Note: Descriptions are shown in the official language in which they were submitted.
BACKGROUND OF THE INVENTION
1. Field of the Invention
.
The present invention relates ~o a power supply
for a gas discharge lamp, and particularly to such a device
employing a switching regulator reflecting a unity power
factor and no third harmonic distortion to the ac line,
an inverter and a resonant network including the lamp load,
the inverter operating at the resonant frequency of the
network to provide a sinusoidal output voltage.
2. Description of the Prior_Art
Various types of gas discharge lamps are widely
used for lighting purposes. These include fluorescent
lamps, high intensity discharge lamps of different types
including ~he metal halide varieties and sodium lamps of
both high and low pressure. A common feature of all these
lamps is that they require some type of ballast for operation.
Ordinarily ballast transformers are used. This approach
has several shortcomings. For operation at line frequency,
the ballast must be of subs~antial physical size and weight,
resulting from the large magnetic transformers and capacitors
that are required. Efficiency is low. The ballast must be
operated at t:he rated line voltage, and any serious deviation
can cause eit:her the ballast to overheat or the lamp to
flicker. Dimming of the lamp is difficult or impossibleO
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One approach of the prior art to overcome these
difficulties has been the use of switching regulators to
provide to the lamp a direct current that is switched on
and off at a high frequencyO While dc will light the lamp
adequately, a specially desiLgned lamp is required if the
lamp lifetime is not to be sacrificed considerably.
Moreover, such circuits require that the ac line voltage
first be rectified and filtered for input to the regulator.
If inductive filtering is used, a very poor power
factor will result, and the inductor may have to be the same
large size as the original ballast. Also, liLne distortion
is created by the combination of the inductor and the bridge
rectifier. If capacitive filterLng is used, all of the
current will be conducted during the peak of the ac line
cycle. This produces "thiird harmonic distortion" which
heats up the pole transformers and requLres extra heavy
wiring between the device and the power source.
Another approach of the prior art is shown in
the U. S. Patent No. 3,999,100. This supply uses a
switching regulator in conjunction with a commutator to
provide power to a metal halide lamp. The commutator is
operated at: or near the ac liLne frequency.
~ e principle object of the pre5ent invention
is to proviLde an improved electronic ballast for a gas
discharge :Lamp. Other objectives are to provide such a
lamp power supply wherein:
~ a) no third harmonic distortion is produced and
the supply has a near unity power factor;
~;3~'~ 7~
~ b) improved efficiency results from supplying
the lamp with high frequency ac power;
(c) dimming is facilitated for both fluorescent
and high intensity gas discharge lamps;
(d) the ballast will operate properly despite line
voltage variation;
(e) all components are lightweight, small in
size and inexpensive;
~ f) sufficiently high output voltage is provided
to start high intensity gas discharge lamps;
(g) circuitry automatically compensates for
changing lamp performance characteristics~ including
during the warm-up period of high intensity lamps;
~ h) sinusoidal output voltage is produced, with
concomitant benefits in ef~iciency, less ~ring~nt performance
requirements and hence lower cost for the circuit switc.hing
transistors~ and substantial elimination of radio frequency
interference;
(i) dimming may be accomplished in response to
ambient light conditions; and
(j) provision is made for battery operation in
the event of ac power line failure.
Yet another object of the invention is to
provide the benefits of (a) (d) (e) for other than
ballast applications.
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~. ,
SUM~R~- OF I-~E lh~'~TION
These and other objectives are achieved in
accordance with the present invention by providing a
power supply for a gas discharge lamp wherein a switching
regulator is used to drive an inverter, the output of which
is supplied to the lamp via a resonant network that includes
the lamp. A feedback circuit maintains the inverter
switching rate at the resonant frequency of the network,
so that a sinusoidal output is produced.
.
The switching regulator is driven by unfiltered
rectified ac line power. The regulator utilizes an output
filter capacitor which is effective at twice the ac line
frequency. The switching duty cycle or rate is responsive
to the regulator output voltage averaged over several hal~
cycles of the ac line frequency. In this way, a unity
power factor is achieved without third harmonic distortion.
The resonant network includes a capacitor
shunted by an inductor in series with the lamp 102d.
