WO1998025442A2

WO1998025442A2 - Industrial voltage ballast circuit with passive power factor correction

Info

Publication number: WO1998025442A2
Application number: PCT/US1997/022144
Authority: WO
Inventors: Mihail S. Moisin
Original assignee: Pacific Scientific Company
Priority date: 1996-12-06
Filing date: 1997-12-05
Publication date: 1998-06-11
Also published as: WO1998025442A3

Abstract

An improved ballast circuit (100) controls the power delivered to a fluorescent lamp for use in industrial plants where the input voltage is on the order of 347 volts. The ballast circuit comprises passive power factor correction (120) to increase the overall power factor of the circuit to above 95 % while also reducing the total harmonic distortion to less than 20 %. The ballast circuit protects the fluorescent lamps from overvoltage when the lamps are nearing the end of their lifetime, or burnout. Adaptations to connect compact fluorescent lamps are also included.

Description

INDUSTRIAL VOLTAGE BALLAST CIRCUIT WITH PASSIVE POWER FACTOR CORRECTION

Background of the Invention Field of the Invention The present invention relates to improved apparatus and methods for operating fluorescent lamps and, in particular, to a method and apparatus to control the power delivered to a fluorescent lamp.

Description of the Prior Art

Fluorescent lamps are conventional types of lighting devices. They are gas charged devices which provide illumination as a result of atomic excitation of a low-pressure gas, such as mercury, within a lamp envelope. The excitation of the mercury vapor atoms is provided by a pair of heater filament elements mounted within the lamp at opposite ends of the lamp envelope. In order to properly excite the mercury vapor atoms, the lamp is ignited or struck by a higher than normal voltage. Upon ignition of the lamp, the impedance decreases and the voltage across the lamp drops to the operating level at a relatively constant current. The excited mercury vapor atoms emit invisible ultraviolet radiation which in turn excites a fluorescent material, e.g., phosphor, that is deposited on an inside surface of the fluorescent lamp envelope, thus converting the invisible ultraviolet radiation to visible light. The fluorescent coating material is selected to emit visible radiation over a wide spectrum of colors and intensities.

As is known to those skilled in the art, a ballast circuit is commonly disposed in electrical communication with the lamp to provide the elevated voltage levels and the constant current required for fluorescent illumination. Typical ballast circuits electrically connect the fluorescent lamp to line alternating current and convert this alternating current provided by the power transmission lines to the constant current and voltage levels required by the lamp. Fluorescent lamps have substantial advantages over conventional incandescent lamps. In particular, the fluorescent lamps are substantially more efficient and typically use 80 to 90% less electrical power than incandescent lamps for an equivalent light output. For this reason, fluorescent lamps have gained use in a wide range of power sensitive applications. However, potential pitfalls exist in the use of fluorescent lamps. In fluorescent lamps near the end of their lifetime or near burnout, the impedance of the lamp increases as the filament breaks down, causing a rapid build up of voltage in the lamp. This voltage buildup may trigger a catastrophic failure of the lamp, which, in extreme cases, results in an explosion within the lamp and the broadcast of shards of glass for several feet.

Further, some industrial plants, typically in Canada, use an input AC voltage of approximately 347 volts. Typical fluorescent ballast circuits may not be used in these plants. In current ballast circuits, voltage doubters and boost circuits are used to amplify the input voltage. However, if the voltage in a ballast circuit exceeds 500 volts, the transistors in the circuit will be damaged. Special high voltage components may be used to prevent this damage, but these components are very costly. Summary of the Invention In the present invention, a ballast circuit accepts high voltage (approximately 347 volts AC) input and converts the high voltage input into useable DC voltage to drive fluorescent lamps. The present invention allows the continued use of inexpensive transistors by not amplifying the input voltage while still achieving a high power factor with low total harmonic distortion.

The ballast circuit of the present invention uses a combination of full-wave rectification and passive power factor correction with the resonant circuit to provide for low total harmonic distortion and for high power factor correction without exceeding the 500 volt limit. In the preferred embodiment, a power factor of greater than 0.95 is achievable. A further aspect of the present invention is end-of-life protection for the circuit. The ballast circuit of the present invention serves to automatically prevent destructive breakdown of the fluorescent lamp by limiting the voltage buildup in the lamp as the lamp impedance increases over its useful life. The voltage in the lamp is never allowed to exceed a predetermined voltage where catastrophic breakdown of the lamp may occur.

