Power supply

Abstract

A power supply (500) configured to provide an alternating output. The power supply (500) includes a positive feedback loop (700) configured to and provide a gate drive signal with a first polarity and change the polarity of the gate drive signal to a second polarity based on the alternating output crossing a threshold. The power supply (500) also includes a negative feedback loop (710) configured to receive a second feedback signal and control, based on an amplitude of the alternating output: (i) a delay between the alternating output crossing the threshold and the change of the polarity; or (ii) a duty cycle of the gate drive signal. The power supply (500) includes an inverter circuit element (530) comprising an inverter switch. The inverter circuit element (530) is configured to operate the inverter switch in a conducting state or a non-conducing state based on the polarity of the gate drive signal.

Description

[0001] 99.173.173746

[0002] Power Supply

[0003] Technical Field

[0004] This disclosure relates to power supplies configured to generate an alternating output (e.g. an alternating current or voltage).

[0005] Background

[0006] A power supply is a device for providing an electric current or voltage. Various applications require a power supply to provide an alternating current or an alternating voltage at a certain frequency. One such application is radio frequency (RF) drying.

[0007] Some RF drying systems are configured to create an alternating electric field between two plates of a capacitor by applying an alternating voltage (at a radio frequency) across the capacitor. Exposure of water molecules to an alternating electric field at this radio frequency causes a heating effect. This heating effect may be used to increase the rate of evaporation of the water molecules, e.g. in order to cook or dry an object.

[0008] The water molecules may be modelled as a resistor arranged in parallel with the capacitor. However, this arrangement may be modelled as a more complex circuit. One of the challenges of designing an RF drying system is that the effective resistance of the water molecules changes as the water molecules evaporate. Thus, it is difficult to design an RF drying system that operates efficiently throughout the drying process.

[0009] It is beneficial for printers that use water-based inks to have a drying system to dry the waterbased ink, e.g. in order to prevent the ink from smudging. Existing systems include the use of traditional heaters that heat the air around the paper to speed up the process of evaporation of the water molecules. Thermal energy is transferred from the hot air to the wet ink in order to speed up the rate at which the ink dries. This process is relatively energy intensive, as it requires heating the air around the wet ink in order to dry the ink, and therefore this process is relatively inefficient. However, it is difficult to design an RF drying system that can be used to heat the water molecules directly, e.g. as the resistance of the wet ink will change as it dries out. It is an object of the invention to provide a signal generator configured to provide an alternating current or an alternating voltage to an attached load, that is able to account for variation of the (e.g. effective) impedance (e.g. resistance) of the attached load.

[0010] Summary

[0011] When viewed from a first aspect, the invention provides a power supply configured to generate an alternating output, the power supply comprising: a positive feedback loop configured to: receive a first feedback signal, which is representative of the alternating output; and provide a gate drive signal based on the first feedback signal; wherein, the positive feedback loop is configured to, for each of a plurality of cycles of the alternating output: provide the gate drive signal with a first polarity; and change the polarity of the gate drive signal from the first polarity to a second polarity based on the alternating output crossing a threshold; the power supply further comprising a negative feedback loop, configured to: receive a second feedback signal, which is representative of the alternating output; and control, based on an amplitude of the alternating output:

[0012] (i) a delay between the alternating output crossing the threshold and the change of the polarity of the gate drive signal; or

[0013] (ii) a duty cycle of the gate drive signal; the power supply further comprising an inverter circuit element, the inverter circuit element comprising an inverter switch, wherein the inverter circuit element is configured to: receive a direct input; receive the gate drive signal; and operate the inverter switch in a conducting state or a non-conducing state based on the polarity of the gate drive signal so as to convert the direct input to an alternating inverter output in order to generate the alternating output.

[0014] The present invention provides a power supply configured to control, based on the amplitude of the alternating output: (i) a phase of operation of the inverter switch relative to the alternating output (by controlling the delay); or (ii) the duty cycle of the inverter switch. Therefore, in use, a power supply according to the present invention may be used to control operation of the inverter switch to account for variation of the amplitude of the alternating output, e.g. arising from variation of a resistance of an attached load.

[0015] Control of the delay between the alternating output crossing the threshold and the change of the polarity of the gate drive signal based on the amplitude of the alternating output may comprise: phase-shifting a signal representative of the alternating output crossing the threshold based on the amplitude of the alternating output. The negative feedback loop may be arranged to phaseshift a signal representative of the alternating output crossing the threshold based on the amplitude of the alternating output; and control the change of the polarity of the gate drive signal based on the phase-shifted signal.

[0016] Thus, a power supply according to the present invention may be used to help ensure efficient operation of the power supply is maintained or a substantially constant amplitude of the alternating output is provided to an attached load, e.g. despite variation of the resistance of the attached load. Variation of the amplitude of the alternating output may (e.g. also) arise from variation of the (e.g. magnitude of the) direct input.

[0017] The duty cycle of the gate drive signal may be understood to mean the proportion of time that the gate drive signal is in an "on" state (e.g. as opposed to an "off") state (e.g. during one cycle of operation). An “on” state may be understood to mean a conducting state. An “off’ state may be understood to mean a non-conducting state. For example, a duty cycle of 60% may correspond to the gate drive signal having an “on” state for 60% of a cycle of the alternating output, and an “off” state for the other 40% of the cycle. The gate drive signal may be in an “off” state when the gate drive signal is provided with the first polarity. The gate drive signal may be in an “on” state when the gate drive signal is provided with the second polarity.

[0018] In some embodiments, the inverter circuit element is configured to operate the inverter switch in a non-conducting state when the gate drive signal has the first polarity. In some embodiments, the inverter circuit element is configured to operate the inverter switch in a conducting state when the gate drive signal has the second polarity.

[0019] The inverter circuit element may be arranged to alternate the conducting state of the inverter switch between a conducting state and a non-conducting state based on the polarity of the gate drive signal to convert the direct input to an alternating inverter output. In some embodiments, the alternating inverter output is the alternating output. In embodiments the alternating output is based on the alternating inverter output, however they may not necessarily be the same.

[0020] The alternating output may comprise an alternating current or an alternating voltage. The direct input may comprise a direct current or a direct voltage. The alternating inverter output may comprise an alternating current or an alternating voltage (which may be the same as or different to the power supply’s alternating output current or voltage).

[0021] Any of the signals (e.g. the first or second feedback signals or the gate drive signal) may be analogue or digital signals. Any of the signals (e.g. the first or second feedback signals or the gate drive signal) may be electrical signals (e.g. comprising a current or a voltage).

[0022] The power supply may be configured to receive the direct input from an attached power source. The inverter circuit element may be configured to receive a (e.g. substantially) constant direct input (e.g. from an attached power source).

[0023] The signal generator may comprise a power source configured to provide the direct input, e.g. to the inverter circuit element. The power source configured to provide a direct input may comprise a direct voltage source or a direct current source. The power source may be configured to provide a substantially constant direct input.

[0024] A feedback signal may be defined as a signal that provides real-time information about the output or state of a system for use in the control of the system (e.g. to control the output). The purpose of such a feedback signal may be to compare the actual output with a desired output (setpoint). Such a comparison may be used to make adjustments (if necessary) to the control of the system.

[0025] The first feedback signal and the second feedback signal are representative of the alternating output. In some embodiments, a frequency of the first and / or the second feedback signal is (e.g. directly) proportional to a frequency of the alternating output. In some embodiments, an amplitude of the first and / or the second feedback signal is (e.g. directly) proportional to the amplitude of the alternating output. In some embodiments, there is a (e.g. constant) phase difference between a phase of the first and / or the second feedback signal and a phase of the alternating output. The first and / or the second feedback signal may have (e.g. substantially) the same frequency, amplitude or phase as the alternating output. The power supply may comprise a (e.g. first) feedback circuit element configured to receive the alternating output and provide the first feedback signal (based on the alternating output) to the positive feedback loop. The power supply may comprise a feedback circuit element (e.g. the first feedback circuit element or a second feedback circuit element) configured to receive the alternating output and provide the second feedback signal (based on the alternating output) to the negative feedback loop. The power supply may comprise a feedback circuit element configured to receive the alternating output and provide both the first feedback signal (to the positive feedback loop) and the second feedback signal (to the negative feedback loop). The (e.g. first and / or second) feedback circuit element may be configured to receive the alternating output in parallel with an attached load. The (e.g. first and / or second) feedback circuit element may be arranged in parallel with an attached load.

[0026] The first feedback signal and the second feedback signal may be the same, or they may be different.

[0027] The positive feedback loop may be configured to change the polarity of the gate drive signal from the first polarity to the second polarity in response to (i) the alternating output crossing a threshold or (ii) the end of a delay following the alternating output crossing the threshold. In embodiments in which the negative feedback loop is configured to control the duty cycle of the gate drive signal, the delay may be (e.g. substantially) constant.

