BALLAST CIRCUIT

Abstract: A lighting control circuit that controls the lighting of particular lamps in response to the toggling of the power switch. The circuit a) connects only with the high (output) side of a lighting system's ballast, b) is completely contained on the high side, and c) with regard to toggling, is dependent upon only a single time period. The circuit can be used with any ballast which makes use of an output tranformer and no change need be made to the original ballast circuitry. Users will find operation of the circuit to be straightforward. A triac driven by a flip-flop via a driver transistor is used to control the high frequency AC power that is used to drive the lamps. A Schmitt trigger sharpens the signal generated by the ballast output transformer in response to the toggling of the light switch which is employed to change the output state of the flip-flop. Operationally, all the lamps driven by the ballast are lit when the power switch is initially turned on. Toggling the power switch once while all of the lamps are lit causes only a predetermined number of the lamps to remain lit. Toggling the power switch while only a portion of the lamps are lit causes all of the lamps to light again. Leaving the power switch off causes all of the lamps to be turned off. The toggling may be performed quickly or leisurely, so long as the entire toggle cycle is completed within a predetermined amount of time.

Description: Ballast circuit. The invention relates to a ballast circuit for operating a lamp comprising ballast means for generating a high frequency lamp current out of a mains supply voltage, control means for controlling the power supplied to the lamp by the ballast means in response to an interruption of the mains supply voltage. Such a ballast circuit is known from GB 2151115 A. In the known ballast circuit the control means inhibit or enable the operation of the ballast circuit in response to an interruption of the mains supply voltage. Switching lamps on and off by interrupting the mains supply voltage is also called the "toggle method". A disadvantage of the known ballast circuit is that when several lamps are operated in parallel by means of the same ballast circuit, all these lamps are either on or off and it is impossible to operate only part of the lamps. The invention aims to overcome this disadvantage and provide a more versatile ballast circuit. A ballast circuit as mentioned in the opening paragraph is therefore characterized in that the control means comprises a switching element that during operation is in series arrangement with the lamp and a control circuit for changing the conductive state of the switching element in response to an interruption of the mains supply voltage. In case the ballast circuit according to the invention is operating only one lamp, this lamp can be switched on and off by means of interruptions of the mains supply. In that way the invention can be used for instance in a room wherein part of the installed ballast circuits are ballast circuits according to the present invention and the remaining part of the ballast circuits is not equipped with control means controlling lamp operation in response to an interruption of the mains supply voltage. Interruption of the mains supply leads to part of the lamps being switched of while the remaining part remains lit. In case, however, the ballast circuit according to the invention is operating a number of lamps in parallel and said switching element is in series arrangement with only part of said number of lamps during lamp operation, interruptions of the mains supply voltage will result in said part of the lamp being switched on and off. If for instance only one ballast circuit is used to operate all the lamps in a room, it is possible to switch part of these lamps on and off using the main switch. Important advantages of the invention are thus that one wall switch can control multiple ballasts and/or multiple lamps and no extra wire or extra switches are required in the installation of ballast circuits according to the invention. Thus the invention provides a low cost solution for light intensity control. Good results have been obtained for ballast circuits according to the invention wherein the switching element is a triac. Preferably the control circuit comprises a flipflop, a transistor (preferably a metal oxide field effect transistor), and a Schmitt trigger. Preferably the control circuit changes the conductive state of the switching element only when the interruption of the mains supply voltage is shorter than a predetermined time interval. When the predetermined time interval is long enough, e.g. 5 seconds the toggling may be performed quickly or leisurely, so long as the entire toggle cycle is completed within a predetermined amount of time. Preferably also, the control circuit comprises reset means for rendering the switching element conductive when the interruption of the mains voltage is longer than said predetermined time interval. When the lamps are first switched on after having been extinguished for longer than said predetermined time interval, all the lamps are lit. The invention will be further explained making use of a drawing. In the drawing: FIG. 1 shows a block diagram of a lighting system which includes an exemplary embodiment of the invention; FIG 2 shows an exemplary embodiment of the invention for a four lamp instant start electronic ballast; FIGs. 3,4, and 5 show how to employ a flip-flop to construct a Schmitt trigger, in accordance with an aspect of the invention; and FIG. 6 shows a