# Patent

## Title
Self-oscillating dimmable electronic ballast

## Abstract

An electronic ballast for driving a light-emitting device, includes a square wave generator having a plurality of switch elements for converting a DC input voltage into a square-wave AC voltage. A transformer has a driving winding and a plurality of inductive windings mutually connected with each other, in which at least a portion of the inductive windings are respectively connected to a control terminal of the switch element. A resonant circuit connects the driving winding and a light-emitting device and converts the square-wave voltage into an AC output voltage to drive the light-emitting device. An auxiliary control unit connected to the transformer regulates a voltage waveform of the driving winding or a voltage waveform of the inductive winding according to a control signal, thereby changing the voltage waveform of the inductive winding connected to the switch element to adjust the switching frequencies of the switch elements.

## Background
FIELD OF THE INVENTION
The invention relates to an electronic ballast, and more particularly to a self-oscillating electronic capable of achieving dimming function, breaking down the light-emitting device to ignite, and preheating the filaments of the light-emitting device.
BACKGROUND OF THE INVENTION
In recent years, with the great advancement of power electronics, the electronic ballasts have replaced the conventional electromagnetic ballasts for driving fluorescent lamps. The electronic ballast is advantageous in terms of the thin and small size, enhanced illuminating efficiency, and improved luminance.
The electronic ballasts have various topologies in practice. The self-oscillating electronic ballast is widely employed as the self-oscillating electronic ballast has a short startup time, high illuminating efficiency, low cost, and simple structure. However, the self-oscillating electronic ballast is difficult to perform dimming control and preheat the filaments of the lamp due to its inherent design limitation. Thus, the lifetime of the lamp driven by the self-oscillating electronic ballast is negatively affected.
It is incline to develop an electronic ballast to address the aforementioned problems encountered by the prior art.
SUMMARY OF THE INVENTION
An object of the invention is to provide an electronic ballast configured in a self-oscillating topology for regulating the switching frequency of its internal switch elements by the windings of a transformer thereof, thereby preheating the filaments of the light-emitting device, breaking down the light-emitting device, and performing dimming control to the light-emitting device. Thus, the inventive electronic ballast can remove the deficiencies encountered by the conventional self-oscillating electronic ballast.
To this end, the invention provide an electronic ballast, which includes a square wave generator receiving a DC input voltage and having a plurality of switch elements for converting the DC input voltage into a square-wave AC voltage according to the switching operations of the switch elements; a transformer having a driving winding and a plurality of inductive windings mutually connected with each other, wherein at least a portion of the inductive windings are respectively connected to a control terminal of the switch element; a resonant circuit connected to the driving winding and the light-emitting device for receiving the square-wave voltage through the driving winding and converting the square-wave voltage into an AC output voltage to drive the light-emitting device; and an auxiliary control unit connected to the transformer for regulating the voltage waveform of the driving winding or the voltage waveform of the inductive winding according to a control signal, thereby changing the voltage waveform of the inductive winding connected to the switch element. Thus, the switching frequencies of the switch elements are adjusted.
Now the foregoing and other features and advantages of the invention will be best understood through the following descriptions with reference to the accompanying drawings, in which:



BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a circuit block diagram showing the electronic ballast according to an exemplary embodiment of the invention;
FIG. 2 shows the partial circuitry of the electronic ballast of FIG. 1;
FIG. 3 shows an alternative example of the circuitry of the electronic ballast of FIG. 2;
FIG. 4 shows the detailed circuitry of the auxiliary control unit shown in FIG. 3;
FIG. 5 is a gain curve diagram depicting the gain curve of the AC output voltage versus the switching frequency of the switch element in the square wave generator before the light-emitting device is broken down and the gain curve of the AC output voltage versus the switching frequency of the switch element in the square wave generator after the light-emitting device is broken down; and
FIG. 6 shows an alternative example of the auxiliary control unit shown in FIG. 4.



