An electrical appliance

Abstract

An electrical appliance 10 includes a passive load 58-62, and a power converter 20 configured to convert supplied electrical power from a mains power supply 18 to a delivered electrical power for deli

Description

BACKGROUND Haircare appliances are typically used to style and / or dry a user’s hair. Some haircare appliances therefore comprise heaters for heating a user’s hair, whether through direct contact of the user’s hair with a surface heated by the heater, or through a heated airflow that is delivered to the user’s hair. Other electrical appliances may also utilise heaters, for example, to heat an airflow for delivery to a user. SUMMARY A first aspect provides an electrical appliance comprising: a passive load; a power converter configured to convert supplied electrical power from a mains power supply to a delivered electrical power for delivery to the passive load; a switching arrangement configured to selectively disconnect the passive load from the power converter; and a power converter controller for controlling the power converter; wherein the power converter controller is configured to control the power converter to be in an off state when the passive load is disconnected from the power converter. If the power converter were to be operational when the passive load is disconnected from the power converter, then there is a risk of gain distortion occurring for the power converter. Such gain distortion may be caused by parasitic components such as resonant inductances and stray capacitances. The gain distortion can lead to an increase in gain, which may in turn lead to an increase in peak voltage and a decrease in energy efficiency of the power converter. The increase in peak voltage may lead to the peak voltage being above an acceptable threshold, for example, such as above the Safety Extra Low Voltage (SELV) threshold. As the power converter controller is configured to control the power converter to be in an off state when the passive load is disconnected from the power converter, the above mentioned increase in peak voltage and decrease in energy efficiency may be mitigated for and / or avoided. The switching arrangement may comprise a switching arrangement switch operable to selectively disconnect the passive load from the power converter. The power converter may comprise a plurality of power converter switches. The power converter controller may be configured to control the plurality of power converter switches to be open to place the power converter in the off state. The passive load may be located in a first appliance housing, and the power converter may be located in a second appliance housing different from the first appliance housing. The first appliance housing may be spaced from the second appliance housing, for example with the first appliance housing and the second appliance housing linked by an electrical cable. The power converter controller may be located within the second appliance housing. There may be no communication between the first appliance housing and the second appliance housing, for example, no communications cable extending between the first appliance housing and the second appliance housing, which may reduce the cost and size of the electrical appliance. The first appliance housing may be a housing of a main unit of the electrical appliance. The first appliance housing may define a handle of the electrical appliance. The electrical appliance may be configured such that the passive load is galvanically isolated from the mains power supply when the haircare appliance is connected to the mains power supply. The power converter may be a resonant power converter. The power converter may be a resonant LLC power converter, which may have different configurations of a rectifier arrangement in its second winding. The power converter may be a full bridge LLC resonant power converter. The power converter may be a split capacitor LLC resonant power converter. The power converter may comprise a primary side, a secondary side, and an actively controlled power converter stage located on the primary side, wherein the actively controlled power converter stage is the only actively controlled power converter stage located on the primary side. By actively controlled is meant that the actively controlled power converter stage, and in particular one or more switches of the actively controlled power converter stage, is actively controlled by the power converter controller. This is in contrast to, for example, a passive stage of the power converter such as a diode bridge rectifier or the like. There may be no boost converter on the primary side of the power converter. The power converter may be configured to provide the delivered electrical power at a root mean square voltage of no more than 30 V, at a peak voltage of no more than 42 V, and at a root mean square current of no less than 50 A. The actively controlled power converter stage may be an LLC resonant power converter stage. The power converter may comprise a galvanic isolator, and the primary side and the secondary side may be opposite sides of the galvanic isolator. The galvanic isolator may be a transformer. The power converter may be configured to provide a rectified output voltage, for example, a rectified output voltage having an approximately sin2 form. The power converter may comprise an energy storage capacity of less than lOOnF. The power converter may be configured to provide a root mean square output voltage that varies by no more than ±5% from a pre-determined root mean square output voltage. The pre-determined root mean square output voltage may be in the region of 20V to 30V. The power converter controller may be configured to obtain current data indicative of current flowing through the power converter, and to control the power converter to be in the off state based on the current data. The current data may be indicative of current flowing through a low side of the power converter. The current data may comprise a current value. The current data may comprise a voltage value that is indicative of a corresponding current value. The power converter controller may be configured to obtain the current data over a time period. The time period may be a time period of the mains electrical half cycle. The power converter controller may be configured to control the power converter to be in the off state based on a mean current value indicated by the current data. The power converter controller may be configured to control the power converter to be in the off state when the current data indicates that current flowing through the power converter is below a pre-determined current threshold. The power converter controller may be configured to control the power converter to be in the off state when the current data indicates that a mean current flowing through the power converter is below a predetermined mean current threshold. The pre-determined current threshold may be determined based on the output power of the power converter and a predetermined time duration from a zero-crossing point of a rectified mains voltage from a mains power supply to which the electrical appliance is connected. The power converter controller may be configured to determine, based on the current data, whether the passive load is connected to the power converter. The power converter controller may be configured to control the power converter based on the determination as to whether the passive load is connected to the power converter. The electrical appliance may comprise a sensor configured to sense the current data, and configured to communicate the current data to the power converter controller. The sensor may comprise a shunt resistor. The sensor may be located on the low side of the power converter. The sensor may be also located on a low side of a resonant tank circuit of the power converter. The electrical appliance may comprise a passive load controller configured to control the switching arrangement, the passive load controller different to the power converter controller. The resistive load controller may be located within the first appliance housing. The passive load controller may be configured to obtain zero-crossing data indicative of a zero-crossing of an output voltage of the power converter, and to control the switching arrangement based on the zero-crossing data. The passive load controller may be configured to control the switching arrangement switch based on the zero-crossing data. The passive load controller may be configured to control power delivery to the passive load based on the zero-crossing data, for example by opening and closing the switching arrangement switch based on a control time period. The passive load controller may be configured to cause the switching arrangement to disconnect the passive load from the power converter when a power demand of the passive load is below a pre-determined threshold, for example when the passive load is in a dwell period. The passive load controller may be configured to cause the switching arrangement to disconnect the passive load from the power converter to regulate a temperature of the passive load. The electrical appliance may comprise an output voltage sensor configured to sense the output voltage of the power converter, and configured to communicate the output voltage of the power converter to the passive load controller. The voltage sensor may be located within the first appliance housing. The passive load controller may be configured to obtain the zero-crossing data based on the output voltage of the power converter. The passive load controller may be configured to control the switching arrangement to control the passive load using burst fire control. The passive load controller may be configured to control the switching arrangement to control the passive load using pulse width modulation in a rectified mains half cycle of the mains power supply to which the electrical appliance is connected. The pulse width modulation may comprise high frequency pulse width modulation. The passive load controller may be configured to control the switching arrangement to control the passive load using a combination of burst fire control and pulse width modulation. The pulse width modulation may comprise high frequency pulse width modulation. The power converter controller may be configured to obtain mains voltage data indicative of a zero-crossing of a voltage of the mains power supply to which the electrical appliance is connected, and to control the power converter based on the mains voltage data. The power converter controller may be configured to control the plurality of power converter switches based on the mains voltage data. The electrical appliance may comprise a mains voltage sensor configured to sense a voltage of the mains power supply. The mains voltage sensor may be located within the second appliance housing. The controller may be configured to receive input voltage data indicative of an input voltage of the power converter, and to control the power converter