Note: Descriptions are shown in the official language in which they were submitted.
CA 02296962 2000-O1-26
OPEN LOOP CURRENT CONTROL BALLAST
FOR LOW PRESSURE MERCURY GERMICIDAL UV LAMPS
This invention relates to an improved low cost open loop current
stabilization circuit for starting and operating ultra violet (UV) lamps, and
more
particularly for starting and operating low pressure germicidal UV lamps used
in water
sterilization apparatus.
The performance of water sterilization apparatus utilizing low pressure
germicidal UV lamps is directly related to absolute current flow in the UV
lamp.
The ballast circuits used in UV water sterilizer applications today are
generally modified versions of ballasts originally developed for fluorescent
lighting
applications. These ballasts generally have no current control function, and
this results
in the current flow in the low pressure mercury UV lamp varying with changes
in the
input supply voltage. In the case of simple magnetic ballasts, current flow in
the low
pressure mercury UV lamp will also vary with frequency of the input AC power.
The reliability of many of the ballasts presently used in UV water
sterilization apparatus, especially when operating from 240 volt AC input
power and
utilizing short arc length lamps, has not been satisfactory.
Various types of ballast circuits utilizing the frequency modulation current
control technique are well known in the art.
US patent 2,928,994 by Widakowitch shows a saturable core variable
frequency DC powered inverter whose frequency varies as a function of the DC
supply
voltage to maintain the current flowing in the lamp relatively constant.
US patent 4,060,751 by Anderson and US patent 4,060,752 by Walker
both describe inverter circuits in which the switching elements are commutated
when
the instantaneous current exceeds a predetermined level. The lamp is coupled
to the
inverter output with a series inductor capacitor network with the lamp
connected across
the capacitor element. Before ignition the lamp appears as a high impedance
shunted
across the capacitor. On power up the inverter output sees only the series
inductor
capacitor network and is forced to operate at the resonant frequency of the
series
inductor capacitor combination. As the resonance voltage builds, the point at
which
lamp ignition is initiated is reached and the negative impedance of the
ignited lamp
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effectively shunts the series capacitor. After the lamp is ignited, the
inverter switches
are commutated when the instantaneous lamp current exceeds the predetermined
current threshold. As the rate of current rise increases, the predetermined
current
reference point will be reached more quickly increasing the frequency of
oscillation of
the inverter and reducing the current flow through the series combination of
the ballast
inductor and UV lamp. As the rate of current rise decreases the amount of time
to reach
the predetermined current switching threshold is extended, lowering the
frequency of
oscillation and allowing more current to flow through the series combination
of the
ballast inductor and UV lamp.
US patent 4,498,031 by Stupp, US patent 4,585,974 by Stupp, US patent
4,698,544 by Stupp and US patent 4,700,113 by Stupp disclose a ballast circuit
consisting of driven push-pull inverter circuit driven by a voltage controlled
oscillator
whose output is divided by two with flip-flop circuit to insure symmetrical
drive to the
inverter switches. In all cases a closed loop current sensing circuit controls
the
frequency of the voltage controlled oscillator. The use of smaller filter
capacitors to filter
the pulsating DC output of the AC to DC rectifier to improve the circuit power
factor is
claimed and implication of improved lamp current crest factor though not
specifically
mentioned is obvious from the operational description. The use of analog,
digital and
hybrid analog digital implementations for the oscillators is discussed.
US patent 4,717,863 by Zieler describes a ballast circuit with a driven
push-pull inverter and non resonant output network. The output frequency of
the
inverter is at the lower limit of operation on start up to provide maximum
preheat current
to the lamp filaments. Control of the lamp current after ignition using closed
loop circuits
based on lamp current or measuring lamp output with a photo-detector circuit
are
described. Additionally, an interface to allow the desired output level to be
programmed
by an external computer input is disclosed.
US patent 4,727,470 by Nilssen describes a ballast circuit utilizing a self
oscillating half-bridge inverter topology using a saturable core transformer
in the feed
back loop. The lamp is connected to the inverter output across the capacitor
of series
inductor capacitor network. On start up the lamp appears as a high impedance
parallel
with the capacitor element of the series inductor capacitor output network and
the
inverter oscillates at the resonant frequency of this network. As the
resonance voltage
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builds, the lamp is ignited and the capacitor is effectively shunted out of
the series
inductor capacitor network by the negative impedance of the lamp. The
operating
frequency of the inverter when the lamp is ignited is determined by the
saturation flux
density of the saturable core transformer feedback element. A technique for
modifying
the saturation point of the saturable core feedback element in both closed
loop and
open loop current control topologies is described. Improved power factor and
lamp
current crest factor are both claimed.
It is an object of the present invention to provide an improved ballast
circuit for starting and operating low-pressure mercury germicidal UV
discharge lamps,
particularly in UV water sterilization apparatus.
