GB2520963A

GB2520963A - Power supply circuit

Info

Publication number: GB2520963A
Application number: GB1321404.4A
Authority: GB
Inventors: Christopher John Smethurst
Original assignee: Harvard Engineering PLC
Current assignee: Harvard Engineering PLC
Priority date: 2013-12-04
Filing date: 2013-12-04
Publication date: 2015-06-10
Anticipated expiration: 2033-12-04
Also published as: GB201321404D0; GB2520963B

Abstract

A power supply circuit comprises: an output terminal 3; an inductor L1; first and second capacitors C1, C2; a resistor R1; first and second diodes D1, D2; a switch S1; control means 1 to control the switch; and a ground rail 2. The diodes are connected in series between the ground rail and a terminal of the second capacitor which is also connected to the output terminal, the other terminal of the second capacitor being connected to the ground rail. The inductor, first capacitor and resistor are connected in series between the ground rail and a node N between the diodes. The switch is connected in series with the first diode, the inductor, the resistor and the first capacitor from the ground rail and back to the ground rail, and is switchable between a first state in which it permits the first capacitor to charge in response to a voltage of a first polarity across the inductor, and a second state, in which it prevents the first capacitor from charging. The control means controls the switch dependent upon a voltage at the output terminal, whereby the first capacitor charges the second capacitor in response to a voltage of a second, opposite polarity across the inductor.

Description

Power Supply Circuit [0001] The present invention relates to power supply circuits, apparatus incorporating such circuits, and in particular, although not exclusively, to auxiliary power supply circuits arranged to power internal circuits, processors, controllers, components, or other elements of electrical and/or electronic apparatus.

BACKGROUND

[0002] In many pieces of electrical and/or electronic equipment it is necessary to supply one or more control circuits from the mains supply, or other AC supply, over a range of input voltages and load conditions. Where the equipment requires multiple supply voltages (e.g. for powering a number of different internal circuits, components, controllers etc.) and is not particularly cost sensitive, a separate auxiliary power supply may be used but, where a limited amount of power is required a simple overwind on a power conversion winding can be used. However, due to variations in the operating conditions this can lead to excessive voltage changes requiring extra regulation and losses.

[0003] Fig. us taken from an Onsemi data sheet (for products MC34262 and MC33262) and shows a prior art circuit for a power factor corrector (PFC) (a 450W Universal Input Power Factor Controller) with inductor T and main switch Qi operating in transition mode such that the circuit provides a 400V dc output from any ac input voltage between 90 and 268 volts by high frequency switching of Q1. A small overwind on T connected to 06 is used to supply the control voltage to pin 8 of the MC33262. The winding has to provide enough voltage under minimum input and output conditions to maintain the supply voltage above the lower operating voltage of MC33262. This means that at high input and load the supply can rise excessively. To prevent damage and maintain proper operation a zener diode is used, shown internal to the MC33262 as 36V which dissipates power and limits the voltage. Thus the circuit shows a power supply, having input terminals for connecting to an AC supply (e.g. mains supply), output terminals for connecting to an external load, a primary inductor connected in series between one of the input terminals and one of the output terminals, and a controller arranged to control a switching device Qi to control supply of power to a connected load via the primary winding (primary inductor). The controller has a supply pin 8 via which it receives power to operate, that power being derived from the secondary winding (secondary inductor) of the transformer T. As will be appreciated, power supply circuits embodying the invention may be incorporated in circuits of this type, resulting in power supplies which themselves embody the invention.

[0004] Fig. 2 is taken from an ST data sheet for product [6561 and shows a circuit similar to that of fig. 1 to derive a supply for the [6561 but this time using an overwind (auxiliary winding of a transformer T comprising a main winding) and charge pump arrangement comprising components C6, R2, D2 and D3. This type of circuit transfers energy on every switching edge in and out of CS and into 02. The diode D2 in the charge pump is a zener diode which dissipates power and so limits the voltage to [6561 to safe values as the operating conditions change.

[0005] Fig. 3 shows waveforms that can be expected with the circuit of fig. 2. On the negative going voltage edge C6 is charged via R2 and EQ and on the positive going edge C6 is discharged via D3 into C2 to provide the supply for the control IC [6561.

[0006] Problems with these prior art arrangements, especially when operation over a wide range of conditions (supply voltages and load conditions, for example) is required, include the dissipation of power (i.e. waste of energy) in voltage regulation to allow for the desired operating range, and the need to use components with very high voltage ratings for very wide operating ranges.

[0007] It is an aim of certain embodiments of the invention to solve, mitigate or obviate, at least partly, at least one of the problems and/or disadvantages associated with the prior art. Certain embodiments aim to provide at least one of the advantages described below.

