CA1265576A - Transient control circuit for fluorescent lamp systems - Google Patents

Abstract

Abstract of the Invention A method and apparatus are provided for operating a fluo-rescent lamp illumination control system which comprises an AC
voltage source for supplying power to an electrical load com-prising a standard transformer-ballast unit driving a fluore-scent lamp or lamps which have externally heated cathodes, and a power controller, which includes a capacitive synchronous switch formed by an electronic switch and shunt capacitor, for controlling the "on" time of the lamp or lamps to thereby vary the luminance output thereof to values less than the nominal rated value. The invention provides heating of the lamp cath-odes prior to arc ignition, provides arc ignition at a lower arc current level than that for full-on operation, and provides for subsequently gradually increasing the arc current after the arc is struck to a value providing the desired illumination level.

Description

~2~5~6 T~NSIENT CONTROL CIRCUIT FO~
~LUO~ESCENT LAMP SYSTEMS
Field of the Invention The present invention relates to control systems or fluorescent lamps and, more particularly, to an improved start-ing method and system for such lamps which reduces the effect of transients and extends the life of the lamp cathodes.
Background of the Invention It is generally recognized by those skilled in the art of electrical dimming control systems for fluorescent lamps that the externally heated cathodes of rapid start type fluorescent lamps must be heated to a temperature that permits the required level of thermionic electron emission to be achieved. For this reason, such fluorescent lamp dimming control systems usually provide for initially turning the lamps "full-on~ so that the rated arc current flows, before dimming, i.e., reduction of the arc current, is undertaken. Such full-on ignition of the lamps is generally accomplished by applying full rated line voltage to the standard transformer-ballast u~ually employed as the lamp driver. This approach is described, for example, in U.S.
patent 3,350,935 (see column 12 lines 27-35) and ~.S. patent 3,352,045 (see column 7 starting at line 5). When full rated A.C. line voltage is applied to the ballast driving the ~luor-escent lamp load, the cathode heating voltaye a3 well as the necessary arc striking voltage appear at the lamp electrodes at the speciiied nominal magnitudes. After a short heating peri-od, the cathode begins to emit electrons, and the arc thereaf-ter i~nites and extinguishes one or more times before the cath-ode reaches the temperature at which the thermionic emission provided is capable of sustaining the arc at the rated cur-.

~26~ 6 rent. This initial arc-on/arc-off operation causes the cathode to ~sputter~ which substantially contrlbutes to cathode wear.
The term "sputtering~ as used here refers to the actual physical emission or giving off of cathode material from the remainder of the cathode caused when arc current flows to the c~hode prior to the temperature of the cathode reaching a value which insures sufficient electron emission. ThuS, the cathode is, in effect, operating in a temperature-limited mode rather than in a space-charge-limited mode as intended.
Cathode wear is the primary determinant of the life of a fluorescent lamp because when the cathode is finally consumed, insufficient emission electrons are available to ignite or main-tain the arc. Nevertheless, this ignition wear phenomena is accepted in the prior art. The lamp manufacturing industry 15, generally rates a standard 40 watt lamp as having a 20,000 hour Mean Time Between Failure (MTBF) life based on a test cycle of three hours "on" and twenty minutes "off". It is also well known that lamp operating life will be e~tended when longer "on" peri-ods are provided between the starting events which cause the cathode wear.

