FIELD OF THE INVENTION

The invention relates to the field of dimmable lamps, and more specifically to a power supply circuit for supplying power to a lamp. The invention further relates to a lighting system, and a method to operate said power supply circuit.

BACKGROUND OF THE INVENTION

A lighting system usually comprises a lamp and a power supply circuit to connect the lamp to electric mains. The lamps comprise light-emitting components like light emitting diodes (LEDs), halogen burners, or compact fluorescent lamps (CFLs). The power supply circuit may comprise components like a rectifier, DC-DC converter, and/or output inverter to alter the electric mains power signal into an output signal suitable for the lamp. The lighting system may also comprise a dimmer for power limiting which may be operated in dependency of a user input. This dimmer is usually introduced between the electric mains and the power supply circuit and is advantageous from energy saving point of view and/or because it provides the possibility to create a certain kind of ambiance in a space comprising such a lighting system.

One way of power limiting is by phase-cutting, which is a method of pulse width modulation applied to AC voltages. In such cases, a thyristor, triac, thyratron, silicon- controlled rectifier (SCL), or other such gated diode-like device is modulated into and out of conduction at a predetermined phase of the applied AC waveform.

An example of a lighting system including such a dimmer is shown in

US2008/030148 Al. A drawback of these lighting systems is that due to the phase-cutting the dimmer may provide a power signal with a voltage step in each period of the phase-cutted mains. Due to these voltage steps, capacitors connected to an input side of the power supply circuit may draw a so-called peak current. It is noted here that a capacitor is connected to the input side if it substantially contributes to the input impedance of the power supply circuit.

The peak currents, which can be as high as 5 A, may have one or more drawbacks of which a few are listed below. The peak currents may: limit the number of lamps/power supply circuits that can be mounted to a dimmer; increase the electromagnetic interference; cause humming due to extreme loading of the capacitor and possibly other components such as inductors; cause extra losses; destroy the dimmer; and pollute the electric mains.

SUMMARY OF THE INVENTION

It would be desirable to provide an improved lighting system in which more power supply circuits can be mounted to a dimmer. It would also be desirable to provide an improved lighting system in which the humming is reduced. It would further be desirable to provide an improved lighting system in which the pollution of the electric mains is minimized.

To better address one or more of these concerns, in a first aspect of the invention a power supply circuit is provided that comprises: an input side to connect the power supply circuit to a dimmer; - an output side to connect the power supply circuit to the lamp; a capacitor connected to the input side; wherein the power supply circuit comprises a charge device in electrical communication with the capacitor to anticipate a voltage step originating from the dimmer by transferring a charge to or from the capacitor prior to the occurrence of the voltage step in order to reduce an electrical current drawn by the capacitor due to the voltage step.

In an embodiment, a lighting system is provided comprising a power supply circuit, said power supply circuit comprising: an input side to connect the power supply circuit to a dimmer; an output side to connect the power supply circuit to the lamp; - a capacitor connected to the input side; wherein the power supply circuit comprises a charge device in electrical communication with the capacitor to anticipate a voltage step originating from the dimmer by transferring a charge to or from the capacitor prior to the occurrence of the voltage step in order to reduce an electrical current drawn by the capacitor due to the voltage step, and wherein said lighting system further comprises a lamp connected to the output side of the power supply circuit.

In another embodiment, a method is provided for operating a power supply circuit that supplies power to a lamp from a dimmer, comprising the step of transferring a charge to a capacitor connected to an input side of the power supply circuit prior to the occurrence of a voltage step originating from the dimmer using a charge device in order to reduce an electrical current drawn by the capacitor due to the voltage step.

These and other aspects of the invention will be more readily appreciated as the same becomes better understood by reference to the following detailed description and considered in connection with the accompanying drawings in which like reference symbols designate like parts.

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 depicts a schematic representation of a lighting system according to an embodiment of the invention;

Figure 2 depicts a schematic representation of a lighting system according to another embodiment of the invention.

