Power supply circuit for an electricity meter

Abstract

A power supply circuit for an electricity meter is disclosed for converting an input AC voltage into an output DC voltage for supply to circuitry of the electricity meter, wherein the power supply circuit comprises a voltage multiplier clamp comprising a primary energy storage capacitance; and a switched DC to DC converter comprising a secondary energy storage capacitance. The voltage multiplier clamp is configured to multiply and rectify the input AC voltage to generate a primary DC voltage across the primary energy storage capacitance whilst also limiting the primary DC voltage. The switched DC to DC converter is configured to convert the primary DC voltage into a secondary DC voltage across the secondary energy storage capacitance for supply as the output DC voltage to the circuitry of the electricity meter. The power supply circuit may be configured to store sufficient energy to maintain a DC output voltage within a predetermined voltage range for a predetermined holdup time after a loss or reduction of an input AC line voltage.

Description

[0001] POWER SUPPLY CIRCUIT FOR AN ELECTRICITY METER

[0002] FIELD

[0003] The present disclosure relates to a power supply circuit for an electricity meter, and in particular though not exclusively, to a power supply circuit for an electricity meter which stores sufficient energy to maintain a DC output voltage within a predetermined voltage range for a predetermined holdup time after a loss or reduction of an input AC line voltage and which does not include any electrostatic double-layer capacitors (EDLCs).

[0004] BACKGROUND

[0005] A known power supply circuit for an electricity meter uses a flyback converter to convert an input AC line voltage into an output DC voltage for supply to electricity meter circuitry. Such a known power supply circuit is configured to store sufficient energy to maintain the DC output voltage within a predetermined voltage range for a predetermined holdup time after a loss or reduction of the input AC line voltage. Specifically, such a known power supply circuit uses one or more high capacitance electrostatic double-layer capacitors (EDLCs) on the secondary low-voltage side of the flyback converter to store sufficient energy to maintain the DC output voltage within a predetermined voltage range for a predetermined holdup time after a loss or reduction of the input AC line voltage. However, use of EDLC capacitors may be undesirable for some applications, for example due to current leakage and / or because the performance and / or lifetime of EDLC capacitors is temperature sensitive. Consequently, it would be desirable not to use EDLC capacitors but to use an alternative type of capacitor such as an electrolytic capacitor. However, electrolytic capacitors have limited capacitance, with the result that the use of an electrolytic capacitor in place of each EDLC capacitor may not store sufficient energy for energy holdup purposes.

[0006] SUMMARY

[0007] According to an aspect of the present disclosure there is provided a power supply circuit for converting an input AC voltage into an output DC voltage for supply to circuitry of an electricity meter, the power supply circuit comprising: a voltage multiplier clamp comprising a primary energy storage capacitance; and a switched DC to DC converter comprising a secondary energy storage capacitance, wherein the voltage multiplier clamp is configured to multiply and rectify the input AC voltage to generate a primary DC voltage across the primary energy storage capacitance whilst also limiting the primary DC voltage, and wherein the switched DC to DC converter is configured to convert the primary DC voltage into a secondary DC voltage across the secondary energy storage capacitance for supply as the output DC voltage to the circuitry of the electricity meter.

[0008] Such a power supply circuit may be used to store sufficient energy in the primary energy storage capacitance on the primary high-voltage side of the switched DC to DC converter to maintain the secondary DC voltage within a desired range for a desired holdup time after a loss or reduction of the input AC line voltage. Even when the primary energy storage capacitance has a smaller capacitance than an EDLC capacitor, multiplication of the input AC line voltage when generating the primary DC voltage across the primary energy storage capacitance allows more energy to be stored in the primary energy storage capacitance because the energy E stored in the primary energy storage capacitance C depends on the voltage V across the primary energy storage capacitance according to E = CV2. Multiplication of the input AC line voltage when generating the primary DC voltage across the primary energy storage capacitance also means that a smaller primary energy storage capacitance is required to store a given quantity of energy in the primary energy storage capacitance i.e. multiplication of the input AC line voltage when generating the primary DC voltage across the primary energy storage capacitance may reduce the number and / or size of primary energy storage capacitors required to store a given quantity of energy.

[0009] Furthermore, the limited primary DC voltage which is generated by the voltage multiplier clamp across the primary energy storage capacitance may be limited such that the voltage across each of the one or more primary energy storage capacitors of the primary energy storage capacitance does not exceed the rated voltage of each primary energy storage capacitor even when the input AC line voltage is higher than expected, for example even when the voltage multiplier clamp is configured to multiply a 120 V input AC line voltage up to a primary DC voltage required to store sufficient energy in the primary energy storage capacitance to maintain the secondary DC voltage within a specified range for a holdup time after a loss or reduction of the input AC line voltage, but where a 240 V input AC line voltage is used in error instead of a 120 V input AC line voltage. Moreover, larger, more complex, switched DC to DC converters are generally required for higher voltages across the primary energy storage capacitance. Consequently, limiting the primary DC voltage across the primary energy storage capacitance also has the advantage that there is no requirement for the switched DC to DC converter to be larger or more complex to withstand a higher voltage across the primary energy storage capacitance when the input AC line voltage is higher than expected.

[0010] Optionally, the switched DC to DC converter comprises a flyback converter, a buck-boost converter, or a buck converter.

