Resonator ballast for arc discharge lamps

[57] ABSTRACT

An alternating current electronic ballast for an arc discharge lamp has two sets of capacitors and diodes arranged so that each set of capacitors can be charged and discharged only during opposite polarities of an AC supply. The ballast can develop a high voltage for starting the lamp and also can efficiently regulate the current flow through the lamp.

Claims, 2 Drawing Figures

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PATENTED JUL 111972

3.676,735

SHEET 1 OF 2

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WILLIAM J. ROCHE INVENTOR

BY l

ATTORNEY

PATENTED JUL 11197?

SHEET 2 OF 2

3.676,735

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3,676,735

RESONATOR BALLAST FOR ARC DISCHARGE LAMPS

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to circuits for starting and operating gaseous discharge devices such as, for example, fluorescent lamps and high pressure mercury vapor lamps.

2. Description of the Prior Art

Arc discharge lamps usually have a so-called negative resistance and hence require a current-limiting device in series with them. This regulates the current flowing through the lamp, thereby preventing run-away of the current or extinguishing of the arc as a result of normal fluctuation in the supply of AC electrical power. Previously, this current-regulating ballast has been primarily an inductance coil or a resistance, with or without a capacitor, the leakage reactance of a transformer some-times being used as the inductance. Ballasts of such types generally require a considerable amount of iron and copper to operate on the usual 60-Hertz power line, and are accordingly bulky, expensive and inefficient. Also, the capacitors used are generally of the oil-filled type because of the AC charge impressed on them, and are also bulky and expensive.

In addition, a higher voltage than the usual 120 or 240 volts is usually necessary to ignite or start such lamps, especially high pressure mercury vapor lamps. This higher voltage is generally supplied by a bulky and expensive transformer, which undesirably dissipates large amounts of electrical power.

Furthermore, the highest voltage obtainable from such a transformer is generally not high enough to restart a high pressure mercury vapor lamp within a very few minutes after the lamp has been extinguished. This refers to the so-called hot restart time and is the length of time required for a lamp to cool down sufficiently for restarting after the lamp has been switched off during operation and results from the fact that a higher voltage is needed to start a hot lamp than one that is cooler.

interval by the above mentioned series-connected diodes on the one hand, and, on the other hand, by the fact that the arc discharge lamp, which is in a discharge path of the capacitors, is unignited and non-conducting.

During the succeeding cycles the second set of capacitors can be made to transfer its charge to the capacitors in the first set, so that the charge thereacross is doubled. In this manner the open circuit voltage across the lamp can be up to 5 times the peak supply voltage depending on the number of diodes ^u and capacitors used in each polarity set, which will depend on the lamp type being used. In this way a proper ignition voltage is generated for the particular lamp type concerned herewith.

After the lamp has ignited, the capacitors, together with a small inductance in series with the lamp, effectively regulate and limit the current flow through the lamp. The capacitors alternately charge from the supply voltage and discharge through the lamp at a frequency greater than the line frequency. The current flow through the lamp is controlled to a large 20 degree by the amount of energy stored in the capacitors at the time they discharge through the lamp; thus, the need for a power-consuming, current-limiting, high impedance in the circuit is eliminated.

The construction of the circuit permits current flow through 25 the lamp in one direction only. Thus the capacitors that control the lamp current differ according to the polarity of the line voltage. That is to say, when the line voltage is, say, positive with respect to ground, one set of capacitors charges from the supply and discharges through the lamp. And when the 30 line voltage is negative, the other set of capacitors charges from the supply and discharges through the lamp.

The rate at which the capacitors charge and discharge, that is to say, the rate at which the circuit resonates, is approxirnately equal to the characteristic frequency of the circuit it35 self.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a circuit diagram of a ballast in accordance with this invention.

FIG. 2 shows waveforms of the rectified supply current, the modulating voltage developed by the resonant circuit, and the resulting lamp current for a ballast circuit embodying the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT

As shown in FIG. 1, a ballast 1 in accordance with this invention can be energized by connecting it across an AC power supply 2, which can be the usual 60-Hertz, 120-volt line supply. An arc discharge lamp 3 can be connected across the output end of ballast 1.

The electronic ballast circuit, includes a full wave bridge rectifier comprising diodes 7, 8, Il and 12. The bridge input 55 terminals, represented by nodes 20 and 21, are coupled across AC supply 2, with node 21 being connected to the power supply through a current limiting inductor 6. Diodes 12 and 8 are shunted by polarized capacitors 4 and 10, respectively. A blocking diode 9, lamp 3, and inductor 13 are serially con¢0 nected across the bridge output terminals, represented by nodes 22 and 23. A third polarized capacitor 5 is connected between node 21 and the junction of diode 9 and lamp 3. The diodes are all oriented to provide unidirectional charging of each of the capacitors.