BRIEF SUMMARY OF THE DISCLOSURE

[0008] In accordance with the present inventions there is provided a power supply circuit comprising: an output terminal for connection to a load to supply power to the load; an inductor; a first capacitor and a second capacitor; a first resistor; a first diode and a second diode; a switching device; control means (e.g. a controller, control circuit, control circuitry) for controlling the switching device; and a ground rail, wherein the first and second diodes are connected in a series arrangement between the ground rail and a second terminal of the second capacitor (for example in a series arrangement which also includes a controllable current path through the switching device, such as a channel in embodiments where the switching device is a field effect transistor), the second terminal of the second capacitor being connected to the output terminal, and a first terminal of the second capacitor being connected to the ground rail. The inductor, first capacitor and first resistor are connected in a series arrangement between the ground rail and a node between the first and second diodes. The switching device (or in other words a controllable current path, such as a channel of semiconductive material, through the switching device) is connected in a series arrangement with the first diode, the inductor, the first resistor, and the first capacitor from the ground rail and back to the ground rail, and is switchable, in response to a control signal from the control means, between a first state in which it permits the first capacitor to charge (through the first diode and the first resistor, and the current path through the switching device itself) in response to a voltage of a first polarity being developed across the inductor, and a second state, in which it prevents the first capacitor from charging in response to a voltage of said first polarity being developed across the inductor (in other words, in the second state the controllable current path through the switching device is closed, i.e. blocked, substantially non-conducting). The control means is connected to the output terminal and the switching device and is arranged to generate the control signal and provide the control signal to the switching device to control said switching between the first and second states. The control means is arranged such that the control signal is dependent upon at least a voltage at the output terminal (and in certain embodiments, that control signal is also dependent upon a reference voltage, for example provided by reference voltage supply or generation means incorporated in the control means). The arrangement is such that the first capacitor charges the second capacitor (through the first resistor and the second diode) in response to a voltage of a second, opposite polarity being developed across the inductor.

[0009] In certain embodiments, the control means comprises a comparator (or comparator circuit, comparator circuitry, or comparator means) connected to the output terminal and the switching device. The comparator may be arranged to compare the voltage (which may therefore be described as the compared voltage) at the output terminal, or a voltage (compared voltage) derived from, corresponding to, indicative of, or otherwise dependent upon the voltage at the output terminal, with a reference voltage and generate the control signal according to the comparison, the control signal being supplied to the switching device to control said switching between the first and second states. In certain embodiments the comparator may directly compare the output voltage (at the output terminal) with a reference voltage. In alternative embodiments, the comparator may be arranged to compare not the output voltage itself, but a voltage or signal derived from, corresponding to, indicative of, or otherwise dependent on, the output voltage with the reference voltage. Thus, in certain embodiments, the comparator comprises delay means or a delay circuit arranged to generate a voltage signal lagging behind the voltage at the output terminal (the delay for example being determined by a time constant of the delay circuit), and the comparator is arranged to compare the delayed voltage signal (i.e. the signal from the delay means) with the reference voltage.

[0010] In certain embodiments, the arrangement of the diodes is such that the first diode has a forward conduction direction from the switching device to the first capacitor, but blocks current flow from the first capacitor through the switching device to ground. The second diode has a forward conduction direction from the first capacitor to the second capacitor, and so blocks current flow from the second capacitor back to the first capacitor.

[0011] Advantageously, the output voltage is substantially maintained at (regulated to) the reference voltage without the undesirable dissipation/waste of energy associated with certain prior art techniques, because, when the output voltage (or a voltage derived from or dependent on that output voltage, for example the voltage from a delay circuit incorporated in the comparator) rises above the threshold the comparator controls the switching device so as to prevent charging of the first capacitor during the relevant part of the duty cycle.

Embodiments of the invention are thus able to operate more efficiently than prior art equipment.

[0012] In certain embodiments the comparator is arranged to control the switching device to switch from the first state to the second state in response to the compared voltage (e.g. the voltage at the output terminal, or a voltage derived from the output terminal) rising above the reference voltage.

[0013] In certain embodiments the comparator is arranged to control the switching device to switch from the second state to the first state in response to the compared voltage (e.g. the voltage at, or derived from, the output terminal) falling below the reference voltage.

[0014] In certain embodiments the switching device is connected between the ground rail and a first terminal of the first diode (so as to provide a controllable current path from ground to the first terminal of the first diode), the second terminal of the first diode being connected to said node, and a first terminal of the second diode being connected to said node.