In one embodiment of the present invention, a ballast is used with a high AC input voltage to power a fluorescent lamp. The ballast comprises an EMI filter stage which receives the high AC voltage input. A rectification stage connected to the EMI filter stage converts the AC voltage to a DC voltage. A passive power factor correction stage receives the DC voltage from the rectification stage and generates a corrected signal. A high frequency resonating stage receives the corrected signal from the passive power factor correction stage and generates a high frequency signal. A load stage receives the high frequency signal from the resonating stage and applies the high frequency signal to light the fluorescent lamp.

The present invention also includes a method of powering a fluorescent lamp by a high AC input voltage. The method comprises the steps of receiving the high AC voltage input; converting the AC voltage to a DC voltage without voltage amplification; generating a corrected signal from the DC voltage; creating a high frequency signal from the corrected signal and applying the high frequency signal to light the fluorescent lamp. Brief Description of the Drawings

Figure 1 is a block diagram of a ballast circuit of one embodiment of the present invention.

Figure 2 is a schematic circuit diagram of a ballast circuit of the present invention with the load stage removed.

Figure 3 is a schematic circuit diagram of a load stage in one embodiment of the ballast circuit of the present invention.

Figure 4 is a schematic circuit diagram of a load stage in an alternative embodiment of the ballast circuit of the present invention.

Figure 5 is a graphical representation of current and voltage waveform patterns generated by prior art ballast circuits. Figure 6 is a graphical representation of current and voltage waveform patterns generated by the ballast circuit of Figure 2.

Detailed Description of the Preferred Embodiment Ballast Circuit for the Three-Wav Switch Figure 1 illustrates the ballast circuit 100 in accordance with one aspect of the present invention. The ballast circuit 100 comprises an EMI filter stage 110, a rectification stage 115, a passive power factor correction stage 120, an active high frequency resonant stage 125, an end-of-life control stage 130 and a load stage 150. The ballast circuit 100 is adapted so that fluorescent lamps connected at the load will operate properly when a high voltage input is applied to the ballast circuit 100. An input AC source is connected to a high voltage input line 102, a ground line 106 and a neutral input line 104. The input lines 102, 104 and 106 are connected to the EMI filter stage 110. The EMI filter stage 110 is connected to the rectification stage 11 . The rectification stage 115 is connected to the passive power factor correction stage 120, which is in turn connected to the active high frequency resonant stage 125. The active high frequency resonant stage 125 is connected to the end-of-life control stage 130 and to the load stage 150. The load stage provides an input to the end-of-life control stage 130. Figure 2 is a schematic representation of the ballast circuit of Figure 1. Each stage of the ballast circuit

100 will be examined in detail below.

EMI filter stage 110

The EMI filter stage 110 supplies high voltage AC power to the ballast circuit 100. The EMI filter stage

110 comprises the high voltage input line 102, the ground line 106, the neutral input line 104, a fuse F1, capacitors C1, C2, and C3 and inductors L1-1 and L1-2. The high voltage input line 102 is connected in series to a first terminal of the fuse F1. A second terminal of the fuse F1 is connected to a first terminal of the inductor L1-1 and to a first terminal of the capacitor C1. A second terminal of the inductor L1-1 is connected to the anode of a diode

D2, to the cathode of a diode D4 and to a first terminal of the capacitor C2. In a specific circuit, the fuse FI is advantageously formed as a fusible link on a printed circuit board (not shown). The neutral input line 104 is connected to a first terminal of the inductor LI -2 and to a second terminal of the capacitor C1. A second terminal of the inductor L1-2 is connected to a second terminal of the capacitor C2, to the anode of a diode D1, to the cathode of a diode D3 and to a first terminal of the capacitor C3. The ground line 106 is connected to a second terminal of the capacitor C3. The inductors LM and L1-2 are connected to the line voltages to protect the line against EMI by preventing high frequency signals from propagating to the lines 102, 104 and 106. In the preferred embodiment, each of the inductors L1-1 and L1-2 is a 2.5 millihenry inductor having 120 turns. The capacitors C1 and C2 are 0.1 microfarad capacitors rated at 400 volts, and the capacitor C3 is a

0.0033 microfarad capacitor rated at 1000 volts.