Component values are selected so that these circuit
elements together exhibit capacitive reactance under all
lamp impedance conditions. Another inductor is connected
in series with these circuit elements to form a series
resonant circuit across the inverter output. A phase
detector or other circuit controls the inverter switching
rate so that it equals the resonant frequency of the network.
The circui~ will work equally well with capacitors replacing
the inductors and an inductor in place of the capacitor
if high harmonies of the inverter frequence can be
tolerated or desired in the load.
f
~3 3~9
Dimm~ng is accomplished either by varying the
duty cycl2 of the inverter or by al~ering the dc ~olta e
leveL pr~vlded to the inYerter from the swit~hing regu~ator.
Mor~ particularly, there i.s provided:
A power supply for a gas discharge lamp comprising:
source of DC voltage;
an inverter connected to receive DC voltaga from said
sonrce comprising a pair of switching transistors for alternately
switching DC voltage from said source;
a resonant network whereby the output of the inverter
as presented to the lamp will be sinusoidal in wave shape;
said inverter includes a transformer, the output o~ said
transformer being connected to said resonant network which is in
turn connected to the lamp;
- an oscillator connected to control the switching rate
of said pair of switching transistors to switch said transistors
at such time as the current flowing in or out of said resonant
circuit shal~ be at or near zero;
a feed-back means which determines that said resonant
network is.at resonance a~d that it presents a resistive load
to said inverter;
a frequency control means for said oscillator connected
to the feedback melns in such a manner as to control the frequency
to maintain resonance in said resonant network.
There is also provided:
... . . . . . .
A power supply for a gas discharge lamp comprising:
a source of DC voltage;
an inverter connected to receive DC from said source
comprising a pair of switching transistors;
3r a resonant network including said lamp wherein said
network is connected to the output of said inverter;
~33~
feeaback means ope~ably connected to said resonant net-
wor~ and to said in~er~er for controlling the switching rate of
~aid inverter so ~hat it is maintai.ned at the resonant frequency
of said resonant networ~ by commutating the voltage applied to
said resonant network at such times as the current flowing in or
out of said resonant circuit shall be at or r.ear zero;
said inverter comprises;
an autotransformer, having an output connected to
said resonant network:
first and second switching transistors c~nnected to
supply DC voltage fr~m said source to said ~uto~ransformer in
respectiYe first and opposite polarity; and
oscillator and drive circuitry connected to said
switching transistors for alternately turning one transistor
on and the ~ther transistor off, said circuitry providiny a
delay af~er each transistor is turned.off before the other is
tllrned on; and
said feedback means controls the frequency of said
oscillator and drive circuitry via phase detection.
Ther~ i~ further provided:
A direct current power supply, comprising:
a source of DC voltage which includes rectification
means for converting AC line power to pulsating DC;
a switching regulator;
connection means allowing the essentially unfiltered
pulsating DC to be the source for said switching regulator;
said switching regulator made deliberately non-
responsive to the! frequency of the pulsating DC supplied by
said rectification means so as not to compensate for the varia-
tion of inpu~ voltage at the pulsating frequency;
-6a-
~ ~3 ~
said swit~hing regulator output i~. filtered by a large
: capacitor to make up for the regulator's non-responsiveness to
the variation in input voltage to said switching regulator
caused by said pulsating DC;
wherein said nonresponsive!ness is accomplished by
ma~ntaining the on to off time ra~io of the said ~wit~hing
regulator essentially constant over the full AC half cycle with
any such ratio changes occurring o~er the average input voltage
such that changes will not appreciably affect said ratio duxing
any one half cycle of AC line power.
BRIEF DESCRIPTION OF ~HE DRAWINGS
The features of th~ present inYentisn which are
bel~eved to be novel are ~et forth with particularity in
th~ appended claims~ The present inventiou7 both as to i~s
osgani~ation and manner of operation3 together with further
ob~ects and a~vantag~s ~hereof, may best be understood by
reference ~o the ~ollowing descrip~ion, ~aken in connec~ion
with the accompanying ~rawings ~n which:
FIGURE 1 is an electrical block diagram of the
i~entive power supply for a gas discharge lamp;
FIGURES 2A and 2B are electrical schema~ic diagrams
of al~ernative swi~ching regula~or circuits that can be
used in the power supply of FIGURE l;
FIGURES 3A, 3B and 3C are graphs illustrating
the effect on power factor of various types of switching
regula~or circuits;
FIGURE 4A is an eiectrical schematic diagram of
the inverter and sesonant network components of the power
supply o~ FIGURE 1;
3~ FIGURE 4B ~s the same as 4A with a different
~nverter conf~Euration.