[0015] In certain embodiments a first terminal of the inductor is connected to the ground rail. In such embodiments, a second terminal of the inductor may be connected to a first terminal of the first capacitor, a second terminal of the first capacitor may be connected to a first terminal of the first resistor, and a second terminal of the first resistor may be connected to said node.

[0016] In certain embodiments the switching device is connected in a series arrangement with the first capacitor and the first resistor between the ground rail and a first terminal of the inductor. In such embodiments the switching device may be connected between the ground rail and a first terminal of the first capacitor. The switching device in such arrangements provides a controllable current path from ground to the first capacitor.

[0017] The power supply circuit may further comprise a third capacitor and a second resistor connected in a series arrangement between the ground rail and the first terminal of the inductor such that the third capacitor charges in response to a voltage of said first polarity being developed across the inductor, irrespective of the state of the switching device, and wherein the third capacitor charges the second capacitor in response to a voltage of said second, opposite polarity being developed across the inductor.

[0018] Another aspect of the invention provides a power supply circuit comprising: an output terminal, for connection to a load to supply power to the load; an inductor; a first capacitor; a switching device connected in a series arrangement (which may also include a first resistor) with the inductor and the capacitor and controllable, with a control signal, to switch between a first state, in which it permits the first capacitor to charge (for example via the first resistor and a controllable current path through the switching device) in response to a voltage of a first polarity being developed across the inductor, and a second state, in which it prevents the first capacitor from charging in response to a voltage of said first polarity being developed across the inductor; a second capacitor arranged to be charged by the first capacitor in response to a voltage of a second, opposite polarity being developed across the inductor, a terminal of the second capacitor being connected to the output terminal; and control means arranged to control the switching device, the control means being connected to the output terminal and the switching device and being arranged to generate said control signal and provide the control signal to the switching device to control said switching between the first and second states, said control signal being dependent upon a voltage at the output terminal.

In certain embodiments the control means comprises a comparator arranged to compare the voltage at the output terminal, or a voltage dependent upon the voltage at the output terminal, with a reference voltage and generate the control signal according to the comparison, the control signal being supplied to the switching device to control said switching between the first and second states.

[0019] In certain embodiments the power supply circuit further comprises a first diode and a second diode connected in a series arrangement between a ground rail and said terminal of the second capacitor, wherein the inductor and first capacitor are connected in a series arrangement (which may also include a first resistor and the controllable conduction path through the switching device) between the ground rail and a node between the first and second diodes, such that said charging of the first capacitor is via the first diode and said charging of the second capacitor is via the second diode. Thus, the forward conduction direction of the first diode may be from ground to the node, and the forward conduction direction of the second diode may be from the node to the second terminal of the second capacitor.

[0020] In certain embodiments the power supply circuit further comprises a third capacitor connected in a series arrangement with the inductor, the third capacitor being arranged to charge in response to a voltage of the first polarity being developed across the inductor, irrespective of the state of the switching device, and to discharge, so as to charge the second capacitor, in response to a voltage of said second polarity being developed across the inductor.

[0021] In certain embodiments the power supply circuit further comprises means for generating an alternating voltage across the inductor. In certain embodiments this means for generating comprises a winding (which may also be described as a further inductor), and means for driving a time-varying current through said winding, the winding being arranged such that at least a portion of the time-varying magnetic flux generated by the winding carrying said time-varying current links said inductor. The means for driving may comprise input terminals, for connection to an AC supply, and rectifying means, the winding being connected (directly or indirectly) to the rectifying means such that connection of an AC supply to the input terminals drives a time-varying current through the winding.

[0022] In certain embodiments the comparator further comprises delay means arranged to generate a delayed voltage signal from the voltage at the output terminal, the comparator being arranged to compare the delayed voltage signal with the reference voltage. In certain embodiments the delay means comprises a delay circuit comprising at least one resistor and at least one capacitor.

[0023] Another aspect of the invention provides a power supply for controlling the supply of power from an AC supply to an external load, the power supply comprising: input terminals for connection to an AC supply; output terminals for connection to an external load; a first inductor (or, in other words, a first winding) connected in a series arrangement between one of said input terminals and one of said output terminals; and a controller arranged to control at least one switching device to control supply of power to a connected external load, the controller comprising a supply terminal for connection to a supply to receive power to operate the controller, the power supply further comprising a power supply circuit in accordance with any one of the above-mentioned aspects or embodiments, wherein said output terminal of the power supply circuit is connected to said supply terminal to power the controller, and said inductor of the power supply circuit is arranged such that an alternating voltage is developed across said inductor of the power supply circuit in response to a time-varying current flowing in said first inductor.