based on the input voltage data. The electrical appliance may comprise a voltage sensor configured to sense the input voltage of the power converter, the voltage sensor in communication with the power converter controller. The power converter controller may be configured to control the plurality of power converter switches based on the input voltage data. The power converter controller may be configured to control, based on the input voltage data, at least one of a frequency of switching of the plurality of power converter switches, and a shape of an input current drawn by the actively controlled power converter stage. The power converter controller may be configured to control the power converter such that a peak output voltage of the power converter is no greater than 42 V. The passive load may comprise at least one of a resistive load, an inductive load, and a capacitive load. Where the passive load comprises a resistive load, the resistive load may be a heater. The heater may be configured to operate at a power greater than 75W. The electrical appliance may comprise an electric motor, for example an electric motor configured to cause rotation of an impeller to generate an airflow through the haircare appliance. The electric motor may be configured to operate at a power in the region of 30W-50W. The power converter may be configured to operate with an output power of at least 75W. The power converter may be configured to operate with an output power of at least 100W, at least 200W, at least 300W, at least 400W, at least 500W, at least 1000W, or at least 1500W. The switching arrangement may comprise one or more metal oxide semiconductor field effect transistors (MOSFETs). The electrical appliance may comprise a haircare appliance. The electrical appliance may comprise a cooking appliance or a heating appliance. BRIEF DESCRIPTION OF THE DRAWINGS Figure lisa schematic illustration of an electrical appliance; Figure 2 is a schematic illustration of a power converter of the electrical appliance of Figure 1; Figure 3is a schematic illustration of an LLC converter stage of the power converter of Figure 2; Figure 4 is a schematic illustration of heater and motor control circuitry of the electrical appliance of Figure 1; Figure 5 is a schematic illustration of operation of the power converter of Figure 2 when operating in a low voltage territory; Figure 6 is a schematic illustration of operation of the power converter of Figure 2 when operating in a high voltage territory; Figure 7 is a schematic illustration of an output voltage waveform of the power converter of Figure 2; and Figure 8 is a plot of a normalised frequency and voltage gain of the power converter of Figure 2 as a function of the input voltage for a given ‘m’ and ‘Q’. DETAILED DESCRIPTION An electrical appliance 10 is illustrated schematically in Figure 1, and has a power supply unit 12 and a main unit 14. The electrical appliance 10 is a haircare appliance 10. The power supply unit 12 has a first housing 16, which may also be referred to as an appliance housing in the context described herein, electrical contacts 18 for connecting to a mains power supply, and a power converter 20 located within the first housing 16. The power converter 20 is shown in isolation in Figure 2. The power converter 20 is a resonant LLC converter. The power converter 20 has a first voltage divider 22, a second voltage divider 23, an EMC filter 24, a third voltage divider 26, a diode bridge rectifier 28, a first inductor LI, a first capacitor Cl, a shunt resistor RI, an LLC converter stage 30, a first zero-cross detection circuit 32, a power converter controller 34, a gate driver 36, an auxiliary power supply unit 38, and an analogue to digital converter 40. The first 22 and second 23 voltage dividers are each formed of a respective pair of resistors R2, R3 and R4, R5. The first 22 and second 23 voltage dividers are connected between the mains power supply MS and the EMC filter 24, and provide a voltage signals from respective high and low sides of the circuit to the first zero-cross detection circuit 32. The EMC filter 24 is located between the voltage-dependent resistor 22 and the third voltage divider 26. The exact form of the EMC filter 24 may vary, and will not be described here for sake of brevity. The third voltage divider 26 is formed of a pair of resistors R6, R7, and is connected between the high and low sides of the circuit, between the EMC filter 24 and the diode bridge rectifier 28. The third voltage divider 26 provides a voltage signal VIN SENSE to the power converter controller 34. The diode bridge rectifier 28 is located after the third voltage divider 26, and is a full bridge rectifier. The first inductor LI is located on the high side of the circuit between the diode bridge rectifier 28 and the LLC converter stage 30, and the shunt resistor RI is located on the low side of the circuit between the diode bridge rectifier 28 and the LLC converter stage 30. The shunt resistor RI provides a signal to the power converter controller 34 via the analogue to digital converter 40. The first capacitor Cl is located between the first inductor LI and the LLC converter stage 30, and between the shunt resistor RI and the LLC converter stage 30, between the high and low sides of the circuit. The LLC converter stage 30 is shown in Figure 3. The LLC converter stage 30 has a first switch SW1, a second switch SW2, a second capacitor C2, a second inductor L2, a third inductor L3, a transformer 42, a first synchronous