# Patent

## Title
Output short circuit protection for electronic ballasts

## Abstract

A continuous mode electronic ballast operates a lamp. A buck converter is configured to generate a direct current (DC) bus voltage at buck converter output. An inverter circuit is operationally coupled to the buck converter output, and configured to receive the DC voltage and convert the DC voltage into an AC voltage to drive the lamp. A short circuit protection circuit is operationally coupled to the buck converter output to sense the DC bus voltage, compare the sensed DC voltage to a predetermined threshold and, based on the comparison, shut down the inverter circuit.

## Background
BACKGROUND
The following relates to electronic ballasts. It finds particular application in conjunction with the high intensity discharge lamps (HID), and will be described with particular reference thereto. However, it is to be appreciated that the following is also amenable to other electronically ballasted lamps such as fluorescent lamps and the like.
A ballast is an electrical device which is used to provide power to a load, such as an electrical discharge lamp, and to regulate its current. The ballast provides high voltage to start a lamp, causing the gas to ionize which begins the process of arc formation. Once the arc is established, the ballast allows the lamp to continue to operate by providing proper controlled current flow to the lamp.
Typically, low frequency, square wave ballasts include a three stage power conversion process. Initially, at stage 1, the AC power line voltage is rectified and filtered. At the intermediate stage 2, the DC voltage is converted to the DC current by a buck converter. At stage 3, the DC current is converted to an AC current by an inverter which includes switches such as MOSFETs to drive the resonant circuit which excites the lamp.
When the lamp is cold, just after ignition, the lamp is characterized by a low impedance. In this condition, the lamp is practically in a short circuit, e.g. the voltage between the lamp terminals is about 20V. Typically, the inverter starts running before the lamp ignites, e.g. the output terminals of the inverter are open prior to ignition. When the lamp ignites, the lamp's impedance quickly drops to about 5% of its stead-state value. As the gas temperature increases in the full arc mode, the lamp voltage increases until it reaches a steady-state voltage. If the output of the inverter is shorted, for example, as the result of a faulty ballast installation process, the inverter MOSFET switches may overheat and also thermally stress the switch of the buck converter of the stage 2.
One approach to prevent the switches from overheating is to use a heatsink. However, such heatsinks are bulky and occupy too much of the ballast space. Other approach is to use one of the thermal management techniques. However, the thermal management techniques are complex and expensive.
The present application contemplates new methods and apparatuses that overcome above referenced problems and others.
BRIEF DESCRIPTION
In accordance with one aspect, a continuous mode electronic ballast for operating a lamp is disclosed. A buck converter is configured to generate a direct current (DC) bus voltage at buck converter output. An inverter circuit is operationally coupled to the buck converter output, and configured to receive the DC voltage and convert the DC voltage into an AC voltage to drive the lamp. A short circuit protection circuit is operationally coupled to the buck converter output to sense the DC bus voltage, compare the sensed DC voltage to a predetermined threshold and, based on the comparison, shut down the inverter circuit.



BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a diagrammatic illustration of a ballast;
FIG. 2 is a diagrammatic illustration of a detailed portion of the ballast;
FIG. 3 is a diagrammatic illustration of another detailed portion of the ballast; and
FIG. 4 is a diagrammatic illustration of another detailed portion of the ballast.