[0024] Another aspect of the invention provides a power supply for controlling the supply of power from an AC supply to an external load, the power supply comprising: input terminals for connection to an AC supply; output terminals for connection to an external load; a transformer having a primary winding and a secondary winding, the primary winding being coupled to the input terminals and the secondary winding being coupled to the output terminals; at least one switching device controllable to control current in the primary winding; and a controller arranged to control said switching device, the controller comprising a supply terminal for connection to a supply to receive power to operate the controller, the power supply further comprising a power supply circuit in accordance with any of the above-mentioned aspects or embodiments, wherein said output terminal of the power supply circuit is connected to said supply terminal to power the controller, and said inductor of the power supply circuit is arranged such that at least a portion of the magnetic flux generated by the primary winding links the inductor of the power supply circuit, whereby an alternating voltage may be developed across said inductor of the power supply circuit in response to a time-varying current flowing in said primary winding.

[0025] In certain embodiments the power supply comprises a transformer comprising a primary winding and a secondary winding, and said first inductor is the primary winding and said inductor of the power supply circuit is the secondary winding.

[0026] Another aspect of the invention provides apparatus comprising a power supply circuit in accordance with any one of the above-mentioned aspects or embodiments.

[0027] It will be appreciated that certain embodiments of the invention provide apparatus and a method of reducing energy losses compared with prior art techniques, whilst maintaining a simple overwind to generate a supply for auxiliary control circuits.

BRIEF DESCRIPTION OF THE DRAWINGS

[0028] Embodiments of the invention are further described hereinafter with reference to the accompanying drawings, in which: Fig. 1 is a circuit diagram of a universal input power factor controller according to the prior art, but which may be modified to incorporate a power supply circuit embodying the present invention; Fig. 2 is a circuit diagram of another circuit in accordance with the prior art, and which may be modified to incorporate a power supply circuit embodying the present invention; Fig. 3 illustrates representative voltage and current wave forms in the circuit of Fig. 2 during operation; Fig. 4 is a circuit diagram of a power supply circuit embodying the invention; Fig. 5 illustrates voltage and current wave forms in the circuit of Fig. 4 during operation; Fig. 6 is a circuit diagram of another power supply circuit embodying the invention; Fig. 7 illustrates voltage and current wave forms developed in the circuit of Fig. 6 during operation; and Fig. 8 illustrates part of a power supply embodying the invention.

DETAILED DESCRIPTION

[0029] Fig 4 shows an embodiment of the invention, FigS shows waveforms in the circuit of fig. 4, and operation is as follows. Li is an inductor (which in certain embodiments is an overwind on the power converter winding as per prior art). At time TO when Vout (at output terminal 3) is below Vref the control means (which in this example is a comparator 1) turns on switch Si enabling Ci, Ri, Di and D2 to operate as a normal charge pump until 02 is charged and Vout exceeds Vref by the comparator hysteresis. When this occurs at time Ti the comparator output turns Si off and Cl can no longer pump charge into C2 as Di is now disconnected. Vout will drop due to the load until time 12 and the comparator turns on Si again and Ci cyclically recharges C2 until time T3 when Si is turned off again and the cycle repeats. In this way Vout is regulated within the hysteresis limits of the comparator with a minimum of loss. Control of the hysteresis controls the Vout ripple. In certain embodiments a comparator time delay, longer than a period of the waveform across Li, may be used, provided, for example, by an RC circuit incorporated in the comparator/comparison means. The SC circuit may be arranged to delay the feedback signal from Vout and so effectively give hysteresis to the switching action. In other words, the delay circuit (SC circuit) may be arranged to receive Vout, and generate a delayed signal Vdel, lagging behind Vout according to the SC time constant. The comparator may then compare Vdel with Vref, to determine when to switch the switching device.

[0030] In fig. 5, it will be apparent that the inductor Li is being exposed to a changing magnetic field (the magnetic flux linking the inductor varying with time in a manner such that an alternating, essentially square wave potential is developed across the inductor).

The first, positive current spike for Ci corresponds to Ci discharging into C2 (i.e. Ci charging C2) in response to the alternating voltage having a second polarity (with the second terminal Li2 of the inductor being positive with respect to its first terminal Li i).

The second, negative current spike for Ci corresponds to Ci charging in response to the polarity of the voltage across Li reversing.

[0031] Referring to Fig. 4 in more detail. this circuit embodying the invention comprises an output terminal 3, an inductor Li, a first capacitor Ci, a second capacitor C2, first and second diodes Di and D2, a first resistor, a comparator 1, a switching device Si and a ground rail 2. The reference numeral Lii denotes a nominal first terminal of inductor Li, Li2 denotes a nominal second terminal of U, and corresponding notation is used to denote the nominal terminals of the other circuit components. Interconnections between the components, in terms of their respective terminals, will be apparent from the figure, as supplemented by the following description. For example, the first terminal Ri 1 of resistor Ri is connected directly to the second terminal Ci2 of the first capacitor Ci in this

example, etc.