[0028] For each of the plurality of cycles of the alternating output, the positive feedback loop may be configured to change the polarity of the gate drive signal from the second polarity back to the first polarity.

[0029] The positive feedback loop may be configured to change the polarity of the gate drive signal from the second polarity back to the first polarity based on the alternating output crossing a (e.g. second) threshold. For example, the positive feedback loop may be configured to change the polarity of the gate drive signal from the second polarity back to the first polarity in response to (i) the alternating output crossing a (e.g. second) threshold or (ii) the end of a (e.g. second) delay following the alternating output crossing a (e.g. second) threshold. The negative feedback loop may be configured to control, based on an amplitude of the alternating output, the (e.g. second) delay between the alternating output crossing the (e.g. second) threshold and the change of the polarity of the gate drive signal. The positive feedback loop may be configured to change the polarity of the gate drive signal from the first polarity to the second polarity based on the alternating output crossing the threshold in a certain direction. In some embodiments, the positive feedback loop is configured to change the polarity of the gate drive signal from the first polarity to the second polarity based on the alternating output crossing from below the threshold to above the threshold.

[0030] The positive feedback loop may be configured to change the polarity of the gate drive signal from the second polarity back to the first polarity based on the alternating output crossing the (e.g. second) threshold in a certain direction. In some embodiments, the positive feedback loop is configured to change the polarity of the gate drive signal from the second polarity back to the first polarity based on the alternating output crossing from above the (e.g. second) threshold to below the (e.g. second) threshold.

[0031] The positive feedback loop may be configured to change the polarity of the gate drive signal from the first polarity to the second polarity based on the alternating output crossing the threshold in a certain (e.g. first) direction and change the polarity of the gate drive signal from the second polarity back to the first polarity based on the alternating output crossing the (e.g. second) threshold in a certain (e.g. second) direction. In some embodiments, the threshold and the second threshold may be the same. In some embodiments, the first and second directions are opposite directions.

[0032] The power supply may be configured to generate a periodic alternating output. In some embodiments, the power supply is configured to maintain (e.g. the frequency, the phase or the amplitude of) a periodic alternating output. Self-oscillating power supplies use positive feedback to help maintain a periodic output. In some embodiments, the power supply is a self-oscillating power supply, wherein the positive feedback loop is configured to provide the gate drive signal (based on the first feedback signal) to maintain a periodic alternating output. In various applications, self-oscillation may provide various (e.g. efficiency) advantages over other control approaches.

[0033] In some embodiments, the negative feedback loop is configured to control (i) the delay or (ii) the duty cycle so as to drive the amplitude of the alternating output towards a desired amplitude. The desired amplitude may be a (e.g. substantially) constant amplitude. Thus, the negative feedback loop may help to maintain a substantially constant amplitude of the alternating output. The negative feedback loop may be configured to control (i) the delay or (ii) the duty cycle so as to change the conducting state of the inverter switch to the non-conducting state when a current through the inverter switch is substantially zero. This is known as zero current switching (ZCS), and is an efficient mode of operation that provides relatively low switching losses. Therefore, in use, the power supply may account for variation of the amplitude of the alternating output (e.g. as a result of variation of a resistance of an attached load) to help maintain ZCS operation of the inverter switch by controlling (i) the delay or (ii) the duty cycle.

[0034] The negative feedback loop may be configured to control (i) the delay or (ii) the duty cycle in order to change the conducting state of the inverter switch to the conducting state when a voltage across the inverter switch is substantially zero. This is known as zero voltage switching (ZVS), and is an efficient mode of operation that provides relatively low switching losses. Therefore, in use, the power supply may account for variation of the amplitude of the alternating output (e.g. as a result of variation of a resistance of an attached load) to help maintain ZVS operation of the inverter switch by controlling (i) the delay or (ii) the duty cycle.

[0035] In some embodiments, the delay is less than one period of the alternating output. A period of the alternating output may be defined as the inverse of the frequency of the alternating output. In some embodiments, the delay is less than half the period of the alternating output.

[0036] In some embodiments, the duty cycle of the gate drive signal is less than 50%. For instance, the gate drive signal may have a polarity that corresponds to a conducting state of the inverter switch for 50% or less of each cycle of the alternating output.

[0037] The positive feedback loop may be configured to change the polarity of the gate drive signal from the first polarity to the second polarity (and optionally back to the first polarity) every cycle of the alternating output. The positive feedback loop may be configured to change the polarity of the gate drive signal from the first polarity to the second polarity (and optionally back to the first polarity) every two or more cycles of the alternating output. Thus, a switching frequency of the gate drive signal may be proportional to (or the same as) a frequency of the alternating output. Hence, a switching frequency of the inverter switch may be proportional to (or the same as) a frequency of the alternating output.

[0038] The positive feedback loop may be arranged to provide a phase delay between the alternating output and the gate drive signal so as to change the conducting state of the inverter switch out of phase with the alternating output. The positive feedback loop may thus help to provide phase-shift oscillation of the power supply, e.g. in order to help to achieve self-oscillation of the power supply.

[0039] The inverter circuit element may comprise a gate driver circuit element configured to receive the gate drive signal from the feedback loop and operate the inverter switch based on the polarity of the gate drive signal.

[0040] The inverter switch may comprise a transitive switch. By way of example, the inverter switch may comprise a field-effect transistor (FET), a metal-oxide field effect transistor (MOSFET), a bipolar junction transistor (BJT), a high-electron-mobility transistor (HEMT) or the like. In various applications, transitive switches provide efficiency or advantages over other types of switches or operate more quickly than other types of switches.

[0041] In some embodiments, the inverter circuit element comprises a Class-E circuit topology. In various applications, a Class-E circuit topology provides high efficiency operation, particularly when using a Class-E inverter circuit element to generate a radio frequency (RF) alternating output.

[0042] The inverter circuit element may comprise a Class-C circuit topology or a Class-D circuit topology. In various applications, these inverter circuit topologies may be suited to generate an RF alternating output.

[0043] The inverter circuit element may comprise an inductor arranged in series with the inverter switch. The inductor may be arranged to store energy (e.g. by building up a magnetic field). The inductor may be arranged to store energy when the inverter switch is operated in a conducting state. The inductor may be arranged to release the stored energy (e.g. as electrical energy). The inductor may be arranged to release the stored energy when the inverter switch changes from the conducting state to the non-conducting state. The inductor may help to maintain a continuous current flow. The power supply may be configured to receive the direct input from an attached power source, and the inductor may be arranged in series between the inverter switch and the attached power source.

[0044] The inverter circuit element’s inductor may be configured to receive the direct input from a (e.g. attached) power source. The inverter circuit element may be configured to provide the alternating inverter output between the inverter circuit element’s inductor and the inverter switch, e.g. in parallel with the inverter switch (and optionally also in parallel with the inverter circuit element’s capacitor).

[0045] In some embodiments, the inverter circuit element comprises a capacitor arranged in parallel with the inverter switch. This is sometimes referred to as a shunt capacitor. The capacitor may be arranged to shape the voltage waveform across the inverter switch. Shaping of the voltage waveform across the inverter switch may help to achieve ZVS.

[0046] The power supply may comprise a resonant tank circuit element, with a resonant frequency, configured to filter the alternating inverter output in order to generate the power supply’s alternating output. In such embodiments, the resonant tank circuit element may be configured to have a resonant frequency that is substantially the same as a frequency of the alternating inverter output. This may help energy transfer to an attached load. Resonance of the resonant tank circuit element may help minimize harmonics, and / or improve the overall efficiency of the power supply.

[0047] The resonant tank circuit element may comprise an inductor and a capacitor. In some embodiments, the resonant tank circuit element’s inductor is arranged in series between the output of the inverter circuit element and the alternating output (e.g. in series with the attached load). The resonant tank circuit element’s capacitor may be arranged in parallel with the alternating output (e.g. in parallel with the attached load).

[0048] In some embodiments, the resonant tank circuit element’s inductor may be arranged to (e.g. inductively) couple to a load so as to provide the alternating output to the (e.g. inductively) coupled load. For instance, the power supply may be arranged to provide inductive heating of a load (e.g. inductively) coupled to the resonant tank’s inductor. The power supply may comprise the (e.g. inductively) coupled load. The (e.g. inductively) coupled load may be arranged to generate plasma. Thus, the disclosure extends to a plasma generator comprising the power supply and the (e.g. inductively) coupled load.

[0049] In some embodiments, the resonant tank’s inductor is the primary winding of a transformer. The primary winding of the transformer may be (e.g. inductively) coupled to a secondary transformer winding. The power supply may comprise the transformer (e.g. both the primary and secondary transformer windings). The secondary transformer winding may be configured to provide the alternating output (e.g. to an attached load). The power supply may comprise the attached load. The transformer may be configured as a step-up, step-down or one-to-one transformer. The power supply may comprise a transformer configured to provide galvanic isolation between the inverter circuit element and the alternating output signal (e.g. between the inverter circuit element and an attached load). The transformer may be configured as a step-up transformer, a step-down transformer or a one-to-one transformer.