The Rectification Stage 115 The rectification stage 115 converts the input AC voltage to a DC voltage and includes rectifying diodes D1, D2, D3 and D4. The anode of the diode D1 is connected to the cathode of the diode D3, to the first terminal of the capacitor C3, to the second terminal of the capacitor C2 and to the second terminal of the inductor L1-2. The cathode of the diode D1 is connected to the positive voltage rail 116. The anode of the diode D3 is connected to the negative voltage rail 118. The anode of the diode D2 is connected to the cathode of the diode D4, to the first terminal of the capacitor C2 and to the second terminal of the inductor LM. The cathode of the diode D2 is connected to the positive voltage rail 116. The anode of the diode D4 is connected to the negative voltage rail 118. Due to the high input voltage (347 volts), no voltage amplification is needed or desired. In fact, any voltage above 500 volts may damage the transistors in the ballast circuit 100. The rectification stage 115 forms a full-wave bridge to convert the input line voltage of the EMI filter stage 110 into DC voltage between the positive voltage rail 116 and the negative voltage rail 118 without voltage amplification.

In the preferred embodiment, each of the diodes D1, D2, D3 and D4 are 1 N4007 diodes. The Passive Power Factor Correction Stage 120

The passive power factor correction stage 120 provides for a passive power factor correction for the ballast circuit 100 and includes two capacitors C4 and C5, three diodes D5, D6, and D7 and a resistor R1. A first terminal of the capacitor C4 is connected to the positive voltage rail 116. A second terminal of the capacitor C4 is connected to the anode of the diode D7 and to the cathode of the diode D6. The anode of the diode D6 is connected to the negative voltage rail 118. The cathode of the diode D7 is connected to a first terminal of the resistor R1. A second terminal of the resistor R1 is connected to the anode of the diode D5 and to a first terminal of the capacitor C5. The cathode of the diode D5 is connected to the positive voltage rail 116. A second terminal of the capacitor C5 is connected to the negative voltage rail 118.

By using the passive power factor correction stage 120 in the circuit, the power factor can be improved to approximately 0.95 without the use of a boost circuit. The increased power factor results in a significant energy cost savings for the overall ballast circuit 100. The passive power factor correction stage 120 receives voltages from both the positive voltage rail 116 and the negative voltage rail 118. A portion of the voltage received from the positive voltage rail is graphically depicted in Figure 5 as a half sine wave 212. If a standard storage capacitor were used in place of the passive power factor correction stage 120, the resultant current delivered to the remainder of the ballast circuit 100 would be approximated by a waveform 210. Because the current surges only during the peak of the voltage cycle 212, a high peak current 215 results which causes a low power factor on the order of 0.60.

By using the passive power factor correction stage 120 instead of storage capacitors, the power factor is improved significantly. A current received from the positive voltage rail 116 first charges the capacitor C4, passes through the diode D7 and the resistor R1, charges the capacitor C5 and then returns to the negative voltage rail 118. Thus, the capacitors C4 and C5 are charged in series. When the voltage on the positive voltage rail 116 passes below a threshold voltage, the diodes D5 and D6 turn on and the capacitors C4 and C5 begin to discharge. With the diodes D5 and D6 on, the capacitors C4 and C5 discharge in parallel. Because a sinusoidal waveform is applied to the passive power factor correction stage 120, this cycle is constantly repeated resulting in a current waveform 310 as shown in Figure 6. The current waveform 310 in Figure 6 more closely approximates the input waveform 312 and has a resultant power factor about 0.95. The total harmonic distortion (THD) of the waveform is also improved, especially due to the use of the resistor R1. By using the resistor R1, the peak charging current is smoothed out resulting in the peak 325 shown in Figure 6. By removing the resistor R1, the peak charging current will tend to spike giving a resultant waveform 320 shown in phantom. With the resistor R1 smoothing out the peak charging current, the THD can be maintained at less than 0.20. In the preferred embodiment, the capacitors C5 and C6 are 33 microfarad capacitors rated at 250 volts.

The diodes D5 and D6 are preferably 1 N4007 diodes. The resistor R1 is a 47Ω resistor and is rated at 2 watts.

