Resonant power supply with self tuning
View Patent ↗This invention relates to current-fed resonant inverters for electrical power applications to change direct current (DC) into alternating current (AC). One application of the invention is to power supplies for inductive power transfer (IPT) systems. There is provided a resonant inverter including an input for supply of current from a DC power source, a resonant circuit including two coupled inductive elements and a tuning capacitance, the inductive elements being arranged to split current from the power source; a first switching means comprising two switching devices operable in substantially opposite phase to alternately switch current from the power source into the inductive elements; and a second switching means to selectively switch one or more control capacitances into or out of the resonant circuit dependent on a power factor of the resonant circuit.
1. A resonant inverter including:
an input for supply of current from a DC power source, a resonant circuit including two electrically or magnetically coupled inductive elements and a tuning capacitance, the inductive elements being arranged to split current from the power source;
a first switching means comprising two switching devices operable in substantially opposite phase to alternately switch current from the power source into the inductive elements;
a second switching means to selectively switch one or more control capacitances into or out of the resonant circuit dependent on a power factor of the resonant circuit; and
a power factor detection circuit arranged to determine the power factor by determining a phase difference between the current of the resonant circuit and a switching signal for driving the switching devices.
2. The resonant inverter according to claim 1 , wherein the second switching means comprises a switching device and a said control capacitance connected in parallel with each of the switching devices of the first switching means.
3. The resonant inverter according to claim 1 , wherein the second switch means is arranged to switch the control capacitance into or out of the resonant circuit at zero voltage crossings of the tuning capacitance.
4. The resonant circuit according to claim 1 , wherein the power factor detection circuit comprises a current transformer coupled to the resonant circuit and an integrating capacitor, a squarer coupled across the integrating capacitor, and a comparator for comparing the output of the squarer with the switching signal.
5. The resonant inverter according to claim 1 , wherein individual or groups of control capacitances of a plurality of control capacitances are arranged to be selectively switched into or out of the resonant circuit in selected cycles of the resonant circuit to control the operation of the inverter.
6. The resonant inverter according to claim 1 , further comprising a buck control circuit coupled between the input and the inductive elements, the buck control circuit having a diode and buck control switch having a duty cycle which is controlled in order to reduce the voltage across the tuning capacitor in response to an over voltage condition.
7. The resonant inverter according to claim 6 , wherein the duty cycle is controlled in order to stop or start the resonant circuit.
8. The resonant inverter according to claim 6 , wherein the duty cycle is controlled in order to reduce the voltage across the tuning capacitor in response to a power factor of the resonating current.
9. The resonant inverter according to claim 1 , further comprising:
a third switching means arranged to selectively switch a control resistor into or out of the resonant circuit in order to start or stop the resonant circuit.
10. The resonant inverter according to claim 1 , wherein individual or groups of control capacitances of a plurality of control capacitances are arranged to be selectively switched into or out of the resonant circuit in selected cycles of the resonant circuit to control operating frequencies of the inverter.
11. The resonant inverter according to claim 1 , wherein the second switch means comprises a plurality of capacitors weighted in a sequence.
12. A method of operating a resonant inverter having an input for supply of current from a DC power source, a resonant circuit including two electrically or magnetically coupled inductive elements and a tuning capacitance, the inductive elements being arranged to split current from the power source, first switching means comprising two switching devices and second switching means; the method comprising:
switching the two switching devices in substantially opposite phase to alternately switch current from the power source into the inductive elements;
switching the second switching means to selectively switch one or more control capacitances into or out of the resonant circuit dependent on a power factor of the resonant circuit; and
determining the power factor by determining a phase difference between the current of the resonant circuit and a switching signal for driving the switching devices.
13. The method according to claim 12 , wherein individual or groups of control capacitances of a plurality of control capacitances are selectively switched into or out of the resonant circuit in selected cycles of the resonant circuit to control the operation of the inverter.
14. The method according to claim 12 , further comprising:
a third switching means arranged to selectively switch a control resistor into or out of the resonant circuit in order to start or stop the resonant circuit.