Information
-
Patent Grant
-
6768274
-
Patent Number
6,768,274
-
Date Filed
Saturday, September 28, 200221 years ago
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Date Issued
Tuesday, July 27, 200420 years ago
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Inventors
-
Original Assignees
-
Examiners
Agents
-
CPC
-
US Classifications
Field of Search
US
- 315 291
- 315 307
- 315 224
- 315 46
- 315 64
- 315 49
- 315 66
- 315 74
- 315 119
- 315 209 R
- 315 213
- 315 246
- 315 274
- 315 278
- 315 279
- 315 282
- 315 312
- 315 324
- 315 219
- 361 42
- 361 59
- 361 35
- 361 38
- 361 71
- 361 75
- 361 93
- 361 94
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International Classifications
-
Abstract
A ballast (100) includes an inverter (140,144,146) and a protection circuit that prevents excessive lamp-to-earth-ground fault current. The protection circuit includes a transformer (202,204,206,208,210) and an inverter disable circuit (300). The transformer measures a first current going out of one set of ballast output terminals (106,108) and a second current going into another set of ballast output terminals (206,208). In response to a substantial imbalance between the first current and the second current, inverter disable circuit (300) terminates inverter switching. Preferably, protection circuit further includes a restart timer circuit (400) that, following termination of inverter switching in response to a fault condition, prevents the inverter from restarting for a predetermined delay period.
Description
FIELD OF THE INVENTION
The present invention relates to the general subject of circuits for powering discharge lamps. More particularly, the present invention relates to a ballast with circuitry for protecting against a lamp-to-earth-ground fault condition.
BACKGROUND OF THE INVENTION
Fluorescent lamps used with electronic ballasts periodically fail and require replacement. In most cases, replacement of a failed lamp is performed while AC power is still applied to the ballast; this practice is sometimes referred to as “live relamping.” Since many newer ballast designs have non-isolated outputs, the possibility exists for high frequency output current to travel from the ballast output, through the lamp, through the person replacing the lamp, to fixture ground. Because an electrical shock may be suffered under such circumstances, safety agencies such as Underwriters Laboratories now require that ballasts be tested for this condition. Thus, standards have been established for the maximum current that is allowed to flow from the ballast output through the lamp to fixture ground. For many ballasts, these standards are readily met. However, for some ballasts, such as those models which are designed to operate with higher line voltages (e.g., 277 volts) or shorter lamp lengths (e.g., 2 foot lamps), these standards can be met only by incorporating special protective circuitry in the ballast.
Some ballast manufacturers have attempted to address the problem of excessive lamp-to-earth-ground current by trying to sense the high frequency leakage current that, in the event of a fault condition, flows out of the ballast output, into the grounded fixture, and back into the ballast via the ballast ground wire that is electrically connected to the fixture during ballast installation. An example of such an approach is described in U.S. Pat. No. 5,363,018. The main problem with this type of detection circuit is that this same type of leakage current normally flows even in the absence of a fault condition, and is actually quite desirable because it aids lamp ignition. Moreover, because the voltage applied to the lamps prior to ignition is much higher than voltage applied after ignition, the magnitude of this “normal” leakage current will be many times higher during the start-up mode than during the steady-state operating mode. Because the magnitude of the normal leakage current that flows into the ballast ground during normal starting conditions can be very close to the magnitude of the undesirable leakage current that flows through the body of a person who accidentally touches the ballast output and fixture ground, the prior art circuits cannot accurately discriminate between “normal” leakage current and the leakage current that occurs due to a true fault condition.
What is needed, therefore, is a ballast with a protection circuit that is capable of more reliably detecting a lamp-to-earth-ground fault condition. A ballast with such a protection circuit would represent a significant advance over the prior art.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1
describes a ballast with a lamp-to-earth-ground fault protection circuit, in accordance with a preferred embodiment of the present invention.
FIG. 2
describes a portion of a ballast adapted to power two gas discharge lamp, in accordance with a preferred embodiment of the present invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
In a preferred embodiment of the present invention, as described in
FIG. 1
, a ballast
100
for powering at least one gas discharge lamp
12
includes an inverter
140
,
144
,
146
,
148
, output connections
106
,
108
,
114
,
116
, and a protection circuit
202
,
204
,
206
,
116
,
210
,
300
,
400
. Preferably, ballast
100
further includes a pair of input connections
102
,
104
adapted to receive a conventional source of alternating current (e.g., 120 VAC at 60 Hertz), a full-wave diode bridge rectifier
120
, a high frequency bypass capacitor
122
, a boost converter
130
, and a bulk capacitance
132
.