The Active High Frequency Resonant Stage 125 As further illustrated in Figure 2, the high frequency resonant stage 125 provides the high frequency required to properly drive the lamps. The high frequency resonant stage 125 comprises resistors R2, R3, R4 and R5, capacitors C6, C7, C8 and C9, diodes D8, D9, D10 and D1 1, a diac D13, a split inductor LR-1, and a pair of transistors Q1 and Q2. A first terminal of the resistor R2 is connected to a first terminal of the capacitor C6, to a first terminal of the diac D13, and to the anode of the diode D8. A second terminal of the resistor R2 is connected to the positive voltage rail 116. A second terminal of the capacitor C6 is connected to the negative voltage rail 118. The cathode of the diode D8 is connected to the anode of the diode D9, to the emitter of the transistor Q1, to a second terminal of the capacitor C7, to the cathode of the diode D 10, to a tap in the inductor LR-1, to the collector of the transistor Q2, to a first terminal of the capacitor C8 and to the cathode of the diode D11. The anode of the diode D11 is connected to the negative voltage rail 118. A second terminal of the capacitor C8 is connected to the negative voltage rail 118. The cathode of the diode D9 is connected to the positive voltage rail 116. The collector of the transistor Q1 is connected to the positive voltage rail 116. The base of the transistor Q1 is connected to a first terminal of the resistor R4, to a first terminal of the resistor R5, to a first terminal of the capacitor C7 and to the anode of the diode D10. A second terminal of the resistor R4 is connected to the positive voltage rail 116. A second terminal of the resistor R5 is connected to a first terminal of the inductor LR-1. A second terminal of the inductor LR-1 is connected to a circuit junction 140, which connects to the lamp load. The base of the transistor Q2 is connected to a first terminal of the capacitor C9, to a first terminal of a resistor R6, to the collector of the transistor Q3 and to a first terminal of the resistor R3. A second terminal of the resistor R3 is connected to a second terminal of the diac D13. A second terminal of the capacitor C9 is connected to the negative voltage rail 118.

In the preferred embodiment, the components of the resonating stage 125 have the following values: the transistors Q1 and Q2 are BUL45 transistors, the diodes D9 and D11 are UF4007 diodes, the diode D8 is a 1 N4007 diode, the diode D10 is a 1IM4148 diode, the diac D13 is a HT-32 diac, the capacitor C6 is a 0.1 μF capacitor rated at 50 volts, the capacitor C8 is a 330 picofarad capacitor rated at 2000 volts, the capacitors C7 and C9 are 0.15 μF capacitors rated at 50 volts, the resistors R2 and R4 are 440 KΩ resistors, the resistor R3 is a 47Ω resistor, and the resistor R5 is a 47Ω resistor and is rated at 2 watts. The value of the inductor LR-1 is dependant on the choice of load stage and is discussed below. Starter Circuit and Start Mode of Operation The capacitor C6, the diac D13 and the current limiting resistor R3 form a starter circuit that initially, at the application of power to the ballast circuit 100, actuates or turns ON the circuit transistor 02 in the active resonant stage 125.

During the start mode of the active resonant stage 125, the switching transistor 02 is actuated by the starter circuit. Specifically, when the capacitor C6 charges to a voltage greater than the reverse breakdown voltage of the diac D13, the diac D13 discharges through the current limiting resistor R3, turning ON the transistor Q2. Once the transistor Q2 is turned on, the switching transistors Q1 and Q2 alternately conduct during each half cycle of the input voltage and are driven during normal circuit operation by energy stored in the second section of the inductor LR-1 and transferred to the secondary windings of the first section of LR-1 and to an inductor LR-2. Therefore, the starter circuit only operates during initial start mode and is not required during the normal operation of the resonant stage 125.

Resonant Mode of Operation During normal or resonant operation, the ballast circuit 100 of Figure 2 is energized by the application of the sinusoidal input voltage having a selected magnitude and frequency to the high voltage input line 102. In the typical embodiment for Canada and other countries where the industrial voltage is 347 volts, the input power has a magnitude of 347 volts. The input voltage is filtered by the EMI filter stage 110, as described above, and produces an input current flow to the rectification stage 115 and to the passive power factor correction stage 120. The output of the passive power factor correction stage 120 is used to power the remainder of the ballast circuit 100. When the transistor Q1 is on, current flows from the emitter of the transistor 01 to the second section of the inductor LR-1 and to the load stage. When the transistor Q1 turns off and the transistor Q2 turns on, current flows from the collector of the transistor 02 to the second section of the inductor LR-1 and to the load stage. When used in combination in the ballast circuit 100, these components produce a current having a selected elevated frequency, preferably greater than 20 Kilohertz, and most preferably approximately 40 Kilohertz, during normal operation of the ballast circuit. This high-frequency operation reduces hum and other electrical noises delivered to the lamp load. Additionally, high-frequency operation of the lamp load reduces the occurrence of annoying flickering of the lamp.

