CA2025988A1 - Solid state electronic ballast - Google Patents

Abstract

ABSTRACT

The electronic ballast for a fluorescent lamp replaces the three main components of a traditional fluorescent lamp ballast, namely th ferro-magnetic ballast, the power factor condenser, the starter and starter holder.
The solid state electronic ballast of the invention comprises a power supply conversion circuit, a start-up circuit and a switching resonant circuit. The power supply conversion circuit consists of a common mode line filter connected with a rectifier bridge circuit and a low harmonic filter. Connected in series follows the start-up circuit with a sawtooth oscillator, a diac trigger and a first transistor. A second transistor forms together with an inductor, a transformer, and a lamp-load the resonant-circuit.

The electronic ballast of the invention safes costs, energy and weight.
(Fig. 7)

Description

202~9gg`

S~ngap~re Solid State Ele~tron1c 9allast . .

Backaround and Summary of the Invention Since their i ntroduction in 1938, fluorescent lamps have largely replaced incandescent lamps as the preferred lighting system for public build~ngs such as schools, offices, factories etc as well as in ~any homes. Since the early 1 g50's, sales of fluorescent lamps have overtaken that of incandescent lamps in most industrlallzed countries.

A fluore~cent lamp consists of a glass tube containing a small amount of mercury and a chemically inert gas at low pressure, usually argon or krypton. Each end of the tube has an electrode which ~s a co11 of tungsten wire with a coat;ng of rare earth oxide. A ferro-magnetic ballast, power factor condenser and starter together with the metal housing, two tube holders, one starter holder an~ wiring system complete the lamp fixture.

2~9 -Qver the past 50 years the three slgn~fl~ant areds o~
improvements to th~s system have been:

~l) Swit~h from a pre-heat starter to a r~pid starter.

~2) Introduction of a warm llght as an alternative to the cool light w~ich has helped somewhat to enhance its aesthetic qualities in certa1n applicatlons.

(3) Ava11abil~ty of a low loss ferro-magnetic ballast which has succeeded in replacin~ about 5-lOX of the conventional ballast market.

A fluorescent lamp requires an element which wlll initially provide the breakdown voltage and thereafter limit the current throu~h the lamp. This element i5 called a ballast and could be constructed wlth an tnductor and a sw1toh. A
more sophtsttcated circuit is constructed by ustng acttve elements. The simple ballast is not very expensive but produces a low eff1c1ency of energy transfer. The sophtst1cated ballast is usually expensive, not very rellable, and liable of producing electromagnetlc interferences.

%~25988 Solid state electronic ballasts are well known in the art.
US-4,689,524 ~Ureche) shows an electron1c fluorescent lamp baltast with a self-adjustin~ frequency of operatlon as a funct10n of lamp impedance var~ations due to age and enables constant lonization with1n the fluorescent lamp.
This i5 achieved by an energy storing component, a controlling component whi ch has two states and two socket elements. They enable the fllaments of the fluorescent lamp to be connected in series with the energy storing component. A constant current flows through a port10n of the energy storing component when the controlling component 1s in a state of producing an "energy latchlng" effeet wh1ch wlll enable the ballast to achieve a very hlgh energy transfer.

In EP 247.529 (Kim) an electron1c bal1ast stab11izer 1s shown compr1s1ng a no1se reduct~on transformer and two capocltors for regulating AC 1nput vottage. A bridge rect1f1er changes the AC input 1nto DC voltage. A nolse reduct10n c1rcu1t compr1ses a resistor and a capacitor through wh1ch plus voltage passes at the time of voltage drop. The positlve votta~e also passes through a resistor and a diode and is applied to the emitters of two ~Z5 9 88`
,_ -- 4 switching transi~tors. The posttive voltage also p~sses through the bases of two reslstors.

A mlnus voltage passes through voltage regulating coils, i5 appl1ed to a collector of constant current transistor and through series and parallel c1rcult comprising resistors, capac1tors and d~ode. A coil 1s connected between the secondary coils of a chopper transformer coupled through noise reduction colls to fi1aments of fluorescent lamps respectively. This eliminates d~sturbing influence on the human eye because the intenslty of 11ght is constant.

~t is one of the objects of this 1nvention to provide an electron1c ballast that repl~ces the three main components of a tr~ditional fluorescent lamp ballast, namely the ~erro-m~gnet1c ball~st, the power factor condenser, the st~rter and starter holder. It is a further ob~ect of the 1nvention to min1mlze th1rd and higher order harmon1c d~stortions of the line current. Another obJect of the invention is to el1m1nate start up fl1cker whlch ~s one of the maJor draw-backs of the fluorescent system. St111 another ob~ect of the invention is to save weight and costs.