This invention concerns a novel apparatus for providlng efficient, long-life operation of the class of fluorescent lamp control system based on the power control techniques disclosed ln my U.S. patent 4,352,045, issued on September 28, 1982, and Cana-dian patent application Serial No. 480,648 filed on May 2, 1985.
In partlcular, the invention is applicable to systems which com-prise an ~.C. voltage source for supplylng pow0r to an ele~trlcal load devlce comprlsing a transform~r-ballust ~ 2~;S~
driving a ~luorescent lamp or lamps having externally heated cathodes, and which use the power control methodology disclosed in the above-identified patent and patent application. Al-though reference is made to the patent an~ patent application ~or a more complete description of this rnethodology, a key ele-ment thereo~ concerns the control of a capacitive synchronous switch, i .e., a synchronously operated switch such as a tran-sistor having a capacitor connected in shunt thereacross.
~ n object of the invention is to reduce the cathode wear discussed above and thus extend lamp life, as well as reduce any deterministic or probabilistic excursions of electric cir-cuit variables which exceed the normal steady state values of system components due to changes in the operating state of the system, e.g., excursions (transients) produced by s~itching of the branch circuit used to implement the A.C. voltage source.
The advantages provided by the invention include a longer oper-ating li~e for the lamps or lamps used and/or for other system components, a more efficient system operation, and an ability to employ relatively low cost semiconductor devlces in the im-plementation of the power controller. In addition, there are energy savings provided by the ~dimrning-up" operation provided by the system of the invention wherein the illumination pro-duced is gradually brought Up to the desired level, as con-trasted with pri.or ark ~ys~eln~ which ~rovide kllLI-or) Lr~i~ial operatioll and then provide dilnlnlny dOWtl to th~ desired level.
It is noted that this latter mode o dimrnlng or visual pur-poses even has negative psycholoyical e~ects, whicll are elilni-nated with the system of the inventlon.
In accordance with a preerred embodirnent o~ the inven-tion, a system is provided or controlling the A.C. power sup-_3_ r ~
~ :6557~~lied fLoln an ~.C. source to an electrical loa~ comprising at least one transformer ballast and at least one fluorescent lamp deiven by the transformer ballast and including externally heated cathodes, the system including a switch connected be-tween the A.C. source and the load, a power controller for con-trolling swltching of the switch in timed relation to the A.C.
source voltage wave so as to control the power supplied to the load, a capacitor connected in shunt across the switch, and control means, connected to the power controller system, for, responsive to energization of the system, controlling the switching operation of the switch provided by said power con-troller so as to initially limit the arc current supplied to the load and thereby provide for ignition of the arc of the at least one fluorescent lamp at an arc current level less than that provided during full on operating condltions while also providing heating of the externally heated cathodes prior to the ignltion of the arc, and so as to thereafter provide gradu-ally increasing arc current up to a predetermined value which produces the de.sired illumination level.
In an exemplary embodiment, the power controller com-~rises a control circuit producing a square wave output for controlling switching of the switch and the control means ini-tially inhibits the square wave output o the control circuit and thereater controls the duration o~ the ~uare wave pulses produced by the control circuit so as to provide a gradual in-crease in the duration of these pulses with time. Advanta-geously, the control circuit includes an operational amplifier and the control means comprises means for supplying a gradually increasing voltage to one input o the operational amplifier.
In a specific preferred embodiment, the voltage supplying means _4_ ~265,~7~i comprises a resistor-capacitor circuit and the gradually increas-ing voltage is produced by charging of the capacitor of the resistor-capacitor circuit.

other features and advantages of the present invention will be set forth, or apparent from, the detailed description of the preferred embodiments which follows.

The present inventlon will be further illustrated by way of the accompanying drawings, in which:-Figure 1 is a block diagram of the basic system inwhich the present invention is incorporated;

Figure 2 is a schematic representation of the waveforms associated of the operation of the inventlon; and Figure 3 ls a schematlc circuit diagram of a lighting control system incorporating the invention.

Referring to Figure 1, which ls a schematic block dia-gram similar to that in my copending Canadian application No.
480,648, there is shown the baslc units or components of a system of the general type to which the invention is applicable. The system of Figure 1 includes a power source which is implemented by a branch circuit A.C. voltage source 10 and a branch circuit switchlng devlce 12, a two-port ~lnput and output) power con-troller 14 and an electrical load 16. The controller 14 requires three wires, with the common wire being either the "hot" or the neutral wire of the branch circuit.
As indlcated ln Figure 1, the electrical load comprises a transformer ballast 18 and a fluorescent lamp 20 havlng a cath_ ode heater lndlcated at 22. The primary winding 18a of the transformer ballast 18 is coupled to a low voltage wlnding 18a , ~ ,~
:
, ~265576 which provides the current necessary to externally heat the elec trodes of lamp 20. It will be appreciated that these elec-~ 2~55~6;rodes operate alternately as cathodes and anodes at the line fre~uency of the A.C. voltage source lO(usually 60Hz in the United States), and that the heater pins of these electrodes are represented schematically by cathode heater 22. It will also be understood that the showing in Figure 1 is highly sche-.
matic and that the transformer ballast secondary winding 18c is connected in a conventional manner to the lamp load. Further, a plurality of transformer ballasts and lamps can be obviously employed.
As shown, power controller 14 comprise~ a switch 24 hav-ing a capacitor 26 connected in shunt thereacross and a syn-chronous switch control (SSC) circuit 28 for controlling switching of switch 24. ;For shorthand purposes switch 24 and capacitor 26 will be referred to collectively as a capacitive synchronous switch (CSS) which is denoted 30.
An important purpose of the invention is to supply at least a minimum heater voltage, denoted Vh, to the cathode heater pins 22 of lamp 20 which is sufficient to provide ex-ternal heating thereof to a design temperature which provides for the level of thermionic emission required for long lamp lie as discussed above. To this end, the CSS 30 is operated under the control of SSC 28 to maintain the RMS (heating) value of the heater voltage Vh above the minimum required to pro-vide long lamp lie throughout all operating ~ates o CSS 30 rorn full "oE" (i.e, the Gwitch open condition) where capacl-tor 26 is connected in serieq with the prirnary winding 18a of transformer ballast 18 to full "on" (the switch closed condi-.tion) whereill the Eull line voltage VAc is applied to pr.irnary winding 18a. It is noted that.for the full ~off~ state referr-ed to above, the RMS voltage applied to the transformer-ballast~