The inverter is preferably implemented as a driven half-bridge
140
,
144
,
146
,
148
. In combination with a direct-coupled series resonant output circuit
160
,
170
, the inverter supplies a high frequency (e.g., greater than 20 kilohertz) alternating current to gas discharge lamp
12
via first, second, third, and fourth output connections
106
,
108
,
114
,
116
. The inverter includes an inverter drive circuit
140
having a voltage supply input
142
for receiving a direct current (DC) supply voltage. Upon initial application of AC power to ballast
100
, capacitor
150
charges up via resistor
152
. Once the voltage across capacitor
150
reaches a predetermined startup threshold (e.g., 10 volts), inverter drive circuit
140
starts and begins to switch inverter transistors
144
,
146
on and off in a substantially complementary manner. Inverter drive circuit
140
continues to provide inverter switching as long as the voltage at voltage supply input
142
remains greater than a predetermined shutdown threshold (e.g., 8 volts), but will cease to provide inverter switching if the voltage at voltage supply input
142
falls below the predetermined shutdown threshold. During normal operation, the voltage at voltage supply input
142
is maintained well above the shutdown threshold by a “bootstrapping” circuit that includes capacitor
172
, zener diode
174
, diode
190
, and resistor
192
.
First and second output connections
106
,
108
are adapted for connection to a first filament
14
of lamp
12
, while third and fourth output connections
114
,
116
are adapted for connection to a second filament
16
of lamp
12
.
Protection circuit
202
,
204
,
206
,
208
,
210
,
300
,
400
, which is coupled to the inverter and the output connections, monitors a first current and a second current. The first current is defined as the absolute value of the difference between the current flowing out of first output connection
106
and the current flowing into second output connection
108
. The second current is defined as the absolute value of the difference between the current flowing out of third output connection
114
and the current flowing into fourth output connection
116
. During normal operation (i.e., when no lamp-to-earth-ground fault condition is present), the first and second currents will be substantially equal. During a fault condition, the first current will not be substantially equal to the second current. Under such a fault condition, the protection circuit will disable the inverter.
The protection circuit includes a transformer T
2
and an inverter disable circuit
300
. Transformer T
2
comprises four primary windings
202
,
204
,
206
,
208
and a secondary winding
210
. First primary winding
202
is coupled in series with first output connection
106
. Second primary winding
204
is coupled in series with second output connection
108
. Third primary winding
206
is coupled in series with third output connection
114
. Fourth primary winding
208
is coupled in series with the fourth output connection
116
. Secondary winding
210
is part of inverter disable circuit
300
. Preferably, first, second, third, and fourth primary windings have the same number of wire turns (e.g., 1 turn). Secondary winding
210
has a number of wire turns (e.g., 30 turns) that is substantially greater than the number of wire turns on the primary windings. The relative orientation or polarity of the four primary windings is indicated by the dots depicted in FIG.
1
.
During normal operation (i.e., when no fault condition is present), the first current is substantially equal to the second current. Correspondingly, the voltages induced in first and second primary windings
202
,
204
are cancelled out by the voltages induced in third and fourth primary windings
206
,
208
. Consequently, the voltage across secondary winding
210
will be substantially zero.
During a lamp-to-earth-ground fault condition, the first current will not be substantially equal to the second current because a portion of the current flowing out of output connections
106
,
108
will be diverted to earth ground and, thus, will not flow back into output connections
114
,
116
. Correspondingly, the voltages induced in first and second primary windings
202
,
204
will not be cancelled out by the voltages induced in third and fourth primary windings
206
,
208
. Consequently, a nonzero voltage will appear across secondary winding
210
. In this way, the voltage across secondary winding
210
indicates the presence of a lamp-to-earth-ground fault condition.
The nonzero voltage that appears across secondary winding
210
during a fault condition is detected by the other circuitry in inverter disable circuit
300
so as to shut down the inverter. More particularly, in response to a nonzero voltage across secondary winding
210
of transformer T
2
, inverter disable circuit
300
terminates inverter switching by coupling the voltage supply input
142
of inverter drive circuit
140
to circuit ground
30
.