DETAILED DESCRIPTION OF EMBODIMENTS Figure 1 depicts a schematic representation of a lighting system according to an embodiment of the invention. The lighting system comprises a lamp L and a power supply circuit PSC for supplying power to the lamp L. The power supply circuit PSC comprises an input side I to connect the power supply circuit to a dimmer D and an output side O to connect the power supply circuit PSC to the lamp L. The power supply circuit PSC further comprises a capacitor C connected to the input side I and a charge device CD in electrical communication with the capacitor C to anticipate a voltage step originating from the dimmer D by transferring a charge to or from the capacitor C prior to the occurrence of the voltage step in order to reduce an electrical current drawn by the capacitor due to the voltage step. The lamp L comprises light-emitting components, such as LEDs, CFLs, and halogen burners. Preferably, the lamp is an energy-saving lamp. The lamp is connected to electric mains M via the series connection of the dimmer D and the power supply circuit PSC.

The dimmer D is able to limit the power from the electric mains M to the power supply circuit PSC by phase-cutting an AC waveform supplied by the electric mains M. This is advantageous from energy point of view and may also create some kind of ambiance in a space in which the lighting system is located.

The capacitor C is connected to the input side I if it substantially contributes to the input impedance of the power supply circuit. A skilled person usually refers to such a capacitor as a capacitor that can be "seen" from the input side I. The capacitor C may thus be directly connected to a phase terminal Ip of input side I, or indirectly via other components such as resistors, inductors, diodes, etc. The capacitor C is depicted here as being connected to the phase terminal Ip only, but it is also directly or indirectly connected to a neutral terminal In of the input side I. The phase and neutral terminal Ip and In should be interpreted broadly here as being any kind of connector capable of transferring power. In this example, the neutral terminal In is connected to ground, and the phase terminal Ip carries a voltage power signal.

Phase-cutting the AC waveform of the electric mains means that a part of the AC waveform from the electric mains M is transmitted by the dimmer D, and another part of the AC waveform is blocked so that the voltage at the phase terminal is equal to the voltage at the neutral terminal. If the transition between transmission of the AC waveform and blocking of the AC waveform does not take place at zero crossings of the AC waveform, voltage steps occur in the voltage waveform at the phase terminal Ip. These voltage steps may introduce peak currents by rapidly charging or discharging capacitor C. The electrical current drawn by the capacitor C due to the voltage step may be reduced by changing an electrical charge stored in the capacitor C with the charge device CD in such a manner that a voltage of the capacitor C is already progressing in a direction towards the "future" voltage step. The voltage difference is then decreased when the voltage step actually occurs and the induced electrical current is smaller than in case no charge was transferred from or to the capacitor by the charge device. The peak currents are therefore reduced, which has the advantage that more lamps and/or power supply circuits may be connected to a dimmer, the humming of the power supply circuit due to extreme loading of the capacitor may be reduced, and/or less pollution of the electric mains occurs.

In this example, the charge device is connected to the capacitors side which is closest to the phase terminal Ip, but it could also be connected to the other side of the capacitor. The charge device may be a passive element like a battery, capacitor, or other current source. In an embodiment, the charge device is actively controlled by a control system in dependency for instance of an output signal of the dimmer D. It is possible to use a setting of the dimmer D to control the amount of charge transferred to or from the capacitor and/or control the timing when this amount of charge is transferred.

Capacitor C may be any type of capacitor connected to the input side I of the power supply circuit PSC. This may include capacitors of DC-DC converters (for instance switched mode DC-DC converters), but also capacitors used in filters, rectifiers and other parts of the power supply circuit PSC.

The charging device can be any kind of charge storing device, and is preferably an already present component, such as a capacitor, which is substantially not connected to the input side of the power supply circuit. The advantage is that this requires the same amount of components.

Figure 2 depicts a schematic representation of a lighting system according to another embodiment of the invention. The lighting system comprises a lamp L, a dimmer D and in between the dimmer D and lamp L a power supply circuit PSC.

The dimmer D comprises a triac T, i.e. the dimmer is a triac dimmer, to limit the power from the electric mains to the power supply circuit by phase-cutting. The triac T is modulated into and out of conduction at a predetermined phase of the applied AC waveform, said predetermined phase usually being set by a user input. The output signal from the dimmer is supplied to the power supply circuit via a phase terminal Ip and a neutral terminal In to an input side I of the power supply circuit. The power supply circuit comprises a rectifier R, such as a full-wave rectifier or half-wave rectifier to convert the alternating voltage supplied by the dimmer D into a direct voltage. In this embodiment, the power supply circuit comprises a step-up converter as a switched mode DC-DC converter, wherein the step-up converter has an output which is higher than the input.