DESCRIPTION OF THE PREFERRED EMBODIMENT
An exemplary embodiment embodying the features and advantages of the invention will be expounded in following paragraphs of descriptions. It is to be realized that the present invention is allowed to have various modification in different respects, all of which are without departing from the scope of the present invention, and the description herein and the drawings are to be taken as illustrative in nature, but not to be taken as a confinement for the invention.
FIG. 1 is a circuit block diagram showing the electronic ballast according to an exemplary embodiment of the invention. As shown in FIG. 1, an electronic ballast 1 is used to receive a DC input voltage VIN and is connected to at least one light-emitting device 9, such as a fluorescent lamp or a light-emitting diode (LED). The electronic ballast 1 is used to convert the DC input voltage VIN into a sinusoidal AC output voltage Vout for driving the light-emitting device 9. The electronic ballast 1 includes a square wave generator 10, a transformer T1, an auxiliary control unit 11, and a resonant circuit 12. The square wave generator 10 is used to receive the DC input voltage VIN and includes a plurality of switch elements, such as a first switch element Q1 and a second switch element Q2 connected with each other in a half-bridge configuration. The square wave generator 10 is used to convert the DC input voltage VIN into a square-wave AC voltage VS according to the switching operations of the first switch element Q1 and the second switch element Q2. Certainly, the square wave generator 10 may include four switch elements connected with each other in a full-bridge configuration.
The transformer T1 includes a driving winding T1-1 and a plurality of inductive windings, such as a first inductive winding T1-2 and a second inductive winding T1-3 which are mutually coupled together. The driving winding T1-1 is connected to the output end of the square wave generator 10 and is used to receive the square-wave AC voltage VS for generating a square-wave control signal (not indicated). The square-wave control signal is coupled to the first inductive winding T1-2 and the second inductive winding T1-3. The first inductive winding T1-2 is connected to the control terminal of the first switch element Q1. The second inductive winding T1-3 is connected to the control terminal of the second switch element Q2. The polarity of the first inductive winding T1-2 is opposite to the second inductive winding T1-3. The first inductive winding T1-2 and the second inductive winding T1-3 are used to control the first switch element Q1 and the second switch element Q2 by the square-wave control signal of the driving winding T1-1, thereby driving the first switch element Q1 and the second switch element Q2 to turn on and off alternately.
The resonant circuit 12 is connected between the driving winding T1-1 and the light-emitting device 9, and may be consisted of a resonant capacitor CR and a resonant inductor LR, the resonant circuit 12 is used to receive the square-wave AC voltage VS through the driving winding T1-1 and convert the square-wave AC voltage VS into an AC output voltage Vout by resonance. Also, during the resonance stage, the resonant circuit 12 will generate a resonant current IR which flows through the driving winding T1-1, so that the driving winding T1-1 can generate the square-wave control signal for controlling the first switch element Q1 and the second switch element Q2 and coupling the square-wave control signal to the first inductive winding T1-2 and the second inductive winding T1-3. Therefore, the switching operations of the first switch element Q1 and the second switch element Q2 are controlled. Furthermore, as the polarity of the first inductive winding T1-2 is opposite to the second inductive winding T1-3, the first switch element Q1 and the second switch element Q2 are alternately turned on and off. Thus, the electronic ballast is termed a self-oscillating electronic ballast.