[0011] Optionally, the voltage multiplier clamp comprises: a voltage multiplier comprising the primary energy storage capacitance and one or more other electronic components; and a voltage clamp, wherein the voltage multiplier is configured to multiply and rectify the input AC voltage to generate the primary DC voltage across the primary energy storage capacitance, and wherein the voltage clamp is configured to limit the primary DC voltage across the primary energy storage capacitance.

[0012] Optionally, the voltage clamp comprises a voltage regulator for limiting the primary DC voltage to a nominal limit voltage.

[0013] Optionally, the voltage regulator and the primary energy storage capacitance are connected in parallel.

[0014] Optionally, the voltage clamp comprises a series resistor connected in series with the voltage regulator between the voltage regulator and the one or more other electronic components of the voltage multiplier.

[0015] Optionally, the voltage clamp comprises a pass transistor, wherein the pass transistor is connected in series with the primary energy storage capacitance between the primary energy storage capacitance and the one or more other electronic components of the voltage multiplier, and wherein a control terminal of the pass transistor is connected to a node between the series resistor and the voltage regulator. Use of such a pass transistor can reduce a variation in the primary DC voltage across the primary energy storage capacitance caused by a variation in a load at an output of the switched DC to DC converter. In use, the primary DC voltage across the primary energy storage capacitance may be equal to a difference between the nominal voltage of the voltage regulator and a threshold voltage of the pass transistor.

[0016] Optionally, the pass transistor comprises a metal-oxide-semiconductor fieldeffect transistor, a bipolar junction transistor, or an insulated-gate bipolar transistor. Optionally, the voltage clamp comprises a series resistor connected in series with the voltage regulator, wherein the voltage clamp further comprises a current source for controlling a current through the series resistor.

[0017] Optionally, the current source comprises a transistor current source.

[0018] Optionally, the transistor current source, the series resistor, and the voltage regulator are connected in series, with the series resistor connected between the transistor current source and the voltage regulator, and with the transistor current source connected between the series resistor and the one or more other electronic components of the voltage multiplier, and wherein a node between the series resistor and the voltage regulator is connected to a control terminal of the transistor current source.

[0019] Optionally, the transistor current source comprises a junction field-effect transistor.

[0020] Optionally, the voltage regulator comprises a Zener diode, wherein the nominal limit voltage comprises a breakdown voltage of the Zener diode.

[0021] Optionally, the voltage multiplier is a non-switching voltage multiplier. A nonswitching voltage multiplier may be less complex and / or smaller in size than a switching voltage multiplier such a boost converter or a flyback converter.

[0022] Optionally, the voltage multiplier is a switching voltage multiplier such as a boost converter or a flyback converter.

[0023] Optionally, the voltage multiplier does not comprise a switch.

[0024] Optionally, the voltage multiplier does not comprise an inductor or a transformer.

[0025] Optionally, the voltage multiplier comprises a network of capacitors and diodes.

[0026] Optionally, the voltage multiplier comprises a half-wave voltage multiplier or a full-wave voltage multiplier.

[0027] Optionally, the voltage multiplier comprises a voltage tripler.

[0028] Optionally, the voltage multiplier comprises a voltage doubler.

[0029] Optionally, the primary energy storage capacitance comprises one or more primary energy storage capacitors.

[0030] Optionally, the primary energy storage capacitance comprises a plurality of primary energy storage capacitors, wherein the plurality of primary energy storage capacitors are connected in series or in parallel. Use of a plurality of primary energy storage capacitors on the primary high-voltage side of the switched DC to DC converter may mean that a given quantity of energy may be stored in the primary energy storage capacitance without exceeding the rated voltage of each primary energy storage capacitor. Although the equivalent capacitance a plurality of primary energy storage capacitors connected in series may be lower than the capacitance of each individual primary energy storage capacitor, the DC voltage across each individual primary energy storage capacitor may be lower than the primary DC voltage across the primary energy storage capacitance. This may be important for higher input AC voltages so that the DC voltage across each individual primary energy storage capacitor does not exceed a rated voltage of each individual primary energy storage capacitor.

[0031] Optionally, the primary energy storage capacitance comprises one or more capacitors of a type other than an EDLC capacitor.

[0032] Optionally, the primary energy storage capacitance comprises one or more capacitors of a type which has a higher rated voltage than an EDLC capacitor.

[0033] Optionally, the primary energy storage capacitance comprises one or more electrolytic capacitors and / or one or more polymer capacitors. Electrolytic or polymer capacitors are less susceptible to current leakage than EDLC capacitors. The performance and lifetime of electrolytic or polymer capacitors are less sensitive to temperature than those of EDLC capacitors. Moreover, an electrolytic or a polymer capacitor generally has a higher rated voltage than an EDLC capacitor.

[0034] Optionally, the secondary energy storage capacitance comprises one or more secondary energy storage capacitors. Optionally, the secondary energy storage capacitance comprises a plurality of secondary energy storage capacitors, wherein the plurality of secondary energy storage capacitors are connected in series or in parallel.

[0035] Optionally, the secondary energy storage capacitance comprises one or more capacitors of a type other than an EDLC capacitor.

[0036] Optionally, the secondary energy storage capacitance comprises one or more electrolytic capacitors and / or one or more polymer capacitors.

[0037] Optionally, the power supply circuit comprises an electromagnetic interference filter and / or surge protector, wherein the electromagnetic interference filter and / or surge protector is configured to receive an input AC line voltage at an input of the electromagnetic interference filter and / or surge protector and to generate the input AC voltage at an output of the electromagnetic interference filter and / or surge protector.