~6~i7~
:imary winding 18a would be near the rated ~alue and this re-quires selecting an appropriate value for capacitor 26 of CSS
30. Typically, a capacitive value of 3 microfarads is useful with a standard 120 volt, 0.8 ampere high power factor trans former-ballast driving two standaed F40 type, 4U watt rapid start fluorescent lamps. The value of capacitor 26 can be de-termined empirically by ad~ing'series capacitance to the bal-last primary 18a until the RMS voltage across the primary wind-ing 18a approaches that of thè A.C. line or the voltage at the cathode heater 22 approaches a nominal 4.0 volts without firing of the lamp arc, this value dropping towards 3.0 volts with lamp loading.
' A characteristic of the power control methodology dis-closed in my previous applications is that switching from the full ~off" state to full ~on" state within a half cycle of the line voltage produces a large transient line current~ ~his is the consequence of the inability of the ferromagnetlc core of the transformer ballast 18 to readily accomrnodate the sudden polarity or phase reversal produced by this off-on switchin~.
Further, if, in addition, there is asynchronous operation, such as is the case during initial turn-on, there will be additional stressing or burdening o the semiconductor device or devices represented by switch 24. These effects canno~ be avoided and thus the conse~uences ~hereo~ must b~ ~imited or eliminated.
A further property or characteristic of the power'control method with which the invention is concerned is that a step change itl the state o the CSS 30 requires a finite number of power line cycles before the resultant line current ~ransient caused by this change subsides to zero and before the'line cur-rent reaches the new steady state value thereof. The minimim '':

, ' . ,:,',; . .

~2~5~76 time constant of the lag represented by this finite number of cycles is dependen~ upon the parasitic resistance and induc-tance of the'ballast transformer 18 when thefcore material is at or near the saturated flux state thereof. The rnechanism providing the decay of the transients is the asymmetries in the positive and negative instantaneous line current waveforms dur-ing a half cycle o~ the operation of CSS 30 acting with the aforementioned parasitics to bring the circuit operating state to the new symmetrical A.C. (Vdc=O) steady state value.
The present invention is concerned with providing a con-tinuous, gradual change in the switching time between the ~ull off and on states of the CSS 30'in a manner such that the transient line currentsjproduced by the polarity'(or phase) re-versals from half cycle to half cycle are limited to a prede-termined value below that which could be harmful to the semi-conductor device(s) used to implement switch 26 of CSS 30. The nvention provides for gradually increasing the 80n" time of the switch 24 until a level is reached where the larnps fire, while providing a prior voltaye which is always sufficient to provide full heating of the lamp cathodes, thereby ameliorating the effects of the current transients and asynchronous opera-tion, while peoviding the require~ cathode heating. This ap-proach preserves the fundam~ntal op~ratincJ charac~eri~ticY o the power con~rol techni~lues oE my earli~r applLcation ~nd pa-tent while providing lamp catllode heating at or above the re-quired minimum for all operating states, i.e~, for both transi-ent (upon starting) and,steady state operation. This mode of operation provided by the invention is indicated in a highly schematic manner in Figure 2 in which the output with time of the SSC circuit 28 used in controlling switch 24 is shown as ~.
.

