WO2015149297A1

WO2015149297A1 - Lamp cathode heating for dimming lamp or step-dimming lamp

Info

Publication number: WO2015149297A1
Application number: PCT/CN2014/074604
Authority: WO
Inventors: Zhu MAO; Bo Zhang; Xin Wang
Original assignee: General Electric Company
Priority date: 2014-04-02
Filing date: 2014-04-02
Publication date: 2015-10-08

Abstract

A ballast circuit including a transformer and a preheating circuit is provided. The transformer provides an output voltage to a lamp. The transformer comprises a primary winding and a secondary winding. The primary winding includes a first pair of windings connected in series with one another, wherein one winding of the first pair of windings operates as a preheating primary windings during preheating. The secondary winding includes a second pair of windings for cathode heating during preheating. The preheating circuit supplies a preheating current from an output of the secondary windings of the transformer to a cathode of the lamp to preheat the lamp cathode.

Description

LAMP CATHODE HEATING FOR DIMMING LAMP OR STEP-DIMMING LAMP

I. Field of the Invention

[0001] The present disclosure relates generally to electroluminescent devices, and in particular to fluorescent lamp ballasts for electroluminescent devices.

II. Background of the Invention

[0002] In recent years, dimming technology has evolved considerably. The use of dimmers initially began as a device for simply matching lighting levels to various tasks and moods. Now, they have emerged as a tool for reducing energy usage.Lighting controls, such as dimmers, have the ability to illuminate where and when it's needed and the power to conserve when illumination is not needed. To accomplish this, lighting controls can provide the right amount of light where it's needed and when it's needed - either automatically or at a user's discretion. Many indoor and outdoor facilities, such as homes, buildings, parking lots, and streets, include light source dimming circuits.

[0003] Dimming is typically quantified using percentages, such as 10% dimming. This percentage can be applied, for example, to lamp voltage, lamp power, or measured lumen output. Therefore, a 750-lumen light source in a 10% dimmed system would produce 75 lumens.

[0004] In the case of a fluorescent lamp, the lamp operates by generating an arc of electric current which is sent across the lamp from one its cathode to another. Then, a phosphor coating inside the light bulb converts the gas into visible light. In order to operate properly, a fluorescent light requires a ballast. The ballast is an electrical device which provides a sufficient starting voltage, which allows for the arc to initiate.

[0005] The ballast also regulates the current that flows through the lamp. Dimming ballasts are configured so that they can receive a signal from a controlling device. This device can change the current that flows through the lamp. The result is that the light emitted from the lamp can be gradually controlled. The duration and intensity of the change in current and light output is controlled by the control signal's characteristics.

[0006] There are several kinds of dimming ballast techniques. Electronic ballasts may be started using one of several starting techniques, including "instant" start, "rapid" start, and "programmed" start.

[0007] The instant start technique starts a lamp without preheating a cathode associated therewith, which results in low cost in ballast design but the lamp cathodes can be degraded rapidly due to the violent nature of the starting method.

[0008] Rapid start ballasts start the ballast and heat the cathode concurrently, resulting in a relatively long start time while mitigating the adverse effects of a cold start on the lamp's cathode.

[0009] Programmed start ballasts apply a relatively low output voltage initially, which is not high enough to begin gas discharge, while the lamp filaments or cathodes are preheated at a relatively high level for a limited period of time. The cathode is heated to emit electrons due to thermic emission. Usually, the heating element is the filament. After the cathodes are preheated, a moderately high voltage is applied to ignite the lamp,and the filament heating power is discontinued. Conventional programmed start ballasts open or short a preheat circuit to stop the preheating power (cathode cut-off).

[0010] Step dimming solutions have developed to allow even further energy savings. Energy consumption can be achieved through the use of two-level or high-low step dimming interfaces to control the inverters in the ballast systems that power the lamps. These systems can be used to switch lighting fixtures from a low-wattage energy saving operation (dimming) to a normal wattage operation thus providing significant cost savings.