[0038] Optionally, the power supply circuit is configured to convert an input AC voltage having a nominal value of 120 V AC or 240 V AC into the output DC voltage.

[0039] Optionally, the power supply circuit is configured to convert an input AC voltage into the output DC voltage in the presence of variations in the input AC voltage in a range of + / - 20% from a nominal value of the input AC voltage. According to an aspect of the present disclosure there is provided an electricity meter comprising: the power supply circuit as described above; and the circuitry of the electricity meter.

[0040] According to an aspect of the present disclosure there is provided a method for converting an input AC voltage into an output DC voltage for supply to circuitry of an electricity meter, the method comprising: multiplying and rectifying an input AC voltage to generate a primary DC voltage across a primary energy storage capacitance whilst also limiting the primary DC voltage; and using a switched DC to DC converter to convert the primary DC voltage into a secondary DC voltage across a secondary energy storage capacitance of the switched DC to DC converter for supply as the output DC voltage to the circuitry of the electricity meter.

[0041] Optionally, the switched DC to DC converter comprises a flyback converter, a buck-boost converter, or a buck converter.

[0042] It should be understood that any one or more of the optional features of any one of the foregoing aspects of the present disclosure may be combined with any one or more of the other foregoing aspects of the present disclosure or the optional features of any one or more of the other foregoing aspects of the present disclosure.

[0043] BRIEF DESCRIPTION OF THE DRAWINGS

[0044] A power supply circuit for an electricity meter will now be described by way of non-limiting example only with reference to the drawings of which:

[0045] FIG. 1 is a schematic block diagram of a power supply circuit for an electricity meter;

[0046] FIG. 2 shows a voltage multiplier clamp of the power supply circuit of FIG. 1 ;

[0047] FIG. 3 shows a first alternative voltage multiplier clamp for use in the power supply circuit of FIG. 1 ;

[0048] FIG. 4 shows a second alternative voltage multiplier clamp for use in the power supply circuit of FIG. 1 ; and FIG. 5 shows a voltage doubler for use in a third alternative voltage multiplier clamp for use in the power supply circuit of FIG. 1 .

[0049] DETAILED DESCRIPTION OF THE DRAWINGS

[0050] Referring initially to FIG. 1 there is shown a power supply circuit generally designated 2 for converting an input AC line voltage from an AC power supply 4 into an output DC voltage for supply to circuitry 6 of an electricity meter. It should be understood that the power supply circuit 2 and the circuitry 6 may both be incorporated into, or form part of, an electricity meter for measuring one or more electrical quantities associated with the AC power supply 4.

[0051] The power supply circuit 2 comprises a voltage multiplier clamp generally designated 10 and a switched DC to DC converter in the form of a flyback converter generally designated 12. The voltage multiplier clamp comprises a primary energy storage capacitance in the form of a primary energy storage electrolytic capacitor Cp. The flyback converter 12 comprises a secondary energy storage capacitance in the form of a secondary energy storage electrolytic capacitor Cs. As will be described in more detail below, the voltage multiplier clamp 10 is configured to multiply and rectify the input AC line voltage to generate a clamped or limited primary DC voltage across the primary energy storage capacitor Cp, and the flyback converter 12 is configured to convert the primary DC voltage into a secondary DC voltage across the secondary energy storage capacitor Cs for supply as the output DC voltage to the circuitry 6 of the electricity meter. In particular, the voltage multiplier clamp 10 limits the primary DC voltage across the primary energy storage capacitor Cp to a manageable level in the approximate range of 350 to 400 V to avoid the need for additional measures to deal with higher primary DC voltages such as capacitor networks with balancing networks, use of high voltage capacitors which are more rare and costly, or limiting the AC voltage at the input to the voltage multiplier clamp 10 to a reasonable level. It should be understood that the voltage multiplier clamp 10 serves to minimize the size of the secondary energy storage capacitor Cs as this is inefficient for energy storage. The load and parameters of the flyback converter 12 would dictate the size of the secondary energy storage capacitor Cs to handle the ripple current. The secondary energy storage capacitor Cs may have a capacitance in the range of 100 to 330 p.F for a polymer capacitor, or 1000 to 3300 LIF for an electrolytic capacitor. The capacitance of the primary energy storage capacitor Cp is sized according to the energy to be stored. For example, the capacitance of the primary energy storage capacitor Cp may be in the range of 22 piF to 220 |iF depending on the load that needs to be supported.

[0052] The power supply circuit 2 further comprises an electromagnetic interference filter and surge protector 14, wherein the electromagnetic interference filter and surge protector 14 is configured to receive the input AC line voltage at an input of the electromagnetic interference filter and surge protector 14 and generate an input AC voltage at an output of the electromagnetic interference filter and surge protector 14 for input to the voltage multiplier clamp 10.