In a preferred embodiment, as described in
FIG. 1
, inverter disable circuit
300
comprises the secondary winding
210
of transformer T
2
, a disable output
302
, a transistor
320
, a first resistor
304
, a diode
310
, a capacitor
316
, a second resistor
318
, and a third resistor
328
. Secondary winding
210
and first resistor
304
are each coupled between a first node
302
and circuit ground
30
. Disable output
302
is coupled to voltage supply input
142
of inverter drive circuit
140
. Transistor
320
has a base
322
, a collector
324
, and an emitter
326
. Emitter
326
is coupled to circuit ground
30
. Diode
310
is coupled between first node
302
and the base
322
of transistor
320
; more specifically, diode
310
has an anode coupled to first node
302
and a cathode coupled to base
322
. Capacitor
316
and resistor
318
are each coupled between base
322
and circuit ground
30
. Finally, third resistor
328
is coupled between disable output
302
and emitter
324
of transistor
320
.
In a prototype ballast configured substantially as shown in
FIG. 1
, inverter disable circuit
300
was implemented with the following component values:
Resistor
304
: 100 kilohms
Diode
310
: 1N4148
Capacitor
316
: 22 micofarads
Resistor
318
: 2.2 kilohms
Transistor
320
: Q2N3904
Resistor
328
: 10 ohms
As previously described, it is preferred that transformer T
2
be implemented with one turn on each of the four primary windings
202
,
204
,
206
,
208
, and with thirty (
30
) turns on secondary winding
210
.
During normal operation (i.e., when no fault condition is present), the voltage across secondary winding
210
is approximately zero. Consequently, little or no voltage is provided at the base
322
of transistor
320
, so transistor
320
is off. Accordingly, in the absence of a fault condition, inverter disable circuit
300
does not affect the normal operation of inverter drive circuit
140
.
If a lamp-to-earth-ground fault condition occurs, a nonzero voltage will develop across secondary winding
210
. The nonzero voltage across secondary winding
210
is peak-detected by diode
310
and capacitor
316
, which causes transistor
320
to turn on. With transistor
320
turned on, resistor
328
is connected between voltage supply input
142
and circuit ground
30
. Because resistor
328
has a very low resistance (e.g., 10 ohms), it quickly discharges capacitor
150
, in spite of the fact that appreciable current continues to be supplied to capacitor
150
from bootstrap power source
172
,
174
via diode
190
and resistor
192
. Consequently, the voltage at voltage supply input
142
rapidly falls below the level necessary to keep inverter drive circuit
140
operating, and inverter switching ceases.
Preferably, the protection circuit further includes a restart timer circuit
400
for keeping the inverter disabled for a predetermined restart period following detection of lamp-to-earth-ground fault condition. Without restart timer circuit (
400
), the inverter will automatically restart after a brief delay period (e.g., on the order of 100-200 milliseconds) after being disabled by inverter disable circuit
300
. In order to ensure that the average rms fault current will be well within safety requirements, it is desirable that the delay period be increased considerably (e.g., to about 1.5 seconds). Restart timer circuit
300
provides such an increased delay.
In a preferred embodiment, as described in
FIG. 1
, restart timer circuit
400
comprises a restart input
402
, a restart output
404
, a transistor
418
, a series combination of a diode
406
and a resistor
408
, a capacitor
412
, a second resistor
414
, a third resistor
416
, and a fourth resistor
426
. Restart input
402
is coupled to the bootstrap power source
172
,
174
of the inverter. Restart output
404
is coupled to voltage supply input
142
of inverter drive circuit
140
. Transistor
418
has a collector
422
, an emitter
424
, and a base
420
. Emitter
424
is coupled to circuit ground
30
. The series combination of diode
406
and resistor
408
is coupled between restart input
402
and a second node
410
; more specifically, diode
406
has an anode coupled to restart input
402
and a cathode coupled to resistor
408
, wherein resistor
408
is coupled to second node
410
. Capacitor
412
is coupled between second node
410
and circuit ground
30
. Second resistor
414
is coupled between second node
410
and base
420
of transistor
418
. Third resistor
416
is coupled between base
420
and circuit ground
30
. Finally, fourth resistor
426
is coupled between restart output
404
and collector
422
of transistor
418
.