DETAILED DESCRIPTION
With reference to FIG. 1, an electronic ballast 10 includes a rectifying circuit 12 which converts alternating current (AC) voltage to direct current (DC) voltage. The rectifying circuit 12 is coupled to an AC power supply 14 via a positive voltage terminal 16 and a neutral terminal 18. Typically, the line frequency of the AC power supply is 50 Hz or 60 Hz. The rectifying circuit 12 converts the AC input voltage to a full wave rectified voltage. The rectifying circuit 12 is connected to a buck converter 20 which is a direct current (DC) voltage to the direct current (DC) voltage converter. A filter capacitor or capacitors 22 are connected across positive and ground input terminals 30, 32 of the buck converter 20. Positive and negative output terminals 34, 36 of the buck converter 20 are coupled to input lines 38, 40 of an inverter circuit 44. The inverter circuit 44 converts DC to AC. A crest factor reduction circuit 46 is coupled to the buck converter positive output terminal 34 and ground to detect an increased rate of a voltage change on the inverter DC bus and to shut down the buck converter 20 for a period of time to reduce the buck converter output current as discussed in detail below. A power control circuit 48 is coupled to the buck converter positive input terminal 30 and ground to control the buck converter operation, and to a sense resistor that measures the amount of current flowing into the buck converter 20. Together, these two signals establish how much power is flowing into the buck converter, ultimately regulating the amount of power flowing into the lamp. A ripple detection circuit 50 is coupled to the buck converter positive input terminal 30 and ground to sense the AC component in the DC voltage. The AC component is twice the power line frequency due to the full wave rectifier 12. The power control circuit 48 attenuates the sensed AC voltage so that the ripple voltage is substantially reduced as discussed in detail below. A short circuit protection circuit 52 is coupled to the buck converter positive output terminal 34 and ground to detect the undervoltage and shut down the inverter circuit 44 when the lamp voltage is below a predetermined threshold as described below. The inverter circuit 44 is connected to an output circuit 56, which typically includes an inductor and a winding to pulse start the lamp. The output circuit 56 is connected to first and second load terminals or electrodes 58, 60 to drive a load 62 such as an HID lamp, a fluorescent lamp, or any other lamp operated by the electronic ballast.
With continuing reference to FIG. 1 and further reference to FIG. 2, the buck converter 20 includes a buck converter controller 64 such as, for example, the controller manufactured by ST Electronics, PN L6562. The buck controller 64 turns ON and OFF a controllably conductive first or buck switch 66. Power is supplied to a buck controller pin 68 via a resistor 70 from the positive terminal 30. When the first switch 66 is turned ON, the input voltage is applied to a first or buck inductor 74, which is connected in series with the first switch 66. Power is delivered to the buck converter output terminals 34, 36. The current in the first inductor 74 is building up. A first or buck charging capacitor 76, which is coupled with a cathode of a first or buck freewheeling diode 78 and the first inductor 74, is charging up through the first inductor 74. When the first switch 66 is turned OFF, the current across the first inductor 74 reverses. The first diode 78 becomes forward biased. The energy stored in the first inductor 74 and first capacitor 76 is delivered to the buck converter output terminals 34,36. Output voltage regulation is obtained by varying the duty cycle of the first switch 66. The power control circuit 48 receives a sense or converter bus voltage signal Vs and a signal proportional to the buck converter input current IS via a resistor 80. By adjusting a set point SP, which is provided by the power control circuit 48 as discussed below, the sense voltage signal Vs can be employed for controlling the power of the lamp 62 to track the set point SP. The voltage sense signal Vs helps to regulate the power applied to the lamp when the power line voltage changes. More specifically, bus current Is is directed to a low pass filter 82 having the resistor 80 and a capacitor 86. An output voltage signal Vo at an output 88 of the low pass filter 82 is representative of the average of the bus current Is and is proportional to the actual output power which is provided to the lamp 62.
An error amplifier 90 receives the output voltage signal Vo at an error amplifier input terminal 92 via a resistor divider 94 which includes serially connected resistors 96, 97 and determines a difference between the output voltage signal Vo and the set point voltage signal SP. A capacitor 98 is connected in series with the resistors 96, 97. A diode 99 is connected between the terminal 92 and ground.