The auxiliary control unit 11 is connected to the driving winding T1-1, the first inductive winding T1-2, or the second inductive winding T1-3 for regulating the voltage waveforms of the driving winding T1-1, the first inductive winding T1-2, or the second inductive winding T1-3 according to a control signal (not indicated), so that the voltage waveforms of the driving winding T1-1, the first inductive winding T1-2, or the second inductive winding T1-3 are commuted beforehand. As the driving winding T1-1, the first inductive winding T1-2, and the second inductive winding T1-3 are mutually coupled, the voltage waveforms of the driving winding T1-1, the first inductive winding T1-2, and the second inductive winding T1-3 will commute together in a direct way or in a indirect way by the electrical connection with the auxiliary control unit 11 or the their mutual coupling. Hence, by the control of the auxiliary control unit 11, the waveforms of the voltages on the first inductive winding T1-2 and the second inductive winding T1-3 that are used to respectively control the first switch element Q1 and the second switch element Q2 can commute beforehand. In other words, the periods of the voltage waveforms of the first inductive winding T1-2 and the second inductive winding T1-3 are shortened, thereby increasing the switching frequencies of the first switch element Q1 and the second switch element Q2. Therefore, the AC output voltage Vout can be varied to achieve the dimming function, break down the light-emitting device 9, and preheat the filaments of the light-emitting device 9.
Referring to FIG. 2 and FIG. 1, in which FIG. 2 shows the partial circuitry of the electronic ballast of FIG. 1. As shown in FIG. 2, the first switch element Q1 and the second switch element Q2 of the square wave generator 10 may be implemented by transistors. In that case, the collector of first switch element Q1 is used to receive the DC input voltage VIN, the emitter of the of first switch element Q1 is connected to the collector of the second switch element Q2, and the emitter of the second switch element Q2 is connected to the ground terminal G.
In this embodiment, the resonant inductor LR is connected between one end of the driving winding T1-1 and the light-emitting device 9. The resonant capacitor CR is connected in parallel with the light-emitting device 9 and connected to the resonant inductor LR. Thus, the resonant inductor LR and the resonant capacitor CR form a parallel resonant circuit. In this embodiment, the resonant capacitor CR may be connected between the resonant inductor LR and the light-emitting device 9, thereby allowing the resonant inductor LR and the resonant capacitor CR to form a series resonant circuit.
In this embodiment, the transformer T1 includes a driving winding T1-1, a first inductive winding T1-2, and a second inductive winding T1-3. Moreover, the driving winding T1-1, the first inductive winding T1-2, and the second inductive winding T1-3 are magnetically coupled with each other. The driving winding T1-1 is connected to the output end of the square wave generator 10, and is connected to the emitter of the first switch element Q1 and the collector of the second switch element Q2 through the output end of the square wave generator 10. The first inductive winding T1-2 is connected to the base of the first switch element Q1 through a first resistor R1, and is connected to the emitter of the first switch element Q1. The second inductive winding T1-3 is connected to the base of the second switch element Q2 through a second resistor R2, and is connected to the emitter of the second switch element Q2.
In this embodiment, the auxiliary control unit 11 is connected to the first inductive winding T1-2, and connected to the base of the first switch element Q1 through a first resistor R1. The auxiliary control unit 11 is configured to directly control the voltage waveform of the first inductive winding T1-2 to commute beforehand according to the control signal received therefrom, thereby adjusting the switching frequency of the first switch element Q1. Also, as the first inductive winding T1-2 and the second inductive winding T1-3 are mutually coupled, the voltage waveform of the second inductive winding T1-3 is indirectly controlled to commute beforehand by the auxiliary control unit 11. Therefore, the switching frequency of the second switch element Q2 is adjusted accordingly. Hence, the AC output voltage Vout can be varied to achieve the dimming function, break down the light-emitting device 9, and preheat the filaments of the light-emitting device 9.