[0032] In this first embodiment the first and second diodes Di and D2 are connected in series between the ground rail 2 and a second terminal C22 of the second capacitor C2, via the controllable current path through switching device Si. The forward conduction direction of Di is from ground to node N, and the forward conduction direction of D2 is from node N to terminal C22 of C2. That second terminal C22 of the second capacitor is connected to the output terminal 3, and a first terminal C2i of the second capacitor is connected to the ground rail 2. The inductor Li, first capacitor Ci and first resistor Ri are connected in a series arrangement between the ground rail 2 and a node N between the first and second diodes (i.e. that node N is a connection between a second terminal Di2 of the first diode and a first terminal D2i of the second diode). The switching device Si is connected between the ground rail 2 and the first terminal Dii of the first diode and when that switching device is in a first state it permits current to flow between a first switch terminal Si 1 and a second switch terminal 312, and hence current can flow through the diode D1. In other words, a controllable current path (e.g. a channel in embodiments where the switching device is an FET) through the switching device is arranged in series between ground and the first diode. When the switch Si is in its second state, conduction between the terminals 311 and 312 is prevented. In other words the switch Si is in an open state. The first state can be regarded as a closed state. The comparator 1 controls operation of the switching device Si by means of an input signal provided from comparator output 13 to switch control input S13. In this example a further terminal of the switching device Si is connected to the ground rail, that terminal being 5i4. The comparator has a first input terminal ii connected to the output terminal 3, and a second input terminal 12 connected to receive (supplied with) a reference voltage Vref. When the output voltage exceeds Vref, the comparator controls the switch Si to switch from the first state to the second state (i.e. to switch from closed to open), thereby disconnecting the first terminal Dii of the first diode from the ground rail and so preventing current flow through Dl.

When the output voltage at terminal 3 falls below the reference voltage, the comparator causes the switch Si to transition from the open to the closed state, thereby enabling current flow from the ground rail 2 through the diode Dl in its forward direction. However, as described above, in certain embodiments the comparator may further comprise a delay circuit arranged to receive Vout and provide a corresponding delayed signal Vdel which is compared with Vref, so as to add hystereisis to the switching cycle.

[0033] In use, the inductor Li is arranged to sense the changing flux generated by changing current in another inductor. In other words, the inductor Li is arranged such that, in use, the magnetic flux linking it varies with time in a manner such that an alternating voltage is developed across the inductor (i.e. between its first inductor terminal Lii and its second inductor terminal Li2). In this example, the inductor Li is arranged such that in use an alternating square voltage is developed across it. When the voltage across Li has a first polarity (i.e. when the voltage at the second terminal Li2 is negative with respect to the voltage at terminal Lii) and the switch Si is closed (i.e. permitting conduction) capacitor Ci will charge through the series arrangement of switching device Si, first diode Di and first resistor Ri. Then, when the voltage across the inductor Li reverses (i.e. changes polarity) such that the potential at the second terminal Li2 is now positive with respect to the voltage at terminal Lii, capacitor Ci discharges through the first resistor Si and second diode D2, thereby charging capacitor C2. In this way, through repeated generation of an alternating voltage across the inductor Li, charge is pumped into capacitor 02, which can therefore supply an output current to a load connected to the output terminal 3.

[0034] When switch Si is in the open (non-conducting) state, then when the voltage across inductor Li has the first polarity mentioned above, capacitor Cl is not able to charge during that part of the duty cycle as the flow of current to charge it through Di and Ri is blocked. Thus, when switch Si is off (non-conducting) the pumping of charge into C2 is prevented, so that C2 can only discharge through the load in the other part of the duty cycle, and hence the output voltage at terminal 3 will drop. This output voltage falls until the comparator again switches the switch Si on again, and then the charge pumping recommences.