-6b-
;,' :
DESCRIPTION OF THE PR~ ERRED EMBODIMENT
The following detailed description is of the
best presently contemplated modes of carrying out the
invention. This description is not to be taken in a
limiting sense~ but is made merely for the purpose of
illustrating the general principles of the invention
since the scope of the invention best is defined by
the appended claims.
Operation characteristics attributed to forms
of the invention first described also shall be attributed
to forms later described, unless such characteristics
obviously are unapplicable or unless specific exception
is made.
Referring to FIGURE 1, the inventive power
supply 10 is used to energize a ga~ discharge lamp 11
which may be of the mercury vapor, metal-halide, sodium
or fluorescent type. To this end, ac power from a source
such as the 60 Hz, 120 volt power lines, is connected
~ia a pair of terminals 12a, 12b to a bridge rectifier 13.
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f~l 33~
Th~ unfiltered output of the bridge rectifier 13
is supplied via the lines 14 and 15 to a switching regulator 16.
No filtering is used at the output of the bridge rectifier 13
to eliminate both (a) the lagging power factor and dist~rtion
which would result if an inductor were used to ~ilter the
rec~ified ac, and ~) the third harmonic distor~ion which
would result if a capacitor we!re used.
The switching regulator 13 provides a substantially
constant dc voltage to a pair of output lines 17, 18. In
accordance with the present invention, a filter capacitor 19
connected across the output lines 17, 18 has a value
sufficiently large so as to filter at twice the ac line
frequency (e.g., at 120 Hz). The duty cycle of the switching
regulator 16 is controlled in response to the output voltage
across the capacitor 19, and therefor will depend on the
average voltage level over several cycles of the rectified
ac supplied to the input of the regulator 16.
This is in contradistinction to the typical prior
art arrangement in which the switching regulator duty cycle
is responsive to changes in the input line voltage and in
which a small output filter capacitor is used . Such a
capacitor filters effectively at the regulator switching
frequency (typically 20k Hz) but not at twice the line
frequency (e.g., at 120 Hz). rnerefor such a prior art
regulator draws more ~urrent from the ac line during the low
voltage portions of the ac cycle and less current during the
high voltage portions. Such regulation produces a lagging
power factor similar to that produced by inductive input
.. . . .
.;
1 ~ 33~
filtering, and hence is undesirable. As noted above, this
~s ellminated by the inventive arrangement in which the
output filter capcitor 19 is sufficiently large so as to
filter over several cycles of the supplied rectified ac voltageO
The regulated dc voltage on the lines 179 18 may
be supplied ~o any load requiring direc~ current in
FIGURE 1 i~ is supplied to an inverter 20 which typically
operates at a nominal frequency of 20~ Hz. Thus the output
oi.-~he inver~er 20 is an ac voltage square wave having a nominal
20k Hz frequency_ This ac v~l~age is ed to the lamp 11 via a
resonant network 21 which includes the lamp 11 as an element
of a resonant circuit. A feedback path 22 carries a control
signal that adjusts the frequency of the inverter 20 to
correspond to the resonant frequency of the network 21. In
this manner, a resistive load is seen by the inverter 20
regardless of the operating condition of the lamp 11, and
slnusoidal voltage is provided to the lamp, resulting in
improved lamp efficiency. The inverter switches at the
current null points of the network 21 output sinusoidal
voltage, since the inverter 20 operates at the network 21
resonant frequency. Switching losses are reduced since the
current through the inverter switching transistors is at or
near zero when switching occurs. Therefor less expensive
~ransistors of lower rating can be used.
Dimming oi. the lamp 11 can be accomplished by
controlling the vol~:age supplied by the regulator 16 to
the inverter 200 To this, an external adjustment 23 is
provided to control the duty cycle and hence the voltage
output of the switching regula~or 160 Alternatively, the
~v~
regulator 16 duty cycle may be controlled by a photocell 24
positioned to sense the light level at a location illuminated
by the lamp 11~ In this way, if sunlight produces a high
amb;ent light level at the location, this will be sensed by
the photocell ~4, resulting in dimming of the lamp 11.