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-r s BRIEF DESCRlPT30N OF THE ORAW~NGS.

r;g. 1 ;~ ~ fune~ona~ block di ~gr~m of tho r~r~ >n1 1nvention;

F19. 2 illustrates a power conversion circuit;

Fig. 3 shows the waveform generated by the bridge circult in the power conversion circuit;

Fig. 4 shows the waveform generated by the low harmon1c f11t~r ~Pnt.inn in the ~wer convers~on circuit;

F19. S 111ustrates the start-up circuit that 1s downstream-connected to the power conYers10n c1rcuit;

Fig. 6 illustrates the sw1tching resonant circuit that ~5 downstream-connected to the start-up circuit of f~g.
5;

F1g, 7 shows the complete scheme of the soltd state electrontc ballast;

g 8 8 _ 6 -Detailed Description of the Preferred En!bodiment ~enerally an electronic ba~last operates as an on/off switch alternating the voltage of the fluorescent 1amp. The basic principle of the solid state electron1c baltast of the invention is to convert the SO ~k 230 V AC mal ns into an AC voltage at a much higher frequency ~approx. 30-40 K~z) to operate the fluorescent lamp. The ballast circuit is composed of three basic function blocks. ~hese are namely.

a power supply conversion circuit l a start-up circu1t 2 and a switching resonant circuit 3, connected to the f1uorescent la~p 4. All three circuits are downstream-connected as shown in fig. l.

The primary role of the power supply circuit l i5 to convert the AC mains power into a varying PC supply voltage for the operation of the fluorescent lamp circuit. The .~ 7 variatton of th1s ~C voltage is deslgned in a manner to minim1~e the third harmonic distortlon of the line current.
Fig. 2 ~hows the deta~ls of the ctrcuit. The inductor Ll and the capacitor C7 for~ a common mode line filter to provide isolation and prevents the high frequency currents of the ballast circu1t from being coupled back to the mains supply.

The br1dge circuit, consisting of diodes D4 and D7, rectiftes the mains supply into a full wave rect1f1ed voltage whtch is a positive s1nuso~dal varying voltage (fig. 3).

If a sing1e capacitor is used tD filter the above voltag~, a hlgh DC voltage with a low ripple voltage is achieved.
However, the consequence is to have high and short current pulses charging the filter capacttor, resulting tn unacceptable levels of thtrd and higher order harmon1c distort10ns to the l~ne current. The design of the filter sect10n 1s to avoid short and sharp line current pulses.
Components ~ 2. D3, Cl and C2 constttute an improved low harmonic filter section ~ftg. 23 resulttng in a filtered voltage w;th a waveform as shown ln fig. 4.

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The operation of the filter circu~t is explalned ~s follows:

The bridge circuit conducts over the period of tl, t2 and t3. During this period, power ta the lamp circuit 1s drawn directly from the mains supply. ~uring the period t2, the lye ~,u-l-lu~, La ~ I i L i UIIU~ LUI I Cllt,a l,U Ull~l y~ r~
capac1tors Cl and C2.

During the period tl, the rectifled voltage exceeds the capacitor voltages of Cl and C2. All d;odes Dl, D2 ~nd D3 ~rs reversed b1ased and the capacitors C~ and C2 are prevented from charging.

I rl L ll ~ y l ~ ll y u ~ . 2, 1, ~ r ~ v u l ~. ~ y ~
now exceed~ the total of voltages of the capaoitûrs Cl and C2. Diode D3 is fDrward biased and connects the capacitors Cl and CZ in serie~ for ch~rging. The capacitors are charged to a maximum of 167 volts (approx.) each at the end of per1od t2. After this, diode D3 1s reversed biased again and d1sconnects the capacitors Cl and C2.

When the rectified voltaye falls below 167 volts, diodes Dl and D2 are forward biased thereby connecting both 2~25988 rapacitors Cl and C2 in par~llel. Hence, over the perlod t~ energy to the lamp circuit ls prov~ded by the two capacttors Cl and C2. The voltases of the capac~tors dlscharge until they equal the rectif~ed voltage.

Downstream the power supply convers10n circuit l and the start-up circuit ~ is connected. The role of thc start-up circuit is to ;nitiate the oscillat10n of the switching resonant circuit. This circuit operates during the initial power~on of the ballast. Once the switching resonance c~rcu1t ~s in operation, this circuit i5 deact1vated. The c1rcu~t operates aga1n in the event that the lamp 1s ~emoved. Hence, the resonant clrcuit will be energ12ed immed1ately when the lamp is replaced.

Th2 start-up circuit 2 is essentially a "sawtooth"
osc1l1ator. W1th reference to fig. 5 capacitor C5 charges toward Vs through res1stor R5 11nearly until 1t reaches 32 volts. This 1s the "turn on" voltage of the d1ac trigger, D9, whlch subsequently sw1tches the trans1stor TRl on.
Currents are then passed through the resonant circuit. this exc1tat~on provides induced voltages at the secondary wlndings of the transfor~er Tl to sustain self-oscilla-tlons. Resistor R6 acts as a load to trans~stor TRl before ~2~sg8~8`

the resonant clrcuit is turned on. Diode D4 dlscharges capaoitor C5 through transist~r TRl and prevents it from charging sufficiently to fire the diac during the normal operation of the resonant circuit.

