Patent Application
20100225239
The present disclosure relates generally to electronic lighting ballasts and, more particularly, to methods and apparatus for high efficiency ballasts operating in a cold cathode mode of operation, wherein the ballast has a with a high power factor that can also be effectively dimmed.
Methods and apparatus for powering dimmable ballast circuits having a high power factor are disclosed, and which operate with various bulbs in a cold cathode mode. Specifically, gas discharge lamps, such as fluorescent lamps, without heating filaments are used with a ballast. The ballast can be operated with a dimmer and various energy savings circuits for added flexibility and efficiency.
A described dimmable ballast circuit includes a power source connected to a first node and a second node, the power source having a current that alternates at a line frequency. The first node and the second node are connected to each other via an energy storage device in the form of a capacitor that stores energy and provides current at a first (high) frequency, which exceeds the line frequency of the power source and presents a high impedance to the line frequency. This capacitor is small enough in capacitance value relative to the load that it does not distort the rectified AC input from the power source. A first switch is operable to selectively couple the energy storage device to a resonant circuit via the first node. The resonant circuit has a resonant frequency and stores energy during a first portion of a cycle of the first frequency thereby causing light to be emitted. A second switch is operable to selectively couple the resonant circuit via the second node to cause energy stored in the resonant circuit to be substantially recycled via the capacitor. When the second switch closes, this reverses the voltage across the lamp during a second portion of the cycle at the first frequency, also causing light to be emitted.
The above ballast can be adapted to provide energy to a resonant circuit, also known as a “tank circuit” that operates cold cathode fluorescent lights (“CCFL”) in a highly efficient manner. Further, this ballast can be used with bulbs without requiring heating of the filaments to facilitate ignition of the bulb.
In the field of light sources (e.g., gas discharge lamps, fluorescent lamps, light emitting diodes, etc.), many light sources can present a negative resistance that causes the power source to increase the amount of current provided. If the current were not limited in some manner during operation, the current would rapidly increase until there was a catastrophic failure of the light source. To limit the current, a ballast circuit is typically provided that controls the amount of current provided to the light source to maintain a steady state, flicker-free generation of light. Initial ballasts were of the magnetic type, which presented a large inductance to the power source with poor secondary coupling. Such ballasts resulted in the current being largely in phase at the load with respect to the voltage provided by the power source, which resulted in a high power factor. However, magnetic ballasts have very poor efficiencies. Magnetic ballasts have other disadvantages including being relatively large and heavy, and are prone to producing an audible humming sound. Further, they are temperature dependent and when cold they may present difficulties in causing ionization in the lamp and therefore generating light. Magnetic ballasts have largely been replaced by quieter, smaller electronic ballasts to provide the proper starting and operating power to fluorescent lamps. Further, electronic ballasts are generally smaller and more compact and can be integrated with a fluorescent bulb (tube) to produce compact fluorescent lamps (“CFLs”). Electronic ballasts rely on electronic switching circuitry to switch the input voltage to produce a high frequency (typically 20 kHz or higher) voltage to the nodes of the fluorescent lamp. Typically, the ballast includes a “tank circuit” (a.k.a resonant circuit) which increases the line voltage to a higher voltage, typically anywhere from 200 to 600 volts, so as to initiate ionization and maintain the light output of the fluorescent lamp during operation.
The power factor is generally defined as the relationship of the real power to the apparent power. However, electronic ballasts often exhibit a lower power factor, which means the current is not in phase with the voltage. A lower power factor means the power company has less efficiency in energy transmission. Further, as the use of fluorescent lighting becomes widespread, a lower power factor in residential applications becomes more of a concern to the power company. Some ballasts have incorporated a power factor correction circuit, which may include an integrated circuit, capacitor, and other components, which monitor and adjust the current flow so as to be in phase with respect to the line voltage, however, such power factor correction circuits generally have poor efficiency caused by losses due to these components and increase the cost of the ballast. Further, such ballast circuits generally include a low temperature rated, high voltage electrolytic capacitor that substantially limits the life of the ballast.
Electronic ballasts are generally relied upon exclusively for compact fluorescent light (“CFL”) because of their smaller size and weight, relative to magnetic ballasts, which allows a CFL to incorporate both a lamp (light source) and a ballast. Hence, a CFL has an integrated ballast with the lamp. In other applications, such as when using “linear” or “tubular” fluorescent bulbs, the ballast is separate from the lamps, allowing the lamp to be replaced separately from the ballast. Many fluorescent lamps have filaments, which are heated to facilitate ionization. Other fluorescent lamps do not, and these are referred to as cold cathode lamps or bulbs. Bulbs without filaments have fewer components and are easier to manufacturer, but the absence of a heating filament requires a higher voltage to obtain ignition or ionization. Further, the heating of the filaments results in lower energy efficiency. Thus, there is a need for a highly efficient ballast capable of operating a bulb in a cold cathode mode of operation.
