NL2005269C2 - High-frequency switching-mode power supply for a dimming circuit. - Google Patents

Description

P92488NL00 TITLE: High-frequency switching-mode power supply for a dimming circuit.

FIELD OF THE INVENTION

5 The invention relates to a high-frequency switching-mode power supply for a dimming circuit.

BACKGROUND OF THE INVENTION

High and Low-power Compact Fluorescent Lamps (CFLs), lately High-and 10 Low Power Light-Emitting-Diode Lamps (LEDLs) and Surface-Mounted Device (SMD) LEDLs are steadily becoming the new standard means of providing lighting in the household, in commercial and industrial sectors.

The old incandescent system is being superseded by new forms of new 15 Energy-Saving lightning transducers, of which the (HP) LEDLs, at present time, are the types most cost-effective in terms of overall performance.

As the general household sector begins to exchange the old trusted 20 incandescent bulbs for the new (HP) LED technology, the users soon take credit of its inherent advantages, namely its efficiency, improved luminosity and cool running. But they soon discover that when attempted to be dimmed the new lamps start to flicker and soon fail altogether. Flicker is brought about by the repeated re-starting of the lamp at low 25 input voltages which, not only progressively degrades the LED lighting transducers irreparably, but also stresses all the driver’s electronic components. Therefore its avoidance will contribute not only to a desired smooth brightness transition throughout the full range of the dimmer, but will greatly help to extend the (HP) LEDL’s safe and reliable working life 30 as well.

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The problem is not in the dimmer as such or the (HP) LEDL itself. The reason for this poor performance resides mainly in the impedance miss-match between the two.

5 This dynamic interaction between the source energy device and its corresponding applied load counterpart is seldom efficient, and there is always certain loss when attempting a practical intercoupling.

When an AC load is not a perfect resistor (as generally is the case in the 10 incandescent-type lamps) the electrical source-energy is not converted into light-energy in its entirety: an AC complex-impedance-load has some resistance and some reactance values. The reactive components of the load are the cause of losses and wasted applied energy, therefore the aim of any good engineering electronics AC design is to keep the resistive/reactive-15 ratio of the load as high as possible in order to obtain the maximum desirable - yet practical - (reads: cost-effective-way) energy-transfer (efficiency) effect. One way to express this important ratio is called: the Power-Factor of the load. Another way (mainly concerning known complex loads) might be: the relative index of the load’s capability to do real work.

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Power-Factor (PF) can alternatively be described as the resultant amount of phase-shift angle between Voltage and Current that can be induced by any load (with some reactive component/s in them) when an AC electrical source-energy is applied to it. The magnitude of the PF is defined as the 25 value of the cosine of said angle, therefore, a unit-less number between 1 and 0.

When the AC load is a pure resistor, practically all the energy is transferred to the load, (as - by definition - there are no reactive 30 components in them), and therefore no phase-shift can occur: PF= 1.0, theoretically.

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Voltage and Current are said to be always in-phase: the work efficiency of the circuit is maximum.

When an AC load is a pure reactance (i.e.: a perfect Capacitor and/or 5 perfect Inductor), the load not only bounces-back all the incoming energy: but the resultant phase shift is theoretically +/- 90 degrees now, and therefore the PF = 0.0. Transfer efficiency is at minimum, as only heat losses are generated.

10 In-between these two extreme examples, the real-world of generally useful electronic applications resides. One of the most ubiquitous is the modern, so called, energy-saving (HP) LEDL.

An (HP) LEDL is composed of a few dozen different electronic components, 15 but basically, can be thought-of as a miniature high-frequency switchingmode power supply (HF-SMPS) driving a solid-state light-transducer. The overall input impedance of the (HP) LEDL is far removed from the theoretical pure-resistive concept, (vis-a-vis incandescent lamps) as their Power Supply Unit (PSU) front-end is generally made purposely highly 20 capacitive, due to the need for a reservoir or smoothing DC capacitor(s) of very significant value, as any good-behavior standard AC/DC/AC/DC (HP) LEDL converter design (theoretically, at least.) must have for its intended stable and reliable long-term operation.