--8-- , ~26SS76 increasing gradually from a zero value at an initial time ~To) to a value at which the lamps fire ~TF) and thereafter to a desired operating value tTD). It should be noted that Figure 2 is highly schematic and a large number of cycles would normally occur before the arc is struck.
Referring to figure 3, a schematic circuit diagram of a light control system incorporating the lnve~ti~n is illustrated.
The circuit shown is basically very similar to that disclosed in my U.S. Patent No. 4,352,045 and my copending Canadian applica-lo tion No. 480,648, and the following description thereof will belargely limited to the portions of the clrcuit used in implemen-ting the invention. The CSS 30 of Figure 1 is baslcally cons-tituted by transistors Q4 and Q5 and the dlode bridge formed by diodes D13, D14, D15 and D16 (corresponding to switch 24 of Fig-ure 1), and capacitor C8 ~corresponding to capacitor 26 of Figure1). It is also noted that detection of the voltage on the switch formed by transistor Q4 Q5 and the diodes, used in inhibiting closing of the switch by control circuit as provided for in Cana-dian Serial No. 480,148 is implemented in this embodiment by the connection to the diode bridge which includes resistors Rll, Rs and R10 and a Zener diode Z2 connected in shunt with resistor R10.

In order to effect the aforementloned slow turn-on of the synchronous switch formed by transistors Q4 and Q5 and the full wave brldge dlodes, a resistor-capacitor network, comprising a series resistor ~7 and a shunt capacitor C3, is connected to the lnput of an operatlonal ampllfler ~1 of the power controller so as to lnhlblt the s~uare wave output of the operatlonal ampli-fier Ql durlng the time after th0 inltial energlzation of thesystem that ls requlred for capacitor C3 to charge to the _ g _ m ~-~
~265576 steady state level thereof. (It is noted that dual operational amplifiers Ql are employed in this speciflc embodiment and re-ference will be made to the first operational amplifier of the dual in the discussion which follows). Initially, capacitor C3 will provide a short circuit, thereby holding the base of oper-ational amplifler Ql to zero volts, and as capacitor C3 charges, operational amplifier Ql will begin produce a time limited square wave output, the duration of which gradually in-creases as discussed above. As explained previously, and is shown schematically in Figure 2, after operational amplifier Ql first produces a s~uare wave output, the duration of the square wave will gradually increase with time until the voltage pro-duced is such as to provide ignition of the arc and to esta-blish equilibrium. This time period from initial energization to arc ignition is typically one or more seconds.
The invention has been described above relative to the application thereof to rapid start fluorescent lamps, but it is to be understood that the invention is also useful in connec-tion with other Eluorescent lamps such as so-called ~preheatn lamps, and that the transient amelioration and upward dimming features of the invention have application to even instant start i-lUoresCen~ lamps.
Although the inventioll has been de~cribe~ relatlve to ex-emplary embodiments thereof, it will be understood by those skilled in the aet that variations and modi~ications can be ef-Eected in the exemplary embodiments without departing from the scope and spirit of the invention.

' ' . :,,

Claims (4)

The embodiments of the invention in which an exclusive ?operty or privilege is claimed are defined as follows:

1. A system for controlling the A.C. power supplied from an A.C. source to an electrical load comprising at least one transformer ballast and at least one fluorescent lamp driven by said transformer ballast and including externally heated cath-odes, said system including a switch connected between the A.C.
source and the load; a power controller for controlling switch-ing of said switch in timed relation to the A.C. source voltage wave so as to control the power supplied to the load, a capaci-tor connected in shunt across said switch, and control means, connected to said power controller, for, responsive to energi-zation of the system, controlling the switching operation of the switch provided by said power controller so as to initially limit the arc current supplied to the load and thereby provide for ignition of the arc of the at least one fluorescent lamp at an arc current level less than that provided during full on operating conditions while still providing heating of said ex-ternally heated cathodes prior to the ignition of said arc and to thereafter provide gradually increasing arc current up to a predetermined value which produces the desired illumination level.

2. A system as claimed in Claim 1 wherein said power con-troller comprises a control circuit for producing a square wave output for controlling switching of said switch and said con-trol means initially inhibits the square wave output of said control circuit and thereafter controls the duration of the square wave pulses produced by said control circuit so as to provide a gradual increase in the duration of said pulses with time.

. A system as claimed in Claim 2 wherein said control cir-cuit includes an operational amplifier and said control means comprises means for supplying a gradually increasing voltage to one input of said operational amplifier.

4. A system as claimed in Claim 3 wherein said voltage sup-plying means comprises a resistor-capacitor circuit and said gradually increasing voltage is produced by charging of the capacitor of the resistor-capacitor circuit.