In the past, using ballasts precluded the ability to dim the light source. It becomes difficult to sustain ionization in the fluorescent tube at low dimming levels with conventional ballasts, causing the lamp to flicker. Newer ballasts now allow the light source to be dimmed to a degree, but still present problems in that the dimming is over a narrow range of light output. Specifically, many ballasts may effectively limit dimming to a narrow range of the light output before the light source is extinguished, or the lamp begins to flicker in an annoying manner. Further, the energy savings is not commensurate with the amount of light that is dimmed. Thus, if the light is dimmed a certain level (e.g., 25% of its output), one would expect the energy savings to be the commensurate (e.g., only 25% energy is used). However, in many cases, only a small fraction of energy is saved given the reduction in light output. Thus, the benefit of saving energy is not fully realized. Consequently, there is a need for a highly efficient and dimmable ballast for lighting applications operation in a cold cathode mode.
a-g illustrate a conventional prior art ballast circuit having a power factor correction circuit and various voltage waveforms produced therein.
a-c illustrate a block diagram of one embodiment of ballast circuit according to the principles of the present invention having a high power factor in accordance with the present invention, along with voltage waveforms produced therein.
a and 4b are schematic diagrams of example circuits that may implement the example process of
c illustrates waveforms of the voltage in conjunction with use of a dimmer.
d illustrates a schematic diagram of another embodiment of the present invention.
a-c illustrates one embodiment of an inductor core used in the tank circuit of the ballast.
a-b illustrate voltage waveforms associated with the cold cathode ballast with a dimmer.
Methods and apparatus for dimmable ballasts with a high power factor are described herein, including operating in a cold cathode configuration. In the described examples, a dimmable ballast circuit having a high power factor is described that directly interfaces a power source with a light source via a single resonant circuit. In addition, the described dimmable ballast includes a high frequency filter capacitor to reduce high frequency energy from entering the power supply during its operation to increase efficiency.
When an element is said to be “coupled” to another element, the elements can be connected or coupled to one another either directly (without intervening elements) or indirectly (with intervening elements). However, if an element is said to be “directly coupled” to another element, no intervening elements are present. If “connected,” then it generally means that no intervening elements are present.
A rectifier 106 comprising a full wave bridge diode assembly rectifies the AC voltage to produce unfiltered, rectified DC voltage. The aforementioned power factor correction circuit 108 may be present, and typically may incorporate a high voltage electrolytic capacitor or other capacitor, integrated circuit, and other components. The switching circuit 110 typically comprises two transistors for switching at a high frequency, and incorporates a self resonant circuit for driving the transistors to switch at a high frequency, typically 20 kHz or higher. A so-called “tank” circuit 112 includes a combination of induction and capacitance values that has a resonant frequency, and which increases the DC line voltage to a higher value and frequency, typically around 200 volts or more. In some contexts, the bulbs can be considered as part of the tank circuit, since the removal of the bulb may disconnect a capacitor impacting the resonant frequency of the tank circuit. However, unless noted otherwise, the tank circuit as referenced herein generally does not include the bulb. However, in the context of a CLF having an integrated lamp, a bulb is presumed to be connected with the tank circuit. In various countries, such as in the U.S., Europe, or Asia, the resistance value of the filaments in the bulbs is respectively standardized to different values.
The voltage waveform produced by the power source 102 is shown in
The voltage waveform 120 is provided to the input into the rectifier circuit of
In prior art ballasts, a large electrolytic capacitor is often incorporated either by itself, or as part of the power factor correction circuitry 108, to filter the 120 Hz ripple. The presence of this type of filter capacitor, which is designed to filter out the 120 Hz ripple in the rectified power wave, produces a waveform 132 shown in
The switching circuit 110 of
In
The function of the tank circuit, which has a resonant frequency and which is tuned to be a slightly lower frequency than the switching frequency, is to re-circulate the energy introduced and “step up” the voltage introduced to around 200-600 volts that is provided to the bulb. This voltage is high enough to initiate ionization on the fluorescent light bulb. The bulb itself, once ionized, serves to limit the voltage across its terminals. Thus,
While this type of prior art circuit does provide suitable light generation in a lamp, it has difficulty in allowing dimming of the light source over a wide range of light output. Further, this type of prior art circuit is not energy efficient when dimmed. If it does not have the power factor correction circuit, then its power factor is low. If the power factor correction circuit is present, then the circuit contains additional components, increasing its cost.
In the example of
The first node 212 and the second node 214 are connected via a high frequency energy storage device, such as a polypropylene capacitor 215, also referred to as a bypass capacitor herein. In the example of
Ballast circuit 200 also includes a regulator 220, (generically referred in the industry as a housekeeping supply circuit) connected to nodes 212 and 214. Regulator 220 generates a substantially constant voltage that exceeds a first threshold (e.g., 10 volts, etc.) to provide power to a driver 225. Because the voltage at nodes 212 and 214 is not filtered, a regulator is required to provide a steady input voltage to the driver 225. The voltage waveform from the rectifier has at each half cycle a “valley” wherein the voltage drops to zero or near-zero, albeit for a short time. In the illustrated example, the driver 225 is configured to alternately actuate one of a first transistor 235 and a second transistor 240 at a high frequency, referred to herein as the switching frequency, typically at a frequency of 20 kHz or more. The example transistors 235 and 240 are both implemented using vertical N-Channel metal oxide semiconductor (NMOS) field effect transistors, although one of ordinary skill in the art would know that the transistors 235 and 240 can be implemented by any other suitable solid state switching device (e.g., a P-channel metal oxide field effect transistor, an insulated gate bipolar transistor (IGBT), a lateral N-channel mode MOS transistor, a bipolar transistors, a thyristor, gate turn off (GTO) device, etc.).
