Abstract:
Disclosed is an electric ballast system. The system comprises a voltage source for supplying power to the electric ballast system; a lamp driving circuit having a first terminal, a second terminal, and a third terminal, the power of the voltage source being supplied through the first terminal to begin the driving of the electric ballast system, and the lamp driving circuit outputting PWM waves through the second and third terminals; a half bridge converter, a first end of which is connected to the second terminal of the lamp driving circuit and a second end of which is connected to the third terminal of the lamp driving circuit, the half bridge converter receiving input through the second and third terminals of the lamp driving circuit, and the half bridge converter performing output of a current which changes flow directions according to the PWM waves output by the lamp driving circuit; a lamp portion, a first end of which is connected to an output end of the half bridge converter, the lamp operating according to the current output by the half bridge converter; and a lamp protector connected between a second end of the lamp and the first terminal of the lamp driving circuit, the lamp protector discontinuing the operation of the lamp driving circuit if there is no bulb installed in the lamp.

Description:
BACKGROUND OF THE INVENTION 
     (a) Field of the Invention 
     The present invention relates to an electric ballast system, and more particularly, to a control circuit for an electric ballast system. 
     (b) Description of the Related Art 
     Electric ballast systems perform the initial driving of a lamp and thereafter supply stable power to the lamp. The conventional electric ballast system is typically comprised of a DC-DC converter for supplying DC power, and a lamp driving circuit for controlling the driving of a lamp. In such a conventional ballast system, power is supplied to the lamp driving circuit from a point at which an upper switch and a lower switch of the DC-DC converter meet. That is, power is supplied from this point without passing through the lamp. 
     However, the supply of power to the lamp driving circuit in the manner described above has many drawbacks. In particular, during a soft start or dimming, driving frequencies of the lamp quicken and a dead time decreases. At this time, a capacitor for supplying power to the lamp driving circuit limits the dead time operation. That is, a voltage of the point between the switches does not become zero during switching as a result of the capacitor connected between the upper and lower switches of the DC-DC converter. Hence, zero voltage switching is not able to occur, a temperature of a switching MOSFET increases, and normal lamp operation becomes difficult. 
     Further, since power is supplied to the lamp driving circuit without passing through the lamp, if there is no bulb in the lamp, power is continuously supplied to the lamp driving circuit, causing the switching MOSFET to burn out. To prevent such a problem, a protection circuit must be additionally installed in the lamp driving circuit. 
     SUMMARY OF THE INVENTION 
     The present invention has been made in an effort to solve the above problems. 
     It is an object of the present invention to provide an electric ballast system which can operate normally without relation to changes in driving frequencies. 
     It is another object of the present invention to provide an electric ballast system which discontinues the operation of a lamp driving circuit when there is no bulb in a lamp such that an additional protection circuit is not required. 
     To achieve the above objects, the present invention provides an electric ballast system comprising a voltage source for supplying power to the electric ballast system; a lamp driving circuit having a first terminal, a second terminal, and a third terminal, the power of the voltage source being supplied through the first terminal to begin the driving of the electric ballast system, and the lamp driving circuit outputting PWM waves through the second and third terminals; a half bridge converter, a first end of which is connected to the second terminal of the lamp driving circuit and a second end of which is connected to the third terminal of the lamp driving circuit, the half bridge converter receiving input through the second and third terminals of the lamp driving circuit, and the half bridge converter performing output of a current which changes flow directions according to the PWM waves output by the lamp driving circuit; a lamp portion, a first end of which is connected to an output end of the half bridge converter, the lamp operating according to the current output by the half bridge converter; and a lamp protector connected between a second end of the lamp and the first terminal of the lamp driving circuit, the lamp protector discontinuing the operation of the lamp driving circuit if there is no bulb installed in the lamp. 
     According to a feature of the present invention, the electric ballast system further comprises a first resistor connected between the voltage source and the first terminal of the lamp driving circuit; a first capacitor connected between a ground and a common terminal of a fourth resistor and the first terminal of the lamp driving circuit, the first capacitor being charged by a current input through the fourth resistor; and a first diode connected between a ground and a common terminal of a third capacitor and the first terminal of the lamp driving circuit, the first diode acting to maintain a charge voltage of the third capacitor above a predetermined potential. 
