Abstract:
A method and apparatus for controlling the operation of a gas discharge lamp including regulation of power provided to the lamp for maintaining a preselected illumination intensity, automatic lamp shut down for preventing a catastrophic failure of the lamp, and automatic selection of operating frequencies for increasing efficiency and extending useful life of the lamp. An appropriate quality factor Q is achieved by including a low pass filter followed by a high pass filter in the lamp network so as to allow deep dimming of the lamp. Preferably, the arc power delivered to the lamp network is sensed and regulated. By sensing the arc power instead of only the lamp current, the illumination intensity of the lamp is accurately regulated. Further, the lamp ballast is automatically shut down near the end of the lamp&#39;s useful life, before operation in partial rectification. Preferably, power to the lamp is shut off based on the arc power entering the lamp network instead of relying only on the voltage of the lamp to avoid unnecessarily shutting down the lamp. The lamp network is automatically operated at an appropriate frequency selected from among a plurality of predetermined frequencies according to the lamp&#39;s present mode of operation including: preheating, starting, and continuous operation. Preferably, each frequency is optimized for the particular lamp and for the particular mode of operation.

Description:
FIELD OF THE INVENTION 
     The invention relates to the field of control circuits for gas discharge lamps. In particular, the invention relates to control circuits for gas discharge lamps that monitor and regulate the power provided to the gas discharge lamps. 
     BACKGROUND OF THE INVENTION 
     Gas discharge lamps, such as conventional fluorescent lamps, offer substantial improvements over incandescent lamps, including higher energy efficiency and longer life. A drawback to fluorescent lamps, however, is that they can be difficult to control. This is due, in part, because they have “negative resistance.” This means that the operating voltage decreases as current through the lamp increases. Therefore, circuits for supplying power to fluorescent lamps generally require a electronic ballast to maintain operating stability of the circuit and to provide an ability to dim the lamp. 
     During a typical manufacturing process for gas discharge lamps, the lamps are optimized to provide a maximum light output with a minimum amount of energy consumption. Different capacity gas discharge lamps having different lumen outputs are each designed for a different optimum voltage level. The benefits of high energy efficiency and long lamp life require that the ballast provide the gas discharge lamp with the optimum lamp voltage and which appropriately control the current for adjusting the light output of the lamp. 
     A conventional non-adjustable ballast provides a fixed lamp voltage and lamp current for a lamp with a specific lumen output. As a gas discharge lamp ages, however, the lamp deteriorates which causes the impedance of the lamp to increase. When such a lamp is operated with a non-adjustable ballast, this deterioration causes the lamp output to become increasingly dim over time. Accordingly, even though the non-adjustable ballast is initially optimized for the particular lamp, over time, the lamp output becomes increasingly dim and efficiency decreases. 
     A prior alternative to a conventional non-adjustable ballast is an adjustable fixed ballast. The adjustable fixed ballast allows the lamp current and lamp voltage to be adjusted by the user in an attempt to optimize a particular gas discharge lamp for a specific light output intensity. This allows gas discharge lamps of different capacities to be used in conjunction with identical ballasts. However, as stated above, the impedance of gas discharge lamps increases over time. Thus, over time, the gas discharge lamp will produce an increasingly dimmer light output and efficiency decreases. Therefore, optimization will be lost unless the user re-adjusts the ballast. 
     An approach to some of the problems associated with an adjustable fixed ballast is an electronic self-adjusting ballast. A common technique by which such a self-adjusting ballast regulates a gas discharge lamp is by sensing and controlling the current in the lamp. One problem with regulating only the lamp current is that the light output of the lamp is more closely related to the arc power of the lamp than to the lamp current. The arc power is equal to the product of lamp current and lamp voltage. Lamp voltage, however, is dependent on the temperature of the lamp. Therefore, if only current is regulated, the arc power and, hence, light output, will vary with the temperature of the lamp. 
     Another problem associated with gas discharge lamps is safety. When the gas discharge lamp is near the end of its useful life, the gas discharge lamp can continue to operate in a condition of partial rectification. When operating in partial rectification, there is a high cathode fall voltage in the region of a depleted cathode. Accordingly, operation in partial rectification causes excessively high power dissipation in the region of the depleted cathode. Further, when only lamp current is regulated, increases in the impedance of the lamp caused by aging results in increased power dissipation. As a result of these factors, portions of the gas discharge lamp can reach excessive temperatures. This can present a dangerous fire hazard and can cause the glass envelope of the lamp to shatter. This can pose an immediate safety hazard for persons in the vicinity of the lamp. 
     Although gas discharge lamps tend to be more efficient than their incandescent counterparts, it is advantageous for gas discharge lamps to operate in a dimmed mode. By operating in a dimmed mode, the light intensity from the gas discharge lamp can be adjusted according to the needs or tastes of the user. Unfortunately, prior control circuits for gas discharge lamps, especially small diameter lamps such as the T 4 , generally cannot operate in a dimmed mode below approximately 40% of the lamps&#39; rated illumination output without the lamp extinguishing itself or flickering excessively. 