Driver 225 and transistors 235 and 240 form a half-bridge topology that is implemented to cause a resonant circuit or “tank circuit” 245 to power a light source 250 in the illustrated example. To form the half-bridge topology, the drain of the first transistor 235 is connected to the first node 212 and the source of the second transistor 240 is connected to the second node 214. Thus, the voltage present on the node 212 and the drain of the first transistor 235 is the rectified voltage waveform 260 shown in
The resonant circuit 245 has a high resonant frequency that is slightly lower than the switching frequency of the transistors. Typically, the lowest frequency operable for practical purposes is 18 kHz, and the upper limit is limited by other practical considerations, but maybe as high as 80 kHz. The resonant circuit is also connected to the second node 214 and a light source 250 (e.g., a gas discharge lamp, a fluorescent lamp, a light emitting diode (LED), etc.).
In particular, a first input 252 is connected to the source and drain of NMOS transistors 235 and 240. A first output 253 of the resonant circuit 245 is connected to a second input 254 of the resonant circuit 245 via a first filament 255 of the light source 250. Further, in the example of
The tank circuit presents a variable input impedance. When the input voltage at node 252 is just rising, such as shown with square wave 270 of
After emitting light from the light source, exemplary process 300 then couples the resonant circuit to the second node (block 330). As a result, the resonant circuit has a voltage with a negative magnitude, and the energy is circulated within the tank circuit, thereby causing the light source connected to ionize the gas and emit light during the second half cycle (block 340). During this time, the bypass capacitor is also charged from the power source. Exemplary process 300 determines if power is still provided by the energy source (block 345). If power is provided, the exemplary process returns to block 305. On the other hand, if power is not provided to the ballast, the exemplary process ends. In the present invention, there is no ionization during a brief time period while the rectified unfiltered DC input voltage is in a “valley.” This point corresponds to the zero crossing point of the AC input line voltage. The time period during which the bulb is not ionized is typically at least 200 microseconds. However, this short time period is not perceivable to the human eye and the bulb may be generating light due to persistence of the phosphor in the bulb.
In the example of
a is a schematic diagram of an exemplary circuit 400 that may implement exemplary process 300 (
The value of capacitor 215 is typically a 0.8-1.5 μF polypropylene capacitor for a 23 watt light source, and 0.22 μF for a 5 watt light source. The value can be adjusted as appropriate for the output load, but typically is 4 μF or less for a typical CFL. The value of capacitor 215 is small enough so as to not impact the output rectified voltage at node 212. Specifically, the value should not preclude the output voltage presented at node 212 from dropping down to 15% or less of its peak voltage of the rectifier output at the end of each half cycle. In other words, the voltage at the bottom of the “valley” should be no more than 10-18 volts.
Voltage regulator 220 is also connected to first and second nodes 212 and 214 and is configured to provide a substantially constant output voltage to the driver circuit. In the illustrated example, voltage regulator 220 is implemented using an NMOS transistor 410 that is connected to the first node 212 via a resistor 412. The drain of NMOS transistor 410 is connected to its respective gate via a resistor 414. The gate of NMOS transistor 410 is further connected to a collector of a transistor 416 via an optional resistor 421, which has its respective base connected to the anode of a zener diode 418. Resistor 421 reduces the gain of the transistor thereby reducing possibility of oscillations in transistor 410. The cathode of zener diode 418 is connected to the source of NMOS transistor 410.
In addition, the base of transistor 416 is connected to second node 214 via resistor 420 and its emitter is connected to the second node 214 via a resistor 422. In the example of
In the illustrated example of
Referring to the driver 225, regulator 220 provides the substantially constant (i.e., regulated) voltage via diode 424, which also isolates voltage regulator 220 from driver 225. Stated differently, diode 424 prevents current from flowing from capacitor 426 into regulator 220 when the voltage of the first node 212 falls below the voltage stored in capacitor 426. In the embodiment of
In the illustrated embodiment of
In the illustrated example, the resistance value of the resistor 436 and the capacitance value of the capacitor 434 configure the driver circuit 428 to produce pulses at a frequency in the range of approximately 20 to 100 KHz. Specifically, the pulses are alternately produced by driver circuit 428 and are output via the high side gate driver output (HO) and the low side gate driver output (LO). Stated differently, during the first half cycle of a period of the switching frequency (i.e., the half of the time period for a single cycle), the high side gate driver output of the driver circuit 428 produces a pulse. During the second half cycle of the period (i.e., the low side of the cycle) of the switching frequency, the low side gate driver output of the driver circuit 428 produces a pulse. Typically, there is a dead time between pulses when neither transistor is turned on, e.g., the time after the first pulse ends and before the second pulse begins.
In the embodiment of
As described above, the source of the NMOS transistor 235 and the drain of the NMOS transistor 240 are connected to the resonant or “tank” circuit 245, which selectively stores a charge therein. In the illustrated example, the resonant circuit 245 includes a capacitor 442 in series with an inductor 444. The capacitor 442 functions in part as a DC blocking capacitor. Its value is in some embodiments is 1/10 the value of capacitor 215 as a rough rule of thumb. However, other ratios can be used, but may not be optimized for the power factor. Typically, the capacitor 442 has a value from 1 μF to 0.01μF.