     According to another feature of the present invention, the lamp driving circuit comprises a reference current generator for generating and outputting a reference current; a lamp drive starter for receiving the power of the voltage source through the first terminal of the lamp driving circuit to begin the operation of the lamp driving circuit; a soft starter receiving a starting signal from the lamp drive starter and the reference current from the reference current generator, and outputting a lamp initial drive current to soft start the lamp; a sawtooth wave oscillator for outputting a sawtooth wave current; an adder receiving the lamp initial drive current from the soft starter and the sawtooth wave current from the sawtooth wave oscillator, and adding the lamp initial drive current to the sawtooth wave current and outputting a resulting output current; a first current source connected to the adder to receive the output current of the adder, the first current source selectively dividing the output current of the adder; a PWM wave generator connected to a common terminal of the adder and the first current source, receiving the output current of the adder, and generating and outputting output PWM waves; and a PWM wave splitter receiving the output PWM waves from the PWM wave generator, and alternatingly splitting and outputting the PWM waves through the second and third terminals of the lamp driving circuit. 
     According to yet another feature of the present invention, the electric ballast system further comprises a second capacitor connected between the soft starter and a ground, the second capacitor determining a soft starting time; a third capacitor connected between a ground and a common terminal of the adder and the PWM wave generator, the third capacitor determining a frequency of the PWM waves; and a second resistor connected between the reference current generator and a ground, the second resistor determining a magnitude of the reference current output by the reference current generator. 
     According to still yet another feature of the present invention, the soft starter comprises a first switch connected between a ground and the second capacitor, the first switch being controlled to ON if the starting signal of the lamp drive starter is generated, thereby reducing a charge voltage of the second capacitor; a subtractor connected to a common terminal of the first switch and the second capacitor, the subtractor obtaining a difference between a reference voltage and the charge voltage of the second capacitor, and outputting an output voltage corresponding to the difference; and a multiplier receiving the output voltage of the subtractor and the reference current of the reference current generator, and multiplying the output voltage of the subtractor to the reference current of the reference current generator. 
     According to still yet another feature of the present invention, the PWM wave generator comprises a first comparator receiving a charge voltage of the third capacitor through a first terminal and a first potential through a second terminal, comparing the charge voltage of the third capacitor with the first potential, and outputting a comparison value; a second comparator receiving the charge voltage of the third capacitor through a second terminal and a second potential through a first terminal, comparing the charge voltage of the third capacitor with the second potential, and outputting a comparison value; and a latch receiving the output value of the first comparator and the output value of the second comparator, and outputting a latching value. 
     According to still yet another feature of the present invention, the half bridge converter comprises a transformer having a primary coil, a first end of the primary coil being connected to the second terminal of the lamp driving circuit and a second end of the primary coil being connected to the third terminal of the lamp driving circuit, and having first and second secondary coils through which the PWM waves of the lamp driving circuit are alternatingly output; a first MOSFET transistor having a source connected to the voltage source, a gate connected to a first end of the first secondary coil of the transformer, and a drain connected to a second end of the first secondary coil of the transformer, the first MOSFET transistor performing switching according to an output waveform of the first secondary coil of the transformer; and a second MOSFET transistor having a drain connected to a common terminal of the drain of the first MOSFET transistor and the first secondary coil of the transformer, a gate connected to a first end of the second secondary coil of the transformer, and a source connected to a second end of the second secondary coil of the transformer, the second MOSFET transistor performing switching according to an output waveform of the second secondary coil of the transformer. 
     According to still yet another feature of the present invention, the electric ballast system further comprises a third resistor connected between the first secondary coil of the transformer and the gate of the first MOSFET transistor, the third resistor preventing an excess current from flowing to the first MOSFET transistor; and a fourth resistor connected between the second secondary coil of the transformer and the gate of the second MOSFET transistor, the fourth resistor preventing an excess current from flowing to the second MOSFET transistor. 
     According to still yet another feature of the present invention, the lamp portion comprises an inductor connected to a common terminal of the first MOSFET transistor and the second MOSFET transistor; a lamp, a first end of which is connected to the inductor; a fourth capacitor connected in parallel to the lamp; a fifth capacitor connected between a second end of the lamp and a common terminal of the voltage source and the first MOSFET transistor; and a sixth capacitor connected between the source of the second MOSFET transistor and a common terminal of the second end of the lamp and the fourth capacitor. 
