Note: Descriptions are shown in the official language in which they were submitted.
2179437
FIELD OF THE INVENTION
This invention is a division of Canadian Patent
application Serial No. 2,062,126-5 filed March 2, 1992.
This invention relates to low-pressure discharge
lamps, particularly fluorescent lamps, and especially to
starting and operating circuity for compact fluorescent
lamps.
BACKGROUND OF THE INVENTION
Various types of operating circuits are known to
start and operate compact fluorescent lamps. One type of
circuit is illustrated in the FIG. 7 schematic of French
Publication No. 0346782 (December 20, 1989). This
schematic is similar, in general principle, to the state
of the art as practised in a lamp sold by the Osram
Company under the registered trademark "DULUX EL" or in
the lamp of the Philips Company which bears the
denomination "PLC 20 Electronic". Using the circuit of
French Publication 0346782 (December 20, 1989) as an
example of such circuits, after the two input terminals of
the DC/AC converter (or oscillator) are energized by a DC
voltage which appears across a filter capacitor, the
starting capacitor C5 charges through a starting resistor
R3 to a voltage which is substantially equal to the
threshold voltage of the threshold element (i.e., the
diac). The threshold element breaks down and supplies a
pulse to the base terminal of transistor T2. As a result,
transistor T2 begins to conduct. A current
21794~7
-2- PATENT APPLICATION
flows through transistor T2 and the load circuit.
Subsequently, this transistor becomes non-conducting
and the other transistor Tl becomes conducting. This
process is then continuously repeated. This leads to
an oscillation, i.e., an alternating current through
the load circuit which includes the discharge tube.
It has been discovered that disadvantages may
appear in some circuits similar to those described
above. For example, when power to the circuit is
removed, a momentary blink or flicker in the lamp may
occur immediately after the tube is extinguished. It
has been observed that when AC power to the circuit is
removed, a voltage initially remains on the filter
capacitor of the DC power supply. This filter
capacitor voltage gradually depletes to a point
(usually greater than the starter threshold voltage)
where the oscillator shuts down. However, the
starting capacitor is allowed to recharge to a point
where the threshold element of the starting circuit
triggers causing the oscillator to conduct for a short-
period of time. Consequently, the discharge tube will
blink or flicker as a result of current from the
filter capacitor flowing through the conducting
transistors and load circuit. This conduction
continues for approximately 10 msecs. until the filter
capacitor voltage is less than the starter's trigger
voltage.
Another disadvantage may appear in circuits
similar to those described above at the end of lamp
life when the emissive material on one or both of the
filament electrodes has depleted. Although a
discharge is unable to be established between the lamp
~179437
-3- PATENT APP~ICATION
electrodes, the oscillator may continue to conduct
current through circuit components causing an
unnecessary consumption of power until, for example,
the AC power source is disconnected or the lamp and
tank capacitor are removed from the load circuit. In
the instances where the lamp is permanently connected,
such as in an integral lamp unit, this latter option
is unavailable.
SUMMARY OF THE INVENTION
It is, therefore, an object of the present
invention to obviate the disadvantages of the prior
art.
It is still another object of the invention to
provide an improved circuit for starting and operating
a discharge lamp.
It is another object of the invention to provide
an improved circuit which does not result in a
momentary blink or flicker in the lamp immediately
following the removal of AC power from the circuit.
It is still another object of the invention to
provide an improved circuit which does not cause an
unnecessary power consumption upon an end-of-life
condition caused by the depletion of emissive material
on one or both of the lamp filament electrodes.
These objects are accomplished in one aspect of
the invention by the provision of a starting and
operating circuit for a discharge lamp comprising a
pair of AC input terminals adapted to receive an AC
signal from an AC supply and a DC power supply coupled
to the AC input terminals for generating a DC
2179437
"_
-4- PATENT APPLICATION
voltage. An oscillator includes a pair of
semiconductor switches and is coupled to the DC power
supply so as to receive the DC voltage. An oscillator
starting circuit is coupled to one of the
S semiconductor switches. The oscillator starting
circuit includes a series circuit comprising a
resistor and a capacitor connected together and a
threshold element connected to the junction point of
the RC starting circuit. The input to the oscillator
starting circuit is coupled to one of the AC input
terminals so that the capacitor means is charged only
on one half cycle of the AC signal. A load, which
includes the discharge lamp, is coupled to the output
of the oscillator.
In accordance with further teachings of the
present invention, the load further includes a tank
capacitor in parallel with the lamp and a tank
inductor in series with the parallel combination of
the tank capacitor and lamp. The tank inductor and
tank capacitor form a resonant circuit having a
resonant frequency greater than the switching
frequency of the oscillator.
In accordance with another aspect of the present
invention, the oscillator further includes a fusible
circuit element to inhibit further operation of the
oscillator upon failure of the lamp due to depletion
of emissive material on at least one of the lamp
electrodes.