[0050] The power supply may comprise a transformer configured to provide galvanic isolation between the resonant tank circuit element and the alternating output (e.g. between the resonant tank circuit element and an attached load). The resonant tank circuit element may be arranged between the inverter circuit element and the transformer. Therefore, the transformer may also provide galvanic isolation between the inverter circuit element and the alternating output (e.g. between the inverter circuit element and an attached load). The transformer may be configured as a step-up transformer, a step-down transformer or a one-to-one transformer.

[0051] The negative feedback loop may be configured to provide the gate drive signal to control (i) the delay or (ii) the duty cycle based on a peak amplitude of the alternating output. A peak amplitude of the alternating output will be understood to mean the peak amplitude of the alternating output signal across one or more cycles (e.g. a plurality of cycles) of the alternating output.

[0052] The negative feedback loop may comprise an amplitude detector circuit element configured to provide an amplitude detector output based on the second feedback signal; wherein the amplitude detector output is representative of (e.g. the peak value of) the amplitude of the alternating output; and wherein the negative feedback loop is configured to control, based the amplitude detector output: (i) the delay or (ii) the duty cycle. The amplitude detector circuit element may be configured to receive the second feedback signal.

[0053] The amplitude detector circuit element may comprise a peak detector circuit element configured to provide a peak detector output based on the second feedback signal, wherein the peak detector output is representative of the peak (e.g. value of the) amplitude of the alternating output; and wherein the amplitude detector output is based on the peak detector output. The peak detector circuit element may be configured to receive the second feedback signal.

[0054] The amplitude detector circuit element may comprise an error amplifier circuit element, configured to receive an error amplifier input based on the second feedback signal, which is representative of the (e.g. peak) amplitude of the alternating output; compare the error amplifier input and an amplitude reference (e.g. an amplitude reference voltage or an amplitude reference value); and provide an error amplifier output based on the comparison, wherein the amplitude detector output is based on the error amplifier output.

[0055] The error amplifier circuit element may be configured to amplify the difference between the error amplifier input and the amplitude reference. The amplitude reference may be representative of a desired amplitude of the alternating output. The error amplifier may thus provide an output that is representative of the difference between the (e.g. peak) amplitude of the alternating output and the desired amplitude. Thus, the negative feedback loop may be configured to use the amplitude detector output (based on the error amplifier’s output) to control (i) the delay or (ii) the duty cycle, to drive the (e.g. peak) amplitude of the alternating output towards the desired amplitude.

[0056] The error amplifier input may be the second feedback signal or the peak detector output. The error amplifier may be arranged to receive the second feedback signal or the peak detector output. The amplitude reference may be provided by a reference source (e.g. a reference voltage source). The power supply may comprise the reference source.

[0057] The positive feedback loop may comprise a (e.g. first) comparator circuit element configured to: receive a (e.g. first) comparator input based on the first feedback signal; and compare the (e.g. first) comparator input to a (e.g. first) reference (e.g. a first reference value or a first reference voltage) to provide a (e.g. first) comparator output. The (e.g. first) comparator output may be representative of whether the alternating output is greater than the threshold (for changing the polarity of the gate drive signal). For instance, the (e.g. first) reference may be representative of the threshold. The gate drive signal may be based on the (e.g. first) comparator output.

[0058] The (e.g. first) comparator may be used to convert the comparator input into a square wave signal. The (e.g. first) comparator circuit element may provide (e.g. impedance) isolation between the comparator input and the comparator output. Isolation (e.g. impedance isolation) of the comparator input and comparator output may help to prevent interference between the input side circuitry and the output side circuitry.

[0059] The first feedback signal may be an analogue signal, and the comparator may be used to convert the comparator input (based on the first feedback signal) to a digital signal. In some embodiments, the (e.g. first) comparator input is the first feedback signal. The positive feedback loop may comprise a modulator circuit element configured to: receive a modulator input based on the first feedback signal; and modulate the modulator input based on the amplitude of the alternating output; wherein the gate drive signal is based on the modulator output. The modulator circuit element may be configured to receive an output of the negative feedback loop, which is based on the amplitude of the alternating output (e.g. the peak detector output or the error amplifier output) and modulate the modulator input based on the output of the negative feedback loop.

[0060] The modulator circuit element may be configured to control (i) a duty cycle of the modulator output or (ii) a delay between the modulator input and the modulator output based on the amplitude of the alternating output. The modulator output may be the gate drive signal. The modulator circuit element may thus provide the means by which the negative feedback loop controls the gate drive signal provided by the positive feedback loop.

[0061] The modulator input may be the first feedback signal or the (e.g. first) comparator output. The output of the negative feedback loop may be the amplitude detector output (e.g. the peak detector output or the error amplifier output).

[0062] The modulator circuit element may be configured to control the delay between the modulator input and the modulator output by adding a delay, e.g. a phase shift, which may be a positive delay (e.g. a positive phase shift) or a negative delay (e.g. a negative phase shift, also known as a phase advance) to the modulator input to provide (e.g. indirectly, for instance via a second comparator) the modulator output. The delay may be based on the output of the negative feedback loop.

[0063] In embodiments in which the positive feedback loop comprises a comparator circuit element and a modulator circuit element, the modulator input may be the comparator output.

[0064] In embodiments in which the negative feedback loop comprises an amplitude detector circuit element (e.g. comprising the peak detector circuit element and / or the error amplifier circuit element), the modulator circuit element may be configured to: receive the amplitude detector output (e.g. the output of the error amplifier circuit element); and modulate the modulator input based on the amplitude detector output.

[0065] In embodiments in which the positive feedback loop comprises a modulator circuit element, the modulator circuit element may comprise a (e.g. second) comparator configured to compare a (e.g. second) comparator input, based on the modulator input, to a (e.g. second) reference (e.g. a second reference voltage or a second reference value), and provide a (e.g. second) comparator output based on the comparison. The modulator output may be based on the (e.g. second) comparator output. In some embodiments, the (e.g. second) comparator output is the modulator output or the gate drive signal. The gate drive signal is used to operate the inverter switch, therefore it may be helpful to provide a digital gate drive signal.

[0066] The modulator circuit element may be configured to control the duty cycle by controlling the (e.g. second) reference. In some embodiments, the modulator circuit element may be configured to set the (e.g. second) reference based on the output of the negative feedback loop. In some embodiments, the modulator circuit element is configured to add an offset (e.g. a voltage offset) to the modulator input to provide the (e.g. second) comparator input. The offset may be based on the output of the negative feedback loop. The offset may be a positive offset or a negative offset.

[0067] The modulator circuit element may be configured to control the delay by adding a delay, e.g. a phase shift, to the modulator input. The delay may be a positive delay or a negative delay. In some embodiments, the modulator circuit element may be configured to add a delay, e.g. a phase shift, to the modulator input based on the output of the negative feedback loop.

[0068] The positive feedback loop may be configured to provide the gate drive signal based on the first feedback signal after initialisation of the signal generator.

[0069] The power supply may comprise a kick starter circuit element configured to provide a kick starter output to initialise operation of the power supply; wherein, during initialisation of the power supply, the gate drive signal is based on the kick starter output. During initialisation of the power supply, the gate drive signal may not be based on the first feedback signal. The power supply may be configured not to provide the gate drive signal based on the kick starter output after initialisation (of the power supply).

[0070] The power supply may be configured to end initialisation (of the power supply) based on the amplitude of the alternating output. In some embodiments, the power supply is configured to end initialisation of the power supply based on (e.g. in response to) the amplitude of (e.g. an average or a peak amplitude of) the alternating output exceeding an initialisation threshold. The kick starter output may have a frequency that is representative of a target frequency of the alternating output. For instance, the kick starter output may have a frequency that is a certain multiple of a target frequency of the alternating output (e.g. quarter, half, twice or four times the target frequency). The kick starter output may have a frequency that is the same as a target output frequency of the alternating output.

[0071] In embodiments in which the power supply comprises a kick starter circuit element and an amplitude detector circuit element (e.g. comprising a peak detector circuit element), the feedback loop may be configured to end initialisation of the power supply based on (e.g. in response to) the amplitude detector output or the peak detector output being greater than an amplitude threshold (which may be representative of the initialisation threshold). In embodiments that include an error amplifier circuit element, the amplitude threshold may be a different amplitude to the amplitude reference used by the error amplifier circuit element.