modified version of the FIG. 2 embodiment of the invention which may be used to insure that a 50% input power reduction will result when half of the lamps are off. FIG. I shows a block diagram of a lighting system which includes an exemplary embodiment of the invention. As shown, wall switch SI controls multiple ballasts B1...BN. In accordance with the principles of the invention, the output of ballast Bl is coupled as an input to each of power switch PSI and control unit CU1. Control unit CU1 determines how many of lamps L1...L4 should be lit as a function of the operation of wall switch SI. Power switch PSI causes the number of lamps determined by control unit CU1 to be lit in response to commands from control unit CU1 and the presence or absence of lamp drive power at the output of ballast Bl. Each ballast and lamp set may be independently controlled by their own control unit and power switch (not shown). In accordance with an aspect of the invention, each control unit and power switch may control which of their lamps are lit independent of any other control units or power switch units, even ones that are connected to the same wall switch. FIG 2 shows an exemplary embodiment of the invention for a four lamp instant start electronic ballast. In this embodiment, lamps LI and L2 are driven by ballast output transformer T21 of ballast Bl via capacitors C1OA and C1OB. Thus, the lighting state of lamps LI and L2 corresponds directly to the output presence of lamp drive power at the of ballast output transformer T21. However, in accordance with an aspect of the invention, the lighting of lamps L3 and L4 is controlled by triac TH101 in conjunction with the output of ballast transformer T21. When triac TH101 is on in the presence of an output voltage supplied by ballast output transformer T21, lamps L3 and L4 are lit. Otherwise, lamps L3 and L4 are off. Note that ballast output transformer T21 has two secondary windings. In more detail, diode D103 and capacitor C104 provide a direct current (DC) voltage for driving triac TH101. Resistor R105 limits the triac drive current. Metal oxide semiconductor field effect transistor (MOSFET) QlOl controls the trigger input of triac TH101. When the gate of MOSFET QlOl has a high voltage supplied as an input thereto, MOSFET QlOl turns on. This, in turn, causes triac TH101 to be turned on as well, resulting in ignition of lamps L3 and L4. When the voltage supply to the gate of MOSFET QlOl is zero, MOSFET QlOl is off, as are triac TH101 and lamps L3 and L4. Thus, the voltage level at the gate of MOSFET QlOl controls the lighting of lamps L3 and L4. MOSFET QlOl is driven, for example, by flip-flop IC1-B, which is half of dual D flip-flop IC1. A dual D flip-flop suitable for use as IC1 is the MC14013. Diode D102 and capacitor C102 provide a DC power supply for dual D flip-flop IC1. Capacitor C103 and resistor R104 provide a narrow pulse which sets flip-flop ICl-B's Q output to high when the DC power supply is ramping up. Since the Q output of flip-flop ICl-B controls MOSFET QlOl, and hence triac TH101, all 4 lamps will turn on when the main power turns on and prior thereto there was insufficient DC power to operate ICl. Advantageously, to drive a MOSFET requires almost no current. Likewise, an MC14013 dual D flip-flop chip, since it is a CMOS integrated circuit, consumes very little current. Thus, the power supply for ICl can sustain itself for a certain amount of time, which mainly is a function of the values of capacitor C102 and resistor R103. The values of capacitor C102 and resistor R103 are selected, for example, such that sufficient DC power is supplied to operate ICl for approximately 5 seconds after the ballast input power is turned off. This means that ICl can perform its normal functions within a 5 second window after the loss of power at the output of ballast transformer T21, which occurs when switch SI is toggled. Since ICl is operable for 5 seconds after power at the output of ballast transformer T21 is turned off, the status of ballast output transformer T21 can be used as the clock signal to drive D flip-flop ICl-B. For example, no output from transformer T21 means a logic "0" and an output from transformer T21 represents a logic " 1". If wall switch SI is turned off and then turned on within 5 seconds, D flip-flop ICl-B will change its output status once, which occurs at the transition from "0" to "1". Doing so causes the on/off status of triac TH101 and lamps L3 and L4 to change. Although using a triac to control alternating current (AC) devices is known in the art, such use is limited to only low frequency applications, e.g., where the AC power frequency is lower than 400Hz. This is because, as is known in the art, a triac controlling high frequency AC power may not operate as desired. For instance, a triac is supposed to turn off automatically when the AC current being controlled by the triac, namely, the AC current through the triac, crosses zero and no trigger signal, which is the control signal for a triac, is present. However, a triac that is controlling high frequency AC power may not do so. Instead, once a triac controlling high frequency AC power turns on, it may stay on when the current which is passing through, and being controlled by, the triac crosses zero and there is no trigger signal, even though it is not supposed to. Such undesired triac operation is known as "commutation failure". Commutation failure occurs when the reverse recovery current, due to unrecombined charge carriers of one of the thyristors in the triac as it turns off, acts as a gate current