In this embodiment, as shown in FIG. 3, the transformer T1 may include a third inductive winding T1-4 that is connected to the ground terminal G and magnetically coupled with the driving winding T1-1, the first inductive winding T1-2, and the second inductive winding T1-3. Also, the auxiliary control unit 11 is connected to the third inductive winding T1-4 instead. The auxiliary control unit 11 is configured to directly control the voltage waveform of the third inductive winding T1-4 to commute beforehand according to the received control signal. As the third inductive winding T1-4 is mutually coupled with the first inductive winding T1-2 and the second inductive winding T1-3, the voltage waveform of the first inductive winding T1-2 and the voltage waveform of the second inductive winding T1-3 will be indirectly controlled by the auxiliary control unit 11 to commute beforehand. In this manner, the switching frequency of the first switch element Q1 and the switching frequency of the second switch element Q2 can be adjusted by the auxiliary control unit 11, thereby allowing the AC output voltage Vout to vary accordingly to preheat the filaments of the light-emitting device 9, break down the light-emitting device 9, and perform the dimming function to the light-emitting device 9.
Next, the detailed circuitry of the auxiliary control unit 11 will be illustrated with reference to the configuration of FIG. 3. Referring to FIG. 4 and FIG. 3, in which FIG. 4 FIG. 4 shows the detailed circuitry of the auxiliary control unit shown in FIG. 3. In this embodiment, the auxiliary control unit 11 includes a clamping circuit 110 connected to the third inductive winding T1-4. The input end of the clamping circuit 110 is used to receive a control signal VDIM that can be inputted from external circuits or generated by internal circuits. The clamping circuit 110 is used to control the voltage waveform of the windings connected to the auxiliary control unit 11 according to the magnitude of the control signal VDIM. In this embodiment, the voltage waveform of the third inductive winding T1-4 is controlled by the clamping circuit 110 according to the magnitude of the control signal VDIM. Also, as the third inductive winding T1-4 is mutually coupled with the first inductive winding T1-2 and the second inductive winding T1-3, the voltage waveform of the first inductive winding T1-2 and the voltage waveform of the second inductive winding T1-3 will also be indirectly controlled by the clamping circuit 11. In this manner, the switching frequency of the first switch element Q1 and the switching frequency of the second switch element Q2 can be adjusted by the control of the clamping circuit 110.
The clamping circuit 110 includes a first NPN bipolar junction transistor B1 and a PNP bipolar junction transistor B2. The base of the first NPN bipolar junction transistor B1 is connected to the input end of the clamping circuit 110 for receiving the control signal YDIM. A third resistor R3 is connected between the base of the first NPN bipolar junction transistor B1 and the emitter of the first NPN bipolar junction transistor B1. The base of the first NPN bipolar junction transistor B1 is connected to the ground terminal G through the third resistor R3. The emitter of the first NPN bipolar junction transistor B1 is connected to the ground terminal G. The collector of the first NPN bipolar junction transistor B1 is connected to the base of the PNP bipolar junction transistor B2. The emitter of the PNP bipolar junction transistor B2 is connected to the third inductive winding T1-4 through a first diode D1. The anode of the first diode D1 is connected to the third inductive winding T1-4. The cathode of the first diode D1 is connected to the emitter of the PNP bipolar junction transistor B2. A fourth resistor R4 is connected between the emitter of the PNP bipolar junction transistor B2 and the base of the PNP bipolar junction transistor B2.
As shown in FIG. 4, the clamping circuit 110 may include a first capacitor C1, a second diode D2, a second capacitor C2, and a voltage divider 1100. The first capacitor C1 is connected between the base of the PNP bipolar junction transistor B2 and the emitter of the PNP bipolar junction transistor B2 for the purpose of filtration. The second capacitor C2 is connected between the base of the first NPN bipolar junction transistor B1 and the emitter of the first NPN bipolar junction transistor B1 for the purpose of filtration. The second diode D2 is connected in parallel with the second capacitor C2 for preventing the second capacitor C2 from being charged to generate a large negative voltage as the voltage on the third inductive winding T1-4 is commuting. The voltage divider 1100 is connected to the input end of the clamping circuit 110, the third inductive winding T1-4, and the base of the first