TRIAC DIMMABLE BALLAST

Abstract: A triac dimmable compact fluorescent lamp including a power feedback circuit. The power feedback circuit helps to maintain the level of current drawn from the triac to at least the level of the triac holding current especially during low dim levels. In one preferred embodiment, the feedback circuit is fed into a junction joining together a pair of fast recovery diodes for converting the high frequency square wave signal into a unidirectional signal supplied to an inverter. Overboost voltages across a buffer capacitor are minimized during low dim levels.

Description: TRIAC DLMMABLE BALLAST. This invention relates to a ballast, comprising: mains input terminals; a first rectifier coupled to the mains input terminals and having rectifier output terminals for converting a low frequency AC signal into a unidirectional signal; an inverter having input terminals coupled to the rectifier output terminals and including a pair of switches joined together at a first junction; a control circuit coupled to the switches for rendering said switches alternately conductive and non-conductive; a dim circuit equipped with a dim input and coupled with the control circuit for controlling the light output of a lamp operated by the ballast in dependency of a dim signal present at the dim input; a circuit part coupled to said dim interface and to the mains input terminals for generating the dim signal that is a function of the shape of the low frequency AC signal; and a power feedback circuit. The invention also relates to a compact fluorescent lamp. Such a ballast is known from WO 98/46050, which was published after the priority date of this patent application. A lamp operated by the known ballast can be dimmed by means of a TRIAC dimmer. The TRIAC dimmer interrupts the mains supply voltage at the beginning of each half period during an adjustable time interval. Circuit part I generates a dim signal that depends on the duration of this time interval and via the dim circuit and the control circuit the light output of the lamp is adjusted at a level that corresponds to the amplitude of the dim signal. The power feedback circuit makes sure that a sufficient amount of current is drawn from the supply to keep the TRIAC conductive. The power feedback in the known ballast connects a capacitive voltage divider comprised in the inverter to the input side of the first rectifier. The invention aims to provide further embodiments of a ballast that offer the advantages of the known ballast and are relatively simple and therefore cheap. 2 This is realized in that in a ballast as described in the opening paragraph a first diode is coupled between a first rectifier output terminal and an inverter input terminal and the power feedback circuit connects the first junction and a second junction that is present between the first diode and the first rectifier output terminal. It has been found that a ballast according to the invention can be dimmed over a wide range making use of a TRIAC dimmer, while the ballast is also very simple and therefore relatively cheap. Preferably the ballast includes a second diode coupled between the first diode and the first rectifier output terminal. In ballasts comprising such a second diode, it is unnecessary for part of the diodes comprised in the first rectifier to be relatively fast. In these ballasts the second junction is between said first diode and said second diode. Good results have been obtained for embodiments of a ballast according to the invention including a buffer capacitor connected in parallel with the serial combination of the pair of switches. The feedback circuit may include a capacitor or even consist of a capacitor. In this way the feedback circuit is realized in a simple and dependable way. A ballast according to the invention is very suitable to be used in a compact fluorescent lamp. Compact fluorescent lamps are used on a very large scale to replace incandescent lamps. A compact fluorescent lamp comprising a ballast according to the invention can be dimmed making use of the same dimmers that have been in use in the past to dim incandescent lamps. For a fuller understanding of the invention, reference is had to the following description taken in connection with the accompanying drawings, in which: FIG. 1 is a partial block diagram and partial schematic of a triac dimmable, single stage compact fluorescent lamp in accordance with a first embodiment of the invention together with a triac dimmer; and FIG. 2 is a schematic of the triac dimmer of FIG. 1 together with a compact fluorescent lamp; FIG. 3 is a partial block diagram and partial schematic of a triac dimmable, single stage compact fluorescent lamp in accordance with an alternative embodiment of the invention together with a triac dimmer; 3 FIG. 4 is a partial block diagram and partial schematic of a triac dimmable, single stage compact fluorescent lamp electrically equivalent to FIG. 1 together with a triac dimmer; and FIG. 5 is a partial block diagram and partial schematic of a triac dimmable, single stage compact fluorescent lamp electrically equivalent to FIG. 3 together with a triac dimmer. As shown in FIG. 1, a compact fluorescent lamp (CFL) 10 is powered from an A.C. power line represented by an A.C. source 20. The desired level of illumination by a lamp 50 is set by a triac dimmer 30 and is communicated to a driver 40 via a circuit part I and a dim circuit π. One type of triac dimmer and driver communication is discussed in WO 98/46050. As shown in FIG. 2, triac dimmer 30 is connected across A.C. source 20 through a pair of lines 21 and 22. Triac dimmer 30 includes a capacitor C5 which is charged