[0072] In some embodiments, the kick starter circuit element comprises a comparator arranged to compare the output of the peak detector circuit element to the amplitude threshold. The power supply may be arranged to stop the kick starter circuit element from providing the kick starter output to initialise operation of the power supply (e.g. by switching off the kick starter circuit element, or otherwise disabling the kick starter circuit element) based on the output of the kick starter’s comparator (e.g. based on whether the output of the peak detector is greater than the amplitude threshold).

[0073] In embodiments in which the positive feedback loop comprises a (e.g. first) comparator circuit element, the (e.g. first) comparator input may be based on the kick starter output (e.g. instead of the first feedback signal) during initialisation of the power supply. In some embodiments, during initialisation of the power supply, the (e.g. first) comparator input is the kick starter output.

[0074] In embodiments in which the positive feedback loop comprises a modulator circuit element, during initialisation of the power supply, the modulator input may be based on the kick starter output (e.g. instead of the first feedback signal).

[0075] It will be appreciated that any of the elements of the positive or negative feedback loops may be implemented in an analogue manner or in a digital manner. Thus, the positive or negative feedback loops may have an analogue implementation, a digital implementation or a hybrid (analogue and digital) implementation. The power supply may be configured to provide a RF alternating output. A RF alternating output may be understood to mean an alternating output having a frequency in the RF range of the electromagnetic spectrum, e.g. a frequency between 20 kHz and 300 GHz. In some embodiments, the alternating output has a frequency between 6 MHz and 30 MHz. In some embodiments, the alternating output has a frequency between 13.68 MHz and 27 MHz.

[0076] The power supply may comprise an output capacitor. The power supply may be configured to provide the alternating output, e.g. an alternating output voltage, across the output capacitor. The power supply may be configured to provide a RF alternating output, e.g. a RF alternating output voltage, across the output capacitor. This arrangement may be used to provide an RF drying system configured to heat, and thus evaporate, water molecules positioned between the plates of the output capacitor.

[0077] In other words, the power supply may be configured to provide a RF alternating output, e.g. a RF alternating output voltage, across an output capacitor in order to heat water molecules positioned between the plates of the output capacitor.

[0078] The first aspect extends to a RF drying system comprising the power supply according to the first aspect, and configured to dry water molecules positioned between the plates of the output capacitor.

[0079] The present invention extends to a printer comprising the RF drying system; wherein the printer is configured to apply printer ink comprising water molecules to a printing medium, e.g. paper or the like; and wherein the printer is configured to position the printing medium between the plates of the output capacitor (of the RF drying system) after the printer ink has been applied to the printing medium to dry the printer ink.

[0080] The water molecules may be modelled as an output resistor arranged in parallel with the output capacitor. Although, it is also possible to model this arrangement as a more complex circuit. In use, the output resistor would have a variable resistance, which would change as the water molecules evaporate. For instance, the effective resistance provided by the water molecules may increase as the water molecules evaporate. In such embodiments, the power supply may account for variation of the amplitude of the alternating output arising from variation of the effective resistance of the water molecules by adjusting the duty cycle of the inverter switch or the delay (between the alternating output crossing the threshold and the change of conducting state of the inverter switch). This may improve the efficiency of the RF drying system or enable the RF drying system to provide a substantially constant output amplitude.

[0081] According to another aspect, the invention provides a method of generating an alternating output using a power supply, the method comprising: receiving a first feedback signal, which is representative of the alternating output; and for each of a plurality of cycles of the alternating output: providing the gate drive signal with a first polarity; and changing the polarity of the gate drive signal from the first polarity to a second polarity based on the alternating output crossing a threshold; the method further comprising: receiving a second feedback signal, which is representative of the alternating output; and controlling, based on an amplitude of the alternating output:

[0082] (i) a delay between the alternating output crossing the threshold and the change of the polarity of the gate drive signal; or

[0083] (ii) a duty cycle of the gate drive signal; wherein the power supply comprises an inverter circuit element, the inverter circuit element comprising an inverter switch; and wherein the method comprises: receiving a direct input; and operating the inverter switch in a conducting state or a non-conducing state based on the polarity of the gate drive signal so as to convert the direct input to an alternating inverter output in order to generate the alternating output.

[0084] The power supply according to the first aspect of the invention may be arranged to carry out the method according to this aspect of the invention.

[0085] Any of the features of the first aspect may apply equally to the second aspect.

[0086] Brief Description of Drawings

[0087] One or more non-limiting examples will now be described, by way of example only, and with reference to the accompanying figures in which:

[0088] Figure 1 shows a RF drying system;

[0089] Figure 2 shows another RF drying system;

[0090] Figure 3 shows a further RF drying system; Figure 4 shows a RF power supply;

[0091] Figure 5 shows a change of resistance of a load attached to a power supply comprising a Class-E driver circuit element;

[0092] Figure 6 shows the power dissipation across a switch of the Class-E driver circuit element if there is no adjustment to operation of the power supply as the resistance of the attached load changes;

[0093] Figure 7 shows the voltage applied to the attached load if there is no adjustment to operation of the power supply as the resistance of the attached load changes;

[0094] Figure 8 shows a change of resistance of a load attached to a power supply comprising a Class-E driver circuit element;

[0095] Figure 9 shows the power dissipation across a switch of the Class-E driver circuit element if operation of the power supply is adjusted to compensate for the change of resistance of the attached load;

[0096] Figure 10 shows the voltage applied to the attached load if operation of the power supply is adjusted to compensate for the change of resistance of the attached load; and Figure 11 shows a method of operating the power supply shown in Figure 4.

[0097] Detailed Description

[0098] Figure 1 shows a RF drying system 10, which is arranged to provide an alternating output voltage Vout_Ac across (e.g. the plates of) a drying capacitor Cp 15. By providing an alternating output voltage Vout_Ac across the drying capacitor Cp 15, the RF drying system is able to heat water molecules positioned between (e.g. the plates of) the drying capacitor Cp 15. The water molecules may be modelled as a resistor R1 16 arranged in parallel with the drying capacitor Cp 15. In use, the effective resistance of the resistor R1 16 will increase as the water molecules evaporate, which presents various challenges. The RF drying system 10 may be incorporated into a printer to dry printer ink.

[0099] The RF drying system 100 will now be explained in more detail, with continued reference to Figure 1.

[0100] A voltage source VCC1 11 is arranged to provide an input voltage, and smoothing capacitors 12 are arranged to filter out AC components of the input voltage to provide a DC voltage VDC.

[0101] A Class-E driver circuit element 13 is arranged to receive the DC voltage VDC and provide an alternating voltage VAC with a radio frequency. The Class-E driver circuit element 13 includes an inductor L1 , a shunt capacitor C11 and a switch M1. In this example, the switch M1 is a metal- oxide-semiconductor field-effect transistor (MOSFET). In use, the output voltage of the Class-E driver circuit element 13 reduces to zero when the switch M1 conducts, and increases to the value of the direct voltage VDC when the switch M1 does not conduct. Thus, it is possible to operate the switch M1 to produce an alternating voltage VAC at the output of the Class-E driver by alternately operating the switch M1 in a conducting or non-conducting state.

[0102] The shunt capacitor C11 in a Class E driver circuit may improve the efficiency and performance of the system by helping to enable zero-voltage switching (ZVS) of the switch M1 , e.g. by shaping the voltage waveform across the switch M1. ZVS operation of the switch M1 may reduce switching losses and associated stress. Shaping the voltage waveform across the switch may help to reduce voltage spikes and overshoots, which can help to protect the switch M1 and / or reduce electromagnetic interference (EMI).

[0103] In this example, the shunt capacitor C11 forms part of a filter (formed by the combination of the shunt capacitor C11 and the inductor L2 14). The filter acts to filter out unwanted harmonics, which improves signal purity. This may help to facilitate high-frequency operation and / or improve the efficiency of power transfer to the load Cp 15.

[0104] The RF drying circuit 10 includes a gate driver circuit element 20, which is configured to control operation of the switch M1 based on a received gate drive signal VGATE.

[0105] An inductor L2 14 is arranged between the Class-E driver circuit element 13 and the drying capacitor Cp 15. The combination of the inductor L2 14 and the drying capacitor Cp 15 forms a resonant tank. In use, the alternating voltage VAC is applied across the resonant tank. The resonant tank is designed to have a resonant frequency close to or the same as the frequency of the AC voltage VAC.

[0106] The RF drying system 10 is configured to control operation of the switch M1 so that the voltage across the switch M1 is substantially zero when the switch M1 is turned on. This is known as zero-voltage switching (ZVS). The RF drying system 10 is configured to control operation of the switch M1 so that the current through the switch is close to zero when the switch M1 is turned off. This is known as zero-current switching (ZCS). The combination of ZVS and ZCS switching help to reduce power losses associated with operation of the switch M1.