Energy is conserved while the desired illumination level is
maintained.
In some applications it may be desirable to control
the output to the lamp 11 in response to the current flowing
to th2 lamp. This can be accomplished by providing a current
sensing resistor 25 in sPries with one of the lines 25, 26,
to the lamp 11, as shown in FIGURE 1. A feedback line 26 is
connected between the resistor 25 and the switching regulator
16 so as to control the output voltage cycle in response
to the sensed output current to the lamp 11,
'
FI&URE 2A shows one type of switching regulator
16A that can be used as the regulator 16 in the power supply lC
A switching transistor 30 is turned on and off by an oscillator
and drive circuit 31 at a rate (typically 30k Hz) that is
above the audio range. When the transistor 30 is on, some
energy is stored in an inductor 32 which maintains current
during the off-time of the transistor 30 via a diode 33. A
small RF filter capacitor 34 prevents RF signals, which may
be generated by the switching transients of the transistor 30
from being conducted back to the ac line.
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~1 33.~
In such a regul~tor L6A, the output voltage is a
direct function of the input supply voltage time the duty
cycle or on-to-off time ratio of the transistor 30. Since
the output of the bridge rectiEier 13 is unfiltered, the
supply voltage to the switching regulator 16 varies be~ween
zero and the peak ac line volt~ge, at twice the input ac
frequency. In a conventional switching regulator, the on-to~of
time ratio normally is varied in response to the input supply
voltage so that the on-time is greatest when the input
voltage is least. As illustrated in FIGURE 3A, this results
in maximum current flow when the input voltage is minimum.
This corresponds to a negative or very lagging power factor,
and is undesirable.
This effect can be o~ercome by reversing the
conventional approach and (a) making the on-time of the
switching transistor 30 a maximum when the input voltage is
greatest, and ~) using an inductor 32 that is sufficiently
large so as to maintain substantially the same current
through the switching transistor 30 regardless of the input
voltage. With this arrangement, the current waveform can
be matched to the voltage waveform, by controlling the on-
time at each portion of the ac line half cycle as illustrated
in FIGURE 3B, thereby creating the effect of a resistive
load or un~ty power ~actor.
An alternative approach is (a) to make the inductor
large with respect to the switching frequency (e.g., 30 kHz)
but small with respect to the ac line frequency ~e.gO, 60 Hz) 9
and ~b) to hold the duty cycle (on-to-off time ratio) of
3~
~he switching transistor 30 constant over a complete
half-cycle of the input ac, but allowing i~ to vary only
with gradual changes in the average input line voltage or required
output ~oltage. In this case, the value of the output capac~tor
lg mu~t be sufficient to filter ~he strong~ tw~ce llne frequency
(e.g., 120 Hz) ripple that will be present at the regulator
output. With this arrangement, the current waveform can
be made very closely to correspond to the input voltage
waveform, as illustrated in FIGURE 3C. The resul~ is a
unity power factor devoid of harmonic distortionO
A different form of switching regulator 16B,
als~ usable as the regulator 16 in the power supply 10, is
shown in FIGURE 2B. Here the inductor 35a is the primary
winding of a ferrite core transformer 35, and is connected
in series with the switching transistor 36 across the input
lines 14, 15 from the bridge rectifier 13. With this
arrangement, the inductor 35a is loaded and unloaded
each time the transistor 36 is turned on and off at he switching
frequency. Output dc ~oltage is taken from the transformer
secondary winding 35b via a diode 37. This circuit offers
the advantage that should the ssitching transistor 36 become
shorted for any reason, input current will not flow to the load.
In the circuit 16B, the output voltage may be regulated
elther by varying the switching frequency or the duty cycle. Wi~h
variable frequency control the oscillator and drive circuit 38
advantageous~y includes a voltage rontrolled or like
oscillator, the nominal frequency of which is controlled by
the external adjustment 23, the photocell 24 or the feedba~
signal on the line 26. Here again, the capacitor 19 should
~12- -
~ ~3~7~
be sufficiently large so as to filter effectively at twice
the ac line frequency. Changes in the filtered output
voltage are used to vary the frequency of the oscillator
and drive circuit 38 thereby to connect the switching frequency
so as to achieve substantially constant output voltage.