rectification switch SRI, a second synchronous rectification switch SR2, a fourth inductor L4 and a third capacitor C3. Each of the first switch SW1 and the second switch SW2 is metal oxide semiconductor field effect transistor (MOSFET) that is controlled by the gate driver 36 in response to signals from the power converter controller 34. In other examples, the first switch SW1 and the second switch SW2 may each be a bi-directional Gallium Nitride (BiGaN switch). Where BiGaN switches are used the diode bridge rectifier 24 is omitted. The first SW1 and second SW2 switches are arranged in a half-bridge configuration. The second capacitor C2, the second inductor L2, and the third inductor L3 define a resonant tank. Collectively, the first switch SW1, the second switch SW2, the second capacitor C2, the second inductor L2, and the third inductor L3, define an actively controlled power converter stage 44. The transformer 42 has a primary winding 46 and a secondary winding 48. Thus, the power converter 20 can generally be considered to have a primary side 50 and a secondary side 52. The actively controlled power converter stage 44 is located on the primary side 50 of the power converter 20, and as will be appreciated from Figures 2 and 3, the actively controlled power converter stage 44 is the only actively controlled power converter stage located on the primary side 50 of the power converter 20. The transformer 42 acts to galvanically isolate and provide user safety to the main unit 14 from the mains power supply MS. The first synchronous rectification switch SRI, the second synchronous rectification switch SR2, the inductor L4 and the third capacitor C3 are located on the secondary side 52 of the power converter 20. The first synchronous rectification switch SRI is located on the high side of the circuit, and the second synchronous rectification switch SR2 is located on the low side of the surface. Each of the first synchronous rectification switch SRI and the second synchronous rectification switch SR2 is an actively controlled metal oxide semiconductor field effect transistor (MOSFET). In other examples GaN devices may be utilised. Controllers of the first synchronous rectification switch SRI and the second synchronous rectification switch SR2 are not shown here for the sake of clarity. The fourth inductor L4 is connected between the common drain terminal of both the first synchronous rectification switch SRI and the second synchronous rectification switch SR2, and the positive terminal of the LLC output. The third capacitor C3 is located after the fourth inductor L4, and in-between the output positive terminal and output negative terminals of the LLC power converter 20. The first zero-cross-detection circuit 32 is any circuit that is capable of receiving voltages from the first 22 and second 23 voltage dividers, and determining when a zero-crossing occurs in the AC voltage from the mains power supply MS. The first zero-cross detection circuit 32 is in communication with the power converter controller 34. The power converter controller 34 is configured to receive a signal from the first zero-cross detection circuit 32, and is also configured to receive a voltage from the third voltage divider 26, and a current from the shunt resistor RI via the analogue to digital converter 40. The power converter controller 34 is configured to control the first SW1 and second SW2 switches of the LLC converter stage 30, by issuing instructions to the gate driver 36, as will be discussed in further detail hereinafter. The auxiliary power supply unit 38 is connected after the diode bridge rectifier 28 and before the LLC converter stage 30, and is configured to power the first zero-cross detection circuit 32,the power converter controller 34, and the gate driver 36. The power supply unit 12 is electrically connected to the main unit 14 by an electrical cable 54. The main unit 14 has a second housing 56, which may also be referred to as an appliance housing in the context described herein, a heater 58, an electric motor 60, and heater and motor control circuitry 62. Although not illustrated in Figure 1 due to the schematic nature of Figure 1, it will be appreciated that the second housing 56 may have an air inlet and an air outlet to enable an airflow to be generated by the electric motor 60 in use. The heater 58 has first 64 and second 66 heating elements, and presents a resistive load to the power converter 20, as will be discussed in further detail hereinafter. The heater is configured to operate at a power of several kW. The electric motor 60 is any electric motor suitable for generating an airflow. An example electric motor is the Dyson V9 Motor produced by Dyson Technology Limited. The motor is configured to operate at a power of around 30W to 50W. The heater and motor control circuitry 62 is shown schematically in Figure 4, with the first 64 and second 66 heating elements and the electric motor 60 in place. The heater and motor control circuitry 62 has a fourth voltage divider 68, third SW3 and fourth SW4 switches, an inverter 70, a second zero-cross detection circuit 72, and a heater and motor controller 74. The fourth voltage divider 68 is formed of a pair of resistors R8, R9. The fourth voltage divider 68 is located on a high side of the heater and motor control circuitry 62, and provides a voltage to the second zero-cross detection circuit 72. The first 64 and second 66 heating elements are connected in parallel with one another. The third switch SW3 is connected in series