CONTROL CIRCUIT OF A DIMMER ASSEMBLY FOR DIMMING AN ENERGY- SAVING LAMP

FIELD OF THE INVENTION

The present invention relates to a control circuit of a dimmer assembly for dimming an energy-saving lamp, in particular a gas discharge lamp, said dimmer assembly comprising a dimmer circuit and said control circuit for controlling the output current of said dimmer assembly.

The present invention further relates to a corresponding control method, a dimmer assembly and an energy-saving lamp assembly.

BACKGROUND OF THE INVENTION An energy-saving lamp, in particular a gas discharge lamp, is usually operated by an electronic ballast comprising a typical half-bridge inverter with a typical multi-resonant impedance matching network with or without a transformer. To allow dimming of such a gas discharge lamp, a TRIAC or any other type of dimmer circuit is used.

Common standard dimmer circuits employ a TRIAC in order to shape an alternating supply voltage such as a mains voltage. TRIACs were originally designed to control the power of resistive types of loads, such as heating resistors or incandescent lamps, which can be directly connected to the AC mains voltage. They are series-connected between the AC mains and the resistive load and are based on the following working principle. The TRIAC is a power switch which turns on when a certain current is supplied to its control gate. The current pulse should have a pulse width which is long enough for the TRIAC current to reach at least the so-termed latching current, i.e. the minimum TRIAC current required at the time the gate current ceases for the TRIAC to stay on. After latching, the TRIAC current continues to flow also without a gate current until it falls below a second threshold, referred to as the holding current, which is typically lower than the latching current. When the instantaneous TRIAC current falls below the holding current, the device turns off.

In an application with an AC input voltage (typically 50Hz or 60Hz), the gate current is supplied by a DIAC in series with the gate, and an additional RC network in parallel with the TRIAC, the mid-point of which is connected to said DIAC element. The time constant of the RC network defines the AC phase angle at which the TRIAC is turned on (leading edge control).

A gas discharge lamp operated by an electronic ballast, commonly employed as an energy-saving lamp, only draws current from the supply in the peaks of the alternating voltage due to the presence of a bridge rectifier followed by a buffer capacitor in the electronic ballast. However, the common TRIAC dimmer circuit is only suitable for use with a resistive load. Therefore, dimming of an electronic energy-saving gas discharge lamp using a TRIAC dimmer circuit generally does not function correctly and additional interfacing circuits are commonly used for dimming with standard wall plug dimmers (or TRIACs).

US 6,452,343 discloses the provision of a circuit having a resistive characteristic, e.g. a resistor, between the input terminals of a ballast circuit of a gas discharge lamp. A TRIAC dimmer circuit is thus provided with a resistive load and may function properly. However, a substantial power loss is thereby generated, because a current flows through the resistive circuit at any time. The resistive load has to be designed in such a way that, at relatively low AC voltages, the current is already sufficient for the TRIAC to latch, i.e. at high AC voltages many unnecessary losses are generated. WO 2006/120629 A2 (PH000441EP1) discloses a method and circuit for reducing these power losses, which circuit draws a relatively large current when the electronic gas discharge lamp and its electronic ballast is not drawing current, in order to charge the timing circuit and bring the TRIAC in a conducting state, and draws a reduced current when only a small current is needed to keep the TRIAC of the dimmer circuit in a conducting state.

When the alternating supply voltage and current increase from zero at the start of a cycle, the TRIAC is in a non-conducting state. To bring the TRIAC to a conducting state, the load should draw sufficient current to charge the timing circuit of the TRIAC and latch it. Since the electronic gas discharge lamp does not draw current at this stage of the cycle of the alternating voltage and current, the current control circuit is designed to draw current, e.g. by providing a resistive load, when the gas discharge lamp and its ballast circuit are not drawing current. When the TRIAC has become conducting, only a small current is needed to keep the TRIAC in its conducting state. The resistance of the load may therefore be increased.

The current control circuit according to WO2006/ 120629 A2 is designed to switch between two branches of a parallel circuit, each branch having a predetermined resistance. The switches are controlled by the voltage level of the supply voltage. When the voltage level is below a predetermined value, the branch having a low resistance is switched to the conducting state. When the voltage level is above the predetermined value, the branch having a high resistance is switched to the conducting state. If the supply voltage is a mains voltage of 230 V at 50 Hz, a suitable predetermined voltage level may be about 50 V.

If the gas discharge lamp and the electronic ballast thereof draw sufficient current to keep the TRIAC in its conducting state, it may not be necessary that an additional circuit draws any current. The current control circuit is therefore designed to control the total current drawn by the lamp and the resistive circuit, e.g. by preventing a current from flowing through the resistive circuit when the assembly of the gas discharge lamp and electronic ballast is drawing sufficient current.