The step-up converter comprises a diode Dl, input capacitor Cl, inductor ZL, diode D2, output capacitor C2, switch S, and controller Co. The controller Co controls the switching of the switch S in dependency of an output voltage of the step-up converter. The working principle of a step-up converter, i.e. a boost converter, is known to a person skilled in the art, and will not be explained in detail here. The input capacitor Cl is connected to the input side of the power supply circuit as it substantially contributes to the input impedance. The output capacitor C2 acts as a buffer capacitor to smooth the output voltage of the switched mode DC-DC converter as is known to a person skilled in the art. In this embodiment, the output capacitor C2 is used as a charge device to transfer charge from the output capacitor to the input capacitor prior to the occurrence of a voltage step. This is done via an impedance Z which connects the output capacitor C2 to the input capacitor Cl. When the dimmer is blocking the AC waveform of the electric mains, the output capacitor C2 is able to pre-charge the input capacitor so that when a voltage step occurs, the electrical current drawn by the input capacitor due to the voltage step is reduced. Preferably, the capacitance of the output capacitor C2 is larger than the capacitance of the input capacitor Cl so that pre- charging of the input capacitor will not drain the output capacitor and influence the smoothing function of the output capacitor.

The amount of pre-charging, which determines the electrical current drawn by the input capacitor when a voltage step occurs, is determined amongst others by the impedance Z. When designing the impedance Z, a compromise is to be found between a high impedance, which results in an efficient step-up converter, but a relatively low pre-charging and thus a high electrical current, and a low impedance resulting in a relatively high pre- charging and thus a low electrical current, but an inefficient step-up converter. In case of a variable dimmer, the impedance Z is preferably also variable, to allow optimization with respect to the setting of the dimmer. It is to be noted here that only the peak currents have to be reduced, it is not the object of the invention to reduce the overall average current, or to reduce the current to zero when a voltage step occurs. It is sufficient to reduce the peak currents to a predetermined value which will be in the order of the maximal current drawn when no voltage steps occur.

The value of the impedance Z is also dependent on the capacitor Cl, which is to be charged via the impedance. For instance, in case the impedance Z comprises a resistor R, the RCl time must be chosen such that there is time enough to charge capacitor Cl to the required level. Alternatively or additionally, the impedance Z may also comprise one or more of the following components: capacitor; a FET as variable resistor; a voltage dependent resistor; PTC; NTC; diode, in particular a Zener diode. The advantage of a diode is that the transference of charge can be limited to one direction, i.e. to or from the capacitor. This minimizes the influence the impedance connection has on the working principle of the power supply circuit, in particular the working principle of the switched mode DC-DC converter. As required, detailed embodiments of the present invention are disclosed herein; however, it is to be understood that the disclosed embodiments are merely exemplary of the invention, which can be embodied in various forms. Therefore, specific structural and functional details disclosed herein are not to be interpreted as limiting, but merely as a basis for the claims and as a representative basis for teaching one skilled in the art to variously employ the present invention in virtually any appropriately detailed structure. Further, the terms and phrases used herein are not intended to be limiting, but rather, to provide an understandable description of the invention.

The terms "a" or "an", as used herein, are defined as one or more than one. The term plurality, as used herein, is defined as two or more than two. The term another, as used herein, is defined as at least a second or more. The terms including and/or having, as used herein, are defined as comprising (i.e., open language, not excluding other elements or steps). Any reference signs in the claims should not be construed as limiting the scope of the claims or the invention. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage.

The term coupled, as used herein, is defined as connected, although not necessarily directly, and not necessarily mechanically.

A single processor or other unit may fulfill the functions of several items recited in the claims.

The terms program, software application, and the like as used herein, are defined as a sequence of instructions designed for execution on a computer system. A program, computer program, or software application may include a subroutine, a function, a procedure, an object method, an object implementation, an executable application, an applet, a servlet, a source code, an object code, a shared library/dynamic load library and/or other sequence of instructions designed for execution on a computer system.

A computer program may be stored and/or distributed on a suitable medium, such as an optical storage medium or a solid-state medium supplied together with or as part of other hardware, but also be distributed in other forms, such as via the Internet or other wired or wireless telecommunication systems.