with the first heating element 64, and the fourth switch SW4 is connected in series with the second heating element 66. First 65 and second 67 temperature feedbacks are associated with respective ones of the first 64 and second 66 heating elements, and provide temperature feedback to the heater and motor controller 74. A Schottky diode can be used for the isolation of the heater output voltage to the electric input of the inverter 70. A capacitor may be connected across the inverter input to store the energy and to supply electrical power input to the inverter 70 for driving the electric motor 60. The size of the capacitor may be rated depending on the acceptable ripple output voltage. The inverter 70 is connected in parallel with a capacitor C4 and the first 64 and second 66 heating elements. The inverter 70 is an appropriate inverter for use with the electric motor 60, and may be a single phase or three phase inverter, as appropriate. The second zero-cross-detection circuit 72 is any circuit that is capable of receiving voltages from the fourth voltage divider 68, and determining when a zero-crossing occurs in a rectified voltage supplied by the power converter 20. The second zero-cross detection circuit 72 is in communication with the heater and motor controller 74. The heater and motor controller 74 is configured to receive signals from the zero-cross detection circuit, and to receive signals from the first 65 and second 67 temperature feedbacks, and to control the heater 58 and the electric motor 60 accordingly, as will be discussed in more detail hereinafter. The inverter 70 is also in communication with the heater and motor controller 74. In use, the haircare appliance 10 draws power from the mains power supply MS. The first 22 and second 23 voltages dividers provide voltage signals to the first zero-cross detection circuit 32, and the first zero-cross detection circuit 32 determines zero-crossing points in the voltage of the mains power supply MS. The first zero-cross detection circuit 32 provides a signal indicative of the zero-crossing points to the power converter controller 34. The third voltage divider 26 provides the voltage signal VIN_SENSE to the power converter controller 34, and this may also be referred to as a feed-forward control signal. The power converter controller 34 utilises the signal from the first zero-cross detection circuit 32, and the voltage signal VIN_SENSE, to provide control signals to the gate driver 36 to control the first SW1 and second SW2 switches of the LLC converter stage 30. The power converter controller 34 controls the first SW1 and second SW2 switches of the LLC converter stage 30 such that the LLC converter stage 30 produces a regulated output root mean square voltage that is within ±5% of an intended output root mean square voltage value of 28.2V, and such that a full load output power of 500W is produced across a range of input voltages corresponding to the line variation for the mains power supply in question. To do so, the time at which the LLC converter stage 30 is operated in the mains half cycle is determined based on the input voltage and the output root mean square voltage is determined from a look-up table by the power converter controller 34. In some examples, the electrical appliance 10 is intended to operate in territories having a low voltage mains power supply. In such examples the power converter 20 provides the full load output power across a pre-determined range of input voltages from 97V to 123 V. Between 80V and 97V, and between 123V and 140V, the power converter 20 derates the output power by 10%. At an input voltage of 97V, the power converter controller 34 controls the first SW1 and second SW2 switches to operate at a switching frequency that is at the resonant frequency of the LLC converter stage 30. At higher voltages within the pre-determined range of input voltages, the power converter controller 34 controls the first SW1 and second SW2 switches to operate at a switching frequency greater than the resonant frequency of the LLC converter stage 30, with the switching frequency increasing as the input voltage increases. This is to compensate for the change in gain response of the LLC converter stage 30 with the change in input voltage. At input voltages below 97V, between 80V and 97V, the power converter controller 34 controls the first SW1 and second SW2 switches to operate at a switching frequency less than the resonant frequency of the LLC converter stage 30, with the switching frequency decreasing as the input voltage decreases. At input voltages above 123V, between 123V and 140V, the power converter controller 34 controls the first SW1 and second SW2 switches to operate at a switching frequency greater than the resonant frequency of the LLC converter stage 30, with the switching frequency increasing as the input voltage increases. Alternatively or additionally to controlling the switching frequency based on the resonant frequency of the LLC converter stage 30, in some examples the power converter controller 34 controls the first SW1 and second SW2 switches of the LLC converter stage 30 such that the LLC converter stage 30 draws a non-sinusoidal input current. This may ensure that a peak output voltage of the LLC converter stage 30 remains within a safety extra low voltage (SELV) limit for the electrical appliance 10. The exact form of the non-sinusoidal input current can vary depending on required circumstances. For example, the LLC converter stage can shape at least one of a