An additional problem imposed by the use of an electronic ballast having a first rectifier and buffer capacitor stage is the huge capacitive inrush currents when the leading edge-controlled TRIAC is turned on. These inrush currents can easily destroy the TRIAC, if not properly designed. For this reason, trailing edge-controlled dimmers based on MOSFET transistors or IGBTs have been introduced on the market. These dimmers suppress the huge inrush currents by avoiding the steep initial rate of voltage change (dV/dt) at dimmer turn-on. These developments have led to a large variety of wall dimmers already installed worldwide, which new electronic ballasts have to comply with. Since most electronic ballasts cannot cope with all of them, the product package usually mentions which types of wall dimmers the energy savers are compatible with. This requires the normal customer to have a level of technical background which is not really realistic, and this is one major obstacle for the widespread use of energy-saving lamps.

US 5,604,411 discloses a dimming ballast for use with a phase control dimmer, and particularly a TRIAC dimmer, which includes an EMI filter selected with a high impedance to avoid excessive voltage and peak currents in the filter due to resonance with the phase- controlled AC waveform at low conduction angles, when the load presented by the lamp is low. The EMI filter includes a filter capacitor connected at the output of a rectifier. The ballast also includes circuitry for sensing the rectified DC voltage and discharging the filter capacitor when the rectified voltage is at or near zero, so as to thereby keep the EMI filter loaded and prevent misfiring of the TRIAC dimmer. In a favorable embodiment, the sensing and discharge function is carried out by a pre-conditioner of the switched-mode type.

OBJECT AND SUMMARY OF THE INVENTION

It is an object of the present invention to provide a control circuit and method for a dimmer assembly for dimming an energy-saving lamp, in particular a gas discharge lamp, which allows dimming of the lamp by using any type of dimmer circuit without introducing additional power loss.

In a first aspect of the present invention, a control circuit is presented, which comprises: a boost converter for converting a DC input voltage into a DC output voltage, said boost converter including a power switch, a control unit for controlling said power switch in such a way that i) during a latch time, the power switch is switched off if a dimmer current drawn from said dimmer circuit exceeds a first upper threshold, and the power switch is switched on if said dimmer current falls below a first lower threshold, and ii) during a subsequent hold time, the power switch is switched off if said dimmer current exceeds a second upper threshold, and the power switch is switched on if said dimmer current falls below a second lower threshold.

In a further aspect of the present invention, a corresponding control method is presented. In a further aspect of the present invention, a dimmer assembly is presented for dimming an energy-saving lamp, in particular a gas discharge lamp, said dimmer assembly comprising a dimmer circuit and a control circuit as defined in claim 1 for controlling the output current of said dimmer assembly.

In a further aspect of the present invention, an energy-saving lamp assembly is presented, including an energy-saving lamp, in particular a gas discharge lamp, a dimmer assembly as defined in claim 8 for dimming said energy-saving lamp, and a ballast circuit.

The present invention is based on the recognition that the control method and the control circuit only need to draw a relatively large initial current which is preferably larger than or equal to the latching current, when the dimmer is turned on in order to bring it to a conducting state, and that, subsequently, only a reduced current needs to be drawn, which is preferably larger than or equal to the holding current for the remainder of the dimmer conduction angle. The latch time is usually significantly smaller than the holding time and is typically of the order of some 10 to 100 μs.

It should be noted that the expression "DC input voltage" also includes a rectified AC input voltage.

Preferred embodiments of the invention are defined in the dependent claims. It should be understood that the control method, dimmer assembly and energy-saving lamp have similar and/or identical preferred embodiments as defined in the dependent claims of the control circuit.

In accordance with preferred embodiments, it is proposed that the control unit is adapted to set said first upper threshold, such that it is equal to or higher than said second upper threshold, and/or to set said first lower threshold, such that it is equal to said second lower threshold, in particular at zero level. The lower limits do not necessarily need to be equal, but in case both are zero, the control is especially simple to implement. This embodiment is easy to implement. It should, however, be ensured that the average value of the first upper and lower thresholds should be higher than the average value of the second upper and lower thresholds. In accordance with a different implementation, a kind of sinusoidal current is provided, which is superposed by a high initial current for latching the power switch (preferably a TRIAC) and by a DC current which equals the holding current of the power switch. A better power factor can be achieved with such an embodiment, which is, however, more costly. In accordance with further preferred embodiments, it is proposed that the control unit is adapted to set said first upper and lower thresholds, such that the average dimmer current during the latch time is equal to or larger than a latching current required for the dimmer circuit to get into a permanent conducting state, and/or to set said second upper and lower thresholds, such that the average dimmer current during the hold time is equal to or larger than a holding current required for the dimmer circuit to stay in its conducting state.