to trigger the other thyristor in the triac into conduction as the voltage rises in the opposite direction. The probability of any triac undergoing commutation failure is dependent on the rate of rise of the reverse voltage (dV/dt) and the rate of decrease of conduction current (dl/dt). The higher the dl/dt, the more unrecombined charge carriers that are left at the instant of turn-off. The higher the dV/dt, the more probable it is that some of these charge carriers will act as a gate current to trigger the triac into conducting. Thus, the commutation capability of a triac, i.e., the limits up to which the triac can be operated before commutation failure will occur, is usually specified in terms of the turn off dl/dt and the re-applied dV/dt that the triac can withstand at any particular junction temperature. For use in controlling the current to lamps L3 and L4 according to the invention, (dl/dt) c = 80 A/mS and (dV/dt) c = 170 V/uS, where c indicates commutation. But for conventional triacs, even ones such as the MAC8N, available from Philips Semiconductors, which are designed to have a high commutation capability, the commutation capability is specified as being only (dI/dt)Dc = 6.5 A/mS and 20 (dV/dt) c = 18V/uS. Clearly, such a commutation capability is insufficient to prevent commutation failure when the triac is used under the conditions which are required in order to control the current to lamps L3 and L4, and one would not expect such a triac to operate properly under such circumstances. The foregoing notwithstanding, in accordance with a principle of the invention, the frequency of the AC power being controlled by triac TH101, namely the output from ballast output transformer T21, is greater than 400 Hz, e.g., 20 KHz or more, and without requiring a snubber network. Indeed, we have recognized that, unlike other prior art triac applications, the undesirable triac behaviour which results from commutation failure is not a problem when a triac is used for lamp control according to the invention. This is because, after the triac is turned on, the triac never has to turn off before the AC power it is controlling is turned off at another point by some other control, e.g. , a switch at a different location. In other words, when the main power to the ballast is turned off, e.g., upon any opening of wall switch SI (FIG. 1). - either to keep all the lamps off or as part of a toggle—, the output of ballast output transformer T21, which is supplying the power being controlled, becomes zero. This in turn causes triac TH101, and hence lamps L3 and L4, to turn off, because there is no longer any current available to pass through the triac. In the case of a toggle, since the triac turned off in response to the wall switch opening, when the wall switch is closed again —thus causing the trigger signal to be removed and high frequency AC power to reappear at the output of ballast output transformer T21— , the triac need merely stay off in the presence of the AC power to keep lamps L3 and L4 off. As such, in accordance with an aspect of the invention, at the high AC power frequency the triac 7 employed need meet only the off-state dV/dt specification. Conventionally, the voltage across the triac is around 600 V,^. As such, it is well below a conventional voltage rating for a triac, which is around 800 V,^. Nevertheless, fast recovery diodes D105 and D 106 are employed to protect triac TH101 against any transient voltage spikes that exceed its rated voltage. Such transient voltage spikes may occur during the turn on stage of ballast Bl. When ICl is implemented as an MC14013, its clock input has a special requirement namely the rise and fall times of the clock input should not exceed 15 microseconds when the DC power supply voltage is 5 volts. Otherwise, flip-flop ICl-B may not operate properly. Unfortunately, the signal from transformer T21, which one would desire to use as the clock input signal, does not meet this requirement. Therefore, its waveform must be cleaned prior to being supplied to the clock input of ICl-B. A conventional method of cleaning a slow signal is to use a Schmitt trigger integrated circuit, such as a 74HC14. The threshold of the Schmitt trigger is employed to guarantee a clean, sharp output waveform. However, to make use of such a Schmitt trigger integrated circuit would require that the system include a second integrated circuit, which would increase the system's cost. Instead of doing so, in accordance with an aspect of the invention, since the MC14103 has two D flip-flops in one package, the other, previously unused D flip-flop of the MCI 4013 is configured to operate as a Schmitt trigger. How this is achieved is shown in FIGs. 3, 4, and 5. FIG. 3 shows the internal configuration of an MC14013. Between Pins 4 and 2 is NOR gate 301 and inverter 303. If the other input, i.e., the one not connected to Pin 4, of NOR gate 301 is held at a logic "0", NOR gate 301 acts as an inverter for the signal supplied to Pin 4. The resulting equivalent circuit of coupled inverters is shown in FIG. 4. Also shown in FIG. 4 are 2 resistors, RA and RB, which are added between Pin 2 and Pin 4 to create a circuit which functions as a Schmitt trigger. The input/output characteristic of the resulting Schmitt trigger circuit is shown in FIG 5. Note that R106 of FIG. 2 corresponds to RA of FIG. 5 and that R107 of FIG. 2 corresponds to RB OF FIG. 5. The output signal of ballast transformer T21 , which is equivalent to the status of wall switch SI (Fig. 1), is rectified by diode DlOl and filtered by capacitor ClOl prior to