Other forms of switching regulator circuits may also
employ the novel concepts claimed he!rein of tayloring the
duty cycle to provide unity power factor without line distortion9
Details of one format of the inverter 20 and the resonant
network 21 are shown in FIGURE 4A. The regulated dc voltage
from the switching regulator lS is supplied alternately to one
or the other half windings 40a, 40b of an auto transformer
40 via a respective transistor 41 or 42. These two transistors
41~ 42 are alternately driven into conduction by an oscillator
and drive circuit 43, one embodiment of which is shown in
FIGURE 4b. Advantageously, the drive circuit 43 switches one
of the transistors 41, 42 completely off before ~urning the
other one on. This insures that both transistors 41, 42 are
never on at the same t~me. If both transistors were to be
on together, very high current would be dr~wn, greatly
reducing circuit efficiency and subjecting the transistors
41, 42 to damage. With the circuit shown in FIGURE 4, the
ac voltage developed across the autotransformer 40 on the
lines 44~ 45 is twice the dc input voltage. If the output
of the regulator 16 is selected to be in the range of about
35 v dc tc 50 v dc, the ac voltage developed across the lines
44, 45 will be on the order to 70 v to 100 v; thus, the
use of the regulator suffering a lower voltage than line
to the inverter allows the additional advantage of the use
of inexpensive low voltage components.
-13-
~ 3 ~ ~
The resonant network 21 includes the lamp 11 in
series with an inductor 45 across a capacitor 46. Since
the gas discharge lamp 11 is a negative impedance, which
operates at relatively constant voltage, the inductor 45
serves in part to limit current flow t~rough the lamp 11
Further, the inductor 45 and the lamp 1~ together represent
an inductive reactance in parallel with ~he capacitor 460
The value of the capacitor 46 is selected so that even when
the lamp 11 appears as a short circuit ~e.g~, during the
warm-up of a high intensity discharge la~p) an inductive
reactance is present, the combined reactance of the
parallel circuit including the lamp 11, the inductor 45
and the capacitor 46 will be capacitive. This capacitive
reactance resonates with a series lnduc~or 47.
The frequency of the inverter 2~ is controlled
such that the reactance of the inductor 47 equals the
capacitive reactance of the series-parallel combination of
the capacitor 46, the inductor 45 and the lamp 11. Under
such resonant condition, the network 21 presents a resistive
load to the inverter 20. A unity power factor is achieved.
Moreover, under such resonant condition, a sinusoidal voltage
is presented to the lamp 11 and a sinusoidal current load
is seen by the in~erter 20. Thus, even though the
transistors 41, 42 are switohed full on and full of in
"square-wave" fashion by the drive circult 43, the current
through the transistors 41~ 42 is at a ~inimum when the
swltching occurs, as the sinusoidal signal across the
resonant network 21 is going through null at this timeO
14-
~ 3 ~ 7 ~
Under various operating conditions of the lamp 11,
the effective impedance of the lamp 11~ and hence the
resonant frequency of the network 21 will change. A feed-
back circuit is used to correct the frequency of the invertor
20 so as to main~ain thP inverter frequency in resonance
with the network 21 when such lamp impedance changes occur,
To this end, a phase detector 50 (FIGURE 4) compares the
phase of the signal across the inductor 479 as sensed by a
sense winding 47, with the phase of the signal across the
transformer 40,as detected by a sense winding 40s~ The phase
of signal from winding 47s may also be compared with the
phase of the drive signal to transistors 41 and 42 with
the same result.
Since in an inductor the current through it is
90 degrees out of phase with the voltage across it and
since the current in inductor 45 ~s equal to that flowing
in the load the voltage on the sense winding 45s on inductor
45 will be 90 degrees out of phase with the current in the
load therefore with the voltage across the load since the
load is not reactive. At the proper frequency for resonance
of the network the voltage input to the network must be in
phase with the voltage across the load for the network to
appear resistive to the inverter. The phase detectox 50
senses whether the actual phase difference is indeed 90
degrees, indicating that the inverter 20 is operating at
the resonant frequency of the network 21. If not, an error
signaL is E~roduced on a line 51 that causes tha oscillator an
drive circuit 43 to alter frequency until the resonant
condition against is achieved.