leading edge and a trailing edge of the input current waveform. In some examples, such shaping of the input current occurs only when the input voltage is above 123V, between 123V and 140V. In other examples, such shaping of the input current also occurs when the input voltage is within the predetermined range of input voltages, between 97V and 123V. In some examples, such shaping of the input current also occurs when the input voltage is at least 103.5VV. In some examples, no shaping of the input current occurs below 97V. It will be appreciated that operation within and outside the pre-determined range of input voltages can be considered to be different modes of operation in the context of the present application. Such modes of operation are illustrated schematically in Figure 5, which also shows the output power derating. In some examples, the electrical appliance 10 is intended to operate in territories having a high voltage mains power supply. This can be alternatively or in addition to being configured to operate in territories having a low voltage mains power supply. In such examples, the power converter 20 provides the full load output power across a predetermined range of input voltages from 214V to 246V. Between 176V and 214V, and between 246V and 264V, the power converter 20 derates the output power by 10%. Similar to operation in low voltage territories described above, at an input voltage of 214V, the power converter controller 34 controls the first SW1 and second SW2 switches to operate at a switching frequency that is at the resonant frequency of the LLC converter stage 30. At higher voltages within the pre-determined range of input voltages, the power converter controller 34 controls the first SW1 and second SW2 switches to operate at a switching frequency greater than the resonant frequency of the LLC converter stage 30, with the switching frequency increasing as the input voltage increases. This is to compensate for the change in gain response of the LLC converter stage 30 with the change in input voltage. At input voltages below 214V, between 176V and 214V, the power converter controller 34 controls the first SW1 and second SW2 switches to operate at a switching frequency less than the resonant frequency of the LLC converter stage 30, with the switching frequency decreasing as the input voltage decreases. At input voltages above 246V, between 246V and 264V, the power converter controller 34 controls the first SW1 and second SW2 switches to operate at a switching frequency greater than the resonant frequency of the LLC converter stage 30, with the switching frequency increasing as the input voltage increases. Similar to operation in low voltage territories described above, in high voltage territories the power converter controller 34 can also cause shaping of the input current. In some examples, such shaping of the input current occurs only when the input voltage is above 246V, between 246V and 264V. In other examples, such shaping of the input current also occurs when the input voltage is within the pre-determined range of input voltages, between 214V and 246V. In some examples, such shaping of the input current also occurs when the input voltage is at least 222.0V. It will be appreciated that operation within and outside the pre-determined range of input voltages can be considered to be different modes of operation in the context of the present application. Such modes of operation are illustrated schematically in Figure 6, which also shows the output power derating. With the LLC converter stage 30 operating in the manner described above, the output voltage of the power converter 20 is regulated. The fourth inductor L4 along with the third capacitor C3 forms a low pass filter and acts to minimise the high-frequency ripple from the output voltage, and an example output voltage waveform of the power converter 20 is shown in Figure 6. As can be seen, the output voltage has a rectified, generally sin2, form. In some examples, the peak output voltage is monitored and fed-back to the power converter controller 34, such that the power converter controller 34 can override it with a previous value of the output voltage of the LLC converter stage 30, if required. Figure 8 shows the normalized frequency, and voltage gain of the power converter 20 as a function of the input voltage for a given ‘m’ and ‘Q’. Here ‘Q’ is the quality factor of the circuit and ‘m’ is the ratio of the total primary inductance to the resonant inductance. In all the modes of the operation, the switches in the power converter 20 operate in ZVS (Zero voltage switching) conditions to maintain the high efficiency. The output of the power converter 20 is passed to the heater and motor control circuitry 62 via the electrical cable 54. The fourth voltage divider 68 passes a voltage signal to the second zero-cross detection circuit 72, and the second zero-cross detection circuit 72 determines zero-crossing points in the rectified output voltage of the power converter 20. The second zero-cross detection circuit 72 provides a signal indicative of the zero-crossing points to the heater and motor controller 74. Using the signal indicative of the zerocrossing points, the heater and motor controller 74 controls the third SW3 and fourth SW4 switches to control the supply of electrical power to the first 64 and second 66 heating elements. The heater and motor controller 74 control the third SW3 and fourth SW4 switches using a combination of high-frequency pulse width modulated signals and