The inductor 444 is generally a gapped core inductor that is capable of handling a large peak current occurring primary at 60 Hz. The choice of the core material of inductor can be selected so as to not saturate the inductor even if a gap is not present. Typically, using conventional ferrite core materials, a gap would be needed to avoid saturation. The inductor is larger than what is used in a typical prior art ballast of the same power, because this inductor processes both the lower line frequency current (e.g., 120 Hz) as well as the higher, switching frequency current (e.g., 20-100 kHz) and must avoid saturation at the lower frequency. This is in contrast to prior art ballasts which process a filtered rectified DC output voltage, resulting in a largely constant DC voltage with little ripple. Hence, the prior art inductors in the tank circuit are not designed to conduct an appreciable amount of current at the line frequency. In
The inductor 444 is connected to the second node 214 via a capacitor 446 to store a charge therein and excite the light source. Further, the inductor and capacitors are a small value in relation to 60 Hz, such that they do not change the phase angle of the current relative to the supply voltage, thereby contributing to the high power factor of the circuit. Further still, the inductor 444, which has a small value relative to the prior art, is connected to a capacitor 448 via the first filament 255 and does not have an appreciable reactance at 120 Hz. The capacitor 448 is also connected to the second node 214 via the second filament 260. The capacitor 448 receives current and stores a charge therein to excite the light source via current flowing across the filaments 255 and 260. The resonant frequency of the example resonant circuit 245 is described by equation 1 below:
where fR is the resonant frequency of the circuit, L444 is the inductance value of the inductor 444, C442 is the capacitance value of the capacitor 442, C446 is the capacitance value of the capacitor 446, and C448 is the capacitance value of the capacitor 448. In the illustrated embodiment, the capacitor 446 is configured to have a different value such that it has a different energy potential than the capacitor 448. In particular, the capacitor 446 provides a larger voltage to allow the lamp 250 (
The values of the components in the circuit vary on the output power of the lamp and the desired resonant frequency. In certain embodiments, values for 120VAC operation of certain components are illustrated in the table below:
In embodiment 1 and 3, the operation is for a CFL bulb, whereas embodiment 2 is for a pair of 4 foot tubular lamp bulbs. For embodiments 1, and 2, the inductor can be made from an Elna bobbin part # CPH-E34/14/9-1S-12PD-Z. For embodiment 3, the inductor can be made from an Elna/Fair-Rite core #9478375002. In the above embodiments, it is possible to use a 1 μF capacitor for output powers of 15 -42 watts.
The other values of the circuit shown in
Those skilled in the art will realize that other values or type of components may be used.
The embodiment of
When operated with a dimmer, the voltage provided to the ballast circuit may not be that as shown as waveform 472 in
Thus, during the time period up to t1 the voltage regulator circuit 220 ensures that the driver circuit still receives a DC operating voltage. If, however, the ballast circuit is never used with a dimmer (or the dimmer itself is never used), then the voltage waveform similar to 474 would never occur, and the voltage at node 212 would always look like waveform 472.
In such cases, the voltage regulator circuit 220 can be simplified to the embodiment shown in
The operation of the example of
In addition, the line filter 401 is configured to prevent high frequency energy from the capacitor 215 from entering back into the power source 205. The filter 401 is not required to be present in commercial products embodying the invention, but typically a filter circuit of some form is included when the ballast is designed to power 40 watt or higher fluorescent lamps. As shown in
Returning to
However, when the voltage across the zener diode 418 exceeds a corresponding breakdown voltage (e.g., about −14.0 volts, etc.), the zener diode 418 enters what is commonly referred to as “avalanche breakdown mode” and allows current to flow from its cathode to its anode. In response, the current flows across the resistor 420 and causes the transistor 416 to have a base-emitter voltage (VBE), thereby having a base-emitter current thereby turning on the transistor 416. The transistor 416 sinks current into the second node 214, which reduces the gate-source voltage of the NMOS transistor 410 and the current through the zener diode 418. Once the current in the zener diode 418 does not exceed the design of the output of the regulator value, the zener diode 418 recovers to the design value and reduces the current from flowing into the resistor 420. That is, as illustrated in the example of
Thus, the example voltage regulator 220 is configured to provide a substantially constant (i.e., regulated) voltage to the driver 225. When the rectified voltage provided via the rectifier 210 falls below a predetermined threshold voltage (VT), the voltage output by the voltage regulator 220 decreases. However, as illustrated in the example of
The driver circuit 428 is configured to generate a signal that alternately actuates one of the transistors 235 and 240 at the switching frequency, which is much higher than the line frequency. In particular, during the first half (or a portion thereof) of a single cycle of the switching frequency, the high side output (HO) of the driver circuit 428 produces a high side pulse to turn on transistor 235 while transistor 240 is turned off. Typically, the high side pulse has a duration that does not exceed half of the time period of a cycle of the switching frequency. When the driver circuit 428 turns on transistor 235, the transistor 235 couples the node 212 to the resonant circuit 245 via a low impedance path.
The example of
During the second half of the time period of the switching frequency, the low side output (LO) of the driver circuit 428 produces a low side pulse to turn on the transistor 240 just after transistor 235 is turned off. When the driver circuit 428 turns on the transistor 240, the transistor 240 couples the node 214 to the resonant circuit 245 via a low impedance path. The second pulse generally has a duration that is less than 50% of the time period of the switching frequency (e.g., less than a half-cycle).