[0035] Fig 6 shows another power supply circuit embodying the invention, and Fig 7 shows the waveforms in the circuit of fig. 6, when an essentially square wave alternating voltage is developed across the inductor Li. This embodiment represents a slightly different implementation in which second resistor R2 and third capacitor 03 are chosen to provide sufficient power at the maximum operating condition, and Ri and Ci, in parallel with R2 and C3, are selected for operation at the minimum operating condition. The switching device Si in this example is mosfet Mi, which is controlled to turn on and off as previously described (under control of the comparator 1, which may, for example include a delay circuit or delay means such that a voltage signal lagging behind Vout is compared with Vref) to maintain Vout, but Mi now only handles the difference energy between the two conditions. The time points TO, Ti, T2, T3 are similar switching times as the Fig. 4 implementation. Thus, the state of the switching device determines whether first capacitor Ci charges or not when the voltage across the inductor Li has a first polarity. However, the arrangement of the third capacitor 03 in series with the second resistor R2, between the ground rail 2 and the first terminal Lii of the inductor means that the third capacitor 03 charges (via Di and R2) when the voltage across the inductor has the first polarity, irrespective of the state of the switching device. If the switch is in the on state (conducting) both the first and third capacitors charge during the charging part of the cycle, and then both discharge (to charge C2) during the pumping part of the cycle. If the switch is in the off state, only the third capacitor is charged and then pumps charge to the second capacitor.

[0036] It will be appreciated that, for a charge pump circuit (as in Fig 4 but with Si linked so that Di anode is connected to ground rail 2) with a fixed Ci Ri, the amount of power supplied to the load increases as either the frequency or peak to peak voltage (or both) of the supply across Li increases, and vice versa. Considering Fig 6, for the condition that the supply to Li is at maximum combination of frequency and voltage, R2 03 is chosen so that no more than the minimum load requirement at Vout is available. For the condition that the supply to Li is at minimum combination of frequency and voltage Ri Ci is chosen so that in parallel with R2 C3 not less than the maximum load requirement at Vout is available. In practice margins are added to the mm/max conditions to ensure control is maintained over component tolerances and operating ranges.

[0037] Another embodiment of the invention provides a circuit (power supply) generally in accordance with fig. 1, with a power supply circuit in accordance with fig. 4 arranged to provide power supply to pin 8, the inductor Li of the circuit being provided by the secondaly winding of transformer T. [0038] Further embodiments of the invention provide circuits generally in accordance with fig. 1 or fig. 2, adapted to incorporate a power supply circuit in accordance with fig. 4 or fig. 6 to power a controller.

[0039] Referring now to fig. 8, this illustrates part of another power supply embodying the invention. The overall power supply is a dimmable LED driver, having input terminals Ii and 12 for connection to a mains supply, and output terminals 01 and 02 for connection to a load in the form of an LED, string of LED5, or LED lighting unit. The power supply drives connected LEDs with a DC current. A transformer T, having a primary winding WP and a secondary winding WS, provides DC isolation between an input side and anoutput side of the supply; only capacitor CT connects the two sides. Rectifying means 20 is connected to the input terminals Ii, 12, and provides a time-varying current through primary winding WP (when a mains supply is connected). A switching device SD is controlled by a controller (IC1) to control the current through WP and so control the current driven through the load.

Switching device SD receives a drive signal from the drive output (pin P7) of the controller IC1. The power supply also comprises a power supply circuit generally in accordance with the embodiment shown in fig. 4, and corresponding components are labelled with the same reference numerals in fig. 8. The power supply circuit generates an output voltage at nominal output terminal 3, which is connected to the voltage supply pin P8 of the chip IC1.

Thus, the power supply circuit powers the controller controlling supply of power to the main load (LED load). In the power supply circuit, the inductor Li is arranged to sense the changing magnetic flux generated by the time-varying current in the primary winding. In other words, the inductor Li is arranged such that at least a portion of the magnetic flux generated by the primary winding links Li. Switching device Si is a MOSFET in this example, receiving a drive input S13 (i.e. a control signal) from control circuitry (which may also be described as a comparator, comparator means, or a comparatol circuit) including transistor 101. A Zener diode Zi provides the reference voltage to the comparator. In this example, the control circuitry includes bipolar transistor iDi, resistors RiO, Ru, and R12, capacitor Clo, and the Zener diode Zi supplying the reference voltage. As will be seen from the figure, a terminal of the Zener diode Zi is connected to the nominal output terminal 3, and hence the voltage at the transistor base is dependent upon the voltage at the output terminal (and on the reference voltage). The base-emitter junction of transistor 101 effectively performs the comparison function; if the base voltage exceeds a threshold, then transistor 101 is in an "on" or conducting state, and the voltage signal S13 is "low". If the base voltage is below the threshold, transistor 101 is "off' (substantially non-conductive), and S13 is then high". RiO, Ru, Ri2 and C10 form an RC delay circuit having an RC time constant such that the voltage at the base of the transistor 101 lags behind the voltage at the output terminal 3. As an alternating voltage is generated across [1, the power supply circuit incorporated in fig. 8 generates a regulated supply voltage to pin P8 in a manner as described above in relation to fig. 4.

[0040] It will be appreciated than in alternative embodiments to those described above, the output voltage may be reversed (made negative) by reversing the diode connections and switching polarity if a mosfet is used as the switching device.