[0053] The flyback converter 12 further comprises a switcher circuit such as a switcher IC 20, a primary inductor 22a, a secondary inductor 22b, a diode 24, and the secondary energy storage capacitor Cs. The primary and secondary inductors 22a, 22b are magnetically coupled. The primary and secondary inductors 22a, 22b have a turns ratio Np : Ns such as 10:1 or 20:1 , or a turns ratio between 10:1 or 20:1 . The switcher IC 20 and the primary inductor 22a are connected in series across the primary energy storage capacitor Cp of the voltage multiplier clamp 10. The secondary inductor 22b and the diode 24 are connected in series across the secondary energy storage capacitor Cs. As will be described in more detail below, the switcher IC 20 is configured to repeatedly connect and disconnect the primary energy storage capacitor Cp to the primary inductor 22a to convert the input AC voltage at the input of the voltage multiplier clamp 10 to the secondary DC voltage across the secondary energy storage capacitor Cs for supply as the output DC voltage to the circuitry 6 of the electricity meter.

[0054] As shown in more detail in FIG. 2, the voltage multiplier clamp 10 comprises a surge resistor R1 , a voltage multiplier in the form of a non-switching voltage tripler generally designated 30 for tripling an input AC voltage V1 when the input AC line voltage supplied by the AC power supply 4 has a nominal value of 120 V AC, and a voltage clamp generally designated 32. As will be appreciated by one of ordinary skill in the art, the voltage tripler 30 includes the primary energy storage capacitor Cp and other electronic components in the form of a network 34 of capacitors C1 , C2 and diodes D1 , D2, D3. The voltage clamp 32 comprises a series resistor R2 and a voltage regulator in the form of a Zener diode D4. The series resistor R2 is connected in series with the Zener diode D4 between the Zener diode D4 and the network 34 of capacitors C1 , C2 and diodes D1 , D2, D3. The Zener diode D4 and the primary energy storage capacitor Cp are connected in parallel.

[0055] In use, the AC power supply 4 supplies a nominal input AC line voltage of 120 V AC to the electromagnetic interference filter and surge protector 14 and the electromagnetic interference filter and surge protector 14 generates an input AC voltage V1 at the input of the voltage multiplier clamp 10. The voltage multiplier clamp 10 is designed so that for a nominal input AC line voltage of 120 V AC, the voltage tripler 30 triples and rectifies the corresponding input AC voltage V1 and the voltage clamp 32 limits the voltage across the primary energy storage capacitor Cp to a breakdown voltage of the Zener diode D4. One of skill in the art will understand that for a nominal input AC line voltage of 120 V AC, a perfect voltage tripler would generate a primary DC voltage of around 510 V DC. In practice, however, the voltage is not as high as this, especially under load. Additionally, the purpose of the Zener diode D4 is to clamp the primary DC voltage to a voltage which is lower than 510 V DC and which is practical to implement with a common electrolytic capacitor voltage e.g. the Zener diode D4 clamps the primary DC voltage to a voltage of approximately 350 V DC.

[0056] The switcher IC 20 repeatedly connects and disconnects the primary energy storage capacitor Cp to the primary inductor 22a of the flyback converter 12 to convert the primary DC voltage across the primary energy storage capacitor Cp to a secondary DC voltage across the secondary energy storage capacitor Cs of the flyback converter 12 for supply as an output DC voltage to the circuitry 6 of the electricity meter. For example, the secondary DC voltage may be on the order of 5 to 15 V DC. Alternatively, the secondary DC voltage may be higher than this - e.g. around 48 V DC - to allow the storage of more energy on the secondary energy storage capacitor Cs according to the relationship E = GV2where E represents the stored energy, C is the capacitance of the secondary energy storage capacitor Cs, and V is the secondary DC voltage.

[0057] It should be understood that the series resistor R2 is sized to limit the load current that flows to and from the primary energy storage capacitor Cp as the switcher IC 20 repeatedly connects and disconnects the primary energy storage capacitor Cp to the primary inductor 22a of the flyback converter 12. In other words, the resistance of the series resistor R2 is sized to ensure the primary energy storage capacitor Cp is fully charged based on the switching frequency of the flyback converter 12 and the load. The series resistor R2 also needs to be rated to accommodate the power dissipated during the heaviest loads.

[0058] It should also be understood that the primary energy storage capacitor Cp and the breakdown voltage of the Zener diode D4 are selected so that for the nominal input AC line voltage, the primary energy storage capacitor Cp can store sufficient energy to maintain a secondary DC voltage across the secondary energy storage capacitor Cs within a predetermined voltage range for a predetermined holdup time after a loss or reduction of the input AC line voltage. Furthermore, the Zener diode D4 is configured so that the breakdown voltage of the Zener diode D4 is less than or equal to a rated voltage of the primary energy storage capacitor Cp. Consequently, even if the input AC line voltage is higher than the nominal input AC line voltage, the primary DC voltage across the primary energy storage capacitor Cp cannot exceed the rated voltage of the primary energy storage capacitor Cp. In particular, for the case where the primary energy storage capacitor Cp and the breakdown voltage of the Zener diode D4 are selected for a nominal input AC line voltage of 120 V AC, but where an incorrect input AC line voltage of 240 V AC is used, the primary DC voltage across the primary energy storage capacitor Cp cannot exceed the rated voltage of the primary energy storage capacitor Cp.

[0059] It should also be understood that the primary DC voltage across the primary energy storage capacitor Cp may vary according to the current flowing through the series resistor R2 and that the R2 / D4 circuit has to be sized such that the Zener diode D4 takes all the demand of the primary energy storage capacitor Cp, when the primary energy storage capacitor Cp is fully charged e.g. if it takes 100 mA to replenish Cp between switching cycles, the switch is on for 50% of the period, and the recharge is done in 20% of the period, then the Zener diode D4 has to sink 100 mA for the remaining 30% of the period. This may lead to a potentially impractical Zener diode D4, and relatively high power dissipation.