25 This relative significant capacitive character, for example, in the order of several microfarads, of the input impedance of the classical front-end of the PSU of any standard energy-saving converter lamp tends not only to lower its PF, but as a direct consequence (due to their added full bridge rectifier-diodes classic configuration and conductive behavior), distorts the 30 incoming Current supply waveform itself, proportionally increasing it’s Low Frequency (LF) Total Harmonics Distortion (LF THD).

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Now, instead of a smooth co-relation between the original sinusoidal inputs Voltage (In-VAC) and Current (In-IAC), an unpredictable situation occurs, in which only burst of Current is drawn from the supply line at irregular intervals within the Voltage waveform reference, depending on 5 the load’s demands.

This bad situation is exacerbated further when attempting to dim the (HP) LEDL with a standard, so called Phase Cut/Triac-based dimmer (the popular-one generally available in the household and commercial sector), 10 as the dimmer itself is a source of distorted power, as it goes on doing it’s operation progressively chopping the input waveforms to the lamp as it’s setting is advanced in order to restrict it’s power output.

Another unwanted sub-product of any standard High-Frequency 15 Switching-Mode PSU (HF-SMPSU) is the nasty interference energy generated all along the HF spectrum (9Khz to 30Mhz) due to its efficient power supply unit’s inherent high-speed switching transition behavior. To comply with ever tightening international Electro-Magnetic-Interference (EMI) limit regulations, some counter-measures are necessary to be added 20 to any advanced design meant for general commercial approval and legal marketplace distribution.

SUMMARY OF THE INVENTION:

The present invention aims to incorporate a novel power supply unit 25 with improved power factor, combined with a power supply front end that has minimal or even substantially zero capacitive character, and in a preferred embodiment lacks any DC electrolytic capacitor.

In accordance with a first aspect of the invention a high-frequency switching-mode power supply is provided for a dimming circuit according 30 to the features of claim 1.

In accordance with some aspects of the invention, a high-frequency switching-mode power supply is provided for a dimming circuit, the power 5 supply comprising: a bridge rectifier section having an AC input; and a bridge rectifier output; a high frequency oscillator circuit including a pair of transistors and a driving coil, coupled to the bridge rectifier output; a back-end circuit galvanically isolated from the oscillator circuit and 5 coupled via a transformer comprising said driving coil, the transformer tuned to transfer electric power from the driving coil into the back-end circuit in a working frequency range that is associated with the working frequency of the oscillator circuit; and a feedback circuit arranged transfer a sample signal from the back end circuit to adjust the transfer 10 efficiency of the transformer by setting a control circuit in the oscillator , in order to change the operation of the oscillator circuit as a function of the sensed sample signal.

Accordingly, relatively bulky electrolytic capacitors can be dispensed with, which has a benefit in that the final lamp’s driver design 15 can be made smaller and more compact as it is drastically reduced in overall volume: update of existing, mechanically-challenging lamps with tight industry-standard enclosures, and newer smaller and trendy lamps format designs can be made possible now. In addition, as no DC reservoir capacitors of any substantial value (typically more than 1 microF) are 20 required, the basic topology can be much more easily scaled (up or down of the range) to be extended to cater for any desired power output required.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will further be elaborated with reference to the following 25 Figures:

Figure 1: Schematic working diagram according to the invention;

Figure 2: exemplary circuit diagram for Figure 1;

Figure 3: Detailed circuit diagram including a dimmable LED lamp according to the block diagram of Figure 2.

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DETAILED DESCRIPTION OF EMBODIMENTS

Turning now to Figure 1, the block diagram of the dimming circuit, coupled to AC mains is divided into four areas: 5 10) Triac-dimmer minimum-load interface.

20) Low-frequency AC to DC rectifier-converter.

30) Passive LRC EMC counter measure network 40) NO electrolytic capacitor required 50) High-frequency DC to AC oscillator 10 60) Galvanic isolation barrier 70) LED1 back-end 80) Opto-isolated feed back

Referring specifically to the 10 - 80 sections of Figure 1, the following is 15 observed: (10) Triac-dimmer minimum-load interface.

Just a simple HV capacitor of calibrated value and rating, placed at the 20 input mains’ port of the (HP) LEDL can make the performance of any phase-cut dimmer more predictable and repeatable. Its important contribution is therefore two-fold: - to help equalize the performance of the very disparate dimmer electronic 25 designs dimming ranges, and the mechanical variations in their “control- pot” geometry span-travels.

