,

the voltage detection circuit detects the voltage of the direct-current voltage signal for each combination of the input alternating-current signals, and outputs a voltage detection signal to the control circuit every time a maximum value of the voltage of the direct-current voltage signal is updated, and

the control circuit determines the combination of the input alternating-current signals which maximizes the voltage of the direct-current voltage signal according to the voltage detection signal, and outputs the input selection signal corresponding to the combination of the input alternating-current signals which maximizes the voltage of the direct-current voltage signal in a normal operation state.

4. The power supply circuit according to claim 1 , comprising a regulator circuit which steps down or steps up the direct-current voltage signal to generate an internal power supply.

5. The power supply circuit according to claim 4 , comprising:

a start-up circuit that is arranged in parallel with the regulator circuit, generates the internal power supply based on an input voltage lower than a voltage input to the regulator circuit, and outputs the internal power supply to an output terminal of the regulator circuit;

a power supply changeover switch that transmits the direct-current voltage signal to one of the regulator circuit and the start-up circuit; and

an internal voltage detection circuit that monitors a voltage of the output terminal of the regulator circuit, and outputs a power supply selection signal to the power supply changeover switch when the voltage of the output terminal of the regulator circuit has reached a predetermined voltage or more, the power supply selection signal instructing transmission of the direct-current voltage signal to the regulator circuit.

6. The power supply circuit according to claim 1 , comprising a voltage detection circuit that detects a voltage of the output alternating-current signal, wherein:

the control circuit switches, by the input selection signal, the combination of the input alternating-current signals to be synthesized by the alternating-current signal synthesis unit,

the voltage detection circuit detects the voltage of the direct-current voltage signal for each combination of the input alternating-current signals, and outputs a voltage detection signal to the control circuit every time a maximum value of the voltage of the direct-current voltage signal is updated, and

the control circuit determines the combination of the input alternating-current signals which maximizes the voltage of the direct-current voltage signal according to the voltage detection signal, and outputs the input selection signal corresponding to the combination of the input alternating-current signals which maximizes the voltage of the direct-current voltage signal in a normal operation state.

7. The power supply circuit according to claim 1 , comprising a phase difference detection circuit that detects a phase difference between the plurality of input alternating-current signals,

wherein the control circuit outputs, based on the phase difference detected in the phase difference detection circuit, the input selection signal which instructs to exclude, from a target to be synthesized, a second input alternating-current signal of the plurality of input alternating-current signals in which a phase difference from a first input alternating-current signal is equal to or larger than a predetermined phase difference.

8. The power supply circuit according to claim 1 , comprising a phase adjustment circuit that is provided between the alternating-current signal synthesis unit and a plurality of alternating-current power supplies that output the plurality of input alternating-current signals and adjusts a phase of an input alternating-current signal in which a phase difference from other input alternating-current signals is determined to be equal to or larger than a predetermined phase difference.

9. The power supply circuit according to claim 1 , wherein the alternating-current signal synthesis unit comprises a transformer or a balun as an alternating-current coupling element.

10. A power supply circuit comprising:

a plurality of alternating-current coupling elements each including a first input terminal and a second input terminal on a primary side to which an input alternating-current signal is input and a first output terminal and a second output terminal on a secondary side, the first output terminal and the second output terminal on the secondary side being connected in series, the plurality of alternating-current coupling elements outputting an output alternating-current signal from between the first output terminal provided in a lowest level and the second output terminal provided in a highest level;

a plurality of input selection switches that are provided in the respective alternating-current coupling elements and provided between the terminals on the primary side or the terminals on the secondary side;

an AC/DC conversion circuit that converts the output alternating-current signal into a direct-current voltage signal; and

a control circuit that generates an input selection signal specifying a combination of the input alternating-current signals to be synthesized so as to maximize the output alternating-current signal or the direct-current voltage signal,

wherein the plurality of input selection switches switch, according to the input selection signal, whether to short-circuit or open the terminals on the primary side and the terminals on the secondary side.

11. The power supply circuit according to claim 10 , wherein the plurality of input alternating-current signals are reception signals generated through an antenna which receives a radio signal.

12. The power supply circuit according to claim 10 , wherein the plurality of alternating-current coupling elements are formed on the same semiconductor substrate on which the plurality of input selection switches, the AC/DC conversion circuit, and the control circuit are formed.

13. The power supply circuit according to claim 10 , wherein the input selection switches each comprise at least one of a primary-side input selection switch provided between the terminals on the primary side and a secondary-side input selection switch provided between the terminals on the secondary side.

14. The power supply circuit according to claim 10 , wherein the alternating-current coupling element is a transformer or a balun.

15. A power supply circuit comprising:

an alternating-current signal synthesis unit that synthesizes a plurality of input alternating-current signals to output the synthesized signal as an output alternating-current signal;

an AC/DC conversion circuit that converts the output alternating-current signal into a direct-current voltage signal; and

a control circuit that generates an input selection signal, the input selection signal specifying a combination of the input alternating-current signals to be synthesized so that the output alternating-current signal or the direct-current voltage signal becomes equal to or larger than a predetermined voltage that is set in advance,

wherein the alternating-current signal synthesis unit synthesizes the input alternating-current signals to be synthesized specified by the input selection signal to output the output alternating-current signal,

wherein the control circuit generates the input selection signal specifying the combination of the input alternating-current signals to be synthesized during an operation configuration processing stage which occurs before a normal operating state, and

wherein the control circuit initially connects all of the plurality of input alternating-current signals during the operation configuration processing stage in order to obtain a DC voltage V DC resulting if all the input alternating-current signals are used.

16. The power supply circuit according to claim 15 , wherein the control circuit then selects and connects one of the input alternating-current signals for comparison with the voltage V DC .

17. The power supply circuit according to claim 16 , wherein the control circuit sequentially connects one of the input alternating-current signals for comparison and retains the higher value voltage for subsequent comparisons with remaining input alternating-current signals.