In accordance with further aspects of the present
invention, each of the semiconductor switches includes
a first resistor connected in series with the output
thereof and a second resistor connected to the input
~17943 ~
-5- PATENT APPLICATION
thereof. Preferably, both the first and second
resistors are fusible type resistors.
Additional objects, advant~ges and novel features
of the invention will be set forth in the description
which follows, and in part will become apparent to
those s~illed in the art upon examination of the
following or may be learned by practice of the
invention. The aforementioned objects and advantages
of the invention may be realized and attained by means
of the instrumentalities and combination particularly
pointed out in the appended claims.
BRIEF DESCRIPTION OF THE DRAWINGS
The invention will become more readily apparent
from the following exemplary description in connection
with the accompanying FIGURE. This FIGURE represents
a schematic diagram of a preferred embodiment of a
starting and operating circuit for a discharge lamp
according to the present invention.
BEST MODE FOR CARRYING OUT THE INVENTION
For a better understanding of the present
invention, together with other and further objects,
advantages and capabilities thereof, reference is made
to the following disclosure and appended claims in
connection with the above-described drawings.
The sole FIGURE represents a schematic diagram of
a preferred embodiment of a starting and operating
circuit for a discharge lamp LP1. Lamp LPl is an arc
discharge lamp such as a low-pressure fluorescent lamp
2179437
-6- PATENT APPLICATION
having a pair of opposing filamentary electrodes El,
E2. Each of the filamentary electrodes is coated
during manufacturing with a quantity of emissive
material. Lamp LP1, which forms part of a load
circuit 16, is ignited and fed via an oscillator 12
which operates as a DC/AC converter. Oscillator 12
receives filtered DC power from a DC power supply 10
which is coupled to a source of AC power. Conduction
of oscillator 12 is initiated by a starting circuit
14. The circuit will be described in more detail
below.
A pair of input terminals INl, IN2 are connected
to an AC power supply such as 10~ to 132 volts, 60
~z. A transient suppressor Rvl is shunted across
input terminals IN1, IN2 in order to absorb any surge
energy that may otherwise cause damage to the
circuit. The AC input power is coupled by way of a
fuse Fl to the input of DC power supply 10 which
consist of diode rectifier bridge D3 and a filter
capacitor C6. Capacitor C6 filters the rectified AC
voltage so that the bus voltage (VBUS) is a DC voltage
with minimal low frequency modulation which serves to
minimize lamp current crest factor. A capacitor C5,
which is connected in parallel with transient
suppressor RV1 and the input to DC power supply 10,
and an inductor L2 connected to the positive output
terminal of DC power supply 10 serve to suppress EMI
generated by oscillator 12.
Oscillator 12, which includes (as primary
operating components) a pair of series-coupled
semiconductor switches, such as bipolar transistors
Ql, Q2 or MOSFETS (not shown), is coupled in parallel
~179~37
._ .
-7- PATENT APPLICATION
with the output of DC power supply 10. The collector
of transistor Q2 is connected to one end of inductor
L2 while the emitter of transistor Q2 is connected to
one end of a resistor R5. The other end of resistor
R5 is connected to the collector of transistor Ql.
The emitter of transistor Q~ is coupled to circuit
ground through a resistor R6. During lamp operation,
emitter resistors R5 and R6 minimize lamp current
variations caused by temperature. More specifically,
as the junction temperatures of transistors Q1 and Q2
increase due to increases in ambient temperature, the
base-emitter voltages of Ql and Q2 tend to decrease.
As a result, the voltage drop across resistors RS and
R6 increases and thereby compensates for the decrease
in the base-emitter voltage. Consequently, the lamp
current will remain relatively constant with
temperature. In addition to compensating for
temperature variations, resistor R6 acts to limit the
current through transistor Q1 at initial startup.
Base drive and switching control for transistors
Q1 and Q2 are provided by secondary windings W2, W3 of
a saturable transformer Tl, base resistors R2 and R3,
and capacitors C2 and C3. The values of resistors R2
and R3 are chosen so that transistor control or base
leads are properly driven.
Oscillator startin~ circuit 14 includes a series
arrangement of a resistor R1 and a capacitor Cl. The
junction point between resistor Rl and capacitor Cl is
connected to a bidirectional threshold element D2
(i.e., a diac). One end of threshold element 12 is
coupled to the base terminal of transistor Q1 through
base resistor R2. As illustrated in the FIGURE, the
~179437
-
-8- PATENT APPLICATION
input of the oscillator starting circuit 12 ti.e., the
upper end of resistor R1) is connected to one of the
terminals (e.g., IN2) applied to the AC power supply.