- to present the dimmer’s gating device (the “Triac” core) with a minimum load current to keep it conducting for a longer angle span, especially at the 30 critical low-brightness dimming settings, when the avoidance of flickering is highly desirable, not only just for aesthetics but also due to sound electronic design principles.

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In the present invention design, a fixed resistor of relative medium K-ohms value may further help to equalize the performance of many disparate dimmer devices, especially at low brightness levels, as it 5 presents a constant and fixed minimum working load to the wide-spread brands and types of light-controlling TRIAC-core products commercially available in the marketplace: their average response becomes smoother, less prone to flickering, less noisy, a more reliable operational life-span can be expected, and their general performance becomes more predictable.

10 (20) Low-frequency AC to DC rectifier-converter.

This is implemented as a high-voltage full diode-bridge rectifier topology BR1 of standard-grade inverse-recovery response.

15 30) Passive LRC EMC counter measure network THD is present and measured with specialized relative low-frequency response analyzers (up to the 40th Harmonic of the fundamental Mains frequency - 50 or 60 Hz - that means roughly up to 25KHz or below). The 20 current Pass-Specification usually keep a special watch for the third and the fifth ones, still within the realm of the very low frequencies, indeed.

Furthermore, any HF-SMPS (High-Frequency, Switching-Mode Power Supply) is a source of HF interfering harmonics also, and every driver of 25 any (HP) LEDL have one due to its more efficient and inherent AC/DC/AC/DC switching frequency-conversion topology.

As the fundamental oscillation of the built-in converter of any (HP) LEDL is already in the region of the HF band, we can expect indeed very high 30 frequencies to be present in the total electrical circuit as well, as the load that the AC source is presented with is a HF converter, a non-linear one, as such.

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If we add to this scenario the possibility of varying the input power to the (HP) LEDL by means of varying the power input to the lamp (and it’s core non-sinusoidal switching driver) with a waveform-chopping device/Phase-5 Cut/Triac-base dimmer, we have in every (HP) LEDL itself a potential interference-generator of some important magnitude indeed.

As the present’s EMC regulations extend a mandatory test for conductive Interference (C-EMI) up to 30MHz, means must be provided to minimize 10 the harmful effect of the HF oscillator’s fundamental frequency and its related HF harmonics.

To this effect, an L-R High Frequency Low-Pass Filter (HF-LPF) is configured between the positive (total DC current’s send) and the negative 15 (total DC current’s return) terminals of the bi-polar to the uni-polar (converter) bridge rectifier.

40) NO electrolytic capacitor required

As disclosed in this application, the (HP) LED lamps presented here do not 20 require any kind of electrolytic capacitors nor any type of PFC to achieve a relative high PF and a cleaner and more efficient transfer of energy between the AC source and the DC load.

This has been achieved by removing as much as possible any input 25 capacitance of significant high value and therefore allowing the input unipolar energy current to flow more freely through an enlarged angle of conduction, relative to the bi-polar AC waveform reference.

A PF of 0.85 or more, is inherently and straight-forwarding possible with a 30 THD less 40% or better, easily and readily achievable.

50) High-frequency DC to AC oscillator 9

Although new specific ICs available for this very purpose have started to appear in the marketplace, variations of tried and tested discrete components topologies can work as well as the emerging new-ones, and 5 can be as good for its perceived price-performance competitiveness.

According to one aspect, no specialized high voltage IC-driver is used but instead a discrete high voltage switched mode power supply.

In this case a DC Variable Voltage to an AC Frequency (V/V/F) driver topology is used to indirectly control the output current into the LEDs 10 through the variable impedance presented to the Galvanic Isolation

Barrier (GIB)/Transformer by the variable-frequency energy-drive. The present V/V/F driver is arranged in such a way as to have an inverse working law: the higher it’s input voltage the lower its output frequency (and therefore the LOWER the inductive-impedance/restriction of the 15 energy through the GIB, and the MORE current is delivered to the LEDs) and vice versa.