     A prior art electronic ballast and network for gas discharge lamps is described in U.S. Pat. No. 5,315,214 and shown in FIG.  1 . FIG. 1 illustrates a prior art circuit which controls the illumination intensity of the lamp by controlling the current passing through the lamp. This prior art circuit also shuts off the lamp circuit when the lamp voltage exceeds a preselected threshold. Further, this prior art circuit utilizes a low pass filter at the output lamp network to allow the lamp to be dimmed. These features of operating of the circuit shown in FIG. 1 are disadvantages for the following reasons. Because the lamp current remains constant, the illumination intensity of the lamp will vary with impedance changes caused by aging of the lamp. Further, by sensing lamp voltage to determine when to shut down, in the case of a removed or unlit lamp, this prior art lamp circuit does not protect the lamp from circumstances when the lamp current remains constant and the lamp voltage rises thus causing excess power to dissipate into the lamp. Finally, this prior art lamp circuit does not allow the lamp, especially a small diameter lamp such as the T 4 , to be dimmed below approximately 40% without extinguishing itself or excessively flickering because of a high quality factor Q lamp network. 
     Therefore, what is needed is a control circuit for a gas discharge lamp that overcomes these disadvantages. 
     SUMMARY OF THE INVENTION 
     The present invention is a method and apparatus for controlling the operation of a gas discharge lamp including regulation of power provided to the lamp for maintaining a preselected illumination intensity, automatic lamp shut down for preventing a catastrophic failure of the lamp, and automatic selection of operating frequencies for increasing efficiency and extending useful life of the lamp. The invention also provides an appropriate quality factor Q for the lamp network so as to allow the lamp to be dimmed to low levels, referred to as “deep dimming” or “architectural dimming,” while maintaining operation of the lamp. An example of a gas discharge lamp is a commercially available fluorescent lamp commonly used in office, factory and commercial retail settings. 
     Preferably, the present invention measures the arc power delivered to the lamp network and regulates the power received by the lamp. By sensing the arc power instead of only the lamp current, the present invention accurately regulates the illumination intensity of the lamp. This is true despite the impedance of the lamp changing due to aging and despite the lamp voltage being affected by temperature changes. Further, the present invention preferably also automatically shuts down the lamp ballast near the end of the lamp&#39;s useful life and before operation in partial rectification occurs. Preferably, power to the lamp is shut off based on the arc power entering the lamp network instead of relying only on the voltage of the lamp. This avoids unnecessarily shutting down the lamp. 
     In addition, the present invention also automatically operates the lamp network at an appropriate frequency selected from among a plurality of predetermined frequencies. The appropriate frequency is selected according to the lamp&#39;s present mode of operation including: preheating, starting, and continuous operation. Preferably, each frequency is optimized for the particular lamp and for the particular mode of operation. 
     Further, the preferred embodiment of the present invention includes a lamp network that has a low pass filter followed by a high pass filter coupled to the lamp in series. As the lamp is dimmed, the lamp goes into a region of high negative resistance and is more prone to being extinguished or excessively flickering. This configuration of the lamp network results in a lower the quality factor Q and allows the lamp to continue operation during deep dimming. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 illustrates a prior art schematic diagram showing an electronic ballast controller integrated circuit. 
     FIG. 2 illustrates a first embodiment of the present invention showing a circuit for regulating power received by a gas discharge lamp to maintain a constant illumination and for shutting down the lamp before partial rectification occurs based on power received by the lamp. 
     FIG. 3 illustrates a second embodiment of the present invention showing a circuit for regulating current received by a gas discharge lamp to maintain a constant illumination and for shutting down the lamp before partial rectification occurs based on power received by the lamp. 
     FIG. 4A illustrates a preferred embodiment of the present invention showing a first portion of a circuit for regulating power received by a gas discharge lamp to maintain a constant illumination, dimming the lamp to a low level, shutting down the lamp before partial rectification occurs, and selecting a proper operating frequency. 
     FIG. 4B illustrates a preferred embodiment of the present invention showing a second portion of the circuit referenced in FIG.  4 A. 
     FIG. 5 illustrates a timing chart showing three different phases of operation for the preferred embodiment. 
     FIG. 6 illustrates the equivalent circuit of the lamp network shown in the preferred embodiment of FIG.  4 B. 
     FIG. 7 shows the equivalent circuit shown in the lamp network of FIG.  6 . 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     FIG. 2 illustrates a first embodiment of a power regulating circuit according to the present invention which controls a level of power provided to a gas discharge lamp  290  for controlling an illumination intensity of the lamp  290 . The level of power provided to the lamp  290  is sensed by providing a regulated voltage to a lamp network  300  and by sensing current leaving the lamp network  300 . The power provided to the lamp network  300  is controlled by adjusting a switching frequency of an inverter which comprises switches  310  and  320 . Further, the power regulating circuit illustrated in FIG. 2 automatically shuts off power to the lamp  290  near the end of the useful life of the lamp  290  so as to avoid a potentially dangerous catastrophic failure of the lamp  290 . This accomplished by disabling a power factor correction circuit (PFC)  200  when regulation of the power provided to the lamp fails to prevent the power from exceeding a predetermined level. These features of power regulation and automatic shutoff are not affected by variations in the temperature of the lamp  290 . 