Capacitors 4, 5 and 10 serve a dual purpose. Firstly, they generate the high voltage required to initially ignite lamp 3 and to re-ignite the lamp at the beginning of each new half cycle of lamp operation, corresponding to the cyclic frequency of the AC power supply 2. Secondly, they form a resonant 70 switching circuit in conjunction with inductor 13 and lamp 3.

With regard to the first function of the capacitors, the build up of the lamp ignition voltage can be described by the following sequence of events. When the 120-volt supply 2 is positive relative to ground, capacitors 4 and 5 become charged to the 75 170-volt peak value of the supply voltage and retain this

Warm-up time is the length of time required for a cold lamp to heat up to about its normal operating temperature and emit its rated amount of light energy. In the usual high pressure mercury vapor ballasts the warm-up time is quite long since the starting current is usually quite low.

Electronic ballasts, so called, have been developed for arc discharge devices but these have generally required the use of gated, bidirectional, semiconductor devices. See, for example, U.S. Pat. No. 3,414,768 issued on Dec. 3, 1968 to S. C. Peek, Jr. Such devices are generally more complex and expensive than the components used in the instant invention.

SUMMARY OF THE INVENTION

An electronic ballast for arc discharge devices, in accordance with this invention, comprises simple, compact and inexpensive components and is smaller, less expensive and more efficient than the prior art ballasts mentioned above. The instant ballast also provides higher open circuit voltages, thereby reducing lamp hot restart time, and higher starting currents, thereby reducing warm-up time.

Because the capacitors used in this invention store charge in only one direction, the use of dry electrolytic capacitors is permitted. Such capacitors are more compact and less expensive than the oil-filled capacitors used in the prior art ballasts.

A ballast in accordance with this invention has a plurality of capacitors, each capacitor being in parallel and in series with diodes that permit the capacitor to be charged and discharged in one direction only. When the ballast is energized, such as by, for example, the usual line power supply of 60 Hertz, 120 volts, a set of one or more capacitors develops a charge thereacross during a one-half cycle of the supply voltage equivalent to the peak voltage thereof. During the next half cycle, which is of opposite polarity, a second set of one or more capacitors develops a similar charge thereacross. The first set of capacitors is prevented from discharging during this

3,676,735 charge until the lamp becomes ignited. The current path for charging capacitor 4 consists of capacitor 4, diode 7 and inductor 6. The path for charging capacitor 5 consists of diode 8, diode 9, capacitor 5 and inductor 6.

When the supply voltage reverses polarity and becomes negative relative to ground, capacitor 10 becomes charged to the 170-volt peak value of the supply. The path for charging capacitor 10 consists of inductor 6, diode 11 and capacitor 10.

When the supply voltage returns through zero each capacitor (4, 5 and 10) will be charged to 170 volts according to their polarity as indicated in the circuit diagram.

When supply voltage 2 again becomes positive relative to ground, capacitor 10 transfers a portion of its voltage (113 volts) to capacitor 5. The transfer current path consists of capacitor 10, diode 9, capacitor 5 and inductor 6. When the supply voltage 2 returns through zero, capacitor 4 is charged to 170 volts, capacitor 10 retains a charge of 57 volts and capacitor 5 will then be charged to 283 volts. When the supply voltage 2 again reverses polarity and becomes negative relative to ground, capacitor 10 again charges to the one hundred seventy peak voltage of the supply. As supply voltage 2 again becomes positive relative to ground, a portion of the 170 volts on capacitor 10 will again transfer to capacitor 5. The value of the voltage transferred from capacitor 10 to capacitor 5 during each cycle of the AC supply is given by the expression

Al·* = %* where:

Δ = the incremental voltage transfer from capacitor 10 to capacitor 5, = 170 volts (the peak value of the 120-volt, 60-Hertz supply), k = the cycle index number, and where the capacitive reactance of the circuit is at least about ten times the inductive reactance.

The cycle of capacitor 10 charging to 170 volts on one half cycle of AC supply 2 and then transferring a portion of this voltage to capacitor 5 on the following half cycle will cause the voltage across capacitor 5 to increase very rapidly according to the expression:

2/3*V„.

where:

— the voltage across capacitor 5,

V_r = 170 volts (the peak value at the 120-volt, 60-Hertz supply), k = the cycle index number, and n ~ the number of cycles incurred by the circuit.

After a total of five cycles of the AC supply (n=5), V₃ will attain 99 percent of its final value of 340 volts. At this time, the voltage across capacitor 5 (K_s) will add with the alternating supply voltage (V_r sin ωί) and the voltage across capacitor 4 (V₄) to yield an output voltage at nodes 23 and 24 which has a peak value of 680 volts, an average value of 510 volts, and a low value of 340 volts. This additive combination of voltages results from the series circuit arrangement of the elements which may be traced from node 24, through capacitor 5, inductor 6, supply 2, and capacitor 4 to node 23. Thus, the open circuit voltage (V„.) at nodes 23 and 24, and hence across lamp 3, may be expressed as:

+ V_t sin art + K, which after capacitor 5 builds up to full charge, using the above values, may be written

F«. = 340 + 170 sin orf + 170 The resulting peak open circuit voltage of 680 volts is more than sufficient to ignite lamp 3.