multicycle burst fire control of the rectified mains. The first 65 and second 67 temperature feedbacks are also used by the heater and motor controller 74 to control the third SW3 and fourth SW4 switches. The output of the power converter 30 is also passed to the inverter 70, with the heater and motor controller 74 controlling operation of the inverter 70 to control operation of the electric motor 60 to generate an airflow through the main unit 14 of the electrical appliance 10. In use of the electrical appliance 10, there may be periods of time in which the power required to be delivered to the first 64 and second 66 heating elements varies. For example, there may be periods of time in which the third SW3 and fourth SW4 switches are turned off such that the first 64 and second 66 heating elements are disconnected from the power converter 20. In such scenarios, the power converter 20 sees no load, and there is a risk of gain distortion occurring for the power converter 20. Such gain distortion may be caused by parasitic components such as resonant inductances and stray capacitances distributed to the high-frequency transformer and synchronous rectifier. The gain distortion can lead to an increase in gain, which may in turn lead to an increase in peak voltage and a decrease in energy efficiency of the power converter 20. If the power converter 20 continues to be operated as normal, then there is a risk that the peak output voltage of the LLC converter stage 30 may exceed the SELV limit for the electrical appliance 10. To mitigate for this risk, the power converter controller 34 receives a current measurement from the shunt resistor RI via the analogue to digital converter 40, and calculates a mean current for a given time period in the mains cycle. In some examples the given time period is 100ps-500ps. When the mean current is less than or equal to a threshold value based on the output power of the power converter 20 and the above-mentioned time period , the power converter controller 34 determines that the third SW3 and fourth SW4 switches are turned off, and hence that there is no load connected to the power converter 20. In such circumstances, the power converter controller 34 controls the first SW1 and second SW2 switches of the LLC converter stage 30 to be open, such that the power converter 20 is turned off. This inhibits the peak output voltage of the LLC converter stage 30 from exceeding the SELV limit and improves the energy efficiency of the LLC power converter stage 30. As the power converter controller 34 is configured to control the power converter 20 to be in an off state when the first 64 and second 66 heating elements are disconnected from the power converter 20, the above-mentioned increase in peak voltage and decrease in energy efficiency may be mitigated for and / or avoided. As the determination made by the power converter controller 34 is based on the current value measured by the shunt resistor RI, there is no need for a communications cable to be present between the power supply unit 12 and the main unit 14, which may reduce cost and / or complexity compared to a similar arrangement where a communications cable is present. When the mean current is greater than the threshold value the power converter controller 34 controls the first SW1 and second SW2 switches in the manner described above to deliver the regulated output voltage to the main unit 14. In the examples described above, the power converter 20 has a single actively controlled power converter stage on the primary side of the power converter 20, in the form of the LLC converter stage 30, and the power converter controller 34 is able to determine, without active communication from the main unit 14, when the power converter 20 sees no load, and to subsequently turn the power converter 20 off. It will be appreciated that examples in which only one of those features is present are also envisaged. For example, examples with a single actively controlled power converter stage on the primary side of the power converter 20, but without the power converter turn of capability discussed above, are envisaged. Similarly, examples with the power converter turn off capability discussed above and more than one actively controlled power converter stage on the primary side of the power converter 20 are also envisaged. Other variations to the electrical appliance described above are also envisaged. For example, the operating parameters of the power converter 20, such as a desired output voltage and desired output power, may be varied, as appropriate, for a given resistive load such as a heater. Although the LLC converter stage 30 is illustrated as a half bridge resonant LLC converter stage, it will be appreciated that full bridge resonant LLC converter stages and split capacitor resonant LLC converter stages are also envisaged. It will further be appreciated that the teachings discussed herein can be applied to other forms of electrical appliance that have a passive load. A passive load can comprise at least one of a resistive load, an inductive load, and a capacitive load. For example, the passive load can comprise a resistive, or resistive dominant, load, such as a heater. For example, the teachings discussed herein may be applied to a fan heater, an induction cooker or the like. More generally, whilst particular examples have been described, it should be understood that these are illustrative examples only and that various modifications may be made without departing from the scope of the claims.