[0011] However, some ballast have been designed to use the same circuitry to apply both the low output voltage to preheat the cathodes or filaments (as described above) and to supply the voltage required to provide sufficient heating current to maintain the proper cathode temperate during dimming or step dimming. The ballast applies a voltage sufficient to operate in low power mode for step dimming. The ballast applies a voltage sufficient to operate at a low level dimming mode for full dimming. This repeated use of the same circuit components in both the preheating mode and the dimming modes can cause the lamp to operate in an unstable condition, which can lead to early failure, especially in T5 lamps.

[0012] During dimming, cathodes in fluorescent lights rely on thermionic emissions to inject electrons. Therefore, simply reducing the supply voltage may not provide sufficient heating current to maintain the proper cathode temperature. Also, the voltage waveform produced by standard phase controlled dimmer switches reacts badly with many fluorescent lighting ballasts making it difficult to maintain an arc in the tube at low power levels. Thus, there is a need for improved cathode heating for dimming a lamp or step dimming a lamp.

[0013] Accordingly, it would be desirable to provide a fluorescent lamp solution wherein the cathode uses differentcircuit components to provide the low voltage to heat the cathode when the lamp is operating in a low power mode or low level dimming mode, rather than directly using the same components that provide the lamp ignition preheating cathode voltage.

ΙΠ. Summary of Embodiments of the Invention

[0014] In certain embodiments, a ballast circuit including a transformer and a preheating circuit is provided. The transformer provides an output voltage to a lamp. The transformer comprises a primary winding and a secondary winding. The primary winding includes a first pair of windings connected in series with one another, wherein one winding of the first pair of windings operates as a preheating primary windings during preheating. The secondary winding includes a second pair of windings for cathode heating during preheating. The preheating circuit supplies a preheating current from an output of the secondary windings of the transformer to a cathode of the lamp to preheat the lamp cathode.

[0015] In certain embodiments, a method for preheating and dimming a lamp is provided, which includes providing a transformer for providing an output voltage to a lamp, the transformer comprising: a primary winding including a first pair of windings connected in series with one another, wherein one winding of the first pair of windings operates as a preheating primary windings during preheating; and a secondary winding including a second pair of windings for cathode heating during preheating; andsupplying a preheating current from an output of the secondary windings of the transformer to a cathode of the lamp to preheat the lamp cathode.

[0016] Further features and advantages of the invention, as well as the structure and operation of various embodiments of the invention, are described in detail below with reference to the accompanying drawings. It is noted that the invention is not limited to the specific embodiments described herein. Such embodiments are presented herein for illustrative purposes only. Additional embodiments will be apparent to persons skilled in the relevant art(s) based on the teachings contained herein.Further features and advantages of the invention, as well as the structure and operation of various embodiments of the invention, are described in detail below with reference to the accompanying drawings. It is noted that the invention is not limited to the specific embodiments described herein. Such embodiments are presented herein for illustrative purposes only. Additional embodiments will be apparent to persons skilled in the relevant art(s) based on the teachings contained herein.

IV. Brief Description of the Drawings

[0017] FIG. 1 illustrates a schematic diagram of an exemplary fluorescent lamp ballast that includes a dimming circuit in accordance with the present disclosure;

[0018] FIG. 2 illustrates a schematic diagram of an exemplary fluorescent lamp ballast that includes a dimming circuit in accordance with the present disclosure; and

[0019] FIG. 3 is a flowchart of an exemplary method of practicing the present invention in accordance with the present disclosure. [0020] The present disclosure may take form in various components and arrangements of components, and in various process operations and arrangements of process operations. The present disclosure is illustrated in the accompanying drawings, throughout which, like reference numerals may indicate corresponding or similar parts in the various figures. The drawings are only for purposes of illustrating preferred embodiments and are not to be construed as limiting the disclosure. Given the following enabling description of the drawings, the novel aspects of the present disclosure should become evident to a person of ordinary skill in the art.

V. Detailed Description of Various Embodiments

[0021] The following detailed description is merely exemplary in nature and is not intended to limit the applications and uses disclosed herein. Further, there is no intention to be bound by any theory presented in the preceding background or summary or the following detailed description.