NPN bipolar junction transistor B1. The voltage divider 1100 may include a fifth resistor R5 and a sixth resistor R6 connected in series with each other. The base of the first NPN bipolar junction transistor B1 is connected between the fifth resistor R5 and the sixth resistor R6. The voltage received by the input end of the clamping circuit 110, i.e. the control signal VDIM and the signal of the inductive winding T1-4, passes the voltage divider 1100 in order to provide a fractional voltage for the base of the first NPN bipolar junction transistor B1. When the first NPN bipolar junction transistor B1 is turned on, the base of the PNP bipolar junction transistor B2 is connected to the ground terminal G through the first NPN bipolar junction transistor B1. Therefore, the PNP bipolar junction transistor B2 is also turned on. In this manner, the voltage waveform of the third inductive winding T1-4 will be pulled to a low state and commute beforehand, thereby shortening its period and elevating its frequency. As the first inductive winding T1-2 and the second inductive winding T1-3 are mutually coupled with the third inductive winding T1-4, the voltage waveforms of the first inductive winding T1-2 and the second inductive winding T1-3 will commute beforehand, thereby shortening the period of the voltages on the first inductive winding T1-2 and the second inductive winding T1-3 and elevating the frequency of the voltages on the first inductive winding T1-2 and the second inductive winding T1-3. In this manner, the first inductive winding T1-2 and the second inductive winding T1-3 will drive the switching frequency of the first switch element Q1 and the switching frequency of the second switch element Q2 to increase, thereby regulating the magnitude of the AC output voltage Vout to perform the dimming function, break down the light-emitting device, and preheat the filaments of the light-emitting device.
Referring to FIGS. 3, 4 and 5, in which FIG. 5 is a gain curve diagram depicting the gain curve of the AC output voltage versus the switching frequency of the switch element in the square wave generator before the light-emitting device is broken down and the gain curve of the AC output voltage versus the switching frequency of the switch element in the square wave generator after the light-emitting device is broken down. As shown in FIG. 5, when the light-emitting device 9 has not been broken down, the gain curve of the AC output voltage Vout versus the switching frequency f of the switch element in the square wave generator 10, such as the first switch element Q1, is labeled as curve A. Referring to curve A, when the electronic ballast 1 is powered on, the magnitude of the control signal VDIM is relatively large. Thus, the operating frequency of the square wave generator 10 is relatively high (as indicated by the frequency f2). The AC output voltage Vout is too low to break down the light-emitting device 9. Hence, the filaments of the light-emitting device 9 can be preheated. After a period of time, the magnitude of the control signal VDIM is going to decline. When the switching frequency of the first switch element Q1 reaches a predetermined frequency f1, the AC output voltage Vout reaches a breakdown voltage VMAX. Under this condition, the light-emitting device 9 is broken down and starts to ignite. Under this condition, the gain curve of the AC output voltage Vout versus the switching frequency f of the first switch element Q1 in the square wave generator 10 is labeled as curve B. It can be known from curve B that the operating frequency of the square wave generator 10 can be further adjusted by changing the magnitude of the control signal VDIM, so as to adjust the luminance of the light-emitting device 9. For example, when the magnitude of the control signal VDIM is increasing, the voltage waveform of the third inductive winding T1-4 can be commuted earlier. Thus, the switching frequency of the first switch element Q1 and the switching frequency of the second switch element Q2 are increased as well, and the AC output voltage Vout is reduced accordingly. Hence, the luminance of the light-emitting device 9 is dimmed.
Referring to FIGS. 5 and 6, in which FIG. 6 shows an alternative example of the auxiliary control unit shown in FIG. 4. As shown in FIG. 6, the auxiliary control unit 11 employs the clamping circuit 110 and the delay circuit 111 to preheat the filaments of the light-emitting device 9 and break down the light-emitting device 9, thereby prolonging the lifetime of the light-emitting device 9.