[0060] FIG. 3 shows a first alternative voltage multiplier clamp 1 10 for use in the power supply circuit 2 of FIG. 1 in place of the voltage multiplier clamp 10 for reducing the current flowing through the Zener diode D4 and for reducing the dependency of the primary DC voltage across the primary energy storage capacitor Cp on the current flowing through the series resistor R2. The voltage multiplier clamp 1 10 comprises a surge resistor R1 , the voltage tripler 30, and a voltage clamp generally designated 132. As will be appreciated by one of ordinary skill in the art, the voltage tripler 30 includes the primary energy storage capacitor Cp and other electronic components in the form of the network 34 of capacitors C1 , C2 and diodes D1 , D2, D3. The voltage clamp 132 comprises a series resistor R2 and a voltage regulator in the form of a Zener diode D4. The series resistor R2 is connected in series with the Zener diode D4 between the Zener diode D4 and the network 34 of capacitors C1 , C2 and diodes D1 , D2, D3. The voltage clamp 132 further comprises a pass transistor in the form of a metal-oxide-semiconductor field-effect transistor (MOSFET) M1 , wherein the pass transistor M1 is connected in series with the primary energy storage capacitor Cp between the primary energy storage capacitor Cp and the network 34 of capacitors C1 , C2 and diodes D1 , D2, D3, and wherein a gate terminal of the pass transistor M1 is connected to a node between the series resistor R2 and the Zener diode D4.

[0061] In use, the AC power supply 4 supplies an input AC line voltage to the electromagnetic interference filter and surge protector 14 and the electromagnetic interference filter and surge protector 14 generates an input AC voltage V1 at the input of the voltage multiplier clamp 1 10. The voltage multiplier clamp 1 10 is designed so that for a nominal input AC line voltage, the voltage tripler 30 triples and rectifies the corresponding input AC voltage V1 and the voltage clamp 132 essentially limits the voltage across the primary energy storage capacitor Cp to a breakdown voltage of the Zener diode D4 less a threshold voltage of the pass transistor M1 . The switcher IC 20 repeatedly connects and disconnects the primary energy storage capacitor Cp to the primary inductor 22a of the flyback converter 12 to convert the primary DC voltage across the primary energy storage capacitor Cp to the secondary DC voltage across the secondary energy storage capacitor Cs of the flyback converter 12 for supply as the output DC voltage to the circuitry 6 of the electricity meter.

[0062] To limit the power dissipation in the MOSFET the capacitors used in the multiplying stages of the voltage tripler 30 can be reduced in scale such that smaller current impulses are passed through the pass transistor M1 at one time. Simulations show that values as low as 2.2uF are viable for the capacitors used in the multiplying stages of the voltage tripler 30, and that such capacitors considerably reduce the power dissipation in the pass transistor M1 even in a worst case condition where the input AC line voltage is 20% higher than a nominal value of the input AC line voltage. The tradeoff is an increased time to charge the primary energy storage capacitor Cp and the secondary energy storage capacitor Cs to the full value for maximum holdup time.

[0063] Use of the pass transistor M1 also means that the voltage clamp 132 essentially limits the voltage across the primary energy storage capacitor Cp to a breakdown voltage of the Zener diode D4 less a threshold voltage of the pass transistor M1 largely independently of the current flowing through the pass transistor M1 . The pass transistor M1 is also able to provide more current to the primary energy storage capacitor Cp. More specifically, the pass transistor M1 allows the R2 / D4 circuit to be biased with a small amount of current and passes current to the primary energy storage capacitor Cp as needed. This may reduce the power dissipation in D4 and allow the use of a more practical Zener diode D4.

[0064] FIG. 4 shows a second alternative voltage multiplier clamp 210 for use in the power supply circuit 2 of FIG. 1 in place of the voltage multiplier clamp 10 The voltage multiplier clamp 210 comprises a surge resistor R1 , a voltage multiplier in the form of the voltage tripler 30, and a voltage clamp generally designated 232. As will be appreciated by one of ordinary skill in the art, the voltage tripler 30 includes the primary energy storage capacitor Cp and other electronic components in the form of the network 34 of capacitors C1 , C2 and diodes D1 , D2, D3. The voltage clamp 232 comprises a JFET current source J1 , a series resistor R2, and a voltage regulator in the form of a Zener diode D4, wherein the JFET current source J1 , the series resistor R2 and the Zener diode D4 are connected in series, with the series resistor R2 connected between the JFET current source J1 and the Zener diode D4, and with the JFET current source J1 connected between the series resistor R2 and the network 34 of capacitors C1 , C2 and diodes D1 , D2, D3. A node between the series resistor R2 and the Zener diode D4 is connected to a gate terminal of the JFET current source J1 to help regulate the current flowing through the Zener diode D4 and thereby maintain the breakdown voltage across the Zener diode D4. The voltage clamp 232 further comprises a pass transistor in the form of a metal-oxide-semiconductor field-effect transistor (MOSFET) M1 , wherein the pass transistor M1 is connected in series with the primary energy storage capacitor Cp between the primary energy storage capacitor Cp and the network 34 of capacitors C1 , C2 and diodes D1 , D2, D3, and wherein a gate terminal of the pass transistor M1 is connected to a node between the series resistor R2 and the Zener diode D4.

