U.S. patent number 3,676,735 [Application Number 04/860,985] was granted by the patent office on 1972-07-11 for resonator ballast for arc discharge lamps.
This patent grant is currently assigned to Sylvania Electric Products Inc.. Invention is credited to William J. Roche.
United States Patent |
3,676,735 |
Roche |
July 11, 1972 |
RESONATOR BALLAST FOR ARC DISCHARGE LAMPS
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.
Inventors: |
Roche; William J. (Merrimac,
MA) |
Assignee: |
Sylvania Electric Products Inc.
(N/A)
|
Family
ID: |
25334559 |
Appl.
No.: |
04/860,985 |
Filed: |
September 25, 1969 |
Current U.S.
Class: |
315/207; 315/243;
315/241R |
Current CPC
Class: |
H05B
41/042 (20130101) |
Current International
Class: |
H05B
41/04 (20060101); H05B 41/00 (20060101); H05b
037/00 (); H05b 039/00 (); H05b 041/14 () |
Field of
Search: |
;315/227,241,242,243,244,200,205,207,97 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Kaufman; Nathan
Claims
I claim:
1. A ballast circuit for operating an arc discharge lamp from an AC
power supply, said circuit comprising:
a first capacitive means;
means coupling said AC power supply to said first capacitive means
for charging said first capacitive means with energy in half cycles
of a first polarity of said AC supply;
first rectifying means shunting said first capacitive means for
restricting the charging thereof to one direction of said first
polarity;
a second capacitive means;
means coupling said AC power supply to said second capacitive means
for charging said second capacitive means with energy in half
cycles of a second polarity of said AC supply;
second rectifying means shunting said second capacitive means for
restricting the charging thereof to one direction of said second
polarity;
a first inductor connected in series with each of said first and
second capacitive means and said AC supply;
a second inductor connected in series with said lamp; and
means coupling each of said first and second capacitive means
across the series combination of said lamp and second inductor for
discharging said first and second capacitive means through the
conducting lamp and second inductor in half cycles of said first
and second polarity, respectively, of said AC supply during normal
lamp operation;
said lamp, second inductor and first capacitive means comprising a
resonant circuit operative during each discharge of said first
capacitive means through the conducting lamp and second inductor to
produce a signal having a resonant frequency greater than twice the
frequency of said AC supply; and
said lamp, second inductor and second capacitive means comprising a
resonant circuit operative during each discharge of said second
capacitive means through the conducting lamp and second inductor to
produce a signal having said resonant frequency greater than twice
the frequency of said AC supply.
2. The ballast circuit of claim 1 further including means
interconnecting said first and second capacitive means for
transferring a portion of the stored energy charge from said second
capacitive means to said first capacitive means during each
charging period of said first capacitive means prior to the
starting of said lamp.
3. The ballast circuit of claim 1 wherein said means coupling said
supply to said first capacitive means includes at least a first
rectifier in series with said first capacitive means, said means
coupling said supply to said second capacitive means includes at
least a second rectifier in series with said second capacitive
means, and said means coupling each of said first and second
capacitive means across the series combination of said lamp and
second inductor includes means for providing unidirectional current
flow through said lamp and second inductor during half cycles of
both said first and second polarity of said supply during normal
lamp operation.
Description
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.
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 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 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 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 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 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 approximately
equal to the characteristic frequency of the circuit itself.
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, 11 and 12. The bridge input
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 connected 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 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 170-volt peak
value of the supply voltage and retain this 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
.DELTA.V.sub.k = 2/3.sup.k V.sub.p, where:
.DELTA.V.sub.k = the incremental voltage transfer from capacitor 10
to capacitor 5,
V.sub.p = 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:
where:
V.sub.5 = the voltage across capacitor 5,
V.sub.p = 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), V.sub.5 will
attain 99 percent of its final value of 340 volts. At this time,
the voltage across capacitor 5 (V.sub.5) will add with the
alternating supply voltage (V.sub.p sin .omega.t) and the voltage
across capacitor 4 (V.sub.4) 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.sub.oc) at nodes 23 and 24, and hence across
lamp 3, may be expressed as:
V.sub.oc = V.sub.5 + V.sub.2 sin .omega.t + V.sub.4
which after capacitor 5 builds up to full charge, using the above
values, may be written
V.sub.oc = 340 + 170 sin .omega.t + 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 5 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 120-Hertz pulsating DC
current wave, illustrated by curve I.sub.o 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. In other words 2C.sub.4 =
2C.sub.5 = C.sub.10, where C.sub.4, C.sub.5 and C.sub.10 are the
values of capacitors 4, 5 and 10, respectively.
The characteristic or resonant frequency of the circuit is (2
.pi..sqroot.LC).sup.-.sup.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.sub.m represents
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.sub.o) and the resonant frequency
(curve V.sub.m) have opposing polarities and therefore the
resulting lamp current waveform will consist of the difference 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.sub.L in FIG. 2. The
effect of this modulation process on the lamp discharge
characteristic is one of alternately switching the 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.sub.L and the zero
crossings of the curve V.sub.m represent the points at which the
lamp is switched.
In going from a positive to a negative discharge characteristic
(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 current to increase also,
as illustrated by each positive-going slope of the curve I.sub.L.
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 negative-going slope of
the curve I.sub.L. 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 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
self-sustaining oscillator circuit.
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 proportional to frequency
(2.pi.fL), 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, 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 ballast were
as follows:
Line Lamp-
__________________________________________________________________________
Voltage 120 v., AC 89.5 v., DC Current 0.51 ampere 0.54 ampere
Power 47.0 watts 42.5 watts Power Factor 0.79 -- Open Circuit
Voltage -- 680 v., DC Current Crest Factor 1.65 1.93 Voltage Crest
Factor 1.41 1.73 Starting Current -- 1.0 ampere Power Transfer
Efficiency -- 0.91 Factor
__________________________________________________________________________
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 (2C.sub.4 =
2C.sub.5 = C.sub.10 = 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.sub.o being
the 120-Hertz output of the full wave rectifier, curve V.sub.m
being the 595-Hertz resonant frequency signal as represented by the
voltage across inductor 13, and curve I.sub.L being the modulated
lamp current.
* * * * *