The End-of-Life Stage 130 The end-of-life stage 130 prevents catastrophic breakdown of the lamps due to overvoltage during the end of a lamps useful life. The end-of-life stage 130 comprises a transistor Q3, a capacitor C10, two resistors R6 and R7, a diac D14, a diode D12, a zener diode Z1 and the inductor LR-2. The emitter of the transistor Q3 is connected to the negative voltage rail 118. The base of the transistor Q3 is connected to a first terminal of the resistor R7, to a first terminal of the diac D14 and to a first terminal of the capacitor C10. A second terminal of the capacitor C10 is connected to the negative voltage rail 118. A second terminal of the diac D14 is connected to the cathode of the diode D12. The anode of the diode D12 is connected to the anode of the zener diode Z1. The cathode of the zener diode Z1 is connected to a first terminal of the inductor LR-2, to a second terminal of the resistor R7 and to a first terminal of the resistor R6. A second terminal of the inductor LR-2 is connected to the negative voltage rail 118. A second terminal of the resistor R6 is connected to the collector of the transistor Q3, to the base of the transistor Q2, to the first terminal of the capacitor C9 and to the second terminal of the resistor R3.

In operation, the end-of-life stage 130 limits the amount of voltage supplied to the load stages, thereby preventing the breakdown of the lamps. As the voltage applied to the load stage increases, this voltage is applied to the end-of-life stage via the inductor LR-2. When the voltage across the inductor LR-2 reaches a predetermined level, as determined by proper selection of component values, the transistor Q3 will turn on. The amount of voltage required to turn on the transistor Q3 is chosen to be below the level where catastrophic breakdown of the lamps is possible. When the transistor Q3 is on, the voltage on the base of the transistor Q2 keeps the transistor Q2 off. When the transistor Q2 is off, the frequency period of the active high frequency resonant stage 125 is shortened and less power is delivered to the load, thereby decreasing the voltage. When the voltage returns to a safe level, the amount of voltage across LR-2 will fall and the transistor Q3 will turn off. With the transistor Q3 off, the transistor Q2 may again turn on, thereby adjusting the symmetry of the high frequency resonating stage 125.

In a preferred embodiment, the components of the end-of-life stage 130 have the following values: the transistor Q3 is a 2N3904 transistor, the diode D12 is a 1 N4148 diode, the diac D14 is a HS-10 diac, the zener diode Z1 is a 1 N5237B diode, the capacitor C10 is a 0.01 μF capacitor rated at 50 volts, the resistor R6 is a 47Ω resistor and is rated at 2 watts, and the resistor R7 is a 2KΩ resistor. The value of the inductor LR-2 is dependant on the choice of load stage and is discussed below.

The Load Stage 150 When compact fluorescent bulbs are desired to be used with the ballast circuit 100, the load stage 150 shown in Figure 3 is used. The load stage 150 comprises a first compact lamp 152 with filaments 157, 161, filament terminals 156, 158, 160 and 162, a second compact lamp 154 with filaments 165, 169, filament terminals 164, 166, 168 and 170, capacitors Cl 1, C12 and C13, and inductors LR-3 and LR-4. (The inductors LR-1 and LR-2, and the zener diode Z1 are also shown in Figure 3 to show the relationship between Figures 2 and 3.) A first terminal of the capacitor C11 is connected to the circuit junction 140, which connects to the second section of the split inductor LR-1. A second terminal of the capacitor C11 is connected to the filament terminal 1 6. The filament terminal is also connected to a first end of the filament 157. The second end of the filament 157 is connected to the filament terminal 158. The filament terminal 158 is also connected to a first terminal of the inductor LR-3. A second terminal of the inductor LR-3 is connected to a first terminal of the capacitor C12. A second terminal of the capacitor C12 is connected to a first terminal of the capacitor C13 and to the filament terminal 170. The second terminal of the capacitor C13 is connected to a first terminal of the inductor LR-4, to the filament terminal 162 and to the filament terminal 166. The second terminal of the inductor LR-4 is connected to the filament terminal 160 and to the filament terminal 164. The filament terminal 162 is connected to a first end of the filament 161. A second end of the filament 161 is connected to filament terminal 160. The filament terminal 166 is connected to the first end of the filament 165. The second end of the filament 165 is connected to the filament terminal 164. The filament terminal 170 is connected to a first end of the filament 169. A second end of the filament 169 is connected to the filament terminal 168. The filament terminal 168 is also connected to the circuit junction 145, which is connected to the second terminal of the inductor LR-2 and to the negative voltage rail 118.