The delay circuit 111 is connected to the input end of the clamping circuit 110 for receiving a control signal such as an auxiliary signal VCC when the electronic ballast 1 is started and the light-emitting device 9 has not been broken down to ignite. The auxiliary signal VCC is generated when the electronic ballast 1 is started for providing the power required by the internal elements of the auxiliary control unit 11. The delay circuit 111 is used to drive the clamping circuit according to the auxiliary signal VCC to start operating to drive the voltage waveform of the winding connected to the control unit, such as the third inductive winding T1-4, thereby allowing to the voltage waveform of the winding connected to the control unit to commute beforehand within a predetermined time period. Thus, the voltage waveform of the first inductive winding T1-2 and the voltage waveform of the second inductive winding T1-3 can commute beforehand within the predetermined time period as a result of the mutual coupling with the third inductive winding T1-4. Accordingly, the switching frequency of the first switch element Q1 and the switching frequency of the second switch element Q2 are increased, thereby outputting an AC output voltage Vout having a voltage level lower than the breakdown voltage VMAX to preheat the light-emitting device 9. It can be known from the curve A of FIG. 5 that the light-emitting device 9 can not be broken down to ignite when the electronic ballast 1 is just started and the voltage level of the AC output voltage Vout has not reached the breakdown voltage VMAX. If the switch elements are regulated, for example, if the switching frequency of the first switch element Q1 and the switching frequency of the second switch element Q2 are increased, the electronic ballast 1 can output an AC output voltage Vout having a low voltage to preheat the light-emitting device 9, thereby prolonging the lifetime of the light-emitting device 9.
In this embodiment, the delay circuit 111 includes a third capacitor C3, a second NPN bipolar junction transistor B3, and a third NPN bipolar junction transistor B4, in which the third capacitor C3 is used to receive the auxiliary signal VCC and is connected to a seventh resistor R7. The third capacitor C3 is connected to the base of the second NPN bipolar junction transistor B3 through the seventh resistor R7. The collector of the second NPN bipolar junction transistor B3 is connected to the base of the third NPN bipolar junction transistor B4 and an eighth resistor R8. The emitter of the second NPN bipolar junction transistor B3 is connected to the ground terminal G. the base of the third NPN bipolar junction transistor B4 is connected to the eighth resistor R8 and is used to receive the auxiliary signal VCC through the eighth resistor R8. The emitter of the third NPN bipolar junction transistor B4 is connected to the ground terminal G. the collector of the third NPN bipolar junction transistor B4 is connected to the input end of the clamping circuit 110.
When the electronic ballast 1 starts operating and the auxiliary signal VCC is generated accordingly, the third capacitor C3 is charged by the auxiliary signal VCC. The auxiliary signal VCC is coupled to the base of the second NPN bipolar junction transistor B3 through the third capacitor C3, thereby turning on the second NPN bipolar junction transistor B3. Under this condition, the base of the third NPN bipolar junction transistor B4 is connected to the ground terminal G through the second NPN bipolar junction transistor B3. Thus, the third NPN bipolar junction transistor B4 is turned off. In the meantime, the base of the first NPN bipolar junction transistor B1 is controlled by the voltage on the third inductive winding T1-4. When the first NPN bipolar junction transistor B1 is turned on, the base of the PNP bipolar junction transistor B2 is connected to the ground terminal G through the first NPN bipolar junction transistor B1. Thus, the PNP bipolar junction transistor B2 is also turned on. In this manner, the voltage on the third inductive winding T1-4 will be pulled to a low level by the ground terminal G and commute beforehand, thereby shortening its period and elevating its frequency. As the first inductive winding T1-2 and the second inductive winding T1-3 are mutually coupled with the third inductive winding T1-4, the voltage on the first inductive winding T1-2 and the voltage on the second inductive winding T1-3 will also commute beforehand so as to shorten their periods and elevate their frequency. Therefore, the switching frequency of the first switch element Q1 and the switching frequency of the second switch element Q2 will increase. Under this condition, the electronic ballast 1 will output an AC output voltage Vout having a small voltage level, thereby preventing the light-emitting device from being broken down and preheating the filaments of the light-emitting device 9.