In the invention, an improved apparatus for starting and operating UV
lamps, comprises an AC line powered circuit, for example a frequency modulated
inverter, and one or more UV lamps. The circuit has a variable frequency AC
source,
for instance comprising an inverter driven by a voltage controlled oscillator
(VCO),
providing current to the UV lamps and a ballasting inductor, connected in
series with
the UV lamps. The inductor and the lamps form a nonresonant coupling sub-
circuit
where a flow of current from the AC source through the UV lamps is limited by
the
ballasting inductor. The output frequency of the VCO is modulated by changes
in the
input voltage.
The UV lamps are preferably low pressure mercury germicidal UV lamps,
and preferably used in a UV water sterilizing apparatus.
The AC source preferably comprises a rectifier filter and an inverter, the
inverter being driven by a frequency modulated VCO circuit.
The frequency modulated inverter is preferably a driven half-bridge
inverter.
The VCO circuit preferably comprises a varactor-tuned voltage controlled
RC oscillator. A drive signal, that commutates the frequency modulated
inverter, is
obtained from the varactor-tuned voltage controlled RC oscillator, so that the
output
frequency of the oscillator is modulated by the variations in the input supply
voltage for
maintaining a substantially constant current flow through the series connected
ballasting inductor and the UV lamps.
The output of the varactor-tuned voltage controlled RC oscillator is
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preferably divided by a flip-flop circuit, to insure symmetry of the drive
signal.
The varactor-tuned voltage controlled RC oscillator preferably comprises
a 555 timer equivalent integrated circuit.
Advantageously, at least a portion of a frequency determining resistance
of the varactor-tuned voltage controlled RC oscillator comprises a variable
resistance,
to enable the effects of component tolerances to be eliminated during the
manufacturing process and during use of the apparatus.
Thus, a preferred circuit of the present invention utilizes a frequency
modulated half bridge inverter whose output is coupled to a UV lamp with a
series
current limiting inductor. The output coupling circuit is non-resonant and
does not
develop dangerously high output voltages if the UV lamp is disconnected or
burnt out,
as would be the case with resonant output circuits unless additional circuit
protection
is implemented.
The half bridge output stage is driven by a varactor diode tuned voltage
controlled oscillator through a divided by two flip-flop circuit to insure
drive symmetry.
The oscillator circuit of the present invention incorporates an adjustable
resistor in the
frequency-determining portion to enable all component tolerances to be
compensated
for when calibrated during manufacture. Additionally the frequency of the
voltage
controlled oscillator is modulated by the value of the input supply voltage in
a
predetermined manner so as to provide the optimum operating conditions for the
lamp
over a wide variation in input supply voltage.
The impedance of the ballast inductor varies with the current frequency,
enabling it to act as a current control element with changes in the inverter
output
frequency. The open loop control circuit of the present invention maintains
the lamp
current at a relatively constant level without the complexity imposed by
closed loop
current control methods.
Further features of the invention will be described orwill become apparent
in the course of the following detailed description.
In order that the invention may be more clearly understood, the preferred
embodiment thereof will now be described in detail by way of example, with
reference
to the accompanying drawings, in which:
Fig. 1 is a functional block diagram of the preferred embodiment of the
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invention; and
Fig. 2 is a schematic diagram of the preferred embodiment of the
invention.
Fig. 1 is a functional diagram of a preferred embodiment of the present
invention. The AC input supply voltage is converted to filtered DC by a
conventional
diode rectifier 12 with filter capacitor(s). A sample of the DC supply voltage
13 is also
routed to a voltage controlled oscillator 14, which provides a drive signal 15
for an
inverter circuit 16, for example a half-bridge inverter. The filaments 17 and
18 of a low-
pressure mercury germicidal UV lamp 9 are connected to the output of the half
bridge
inverter 16 in series with a ballasting inductor 10. The impedance of the
ballasting
inductor varies with the frequency, and can therefore be used to control the
current
flowing in the circuit if the output frequency of the half-bridge inverter 16
can be varied.
The tuning characteristics of an oscillator 14, for example a varactor tuned
voltage
controlled oscillator, are designed so that the output frequency of the half-
bridge
inverter 16 will result in a substantially constant current flow in the
ballast inductor 10
over the full allowable range of input supply voltages. When power is first
applied to the
ballast circuit the impedance of the low pressure mercury germicidal UV lamp 9
is very
high and the inverter output is forced to flow through the filaments 17 and 18
and pre-
heat capacitor C3 (see Fig. 2). The value of the pre-heat capacitor C3 is
selected so
that it will not form a resonant condition with ballasting inductor 10 under
any condition.
Once the filaments 17,18 have reached the critical temperature to enable the
lamp to
ignite, the impedance of the pre-heat capacitor C3 is effectively bypassed
from the
circuit by the negative impedance of the ignited low pressure mercury
germicidal UV
lamp 9.