through the serial combination of an inductor L2 and a variable resistor Vr. A diac DC is connected to the gate of a triac TR. When the voltage on capacitor C5 reaches the voltage breakdown of diac DC, triac TR fires. Current (i.e. latching current of triac TR) is supplied to CFL 10 through inductor L2 and triac TR. At the end of the 60 Hz, l A wave cycle, the level of current in triac TR decreases below its holding current (i.e. minimum anode current necessary to sustain conduction of triac TR). Triac TR turns off. The firing angle, that is, the angle between 0 and 180 degrees at which triac TR first conducts, can be adjusted by changing the resistance of variable resistor Vr. Variable resistor Vr can be, but is not limited to, a potentiometer. The maximum firing angle is limited by the breakdown voltage of diac DC. Inductor L2 limits the rise or fall time of di/dt and thus protects triac TR from a sudden change in current. A capacitor C6 serves as a snubber and prevents flicker. An electromagnetic (EMF) filter formed by the serial combination of a choke LI and capacitors Cl and C2, significantly dampens harmonics (oscillations) generated by an inverter of CFL 10. A junction 31, which joins capacitors Cl and C2 together, is connected to a reference level (e.g. ground). The output of triac dimmer 30 is connected to choke LI. The junction joining inductor LI to capacitor Cl is connected to a first input terminal 33 of a diode bridge. Capacitor C2 is connected to the junction joining A.C. source 20 to a second input terminal 34 of the diode bridge. The diode bridge is formed by four diodes Dl, D2, D3 and D4. The junction joining the anodes of diodes D2 and D4 together is connected to the reference level (e.g. ground). A capacitor C3 connected between the junction joining the anodes of diodes D2 and 4 D4 together and the junction joining the cathodes of diodes Dl and D3 together further dampens harmonics (oscillations) generated by the inverter. The diode bridge rectifies the low frequency, sinusoidal voltage provided by the damped EMI filter resulting in a D.C. voltage with ripple at a pair of output terminals 35 and 36. A buffer capacitor Cb filters so as to make the sinusoidal voltage into a substantially constant D.C. voltage supplied to the inverter. The inverter is configured as a half-bridge and includes a pair of input terminals 65 and 66, a pair of switches (e.g. power MOSFETs) 60 and 70 which are serially connected across buffer capacitor Cb and a DC blocking capacitor C4. Switches 60 and 70 are joined together at a junction 80 and commonly identified as forming a totem pole arrangement. The MOSFETs serving as switches 60 and 70 have a pair of gates Gl and G2, respectively. Driver 40 controls the switching states of switches 60 and 70 and is connected to gates Gl and G2, respectively. Mains input terminal 23 is connected to an input terminal of a circuit part I for generating a dim signal in dependency of the shape the voltage present at the output of the triac dimmer. An output terminal of circuit part I is connected to a dim input terminal of dim circuit II for controlling the light output of a lamp operated by the ballast in dependency of a dim signal that is present at the dim input. An output terminal of dim circuit II is therefore connected to an input terminal of driver 40. A primary winding Lr of a transformer 70 and a capacitor Cr form a resonant circuit. DC blocking capacitor C4 is connected between primary winding Lr and junction 80. Lamp 50 is connected in parallel with capacitor Cr. A pair of secondary windings 76 and 77 of transformer 70 are coupled to primary winding Lr for application of voltages across the filaments of lamp 50 in conditioning the latter during the preheat operation and when operating lamp load 50 at less than full light output. A power feedback circuit formed by a capacitor Cf and fast switching diodes D5 and D6 advantageously permits CFL 10 to operate at a power factor far less than 1.0 (e.g. about 0.7). When the power factor is about 1.0, there is far greater stress on the components within inverter 60 and load 70 than at a lower power factor. The power feedback circuit raises the power factor sufficiently to the minimal level of about 0.7 necessary to sustain conduction of triac 35. The recovery time (i.e. time required to remove charge from the junction of a diode so as to fully turn off the diode) of diodes D5 and D6 can be upto about 100 nanoseconds. In contrast thereto, slow recovery diodes D1-D4 of the diode bridge have recovery times of about 500 nanoseconds. Capacitor Cf is connected between junction 80 and a junction 81 joining the anode of diode D5 to the cathode of diode D6. The anode of diode D6 and the cathode of 5 diode D5 are connected to the reference level (e.g. ground). Capacitor Cf feeds back a high frequency (e.g. 50 kH), square wave signal from the output of the inverter (i.e. from junction 80) to the diode bridge. The high frequency square wave signal applied to capacitor Cf creates a current path sufficient to sustain triac dimmer operation for entire dimming range. In an alternative embodiment of the invention as shown in FIG. 3, diode D5 can be removed with junction 81 connected directly to input terminal 36. To accommodate the necessary switching of the high frequency square wave signal being fed back to the diode bridge, however, diodes D2 and D4 would need to have fast recovery times as discussed above. The electrical equivalent of FIG. 1 is shown in FIG. 4 in which diodes D5 and D6 are in serial combination between output terminal 35 of the diode bridge and input terminal 65 of the inverter. Similarly, the electrical equivalent