60) Galvanic isolation barrier

The Galvanic Isolation Barrier (GIB), in the form of a "transformer" acts 20 as a “ballast” or limiter for the safe transfer of current towards the low-impedance LEDLs transducers and, due to its primary to secondary turns ratio, helps to resolve the voltage gap between the HV input energy source and its LV output load.

25 In the case of dimmable (HP) LEDLs, some variable parameter must change with the variable power delivery of the operation of its associated dimmer.

This variable parameter might be voltage, current and/or frequency of 30 operation, but in all the cases the varying (generally switching non-sinusoidal) HF driving energy will interact with the non-linear LED

10 transducers in such a way as to generate unwanted and substantial spurious HF terms, high above the original and fundamental frequency.

That unuseful and unwanted energy could be reflected back into the 5 driver’s DC PSU and even further back into the AC supply lines and its limits are the ones that the standard test for C-EMI is concerned with.

In the design of the present invention, the HF drive signal is coupled to the LED lighting transducers/load by means of an isolation transformer.

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This effective galvanic isolation barrier - although not contributing directly to enhance the lamp’s overall power-input/power-output efficiency equation - indeed greatly helps in reducing the coupling of unwanted energy back towards the lamp’s input PSU.

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This galvanic input-output isolation helps indirectly to avoid the HF harmonics terms mixing with the LF spurious terms generated by the nonlinear PSU, thus helping to reduce the overall THD of the (HP) LEDLs.

20 Indirectly, less energy wastage leads to a desirable better reliability and dependability, as well as to an improved overall better operation, perceptions and realities, all so important in a new dimmable lighting device product.

25 70) (HP) LED1 backend

Basically, a LED1 is a current-driven energy-transducer device and can be driven electrically only by a direct (unipolar or single polarity - generally labelled as DC -) energy-source. Its maximum normalized current has a tight specification and can not otherwise be exceeded without a predictable 30 sure failure: in electronic terms: it has a very low dynamic impedance. Therefore the control of its current is an important requirement for the associated electronic driver.

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Its normal operating voltage is relatively very low (only a few volts) and once it is progressively achieved (i.e.: as a slow rise from 0 Volts can testify), it will remain relatively constant. Thereafter it will revert to its 5 constant-current drive requirement, so long as it is operated within the range of its maximum specified current limit.

80) Isolated feedback A feedback circuit is provided to transfer a sample signal (proportional to 10 the load current of the LEDs) through the Galvanic Isolation Barrier (GIB) 60 back to the DC voltage-controlled variable oscillator driver 50 in the input side, as to complete and stabilize the control loop 90, in order to change the operation of the V/V/F (variable voltage to frequency) converter 50, in a smooth and predictable fashion, as the dimmer is exercised up or 15 down, in order to control the smoothness of the variation of the HF driver’s energy delivery in direct proportion of the AC input power to the lamp.

Figure 2 shows an exemplary circuit diagram for Error! Reference source not found, further detailing a feedback circuit 80. In the 20 galvanically isolated back-end 70 of the present embodiment, a (HP) LED lamp 71 is provided that is powered by a current derived from the galvanic isolation barrier in the form of transformer assembly 60. Many variations of self-oscillating networks are possible exploiting the working principle of a galvanic isolation barrier 60 with feedback circuit 80 as presently 25 disclosed.

Essentially, the galvanic isolation barrier 60 only has a limited frequency acceptance, and will fail to resonate in the lower and higher frequencies that are associated with THD and EMC specifications. Ideally, the galvanic isolation barrier 60 is tuned to be active in a limited working 30 range that is associated with the working frequency of the oscillator 50. The feedback circuit is preferably provided as including an opto-coupler 80. This is an electrical device known in the art and essentially comprises 12 a coupled LED device 81, 82. Light generated in LED device 82 activates the transistor of device 81, so that device 81 will act as a tunable resistor with a resistive value indicative of the amount of power received in device 82. In addition to the transformer 60 providing a galvanic separation 5 barrier between the DC-HV section of the oscillator 50 and the back-end 70 the feedback circuit 80 is also galvanically isolated. A further advantage of a feedback circuit in the form of opto-coupler 80 is that it can be tuned to have a limited frequency acceptance so that it is active in a bandwidth that will prevent cross-over of high frequency jitter from the 10 back end 70.