The set point voltage signal SP is provided by a set point amplifier 100 via a set point amplifier output line 101 and a resistor 102. More specifically, the set point amplifier 100 receives as an input a voltage signal Vb via a first set point amplifier input line 104 through a resistor 106. A reference voltage signal VR is provided to the set point amplifier 100 via a second set point amplifier input line 108. By employing a feedback from the input DC voltage Vb, the set point voltage signal SP is adjusted in accordance with the actual input line voltage Vb to reduce variations in the operational voltage of the lamp 62. A control voltage signal Vx for the buck controller 64 is derived from the set point voltage signal SP and supplied to an input 112 of a buck controller multiplier 113 via a line 114 from a resistor divider 116 which includes serially connected resistors 118, 119.
The error amplifier 90 generates an amplified error signal Vc in an error amplifier output line 120 that is proportional to the determined difference between the output voltage signal Vo and the set point voltage signal SP. The amplified error signal Vc or the error amplifier output is supplied to an inverting input pin 122 of the buck controller 64 via a resistor divider 124 including serially connected resistors 126, 128. The amplified error signal Vc is also supplied to a compensation input pin 130 of the buck controller 64 via the resistors 126, 128. A compensation network 131 is placed between the inverting and compensation pins 122, 130 to achieve stability of the voltage control loop and ensure high power factor.
A voltage output 132 of the buck switch 64 outputs a pulse width modulated signal VPWM. The pulse width modulated signal is supplied to the buck switch 66 via a resistor 134. A comparator non-inverting input 136 receives the PWM voltage signal VPWM from a resistor 138 serially connected to the buck switch 66. The PWM voltage signal VPWM is proportional to the current flowing through the buck switch 66 and first inductor 74 during the conduction period of the buck switch 66. The voltage signal VPWM is compared to the internal reference voltage signal, which is determined by the control voltage signal Vx. When the voltage signal VPWM is equal to the internal reference voltage signal, the buck controller 64 turns the buck switch 66 OFF.
As a result, the PWM voltage signal VPWM determines the peak current through the buck switch 66, which establishes how much current is fed into the inverter 44.
With continuing reference to FIG. 1 and further reference to FIG. 3, the inverter 44 is connected to the output terminals 34, 36 of the buck converter 20 for inverting the DC voltage supplied by the buck converter 20 into the AC voltage and providing the AC current to drive the lamp 62. The inverter 44 includes first and second drivers 180, 182 such as, for example, manufactured by ST Electronics, PN L6269A. Each driver 180,182 includes a corresponding pair of first and second low and high side buffers 188, 190, 192, 194. First lower and upper switches 196, 198 is each connected to a corresponding first low, high side buffer 188, 190 through a respective resistor 200, 202. Second lower and upper switches 204, 206 is each connected to a corresponding second low, high side buffer 192, 194 through a respective resistor 208, 210.
Each buffer pair drives in a complementary fashion corresponding first lower and upper switches 196, 198 and second lower and upper switches 204, 206. The first and second lower and upper switches 196, 198, 204, 206 are controllably conductive devices such as, for example, MOSFETs. The first lower switch 196 is connected in series to the first upper switch 198 which is connected to the first high side buffer 190. The second lower switch 204 is connected in series to the second upper switch 206 which is connected to the second high side buffer 194. When the first and second lower switches 196, 204 are ON, the power to corresponding first and second high side buffers 190, 194 is supplied. When the first and second lower switches 196, 204 are OFF, the power to the first and second high side buffers 190, 194 is supplied through corresponding first and second side charging capacitors 220, 222. The first and second lower switches 196, 204 and first and second upper switches 198, 206 are turned ON alternatively to replenish charge on the corresponding charging capacitor 220, 222.
The converter bus voltage Vs to the first and second drivers 180, 182 is supplied via a power line resistor 224 via the terminal 34 to corresponding first and second power pins 230, 232. The resistor 224 is connected in series with corresponding first and second electrolytic storage capacitors 234, 236. The resistor 224 provides the initial power to the drivers 180 and 182. The capacitors 234 and 236 charge via the resistor 224 and the DC bus via the terminal 34. When the voltage at the first power pin 230 surpasses the undervoltage lock-out voltage of the drivers 180 and 182, the oscillator of the second driver 182 begins to operate. An oscillator timing resistor 250 is connected to an oscillator timing resistor pin 252 of the second driver 182. An oscillator timing capacitor 254 is