The resonating storage capacitors C12 and C13 store a selected elevated voltage, preferably equal to or greater than 300 volts rms, which is required to start or ignite the fluorescent lamps 152 and 154. Once the lamps 152 and 154 are struck, the circuit operating voltage is reduced to a value slightly greater than the input voltage.

In the preferred embodiment, the capacitor Cl 1 is a 0.1 μF capacitor rated at 400 volts, the capacitor C12 is a 33 picofarad capacitor, the capacitor C13 is a 0.033 μF capacitor rated at 2000 volts, and the inductor LR-1 is a 4.0 millihenry inductor having 3 turns on the first section and 200 turns on the second section. The inductor

LR-2 is 3 turns on the core of the inductor LR-1, the inductor LR-3 is 20 turns on the core of the inductor LR-1, and the inductor LR-4 is 2 turns on the core of the inductor LR-1.

The Load Stage 180 When regular fluorescent bulbs are desired to be used with the ballast circuit 100, the load stage 180 shown in Figure 4 is used. The load stage 180 comprises a first lamp 182 with filaments 187, 191, filament terminals 186, 188, 190 and 192, a second lamp 184 with filaments 195, 199, filament terminals 194, 196, 198 and 200, capacitors C14, C15, C16, C17, C18 and C19, diodes D15 and D16, a diac D17, a transformer T1 having a primary winding 175, a secondary winding 177 and a tertiary winding 179, and an inductor LR-5. (The inductors LR-1 and LR-2, and the zener diode Z1 are also shown in Figure 4 to show the relationship between Figures 2 and 4.) The cathode of the diode D16 is connected to the circuit junction 135, which connects to the positive voltage rail 116 in Figure 2. The anode of the diode D16 is connected to the cathode of the diode D15 and to the tap of the primary winding 175 of the transformer T1. The anode of the diode D15 is connected to a first terminal of the capacitor C14 and to the circuit junction 145, which connects to the second terminal of the inductor LR-2 and to the negative voltage rail 118. A second terminal of the capacitor C14 is connected to a first terminal on the primary winding 175 of the transformer T1. A second terminal on the primary winding 175 of the transformer T1 is connected to the circuit junction 140, which connects to the second terminal of the inductor LR-1 in Figure 2. A first terminal on the secondary winding 177 of the transformer T1 is connected to a first terminal of the capacitor C16, to a first terminal of the capacitor C17 and to the filament terminal 188. A second terminal of the capacitor C16 is connected to a first tap on the secondary winding 177 of the transformer T1 and to the filament terminal 186. The filament terminal 188 is connected to a first end of the filament 187. A second end of the filament 187 is connected to the filament terminal 186. A second terminal of the capacitor C17 is connected to a first terminal of the inductor LR-5. A tap of the inductor LR-5 is connected to a first terminal of the capacitor C18. A second terminal of the capacitor C18 is connected to a first terminal of the diac D17. A second terminal of the diac D17 is connected to a first terminal of the tertiary winding 179 of the transformer T1, to the filament terminal 190 and to the filament terminal 194. A second terminal of the tertiary winding 179 of the transformer T1 is connected to the filament terminal 192 and to the filament terminal 196. The filament terminal 190 is connected to a first end of the filament 191. A second end of the filament 191 is connected to the filament terminal 192. The filament terminal 194 is connected to a first end of the filament 195. A second end of the filament 195 is connected to the filament terminal 196. A second terminal of the inductor LR-5 is connected to a first terminal of the capacitor C19. A second terminal of the capacitor C19 is connected to the filament terminal 200, to a first terminal of the capacitor C15 and to a second terminal on the secondary winding 177 of the transformer T1. The filament terminal 200 is connected to a first end of the filament 199. A second end of the filament 199 is connected to the filament terminal 198. A second terminal of the capacitor C15 is connected to the filament terminal 198 and to a second tap on the secondary winding 177 of the transformer T1.