During normal lamp operation, oscillator starting
S circuit 14 is rendered inoperable due to a diode
rectifier Dl connected to the junction point of
resistor R1 and capacitor Cl. During lamp operation,
diode rectifier D1 holds the ~oltage across starting
capacitor C1 at a level which is lower than the
threshold voltage of threshold element D2. The time
constant of Rl and C1 should be longer than the
operating frequency of the oscillator in order to
insure that capacitor C1 does not recharge during
normal operation to the threshold voltage of element
D2.
Series capacitors C9 and C10 form one of the two
legs of the half-bridge topology. The other leg being
formed by the series coupled transistors Q1 and Q2.
Unlike conventional half-~ridge circuits in which the
two capacitors are the main energy reservoirs, in the
present circuit capacitors C9 and C10 function as a
voltage divider and help shunt EMI/RFI noise generated
by transistors Ql and Q2.
Load circuit 16 comprises a series combination of
a primary winding Wl of transformer Tl and an inductor
Ll connected in series with a parallel combination
formed by lamp LP1 and a capacitor C4. Inductor Ll
comprises the principle ballasting element for the
lamp. The saturation of transformer Tl influences the
switching fre~uency of transistors Ql and Q2.
Typically, the transistor switching frequency is from
about 25 Khz to 39 Khz. Preferably, the switching
~179437
-9- PATENT APPLICATION
frequency is about 30 Khz. During lamp operation, the
impedance of capacitor C4 is much higher than the
impedance of the lamp, so capacitor C4 acts as an open
circuit. The total load impedance is the sum of the
impedance of inductor L1 and the lamp impedance in
series, which will make the lamp current a sawtooth
waveform. The resonant frequency during normal
operation is very different from the resonant
frequency during startup. In one example, the
resonant frequency during startup is about 46 Khz, but
during operation the resonant frequency is about .25
Hz.
As is conventional in instant-start lamp
operation, the two terminals from each lamp electrode
are connected together. Load circuit 16 further
includes a capacitor C8, connected across the circuit
arrangement of primary winding Wl, inductor Ll, lamp
LPl and capacitor C4. Capacitor C8 forms a single
element snubber circuit which reduces the rise time
and thus the switching losses of transistors Q1 and
Q2. As a result of the reduction in rise time (or
equivalent reduction in dVCe/dt) of transistors Q1
and Q2, high voltage spikes which normally generate
EMI/RFI noise are reduced.
The operation of the circuit will now be
discussed. When terminals IN1 and IN2 are connected
to a suitable AC power source, DC power source 10
rectifies and filters the AC signal and develops a DC
voltage (VBUS) across capacitor C6. Simultaneously,
during the negative half cycle of the AC input signal,
starting capacitor C1 begins to charge through
resistor Rl to a voltage which is substantially equal
~179437
-10- PATENT APPLICATION
to the threshold voltaqe of the threshold element D2.
Upon reaching the threshold voltage (e.g., 32 volts),
the threshold element breaks down and supplies a pulse
to the input or base terminal of transistor Q1. As a
result, current from the Vbus supply flows to circuit
ground through inductor L2, capacitor C9, capacitor
C4, ballast inductor Ll, primary winding W1 of
transformer T1, the collector-emitter junction of
transistor Q1 and emitter resistor R6. Since the lamp
is essentially an open circuit during starting, no
current flows through the lamp at this time. Current
flowing through primary winding W1 causes saturation
of the core of transformer Tl which forces the
inductance of the transformer Tl to drop to zero. A
resulting collapse in the magnetic field in
transformer T1 result in a reverse in polarity on
secondary windings W2 and W3 of transformer T1. As a
result, transistor Ql is turned off and transistor Q2
is turned on. Current now flows to ground through
inductor L2, the collector-emitter junction of
transistor Q2, emitter resistor R5, primary winding Wl
of transformer T1, ballast inductor Ll, capacitors C4
and C10. This process is repeated causing a high
voltage to be developed across capacitor C4 (and lamp
LP1) as a result of a series resonant circuit formed
by capacitors C4, C9, C10 and ballast inductor L1.
The high voltage developed across capacitor c4 is
sufficient to ignite lamp LP1. In addition to
igniting lamp LPl, capacitor C4 improves lamp current
crest factor.
During normal lamp operation, oscillator starting
circuit 14 is rendered inoperable due, in part, to
~179437
-11- PATENT APPLICATION
rectifier Dl which holds the voltage across starting
capacitor Cl at a level which is lower than the
threshold voltage of threshold element D2. Any charge
developed across starting capacitor Cl during this
period is continuously discharged to circuit ground
through diode Dl, the collector-emitter junction of
transistor Ql and emitter resistor R6. In addition,
the time constant of Rl and Cl is selected to be
longer than the operating frequency of the oscillator
so that capacitor Cl will not recharge through
resistor R1 to a level to retrigger diac D2.