[0022] In various embodiments, the present disclosure provides a fluorescent lamp solution wherein the cathode uses different circuit components to provide dimming of a lamp, rather than directly using the same circuit components that provide the lamp ignition preheating cathode voltage.In various embodiments, the circuit has the capability to operate in a low power mode to perform step dimming. In various embodiments, the circuit also has the capability to operate in a low level dimming mode to perform full dimming.

[0023] In various embodiments, a ballast includes a transformer, which comprises a primary winding and a second winding, wherein different components of the transformer are used to provide the voltage for the ignition preheating energy than the voltage for the low power mode for step dimming or the low level mode for full dimming.

[0024] In various embodiments, a transformer comprises a primary winding having a first pair of windings and a secondary winding having a second pair of windings.

[0025] In various embodiments, a transformer consisting of primary windings Tl -1 and Tl -2. Winding Tl-1 is in series with winding Tl-2. The transformer also includes secondary windings Tl -3 and Tl -4. Secondary windings Tl -3 and Tl-4 are connected in parallel with cathode, such that the windings Tl-3 and Tl-4, respectively are connected across the terminal end of cathode of lamp 114 for supplying cathode heating. In a low level dimming cathode heating mode, winding Tl -1 serves as a preheating primary winding to perform the program start cathode heating. Transformer Tl -1 and Tl-2 functions as a primary winding for low level cathode dimming. In a low power mode, winding Tl-1 serves as a preheating primary winding to perform the program start cathode heating. Transformer Tl -1 and Tl -2 functions as a primary winding for the low power mode. Winding Tl -1 and Tl-2 have the same magnetic flux.

[0026] FIG. 1 illustrates a schematic diagram of an exemplary fluorescent lamp ballast 100 that includes a dimming control circuit 106. In the present examples, a program start ballast 100 includes a dimming control circuit 106 that controls a half bridge driver circuit 104, the output of which is applied to output and load circuit 108. The half bridge driver circuit 104 is composed of DC voltage, the half bridge controller 110, dimming control circuit 106 and driver MOSFET Ql and Q2. The DC voltage, which is provided by the DC bus 112, supplies the power for the half bridge driver circuit 104. The half bridge driver circuit sets a frequency for output according to the drivers MOSFET Ql and Q2 and the dimming control circuit 106.

[0027] The half bridge powers the MOSFET Ql and Q2. MOSFET Ql and Q2 serve as the power switching device of the half bridge.

[0028] The half bridge configuration under the control of the half bridge controller 110 provides high frequency substantially square wave output voltage to the output and load circuit 108. The output and load circuit 108 is composed of limit inductor L3, block capacitor C3, and a lamp load 114.

[0029] For the conservation of energy, the electronic ballast 100 is capable of dimming control. When the ballast 100 is operated at various dimming conditions, the heating power to the filament of lamp 114 is adjusted accordingly to ensure a normal life of the filament. Accordingly, ballast 100 provides both variable dimming control 106 and apreheating circuit 116 for heating the lamp cathode.

[0030] The dimming control 106 receives a dimming level setpoint, such as a signal or value and operates to dim lamp 114. The dimming control 106 receives an interface control signal, which is provided externally by an interface device (not shown), as a dimming input. In FIG. 1, the interface control signal is a low level dimming signal, typically 0-10V DC, to set the dimming level. In 0-10V dimming, the light source has a 100% output at 10V DC and a minimum output at IV DC. The dim level signal from the dimming control circuit 106 is input into the half bridge controller 104.

[0031] In various embodiments, the low level dimming signal can be, for example, a 0-10V control signal, a wireless control signal or a photosensitive signal. Various embodiments relate to universal interface of a variety of sensors to the ballast 100. As such, the interface device (not shown) may include one or more sensors, such as light sensors and occupancy sensors, to control the ballast to optimize the light output and energy consumption. For example, if the occupancy sensor detects no occupants in a room, it outputs a sensor control signal to the dimming control 106 to switch off the lamp 114.