     In FIG. 2, the PFC circuit  200  has two output terminals across which a regulated direct-current voltage VDC 1  is provided for supplying the lamp network  300  and the lamp  290  with power. The voltage VDC 1  is preferably between 380 and 460 volts. An AC power source (not shown) supplies power to the PFC circuit  200 . A first output terminal of the PFC circuit  200  is coupled to a first terminal of the switch  310 . A second terminal of the switch  310  is coupled to an input terminal of the lamp network  300  and to a first terminal of the switch  320 . A second terminal of the switch  320  is coupled to a first terminal of a resistor  210  and to a first terminal of a resistor  220 , thereby forming a node N 1 . A second terminal of the resistor  210  is coupled to a second output terminal of the PFC circuit  200  and to a ground node. A second terminal of the resistor  220  is coupled to a positive terminal of a capacitor  230 , to a positive input terminal of a comparator  240  and to a negative input of an amplifier  270 , thereby forming a node N 2 . A negative terminal of the capacitor  230  is coupled to the ground node. 
     The lamp network  300  has two output terminals. A first output terminal of the lamp network  300  is coupled to a first terminal of the lamp  290 . Additionally, a second output terminal of the lamp network  300  is coupled to a second terminal of the lamp  290 . 
     As mentioned, the present invention senses the current received by the lamp network  300  for regulating a level of power provided to the lamp  290  and for providing automatic shut off of power to the lamp  290  when the lamp  290  reaches the end of its useful life. Because the voltage VDC 1  is regulated, the current from the lamp network  300  which flows through the resistor  210 , is representative of a level of power provided to the lamp network  300 . 
     Accordingly, a voltage V N1  developed across the resistor  210  at the node N 1  is representative of the level of instantaneous power provided to the lamp network  300 . The voltage V N1 , however, is affected by the present condition of the switches  310  and  320 . 
     In combination, the capacitor  230  and the resistor  220  form a low pass filter such that the resulting voltage V N2  at the node N 2  represents the average value or DC value of the voltage at the node N 1 . Accordingly, the voltage V N2  is representative of the level of power provided to the lamp network  300  averaged over several cycles of the switches  310  and  320  (i.e. power delivered to the lamp=VDC 1 *V N2 /R 210 ). 
     A user-adjustable attenuator  260  is coupled to a positive terminal of the amplifier  270 . 
     The attenuator  260  preferably provides a voltage in a range from 0 to 10 volts. As explained herein, adjustment of the voltage provided to the amplifier  270  by the attenuator  260  adjusts the illumination intensity of the lamp  290 . 
     An output from the amplifier  270  is coupled to an input terminal of a voltage controlled oscillator (VCO)  280 . The VCO  280  has two output terminals, OUTA and {overscore (OUTA)}. 
     A first output terminal OUTA of the VCO  280  is coupled to control the switch  310 . A second output terminal {overscore (OUTA)} of the VCO  280  is coupled to control the switch  320 . The voltage levels at the terminals OUTA and {overscore (OUTA)} are complementary in that one, and only one, of the switches  310  and  320  is on (closed) at any one time while the other is off (open). 
     The power provided to the lamp network  300  is regulated according the frequency at which the VCO  280  operates the switches  310  and  320 . More particularly, the power is inversely related to the frequency over a certain range of frequencies. The frequency of the VCO  280  is controlled according to a difference between the voltage level V N2  at the node N 2  and a voltage provided by the attenuator  260 . Further, a feedback loop is formed by the amplifier  270 , the voltage controlled oscillator  280 , and the switches  310  and  320  for regulating the voltage V N2  at the node N 2 . Thus, by controlling the voltage at the node N 2  in a feedback loop, the power provided to the lamp  290  is controlled such that the user selected illumination intensity output for the lamp  290  is maintained despite variations in temperature of the lamp or impedance changes caused by aging. 
     A negative input terminal of the comparator  240  is biased to a predetermined threshold voltage VTH 1 . An output terminal of the comparator  240  is coupled to set input terminal S of an RS flip flop  250 . An output terminal Q of the RS flip flop  250  is coupled to a disable switching in the PFC circuit  200  and the VCO  280  thereby disabling the PFC circuit  200  and the VCO  280 . A reset input R on the RS flip flop  250  is coupled to an under-voltage (UV) signal for re-setting the flip flop  250 . The RS flip flop  250  delivers lamp shut off signal DISABLE 1  to the PFC  200  and the VCO  280  when the voltage at the positive terminal of the comparator  240  exceeds the predetermined threshold voltage VTH 1 . 