[0107] Given certain component values of the drying capacitor Cp 15 and the resistor R1 16, it may be possible to choose a component value of the inductor L2 14 to achieve ZVS / ZCS operation for a certain switching frequency and duty cycle of the switch M1. However, as previously mentioned, the resistance of the resistor R1 15 varies in use as the water molecules evaporate. Therefore, it is challenging to design an RF drying circuit 10 that helps to maintain ZVS I ZCS operation of the switch M1 as the water molecules evaporate. It may also be challenging to deliver a substantially constant amplitude of voltage to the water molecules, even as their effective resistance changes.

[0108] Figure 2 shows a RF drying system 100. The RF drying system 100 includes a voltage source VCC1 110, smoothing capacitors 120, a Class-E driver circuit element 130, an inductor L2 140, a drying capacitor Cp 150, and a gate driver circuit element 200. These components are similar to their counterparts in the RF drying system 10 shown in Figure 1.

[0109] The RF drying system 100 shown in Figure 2 also includes various circuit elements which, in combination, are configured to provide a gate driver signal VGATE to the gate driver circuit element 200. The RF drying system 100 shown in Figure 2 is configured to control operation of the switch M1 to provide ZVS I ZCS operation of the switch M1 , as the resistance of the resistor R1 160 changes (owing to evaporation of the water molecules between the plates of drying capacitor Cp 150). The RF drying system 100 is also configured to provide a substantially constant amplitude of voltage to the resistor R1 160, again, as the resistance of the resistor R1 160 changes.

[0110] A kick starter circuit element 180 is shown in Figure 2, which is configured to initialise the RF drying system 100 by providing an alternating voltage at a radio frequency. The kick starter circuit element 180 comprises a voltage source Vstart arranged in series with a resistor R9 and a capacitor C9. Together, the capacitor C9 and the resistor R9 set the load impedance, and the capacitor C9 provides AC coupling between the output of the kick starter circuit element 180 and the output of the feedback circuit element 250.

[0111] A feedback circuit element 250 is arranged to receive the alternating output voltage VAC_OUT across the resistor R1 160. The feedback circuit element 250 is configured to provide a first feedback signal VFBI to a first comparator 230, and configured to provide a second feedback signal VFB2 to a peak detector circuit element 170. In use, both the first feedback signal VFBI and the second feedback signal VFB2 are based on the voltage VAC_OUT across the resistor R1 160.

[0112] A first comparator 230 is configured to provide a certain output voltage (which in this example is five volts) when the input to the first comparator 230 is greater than a reference voltage Vr. Otherwise, the first comparator 230 is configured to provide zero volts at the output. The first comparator 230 is used to convert the relatively small amplitude signal VFB2 derived from the alternating output voltage VOUT_AC into a known amplitude square wave signal. This may help to reduce the phase shift circuit element 190 effecting the characteristics of the feedback circuit element 250. Thus, the first comparator 230 acts as a buffer and provides square wave conversion.

[0113] A phase shift circuit element 190 is configured to receive the output from the first comparator 230 and provide a phase-shifted output. The phase shift applied to the output of the comparator 230 is based on the output of a phase modulator circuit element 260.

[0114] A second comparator 240 is configured to compare the output from the phase shift circuit element 190 to a reference voltage Vr, and to provide a certain output voltage (which in this example is five volts) when the output of the phase shift circuit element 190 is greater than the reference voltage Vr. Otherwise, the second comparator 240 is configured to provide zero volts at the output. The second comparator 240 provides an output with a binary value, and the output of the second comparator 240 is used to operate the switch M1.

[0115] A gate driver circuit element 200 is configured to operate the switch M1 in a conducting state when the second comparator’s output is high, and operate the switch M1 in a non-conducting state when the second comparator’s output is low. Thus, it will be appreciated that square wave conversion of the output of the phase shift circuit element 190 may help to control operation of the switch M 1.

[0116] The peak detector circuit element 170 is configured to receive the second feedback signal VFB2 from the feedback circuit element 250 and provide an output voltage VPEAK that is representative of the peak amplitude of the voltage VOUT_AC across the resistor R1 160.

[0117] A kick starter feedback circuit element 210 is configured to receive the output VPEAK of the peak detector circuit element 170, and disable the kick starter circuit element 180 when the output VPEAK of the peak detector circuit element 170 is greater than a threshold.

[0118] An error amplifier circuit element 220 is also arranged to receive the output VPEAK of the peak detector circuit element 170, and is configured to amplify the difference between this voltage VPEAK and a reference voltage. The phase modulator circuit element 260 is arranged to receive the output of the error amplifier circuit element 220, and is configured to provide an output to the phase shift circuit element 190. The output to the phase shift circuit element 190 is based on the received input. As mentioned previously, the phase shift circuit element 190 is configured to apply a phase shift based on the received signal from the phase modulator circuit element 260.

[0119] Initialisation of the RF drying system 100 will now be described, with continued reference to Figure 2.

[0120] During initialisation of the RF drying system 100, the input to the first comparator 230 is the alternating voltage provided by the kick starter circuit element 180. This causes the first comparator 230 to output a square wave voltage which has the same frequency as the alternating voltage from the kick starter circuit element 180.

[0121] The output of the phase modulator circuit element 260 is substantially constant during initialisation. Therefore, the phase-shift circuit element 190 is configured to apply a substantially constant phase shift is to the output of the first comparator 230 of the RF drying system 100 during this period. Hence, the output of the phase shift circuit element 190 is an alternating voltage which has the same frequency as the output of the kick starter circuit element 180 and is phase shifted relative to it.

[0122] The alternating voltage provided by the phase-shift circuit element 190 alternates between a voltage that is greater than the reference voltage Vr of the second comparator 240 and a voltage that is lower than the reference voltage Vr of the second comparator 240. Therefore, during initialisation, the output VGATE of the second comparator 240 is a square wave voltage which alternates between a high voltage (which, in this example, is five volts) and zero volts at the same frequency and phase as the output of the phase shift circuit element 190.

[0123] The gate driver circuit element 200 receives the alternating voltage VGATE from the second comparator 240 and operates the switch M1 based on this alternating voltage VGATE. AS explained, the voltage VGATE during initialisation is a square wave voltage which has the same frequency as the output of the kick starter circuit element 180. Therefore, during initialisation of the RF drying system 100, the gate driver circuit element 200 is configured to operate the switch M1 with a switching frequency corresponding to the frequency of the signal from the kick starter circuit element 180. Operation of the RF drying circuit following initialisation will now be described with continued reference to Figure 2.

[0124] Following initialisation of the RF drying system 100, the feedback circuit element 250 provides the first feedback signal VFBI and the second feedback signal VFB2, which are both based on the voltage VOUT_AC across the resistor R1 160.

[0125] The second feedback signal VFB2 is provided to the peak detector circuit element 170 which outputs a signal VPEAK which is representative of the amplitude of the voltage VOUT_AC across the resistor R1 160.

[0126] The kick starter feedback circuit element 210 receives the signal VPEAK from the peak detector circuit element 170, and disables the kick starter circuit element 180 when this is greater than a threshold. When the signal VPEAK from the peak detector circuit element 170 is greater than this threshold, this is indicative that the first feedback signal VFBI is sufficient to drive the circuit without the signal from the kick starter circuit element 180.

[0127] The first feedback signal VFBI is an alternating signal which is in phase with the voltage VOUT_AC across the output resistor R1 160 and has the same frequency. By introducing a controllable delay (using the phase shift circuit element 190) it is possible to drive the switch M1 based on this signal VFBI in order to help ensure ZVS I ZCS operation.

[0128] The first comparator 230, the phase shift circuit element 190 and the second comparator 240 form a positive feedback loop, which is configured to control operation of the inverter switch M1 based on the voltage first feedback signal VFBI . AS the first feedback signal VFBI is based on the output voltage VOUT_AC, thus the operation of the switch M1 is controlled based on the output voltage VOUT_AC using self-oscillation.

[0129] For a constant resistance of resistor R1 160, it may be possible to select component values for the phase-shift circuit element 190 to provide an appropriate delay to help ensure ZVS I ZCS operation. However, the necessary delay to help ensure ZVS I ZCS operation will depend on the resistance of the resistor R1 160 and, as previously mentioned, the resistance of the resistor R1 160 changes in use as evaporation of the water molecules between the plates of drying capacitor Cp 150 takes place. Therefore, it is necessary to adjust operation of the RF drying system 100 to help ensure ZVS I ZCS operation as the resistance of the resistor R1 160 changes. Additionally, the amplitude of the output voltage VOUT_AC also depends on the resistance of the resistor R1 160. Therefore, it is also necessary to adjust operation of the RF drying circuit 100 to help maintain an output voltage VOUT_AC with a substantially constant amplitude as the resistance of the resistor R1 160 changes.