The example of
As described above, by turning on the transistor 840, the resonant circuit is connected to the second node 814 via a low impedance path. In response, the capacitors 842, 846 and 848 discharge the voltage therein as currents denoted by reference numerals 806a, 802 and 804, respectively. The currents 802 and 804 flow into the inductor 844 and charge the capacitor 842 as a voltage, thereby causing the resonant circuit 245 to have a negative voltage with respect to the second node 814. As a result of current leaving the capacitors 846 and 848, the light source 850 is actuated to visually emit light. After a delay, the capacitor 842 discharges producing a current as denoted by reference numeral 806, which flows into the node 814. At the end of the second half cycle of the carrier frequency, the resonant circuit stores substantially no energy and all the energy is stored in the inductor, with very little, if any, current flowing. Thus, the driver circuit is continually driving switches 835 and 840 even when there is no current flowing through the switches.
Thus, in
The illustrated voltage waveform of
Each half line cycle in time period A 906 shows a similar pattern. In time period B 900, which occurs at the beginning of the half cycle, the switch 735 of
This process builds up voltage across the tube until ionization occurs (around 20-35 volts of the input voltage to the resonant circuit), which occurs at the beginning of time period C 902. The tube acts as a voltage clamping regulator to keep the voltage constant across it (that is, the magnitude or absolute value of the voltage, recognizing it is either positive or negative in value), which is shown as an average ionization voltage level 910 in
The voltage change over the beginning, peak and falling voltage edges of the rectified AC input to the tank (which is switched by transistors 735 and 740) and the constant ionization voltage of the bulb causes a large change in current to be linearly processed by capacitor 742 and inductor 744. As compared to a traditional ballast with a filtered DC supply, this change in current causes a large change in Q.
Thus, there is short time period at the beginning of a half cycle and the end of the half cycle shown as period E 908, where ionization does not occur in the tube, and there is no light generated as a result of ionization. Consequently, unlike the prior art which initiates ionization in the tube and maintains the ionization during normal operation (e.g., while power is applied to the ballast), the present invention causes ionization to initiate every half cycle, or 120 time per second. Further, there is a time period every half cycle where light due to ionization stops and is not generated. However, the time period when the voltage is too low to generate ionization is very short, and does not create a perceptible condition for humans.
The current flowing into the resonant circuit at the line frequency is largely maintained as a sine wave, which means that the current load is largely in phase with the voltage at the line frequency from the power source. Further, the resonant circuit does not store any significant energy (inductive or capacitive) to distort the low frequency current during the time period between the half cycles, thereby causing the resonant circuit to appear as a resistive load to the power supply. Thus, the present circuit maintains a high power factor during operation. In particular, because the current flowing through the resonant circuit is substantially similar to a sine wave, the crest factor of the illustrated example is approximately the square root of 2 (e.g., about 1.5), which close to an ideal crest factor. Contrast this to the prior art ballasts which require a dedicated power factor correction circuit to obtain a suitable crest factor.
In addition, the example ballast circuit of present invention does not require nor uses a large, high voltage electrolytic capacitor as used in conventional ballasts to store substantial amounts of low frequency energy because the high frequency energy is continually recycled by a non-electrolytic bypass capacitor. Further, the impedance presented to the power source 205 is modified only by the resonant circuit and the example circuit 400 contains only a single inductor. As a result, the embodiments described herein are able to realize a high power factor (typically above 0.9) with a single stage of processing with respect to the power source without incorporating the components found in a traditional power factor correction circuit. In addition, because the described examples do not require a large, high voltage, low temperature electrolytic capacitor, the lifespan of ballasts of the present invention is substantially increased.
Other benefits of the invention include the ability to effectively dim the light source over a predictable and wider range. Although the ballast itself does not provide any dimming and requires interaction with a dimmer circuit to do so, the ballast circuit can be effectively used with the dimmer disclosed in U.S. patent application Ser. 12/205,564 filed on Sep. 5, 2008, which in turn claims the benefit under 35 U.S.C. §119(e) to U.S. Provisional Patent Application entitled “Two-Wire Dimmer Switch for Dimmable Fluorescent Lights” filed on Feb. 8, 2008, bearing Ser. No. 61/006,967, both of which are herein incorporated by reference for all that each teaches. The charging of the housekeeping electrolytic capacitor in the voltage regulator is performed at the very beginning of the voltage waveform produced from the output from the dimmer which dissipates the stored inductance in the house wiring created when the phase controlled dimmer has turned on charging the input bypass capacitor of the ballast. This would normally cause a ringing of current of the input bypass capacitor if it were not damped by the load presented by the series regulator at this precise time during the charging of the house keeping capacitor.
The aforementioned ballast circuitry can be adapted in another variation for providing power to a fluorescent lamp in a cold cathode fluorescent lamp (CCFL) configuration or mode of operation. This arrangement can be used for a variety of fluorescent lamp types, including compact fluorescent lamps (“CFLs”), linear tubular (removable) lamps, and tubular arrangements of other shapes. Advantageously, this arrangement can be used with an integrated lamp and ballast combination, such as a CFL.