     These elements of the present invention automatically shut off the lamp  290  when the lamp nears the end of its useful life. For example, operation in partial rectification can trigger shut down of the lamp  290 . To implement this function, the DC voltage at the node N 2  is supplied to the comparator  240 . When the voltage V N2  at the node N 2  exceeds the predefined threshold voltage VTH 1 , this indicates that the power provided to the lamp  290  can no longer controlled to an appropriate level due to deterioration of the lamp  290 . Accordingly, the comparator  240  sets the RS flip flop  250 , which in turn deactivates the PFC  200  and the VCO  280 , thereby shutting off power to the lamp  290 . The predefined threshold voltage VTH 1  is preferably set at a level higher than a typical, normal voltage at the node N 2  during safe operation of the lamp  290  such that the comparator  240  provides the signal to shut off power to the lamp  290  only when the voltage at the node N 2  reaches unsafe levels. 
     In the preferred embodiment, the output terminal Q of the RS flip flop  250  disables the PFC  200  and the VCO  280  by disabling a clock signal (not shown) utilized for controlling switching in the PFC circuit  200  and the VCO  280 . When the PFC  200  and the VCO  280  are shut down, the voltage VDC 1  falls to low level and little or no power is supplied to the lamp network  300  or to the lamp  290 . 
     FIG. 3 illustrates a second embodiment of a power regulating circuit according to the present invention which controls a level of power provided to a gas discharge lamp  510  for controlling an illumination intensity of the lamp  510 . This feature of power regulation is accomplished by measuring the current received by the lamp  510  via a diode  500 . Similar to FIG. 2, FIG. 3 also incorporates an automatic shutoff feature which prevents the lamp  510  from operating in partial rectification so as to avoid catastrophic failures of the lamp  510  toward the end of its useful life. The feature of automatic shut off is accomplished by measuring the power consumed by the lamp  510  by sensing the current leaving a lamp network  520  and the corresponding voltage. These features of power regulation and automatic shutoff are not affected by variations in the temperature of the lamp  290 . 
     In FIG. 3, the PFC circuit  400  has two output terminals across which a regulated direct-current voltage VDC 2  is provided for supplying the lamp network  520  and the lamp  510  with power. The voltage VDC 2  is preferably between 380 and 460 volts. An AC power source (not shown) supplies power to the PFC circuit  400 . A first output terminal of the PFC circuit  400  is coupled to a first terminal of the switch  530 . A second terminal of the switch  530  is coupled to an input terminal of the lamp network  520  and to a first terminal of the switch  540 . A second terminal of the switch  540  is coupled to a second terminal of the lamp network  520 , to first terminal of a resistor  410  and to a first terminal of a resistor  420 , thereby forming a node N 3 . A second terminal of the resistor  410  is coupled to a second output terminal of the PFC circuit  400  and to a ground node. A second terminal of the resistor  420  is coupled to a positive terminal of a capacitor  430 , to a positive input terminal of a comparator  490 , thereby forming a node N 4 . A negative terminal of the capacitor  430  is coupled to the ground node. 
     The lamp network  520  has two output terminals. A first output terminal of the lamp network  520  is coupled to a first terminal of the lamp  510 . Additionally, a second output terminal of the lamp network  520  is coupled to a second terminal of the lamp  510 . 
     An anode terminal of a diode  500  is coupled to the second terminal of the lamp  510  via a current transformer such that a voltage associated with the second terminal of the lamp  510  is not shared with the anode terminal of the diode  500 . Instead, the anode terminal of the diode  500  receives a current representative of a current that flows through the lamp  510 . 
     A cathode terminal of the diode  500  is coupled to a positive terminal of a capacitor  440 , to a first terminal of a potentiometer  450 , and to a negative terminal of an amplifier  460 , thereby forming a node N 5 . A negative terminal of the capacitor  440  and a second terminal of the variable resistor  450  are coupled to the ground node. A current through the lamp  510  develops a voltage across the potentiometer  450 , thereby forming a voltage V N5  at the node N 5 . The voltage V N5  is smoothed by the capacitor  440  and potentiometer  450  and is, therefore, representative of a level of current supplied to the lamp  510  over several cycles of the switches  530  and  540 . This potentiometer  450 , however, is user adjustable so as to vary this voltage level. Because the voltage VDC 2  is regulated, the voltage V N5  is representative of a level of power provided to the lamp  510 . 
     A positive terminal of the amplifier  460  is biased to a voltage VC. Preferably, the voltage VC is approximately 1 volt. An output terminal of the amplifier  460  is coupled to an input terminal of a voltage controlled oscillator (VCO)  470 . The VCO  470  has two output terminals, OUTB and {overscore (OUT)}B. A first output terminal OUTB and is coupled to control the switch  540 . Further, a second output terminal {overscore (OUT)}B is coupled to control the switch  530 . The voltage levels at the terminals OUTB and {overscore (OUT)}B are complementary such that one, and only one, of the switches  530  and  540  is on (closed) at any one time while the other is off (open). 
     The power provided to the lamp network  520  is regulated according the frequency at which the VCO  470  operates the switches  530  and  540 . More particularly, the power is inversely related to the frequency over a certain range of frequencies. The frequency of the VCO  470  is controlled according to a difference between the voltage level V N5  at the node N 5  and a voltage VTH 2 . Thus, the illumination intensity of the lamp  510  is adjustable by the user adjusting the potentiometer  450 . Further, a feedback loop is formed by the amplifier  460 , the VCO  470 , and the switches  530  and  540  for regulating the voltage V N5  at the node N 5 . Thus, by controlling the voltage at the node N 5  in a feedback loop, the power provided to the lamp  510  is controlled such that the user selected illumination intensity output for the lamp  510  is maintained despite variations in temperature of the lamp or impedance changes caused by aging. 