Reignition of the lamp at the commencement of a particular half cycle of operation is accomplished in a somewhat similar manner. When the supply voltage is positive at node 20 with respect to ground, capacitor 4 charges through diode 7 and inductor 6, and capacitor S charges through diodes 8 and 9 and inductor 6, as before. Capacitor 10 will have retained a residual voltage with the indicated polarity. This residual voltage corresponds to the extinction voltage of the lamp following the previous half cycle of operation and will add with the voltage across capacitor 4 to produce a voltage which is positive at node 24 and negative at node 23.

The voltage necessary to re-start the lamp on each half cycle is always less than the original ignition voltage of the lamp due to the residual charge remaining in the lamp between half cycles of operation. This residual charge occurs as a result of the lamp wall current (charge moving outward to the wall of the lamp, perpendicular to the axis of the lamp) decaying to zero at a slower rate than the axial lamp current which goes to zero between half cycles. This persistent wall current assures that there will be free charge present in the lamp at the commencement of each half cycle of lamp operation. Hence, the value of the reignition voltage will be determined by the peculiar characteristics of the lamp being operated in the circuit. These peculiarities, such as mercury vapor pressure, fill gas pressure, and tube diameter and length, will determine the initial voltage across capacitor 10. The voltage buildup across capacitor 4 will continue until the voltage appearing between nodes 23 and 24 reaches the reignition voltage required for a particular lamp.

The above discussion refers to the reignition process when the supply voltage is positive at node 20 with respect to ground. A similar process occurs when the supply voltage is negative at node 20 with respect to ground. This time, however, capacitor 4 will contain the residual lamp extinction voltage and capacitor 10 will charge through inductor 6 and diode 11 from supply 2 to produce the required reignition voltage appearing between nodes 23 and 24.

Once the lamp reignites, energy will be supplied to the lamp at two distinct pulse frequencies. Approximately 50 percent of the energy delivered to the lamp is supplied through a 120Hertz pulsating DC current wave, illustrated by curve I, in FIG. 2. This pulse train is the result of the rectification process performed on the 60-Hertz AC input current by the full wave bridge comprising diodes 7, 8, 11 and 12. Specifically, when AC supply 2 is positive with reference to ground the lamp is supplied current through diode 8, diode 9, lamp 3, inductor 13, diode 7 and inductor 6. On the alternate AC half cycle the current path consists of inductor 6, diode 11, diode 9, lamp 3, inductor 13 and diode 12. Although the current in inductor 6 is seen to reverse polarity every half cycle of the AC supply, the lamp current remains unidirectional and has a pulse frequency of 120 Hertz.

The remaining 50 percent of the energy delivered to the lamp is supplied through a higher frequency pulsating D.C. current wave having a frequency that depends on the resonant characteristic of the inductive-capacitive circuit comprising inductor 13, and the equivalent inductive element in the lamp.

This formation of a resonant switching circuit comprises the second function of capacitors 4, 5 and 10. The capacitors alternate in this function on every half cycle of supply 2 by the discharge of stored energy following lamp reignition. More specifically, when the supply voltage is positive at node 20 with respect to ground, the resonant switching circuit comprises lamp 3, inductor 13, and the parallel combination of capacitors 4 and 5. In particular, the discharge path of capacitor 4 comprises: capacitor 4, diode 8, diode 9, node 24, lamp 3, inductor 13, and node 23; and the discharge path of capacitor 5 comprises: capacitor 5, node 24, lamp 3, inductor 13, node 23, diode 7 and node 21. When the supply voltage is negative at node 20 with respect to ground, the resonant switching circuit comprises lamp 3, inductor 13, and capacitor 10. In particular, the discharge path of capacitor 10 comprises: node 22, diode 9, node 24, lamp 3, inductor 13, node 23, and diode 12. For symmetrical lamp operation during both half cycles of the supply voltage, the parallel capacitance of capacitors 4 and 5 is made equal to the value of capacitor 10.

3,676,735

6

In other words 2C, — 2C₅ — C_M, where C₄, C, and C_I0 are the values of capacitors 4,5 and 10, respectively.