[0130] The amplitude of the voltage VOUT_AC across the resistor R1 160 varies based the resistance of the resistor R1 160. Hence, the output VPEAK of the peak detector circuit element 170 (which is representative of the amplitude of the voltage VOUT_AC across the resistor R1 160) is also representative of the resistance of the resistor R1 160.

[0131] A negative feedback loop is formed by the peak detector circuit element 170, the error amplifier 220 and the phase modulator circuit element 260. This negative feedback loop is configured to adjust operation of the positive feedback loop (and in particular to adjust the delay introduced by the phase shift circuit element 190) based on the peak amplitude of the alternating output voltage VOUT_AC. Thus, the negative feedback loop can adjust operation of the RF drying system 100 to account for variation of the resistance of the resistor R1 160.

[0132] The error amplifier circuit element 220 amplifies the difference between the peak detector output VPEAK and a reference voltage, that is representative of a desired peak amplitude of the alternating output voltage VOUT_AC.

[0133] The phase modulator circuit element 260 receives the output from the error amplifier circuit element 220 and provides an input to the phase shift circuit element 190 (based on the output of the error amplifier circuit element 220) to control the delay introduced by the phase shift circuit element 190. Thus, the error amplifier circuit element 220 and the phase modulator circuit element 260 act in combination to control the delay introduced by the phase shift circuit element 190 based on the output VPEAK of the peak detector circuit element 170. Hence, a delay is introduced which is based on the voltage VOUT_AC across the resistor R1 160, and therefore also based on the resistance of the resistor R1 160. This negative feedback drives the peak amplitude of the alternating output voltage VOUT_AC towards the desired peak amplitude.

[0134] By introducing a delay based on the resistance of the resistor R1 160, it may be possible to help maintain ZVS I ZCS operation of the switch M1 even as the resistance of the resistor changes (owing to evaporation of the water molecules between the plates of drying capacitor Cp 150). It is also possible to help maintain a substantially constant output voltage VOUT_AC even as the resistance of the resistor R1 160 changes. Hence, it may be possible to provide a RF drying system 100 which is able to: (i) operate efficiently throughout the drying process; and / or (ii) provide a substantially constant amplitude of output voltage VOUT_AC even as the effective resistance R1 160 of the water molecules changes.

[0135] Figure 3 shows another RF drying system 300. The RF drying 300 system includes various components that are similar to their counterparts in the RF drying system 100 shown in Figure 2. These components include: a voltage source VCC1 310, smoothing capacitors 320, a Class- E driver circuit element 330, an inductor L2 340, a drying capacitor Cp 350, a peak detector circuit element 370, a kick starter circuit element 380, a gate driver circuit element 400, a kick starter feedback circuit element 410, an error amplifier 420, a first comparator 430, a second comparator 440, and a feedback circuit element 450.

[0136] As in previous embodiments, in Figure 3 the effective resistance provided by the water molecules between the plates of drying capacitor Cp 350 is modelled as a resistor R1 360 arranged in parallel with the drying capacitor Cp 350.

[0137] A phase shift circuit element 390 is different from its counterpart in Figure 2 in that it is configured to apply a substantially constant phase shift to a received input from the first comparator 430. However, the phase shift circuit element is also arranged to apply a voltage offset to the received input from the first comparator 430. The voltage offset is based on the output of a voltage offset modulator circuit element 460. As will be explained in further detail, this voltage offset may be used to control the duty cycle of the switch M1.

[0138] The voltage offset modulator circuit element 460 is configured to provide an output based on a received output from the error amplifier circuit element 420. The output from the error amplifier circuit element 420 is representative of the difference between the peak amplitude of the voltage VOUT_AC applied to the resistor R1 360 and a desired peak amplitude. The difference between the desired peak amplitude and the actual peak amplitude of the voltage VOUT_AC applied to the resistor R1 360 arises (at least in part) due to variation of the resistance of the resistor R1 360. Thus, the voltage offset modulator circuit element 460 is configured to provide an output that can be used to control the RF drying system 300 to account for variation of the effective resistance of the water molecules between the plates of the drying capacitor Cp 350.

[0139] Initialisation of the RF drying system 300 of Figure 3 is similar to initialisation of the RF drying system 100 shown in Figure 2, which has been previously explained. Various aspects of operation of the RF drying system 300 of Figure 3 are also similar to operation of the RF drying system 100 of Figure 2.

[0140] Operation of the RF drying system 300 of Figure 3 following initialisation will now be described, with continued reference to Figure 3.

[0141] Following initialisation of the RF drying system 300 of Figure 3, operation of the switch M1 is controlled based on the first feedback signal VFBI and the second feedback signal VFB2 (provided by the feedback circuit element 450), e.g. instead of the signal from the kick starter circuit element 380. The first feedback signal VFBI and the second feedback signal VFB2 are both based on the voltage VOUT_AC across the resistor R1 360.

[0142] The second feedback signal VFB2 is provided to the peak detector circuit element 370 which outputs a signal VPEAK which is representative of the peak amplitude of the voltage VOUT_AC across the resistor R1 360.

[0143] The kick starter feedback circuit element 410 receives the signal VPEAK from the peak detector circuit element 370, and shorts the kick starter circuit element 380 when this is greater than a threshold. At this point, the first feedback signal VFBI is used to drive the circuit instead of the signal from the kick starter circuit element 380.

[0144] As previously mentioned, if the resistance of the resistor R1 was constant, it may be possible to design the RF drying system to operate using ZVS I ZCS of the switch M1 (e.g. without requiring any negative feedback to adjust operation of the switch M1 as the resistance of the resistor R1 changes). However, due to variation of the resistance of the resistor R1 360, it is necessary to adjust operation of the RF drying system 300 to help ensure ZVS I ZCS operation.

[0145] As previously mentioned, variation of the resistance of the resistor R1 360 may result in variation of the amplitude of the output voltage VOUT_AC. Therefore, it may be necessary to adjust operation of the RF drying circuit 300 to help maintain an output voltage VOUT_AC with a substantially constant amplitude as the resistance of the resistor R1 360 changes.

[0146] During operation, the RF drying system 300 is configured to adjust the duty cycle of the switch M1 to compensate for variation of the resistance of the resistor R1 360. This approach may be used instead of (or in combination with) adjustment of the phase of the signal used to drive the switch M1. The approach for adjustment of the duty cycle of the switch M1 will now be explained.

[0147] The amplitude of the voltage VOUT_AC across the resistor R1 360 varies based the resistance of the resistor R1 360. Hence, the output VPEAK of the peak detector circuit element 370 (which is representative of the amplitude of the voltage VOUT_AC across the resistor R1 360) is (e.g. also) representative of the resistance of the resistor R1 360.

[0148] A negative feedback loop is formed by the peak detector circuit element 370, the error amplifier 420 and the voltage offset modulator circuit element 460. This negative feedback loop is configured to adjust operation of the positive feedback loop (and in particular to adjust the duty cycle of the gate drive signal by controlling the voltage offset introduced by the phase shift circuit element 390) based on the peak amplitude of the alternating output voltage VOUT_AC. Thus, the negative feedback loop can adjust operation of the RF drying system 100 to account for variation of the resistance of the resistor R1 160.

[0149] The error amplifier circuit element 420 amplifies the difference between the peak detector output VPEAK and a reference voltage, that is representative of a desired peak amplitude of the alternating output voltage VOUT_AC.

[0150] The voltage offset modulator circuit element 460 receives the output from the error amplifier circuit element 420 and provides an input to the phase shift circuit element 390 (based on the output of the error amplifier circuit element 220) to control the voltage offset introduced by the phase shift circuit element 390. Thus, the error amplifier 420 and the voltage offset modulator circuit element 460 act in combination to control a voltage offset applied to the output of the first comparator 430 (by the phase shift circuit element 390) based on the output VPEAK of the peak detector circuit element 370. Thus, a voltage offset is introduced which is based on the voltage VOUT_AC across the resistor R1 360, and therefore also based on the resistance of the resistor R1 360.

[0151] The output of the phase shift circuit element 390 is provided to the second comparator 440 and compared with a reference voltage. When a higher voltage offset is applied by the phase shift circuit element 390, the portion of time that the output of the phase shift circuit element 390 is greater than the reference voltage is increased. Conversely, when a lower voltage offset is applied by the phase shift circuit element 390, the portion of time that the output of the phase shift circuit element 390 is greater than the reference voltage is decreased. Thus, the duty cycle of the output VGATE of the second comparator 440 is controlled based on the voltage VOUT_AC across the resistor R1 360, and therefore also based on the resistance of the resistor R1 360.