     A negative input terminal of the comparator  490  is biased to a predetermined threshold voltage VTH 2 . An output terminal of the comparator  490  is coupled to set input terminal S of an RS flip flop  480 . An output terminal Q of the RS flip flop  480  is coupled to a disable switching in the PFC circuit  400  and VCO  470  thereby disabling the PFC circuit  400  and the VCO  470 . A reset input R on the RS flip flop  480  is coupled to an under-voltage (UV) signal for re-setting the flip flop  480 . The RS flip flop  480  delivers lamp shut off signal DISABLE 2  to the PFC  400  and the VCO  470  when the voltage at the positive terminal of the comparator  240  exceeds the predetermined threshold voltage VTH 2 . 
     These elements of the present invention automatically shut off the lamp  510  when the lamp nears the end of its useful life. For example, operation in partial rectification can trigger shut down of the lamp  510 . To implement this function, the DC voltage at the node N 4  is supplied to the comparator  490 . When the voltage V N4  at the node N 4  exceeds the predefined threshold voltage VTH 2 , this indicates that the power provided to the lamp  510  can no longer controlled to an appropriate level due to deterioration of the lamp  510 . Accordingly, the comparator  490  sets the RS flip flop  480 , which in turn deactivates the PFC  400  and the VCO  470 , thereby shutting off power to the lamp  510 . The predefined threshold voltage VTH 2  is preferably set at a level higher than a typical, normal voltage at the node N 4  during safe operation of the lamp  510  such that the comparator  490  provides the signal to shut off power to the lamp  510  only when the voltage at the node N 4  reaches unsafe levels. 
     In the preferred embodiment, the output terminal Q of the RS flip flop  480  disables the PFC  400  and the VCO  470  by disabling a clock signal (not shown) utilized for controlling switching in the PFC circuit  400  and the VCO  470 . When the PFC  400  and the VCO  470  are shut down, the voltage VDC 2  falls to low level and little or no power is supplied to the lamp network  520  or to the lamp  510 . 
     A circuit, shown in FIGS. 4A and 4B, which in addition to the functions of power regulation and automatic power shut off, implemented by the circuits illustrated in FIGS. 2 and 3, operates gas discharge lamps  620 ,  624  more efficiently by preferably utilizing one of a plurality of predetermined switching frequencies for switches  602  and  604 . Preferably, each of these predetermined frequencies is designed for a different mode of lamp operation, such as preheating, starting or continuous operation. Additionally, each of these frequencies which is associated with a corresponding mode of lamp operation is preferably adjustable to maximize the lamp&#39;s efficiency and longevity. Further, FIGS. 4A and 4B also display a circuit which operates the attached lamp in the continuous operation mode at as low as 5% or lower of it&#39;s rated light output. This feature is referred to as “deep dimming” or “architectural dimming” and provides increased flexibility and efficiency for the lamp user. 
     In FIG. 4A, a power factor corrector (PFC) circuit  600  has two output terminals across which a regulated direct-current voltage VDC 3  is provided for supplying a lamp network  601  (FIG. 4B) and the lamps  620 ,  624  (FIG. 4B) with power. A first output terminal of the PFC  600  is coupled to a first terminal of a switch  602 . A second terminal of the switch  602  is coupled to a first terminal of a switch  604  and to a node A which also corresponds to the node A located in FIG.  4 B. 
     A node B in FIG. 4A corresponds to the node B located in FIG.  4 B. The node B is coupled to a second terminal of the switch  604 , a first terminal of a resistor  628 , and a first terminal of a resistor  630 , thereby forming a node N 10  in FIG. 4A. A second terminal of the resistor  628  is coupled to a second output terminal of the PFC  600  and to a ground node. A second terminal of the resistor  630  is coupled to a positive input terminal of a comparator  634 , a negative input terminal of an amplifier  638 , and a positive terminal of a capacitor  632 , thereby forming a node N 12 . A negative terminal of the capacitor  632  is coupled to ground. A negative terminal of the comparator  634  is biased to a voltage VTH 3 . The current from the lamps  620 ,  624  flow through the resistor  628  and establishes a voltage V N10  at a node N 10 . The resistor  630  and the capacitor  632  form a low pass filter. As a result of this low pass filter, the voltage V N12  at node N 12  is a DC or average voltage. The positive terminal of the comparator  634  and the negative terminal of the comparator  638  both sense V N12 . Because the voltage VDC 3  is regulated, the voltage V N12  is representative of a level of power provided to the lamps  620 ,  624 . 
     An output terminal of the comparator  634  is coupled to set input terminal S of a flip flop  636 . A reset terminal R of the flip flop  636  is coupled to a voltage UV. An output terminal Q of the flip flop  636  is coupled to a terminal “C” to disable switching in the PFC  600 . 
     An attenuator  640  is coupled to a positive terminal of the amplifier  638 . The attenuator  640  is preferably configured to supply from 0 to 10 volts. An output terminal of the amplifier  638  is coupled to an input terminal of a voltage-to-current converter  642 . An output terminal of the voltage-to-current converter  642  is coupled to a first terminal of a switch  644 . The voltage to current converter  642  takes a voltage V at the input terminal of converter  642  and provides a current I at the output terminal of converter  642  where the current I is inversely proportional to the voltage V. A second terminal of the switch  644  is coupled to a control terminal of an oscillator  646 , a first terminal of a switch  650 , a first terminal of a resistor  652 , a positive terminal of a capacitor  660 , and a first terminal of a switch  658 . 