A sense circuit 72 in back-end 70 measures the amount of power transferred through the galvanic isolation barrier. In particular, an output DC sample signal (proportional to the load current of the LEDs) may be derived and provided to an opto-isolated feedback 80.

15 The feedback circuit 80 sets the transfer efficiency of the galvanically isolated barrier 60 by setting a control circuit 51 in the oscillator 50, to control the stable operation of backend LED section 70.

Error! Reference source not found, gives a detailed circuit diagram of 20 a dimmable LED lamp according to the block diagram of Figure 2.

Transistor pair 500, 501 is switched to provide an oscillating circuit 50 with a normal period determined by the RC combination 502, 503. Accordingly, a net DC voltage in the form rectified and chopped wave forms received from the triac-dimmer minimum-load interface and 25 smoothened by the LRC network 20 are alternatingly provided to driving coil 61 in the active phase of transistor 501, which is deactivated by transistor 502 offering a current flow through resistors 504 alternative to the driving coil 61.

In galvanically isolated back-end section 70 a diode circuit including a 30 Schottky diode 73 is provided to deliver an unidirectional high power current through the LED 71, which is smoothened by relative small capacitors 74 in parallel to the LED 71. Back-end 70 further comprises a 13 sensing circuit 72 includes a Zener diode and opto-coupler 80 switched to detect a sensing current indicative of the amount of current generated in the back-end transformer coil 62.

As an example of a settable transfer efficiency, the frequency of the 5 oscillator 50 can be tuned by tuning circuit 51. This circuit 51 comprises a power supply 510 by charging a capacitor through a secondary coil 63. By selectively charging of the capacitor 503 via the opto-coupler 80 powered by power supply 510, the set frequency of the oscillating transistor pair 500, 501 can be increased. An additional voltage from power circuit 510 is 10 transferred to the base of transistor 500 when it is activated.

Accordingly opto-coupler 80 serves to set the working frequency of the oscillator circuit 50 dependent on the transfer efficiency detected in the back-end 70 by sensing circuit 72. In this way the working frequency of the oscillator is changed to a less effective transfer frequency if the 15 amount of sensed power is too high in the back-end 70. A spike circuit 52 is provided to protect the transistor 500 from high voltages in the driving coil 61.

While the invention has been illustrated and described in detail in the drawings and foregoing description, such illustration and description are 20 to be considered illustrative or exemplary and not restrictive; the invention is not limited to the disclosed embodiments.

A high frequency mode power supply for a dimming circuit is disclosed including a bridge rectifier section is provided having an AC input, a DC bridge rectifier output and no reservoir capacitor of any significant value 25 in parallel with the DC rectifier output. A high frequency oscillator circuit includes a transformer assembly 60 coupled to the DC bridge rectifier output with a primary 61 and a secondary sides 62, and further comprises a power factor correction device arranged as a frequency modulated electronic switch 80 which drives the primary-side 61 of the said 30 transformer 60. The frequency modulator circuit is controlled accordingly to the power to be delivered to a load connected in parallel to the secondary-side of said transformer. An isolated feedback circuit 80 is 14 provided from the load side 70 of the transformer 60 to the oscillator-driver side 50 of the transformer in order to close the control loop.

Other variations to the disclosed embodiments can be understood and effected by those skilled in the art in practicing the claimed invention, 5 from a study of the drawings, the disclosure, and the appended claims. In the claims, the word "comprising" does not exclude other elements or steps, and the indefinite article "a" or "an" does not exclude a plurality. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measured 10 cannot be used to advantage. Any reference signs in the claims should not be construed as limiting the scope.