[0152] The output VGATE of the second comparator 440 is used (by the gate driver circuit element 400) to control operation of the switch M1. Therefore, control of the duty cycle of the output VGATE of the second comparator 440 also controls the duty cycle of the switch M1. Hence, the RF drying system 300 is configured to control operation of the switch based on the voltage VOUT_AC across the resistor R1 360, and therefore also based on the resistance of the resistor R1 360. Using this approach, it is possible to adjust operation of the switch M1 to account for variation of the resistance of the resistor R1 360 to help maintain ZVS I ZCS operation and / or to help maintain a substantially constant amplitude of output voltage VOUT_AC to the resistor R1 360.

[0153] While various embodiments have been described with reference to an RF drying system, the skilled person will understand that the approach described herein could be applied to any system with a variable output impedance (e.g. output resistance) to maintain efficient operation (e.g. ZCS I ZVS operation) as the output impedance varies and / or to maintain a substantially constant output voltage to the load.

[0154] Figure 4 shows a RF power supply 500, configured to apply an alternating voltage VOUT_AC (which has a radio frequency) across a load RL 550. Any of the RF drying systems 10; 100; 300 shown in Figures 1 to 3 may comprise the RF power supply 500 shown in Figure 4.

[0155] The RF power supply includes a voltage source Vs 510, which is configured to provide a DC voltage to a Class-E driver circuit element 530. The Class-E driver circuit element 530 includes a switch, which is configured to operate based on a signal from a gate driver circuit element 600. The gate driver circuit element 600 is configured to alternate the switch between a conducting state and a non-conducting state to cause the Class-E driver circuit element 530 to provide an alternating voltage to a resonant tank circuit element 540.

[0156] The resonant tank circuit element 540 is configured to regulate the alternating voltage from the Class-E driver circuit element 530 to provide an alternating output voltage VOUT_AC to a load RL 550. In use, the resistance of the load RL 550 varies.

[0157] While not the case in this example, in some other examples the resonant tank circuit element’s inductor Ls may couple into the load directly. For instance, the inductor Ls may be arranged to (e.g. inductively) couple to a load so as to provide inductive heating. The inductor Ls may be (e.g. inductively) coupled to a load that is used to generate plasma (e.g. inductive plasma generative).

[0158] In further examples, the inductor Ls may represent the primary winding of a transformer. This primary winding may be coupled to a secondary transformer winding. In such examples, the secondary transformer winding may be arranged to provide power to an attached load.

[0159] While not present in this example, in other examples there is a transformer between the resonant tank circuit element 540 and the load RL 550. This may provide galvanic isolation between these circuit elements, and optionally step-up or step-down the voltage provided by the resonant tank circuit element 540 depending on how the transformer is configured.

[0160] The RF power supply 500 includes a feedback circuit element 560 configured to provide a first feedback signal VFBI and a second feedback signal VFB2, which are both based on the alternating voltage VOUT_AC applied to the load RL 550.

[0161] The RF power supply 500 includes a positive feedback loop 700, which is configured to receive the first feedback signal VFBI and provide a signal to the gate driver circuit element 600 to control operation of the switch of the Class-E driver circuit element 530 based on the first feedback signal VFBI .

[0162] The RF power supply also includes a negative feedback loop 710, which is configured to receive the second feedback signal VFB2 and adjust operation of the positive feedback loop 700 based on the second feedback signal VFB2.

[0163] The positive feedback loop comprises a first comparator 630 and a modulator 590. The first comparator 630 is configured to compare a comparator input (e.g. the output of the kick starter circuit element 580 or the first feedback signal VFBI) to a reference, and provide a binary output based on the comparison. The modulator 590 is configured to modulate the comparator output based on an output of the negative feedback loop 710.

[0164] The modulator 590 is configured to modulate the comparator output by modulating the phase of the output of the first comparator 630 or by applying an offset to the output of the comparator 630. The modulator 590 includes a second comparator arranged to compare the modulated signal to a reference and provide a binary output based on the comparison. The gate driver circuit element 600 is configured to operate the switch of the Class-E driver circuit element 520 based on the output of the modulator circuit element 590.

[0165] The RF power supply 580 includes a kick starter circuit element 580 configured to provide a signal to the first comparator 630 during initialisation of the RF power supply 500, and configured to stop providing this signal to the comparator 630 once the peak amplitude of the output voltage VOUT_AC is greater than a threshold. During initialisation, the RF power supply 500 is configured to use the signal from the kick starter circuit element 580 to drive the power supply 500. After initialisation, the RF power supply 500 is configured to use the first feedback signal VFBI to drive the power supply 500 instead.

[0166] The negative feedback loop 710 includes a peak detector circuit element 570, configured to receive the second feedback signal VFB2 and provide an output signal VPEAK which is representative of the amplitude of the voltage VOUT_AC applied to the load RL 550.

[0167] The output VPEAK of the peak detector circuit element 570 is provided to the kick starter circuit element 580 for comparison with a reference to determine whether the peak amplitude of the output voltage VOUT_AC is greater than the threshold for ending initialisation of the RF power supply 500.

[0168] The negative feedback loop 710 also includes an error amplifier 620, configured to amplify the difference between the output VPEAK of the peak detector circuit element 570 and a reference Vref. The reference is representative of a desired peak amplitude of the alternating output voltage Vout_Ac, and therefore the output of the error amplifier 620 is representative of the difference between a current peak amplitude of the alternating output voltage Vout_Ac and a desired amplitude. In this example, the output of the negative feedback loop 710 is the output of the error amplifier 620.

[0169] The resistance of the load RL 550 varies in use. In most applications, it is important to control operation of the RF power supply 500 to help maintain efficient operation as the resistance of the load R 550 varies. For instance, it may be important that switching losses associated with changing the conductive state of the switch of the Class-E driver circuit 530 continue to be acceptable as the resistance of the load RL 550 varies. In some applications, it is also important that the amplitude of the output voltage VOUT_AC to the load RL 550 remains substantially constant.

[0170] Modulation of the signal used to drive the switch of the Class-E driver 530 based on the resistance of the resistor RL 550 may help to allow efficient operation to be maintained even as the resistance of the resistor RL 550 varies. Modulation of the signal used to drive the switch of the Class-E driver 530 based on the resistance of the resistor RL 550 may (e.g. also) help to allow a substantially constant output voltage VOUT_AC to be maintained even as the resistance of the resistor RL 550 varies. Modulation of the signal used to drive the switch of the Class-E driver 530 based on the resistance of the resistor RL 550 may help to reduce power losses associated with operation of the RF power supply 500 under no load condition.

[0171] The functional elements 560, 570, 620, 580, 630, 590 used to control operation of the RF power supply 500 may be implemented digitally or electronically. Examples of electronic implementations of these functional elements are provided in Figures 2 and 3.

[0172] While embodiments of the invention have been described with reference to a Class-E driver circuit, it will be apparent that this approach may be adapted to driver circuits of other types. For instance, the approach may be adapted to control operation of a Class-C driver circuit, a Class- D driver circuit or the like.

[0173] Figure 5 shows the change of resistance of the load RL 550 from a low resistance (which in this example is 0.1 MO) to a high resistance (which in this example is 10 MO). In this example, a step-change of the resistance is shown, however it will be appreciated that the disclosed approach is equally applicable to a gradual change of the resistance of the load RL 550.

[0174] Figure 6 shows the power that would be dissipated across the switch of the Class-E driver circuit element 530 if there is no adjustment to operation of the RF power supply 500 as the resistance of the load RL 550 changes.

[0175] Figure 7 shows the output voltage VOUT_AC that would be applied to the load RL 550 if there is no adjustment to operation of the RF power supply 500 as the resistance of the load RL 550 changes. As can be seen from Figure 7, the amplitude of the output voltage VOUT_AC that would be applied to the load RL 550 would increase (from approximately 5kV to approximately 9kV) if no adjustment to operation of the RF power supply 500 takes place as the resistance of the load RL 550 increases. Figures 8, 9 and 10 are counterparts to Figures 5, 6 and 7, respectively, which show the impact of adjusting operation of the RF power supply 500 to account for the change of resistance of the load.

[0176] As can be seen by comparing Figures 6 and 9, by adjusting operation of the RF power supply 500 to account for the change of resistance of the load RL 550, it is possible to reduce the power dissipated across the switch of the Class-E driver circuit element 530. In this example, the output power dissipated across the switch when the resistance of the load RL 550 has increased is reduced from approximately 75W, as shown in Figure 6, to approximately 15W, as shown in Figure 9. As well as improving the efficiency of the RF power supply 500, this may help to reduce heating of the switch, and therefore reduce the need for associated cooling. This may also help to reduce the risk of the switch failing as a result of over-heating.

[0177] As can be seen by comparing Figures 7 and 10, it may be possible to maintain a substantially constant amplitude of output voltage (which in this example is approximately 5kV) by adjusting operation of the RF power supply 500 as the resistance of the load RL 550 changes. In certain applications, it may be preferable, or even necessary, to help maintain a substantially constant amplitude of output voltage VOUT_AC to the load RL 550.