CCFLs do not rely on a filament to be heated when started (nor in normal operating mode). Pre-heating is used to reduce the required ionization voltage of lamps using filaments. Thus, the initial voltage needed to ionize the tube in a CCFL mode of operation is typically higher relative to ballasts that power filaments in the fluorescent lamp. However, fluorescent lamps that rely on a filament are typically not as efficient because the heat in the filaments does not generate light. Further, the operation of a bulb can be adversely impacted if a filament is broken or degraded in some manner. Further, filaments represent an additional component cost and manufacturing cost to the lamp. While the required starting voltage to initiate ionization in a CCFL configuration is higher than a lamp using filaments, ionization occurs faster in the present invention during initial startup because in part there are no filaments to heat. In the CCFL configuration, a high voltage sufficient to cause ionization is applied to the ends of the tube. Because the tank circuit provides the required ionization voltage very quickly, the bulb quickly ionizes. Once ignited, the tube presents a lower impedance (e.g., negative value) and thus a ballast is required to limit the current. This is true regardless of whether filaments are used. Once ignited, there is no significant difference in the voltage required to maintain ionization in a lamp having filaments as compared to a lamp without filaments.
It is possible to also operate a fluorescent bulb having filaments in a CCFL configuration, i.e., without heating the filaments. In this configuration, the ends of the filaments can be simply shorted together, and they are not relied upon for starting the lamp. In other embodiments, only one terminal of each filament may be connected to the tank circuit, with the other terminal of each filament not connected. From an electrical perspective, shorting the filaments can be considered equivalent to removing the filaments because the filament resistance is reduced to zero. Hence, the present invention can be adapted to function with conventional four-pin fluorescent bulbs, as well as two-pin linear bulbs. Consequently, a “CCFL” bulb as used herein refers to a bulb used in a cold cathode mode—e.g., there is no filament in a bulb that is heated. Thus, a CCFL may have a filament, but if present, it is not heated. The present invention can also be adapted to CFLs having integrated ballasts, and avoids the need for incorporating filaments in the bulbs of CFLs. This reduces component cost and manufacturing complexity.
However, the tank circuit is different compared to previous embodiments and the tank circuit 1150 comprises capacitors 1172 and 1175, an inductor 1174, and lamp 1188. In this embodiment, lamp 1188 is illustrated as having two filaments 1186a and 1186b (e.g., a four-pin gas discharge tube), but each filament has its corresponding leads (1180a, 1180b, and 1182a, 1182b) connected together. Thus, the potential across each filament is zero volts. In other embodiments, a two-pin, filament-less tube can be used. The use of the bulb with filaments in
In this embodiment, the inductor 1174 is configured as a tapped inductor. One portion 1174a (to the left of the tap) comprises about half of the total inductance and the other portion 1174b (to the right of the tap) comprises the other half. From an implementation perspective, the first portion comprises about ¾ of the total number of windings and the second portion comprises about ¼ of the number of windings. This demarcation point occurs typically at a center tap of the inductance value (not a center tap of the number of turns). These portions will be referred to herein as the “right portion” 1174b and “left portion” 1174a, and is merely convenient nomenclature to illustrate the invention in light of
The two windings on the inductor are mutually electromagnetically coupled so as to create an interaction, a so-called ‘transformer action.’ Thus, the inductor can also be viewed as acting as a transformer (e.g., an “autotransformer”). The use of a tapped inductor can be viewed as functionally equivalent to a transformer having a specified inductance on the primary winding. Thus, it may be possible to implement the aforementioned tank circuit using components other than a tapped inductor, but which function equivalent to the tapped inductor.
The tap is connected to node 1193, so that a resonant circuit is formed from node 1151, through capacitor 1172, the left portion of inductor 1174a, to node 1193, and then to node 1153. This portion forms an LC circuit that resonates having a sinusoidal voltage when a square wave—like voltage is provided to the inputs of the tank circuit from the ballast portion 1101. The portion of the inductor to the right of the tap 1174b does not contribute its inductance to the resonant circuit. Specifically, because node 1193 is tapped within the inductor, the right side inductance of the right portion 1174b of inductor 1174 is not used to determine the L value in the resonant circuit.
The inductance associated with the left portion of the inductor, along with the capacitor 1172, determines the resonance of the tank circuit. Thus, the inductor 1174 can be viewed as having a transformer action with respect to generating a voltage for the bulb, but also as having an inductance value for purposes of determining the resonance of the tank circuit.
The inductor value 1174a should be selected (along with the capacitor value of capacitor 1175) so that the resonant frequency of the tank circuit is less than the frequency of the incoming alternating voltage at nodes 1151 and 1153. Further, the value of the inductance of the entire inductor should be such that the inductor operates in a non-saturated or a limited saturated mode of operation. This can be accomplished by use of an inductor using certain materials, core size, and gapping to produce the appropriate inductance value as previously disclosed. Specifically, the presence of a 60 Hz rectified sinusoidal component in the input voltage at nodes 1151 and 1153 should result in no or limited saturation of the inductor. Avoiding saturation of the inductor requires using a typically larger inductor in the tank circuit than is found in the tank circuits of the prior art.
In this embodiment, capacitor 1172 in conjunction with capacitor 1175 determines the total capacitance of the tank circuit, and therefore determines the resonance frequency of the tank circuit (obviously, the inductance value of the inductor also plays a part in determining the resonance frequency). However, the capacitance of the tank resonant circuit is largely determined by the capacitor 1175 as it is smaller in value. Capacitor 1172 also acts as a DC blocking capacitor and removes any DC component in the input square wave provided to the tank circuit by ballast portion 1101. This capacitor ensures a symmetrical (balanced) current is provided to the lamp. Thus, capacitor 1172 electrically isolates the inductor and the bulb from the DC component in the input voltage waveform. Further, capacitor 1172 also limits the current that would otherwise saturate the inductor from the rectified power line frequency (e.g., 120 Hz) present on the input voltage waveform.