[0065] In use, the AC power supply 4 supplies an input AC line voltage to the electromagnetic interference filter and surge protector 14 and the electromagnetic interference filter and surge protector 14 generates an input AC voltage V1 at the input of the voltage multiplier clamp 210. The voltage multiplier clamp 210 is designed so that for a nominal input AC line voltage, the voltage tripler 30 triples and rectifies the corresponding input AC voltage V1 and the voltage clamp 232 essentially limits the voltage across the primary energy storage capacitor Cp to a breakdown voltage of the Zener diode D4 less a threshold voltage of the pass transistor M1 . The switcher IC 20 repeatedly connects and disconnects the primary energy storage capacitor Cp to the primary inductor 22a of the flyback converter 12 to convert the primary DC voltage across the primary energy storage capacitor Cp to the secondary DC voltage across the secondary energy storage capacitor Cs of the flyback converter 12 for supply as the output DC voltage to the circuitry 6 of the electricity meter. Use of the pass transistor M1 means that the voltage clamp 232 essentially limits the voltage across the primary energy storage capacitor Cp to a breakdown voltage of the Zener diode D4 less a threshold voltage of the pass transistor M1 largely independently of the current flowing through the pass transistor M1 . Moreover, use of the JFET current source J1 at least partially reduces any variation in the current flowing through R2 and therefore also at least partially reduces any variation in voltage across the Zener diode D4, and therefore also in the primary DC voltage across the primary energy storage capacitor Cp compared with the first alternative voltage multiplier clamp 110 described above with reference to FIG. 3.

[0066] The voltage multiplier clamps 10, 110, 210 described above each comprise a voltage multiplier in the form of a voltage tripler 30 for tripling an input AC voltage V1 when the input AC line voltage supplied by the AC power supply 4 has a nominal value of 120 V AC, wherein the voltage tripler 30 comprises the primary energy storage capacitor Cp and the network 34 of capacitors C1 , C2 and diodes D1 , D2, D3. In a variant of each of the voltage multiplier clamps 10, 110, 210, a different voltage multiplier may be used when the input AC line voltage supplied by the AC power supply 4 is greater than 120 V AC, for example when the input AC line voltage supplied by the AC power supply 4 is 240 V AC. For example, with reference to FIG. 5, there is shown a voltage doubler for a third alternative voltage multiplier clamp for use in the power supply circuit of FIG. 1 . Such a voltage doubler may be used depending on at least one of a nominal value of the input AC line voltage, an estimated variation of the input AC line voltage around the nominal value of the input AC line voltage, the rated voltage of the primary energy storage capacitor Cp, the capacitance of the primary energy storage capacitor Cp, a required range of the secondary DC voltage across the secondary energy storage capacitor Cs, and the required holdup time during which the secondary DC voltage across the secondary energy storage capacitor Cs is to be maintained after a loss or reduction of the input AC line voltage. Although not shown in FIG. 5, it should be understood that a voltage clamp such as any of voltage clamps 32, 132 or 232 may be used in conjunction with the voltage doubler 330 shown in FIG. 5 to clamp the multiplied primary DC voltage generated across the primary energy storage capacitor Cp.

[0067] One of ordinary skill in the art will also understand that various modifications are possible to the power supply circuit embodiments described above. For example, although all of the voltage multiplier clamps described above use a voltage regulator in the form of a Zener diode, other voltage multiplier clamps may use a different type of voltage regulator. For example, other voltage multiplier clamps may use a voltage source of any kind e.g. a comparator / op-amp circuit could be used to compare the primary DC voltage across the primary energy storage capacitor Cp to a reference voltage and control a transistor or a pulse transformer that in turn controls a MOSFET to control the current flowing onto the primary energy storage capacitor Cp (this would be hysteretic control). A high voltage regulator or reference could be used. A variable load could be used to discharge the primary energy storage capacitor Cp utilizing a comparator / op-amp circuit. Another approach would be to monitor the primary DC voltage across the primary energy storage capacitor Cp and disable the multiplier to prevent charging, e.g. disconnecting C1 . This could be done directly with a transistor. This would not disconnect the input AC voltage V1 from the primary energy storage capacitor Cp, but would inhibit the multiplication and would therefore prevent a further increase in the primary DC voltage across the primary energy storage capacitor Cp.

[0068] Voltage multiplier clamps 110, 210 are described above which comprise voltage clamps 132, 232, wherein the voltage clamps 132, 232 comprise a pass transistor M1 in the form of a (MOSFET). In a variant of each of the voltage clamps 132, 232, a bipolar junction transistor or an insulated-gate bipolar transistor may be used as the pass transistor instead of the MOSFET M1 .

[0069] Voltage multiplier clamps 10, 1 10, 210 are described above which comprise a voltage multiplier in the form of a voltage tripler 30, wherein the voltage tripler 30 comprises the primary energy storage capacitor Cp and the network 34 of capacitors C1 , C2 and diodes D1 , D2, D3. In a variant of each of the voltage multiplier clamps 10, 110, 210, a different non-switching voltage multiplier may be used. For example, depending on at least one of the input AC line voltage, the rated voltage of the primary energy storage capacitor Cp, the capacitance of the primary energy storage capacitor Cp, the specified range of the secondary DC voltage across the secondary energy storage capacitor Cs, and the holdup time during which the secondary DC voltage across the secondary energy storage capacitor Cs is to be maintained after a loss or reduction of the input AC line voltage, the voltage multiplier may comprise a voltage doubler.