The load stage 180 accepts power from the active high frequency resonant stage 125 and applies that power across the transformer T1 to light the lamps 182, 184. The transformer T1 is required by Underwriter Laboratory (UL) regulations when using standard fluorescent lamps. To prevent overvoltage of the load stage 180, the clamping diodes D15 and D16 work in conjunction with the end-of-life stage 130. In normal operation, the voltage at the tap 181 should be less than half the voltage across the positive voltage rail 116. When the voltage at the positive voltage rail 116 is low, the clamping diodes D15 and D16 are very effective to maintain the voltage at the tap 181. However, as the voltage on the positive voltage rail 116 increases towards an end-of-life situation, the clamping circuit formed by the diodes D15 and D16 can no longer control the voltage at the tap 181, and the circuit tends to surge power. In this situation, the voltage across the inductor LR-2 increases and the end-of-life stage 130 limits the power delivered to the load as previously described. The power delivered to the load stage 180 is used to ignite the lamps 182, 184. Once the lamps are ignited, the circuit operating voltage is reduced to a value slightly greater than the input voltage.

In the preferred embodiment, the capacitor C14 is a 0.1 μF capacitor rated at 250 volts, the capacitors C15 and C16 are 0.1 μF capacitors rated at 63 volts, the capacitors C17 and C19 are 0.0022 μF capacitors rated at 1000 volts, the capacitor C18 is a 0.1 μF capacitor rated at 400 volts, the diodes D15 and D16 are UF4007 diodes, the diac D17 is a HT-22 diac, and the inductor LR-1 is a 3.1 millihenry inductor having 4 turns on the first section and 230 turns on the second section. The inductor LR-2 is 4 turns on the core of the inductor LR-1, the inductor LR-5 is 120 turns on the core of the inductor LR-1 with each section of LR-5 having 60 turns, and the transformer T1 is an AC2007 transformer.

Numerous variations and modifications of the invention will become readily apparent to those skilled in the art. Accordingly, the invention may be embodied in other specific forms without departing from its spirit or essential characteristics. The detailed embodiment is to be considered in all respects only as illustrative and not restrictive and the scope of the invention is, therefore, indicated by the appended claims rather than by the foregoing description. All changes which come within the meaning and range of equivalency of the claims are to be embraced within their scope.

Claims

WHAT IS CLAIMED IS:

1. A ballast for use with high AC input voltage to power a fluorescent lamp, said ballast comprising: an EMI filter stage which receives the high AC voltage input; a rectification stage connected to the EMI filter stage, said rectification stage comprising a full-wave bridge which converts the AC voltage to a DC voltage; a passive power factor correction stage which receives said DC voltage from said rectification stage and generates a corrected signal; a high frequency resonating stage which receives said corrected signal from said passive power factor correction stage and generates a high frequency signal; and a load stage which receives said high frequency signal from said resonating stage, wherein said load stage applies said high frequency signal to the fluorescent lamp.

2. The ballast of Claim 1, further comprising an end-of-life stage which limits the voltage delivered to said load stage when the fluorescent lamp is nearing burnout.

3. The ballast of Claim 2, where said end-of-life stage limits the voltage by adjusting the symmetry of said high frequency resonating stage.

4. The ballast of Claim 1, wherein said corrected signal generated by said passive power factor correction stage creates a power factor of approximately 0.95.

5. The ballast of Claim 4, wherein said passive power factor correction stage further includes a resistor to smooth out said corrected signal thereby lowering total harmonic distortion.

6. The ballast of Claim 1, wherein said passive power factor correction stage comprises a plurality of diodes and a plurality of capacitors, wherein said diodes and said capacitors are connected so that said capacitors charge in series and discharge in parallel.

7. The ballast of Claim 1, wherein the AC input voltage is approximately 347 volts and the DC voltage does not exceed 500 volts.

8. A method of powering a fluorescent lamp by a high AC input voltage, the method comprising the steps of: receiving the high AC input voltage; converting the AC input voltage to a DC voltage without voltage amplification; generating a corrected signal from the DC voltage; creating a high frequency signal from the corrected signal; and applying the high frequency signal to the fluorescent lamp.

9. The method of Claim 8, wherein the corrected signal causes the AC input voltage to have a power factor of approximately 0.95.

10. A ballast for use with high AC input voltage to power a fluorescent lamp, said ballast comprising: means for filtering the high AC voltage input to produce a filtered AC input voltage; means for rectifying said filtered AC input voltage and converting said filtered AC input voltage to a DC voltage; passive power factor correction means which receives said DC voltage from said rectification means and generates a corrected signal; high frequency resonating means for receiving said corrected signal from said passive power factor correction means and generating a high frequency signal; and means for receiving said high frequency signal from said resonating means and applying said high frequency signal to the fluorescent lamp.