In a prototype ballast configured substantially as shown in
FIG. 1
, restart timer circuit
400
was implemented with the following component values:
Diode
406
: 1N4148
Resistor
408
: 4.7 kilohms
Capacitor
412
: 10 micofarads
Resistor
414
: 100 kilohms
Resistor
416
: 22 kilohms
Transistor
418
: Q2N3904
Resistor
426
: 3.3 kilohms
The detailed operation of restart timer circuit
400
is now described with reference to
FIG. 1
as follows.
During normal operation (i.e., when no fault condition is present), capacitor
412
remains charged, via bootstrap power source
172
,
174
and the series combination of diode
406
and resistor
408
, at a voltage of approximately 15 volts. A portion of the voltage across capacitor
412
is applied (via resistors
414
,
416
) to transistor
418
, which turns on and connects restart output
404
(and thus voltage supply input
142
of inverter drive circuit
140
) to circuit ground
30
via resistor
426
. When the inverter is operating normally, the loading introduced by having voltage supply input
142
connected to circuit ground
30
via resistor
426
has no effect because resistor
426
is selected to be suitably large (e.g., 3.3 kilohms) and bootstrap power source
172
,
174
(which supplies operating current to inverter drive circuit
140
via diode
190
and resistor
192
) is a low impedance current source that is more than capable of supplying the additional current required by the introduction of resistor
426
while the inverter is operating. Thus, during normal conditions, restart timer circuit
400
does not affect the operation of the inverter.
When inverter drive circuit
140
is shut down by inverter disable circuit
300
in response to fault condition, the connection of resistor
426
between voltage supply input
142
and circuit ground
30
will prevent drive circuit
300
from restarting for as long as the voltage across capacitor
412
is sufficient to keep transistor
418
turned on. More specifically, with resistor
426
present, capacitor
150
will be prevented from charging up (via resistor
152
) to a level sufficient (e.g., 10 volts, which is the typical turn-on threshold of inverter drive circuit
140
) to restart inverter drive circuit
140
. With inverter drive circuit
140
disabled, bootstrap power source
172
,
174
no longer supplies current to capacitor
412
, so the voltage across capacitor
412
will begin to decrease at a rate determined by the capacitance of capacitor
412
and the resistances of resistors
414
,
416
. Once the voltage across capacitor
412
falls below a certain level (e.g., a few volts), transistor
418
will turn off and allow capacitor
150
to charge up (via startup resistor
152
) to a level sufficient (e.g., 10 volts) to restart inverter drive circuit
140
. If a lamp-to-earth-ground fault condition is still present, inverter disable circuit
300
will promptly shut down the inverter once again, and the aforementioned cycle will repeat itself for as long as a fault condition is present.
It is preferred that capacitor
412
and resistors
414
,
416
be sized such that transistor
418
will remain on for about 1.5 seconds after inverter drive circuit
300
is disabled in response to a fault condition; in a prototype ballast configured substantially as shown in
FIG. 1
, the preferred restart delay of about 1.5 seconds was achieved with capacitor
412
set at 10 microfarads, resistor
414
set at 100 kilohms, and resistors
416
set at 22 kilohms. Although the inverter will be allowed to restart every 1.5 seconds even if an uncorrected fault condition remains present, the duty cycle (and, thus, the resulting rms value of the ground fault current) will be quite low because the inverter will be promptly shut down by inverter disable circuit
300
.
Although the ballast
100
described in
FIG. 1
has been shown as operating a single lamp
12
, it should be appreciated that the principles of the present invention are readily extended to a ballast that operates multiple lamps connected in series. For example, as described in
FIG. 2
, the circuitry detailed in
FIG. 1
may be adapted to a ballast for powering two lamps
12
,
22
simply by adding an additional filament winding
164
(on transformer T
1
), an additional current-limiting capacitor
184
, and additional output connections
110
,
112
. As illustrated in
FIG. 2
, output connections
110
,
112
are coupled to both the second filament of lamp
12
and a first filament of lamp
22
. Output connections
114
,
116
are coupled to a second filament of lamp
22
. Along similar lines, ballast
100
may be further adapted to power three of four series-connected lamps. For each additional lamp, an additional filament winding, current-limiting capacitor, and pair of output connections is required.
Although the present invention has been described with reference to certain preferred embodiments, numerous modifications and variations can be made by those skilled in the art without departing from the novel spirit and scope of this invention.