Claims (10)

1. Een hoogfrequente schakelmodusvoeding (100) voor een dimschakeling, welke voeding (100) omvat: 5. een bruggelijkrichter gedeelte (20) omvattende een AC invoer; en een bruggelijkrichter uitvoer; - een hoogfrequent oscillatorcircuit (50) omvattende een paar transistoren (500, 501) en een aandrijfspoel (61), gekoppeld aan de bruggelijkrichter uitvoer; 10. een back-end circuit (70), galvanisch geïsoleerd van het oscillatorcircuit (50) en gekoppeld via een transformator (60), omvattende genoemde aandrijfspoel (61), welke transformator (60) is ingesteld om elektrisch vermogen over te dragen van de aandrijfspoel (61) naar het back-end circuit (70) in een 15 werkfrequentiebereik dat is geassocieerd met de werkfrequentie van het oscillatorcircuit (50); en - een feedback circuit (80) ingericht om een samplesignaal over te dragen van het back-end circuit (70) om de overdrachtsefficiëntie van de transformator (60) aan te passen door het instellen van een 20 regelcircuit (51) in de oscillator (50), om de werking van het oscillatorcircuit (50) te veranderen als functie van het waargenomen samplesignaal.A high frequency switching mode power supply (100) for a dimming circuit, which power supply (100) comprises: 5. a bridge rectifier section (20) comprising an AC input; and a bridge rectifier output; - a high frequency oscillator circuit (50) comprising a pair of transistors (500, 501) and a drive coil (61) coupled to the bridge rectifier output; 10. a back-end circuit (70) galvanically isolated from the oscillator circuit (50) and coupled via a transformer (60) comprising said drive coil (61), which transformer (60) is adapted to transfer electrical power from the drive coil (61) to the back-end circuit (70) in an operating frequency range associated with the operating frequency of the oscillator circuit (50); and - a feedback circuit (80) adapted to transmit a sample signal from the back-end circuit (70) to adjust the transfer efficiency of the transformer (60) by setting a control circuit (51) in the oscillator ( 50), to change the operation of the oscillator circuit (50) as a function of the sensed sample signal. 2. De voeding volgens conclusie 1, waarin het feedback circuit een 25 optische koppelaar (80) omvat, ingericht om een werkfrequentie in te stellen van het oscillerende transistorpaar (500, 501) door het instellen van een weerstandswaarde in het regelcircuit (51) met het samplesignaal gegenereerd in de optische koppelaar (80).2. The power supply according to claim 1, wherein the feedback circuit comprises an optical coupler (80) arranged to set an operating frequency of the oscillating transistor pair (500, 501) by setting a resistance value in the control circuit (51) with the sample signal generated in the optical coupler (80). 3. De voeding volgens conclusie 1, verder omvattende een weerstandsnetwerk (10) parallel gekoppeld aan de DC bruggelijkrichter invoer.The power supply of claim 1, further comprising a resistor network (10) coupled in parallel to the DC bridge rectifier input. 4. De voeding volgens conclusie 1, verder omvattende een inductief netwerk (30), parallel gekoppeld aan de DC bruggelijkrichter uitvoer.The power supply of claim 1, further comprising an inductive network (30) coupled in parallel to the DC bridge rectifier output. 5. De voeding volgens conclusie 1, waarin de dimschakeling van een phase-cut off type is. 10The power supply according to claim 1, wherein the dimming circuit is of a phase cut off type. 10 6. De voeding volgens conclusie 1, waarin de dimschakeling een triac omvat.The power supply according to claim 1, wherein the dimming circuit comprises a triac. 7. Een dimbare spaarlamp omvattende een verlichtingsapparaat en 15 een hoogfrequente schakelmodusvoeding (100) volgens een der conclusies 1-5.7. A dimmable energy-saving lamp comprising a lighting device and a high-frequency switching mode power supply (100) according to any one of claims 1-5. 8. Een dimbare spaarlamp volgens conclusie 7, waarin het verlichtingsapparaat een HP-LED apparaat is. 20A dimmable energy-saving lamp according to claim 7, wherein the lighting device is an HP-LED device. 20 9. Een dimbare spaarlamp volgens conclusie 7, waarin het verlichtingsapparaat een Surface Mounted Device (SMD) LED is.A dimmable energy-saving lamp according to claim 7, wherein the lighting device is a Surface Mounted Device (SMD) LED. 10. Een dimcircuit omvattende: 25. een phase-cut off dimmer gedeelte dat gekoppeld wordt aan het AC lichtnet; en - een hoogfrequente schakelmodusvoeding (100) volgens een der conclusies 1 — 5.10. A dimming circuit comprising: 25. a phase-cut off dimmer portion that is coupled to the AC mains; and - a high frequency switching mode power supply (100) according to any of claims 1 to 5.