When the third capacitor C3 is fully charged by the auxiliary signal VCC as the predetermined time period is elapsed, the auxiliary signal VCC can not be coupled to the base of the second NPN bipolar junction transistor B3. Under this condition, the second NPN bipolar junction transistor B3 will turn off. In the meantime, the base of the third NPN bipolar junction transistor B4 will receive the auxiliary signal VCC through the eighth resistor R8, thereby turning on the third NPN bipolar junction transistor B4. Under this condition, the base of the first NPN bipolar junction transistor B1 is grounded through the input end of clamping circuit 110 and the third NPN bipolar junction transistor B4. Thus, the first NPN bipolar junction transistor B1 is turned off and the PNP bipolar junction transistor B2 is also turned off. Therefore, the voltage on the third inductive winding T1-4 will be stopped from being pulled to a low level by the ground terminal G. Hence, the switching frequency of the first switch element Q1 and the switching frequency of the second switch element Q2 will return to the normal value and the light-emitting device 9 will be broken down by the resonance of the resonant circuit 12.
In this embodiment, the delay circuit 111 will drive the clamping circuit 110 to start operating or stop operating according to the auxiliary signal VCC, thereby preheating the filament and breaking down the light-emitting device. The capacitance of the third capacitor C3 and the resistance of the seventh resistor R7 and the resistance of the ninth resistor R9 will determine the duration of the time for preheating.
In this embodiment, the delay circuit may further include a fourth capacitor C4, a fifth capacitor C5, and a ninth resistor R9, in which the fourth capacitor C4 is connected between the base of the second NPN bipolar junction transistor B3 and the emitter of the second NPN bipolar junction transistor B3 for the purpose of filtration. The ninth resistor R9 is connected in parallel with the fourth capacitor C4 between the base of the second NPN bipolar junction transistor B3 and the emitter of the second NPN bipolar junction transistor B3. The fifth capacitor C5 is connected between the base of the third NPN bipolar junction transistor B4 and the emitter of the third NPN bipolar junction transistor B4 for the purpose of filtration.
In conclusion, the inventive electronic ballast is configured to mutually couple the driving winding and the inductive windings of a transformer together and connect a portion of the inductive windings to the control terminal of the switch elements in the square wave generator, in order to control the switching operations of the switch elements. Hence, the voltage waveforms of the inductive windings connected to the control terminals of the switch elements can be controlled by adjusting the voltage waveform of the driving winding or by adjusting the voltage waveform of any one of the inductive windings. In this manner, the switching frequency of the switch elements can be adjusted for providing different AC output voltage for the light-emitting device. Therefore, the filaments of the light-emitting device can be preheated, the light-emitting device can be broken down, and the luminance of the light-emitting device can be dimmed under the self-oscillating topology.
While the invention has been described in terms of what are presently considered to be the most practical and preferred embodiments, it is to be understood that the invention need not be restricted to the disclosed embodiment. On the contrary, it is intended to cover various modifications and similar arrangements included within the spirit and scope of the appended claims which are to be accorded with the broadest interpretation so as to encompass all such modifications and similar structures. Therefore, the above description and illustration should not be taken as limiting the scope of the invention which is defined by the appended claims.