Capacitor 1175 is also part of the resonant circuit and is present between node 1193 and node 1153. Capacitor's 1175 main purpose is to act as a resonant capacitor for the inductor in the resonant circuit. In this embodiment, the tank circuit can be viewed as having an LC resonant circuit within it, with a portion of the tapped inductor (e.g., the right side) that is outside the resonant circuit, but still part of the tank circuit. Capacitor 1175 also adjusts for any voltage imbalance in the lamp.
In one embodiment of the invention corresponding to
When the tank circuit resonates, the voltage across nodes 1191 and 1153 increases and is presented to the ends of the lamp 1188. Although these nodes are attached to the filaments, the presence of the filaments is insignificant to the analysis of the circuit, because they are connected together. The voltage across the lamp is based on the whole of inductor 1174, not just a portion of it. In other words, even though inductor portion 1174a is in the resonating portion of the tank circuit (and inductor portion 1174b is not), the voltage generated and presented to the lamp is based on both inductor portions 1174a, 1174b. Thus, the voltage is “boosted” by the second set of windings (and hence, these windings may be referred to as “boost windings” or as a “tertiary winding”). The presence of the additional inductor portion 1174b results in a higher voltage to the lamp than what is generated at the tap (which is node 1193). Thus, the right side portion of the inductor 1174b creates an added voltage to the voltage produced at node 1193. This added voltage is designed so that it is sufficient to initiate ionization. The peak voltage at node 1193 (which is the inductor tap) is less (by approximately by 25%-33%, which is the ratio of the windings for 1174b) than the peak voltage at node 1191 during the ramp-up leading to ionization. The voltage generated by the tank circuit and supplied to the bulb results the energy in the inductor being ‘pushed’ into the lamp. Further, the transformer action of the tapped inductor reduces the peak current through the bulb caused by the low frequency voltage (e.g., 120 Hz) compared to other embodiments previously described (e.g., non-CCFL mode of operation).
Once ionization occurs, the voltage across the lamp is reduced. Recall that the nature of an ionized lamp is that it clamps or limits an applied voltage. Thus, once ionized, the voltage across the lamp will not exceed a certain value (depending on the lamp and other factors) and this clamps the voltage at node 1191 to typically around 100 volts. During ionization, the peak voltage at node 1193 (which is the inductor tap) is less (by approximately by 25%-33%, which is the ratio of the windings for 1174b) than the peak voltage at node 1191.
When the bulb ionizes, the bulb forces a reduction in voltage that causes a current surge from the tube. Because the inductor portion 1174b is in series with the current passing through the lamp, the inductor portion 1174b serves to limit the rate of change of current flowing through the lamp. There is a leakage inductance associated with the inductor 1174b, that limits the current. The leakage inductance could be modeled as a separate inductor in series with the inductor, and which is represented as being part of inductor 1174b in
Unlike prior art systems, capacitor 1175 does not discharge as much energy through the lamp at high voltage. The peak voltage across the capacitor at node 1193 is lower than the peak voltage at node 1191, which is the voltage across the lamp. Thus, the capacitor typically discharges 30-60% less energy than prior art ballasts having a capacitor across the lamp. Thus, the voltage across capacitor 1175 peaks typically around 67-70 volts for 120 VAC operating, and is typically less than the 80-100 volts at node 1191, which is the voltage after ionization of the lamp.
Although the bulb is ionized each half cycle of the line power input frequency, the presence of the inductor portion 1174b and capacitor 1175 aid in the longevity of the bulb. First, the inductor portion 1174b ‘cushions’ the current generated by the bulb during ionization by limiting the rate of change (di/dt) of the current, and second, the two-part inductor results in a lower voltage at node 1193, which is the voltage across capacitor 1175. When capacitor 1175 discharges, it does so at a lower voltage and energy level compared to the prior art. In other words, the presence of the boost windings of 1174b increase the voltage to the bulb, and requires less current in the tank to reach the ionization voltage. Hence, capacitor 1175 is smaller, and is required to discharge less energy by the bulb during initial ionization. This may allows use of smaller and less expensive capacitors.
The tank circuit of
In this embodiment, the lamp is re-ionized every 1/120 of a second, which is every half cycle of the input power frequency (at 60 Hz). The voltage waveform across the lamp is illustrated in
The time leading up to ionization is illustrated as Time Period B 1206. In the tank circuit embodiment of
Once the voltage at the bulb reaches an ionization level 1214, the bulb ionizes, and clamps the voltage to a lower level (typically around 100 volts), shown as the ionization voltage Vi 1225. The time period of ionization is illustrated as Time Period C 1208. During this time, light is being generated by the lamp.
Eventually, the AC voltage continues to drop and tank circuit is no longer able to sustain ionization, and Time Period D 1210 is entered. This time period reflects that ionization of the bulb is no longer maintained, and the tank voltage begins to drop.