[0041] Throughout the description and claims of this specification, the words "comprise" and "contain" and variations of them mean including but not limited to", and they are not intended to (and do not) exclude other moieties, additives, components, integers or steps.

Throughout the description and claims of this specification, the singular encompasses the plural unless the context otherwise requires. In particular, where the indefinite article is used, the specification is to be understood as contemplating plurality as well as singularity, unless the context requires otherwise.

[0042] Features, integers, or characteristics described in conjunction with a particular aspect, embodiment or example of the invention are to be understood to be applicable to any other aspect, embodiment or example described herein unless incompatible therewith.

All of the features disclosed in this specification (including any accompanying claims, abstract and drawings), and/or all of the steps of any method or process so disclosed, may be combined in any combination, except combinations where at least some of such features and/or steps are mutually exclusive. The invention is not restricted to the details of any foregoing embodiments. The invention extends to any novel one, or any novel combination, of the features disclosed in this specification (including any accompanying claims, abstract and drawings), or to any novel one, or any novel combination, of the steps of any method or process so disclosed.

[0043] It will be also be appreciated that, throughout the description and claims of this specification, language in the general form of "X for Y" (where Y is some action, activity or step and X is some means for carrying out that action, activity or step) encompasses means X adapted or arranged specifically, but not exclusively, to do Y. [0044] The reader's attention is directed to all papers and documents which are filed concurrently with or previous to this specification in connection with this application and which are open to public inspection with this specification, and the contents of all such papers and documents are incorporated herein by reference.