.
~1 33~
At resonance, a voltage gain is achieved by the
resonant network 21. The voltage developed across the
capacitor 46, between the point 53 and the line 44, is
related to the Q of the network 21 l:imes the input voltage
from the ~nverter 20. This developed ~oltage is normally
selected to be on the order of 700 v to 800 v ac, whlch is
some three to four times greater than that required to sustain
normal operation of a typical gas diischarge lamp. Since the
lamp and network 21 at resonance appear as a resistlve load,
and since the ef~ective reactance of the network 21 components
i high with respect to the lamp 11, substantially constant
current will flow through the lamp 11, even though the voltage
across the lamp will change under different operating conditions.
These operating characteristics make the inventive
power supply 10 ideal for driving both high intensity discharge
~HID) and fluorescent lamps. Eor metal halide, sodium and
other HID lamps, the supply will provide proper drive for
starting, during the warm-up period and for normal operation.
For ~luorescent lamps, proper drive and flickerless dimming
is possible~
Typical HID lamps require a starting vol~age that is
substantially higher than the operating voltage for the same
lamp after it has heated up. Until actually started, the
impedance of an HID lamp is infinite, that is, it appears as
an open circuit. In this start-up mode, the inductor 45
~FIGURE 4) is effectively out of the circuit, and the resonant
network 21 consists effectively of the capacitor 46 in series
with the inductor 47~ Since these are relatively lossless
elements, the network 21 will exhibit a very high Q~ The
voltage developed across the capacitor 46,rand hence across the
-
capacitor 46~ and hence across the lamp 11, will become
16-
' ~ :
~ ~3~
very high, potentially reaching over ~ thousand volts~ This
high voltage will insure initiation of conduction of the lamp,
regardless of the lamp 15 condition or age. In many cases,
the supply 10 will start a lamp ll that has deteriorated to
a point where it cannot be started with a conventional ballast
the result is improved useful lifetime for the lamp.
With no load currenlt flowing there is no voltage
drop across inductor 4~ t~us no feedback on sense winding
45s there fore for open load conditions voltage is sensed at
the junction 53 of capacitor 46 and inductor 47 and supplied
to phase detector 50 by line 55. Under such open load
condi~ions the voltage will contin~e to rise at point 50 as
long as the regulator will supply the necessary current to
the inverter. Therefore, current to the in~erter is sensed
by the voltage across resistor 56 and fed back to the
regulator 16 on line 26 to control the input to ~he inverter
and therefore the maximum voltage permissable at point 53O
During warm-up, an HID lamp typically exhibits a
very low impedance and thus presents virtually a short circuit
load. In a conventional system this would place a very high
current demand on the power supply. In the inventive power
supply 10 however, the actual short circuit of the lamp 11
is not seen as a shortened load. Rather, the "shor~ circuit"
effectively inserts the inductor 45 across the capacitor 46,
thereby changing the resonant frequency of the network 21 but
not shorting the supply. The feedback circuit will shift the
frequency of the in~erter 20 to the new resonant frequency of
the network 21, and the current limiting characteristic of
this network will limit the current flow through the lamp 11
as it warms up. Subsequent to warm-up, when normal lamp
operation is achieved and the lamp impedance is finite but
not short-circuited, proper voltage with substantially
constant current will continue to be provided by the supply 11
The inverter 20 frequency will automatically be set
somewhere between the extremes corresponding to the lamp
start and lamp warm-up conditions at the new resonant
frequency of the network 21. Dimming of the HID lamp
can be achieved either by changing the dc voltage supplied
to the inverter 20 by the switching regulator 16 or by
altering the duty cycle of the inverter 20.
~ mong the benefits provided by the inventive
power supply 10 are improved efficiency and the elimination
of flicker in a metal arc lamp. The high frequency
~typically 20 kHz) drive provided by the inverter 20 tends
to create a smaller diameter ion stream in a HID lamp then
if the same lamp were driven at the ac line frequency.