Claims
- 1. A ballast for powering at least one gas discharge lamp, comprising;first, second, third, and fourth output connections adapted for connection to the gas discharge lamp, wherein the first and second output connections are adapted for connection to a first filament of the lamp, and the third and fourth output connections are adapted for connection to a second filament of the lamp; an inverter for supplying a high frequency alternating current to the gas discharge lamp, the inverter comprising: an inverter drive circuit having a voltage supply input for receiving a supply voltage, the inverter drive circuit being operable to: (i) provide inverter switching as long as the supply voltage is greater than a predetermined shutdown voltage; and (ii) cease to provide inverter switching when the supply voltage falls below the predetermined shutdown voltage; and a bootstrap power source that is operable, while inverter switching is occurring, to provide power to the inverter drive circuit; and a protection circuit, comprising: a transformer, comprising: a first primary winding coupled in series with the first output connection; a second primary winding coupled in series with the second output connection; a third primary winding coupled in series with the third output connection; a fourth primary winding coupled in series with the fourth output connection; and a secondary winding; an inverter disable circuit, comprising: a disable output coupled to the voltage supply input of the inverter drive circuit; a transistor having a base, a collector, and an emitter, wherein the emitter is coupled to circuit ground; the secondary winding of the transformer, the secondary winding being coupled between a first node and circuit ground; a first resistor coupled between the first node and circuit ground; a diode coupled between the first node and the base of the transistor; a capacitor coupled between the base of the transistor and circuit ground; a second resistor coupled between the base of the transistor and circuit ground; and a third resistor coupled between the disable output and the collector of the transistor; and a restart timer circuit, comprising: a restart input coupled to the bootstrap power source of the inverter; a restart output coupled to the voltage supply input of the inverter drive circuit; a transistor having a collector, an emitter, and a base, wherein the emitter is coupled to circuit ground; a series combination of a diode and a first resistor coupled between the restart input and a second node; a capacitor coupled between the second node and circuit ground; a second resistor coupled between the second node and the base of the transistor; a third resistor coupled between the base of the transistor and circuit ground; and a fourth resistor coupled between the restart output and the collector of the transistor.
- 2. A ballast for powering at least one gas discharge lamp, comprising:an inverter for supplying a high frequency alternating current to the gas discharge lamp, the inverter including an inverter drive circuit having a voltage supply input for receiving a supply voltage, the inverter drive circuit being operable to: (i) provide inverter switching as long as the supply voltage is greater than a predetermined shutdown voltage; and (ii) cease to provide inverter switching when the supply voltage falls below the predetermined shutdown voltage; first, second, third, and fourth output connections adapted for connection to the gas discharge lamp, wherein the first and second output connections are adapted for connection to a first filament of the lamp, and the third and fourth output connections are adapted for connection to a second filament of the lamp; and a protection circuit coupled to the inverter and the first, second, third, and fourth output connections, the protection circuit comprising: a transformer comprising: a first primary winding coupled in series with the first output connection; a second primary winding coupled in series with the second output connection; a third primary winding coupled in series with the third output connection; a fourth primary winding coupled in series with the fourth output connection; and a secondary winding operably coupled to the inverter; an inverter disable circuit chat includes the secondary winding of the transformer and that is coupled to the voltage supply input of the inverter drive circuit, the inverter disable circuit being operable, in response to a nonzero voltage across the secondary winding of the transformer, to terminate inverter switching by coupling the voltage supply input to circuit ground; and a restart tuner circuit coupled to the inverter, the restart timer circuit being operable, following termination of inverter switching, so prevent the inverter from resuming inverter switching for at least a predetermined restart period.
- 3. The ballast of claim 2, wherein the inverter disable circuit comprises:a disable output coupled to the voltage supply input of the inverter drive circuit; a transistor having a base, a collector, and an emitter, wherein the emitter as coupled to circuit ground; the secondary winding of the transformer, the secondary winding being coupled between a first node and circuit ground; a first resistor coupled between the first node and circuit ground; a diode coupled between the first node and the base of the transistor; a capacitor coupled between the base of the transistor and circuit ground; a second resistor coupled between the base of the transistor and circuit ground; and a third resistor coupled between the disable output and the collector of the transistor.