[0070] Moreover, one of skill in the art will understand that there are many ways of implementing a voltage doubler or a voltage tripler and that the present invention is not limited to the specific voltage doublers or the specific voltage triplers described above. The voltage multiplier may comprise a network of capacitors and diodes. The voltage multiplier may comprise a half-wave voltage multiplier or a full-wave voltage multiplier. The voltage multiplier may be a non-switching voltage multiplier, wherein the voltage multiplier does not comprise a switch. The voltage multiplier may be a non-switching voltage multiplier, wherein the voltage multiplier does not comprise an inductor or a transformer. The use of a non-switching voltage multiplier may be less complex and / or smaller in size than a switching voltage multiplier such a boost converter or an additional flyback converter. In other variants, the voltage multiplier may be a switching voltage multiplier such as a boost converter or an additional flyback converter.

[0071] In the power supply circuit embodiments described above, the switched DC to DC converter comprises a flyback converter. In other power supply circuit embodiments, the switched DC to DC converter may comprise a buck-boost converter, a buck converter, or any other kind of switched DC to DC converter which is capable of converting a DC input voltage to a DC output voltage which is less than the DC input voltage.

[0072] In the power supply circuit embodiments described above, the primary energy storage capacitance is described as a primary energy storage capacitor. In other power supply circuits, the primary energy storage capacitance may comprise a plurality of primary energy storage capacitors, wherein the plurality of primary energy storage capacitors are connected in series or in parallel. Although the equivalent capacitance a plurality of primary energy storage capacitors connected in series may be lower than each the capacitance of each individual primary energy storage capacitor, the DC voltage across each individual primary energy storage capacitor may be lower than the primary DC voltage across the primary energy storage capacitance. This may be important for higher input AC voltages so that the DC voltage across each individual primary energy storage capacitor does not exceed a rated voltage of each individual primary energy storage capacitor.

[0073] The primary energy storage capacitance may comprise one or more capacitors of a type other than an EDLC capacitor. The primary energy storage capacitance may comprise one or more capacitors of a type which has a higher rated voltage than an EDLC capacitor.

[0074] In the power supply circuit embodiments described above, the primary energy storage capacitance is described as one or more electrolytic capacitors. In other power supply circuit embodiments, the primary energy storage capacitance may comprise one or more electrolytic capacitors and / or one or more polymer capacitors. Electrolytic or polymer capacitors are less susceptible to current leakage than EDLC capacitors. The performance and lifetime of electrolytic or polymer capacitors are less sensitive to temperature than EDLC capacitors. Moreover, an electrolytic or polymer capacitor generally has a higher rated voltage than an EDLC capacitor.

[0075] In the power supply circuit embodiments described above, the secondary energy storage capacitance is described as a secondary energy storage capacitor. In other power supply circuit embodiments, the secondary energy storage capacitance may comprise a plurality of secondary energy storage capacitors, wherein the plurality of secondary energy storage capacitors are connected in series or in parallel. The secondary energy storage capacitance may comprise one or more capacitors of a type other than an EDLC capacitor. The secondary energy storage capacitance may comprise one or more electrolytic capacitors and / or one or more polymer capacitors. Electrolytic or polymer capacitors are less susceptible to current leakage than EDLC capacitors. The performance and lifetime of electrolytic or polymer capacitors are less sensitive to temperature than EDLC capacitors.

[0076] Although a power supply circuit has been described in terms of specific embodiments as set forth above, it should be understood that these embodiments are illustrative only and that the claims are not limited to those embodiments. Those skilled in the art will be able to make modifications and alternatives to the described embodiments in view of the disclosure which are contemplated as falling within the scope of the appended claims. Each feature disclosed or illustrated in the present specification may be incorporated in any embodiment, whether alone or in any appropriate combination with any other feature disclosed or illustrated herein. In particular, one of ordinary skill in the art will understand that one or more of the features of the embodiments of the present disclosure described above with reference to the drawings may produce effects or provide advantages when used in isolation from one or more of the other features of the embodiments of the present disclosure and that different combinations of the features are possible other than the specific combinations of the features of the embodiments of the present disclosure described above.

[0077] The skilled person will understand that in the preceding description and appended claims, positional terms such as ‘above’, ‘along’, ‘side’, etc. are made with reference to the accompanying drawings. These terms are used for ease of reference but are not intended to be of limiting nature. These terms are therefore to be understood as referring to an object when in an orientation as shown in the accompanying drawings.

[0078] Use of the term "comprising" when used in relation to a feature of an embodiment of the present disclosure does not exclude other features or steps. Use of the term "a" or "an" when used in relation to a feature of an embodiment of the present disclosure does not exclude the possibility that the embodiment may include a plurality of such features.

[0079] The use of reference signs in the claims should not be construed as limiting the scope of the claims.

Claims

CLAIMS1 . A power supply circuit for converting an input AC voltage into an output DC voltage for supply to circuitry of an electricity meter, the power supply circuit comprising: a voltage multiplier clamp comprising a primary energy storage capacitance; and a switched DC to DC converter comprising a secondary energy storage capacitance, wherein the voltage multiplier clamp is configured to multiply and rectify the input AC voltage to generate a primary DC voltage across the primary energy storage capacitance whilst also limiting the primary DC voltage, and wherein the switched DC to DC converter is configured to convert the primary DC voltage into a secondary DC voltage across the secondary energy storage capacitance for supply as the output DC voltage to the circuitry of the electricity meter.