When AC input power to the circuit is removed,
starting capacitor C1 is unable to receive energy from
filter capacitor C6 since the input to the starting
circuit (i.e., the upper end of resistor Rl) is
connected to one of the input terminals IN2. As a
result, current from the filter capacitor will be
unable to flow through the conducting transistors and
load circuit to otherwise cause the lamp to
momentarily blink or flicker after the lamp has
extinguished.
It is noted that capacitor C1 charges only when
the AC input voltage on input terminal IN2 is positive
with respect to input terminal INl. During this half
cycle of the AC supply, current flows from input
terminal IN2, through fuse F1, resistor R1, capacitor
C1, diode leg D3A (of diode rectifier bridge D3) to
input terminal IN1. No charge path for capacitor C1
is provided when the AC input voltage on input
terminal IN2 is negative with respect to input
terminal IN1. In addition to preventing the
momentarily blink or flicker discussed above, the
2179437
-12- PATENT APPLICATION
power dissipated by resistor R1 is reduced since
resistor Rl only sees 60 Hz half wave voltage.
In circuits similar to those described above,
power may continue to be consumed in the oscillator
upon an end-of-life condition caused by the depletion
of emissive material on one or both of the lamp
filament electrodes if the electrodes and lamp
- envelope remain intact. When the emissive material on
the lamp electrode is depleted, the lamp acts as an
open circuit element. The circuit will then run in a
series resonant mode with resonant elements of
inductor L1 and capacitor C4. By the nature of any
series resonant circuit, the combined impedance of
inductor Ll and capacitor C4 is zero. The only
noticeable impedance in the circuit is the emitter
resistor, the winding resistance of inductor L1 and
the collector-emitter resistance. The combination of
these resistances is very small (i.e., smaller than 10
ohms). Basically, the circuit is in a short circuit
mode. The short circuit current of transistors Ql and
Q2 will be very high. For example, when Vbus is equal
to 169 VDC and the short circuit resistance is equal
to 10 ohms, the short circuit current will be
169/10=16.9 amps.
In accordance with the teachings of the present
invention, base drive resistors R2 and R3 are fusible
type resistors. Emitter resistors R5 and R6 may also
be fusible type resistors. As a result of the
increased current flow, one of the fusible resistors
creates an open circuit and there~y inhibit operation
of the oscillator.
~179~37
-13- PATENT APPLICATION
As a specific example but in no way to be
construed as a limitation, the following components
are appropriate to an embodiment of the present
disclosure, as illustrated by the FIGURE:
s
_______________________________________________________
Item Description Value
__ ____________________________________________________
Cl Capacitor 0.047MFD, 50VDC
C2 Capacitor 0.047MFD, 50VDC
C3 Capacitor 0.047MFD, 50VDC
C4 Capacitor O.OlMFD, 630VDC/220VAC
CS Capacitor 0.022MFD, 250VAC
C6 Capacitor 47 MFD, 200VDC
C8 Capacitor 2200PFD, 250VDC/160VAC
C9 Capacitor 0.22MFD, 16OVDC/lOOVAC
C10 Capacitor 0.22MFD, 160VDC/lOOVAC
D1 Diode l.OA, 600V
D2 Diac 2A, 32V (BR100/03)
D3 Bridge 1.5A, 600V (lN4005)
Rl Resistor 510 Kohm, 1/4W
R2 Resistor 10 ohm, 1/4W Fusible (Philips NFR25)
R3 Resistor 10 ohm, 1/4W Fusible (Philips NFR25)
R5 Resistor 1.3 ohm, 1/4W Fusible (Philips NFR25)
R6 Resistor 1.3 ohm, 1/4W Fusible (Philips NFR25)
RV1 MOV 150VAC, 1200A V150LA2
Fl Fuse 2.5A, 125V
Q1 Transistor 5.OA, 850V (B W46)
Q2 Transistor 5.OA, 850V (BW46)
Ll Inductor 1.2mH
L2 Inductor l.OmH
T1 Transformer 6 turns prim. 5 turns each sec.
_ _ _____________________
2179437
-14- PATENT APPLICATION
There has thus been shown and described a circuit
for starting and operating an arc discharge lamp. The
invention does not result in a momentary blink of the
lamp immediately following the removal of power from
the circuit. Also, the circuit does not cause an
unnecessary use of energy due to continued oscillator
operation upon an end of lamp life condition caused by
the depletion of emissive material on one of the lamp
filament electrodes.
While there have been shown and described what are
at present considered to be the preferred embodiments
of the invention, it will be apparent to those skilled
in the art that various changes and modifications can
be made herein without departing from the scope of the
invention. Therefore, the aim in the appended claims
is to cover all such changes and modifications as fall
within the true spirit and scope of the invention.
The matter set forth in the foregoing description and
accompanying drawings is offered by way of
illustration only and not as a limitation. The actual
scope of the invention is intended to be defined in
the following claims when viewed in their proper
perspective based on the prior art.