## Claims

1. An electronic ballast for driving at least one light-emitting device, comprising:
a square wave generator receiving a DC input voltage and having a plurality of switch elements for converting the DC input voltage into a square-wave AC voltage according to switching operations of the plurality of switch elements;
a transformer having a driving winding and first and second inductive windings mutually connected with each other, wherein the first and second inductive windings are respectively connected to a control terminal of the corresponding one of the plurality of switch elements, the transformer further including a third inductive winding that is magnetically coupled to the first and second inductive windings and to the drive winding;
a resonant circuit connected to the driving winding and the at least one light-emitting device for receiving the square-wave voltage through the driving winding and converting the square-wave voltage into an AC output voltage to drive the at least one light-emitting device; and
an auxiliary control unit connected to the third inductive winding of the transformer to directly control a voltage waveform of the third inductive winding which, due to the magnetic coupling, indirectly controls a voltage waveform of the first and second inductive windings according to at least one control signal, thereby changing the voltage waveform of the first and second inductive windings for adjusting switching frequencies of the plurality of switch elements.

2. The electronic ballast according to claim 1 wherein the electronic ballast is a self-oscillating electronic ballast.

3. The electronic ballast according to claim 1 wherein the at least one light-emitting device is a fluorescent lamp.

4. The electronic ballast according to claim 1 wherein the resonant circuit is connected in parallel with the at least one light-emitting device and includes a resonant inductor and a resonant capacitor, and wherein the resonant capacitor is connected in parallel with the at least one light-emitting device and is connected to the resonant inductor to form a parallel resonant circuit.

5. The electronic ballast according to claim 1 wherein the driving winding and the first and second inductive windings are magnetically coupled with each other.

6. The electronic ballast according to claim 1 wherein the square wave generator includes a first switch element and a second switch element, the first inductive winding and the second inductive winding having opposite polarities, and wherein the first inductive winding is connected to a control terminal of the first switch element through a first resistor and the second inductive winding is connected to a control terminal of the second switch element through a second resistor.

7. The electronic ballast according to claim 1 wherein the auxiliary control unit includes a clamping circuit connected to the third inductive winding for receiving the at least one control signal.

8. The electronic ballast according to claim 7 wherein the luminance of the at least one light-emitting device is dimmed with the increase of the voltage level of the at least one control signal.

9. The electronic ballast according to claim 7 wherein the clamping circuit includes:
a first NPN bipolar junction transistor having a base for receiving the at least one control signal, an emitter connected to a ground terminal, and a collector;
a PNP bipolar junction transistor having a base connected to the collector of the first NPN bipolar junction transistor, an emitter connected to the third inductive winding connected to the clamping circuit, and a collector connected to the ground terminal.

10. The electronic ballast according to claim 9 wherein the clamping circuit further includes:
a first diode having an anode connected to the third inductive winding connected to the clamping circuit and a cathode connected to the emitter of the PNP bipolar junction transistor;
a first capacitor;
a third resistor connected between the base of the first NPN bipolar junction transistor and the emitter of the first NPN bipolar junction transistor;
a fourth resistor connected in parallel with the first capacitor between the base of the PNP bipolar junction transistor and the emitter of the PNP bipolar junction transistor; and
a voltage divider connected to the base of the first NPN bipolar junction transistor, and including a fifth resistor and a sixth resistor connected in series with each other.

11. The electronic ballast according to claim 9 wherein the clamping circuit further includes:
a second capacitor connected between the base of the first NPN bipolar junction transistor and the emitter of the first NPN bipolar junction transistor; and
a second diode connected in parallel with the second capacitor for preventing the second capacitor from generating a large negative voltage during charging.

12. The electronic ballast according to claim 9 wherein the clamping circuit further includes a delay circuit connected to the clamping circuit for receiving an auxiliary signal generated when the electronic ballast is started and driving the clamping circuit according to the auxiliary signal, thereby driving the clamping circuit to regulate a voltage waveform of the third inductive winding.

13. The electronic ballast according to claim 12 wherein the delay circuit includes:
a third capacitor for receiving the auxiliary signal;
a second NPN bipolar junction transistor having a base, an emitter connected to the ground terminal, and a collector;
a seventh resistor connected between the third capacitor and the base of the second NPN bipolar junction transistor; and
a third NPN bipolar junction transistor having a base connected to the collector of the second NPN bipolar junction transistor for receiving the auxiliary signal, an emitter connected to the ground terminal, and a collector connected to the base of the first NPN bipolar junction transistor.

14. The electronic ballast according to claim 12 wherein the delay circuit further includes:
a fourth capacitor;
a fifth capacitor connected between the base of the third NPN bipolar junction transistor and the emitter of the third NPN bipolar junction transistor;
an eighth resistor for receiving the auxiliary signal and connected to the base of the third NPN bipolar junction transistor; and
a ninth resistor connected in parallel with the fourth capacitor between the base of the second NPN bipolar junction transistor and the emitter of the second NPN bipolar junction transistor.

15. The electronic ballast according to claim 14 wherein a capacitance of the third capacitor, the resistance of the seventh resistor, and the resistance of the ninth resistor determines the duration of the time for preheating filaments of the at least one light-emitting device.