being supplied to the Schmitt trigger input. The output of the Schmitt trigger is supplied to the clock input of D flip-flop ICl-B. Conventionally, the output of a ballast transformer is not an ideal voltage source. When the output load is heavy, the output voltage will drop. Thus, in the embodiment of the invention shown in FIG. 2, the light output of lamps LI and L2 will increase if lamps L3 and L4 are turned off. This means that the main power which is input to the ballast may not be reduced by 50% when half of the lamps are off. To be certain that a 50% input power reduction will result when half of the lamps are off, a modified version of the FIG. 2 embodiment of the invention may be used. Such a modified embodiment of the invention is shown in FIG. 6. In particular, triac TH102 and capacitor C101E are added to the Fig. 2 embodiment of the invention. As with triac TH101, triac TH102 is also controlled by MOSFET QlOl, so that triacs TH101 and TH102 both turn on or off at the same time. To give each of triacs TH101 and TH102 substantially equal trigger currents, resistor R105 of FIG. 2 is divided into resistors R105A and R105B of FIG. 6. Operationally, when triacs TH101 and TH102 are on, capacitor CIOE is shorted and each of lamps LI, L2, L3 and L4 have substantially the same drive voltage. When triacs TH101 and TH102 are off, lamps L3 and L4 are both off and capacitor CIOE is connected in series with capacitors CIOA and CIOB. Careful selection of the value of CIOE will meet the 50% power reduction requirement. For a rapid start ballast, the configuration of FIG. 6 can be simplified by a) removing resistor R1O5B, b) removing triac TH101 (short THlOl 's anode and cathode), and c) selecting a proper value for capacitor C10E. Advantageously, all 4 lamps can be dimmed to a desired lower level. The four lamps are fully lighted when TH102 turns on, otherwise the 4 lamps are dimmed to a desired lower level because of current limiting by C10E when TH102 turns off. Table I is a listing of exemplary components that can be used to implement the invention. The components are listed in association with their reference identifier. PC17IB97/00467 By applying the principles of the invention and employing additional logic circuitry, e.g., counters, gates, and the like, as well as additional triacs and drive transistors, those of ordinary skill in the art will recognize how to create a lamp control circuit for connection to a single ballast which displays, as the power switch is toggled, a sequence of lamp lighting patterns on the multiple lamps driven by the ballast. Also, several ballasts that are connected to a single power switch may have additional logic in their lamp control circuits according to the invention so that the circuits are programmable, e.g, using one or more jumpers in each circuit, as to their individual lamp lighting pattern sequence. Consequently, as the power switch is toggled multiple times an overall sequence of lamp lighting patterns results. This sequence is changeable by changing the programming of one or more of the lamp control circuits. In one such embodiment, upon each completed toggle the number of toggles that have taken place is counted by the circuit of each ballast, e.g., on a modulo basis, and then each circuit makes an individualized determination, as a function of the number of toggles and its jumper settings, regarding which of its lamps it lights. The foregoing merely illustrates the principles of the invention. It will thus be appreciated that those skilled in the art will be able to devise various arrangements which, although not explicitly described or shown herein, embody the principles of the invention and are thus within its spirit and scope.

Claims: CLAIMS: 1. Ballast circuit for operating a lamp comprising ballast means for generating a high frequency lamp current out of a mains supply voltage, control means for controlling the power supplied to the lamp by the ballast means in response to an interruption of the mains supply voltage. 2. Ballast circuit according to claim 1 , wherein the ballast circuit is suitable for operating a number of lamps in parallel and wherein said switching element is in series arrangement with only part of said number of lamps during lamp operation. 3. Ballast circuit according to claim 1 or 2, wherein said control circuit comprises a flip-flop. 4. Ballast circuit according to claim 1, 2 or 3, wherein the control circuit comprises a transistor. 5. Ballast circuit according to claim 4, wherein the transistor is a metal oxide field effect transistor. 6. Ballast circuit according to one or more of the previous claims, wherein the control circuit changes the conductive state of the switching element only when the interruption of the mains supply voltage is shorter than a predetermined time interval. 7. Ballast circuit according to claim 6, wherein the control circuit comprises reset means for rendering the switching element conductive when the interruption of the mains voltage is longer than said predetermined time interval. 8. Ballast circuit according to one or more of the previous claims, wherein the control circuit comprises a Schmitt trigger. 9. Ballast circuit according to one or more of the previous claims, wherein the switching element is a triac.

Inventors: CHOU WEI FAN JOHN, LI QUN RICHARD, SIEPKES ROLAND, XIA YONGPING, CHOU, WEI, FAN, JOHN, LI, QUN, RICHARD, SIEPKES, ROLAND, XIA, YONGPING
Applicants: PHILIPS ELECTRONICS NV, PHILIPS NORDEN AB, PHILIPS ELECTRONICS N.V.
Application year: 1997
Main classification: H05B41
IPC codes: H05B37, H05B41