connected to the oscillator timing capacitor pin 256 of the second driver 182. The oscillator timing resistor and capacitor 250, 254 cooperate to determine the oscillating frequency of the second driver 182. A resistor 258 is connected between a capacitor 259 and the oscillator output of the second driver 182. The capacitor 259 and resistor 258 provide a slight delay to prevent the low and high side buffers 188, 190 of the first driver 180 from conducting simultaneously, thus preventing the first lower and upper switches 196 and 198 from turning ON simultaneously. This prevents the DC bus from being shorted by the first lower and upper switches 196 and 198. The resistor 250 and capacitor 254 of the oscillator circuitry of the second driver 182 set the frequency at which the lamp 62 is operated such as about 130 Hz, which is a substantially slower frequency than the switching frequency of the buck stage. First and second snubber capacitors 260, 262 are connected in parallel to corresponding first and second lower switches 196, 204 to allow the inverter 44 to operate with zero voltage switching.
A first inductor 264 is mutually coupled to a second inductor 265. The first inductor 264 is connected to the first upper switch 198 and the first output lamp terminal 58. The second inductor 265 is connected to the first upper switch 198 and to the second upper switch 206 via serially connected output circuit resistor 266, element 272 and capacitor 274. The second upper switch 206 is connected in series to the second output lamp terminal 60. An output circuit resistor 268 and a serially connected output circuit diode 270 are connected in parallel to the first and second inductors 264, 265. A capacitor 276 is connected in parallel to the lamp outputs 58, 60. The elements of the output circuit 56 cooperate to ignite the lamp 62 and to provide the initial warm up current and a predetermined alternating current voltage during normal lamp operation. The inductor 264 also attenuates the high frequency ripple current produced from the previous buck stage.
With continuing reference to FIGS. 1 and 3 and further reference to FIG. 4, the short circuit protection circuit 52 is connected between the buck converter 20 and inverter 44 to detect the converter bus voltage Vs and shut down the inverter 44 when a low voltage condition, such as about 20V, is detected. Although only the first driver 182 is illustrated for simplicity, it is contemplated that the short circuit protection circuit 52 controls the first and second drivers 182, 184 in the like manner. When the lamp 62 is cold, just after ignition, the lamp 62 is characterized by a low impedance. In this condition, the lamp 62 is practically in a short circuit, e.g. the voltage between the lamp terminals 58, 60 can be about 20V. Typically, the inverter 44 starts running before the lamp 62 ignites, e.g. the output terminals of the inverter are open prior to ignition. When the lamp ignites, the lamp's impedance quickly drops to about 5% of its stead-state value. Generally, activation of the short-circuit protection circuit is not desirable during the low output voltage mode. As the gas temperature increases in the full arc mode, the lamp voltage increases until it reaches a steady-state voltage. If the output of the inverter 44 is shorted, for example, as the result of a faulty ballast installation process, the first and second lower and upper switches 196, 198, 204, 206 may overheat and also thermally stress the first switch 66 of the buck converter 20. The short circuit protection circuit 52 detects the undervoltage, e.g., the voltage which is, for example, less than 20V, and shuts down the inverter 44 thus eliminating the effect of the short circuit that causes the switches 66, 196, 198, 204, 206 to overheat. The inverter is shut down by shorting the supply voltage pins 230, 232 of the drivers 180 and 182.
More specifically, the short circuit protection circuit 52 includes a latch 280 including first and second latch transistors 282, 284. The latch 280 senses the converter bus voltage Vs, which is supplied to the latch 280 via the power line resistor 224.
During the normal lamp operation, the first and second drivers 180, 182 drive the lower and upper switches 196, 198, 204, 206. If the converter bus voltage Vs drops below a predetermined threshold, such as 15V or 20V, current is drawn from a base 286 of the first latch transistor 282. A collector 287 of the second transistor 284 is connected to the base 286 of the first transistor 282. A base 288 of the second transistor 284 is connected to a collector 289 of the first transistor 282. When the current is drawn from the first transistor base 286, the current is also drawn from the second transistor base 288. The latch 280 is triggered. E.g., the first and second latch transistors 282, 284 are turned ON via a regenerative process.