Should the oscillations of the resonant circuit be halted, e.g. in the event of the lamp removal, transistor TRl ls turned off and capacitor C5 is allowed to charge to 3~
volts to fire the dlac trigger ~9 again. I~ the resonant clrcuit can not be exc~ted, the cycte repeats 1tse~f.

Downstream-connected with the start-up c~rcuit 2 ls the switching resonance clrcu;t 3. The resonance clreu1t 3, i~lustrated ~n fig. 6, consists of capacitor C3, lamp load in parallel with a capacltor C4, an inductor L2, a transfor~er Tl and transistors TRl and TR2. Transist~rs TRl and TR2 conduct alternotely and are swltched by a feedbock mechan1sm provided by the secondary wlndings of the transformer Tl. The resonant c1rcuit is series ~onnected across the supply when the first transist~r TRl turns on.
It ~s connected as a loop c1rcuit when the second translstor TR2 turns on. Reslstors Rl and R4 ~lmlt the current through the re~onant circuit and may be used to - il2~25~88 manipulate the luminescence level of the fluorescent lamp.
Resistors R2 and R3 llm~t ~he currents ln the base circu~t of the translstors. The transformer Tt may have a torold core.

On power up, the first transistor TRI is turned on when the diac D9 flres. The resonant circuit ls connected across the po~er supply. The operation of the first half cycle 1s as fotlows:

CUrrQnt flows through the circu1t and energy is stored ln the reactlve components. Power to the lamp is drawn from t~e supply. Induced voltages on the secondary windtngs of t~e tr~nsformer Tl keep the trans1stor TRl turned on and hold the second trans1stor ~2 off until t~e r~sonant circu1t current reaches ~ts peak and begins to fal1.

The second half cycle of oscll1atlons begins when the resonant clrcult current falls. The lnduced voltages are reversed to turn the flrst transistor TRI off and to switch the second trans1stor TR2 on. ~he resonant clrcult loop ls closed by the second trans;stor TR2 and oscillates for the next half cycle. Energy stored ln the reactive components are released to the lamp. When the current reverse~

2~988 direction, transistor TRl is turned on once again and the next cycle beglns. In this way osc111attons ~re self-sustaining.

Under abnorm~l conditions, such as a faulty lamp, an elec-tron~c shut-down mechanism ls tr~ggered. ~he ballast shut-down results from the saturation of the toroid core and the disablllty of the feedback mechan~sm that sustalns oselllatlons. Thts is accomplished by an addltional secondary w1nd1ng that ls shortened by an electronic device when abnormal power level ~s detected.

It 1s obvious that the novel circu~t arrangement using fewer components results ~n lower ~osts and less welght pér ~nlt in comparlson wlth a tradit~onal electron1c lamp ballast. It 1s found that the ballast of the 1nvention uses Y 1PCC Pn~rnv thAn th~ nhn~nt.innAl fRrrn-mannet~c ball~st and 10-12X less than a low-loss ferro-magnetic ballast~

Claims (6)

1. A solid state electronic ballast for a fluorescent lamp with a power supply conversion circuit (1) a start-up circuit (2) and a switching resonant circuit (3), characterised in that the power supply conversion circuit incorporates a low harmonic filter consisting of a first capacitor (C1) and a first diode (D1) and a second capacitor (C2) and a second diode (D2) linked by a third diode (D3) so that all said diodes (D1-3) are reversed biased when the rectified voltage exceeds the voltage of said capacitors and that said capacitors are connectes in series when third diode (D3) is forward biased, and are disconnected when said third diode is reversed biased, while said and second capacitors are parallel connected when said first and second diodes are forward biased.

2 A solid state electronic ballast as claimed in claim 1, characterised in that the switching resonant circuit has a transformer (T1) with additional secondary winding shortened by a power level detection circuit and that a shutdown results from the saturation of the core of said transformer stopping the feedback mechanism that sustains oscillations.

3. The solid state electronic ballast of claim 1 wherein said start-up circuit includes a resistor (R?) that acts as a load to said first transistor (TR1) before said resonant circuit is turned on.

4. The solid state electronic ballast of claim 1 wherein said transformer of said switching resonant circuit having secondary windings that are connected with said first and second transistores (TR1,TR2) keeping said first transistor turned on and said second transistor turned off until the current of the resonant circuit reaches its peak and begins to fall.

5. The solid state electronic ballast of claim 1 wherein two resistors (R1,R4) each connected to the emitter of one of said transistors (TR1,TR2) are variable to mani-pulate the luminescence level of the fluorescent lamp.

6. The solid state electronic ballast of claim 2 wherein said core of said transformer (T1) is preferably a toroid core.