The power to the load can be generally controlled in different ways: i) the average current during the latch time and/or the hold time is regulated to match the power drawn from the AC source to the power of the load; ii) by aligning the current flow time with the actual on-time of the power switch (TRIAC) in one AC half period time (10ms or a 50Hz AC voltage); iii) by stopping to draw current from the AC voltage already prior to the end of the AC half period; iv) a combination of the methods i) to iii).

An advantageous embodiment of the boost converter comprises

a boost inductor through which said dimmer current flows, a transistor representing said power switch coupled in series with said boost inductor, a rectifier diode, the anode of which is coupled to the connection point of the series-connected boost inductor and transistor, and an output capacitor coupled in series with said rectifier diode, said rectifier diode and said output capacitor being coupled in parallel with said transistor.

This is a very inexpensive embodiment. The boost converter preferably further comprises a shunt resistor coupled in series with said transistor. The resistor serves for measuring the current through the power switch at least occasionally, and for allowing a better control.

In a preferred, simple embodiment, the holding current is constant and equal to the latching current. However, different (and also variable) current amplitudes are also conceivable, as long as they fulfil the afore-mentioned current constraints.

In the most general case, the power that flows to a half-bridge inverter of the ballast circuit of the energy-saving lamp assembly and consequently to the connected lamp can be controlled by the dimmer conduction time as well as by the current amplitudes of both the latching and the holding current.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other aspects of the invention are apparent from and will be elucidated with reference to the embodiment(s) described hereinafter. In the drawings,

Fig. 1 shows a conventional TRIAC dimmer circuit,

Fig. 2 shows typical voltage and current waveforms of conventional dimmer circuits, Fig. 3 shows a combination of a TRIAC dimmer circuit comprising a current control circuit and an energy-saving lamp, Fig. 4 shows a control circuit according to the present invention,

Fig. 5 shows a signal diagram illustrating the control method according to the present invention.

DESCRIPTION OF EMBODIMENTS

Fig. 1 shows a conventional TRIAC dimmer circuit suitable for use with the method and circuit according to the present invention. The TRIAC dimmer circuit 1 comprises a resistor 2 having an adjustable resistance, a capacitor 3, a DIAC 4 and a TRIAC 5. A load 6 such as a lamp is connectable between the terminals 7 and 8. The load 6 and the TRIAC dimmer circuit 1 are series-connected to an AC power supply. It is noted that the resistor 2 may comprise a resistor having a static resistance and a resistor having a user- adjustable resistance, as is known in the art.

As is shown in the Figure, the capacitor 3 and the resistor 2 are connected in series between the terminals of the TRIAC dimmer circuit 1. The TRIAC 5 is connected in parallel with the series connection of the resistor 2 and the capacitor 3. The DIAC 4 is connected between a control gate of the TRIAC 5 and a node between the capacitor 3 and the resistor 2. The resistor 2 and the capacitor 3 form the timing circuit of the TRIAC dimmer circuit 1. In operation, when the power supply 6 has a zero voltage, the DIAC 4 and the

TRIAC 5 are in a non-conducting state. With an increasing voltage supplied by the AC power supply 6, the voltage across the capacitor 3 increases. When the voltage across the capacitor 3 reaches the breakover voltage of the DIAC 4, the capacitor 3 is partially discharged by the DIAC 4 into the TRIAC gate. As a result of the current supplied to this TRIAC gate, the TRIAC 5 becomes conducting. The TRIAC 5 stays conducting as long as a current larger than or equal to the holding current flows through it. When the current supplied by the power supply 6 reaches zero again, the TRIAC 5 becomes non-conducting again.

Those skilled in the art readily understand from the above description of the operation that the load needs to draw a current from the TRIAC dimmer circuit 1, i.e. through the series connection of the resistor 2 and the capacitor 3 in order to charge the capacitor 3 when the TRIAC 5 is not conducting, in order to bring the TRIAC 5 to a conducting state. Fig. 2 shows typical dimmer types, namely R, RL and RC, and their typical voltage and current waveforms.

Fig. 3 schematically shows a TRIAC dimmer circuit assembly 10 connected to an electronic gas discharge lamp assembly 20. The electronic gas discharge lamp assembly 20 comprises an electronic ballast circuit having a rectifier circuit 21, a buffer capacitor 22 and an inverter circuit 23 connected to a gas discharge lamp 24. The TRIAC dimmer circuit assembly 10 comprises a standard common TRIAC dimmer circuit 11 for dimming a lamp having a resistive characteristic, a rectifier circuit 12, a current control circuit 13 and adiode 14.