When in conduction, the first and second latch transistors 282, 284 discharge the energy of the first and second storage capacitors 234, 236, causing the under-voltage lockout circuit of the first and second drivers 180, 182 to engage, thus shutting off the inverter 44. When the storage capacitors 234, 236 are almost completely discharged to about 1 or 2V, the latch 280 opens. Since the inverter is being shut-off, the converter bus voltage Vs at this time is at high voltage, and the storage capacitors 234, 236 are charging via the power line resistor 224. When the storage capacitors 234, 236 charge to the voltage at which the first and second drivers 180, 182 are activated, about 8 to 9V, the drivers 182, 184 are turned ON and start operating the switches 196, 198, 204, 206 thereby causing the converter bus voltage Vs to discharge into the output short circuit or the low impedance that causes the converter bus voltage Vs to drop below 15V or 20V. The latching process repeats, shutting off the inverter 44 and protecting the switches 66, 196, 198, 204, 206. The duty cycle of this process is essentially determined by how long it takes to charge up the storage capacitors 234, 236 via the power line resistor 224. In one embodiment, the short circuit protection circuit 52 has a very short duty cycle. In such circuit, the ON time of the inverter under this condition is very short compared with period of process. When the short circuit is removed, the inverter restarts. The process of ignition, warm-up and steady-state control of the lamp power resumes.
A resistor 290, connected between the second transistor base 288 and ground, determines the level of current to trip the latch 280. Capacitors 292, 294 help to eliminate false triggering by acting as a low pass filter. A resistor 296 is connected in series with the storage capacitors 234, 236 to limit the current to the latch 280. In one embodiment, a diode 298 is connected in parallel with the storage capacitors 234, 236 to prevent the base-emitter junction of the first latch transistor 282 from breaking down.
With reference again to FIG. 2, the crest factor reduction circuit 46 detects a rate of the voltage change of the converter bus. More specifically, as the sense voltage Vs increases during the inverter's transition intervals, a pulse is applied to a capacitor 300, which is coupled to the buck converter positive terminal 34. The capacitor 300, which is coupled to a base of the transistor 302, turns the transistor 302 ON. The transistor 302 pulses the control voltage signal Vx at the buck converter controller pin 112 via a resistor 304 to nearly zero volts, thus blanking the voltage set point to the buck converter 20 before the voltage substantially rises. If no positive going transient occurs at the output of the buck converter 20, i.e. no positive going transition of the converter bus voltage Vs, the transistor 302 does not turn ON. The converter bus voltage Vs stays undisturbed, thus providing the required set point voltage to the buck converter 20 to achieve the correct output current. Resistors 306, 308 are serially connected between the capacitor 300 and a base of the transistor 302. A diode 310 is coupled to the resistor 306 and an emitter of the transistor 302. A collector of the transistor 302 is connected to a power output 312 of the error amplifier 90.
In this manner, blanking or modulating the control voltage signal Vx at the buck converter controller pin 112 during the transition intervals of the inverter 44 blanks the output current of the buck converter 20 before the bus voltage rises, thus reducing the current provided to the inverter 44 until the inverter's transition is over. E.g., the higher rate of the voltage change is detected before the voltage substantially changes. This greatly reduces the overshoot of the buck converter output voltage, thus substantially reducing the lamp current crest factor from about 1.0 to about 1.5.
The ripple detection circuit 50 measures an AC component in the converted DC voltage. As described above, the set point amplifier 100 receives the input voltage signal Vb, which is together with the provided reference voltage signal VR determines the voltage set point SP for the buck converter 20 and consequently, how much power is drawn from the DC bus. The ripple detection circuit 50 includes a resistor 400 connected in series with a capacitor 402. A resistor 404 is connected in parallel with the resistor 400 and capacitor 402. The resistor 102 is connected in series with the resistor 400 and capacitor 402. The resistors 102, 400, 404, capacitor 402 and set point amplifier 100 cooperate to measure the AC component in the input DC voltage Vb and modulate the buck converter controller 64 via the control voltage signal Vx at the buck converter controller pin 112 so that the correct level and phase of the modulation to reject the AC component of the DC voltage are provided to the buck converter controller 64. In this manner, the AC component is measured and attenuated.
The application has been described with reference to the preferred embodiments. Obviously, modifications and alterations will occur to others upon reading and understanding the preceding detailed description. It is intended that the application be construed as including all such modifications and alterations.