The transformer action of tapped inductor 1174 provides a brief current flow to the tank circuit at the end of ionization, thereby extending the time which the bulb is ionized. Consequently, with both the ionization Time Period B 1206 and the discharge Time Period D 1210 shortened relative to non-CCFL embodiments, the time period of ionization (Time Period C 1208) is longer. Because the ionization period is longer, the CCFL embodiment generates light longer than without the tapped inductor.
Further, during Time Period D, the residual energy in the tank diminishes, but does not completely dissipate before the next half cycle begins. Thus, the lamp voltage typically does not reach zero volts during the ‘non-ionization time’ (Time Period E 1212). The non-ionization time is the time which the bulb is not ionized, and comprises Time Period B and Time Period D. Although the bulb may not be ionized, that does not necessarily mean that light is not being generated from the bulb. A typical fluorescent bulb comprises a phosphorous coating which persists in generating light. Thus, it is not obvious from
Although the tank circuit 1150 can be used with other ballast designs, using the tank circuit with the ballast portion 1101 results in a highly efficient ballast, having a high power factor with long bulb life. The presence of the bypass capacitor 1102 (which is selected to be suitable with the load of the lamp) aids in achieving a high power factor, and the presence of resistor 1103 (around 3-5 ohms) reduces noise when the ballast is operated with prior art dimmer circuits and which may be necessary to function with prior art dimmers. The operation of the ballast can be combined with the dimmer circuit as disclosed in U.S. patent application Ser. No. 12/353,551, filed on Jan. 14, 2009, entitled Method and Apparatus for Dimming Light Sources, the content of which is incorporated herein by reference. When the dimmer circuitry is combined with the ballast 1101 and tank circuit 1150, the combination provides a highly efficient, high power factor, long lasting lighting system that is also dimmable.
The dimmer acts to limit the incoming power to the ballast by modifying each half cycle of power to the ballast. The dimmer circuit can be viewed as “slicing off” or controlling the phase angle of the input power for a portion of the input power half cycle as shown in
The circuit diagram of the ballast 1105 connected to a dimmer is shown in
The impact of dimming on the voltage across the lamp is illustrated in
When the dimmer is activated, it blocks a beginning portion of each AC input voltage half cycle from being passed to the ballast. The length of this portion is based on the setting of the dimmer. The effect of this is shown in
Once the dimmer allows the input voltage to pass to the ballast, the voltage is significantly above zero volts, and the result is that the tank circuit generates a very high and short spike during Time Period B 1470, which causes the lamp to ionize. During Timer Period C, the lamp is ionized until the input AC voltage drops in value, and Time Period D 1474 is entered. The end of Time Period D represents the end of the input voltage period. The time periods overlaid on the voltage waveforms are not to scale, and hence the end of Time Period D is approximately indicated. During this time period, the tank circuit is still resonating, and not all of the energy has dissipated, hence there is some voltage across the tube during Time Period F 1460 even though no light is being generated.
In prior art ballasts, the presence of non-ionization time is problematic because prior art ballasts are designed to continuously ionize the bulb. Prior art ballasts typically ionize a bulb once (when it is started) and are not designed to re-ionize the bulb at each half cycle. Thus, many prior art ballasts are not dimmable. Recall that prior art ballasts may incorporate a filament to facilitate initial starting and may maintain power to the filament during normal operation. When the bulb has been running, it is easier to restart a bulb after ionization is interrupted, because the gases in the bulb have been already heated. Thus, in the prior art, if the ballast is running, a certain amount of non-ionization time can be tolerated if the ballast is operated with a dimmer because the temperatures of the lamp have risen during operation and the bulb can be easily re-ionized. However, if the non-ionization time is too long, the bulb becomes difficult to re-ionize the bulb and flickering of the bulb occurs or at worst, the lamp goes out. In some prior art ballasts, when the bulb is dimmed, the ballast also reduces the current flowing in the filament. This requires a higher ionization voltage in the lamp, which the ballast may not be able readily provide. Thus, many prior art ballasts are not dimmable, or have a narrow dimming range and quickly begin to flicker when dimmed. In some cases ionization stops completely and the lamp goes out. Even if the prior art ballast is configured to quickly re-ionize the bulb, the presence of the current surge created by the bulb during ionization, along with a capacitor discharging at a high voltage level, contributes to shortening the life of the bulb. Hence, many conventional ballasts are not designed to be dimmed, or if they are, the reliability of the bulb can be adversely impacted by dimming.
In contrast, the present invention does not have these adverse impacts because the ballast is designed to re-ionize the bulb every half cycle during normal operation. Thus, the voltage waveform in
Further, use of the aforementioned dimmer circuit in
The tank circuit of
One embodiment described in the aforementioned patent application (appl. Ser. No. 12/366,886) that can be adapted to
However, application of the current detection circuit to the tank circuit of
One approach to detecting the removal a bulb is shown in
The voltage at node 1695 can be also considered a signal voltage provided as input to the driver integrated circuit, as disclosed in the aforementioned patent application. This is used to set the switching frequency of the ballast. Thus, a change in the signal voltage can alter the switching frequency and lower the voltages produced in the tank circuit, creating a safer condition.
Further, the same voltage at node 1695 in
Although certain methods, apparatus, systems, and articles of manufacture have been described herein, the scope of coverage of this patent is not limited thereto. To the contrary, this patent covers all methods, apparatus, systems, and articles of manufacture fairly falling within the scope of the appended claims either literally or under the doctrine of equivalents.