The electronic gas discharge lamp assembly 20 as shown in Fig. 3 is a commonly available energy-saving lamp which may be connected directly to a mains voltage and is not dimmable when using a standard TRIAC dimmer circuit. The TRIAC dimmer circuit assembly 10 comprises such a standard TRIAC dimmer circuit 11 and further comprises a current control circuit 13 according to the present invention, e.g. as shown in Fig. 4. For proper operation of the current control circuit 13, a rectifier circuit 12 and the diode 14 are also provided in the TRIAC dimmer circuit assembly 10. A simple TRIAC dimmer circuit assembly 10 may thus be provided, with the use of which a common energy- saving lamp assembly 20 having an electronic ballast circuit may be dimmed. The rectifier circuit 21 comprised in the electronic gas discharge lamp assembly

20 is redundant in the circuit assembly of Fig. 3, because the voltage supplied to the lamp assembly 20 is already rectified by the rectifier circuit 12 of the dimmer assembly 10. Hence, the dimmer circuit assembly 10 may also be employed in combination with an energy-saving lamp assembly 20 having an electronic ballast without the rectifier circuit 21. Particular implementations of the dimmer circuit 11, the rectifier circuit 12 and the ballast circuit including units 21, 22, 23 are generally known to the skilled person and are shown in, for example, the above-mentioned documents US 5,604,411 and WO 2006/120629 A2.

Fig. 4 is a circuit diagram of the current control circuit 13 according to the present invention. It comprises a first series inductor L, a power switch T (typically implemented by a bipolar or MOSFET transistor), a rectifier diode D, a filtering and storage capacitor C, a control circuit Co and a shunt resistor Rs. Inductor L, transistor T, diode D and capacitor C form a well-known boost or up-converter for converting the DC input voltage Viriac into a DC output voltage Vb _us - The operation of this control circuit 13 will be explained with reference to the signal diagrams shown in Fig. 5, which illustrate the basic control strategy according to the present invention, applied to the standard boost converter of Fig. 4. In particular, the input voltage Viπac and the inductor current iL are shown in Fig. 5.

During the leading time Ti _ea d, which can be directly translated into the phase control angle alpha of a TRIAC dimmer, the dimmer is in its OFF state. During this time, the transistor T is in its ON state to provide a low impedance path required by the TRIAC dimmer 11 to work properly. When the dimmer is turned ON, the current in the inductor L builds up until a certain first upper threshold Sl is reached. At this point in time, the transistor T is turned OFF and the energy stored in the inductor L is fed to the output capacitor C until the inductor current i^ reaches zero again (or, more generally, until the

inductor current iL falls below a first lower threshold SO which is zero in this example). Subsequently, the whole switching cycle repeats. This mode of operation is usually called boost converter transition mode (boundary between continuous and discontinuous conduction of the inductor current ^). The afore-mentioned first upper threshold S 1 is characterized in that the average inductor current iL,av _g , and consequently the average dimmer current, is larger than or equal to the thyristor or triac latching current i _la tch- The latching current i _latc h is the minimum thyristor or triac current required for the device to stay ON once the gate trigger current has ceased. After a predefined, but not necessarily constant, latching time Ti _atc h, the first upper current threshold Sl is changed to the second upper current threshold S2 for the remainder of the dimmer conduction angle, i.e. for the holding time Thoid- The second lower threshold is equal to the first lower threshold SO, i.e. zero, but may also be different. The second upper current threshold S2 is characterized in that it is higher than or equal to the thyristor or triac holding current ihoid- The holding current ihoid is the minimum current required by the thyristor or triac in order to stay ON after having latched.

The minimum amount of power that can be drawn from the AC mains voltage based on this control strategy depends on the mains voltage, the leading time Ti _ea d (dimmer control angle alpha), the minimum latching current i _la tch, the latching time Ti _atc h and the minimum holding current ihoid-

In summary, according to the present invention, a control circuit and device are proposed, which allow dimming of an energy-saving lamp, in particular a gas discharge lamp, using any type of dimmer circuit, such as dimmers based on leading or trailing edge control. The proposed method and circuit do not introduce any additional power loss. The method and circuit advantageously draw a relatively large initial current which is larger than or equal to the TRIAC latching current when the dimmer is turned on in order to bring it to a conducting state, and draws a second (lower) TRIAC current which is larger than or equal to the TRIAC holding current for the remainder of the dimmer conduction angle. The latching time is usually significantly smaller than the TRIAC holding time and is typically of the order of lms. In the simplest embodiment, the holding current is constant and equal to the latching current; however, different (and also variable) current amplitudes are also conceivable as long as they fulfil the afore-mentioned constraints.

While the invention has been illustrated and described in detail in the drawings and the foregoing description, these drawings and description are to be considered illustrative

or as examples only and are not restrictive; the invention is not limited to the disclosed embodiments. Other variations of the disclosed embodiments can be understood and effected by those skilled in the art in practicing the claimed invention, from a study of the drawings, the disclosure, and the appended claims. In the claims, use of the verb "comprise" and its conjugations does not exclude other elements or steps, and the indefinite article "a" or "an" does not exclude a plurality. A single element or other unit may fulfil the functions of several items recited in the claims. 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. Any reference signs in the claims should not be construed as limiting the scope.