Increased efficiency results since concentration of ~he
current into a smaller area creates a higher energy level
in the gas molecules, thereby inducing more light output~
In prior art high frequency drive sources,
flicker was produced in the metal arc lamp. Since the
lamp tube diameter provided for a larger ion stream than
that which is present with high frequency drive, the
smaller diameter ion stream is free to wander or move
about in the unconfined area within the tube. This moving
is referred to as "flicker". It sometimes appear~ as
slowly moving or rotating standing wav~s in the ion stream.
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. ~
~i~3 ~ ~
It has been found that flicker is eliminated
by the frequency control employed in the inventive power
supply 10. The movement of the ion stream is accompanied
by a slight change in la~p impedance. As discu~sed
above, this results in a concomitant change in the resonant
frequency of the network 21 and hence in a corrective
change in the frequency of the inverter 20. This change
in drive ~requency has been found to eliminate flicker.
A benefit o~ the inventive system is that
conventional, inexpensive components can be used. In
the inverter 20, the transistors 41, 42 need exhibit only
a reasonable switching time, because of the modest switching
frequency of about 20 kHz, and since the drive circuitry
43 provides for a quiescent time between turn-off of one
transistor 41 or 42 and turn-on of the other. Use of a
dc supply voltage from the regulator 16 of between 35 v
dc and 50 v dc permits use of transistors 41, 42 rated at
about 100 volts. The use of a sinusoidal output signal
also reduces the performance requirements and hence the
cost of the switching transistors 41 and 42.
FIGURE 4B shows another inverter configuration
that eliminates transformer 40 and adds capacitors 57 and
58. Besides the advantages of simplicity and size this
circuit puts even less voltage strain on transistors 41
and 42. Higher voltage from the regulator may be used when
desired with transistors having the same voltage rating
as shown in FIGURE 4A. Also, voltage spikes associated
with transformer drive are eliminated. Inductor 47 will
increase in size somewhat howeverO
-19-
33~
~ n optional feature of the present invention
is that it facilitates battery opera~ion of a gas discharge
lamp, particularly during periods of power failure of
the ac line source. To this end, a lbattery 70 may be
connected in series with a diode 71 across the output
lines 17, 18 from the regulator 16, ias shown in FIGURE lo
During normal operation, the diode 71 effectively disconnects
the battery 70 from the circuit. However, in the even~
of an ac power failure, the battery 70 will provide
~nput ~oltage to the inverter 20, maintaining the lamp 11
lit. If dimming is required9 the ~oltage of the battery 70 must
be equal to the lowest desired output of the regulator lÇ. In
such ~nstan~e, the ~amp light output will be dimmed under battery
operation, but will be remain illuminated, e.g.~ for emergency
e~acuation purposes. The lower battery voltage will result
in longer operation. A charging circuit 72 may be
provided to charge the battery 72 from the dc voltage
present at the regulator 16 output during periods of
normal operation.
Use of the standby battery 70 is particularly
valuable with the HID lamps. If an HID lamp is extinguished~
it must first be cooled before it can be relit. With
standby battery operation, the HDD lamp will not be
extinguished during short power outages, and therefore
will come back on ~mmediately to the set intensity when
the power is resto~red.
~20
' ~,-, . ~ - .
~ 33~9
As noted~ the inventive circuit provides for
dimming the lamps. To accomplish this, the input dc
voltage from the switching regulator can be varied~
This dc voltage level can be adjusted manually using the
external adjustment 23 or in eesponse to some other
condition, as for example the ambient light level sensed
by the photocell 24. Dimming is effec~uated since the
voltage developed across the capacitor 46 (FIGURE 4)
is a direct multiple of the voltage supplied to the network
21 which itself is twice the dc input vol~age from the
regulator 16.
By altering the regulator 16 output, the
voltage across the capacitor 46 will be altered accordingly,
Since the inductor 45 is the principal source of impedance
in the series connection of the inductor 45 and the lamp 11~
the current through the lamp will be related almost directly
to ~he voltage across ~he capacitor 46. Thus a change in
input voltage will cause the lamp 11 current to drop
proportionately. The lamp will remain lit at a changed
intensity, The high voltage available across the capacitor
4~ will keep the lamp 11 lit even at lower intensity levels~
despite the negative resistance characteristics of the lamp,
thereby insuring flickerless dimming performance
.
Intending to claim all novel ~eatures shown
and described, the inventor:
-21-