Claims

1. An electrical appliance comprising:a passive load;a power converter configured to convert supplied electrical power from a mains power supply to a delivered electrical power for delivery to the passive load;a switching arrangement configured to selectively disconnect the passive load from the power converter; anda power converter controller for controlling the power converter;wherein the power converter controller is configured to control the power converter to be in an off state when the passive load is disconnected from the power converter.

2. An electrical appliance as claimed in Claim 1, wherein the passive load is located in a first appliance housing, and the power converter is located in a second appliance housing different from the first appliance housing.

3. An electrical appliance as claimed in Claim 1 or Claim 2, wherein the electrical appliance is configured such that the passive load is galvanically isolated from the mains power supply when the electrical appliance is connected to the mains power supply.

4. An electrical appliance as claimed in any one of the preceding claims, wherein the power converter is a resonant power converter.

5. An electrical appliance as claimed in any one of the preceding claims, wherein the power converter comprises a primary side, a secondary side, and an actively controlled power converter stage located on the primary side, wherein the actively controlled power converter stage is the only actively controlled power converter stage located on the primary side.

6. An electrical appliance as claimed in any one of the preceding claims, wherein the actively controlled power converter stage is an LLC resonant power converter stage.

7. An electrical appliance as claimed in any one of the preceding claims, wherein the power converter controller is configured to obtain current data indicative of current flowing through the power converter, and to control the power converter to be in the off state based on the current data.

8. An electrical appliance as claimed in Claim 7, wherein the power converter controller is configured to control the power converter to be in the off state when the current data indicates that current flowing through the power converter is below a predetermined current threshold.

9. An electrical appliance as claimed in Claim 7 or Claim 8, wherein the electrical appliance comprises a sensor configured to sense the current data, and configured to communicate the current data to the power converter controller.

10. An electrical appliance as claimed in any one of the preceding claims, wherein the electrical appliance comprises a passive load controller configured to control the switching arrangement, the passive load controller different to the power converter controller.

11. An electrical appliance as claimed in Claim 10, wherein the passive load controller is configured to obtain zero-crossing data indicative of a zero-crossing of an output voltage of the power converter, and to control the switching arrangement based on the zerocrossing data.

12. An electrical appliance as claimed in Claim 11, wherein the electrical appliance comprises an output voltage sensor configured to sense the output voltage of the power converter, and configured to communicate the output voltage of the power converter to the passive load controller.

13. An electrical appliance as claimed in any one of Claims 10 to 12, wherein the passive load controller is configured to control the switching arrangement to control the passive load using burst fire control14. An electrical appliance as claimed in any one of Claims 10 to 13, wherein the passive load controller is configured to control the switching arrangement to control the passive load using pulse width modulation in a rectified mains half cycle of the mains power supply to which the electrical appliance is connected.

15. An electrical appliance as claimed in any one of the preceding claims, wherein the power converter controller is configured to obtain mains voltage data indicative of a zerocrossing of a voltage of the mains power supply to which the electrical appliance is connected, and to control the power converter based on the mains voltage data.

16. An electrical appliance as claimed in any one of the preceding claims, wherein the power converter controller is configured to receive input voltage data indicative of an input voltage of the power converter, and to control the power converter based on the input voltage data.

17. An electrical appliance as claimed in any one of the preceding claims, wherein the power converter controller is configured to control the power converter such that a peak output voltage of the power converter is no greater than 42 V.

18. An electrical appliance as claimed in any one of the preceding claims, wherein the passive load comprises at least one of a resistive load, an inductive load, and a capacitive load19. An electrical appliance as claimed in any one of the preceding claims wherein the switching arrangement comprises one or more metal oxide semiconductor field effect transistors (MOSFETs) .

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