[0178] Figure 11 shows a method 20 of operating the power supply 500 shown in Figure 4. While the steps of the method 20 are described in a certain order, it will be understood that various of the method steps may be optional and / or performed in a different order to the order described.

[0179] The method includes a first step 21 of providing the gate drive signal to the gate driver circuit element 600 with a first polarity. The gate drive signal is used to control operation of the Class E Driver circuit 530, and in particular the switch of the Class E Driver circuit 530. The conducting state of the switch is based on the polarity of the gate drive signal.

[0180] The method 20 includes a second step 22 of receiving the first feedback signal VFBI from the feedback circuit element 560. The first feedback signal VFBI is representative of the alternating Output Vout_AC.

[0181] The method 20 includes a third step 23 of changing the polarity of the gate drive signal from the first polarity to a second polarity based on the alternating output crossing a threshold. In this example, the comparator 630 compares the first feedback signal VFBI to a reference to determine when the alternating output crosses the threshold.

[0182] The method 20 includes a fourth step 24 of receiving a second feedback signal VFB2 which is representative of at least the amplitude of the alternating output.

[0183] The method 20 includes a fifth step 25 of controlling, based on an amplitude of the alternating output (i) a delay between the alternating output crossing the threshold and the change of the polarity of the gate drive signal; or (ii) a duty cycle of the gate drive signal.

[0184] The method 20 includes a sixth step 26 of receiving a direct input Vs from the voltage source 510. The method 20 includes a seventh step 27 of operating the switch in a conducting state or a non-conducing state based on the polarity of the gate drive signal, so as to alternate the conducting state of the switch to convert the direct input to an alternating output of the Class E Driver circuit element 530.

[0185] The method 20 includes an eighth step 28 of generating an alternating output VOUT_AC based on the alternating output from the Class E driver circuit element 530. This alternating output VOUT_AC is used to provide the first and second feedback signals to perpetuate self-oscillation of the power supply 500.

[0186] While not shown in Figure 11, it will be apparent that the method 20 may include initialisation of the power supply 500 using the kick starter circuit element 580. The method 20 may also include various other steps (e.g. filtering, by the resonant tank 540, the output of the Class E Driver circuit 530 to provide the alternating output VOUT_AC).

[0187] It will be appreciated by those skilled in the art that this disclosure has been illustrated by describing one or more specific examples thereof, but is not limited to these examples; many variations and modifications are possible, within the scope of the accompanying claims.

[0188] Features of any aspect or embodiment described herein may, wherever appropriate, be applied to any other aspect or embodiment described herein. Where reference is made to some embodiments, it should be understood that these are not necessarily distinct embodiments but may overlap.

Claims

1. Claims1. A power supply configured to generate an alternating output, the power supply comprising: a positive feedback loop configured to: receive a first feedback signal, which is representative of the alternating output; and provide a gate drive signal based on the first feedback signal; wherein, the positive feedback loop is configured to, for each of a plurality of cycles of the alternating output: provide the gate drive signal with a first polarity; and change the polarity of the gate drive signal from the first polarity to a second polarity based on the alternating output crossing a threshold; the power supply further comprising a negative feedback loop, configured to: receive a second feedback signal, which is representative of the alternating output; and control, based on an amplitude of the alternating output:(i) a delay between the alternating output crossing the threshold and the change of the polarity of the gate drive signal; or(ii) a duty cycle of the gate drive signal; the power supply further comprising an inverter circuit element, the inverter circuit element comprising an inverter switch, wherein the inverter circuit element is configured to: receive a direct input; receive the gate drive signal; and operate the inverter switch in a conducting state or a non-conducing state based on the polarity of the gate drive signal so as to convert the direct input to an alternating inverter output in order to generate the alternating output.

2. The power supply as claimed in claim 1, wherein the inverter circuit element comprises a Class-E circuit topology.

3. The power supply as claimed in claim 1 or 2, wherein the negative feedback loop comprises an amplitude detector circuit element configured to provide an amplitude detector output based on the second feedback signal;wherein the amplitude detector output is representative of the amplitude of the alternating output; and wherein the negative feedback loop is configured to control (i) the delay or (ii) the duty cycle based on the amplitude detector output.

4. The power supply as claimed in claim 3, wherein the amplitude detector circuit element comprises a peak detector circuit element configured to provide a peak detector output based on the second feedback signal; wherein the peak detector output is representative of a peak amplitude of the alternating output; wherein the amplitude detector output is based on the peak detector output; and wherein the negative feedback loop is configured to control (i) the delay or (ii) the duty cycle based on the peak amplitude of the alternating output.

5. The power supply as claimed in claim 3 or 4, wherein the amplitude detector circuit element comprises an error amplifier circuit element, configured to: receive an error amplifier input based on the second feedback signal, which is representative of the amplitude of the alternating output; compare the error amplifier input with an amplitude reference; and provide an error amplifier output based on the comparison; wherein the amplitude detector output is based on the error amplifier output.

6. The power supply as claimed in claim 5, wherein the reference signal is representative of a desired amplitude of the alternating output; and wherein the negative feedback loop is configured to control (i) the delay or (ii) the duty cycle using the amplitude detector output so as to drive the amplitude of the alternating output towards the desired amplitude.

7. The power supply as claimed in any one of the preceding claims, wherein the positive feedback loop comprises a comparator circuit element configured to: receive a comparator input based on the first feedback signal; and compare the comparator input to a reference to provide a comparator output; wherein the gate drive signal is based on the comparator output; and wherein the comparator output is representative of whether the alternating output signal is greater than the threshold.

8. The power supply as claimed in one of the preceding claims, wherein the positive feedback loop comprises a modulator circuit element configured to: receive a modulator input based on the first feedback signal; receive an output of the negative feedback loop, which is based on the amplitude of the alternating output; and modulate the modulator input based on the output of the negative feedback loop; wherein the gate drive signal is based on the modulator output.

9. The power supply as claimed in claim 8, wherein the modulator circuit element is configured to control, based on the amplitude of the alternating output:(i) a duty cycle of the modulator output; or(ii) a delay between the modulator input and the modulator output.

10. The power supply as claimed in claim 9, wherein the modulator circuit element is configured to add a delay to the modulator input to provide the modulator output; wherein the delay is based on the output of the negative feedback loop.

11. The power supply as claimed in claim 8, 9 or 10, wherein the modulator circuit element comprises a second comparator configured to compare a second comparator input, based on the modulator input, to a reference; and provide a second comparator output based on the comparison; wherein the modulator output is based on the second comparator output.

12. The power supply as claimed in claim 11, wherein the modulator circuit element is configured to add an offset to the modulator input to provide the second comparator input; wherein the offset is based on the output of the negative feedback loop.

13. The power supply as claimed in claim 11 or 12, wherein the modulator circuit element is configured to add a delay to the modulator input to provide the second comparator input; wherein the delay is based on the output of the negative feedback loop.

14. The power supply as claimed in any one of the preceding claims, wherein the power supply comprises a kick starter circuit element configured to provide a kick starter output to initialise operation of the power supply; wherein, during initialisation of the power supply, the gate drive signal is based on the kick starter output.

15. The power supply as claimed in claim 14, wherein the power supply is configured to end initialisation of the power supply based on the amplitude of the alternating output exceeding an initialisation threshold.

16. The power supply as claimed in any one of the preceding claims, wherein the power supply is configured to provide an alternating output with a frequency in the RF range of the electromagnetic spectrum.

17. The power supply as claimed in any one of the preceding claims, wherein the power supply comprises an output capacitor; and wherein the power supply is configured to provide the alternating output across the output capacitor.

18. A RF drying system comprising the power supply as claimed in claim 17, wherein the RF drying system is configured to dry water molecules positioned between the plates of the output capacitor.

19. A printer comprising the RF drying system as claimed in claim 18, wherein the printer is configured to apply a printer ink comprising water molecules to a printing medium; and wherein the printer is configured to position the printing medium between the plates of the output capacitor to dry the printer ink.

20. A method of generating an alternating output using a power supply, the method comprising: receiving a first feedback signal, which is representative of the alternating output; and for each of a plurality of cycles of the alternating output: providing the gate drive signal with a first polarity; and changing the polarity of the gate drive signal from the first polarity to a second polarity based on the alternating output crossing a threshold; the method further comprising: receiving a second feedback signal, which is representative of the alternating output; and controlling, based on an amplitude of the alternating output:(i) a delay between the alternating output crossing the threshold and the change of the polarity of the gate drive signal; or(ii) a duty cycle of the gate drive signal; wherein the power supply comprises an inverter circuit element, the inverter circuit element comprising an inverter switch; and wherein the method comprises: receiving a direct input; and operating the inverter switch in a conducting state or a non-conducing state based on the polarity of the gate drive signal so as to convert the direct input to an alternating inverter output in order to generate the alternating output.

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