2. The power supply circuit as claimed in claim 1 , wherein the voltage multiplier clamp comprises: a voltage multiplier comprising the primary energy storage capacitance and one or more other electronic components; and a voltage clamp, wherein the voltage multiplier is configured to multiply and rectify the input AC voltage to generate the primary DC voltage across the primary energy storage capacitance, and wherein the voltage clamp is configured to limit the primary DC voltage across the primary energy storage capacitance.

3. The power supply circuit as claimed in claim 2, wherein the voltage clamp comprises a voltage regulator for limiting the primary DC voltage to a nominal limit voltage.

4. The power supply circuit as claimed in claim 3, wherein the voltage regulator and the primary energy storage capacitance are connected in parallel.

5. The power supply circuit as claimed in claim 3, wherein the voltage clamp comprises a series resistor connected in series with the voltage regulator between thevoltage regulator and the one or more other electronic components of the voltage multiplier.

6. The power supply circuit as claimed in claim 5, wherein the voltage clamp comprises a pass transistor such as a metal-oxide-semiconductor field-effect transistor, a bipolar junction transistor, or an insulated-gate bipolar transistor, wherein the pass transistor is connected in series with the primary energy storage capacitance between the primary energy storage capacitance and the one or more other electronic components of the voltage multiplier, and wherein a control terminal of the pass transistor is connected to a node between the series resistor and the voltage regulator.

7. The power supply circuit as claimed in claim 3, wherein the voltage clamp comprises a series resistor connected in series with the voltage regulator and the voltage clamp further comprises a current source for controlling a current through the series resistor.

8. The power supply circuit as claimed in claim 7, wherein the current source comprises a transistor current source such as a junction field-effect transistor current source.

9. The power supply circuit as claimed in claim 8, wherein the transistor current source, the series resistor, and the voltage regulator are connected in series, with the series resistor connected between the transistor current source and the voltage regulator, and with the transistor current source connected between the series resistor and the one or more other electronic components of the voltage multiplier, and wherein a node between the series resistor and the voltage regulator is connected to a control terminal of the transistor current source.

10. The power supply circuit as claimed in any one of claims 3 to 9, wherein the voltage regulator comprises a Zener diode and the nominal limit voltage comprises a breakdown voltage of the Zener diode.1 1 . The power supply circuit as claimed in any one of claims 2 to 10, wherein the voltage multiplier is a non-switching voltage multiplier and / or wherein the voltage multiplier comprises a network of capacitors and diodes.

12. The power supply circuit as claimed in any one of claims 2 to 11 , wherein the voltage multiplier comprises a voltage tripler or a voltage doubler.

13. The power supply circuit as claimed in any preceding claim, wherein at least one of: the primary energy storage capacitance comprises one or more primary energy storage capacitors; the primary energy storage capacitance comprises a plurality of primary energy storage capacitors, wherein the plurality of primary energy storage capacitors are connected in series or in parallel; the primary energy storage capacitance comprises one or more capacitors of a type other than an EDLC capacitor; the primary energy storage capacitance comprises one or more capacitors of a type which has a higher rated voltage than an EDLC capacitor; or the primary energy storage capacitance comprises one or more electrolytic capacitors and / or one or more polymer capacitors.

14. The power supply circuit as claimed in any preceding claim, wherein at least one of: the secondary energy storage capacitance comprises one or more secondary energy storage capacitors; the secondary energy storage capacitance comprises a plurality of secondary energy storage capacitors, wherein the plurality of secondary energy storage capacitors are connected in series or in parallel; the secondary energy storage capacitance comprises one or more capacitors of a type other than an EDLC capacitor; or the secondary energy storage capacitance comprises one or more electrolytic capacitors and / or one or more polymer capacitors.

15. The power supply circuit as claimed in any preceding claim, comprising an electromagnetic interference filter and / or surge protector, wherein the electromagnetic interference filter and / or surge protector is configured to receive an input AC line voltage at an input of the electromagnetic interference filter and / or surge protector and togenerate the input AC voltage at an output of the electromagnetic interference filter and / or surge protector.

16. The power supply circuit as claimed in any preceding claim, wherein the power supply circuit is configured to convert an input AC voltage having a nominal value of 120 V AC or 240 V AC into the output DC voltage.

17. The power supply circuit as claimed in any preceding claim, wherein the power supply circuit is configured to convert an input AC voltage into the output DC voltage in the presence of variations in the input AC voltage in a range of + / - 20% from a nominal value of the input AC voltage.

18. The power supply circuit as claimed in any preceding claim, wherein the switched DC to DC converter comprises a flyback converter, a buck-boost converter, or a buck converter.

19. An electricity meter comprising: the power supply circuit as claimed in any preceding claim; and the circuitry of the electricity meter.

20. A method for converting an input AC voltage into an output DC voltage for supply to circuitry of an electricity meter, the method comprising: multiplying and rectifying an input AC voltage to generate a primary DC voltage across a primary energy storage capacitance whilst also limiting the primary DC voltage; and using a switched DC to DC converter to convert the primary DC voltage into a secondary DC voltage across a secondary energy storage capacitance of the switched DC to DC converter for supply as the output DC voltage to the circuitry of the electricity meter.

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