## Claims

1. A continuous mode electronic ballast for operating a lamp comprising:
a buck converter configured to generate a direct current (DC) bus voltage at buck converter output;
an inverter circuit operationally coupled to the buck converter output, configured to receive the generated DC voltage and convert the DC voltage into an AC voltage to drive the lamp;
a short circuit protection circuit which is operationally coupled to the buck converter output to sense the DC bus voltage, compare the sensed DC bus voltage to a predetermined threshold and, based on the comparison, shut down the inverter circuit;
a power line resistor which is operationally coupled to the buck converter output and which receives the DC bus voltage;
a storage capacitor operationally coupled to the power line resistor and inverter circuit, which storage capacitor accumulates a charge through the power line resistor and supplies power to the inverter circuit after initial ignition of the lamp; and
a latch, operationally connected to the storage capacitor, which latch is in an “ON” state when the sensed voltage is less than the predetermined threshold, and which discharges the storage capacitor and shuts down the inverter circuit.

2. The ballast as set forth in claim 1, wherein the latch includes:
a first transistor which includes:
a base operationally connected to the power line resistor, wherein current is being drawn from the first transistor base when the sensed voltage is less than the predetermined threshold.

3. The ballast as set forth in claim 2, wherein the latch further includes:
a second transistor which includes:
a collector operationally connected to the base of the first transistor, and
a base operationally connected to a collector of the first transistor, wherein current is being drawn from the second transistor base when the sensed voltage is less than the predetermined threshold, the first and second transistors cooperate to discharge the storage capacitor.

4. The ballast as set forth in claim 1, wherein the latch stops conducting when the storage capacitor is substantially discharged.

5. The ballast as set forth in claim 4, wherein the power line resistor resumes charging the storage capacitor once the latch stops conducting so that the storage capacitor restarts the inverter circuit.

6. The ballast as set forth in claim 1, wherein the short circuit protection circuit further includes:
a resistor operationally connected in series with the storage capacitor to limit current to the latch.

7. A continuous mode electronic ballast for operating a lamp comprising:
a buck converter configured to generate a direct current (DC) bus voltage at buck converter output;
an inverter circuit operationally coupled to the buck converter output, configured to receive the generated DC voltage and convert the DC voltage into an AC voltage to drive the lamp;
a short circuit protection circuit which is operationally coupled to the buck converter output to sense the DC bus voltage, compare the sensed DC bus voltage to a predetermined threshold and, based on the comparison, shut down the inverter circuit;
a power line resistor which is operationally coupled to the buck converter output and which receives the DC bus voltage;
a storage capacitor operationally coupled to the power line resistor and inverter circuit, which storage capacitor accumulates a charge through the power line resistor and supplies power to the inverter circuit after initial ignition of the lamp; and
a latch, operationally coupled to the storage capacitor, wherein the power line resistor and latch cooperate to facilitate discharging of the storage capacitor when the sensed voltage is less than the predetermined threshold and shut down the inverter circuit.

8. The ballast as set forth in claim 7, wherein the power line resistor, storage capacitor and latch cooperate to restart the inverter circuit after shut down.

9. A continuous mode electronic ballast for operating a lamp comprising:
a buck converter configured to generate DC bus voltage at buck converter output;
an inverter circuit operationally coupled to the buck converter output, configured to receive the generated DC voltage and convert the DC voltage into an AC voltage to drive the lamp;
a short circuit protection circuit including a latch, which is operationally coupled to the buck converter output and which senses the DC bus voltage, compares the sensed voltage to a predetermined threshold and, based on the comparison, shuts down the inverter circuit;
a power line resistor which is operationally coupled to the buck converter output and which receives the generated DC bus voltage; and
a storage capacitor operationally coupled to the power line resistor and latch, which storage capacitor charges through the power line resistor and supplies power to the inverter circuit after initial ignition of the lamps,
wherein the latch is in an “ON” state when the sensed voltage is less than the predetermined threshold, and discharges the storage capacitor, the inverter circuit being shut down when the storage capacitor is substantially discharged.

10. The ballast as set forth in claim 9, wherein the power line resistor resumes charging the storage capacitor once the latch stops conducting, and wherein the storage capacitor and latch cooperate to restart the inverter circuit.

11. The ballast as set forth in claim 9, wherein the short circuit protection circuit further includes:
a resistor operationally connected in series with the storage capacitor to limit current to the latch.

12. The ballast as set forth in claim 9, wherein the latch includes:
a first transistor which includes:
a base operationally connected to the power line resistor, wherein current is being drawn from the first transistor base when the sensed voltage is less than the predetermined threshold.

13. The ballast as set forth in claim 12, wherein the latch further includes:
a second transistor which includes:
a collector operationally connected to the base of the first transistor, and
a base operationally connected to a collector of the first transistor, wherein current is being drawn from the second transistor base when the sensed voltage is less than the predetermined threshold, the first and second transistors cooperate to discharge the storage capacitor.

14. The ballast as set forth in claim 13, wherein the first and second transistors stop conducting when the storage capacitor is substantially discharged.

15. The ballast as set forth in claim 9, wherein the latch is operationally connected with the storage capacitor, and wherein the power line resistor and latch cooperate to discharge the storage capacitor when the sensed voltage is less than the predetermined threshold and shut down the inverter circuit when the storage capacitor is substantially discharged.

16. The ballast as set forth in claim 15, wherein the power line resistor, storage capacitor and latch cooperate to restart the inverter circuit after shut down.