- 4. The ballast of claim 2, wherein:the inverter further comprises a bootstrap power source that is operable, while inverter switching is occurring, to provide power to the inverter drive circuit and the restart timer circuit; and the restart timer circuit comprises: a restart input coupled to the bootstrap power source of the inverter; a restart output coupled to the voltage supply input of the inverter drive circuit; a transistor having a collector, an emitter, and a base, wherein the emitter is coupled to circuit ground; a series combination of a diode and a first resistor coupled between the restart input and a second node; a capacitor coupled between the second node and circuit ground; a second resistor coupled between the second node and the base of the transistor; a third resistor coupled between the base of the transistor and circuit ground; and a fourth resistor coupled between the restart output and the collector of the transistor.
- 5. A ballast for powering at least one gas discharge lamp, comprising:an inverter for supplying a high frequency alternating current to the gas discharge lamp; first, second, third, and fourth output connections adapted for connection to the gas discharge lamp, wherein the first and second output connections are adapted for connection to a first filament of the lamp, and the third and fourth output connections are adapted for connection to a second filament of the lamp; and a protection circuit coupled to the inverter and the first, second, third, and fourth output connections, the protection circuit comprising: a transformer, comprising: a first primary winding coupled in series with the first output connection; a second primary winding coupled in series with the second output connection; a third primary winding coupled in series with the third output connection; a fourth primary winding coupled in series with the fourth output connection; and a secondary winding operably coupled to the inverter, the secondary winding having a voltage that is: (i) substantially zero in the absence of a lamp-to-earth-ground fault condition; and (ii) nonzero in the presence of a lamp-to-earth-ground fault condition.
- 6. The ballast of claim 5, wherein the first, second, third, and fourth primary windings have the same number of wire turns.
- 7. The ballast of claim 6, wherein the secondary winding has a number of wire turns that is substantially greater than the number of wire turns on the first, second, third, and fourth primary windings.
- 8. The ballast of claim 5, wherein the protection circuit comprises a transformer having:a first primary winding coupled in series with the first output connection; a second primary winding coupled in series with the second output connection; a third primary winding coupled in series with the third output connection; a fourth primary winding coupled in series with the fourth output connection; and a secondary winding operably coupled to the inverter, the secondary winding having a voltage that is: (i) substantially zero when the first current is substantially equal to the second current; and (ii) nonzero when the first current is no; substantially equal to the second current.
- 9. The ballast of claim 8, wherein:the inverter includes an inverter drive circuit having at voltage supply input for receiving a supply voltage, the inverter drive circuit being operable to: (i) provide inverter switching as long as the supply voltage is greater than a predetermined shutdown voltage; and (ii) cease to provide inverter switching when the supply voltage falls below the predetermined shutdown voltage; and the protection circuit further includes an inverter disable circuit that includes the secondary winding of the transformer and that is coupled to the voltage supply input of the inverter drive circuit, the inverter disable circuit being operable, in response to a nonzero voltage across the secondary winding of the transformer, to terminate inverter switching by coupling the voltage supply input to circuit ground.
- 10. The ballast of claim 9, wherein the inverter disable circuit comprises:a disable output coupled to the voltage supply input of the inverter drive circuit; a transistor having a base, a collector, and an emitter, wherein the emitter is coupled to circuit ground; the secondary winding of the transformer, the secondary winding being coupled between a first node and circuit ground; a first resistor coupled between the first node and circuit ground; a diode coupled between the first node and the base of the transistor; a capacitor coupled between the base of the transistor and circuit ground; a second resistor coupled between the base of the transistor and circuit ground; and a third resistor coupled between the disable output and the collector of the transistor.
- 11. The ballast of claim 9, wherein the protection circuit further comprises a restart timer circuit coupled to the inverter, the restart timer circuit being operable, following termination of inverter switching, to prevent the inverter from resuming inverter switching for at least a predetermined restart period.
- 12. The ballast of claim 11, wherein:the inverter further comprises a bootstrap power source that is operable, while inverter switching is occurring, to provide power to the inverter drive circuit and the restart timer circuit; and the restart timer circuit comprises: a restart input coupled to the bootstrap power source of the inverter; a restart output coupled to the voltage supply input of the inverter drive circuit; a transistor having a collector, an emitter, and a base, wherein the emitter is coupled to circuit ground; a series combination of a diode and a first resistor coupled between the restart input and a second node; a capacitor coupled between the second node and circuit ground; a second resistor coupled between the second node and the base of the transistor; a third resistor coupled between the base of the transistor and circuit ground; and a fourth resistor coupled between the restart output and the collector of the transistor.
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