Claims

CLAIMS1. A power supply circuit comprising: an output terminal for connection to a load to supply power to the load; an inductor; a first capacitor and a second capacitor; a first resistor; a first diode and a second diode; a switching device; control means arranged to control the switching device; and a ground rail, wherein the first and second diodes are connected in a series arrangement between the ground rail and a second terminal of the second capacitor, the second terminal of the second capacitor being connected to the output terminal, and a first terminal of the second capacitor being connected to the ground rail, the inductor, first capacitor and first resistor being connected in a series arrangement between the ground rail and a node between the first and second diodes, the switching device being connected in a series arrangement with the first diode, the inductor, the first resistor, and the first capacitor from the ground rail and back to the ground rail, and being switchable, in response to a control signal from the control means, between a first state in which it permits the first capacitor to charge in response to a voltage of a first polarity being developed across the inductor, and a second state, in which it prevents the first capacitor from charging in response to a voltage of said first polarity being developed across the inductor, the control means being connected to the output terminal and the switching device and being arranged to generate said control signal and provide the control signal to the switching device to control said switching between the first and second states, said control signal being dependent upon a voltage at the output terminal, whereby the first capacitor charges the second capacitor in response to a voltage of a second, opposite polarity being developed across the inductor.
2. A power supply circuit in accordance with claim 1, wherein the control means comprises a comparator connected to the output terminal and the switching device and being arranged to compare a voltage at the output terminal, or a voltage dependent upon the voltage at the output terminal, with a reference voltage and generate said control signal according to the comparison.
3. A power supply circuit in accordance with claim 2, wherein the comparator is arranged to control the switching device to switch from the first state to the second state in response to the compared voltage rising above the reference voltage.
4. A power supply circuit in accordance with claim 2 or claim 3, wherein the comparator is arranged to control the switching device to switch from the second state to the first state in response to the compared voltage falling below the reference voltage.
5. A power supply circuit in accordance with any preceding claim, wherein the switching device is connected between the ground rail and a first terminal of the first diode, the second terminal of the first diode being connected to said node, and a first terminal of the second diode being connected to said node.
6. A power supply circuit in accordance with claim 5, wherein a first terminal of the inductor is connected to the ground rail.
7. A power supply circuit in accordance with claim 6, wherein a second terminal of the inductor is connected to a first terminal of the first capacitor, a second terminal of the first capacitor is connected to a first terminal of the first resistor, and a second terminal of the first resistor is connected to said node.
8. A power supply circuit in accordance with any one of claims ito 4, wherein the switching device is connected in a series arrangement with the first capacitor and the first resistor between the ground rail and a first terminal of the inductor.
9. A power supply circuit in accordance with claim 8, wherein the switching device is connected between the ground rail and a first terminal of the first capacitor.
10. A power supply circuit in accordance with claim 8 or claim 9, further comprising a third capacitor and a second resistor connected in a series arrangement between the ground rail and the first terminal of the inductor such that the third capacitor charges in response to a voltage of said first polarity being developed across the inductor, irrespective of the state of the switching device, and wherein the third capacitor charges the second capacitor in response to a voltage of said second, opposite polarity being developed across the inductor.
11. A power supply circuit comprising: an output terminal, for connection to a load to supply power to the load; an inductor; a first capacitor; a switching device connected in a series arrangement with the inductor and the capacitor and controllable with a control signal to switch between a first state, in which it permits the first capacitor to charge in response to a voltage of a first polarity being developed across the inductor, and a second state, in which it prevents the first capacitor from charging in response to a voltage of said first polarity being developed across the inductor; a second capacitor arranged to be charged by the first capacitor in response to a voltage of a second, opposite polarity being developed across the inductor, a terminal of the second capacitor being connected to the output terminal; and control means arranged to control the switching device, the control means being connected to the output terminal and the switching device and being arranged to generate said control signal and provide the control signal to the switching device to control said switching between the first and second states, said control signal being dependent upon a voltage at the output terminal.
12. A power supply circuit in accordance with claim 11, wherein the control means comprises a comparator arranged to compare the voltage at the output terminal, or a voltage dependent upon the voltage at the output terminal, with a reference voltage and generate said control signal according to the comparison.
13. A power supply circuit in accordance with claim 11 or claim 12, further comprising a first diode and a second diode connected in a series arrangement between a ground rail and said terminal of the second capacitor, wherein the inductor and first capacitor are connected in a series arrangement between the ground rail and a node between the first and second diodes, such that said charging of the first capacitor is via the first diode and said charging of the second capacitor is via the second diode.
14. A power supply circuit in accordance with claim 13, further comprising a third capacitor connected in a series arrangement with the inductor, the third capacitor being arranged to charge in response to a voltage of the first polarity being developed across the inductor, irrespective of the state of the switching device, and to discharge, so as to charge the second capacitor, in response to a voltage of said second polarity being developed across the inductor.
15. A power supply circuit in accordance with claim 2 or claim 12, or any one of claims 3 to 10, or claims 13 or 14, as depending from claim 2 or claim 12, wherein the comparator further comprises delay means arranged to generate a delayed voltage signal from the voltage at the output terminal, the comparator being arranged to compare the delayed voltage signal with the reference voltage.
16. A power supply circuit in accordance with any preceding claim, further comprising means for generating an alternating voltage across the inductor.
17. A power supply circuit in accordance with claim 16, wherein said means for generating comprises a winding and means for driving a time-varying current through said winding, the winding being arranged such that at least a portion of the magnetic flux generated by the winding links said inductor.
18. A power supply for controlling the supply of power from an AC supply to an external load, the power supply comprising: input terminals for connection to an AC supply; output terminals for connection to an external load; a first inductor connected in a series arrangement between one of said input terminals and one of said output terminals; and a controller arranged to control at least one switching device to control supply of power to a connected external load, the controller comprising a supply terminal for connection to a supply to receive power to operate the controller, the power supply further comprising a power supply circuit in accordance with any one of claims 1 to 15, wherein said output terminal of the power supply circuit is connected to said supply terminal to power the controller, and said inductor of the power supply circuit is arranged such that at least a portion of the magnetic flux generated by the first inductor links the inductor of the power supply circuit, whereby an alternating voltage may be developed across said inductor of the power supply circuit in response to a time-varying current flowing in said first inductor.
19. A power supply in accordance with claim 18, the power supply comprising a transformer comprising a primary winding and a secondary winding, and wherein said first inductor is the primary winding and said inductor of the power supply circuit is the secondary winding.
20. A power supply for controlling the supply of power from an AC supply to an external load, the power supply comprising: input terminals for connection to an AC supply; output terminals for connection to an external load; a transformer having a primary winding and a secondary winding, the primary winding being coupled to the input terminals and the secondary winding being coupled to the output terminals; at least one switching device controllable to control current in the primary winding; and a controller arranged to control said switching device, the controller comprising a supply terminal for connection to a supply to receive power to operate the controller, the power supply further comprising a power supply circuit in accordance with any one of claims ito 15, wherein said output terminal of the power supply circuit is connected to said supply terminal to power the controller, and said inductor of the power supply circuit is arranged such that at least a portion of the magnetic flux generated by the primary winding links the inductor of the power supply circuit, whereby an alternating voltage may be developed across said inductor of the power supply circuit in response to a time-varying current flowing in said primary winding.
21. A power supply in accordance with any one of claims 18 to 20, wherein the power supply is an LED driver.
22. Apparatus comprising a power supply circuit in accordance with any one of claims ito 17.
23. A power supply circuit, a power supply, or apparatus substantially as hereinbefore described with reference to figures 4 to 8 of the accompanying drawings.