The characteristic or resonant frequency of the circuit is (2 ir LC)*¹, where L=the inductance of lamp 3 plus that of inductor 13 and C=the capacitance of the parallel combination of capacitors 4 and 5 during the half cycle that the supply voltage is positive at node 20 with respect to ground, and the capacitance of capacitor 10 during the half cycle that the supply voltage is negative at node 20 with respect to ground. The resonant frequency is preferably several times that of the supply frequency, as illustrated in FIG. 2, where curve V_mrepresents the resonant frequency signal as obtained by observing the voltage across inductor 13. The exponentially damped sinusoid appearing every half cycle of the supply represents the ringing voltage across the inductor 13 during discharge of one of said capacitor sets.

The 120-Hertz frequency (curve I,) and the resonant frequency (curve V.) have opposing polarities and therefore the resulting lamp current waveform will consist of the dif- -ference in their magnitudes. This produces a lamp current comprised of a 120-Hertz pulsating DC waveform modulated by the resonant circuit frequency signal, as illustrated by curve I_t in FIG. 2. The effect of this modulation process on the lamp discharge characteristic is one of alternately switching the 25 discharge mode from a positive to a negative characteristic (i.e. from a low to a high conducting state). The peaks of the curve I_t and the zero crossings of the curve V„ represent the points at which the lamp is switched.

In going from a positive to a negative discharge charac- 30 teristic (i.e. from a low to high conduction), the lamp can be thought of as representing an ON-OFF switch in the circuit of FIG. 2. When the lamp discharge characteristic is negative (i.e. conductance is high), the switch is turning ON, in that the rate of ionization in the lamp is increasing, causing the lamp 35 current to increase also, as illustrated by each positive-going slope of the curve V. When the lamp discharge characteristic is positive (i.e. low conductance), the switch is turning OFF since the rate of ionization within the lamp is decreasing with the lamp current likewise decreasing, as illustrated by each 40 negative-going slope of the curve I_t. It is this switching action within the lamp that generates the circuit perturbations which are tuned by the resonant circuit, composed of inductor 13 and the appropriate capacitors (as described above), to produce the lamp current modulation. The modulation in turn 45 switches the lamp. The process is seen to be regenerative in nature with inductor 13, the appropriate capacitors (i.e. parallel capacitors 4 and 5 on the positive half cycle, and capacitor 10 on the negative half cycle) and lamp 3 constituting a selfsustaining oscillator circuit. 50

There is a distinct advantage in generating a lamp current waveform of this type. By supplying a significant portion of the energy to the lamp at a higher frequency, a portion of the inductive ballasting element can be reduced in both size and cost. Since the inductive ballast impedance is directly propor- ³³tional to frequency (2ir/L), a small inductance, (inductor 13) can exercise a proportionately greater degree of current control at a higher frequency than the larger inductive element, (inductor 6) operating at a lower frequency. In one specific example of an electronic ballast of this invention used to operate a 40-watt high pressure mercury vapor lamp, capacitors 4, 5 and 10 were dry, electrolytic capacitors having ratings of 1 microfarad — 200 volts, 1 microfarad — 400 volts and 2 microfarads — 200 volts, ¢5 respectively. Diode 7, 8, 9,11 and 12 each had ratings of 1.0 ampere, 200 peak inverse volts. Inductors 6 and 13 had iron cores and were rated at 220 and 11 millihenries, respectively. Supply 3 was a 60-Hertz, 120-volt AC supply.

Electrical measurements made during operation of the bal- -jq last were as follows:

Line Lampterconnecting said first and second capacitive means for transVoltage 120 v., AC 89.5 v., DC 75 ferring a portion of the stored energy charge from said second

Current Power Power Factor Open Circuit Voltage Current Crest Factor Voltage Crest Factor Starting Current Power Transfer Efficiency Factor

0.51 ampere 0.54 ampere

47.0 wen* 42.5 watts

0.79 — 680v„ DC

1.651.93

1.411.73 — 1.0 ampere

-0.91

The current and voltage crest factors refer to the ratio of the peak values thereof to the average values. For the purpose of lamp efficiency, the lamp crest factor is preferably less than 2. Said crest factor can be reduced by increasing the inductance of inductor 13, but the impedance losses therein will be increased.

The equivalent inductance for a 40-watt lamp of this type is about 25 millihenries. The resonant characteristics of the ballast then are based on the inductance of lamp 3 (25 millihenries), the inductance of inductor 13 (11 millihenries) and the capacitance of the parallel combination of capacitors 4 and 5 on the positive half cycle and of capacitor 10 on the negative half cycle (2C₄ = 2C, = C₁₀ = 2 microfarads). Thus, the resonant frequency of the ballast is 595 Hertz and the power supplied to the lamp consists of the difference in magnitude between the 595-Hertz waveform and the 120-Hertz pulsating DC waveform. The waveforms of FIG. 2 were obtained from this ballast circuit, curve I» being the 120-Hertz output of the full wave rectifier, curve V„ being the 595-Hertz resonant frequency signal as represented by the voltage across inductor 13, and curve I_t being the modulated lamp current.