of FIG. 3 is shown in FIG. 5 in which diode D6 is connected directly to output terminal 35 of the diode bridge and input terminal 65 of the inverter. To accommodate the necessary switching of the high frequency square wave signal being fed back to the diode bridge, however, diodes Dl and D3 would need to have fast recovery times. The cut-in angle of triac dimmer 30 ranges from about 30 to 150 degrees. The value of the capacitor Cf is set to sustain triac dimmer operation for the maximum cut-in angle. The feedback scheme is compatible for use with any conventional triac dimmer in combination with different types of lamps. Such compatibility is provided by the peak to peak feedback voltage applied to capacitor Cf (i.e. of the square wave voltage applied to capacitor Cf) being equal to at least the voltage across buffer capacitor Cb. Consequently, the serial combination of diodes D5 and D6 in FIGS. 1 and 4 and diode D6 in FIGS. 2 and 5 should be connected between the diode bridge and the inverter. Current is drawn through triac dimmer 30 except at the zero voltage crossings of switches 60 and 70. As can now be readily appreciated, the invention permits the buffer capacitor and resonant tank circuit (i.e. primary winding Lr and capacitor Cr) to be significantly reduced in size since there is no need to maintain a power factor of near unity. The power feedback circuit of the invention raises the power factor sufficiently to the minimal level necessary to sustain conduction of triac 35. Of particular importance, current at or above the triac holding current is drawn through triac 30 at low dim levels so as to maintain triac conduction. Unacceptable overboost voltages occurring across capacitor Cb at low dim levels are avoided. Compatibility for use with any conventional triac dimmer in combination with different types of lamps is achieved by the peak to peak feedback voltage applied to capacitor Cf equal to at least the voltage across buffer capacitor Cb. 6 It will thus be seen that the objects set forth above and those made apparent from the preceding description are efficiently attained .

Claims: CLAIMS: 1. A ballast a triac dimmer comprising a triac (30) having a holding current, comprising: a first rectifier (Dl, D2, D3, D4) having output terminals (35, 36) for converting a low frequency AC signal into a unidirectional signal; an inverter (60, 70, C4) having input terminals and including a pair of switches (60, 70) joined together at a first junction (80); a serial combination of a first diode (D5) and a second diode (D6) joined together at a second junction (81), the first diode and second diode being connected to an output terminal of the first rectifier (35, 36) and an input terminal of the inverter (65, 66); and a single feedback circuit (Cf) carrying a high frequency square wave signal produced by the inverter and connected to the first junction and the second junction ; wherein the first and second diodes convert the high frequency square wave signal supplied by the single feedback circuit into a unidirectional signal fed back to an input terminal of the inverter, characterized in that the ballast further includes a triac dimmer comprising a triac having a holding current and in that said single feedback circuit during operation maintains the level of the current drawn from the triac to at least the level of the triac holding current. 2. The ballast of claim 1, further including a buffer capacitor (Cb) connected in parallel with the serial combination of the pair of switches. 3. The ballast of claim 1 , wherein the single feedback circuit includes a capacitor (Cf). 4. The ballast of claim 2, wherein the single feedback circuit consists of a capacitor (Cf). 8 5. The ballast of claim 1 , wherein the ballast and triac are integrated within a compact fluorescent lamp (10). 6. The ballast of claim 2, wherein the ballast and triac are integrated within a compact fluorescent lamp (10). 7. A ballast in combination with a triac (30) having a holding current, comprising: a first rectifier (Dl, D2, D3, D4) having output terminals for converting a low frequency AC signal into a unidirectional signal; an inverter (60, 70, C4) having input terminals and including a pair of switches (60, 70) joined together (80) at a first junction; a diode (D6) connected to an output terminal (35, 36) of the first rectifier and an input terminal (65, 66) of the inverter; and a single feedback circuit (Cf) carrying a high frequency square wave signal produced by the inverter and connected to the first junction and output terminal of the first rectifier for maintaining the level of current drawn from the triac to at least the level of the triac holding current; wherein one of either the diode and rectifier converts the high frequency square wave signal supplied by the single feedback circuit into a unidirectional signal fed back to an input terminal of the inverter. 8. The ballast of claim 7, further including a buffer capacitor (Cb) connected in parallel with the serial combination of the pair of switches. 9. The ballast of claim 7, wherein the single feedback circuit includes a capacitor (Cf). 10. The ballast of claim 8, wherein the single feedback circuit consists of a capacitor (Cf). 11. The ballast of claim 7, wherein the ballast and triac are integrated within a compact fluorescent lamp (10). 9 12. The ballast of claim 8, wherein the ballast and triac are integrated within a compact fluorescent lamp (10).

Inventors: JANCZAK JERZY, JANCZAK, JERZY
Applicants: KONINKL PHILIPS ELECTRONICS NV, PHILIPS SVENSKA AB, KONINKLIJKE PHILIPS ELECTRONICS N.V., PHILIPS AB
Application year: 1999
Main classification: H05B41
IPC codes: H05B41