[0032] In another example, the interface device may be a daylight sensor mounted so as to measure a total light intensity in the space around the daylight sensor. The daylight sensor is responsive to a total light intensity measured by an internal photosensitive circuit. The photosensor device, which is linked to the dimming controller, varies the lighting in response to natural light, for example, from windows and skylights. The dimming controller 106 may control the ballast 100 to decrease the lighting intensity of the lamp 1 14 in response to increases in the total lighting intensity measured by the daylight sensor.

[0033] In various embodiments, the ballast 100 may be coupled to the dimming control 106 via a dedicated wired digital communication, such as a Digital Addressable Lighting Interface (DALI) communication link to control the ballast dimming. The DALI bus 122 is a lighting interface that provides a bus connection among the luminaires, sensors, switches and the controller as defined by the DALI protocol International Electrotechnical Commission (IEC) 60929/IEC 62386 standard. DALI is a commonly used technical standard for lighting control in buildings and houses, which is based on a network bus. The DALI bus 122 is a lighting interface that provides a bus connection among the luminaires, sensors, switches and the controller as defined by the DALI protocol International Electrotechnical Commission (IEC) 60929/IEC 62386 standard. DALI is a commonly used technical standard for lighting control in buildings and houses, which is based on a network bus.

[0034] In addition to the wired interface provided by the DALI bus 122, in various embodiments, the commands can also be carried through a wireless interface, such as the ZigBee wireless communication protocol. The ZigBee network is a lighting interface that provides a wireless network connection among the luminaires, sensors, switches and controllers as defined by the ZigBee protocol IEEE standard 802.15.4.

[0035] The ballast 100 includes the preheat circuit 116 to control and deliver a predetermined current to filaments oflamp 114 for a predetermined period of time before igniting the lamp to extend the life of the lamp. The preheat time can vary depending upon type of lamp or application. Typically, the cathode preheating time is less than 2 seconds.

[0036] The preheat circuit 116 generates a DC signal indicative of (or proportional to) a desired current setting for filament preheat. The ballast includes a transformerconsisting of two windings, a primary winding and a secondary winding. The coil of the transformer that receives the electrical input energy is the primary winding, while the output coil is the secondary winding. In FIGS. 1 and 2, the transformer consists of primary windings Tl -1 and Tl -2. Winding Tl-1 is in series with winding Tl -2. The transformer also includes secondary windings Tl-3 and Tl-4, which provide current for heating the cathodes. [0037] Capacitor CI is connected in series between MOSFET Q3 and the primary winding Tl -2. Capacitor C2 and MOSFET Q4 are coupled in series, and capacitor C2 is connected between MOSFET Q4 and primary winding Tl -1.

[0038] During cathode preheating, MOSFET Q4 will be turned on, and the voltage is supplied through capacitor C2 to transformer primary winding Tl-1. Transformer winding Tl-1 will be energized to have a voltage. Thus, applying thevoltage and current via transformer primary winding Tl -1, inductor L3 and capacitor C3, secondary winding Tl -3 and Tl-4 heats the cathode of lamp 1 14. The secondary windings Tl-3 and Tl -4 are in parallel with the lamp cathode at each end of lamp 114. Other configurations of preheating secondary windings are possible.

[0039] The preheat circuit 1 16 includes a preheat timer that starts a timing cycle when power is applied to the ballast 100. During this preheating time period, the current provided through transformer winding Tl -1 conducts through the secondary transformer windings Tl-3 and Tl-4. Thus, preheating current flow through the preheating secondary windings Tl-3 and Tl -4 causing current to begin to flow via the filaments, which causes preheating of the light source cathodes. The current through the filaments causes them to heat up and emit electrons into the tube gas by thermionic emission.

[0040] After the preheat time expires, preheating of the cathode ends. The ballast opens or shorts the preheat circuit 1 16 to short the preheating power (cathode cut off) by switching off MOSFET Q4 such that transformer winding Tl-1 receives no voltage.