A preferred embodiment of a ballast and starting circuit of the present
invention is shown schematically in Fig. 2. Direct current to operate the
inverter is
obtained from the AC line input voltage from a rectifier, filter capacitor
arrangement
consisting of diodes D1, D2, D3, D4 and filter capacitors C1 and C2.
The rectifier arrangement of the preferred embodiment of Fig. 2 can be
operated as a full wave bridge rectifier consisting of diodes D1, D2, D3, and
D4 when
a 240 volt input supply voltage is connected to terminals 19 and 20 with the
filter
capacitors C1 and C2 acting as both a filter capacitor and a capacitive
voltage divider.
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For operation from a 120 volt AC input supply the input supply is connected to
terminals
19 and 21. In this configuration the circuit consisting of diodes D2, D3 and
filter
capacitors C1 and C2 operates as a conventional voltage doubter. This
rectifier, filter
capacitor arrangement enables the use of the same circuit and components for
both
voltage ranges.
The commutation of the half bridge output circuit of the preferred
embodiment described in Fig. 2 is provided by integrated circuit 11, which
combines
both the voltage regulator, oscillator and frequency divider functions as well
as drivers
for the FET switching elements Q1 and Q2. The integrated circuit 11 power
supply
regulator consisting of a zener diode connected to pin 1, is powered from the
rectifier
filter circuit via voltage dropping resistor R1 and filter capacitor C9. The
DC supply
voltage of the internal voltage regulator is bootstrapped to the high side
driver portion
of the integrated circuit 11 by diode D5 and capacitor C8. The oscillator
function of the
integrated circuit 11 is an RC oscillator similar to that found in the
familiar 555 timer
integrated circuit. The oscillator timing components consists of resistor R6
in series with
variable resistor R5, the parallel arrangement of capacitor C6 and the
capacitance of
varactor diode D6 determine the oscillatorfrequency of oscillation. The value
of variable
resistance R5 is selected such that it is capable of calibrating out any
errors in output
frequency due to component tolerance during manufacturing. The bias on the
varactor
tuning diode is provided by voltage divider of resistor R2 and R3. One side of
resistor
R2 is connected to the positive DC voltage from the rectifier, filter circuit
22 with
remaining side of resistor RZ connected to one side of resistor R3. The
remaining side
of resistor R3 is connected to the DC supplies negative voltage of the
rectifier, filter
circuit 23. The values of resistors R2 and R3 are selected so as to bias the
varactor
diode at the center of its range under nominal input AC supply voltage
conditions.
Capacitor C7 isolates the RC oscillatorfunctions of the integrated circuit 11
from the DC
bias voltage applied to the varactor diode D6. The capacitance value of
capacitor C7
is much greater than the total value of the sum of capacitors C6 and varactor
diode D6
so as to have a negligible effect on the frequency of oscillation of the RC
oscillator. The
value of the RC oscillator timing components, resistor R6, R5 and capacitor C6
in a
parallel arrangement with the varactor diode D6, are selected so that the
change in
oscillator frequency with variations in input voltage results in a
substantially constant
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current flowing through the series arrangement of the low pressure mercury UV
lamp
9 ballasting inductor 10 and coupling capacitor C4. Coupling capacitor C4
eliminates
any DC component from the voltage applied to the low pressure mercury
germicidal UV
lamp. The commutation drive to the switching elements of the half-bridge
inverter FET
Q1 and 42 is supplied by an high side driver circuit on the integrated circuit
11 via
resistors R7 and R8.
The low pressure mercury germicidal UV lamp 9 in the preferred
embodiment of Fig. 2 is a preheat type of lamp and is connected to the ballast
circuit
via terminals 24,25,26 and 27. Starting of the low pressure mercury germicidal
UV lamp
9 is assisted by a preheat current flowing through the lamp filaments 17 and
18 through
capacitor C3, which is connected to terminals 26 and 27 of the ballast
circuit. Once the
low pressure mercury germicidal UV lamp is ignited, the capacitor C3 is
effectively
shunted from the lamp circuit by the negative impedance of the low pressure
mercury
germicidal UV lamp 9. The preferred embodiment of Fig. 2 is specifically
designed for
use with preheat lamps, but the present invention is not limited to only this
type of lamp.
For example, if used with short arc length lamps, no starting circuit at all
would be
required and use of instant start type lamps could be accommodated by
increasing the
level of the DC supply voltage to the half-bridge inverter.
While a preferred embodiment of the invention has been described in
detail many modifications may be effected by those skilled in the art.
Accordingly it is
intended by the appended claims to cover all such claims as fall within the
true spirit
and scope of the invention.
It will be appreciated that the above description relates to the preferred
embodiment by way of example only. Many variations on the invention will be
obvious
to those knowledgeable in the field, and such obvious variations are within
the scope
of the invention as described and claimed, whether or not expressly described.
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