     An output terminal OUTC of the oscillator  646  is coupled to control the switch  604 . An output terminal {overscore (OUT)}C of the oscillator  646  is coupled to control the switch  602 . The voltage levels of OUTC and {overscore (OUT)}C are complementary in that one, and only one, of the switches  602  and  604  is on (closed) at any one time while the other is off (open). An input terminal of a current source  648  is coupled to a voltage VCC. An output terminal of the current source  652  is coupled to a second terminal of the switch  650 . Additionally, a second terminal of the resistor  652  is coupled to a first terminal of a resistor  654 . A second terminal of the resistor  654  is coupled to a first terminal of a switch  656 . A second terminal of the switch  656  is coupled to a second terminal of the switch  658 . 
     Finally, an input terminal of a current source  662  is coupled to the voltage VCC. An output terminal of the current source  662  is coupled to a negative input terminal of a comparator  668 , a negative input terminal of a comparator  670 , a first terminal of a resistor  666 , and a positive terminal of a capacitor  664 . A negative terminal of the capacitor  664  and a second terminal of the resistor  666  are coupled to ground. A first positive input terminal of the comparator  668  is preferably biased to 4.75 volts. Additionally, a second positive input terminal of the comparator  668  is also preferably biased to 1.25 volts. A first positive input terminal of the comparator  670  is preferably biased to 6.75 volts. Additionally, a second positive input terminal of the comparator  670  is also preferably biased to 1.25 volts. A first output terminal of the comparator  668  is coupled to control line the switch  650 . A second output terminal of the comparator  668  is coupled to control line the switch  658 . The second output terminal of the comparator  668  produces signals that are complementary to signals produced by the first output terminal of the comparator  668 . A first output terminal of the comparator  670  is coupled to a control line for the switch  656 . A second output terminal of the comparator  670  is coupled to a control line for the switch  644 . The second output terminal of the comparator  670  produces signals that are complementary to signals produced by the first output terminal of the comparator  668 . 
     In FIG. 4B, the node A is coupled to a first terminal of an inductor  606 . A second terminal of the inductor  606  is coupled to a first terminal of a capacitor  608  and a first terminal of a capacitor  614 . A second terminal of the capacitor  608  is coupled to a center tapped lead of an autotransformer T 2 . A first terminal of a capacitor  612  is coupled to a first end terminal of the autotransformer T 2 . A first terminal of a primary winding  617  of a filament transformer T 1  is coupled to a second end terminal of the capacitor  612 , a first terminal of a capacitor  680 , a first terminal of a first secondary winding  618  of the transformer T 1 , and a first terminal of the lamp  620 . A second terminal of the first secondary winding  618  is coupled to a second terminal of the lamp  620 . A second terminal of a third secondary winding  623  of the transformer T 1  is coupled to a first terminal of a lamp  624 , a second end terminal of the autotransformer T 2 , a second terminal of the capacitor  614 , and the node B which corresponds to the node B found in FIG.  4 A. 
     A second terminal of a second secondary winding  622  is coupled to a third terminal of the lamp  620  and a third terminal of the lamp  624 . A second terminal of the capacitor  680  is coupled to a first terminal of a capacitor  682 . A second terminal of the capacitor  682  is coupled to a first terminal of the second secondary winding  622  of the transformer T 1 , a fourth terminal of the lamp  620 , and a fourth terminal of the lamp  624 . A first terminal of a third secondary winding  623  of the transformer T 1  is coupled to a second terminal of the lamp  624 . Further, a first terminal of a capacitor  619  is coupled to a second terminal of the primary winding  617  of the transformer T 1 . 
     By the oscillator  646  controlling the frequency of opening and closing the switches  602  and  604 , the power to a lamp network  601  is regulated. Further, a feedback loop is formed by the amplifier  638 , the oscillator  646 , and switches  602  and  604 . Thus, by monitoring the current flowing through the lamp network  601  which is sensed at the node N 10 , the oscillator  646  automatically maintain the user selected illumination intensity output from the lamps  620 ,  624 . 
     The circuit in FIG. 4A also automatically shuts off the lamps  620 ,  624  when the lamps  620 ,  624  near the end of their useful lives. To achieve this function, the DC voltage V N12  at the node N 12  is supplied to the comparator  634 . When the voltage V N12  at the node N 12  exceeds the predefined threshold voltage VTH 3 , the comparator  634  sets the RS flip flop  636 , which in turn deactivates the PFC  600  and the oscillator  646  thereby shutting off power to the lamps  620 ,  624 . The predefined threshold voltage VTH 3  is preferably set at a level higher than a typical, normal voltage at the node N 12  during safe operation of the lamps such that the comparator  634  gives the signal to shut off power when the voltage at the node N 12  only reaches unsafe levels. The output terminal Q of the RS flip flop  636  disables the PFC  600  and the oscillator  646  by disabling a clock signal (not shown) utilized for switching in the PFC  600  and the oscillator  646 . With the clock shut down, little or no power is supplied to the lamp network  601  or the lamps  620 ,  624 . 
     Three modes of operation for the circuit disclosed in FIG. 4A are graphically shown on the chart of FIG.  5 . Below, Table  1  shows the corresponding state of the switches  650 ,  658 ,  656  and  644  relative to the three operating modes of the lamps  620 ,  624 . 
     