[0041] After the cathodes are preheated, the lamp 114 is turned on and the filament heating power is discontinued. To turn on lamp 114, a moderately high voltage is applied to ignite the lamp to provide a sufficient heating current to maintain the proper cathode temperature. When the preheat timer expires, the current through the filament is abruptly interrupted leaving the voltage applied between the filaments at the ends of the tube and generating an inductive kick, which provides the high voltage needed to start the lamp. The lamp will fail to strike if the filaments are not hot enough, in which case the cycle repeat. Several cycles may be needed to ignite the lamp, which causes flickering and clicking during starting. Thus, the voltage supplied to the cathode must be high enough to heat the filament to keep the lamp operating in a stable condition and to prolong the life of the lamp.

[0042] The ballast 100 also provides dimming capabilities, such as low level dimming for full dimming (FIG. 1) and low power mode for step dimming (FIG. 2).

[0043] The exemplary ballast 100 of FIG. 1 also includes a low level dimming cathode heating control circuit 102 that is operatively coupled for providing full dimming capabilities. Because of the low lamp current in both the low level dimming and the low power mode, there is not sufficient current to keep the filament hot enough to emit electrons in order to keep the lamp stable or extend the lamp life. Otherwise, the lamp will operate in an unstable condition and suffer an early lamp failure. Thus, as in some conventional lamps, if the lamp performs initial cathode preheating, then when the ballast operates in either the low level dimming mode (FIG. 1) or low power mode (FIG. 2), the lamp may suffer an early lamp life failure.

[0044] This premature lamp failure is a result of the same circuit components being used during preheating and dimming, especially for T5 lamps. To address this problem, the cathode according to the present disclosure uses different configurations of the components of the transformer to provide a low voltage to heat the cathode when the lamp operates either in a low level dimming mode (FIG. 1) or a low power mode (FIG. 2).

[0045] Fig. 1 illustrates an exemplary ballast 100 that provides a low voltage in a low dimming cathode heating mode 102, wherein the low voltageis suppliedby a different circuitry configuration than the preheating ignition voltage. When the ballast 100 operates in a low level dimming mode in response to a low level dimming signal, MOSFET Q3 will be turned on. The current flows through capacitor CI and thus applying DC voltage and current via transformer primary winding Tl-2 such that primary winding Tl -1 will be energized to have a voltage. Transformer primary winding Tl-1 will be in series with primary winding Tl-2. Thus, applying the voltage and current via transformer primary winding Tl-1 , inductor L3 and capacitor C3, secondary winding Tl - 3 and Tl -4 heats the cathode of lamp 114.

[0046] In FIG. 1 , when the ballast operates at a high power level, MOSFET Q3 will be switched off. Therefore, there will be no voltage across the transformer primary windings Tl -1 and Tl -2, and as a result, no voltage across the secondary windings Tl-3 and Tl -4 of the transformer. No voltage across the secondary windings will cut off the cathode heating.

[0047] In the low level dimming mode, the cathode preheating is performed according to the program start cathode heating, as described above. In this embodiment, because transformer Tl-1 and Tl-2 serve as primary windings, the transformer secondary winding Tl-3 and Tl -4 is not a very high voltage. This is due to the fact that the program start cathode heating uses only transformer Tl -1 as the primary winding.

[0048] FIG. 2 illustrates an exemplary ballast 200 that provides a low voltage in a low power mode control circuit202, wherein the low voltage supplied by a different circuitry configuration than the preheating ignition voltage. FIG. 2 is similar to FIG. l, except that ballast includes a step-dimming control circuit 206 and a low power mode control circuit 202. The low power control circuit 202 is controlled based on a data input line received at step-dimming control circuit 206. The low power control circuit is initiated when the data input line provides a low power mode dimming signal for step dimming. The step dimming control circuit receives an input line for step dimming, which is provided externally by a remote source (not shown).

[0049] In order to keep the lamp operating in a stable condition which extends the lamp's life, the cathode must supply a voltage sufficient to heat the filament hot enough to emit electrons. When the ballast 200 operates in a low power mode in response to a low power signal, MOSFET Q3 will be turned on. Transformer primary winding Tl -1 is in series with primary winding Tl-2. The transformer secondary windings Tl -3 and Tl-4 will heat the cathode.