       
         
               
               
               
               
             
               
               
               
               
               
             
           
               
                   
                 TABLE 1 
               
               
                   
                   
               
               
                   
                 Preheating 
                 Starting 
                 Continuous Operation 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 Switch 650 
                 ON 
                 OFF 
                 OFF 
               
               
                   
                 Switch 658 
                 OFF 
                 ON 
                 ON 
               
               
                   
                 Switch 656 
                 ON 
                 ON 
                 OFF 
               
               
                   
                 Switch 644 
                 OFF 
                 OFF 
                 ON 
               
               
                   
                   
               
             
          
         
       
     
     When the circuit illustrated in FIGS. 4A and 4B is off, the current source  662  is off. Accordingly, a voltage V C664  across the capacitor  664  is discharged through the resistor  666  to a level below 1.25 volts. Upon start-up at time t 0 , the current source  662  turns on, which slowly increases the voltage across the capacitor  664 . Eventually, at the time (t 1 ), the voltage V C664  reaches 4.75 volts. Thus, between the times t 0  and t 1  (preheating mode), the comparators  668 ,  670  control the switches  650 ,  656  to be on (closed), and the switches  658 ,  644  to be off (open). Under these conditions, the current source  648  charges the timing capacitor  660  at a rate appropriate to set the frequency of the oscillator  646  for preheating the lamps  620 ,  624 . Note that the timing resistor  652  affects this preheating frequency as does a dead time characteristic of the oscillator  646 . Because the switch  658  is open, however, the resistor  654  does not affect the preheating frequency. 
     During preheating, the filaments inside the lamps  620 ,  624  are warmed to their emission temperature while, the voltage supplied to the lamps  620 ,  624  is sufficiently low to prevent the lamps from igniting. Preheating the lamps  620 ,  624  prior to ignition is important to prolong the useful life of the lamps  620 ,  624 . 
     Eventually, at the time t 2 , the voltage V C664  reaches 6.75 volts. Thus, between the times t 1  and t 2  (starting mode), the frequency of the oscillator  646  is no longer influenced by the current source  648 . Rather, because the switches  656  and  658  are both closed, the frequency of the oscillator  646  is influenced by the resistor  654 . As a result, during the starting mode the frequency at which the switches  602  and  604  are operated is reduced significantly. This significantly increases the voltage level supplied to the lamps  620 ,  624  so as to ensure ignition. 
     Then, once the voltage V C664  has exceeded 6.75 volts, (after the time t 2 ), the continuous operation mode is entered in which the comparators  668 ,  670  control switches  644 ,  658  to be closed and the switches  656 ,  650  to be open. Under these conditions, the frequency of the oscillator  646  is no longer influenced by the resistor  654 . Rather, because the switch  644  is closed, the frequency of the oscillator  646  is influenced by a feedback signal I operate  which is provided to the oscillator  646  by the voltage-to-current converter  642 . The continuous operation frequency results in a lower power being provided to the lamps  620 ,  624  in comparison to the starting mode, so that the lamps  620 ,  624  draw an appropriate level of power to keep the illumination intensity at the preselected level desired by the user. 
     In the preferred embodiment, the capacitor  660  has a value of 1.5 nF, the resistor  652  is 14.5 Kohms and the resistor  654  is 73.1 Kohms. Further, during the preheating mode, the switches  602 ,  604  are preferably operated at 70 KHz. During the starting mode, the switches  602 ,  604  are preferably operated at 50 KHz. In addition, during the continuous operation mode, the switches  602 ,  604  are operated between 42.3 KHz for maximum intensity to a frequency which results in deep dimming to 5% or less of the maximum rated output for the lamps  620 ,  624 . It will be apparent, however, that other component values and frequencies can be selected. 
     Returning to FIGS. 4A and 4B, another important feature of this circuit allows the lamps  620 ,  624  to operate when they are deeply dimmed down to 5% or less of the lamps&#39; rated illumination output. Compact lamps are known for their characteristic of driving themselves into an area of high negative resistance when dimmed and causing an associated lamp network to have an increased quality factor Q. This increased quality factor Q caused the lamps to extinguish or flicker excessively when they were dimmed. As stated herein, prior circuits attempted to solve this problem by utilizing a low pass network to allowing dimming down to 40%. 
     Recall the lamp network  601  illustrated in FIG. 4B includes the capacitors  608  and  612 ; the transformer T 1 ; the autotransformer T 2 ; and the inductor  606 . The configuration of this lamp network as shown in FIG. 4B provides a lower quality factor Q than the prior art while the attached lamps are being dimmed. In fact, the lamps can be deeply dimmed down to 5% or lower and still operate without excessively flickering or extinguishing. The lamp network  601  provides a low pass filter followed by the autotransformer T 2  and capacitors  608 ,  612  which acts as a high pass filter. This network combination of first the low pass filter, followed by the high pass filter, allows the lamp network  601  to have a lower quality factor Q while the coupled lamps  620 ,  624  are being dimmed. For example, as seen through the nodes A and B of the lamp network  601 , the capacitor  614  and inductor  606  are configured to act as a low pass filter which is followed by the autotransformer T 2  acting as a high pass filter. As a result of the lamp network  601 , the lamps  620  and  624  are configured to have signals pass first through a low pass filter and then through an autotransformer acting as a high pass filter. This allows the lamps  620 ,  624  to be dimmed down to less than 5% of their rated illumination and still operate satisfactorily. 
     FIG. 6 illustrates an equivalent circuit for the lamp network  601  described above and illustrated in FIG.  4 B. Where appropriate, the same reference numbers are utilized to describe common elements. An impedance R L    700  replaces the lamps  620  and  624 ; the capacitors  619 ,  680 , and  682 ; and the transformer T 1  which are outside the lamp network  601  and found in FIG.  4 B. As stated before, the unique lamp network  601  shown in FIG. 4B retains a low quality factor Q even while the lamps are deeply dimmed. 
     A transformer T 3  is shown as a conventional transformer with a primary winding  609  and a secondary winding  611 . In FIG. 6, the transformer T 3  produces an equivalent result as the autotransformer T 2  (FIG. 4B) and is merely substituted for the autotransformer T 2  as shown in FIG.  4 B. To overcome the shortcomings of the prior art, the transformer T 3  is a part of a high pass filter which follows a low pass filter formed by the inductor  606  and the capacitor  614 . 
     There is a large increase in voltage across the lamps when they are dimmed which also indicates an increase in quality factor Q when operated from prior art circuits. However, to supply this increasingly large voltage to the lamps as they are dimmed, a low quality factor Q is needed. To overcome this performance contradiction, the high pass filter formed by the transformer T 3  and capacitors  608  and  612  follows the low pass filter formed by the inductor  606  and the capacitor  614 . It will be apparent, however, that this transformer T 3  of the high pass filter can be substituted for another element which has the necessary inductive reactance to act as the shunt element for the high pass filter. 
     An input quality factor Q in  of the lamp network  601  is seen through the nodes A and B. The high pass filter formed by the transformer T 3  and capacitors  608  and  612  lowers the output quality factor Q out  of the lamp network  601 . Instead of being driven directly by the inductor  606  and the capacitor  614 , the lamps, which are represented by the impedance R L    700 , are driven by the series capacitor  612  which decreases its Q as the lamps are dimmed. To appropriately shape the frequency response of the lamp network  601 , the input quality factor Q in  of the low pass filter is made larger than the output quality factor Q out  of the high pass filter. 
     As the lamps  620  and  624  are dimmed, the output quality factor Q out  decreases. The transformation between parallel capacitive reactance and series capacitive reactance is shown in Eq. 1 below.                X     para                 C       =       X     series                 C       ⋆     (     1   +     1     Q   out   2         )               Eq. (1)                                
     Accordingly, an equivalent parallel reactance X ParaC612  of the capacitor  612  in series becomes larger. 
     The parallel reactance X ParaC612  of the capacitor  612  is then combined with a reactance of the secondary winding  611  of the transformer T 3  and then transformed onto a side of the primary winding  609 . The parallel reactance X ParaC612  combined with the reactance of the secondary winding  611  of the transformer T 3  and then transformed onto the same side as the primary winding  609  is shown in FIG.  7  and labeled reactance X T . 
     An input quality factor Q M  of the high pass filter is shown below in Eq. 2 and given by:                Q   M     =       (         R   PRI       R   M       -   1     )       1   /   2               Eq. (2)                                
     The input quality factor Q M  of the high pass filter is very small. The reactance of the capacitor  608  is given by Eq. 3: 
     