[0050] In FIG. 2, when the ballast operates at a high power level, MOSFET Q3 will be switched off. Due to the closure of switch MOSFET Q3, there can be no voltage across the transformer primary windings Tl -1 and Tl -2, and as a result, no voltage across the secondary windings Tl -3 and Tl -4 of the transformer. No voltage across the secondary windings will cut off the cathode heating. [0051] In the low power mode, because transformer windings Tl-1 and Tl-2 serve as primary winding, the transformer secondary windings Tl-3 and Tl-4 will not be a very high voltage.In various embodiments, winding Tl -1 and Tl -2 have the same magnetic flux.

[0052] Although a single lamp is shown in FIGS. 1 and 2, in various embodiments, the ballast may include multiple lamps. The present invention is equally applicable to preheating, starting and dimming any number of fluorescent lamps. In a multiple lamp configuration, an additional secondary winding will be added for each respective lamp.

[0053] In various embodiments, the controller may be implemented as a processor- based system having a microprocessor, microcontroller, or other programmable or configurable processing or logic components. The controller and the components thereof can be implemented in software, firmware or combinations of various hardware, software, etc. in a single control device or in distributed fashion with one or more functions being implemented separately from others.

[0054] FIG. 3 is a flowchart of an exemplary method 300 of implementing lamp cathode heating for dimming a lamp or step dimming a lamp. In step 302, power is applied to the control circuit of the ballast. In step 304, switching device MOSFET Q4 is opened to provide a voltage to secondary windings Tl -3 and Tl-4 to preheat the cathode of the lamp. The programmed start ballast applies a relatively low output voltage initially, which is not high enough to ignite the lamp, while the lamp filaments are preheated at a relatively high level for a limited period of time. [0055] In step 306, a determination is made as to whether the preheating time has expired. If not ("NO" at step 306), the process remains in the preheating mode. Once the preheating time has expired ("YES" at step 306), the switching device MOSFET Q4 is closed at step 308. Full power is applied to ignite the lamp at step 310.

[0056] With the ballast at full power, a determination is made at step 312 as to whether a dimming signal or command has been received. If "no" at step 312, the process continues monitoring for the dimming signal or command. If "yes" at step 312 and the dimming signal is a full dimming signal, the process proceeds to step 314 and operates in a low level dimming mode as described above with respect to FIG. 1. If "yes" at step 312 and the dimming signal is a step dimming signal, the process proceeds to step 316 and operates in a low power mode as described above with respect to FIG. 2.

[0057] Alternative embodiments, examples, and modifications which would still be encompassed by the disclosure may be made by those skilled in the art, particularly in light of the foregoing teachings. Further, it should be understood that the terminology used to describe the disclosure is intended to be in the nature of words of description rather than of limitation.

[0058] Those skilled in the art will also appreciate that various adaptations and modifications of the preferred and alternative embodiments described above can be configured without departing from the scope and spirit of the disclosure. Therefore, it is to be understood that, within the scope of the appended claims, the disclosure may be practiced other than as specifically described herein. Parts List 2691 16

Figure 1

C1 - Capacitor

C2- Capacitor

C3- Capacitor

L3- inductor

Q1 - MOSFET

Q2- MOSFET

Q3- MOSFET

Q4- MOSFET

T1 -1 - winding. Transformer

T1 -2- winding, Transformer

T 1 -3- Secondary windings

T1 -4- Secondary windings

100-ballast

102-cathode heating control circuit,

104-half bridge driver circuit

106-dimming control circuit

108-load circuit

1 1 0-half bridge controller

1 12- DC bus

1 14-lamp,

1 16-preheating circuit

Figure 2

104-half bridge driver circuit

108-load circuit

1 0-half bridge controller

1 12- DC bus

1 14-lamp,

1 16-preheating circuit 200-ballast

202-low power mode control circuit

206- step-dimming control circuit

C1 - Capacitor

C2- Capacitor

C3- Capacitor

L3- inductor

Q1 - MOSFET

Q2- MOSFET

Q3- MOSFET

Q4- MOSFET

T1 -1 - winding, Transformer

T1 -2- winding, Transformer

T1 -3- Secondary windings

T1 -4- Secondary windings

Figure 3

300-method

302-step

304-step

306-step

308-step

310-step

312-step

314-step

316-step

In the spec:

122- DALI bus

T-5- lamps

Claims

IN THE CLAIMS; We claim:

1. A ballast circuit comprising:

a transformer for providing an output voltage to a lamp, the transformer comprising:

a primary winding including a first pair of windings connected in series with one another, wherein one winding of the first pair of windings operates as a preheating primary windings during preheating; and

a secondary winding including a second pair of windings for cathode heating during preheating; and

a preheating circuit for supplying a preheating current from an output of the secondary windings of the transformer to a cathode of the lamp to preheat the lamp cathode.

2. The circuit of claim 1, further comprising a dimming circuit for controlling luminance of the lamp in response to a dimming control signal.

3. The circuit of claim 2, wherein the dimming circuit comprises a low level dimming circuit.

4. The circuit of claim 3, wherein, during a low level dimming cycle, the first pair of windings of the primary windings are connected in series with each other and operates as low level dimming primary windings.

5. The circuit of claim 7, wherein primary winding Tl-1 and primary winding Tl-2 have the same magnetic flux.

6. The circuit of claim 2, wherein the dimming circuit comprises a step dimming circuit.

7. The circuit of claim 6, wherein, during a step dimming cycle, the first pair of windings of the primary windings are connected in series with each other and operates as step dimming primary windings.

8. The circuit of claim 7, wherein primary winding Tl-1 and primary winding Tl-2 have the same magnetic flux.

9. The circuitof claim 1, further comprises a first switch being responsive to a preheat time signal to open the switch during a preheat time and to close the switch after the preheat time expires and wherein the preheating primary windings are operably coupled to the secondary windings to provide filament power to the lamp filaments during the preheat time.

10. The circuit of claim 9, wherein the switch comprises a capacitor in series with a MOSFET.

11. The circuit of claim 4, further comprises a second switch being responsive to a dimming signal to open the switch and to close the switch and wherein the low level dimming primary windings are operably coupled to the secondary windings to control the luminance of the lamp.

12. The circuit of claim 9, wherein the switch comprises a capacitor in series with a MOSFET.

13. The circuit of claim 6, further comprises a second switch being responsive to a dimming signal to open the switch and to close the switch and wherein the step dimming primary windings are operably coupled to the secondary windings to control the luminance of the lamp.

14. The circuit of claim 13, wherein the switch comprises a capacitor in series with a MOSFET.

15. The circuit of claim 1, wherein the lamp comprises a fluorescent lamp.

16. The circuit of claim 1, wherein the ballast comprises a program start ballast.

17. A method for preheating and dimming a lamp, comprising: providing a transformer for providing an output voltage to a lamp, the transformer comprising: a primary winding including a first pair of windings connected in series with one another, wherein one winding of the first pair of windings operates as a preheating primary windings during preheating; and a secondary winding including a second pair of windings for cathode heating during preheating; and supplying a preheating current from an output of the secondary windings of the transformer to a cathode of the lamp to preheat the lamp cathode.

18. The method of claim 17, further comprising preheating the cathode lamp by controlling a first switch in response to a preheat time signal by opening a first switch during a preheat time and closing the switch after the preheat time expires and wherein the preheating primary windings are operably coupled to the secondary windings to provide filament power to the lamp filaments during the preheat time.

19. The method of claim 18, further comprising low level dimming by controlling a second switch in response to a low level dimming signal and wherein low level primary dimming windings are operably coupled to the secondary windings to control luminance of the lamp.

20. The method of claim 18, further comprising step dimming by controlling a second switch in response to a step dimming signal and wherein step primary dimming windings are operably coupled to the secondary windings to control luminance of the lamp.