       
           X   C608   =Q   M   *R   M   Eq. (3)  
       
     
     According to the above Eqs. 2 and 3, a reactance X C608  of the capacitor  608  is also very small which allows the reactance X T  to be positioned in parallel with the capacitor  614  as seen in FIG.  7 . 
     As the impedance  700  gets larger and the reactance X T  becomes more inductive, the combined reactance of X T  and the parallel reactance X ParaC612  becomes larger thus causing the input quality factor Q in  of the lamp network  601  to be lowered. 
     As a result of the low pass filter created by the inductor  606  in conjunction with the capacitor  614  followed by the high pass filter created by the transformer T 3 , the overall quality factor Q of the lamp network preferably remains low during deep dimming. It will be apparent to those skilled in the art to select components such as resistors, capacitors, and inductors with appropriate values depending on the desired response for the overall quality factor Q for the lamp network  601 . 
     The present invention has been described in terms of specific embodiments incorporating details to facilitate the understanding of the principles of construction and operation of the invention. Such reference herein to specific embodiments and details thereof is not intended to limit the scope of the claims appended hereto. It will be apparent to those skilled in the art that modifications may be made in the embodiment chosen for illustration without departing from the spirit and scope of the invention. 
     Specifically, it will be apparent to one of ordinary skill in the art that the device of the present invention could be implemented in several different ways and the apparatus disclosed above is only illustrative of the preferred embodiment of the invention and is in no way a limitation. For example, it would be within the scope of the invention to vary the values of the various components and voltage levels disclosed herein.