     According to still yet another feature of the present invention, the lamp protector comprises a seventh capacitor, one end of which is connected to a common terminal of the fifth capacitor and the sixth capacitor; a second diode having a cathode connected to the sixth capacitor, and an anode connected to a common terminal of the source of the second MOSFET transistor and the fifth capacitor; and a third diode having an anode connected to a common terminal of the sixth capacitor and the cathode of the second diode, and a cathode connected to the first terminal of the lamp driving circuit. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The accompanying drawings, which are incorporated in and constitute a part of the specification, illustrate an embodiment of the invention, and, together with the description, serve to explain the principles of the invention: 
     FIG. 1 is a circuit diagram of an electric ballast system according to a preferred embodiment of the present invention; 
     FIG. 2 is a detailed circuit diagram of a lamp driving circuit shown in FIG. 1; and 
     FIGS. 3 a  and  3   b  are a waveform diagram of an operation of the lamp driving circuit shown in FIG.  1 . 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Preferred embodiments of the present invention will now be described in detail with reference to the accompanying drawings. 
     FIG. 1 shows a circuit diagram of an electric ballast system according to a preferred embodiment of the present invention. 
     As shown in the drawing, the electric ballast comprises a voltage source Vin; a half bridge converter  100 ; a lamp portion  200 ; a lamp driving circuit  300 ; a lamp protector  400 ; resistors R 1  and R 5 ; capacitors C 1 , C 2  and C 3 ; and a diode Z 1 . 
     A first end of the resistor R 1  is connected to the power source Vin; the half bridge converter  100  is also connected to the power source Vin; the lamp portion  200  is connected to the half bridge converter  100 ; the lamp protector  400  is connected to the lamp portion  200 ; and the lamp driving circuit  300  has six terminals ( 1 ) through ( 6 ), the terminals ( 5 ) and ( 6 ) being respectively connected to each end of the half bridge converter  100 , and the terminal ( 4 ) being connected to both the resistor R 1  and the lamp protector  400 . Further, the capacitor C 2  is connected to the terminal ( 1 ); the capacitor C 3  is connected between the terminal ( 2 ) and the capacitor C 2 ; the resistor R 5  is connected between the terminal ( 3 ) and a common terminal of the capacitors C 2  and C 3 ; the diode Z 1  is connected between a common terminal of the terminal ( 4 ) and the resistor R 1  and a common terminal of the capacitors C 2  and C 3  and the resistor R 5 ; and the capacitor C 1  is connected in parallel to the diode Z 1 . 
     The half bridge converter  100  includes resistors R 2  and R 3 , a transformer T 1 , and transistors Q 1  and Q 2 . The transformer T 1  has two secondary coils—an upper secondary coil and a lower secondary coil. Also, a first end and a second end of a primary coil of the transformer T 1  are respectively connected to the terminal {circle around ( 5 )} and the terminal {circle around ( 6 )} of the lamp driving circuit  300 . The resistor R 2  is connected to the upper secondary coil of the transformer T 1  and the resistor R 3  is connected to the lower secondary coil of the transformer T 1 . Further, a source of the transistor Q 1  is connected to the voltage source Vin, a gate of the transistor Q 1  is connected to the resistor R 2 , and a drain of the transistor Q 1  is connected to the lower secondary coil of the transformer T 1 . In addition, a drain of the transistor Q 2  is connected to a common terminal of the drain of the transistor Q 1  and the upper secondary coil of the transformer T 1 , a gate of the transistor Q 2  is connected to the resistor R 3 , and a source of the transistor Q 2  is connected to the lower secondary coil of the transformer T 1 . 
     The lamp portion  200  includes an inductor L 1 ; capacitors C 5 , C 6  and C 7 ; and a lamp Lamp 1 . A first end of the inductor L 1  is connected to a common terminal of the transistor Q 1  and the transistor Q 2 , and a second end of the inductor L 1  is connected to the lamp Lamp 1 . The capacitor C 5  is connected in parallel to both ends of the lamp Lamp 1 , the capacitor C 6  is connected between the voltage source Vin and a common terminal of the lamp Lamp 1  and the capacitor C 5 , and the capacitor C 7  is connected between the common terminal of the lamp Lamp 1  and the capacitor C 5  and the source of the transistor Q 2 . 
     The lamp protector  400  includes a capacitor C 4  and diodes D 1  and D 2 . The capacitor C 4  is connected to a common terminal of the capacitor C 6 , the capacitor C 7 , and the lamp Lamp 1 . Also, a cathode of the diode D 1  is connected to the capacitor C 4 , an anode of the diode D 1  is connected to the source of the transistor Q 2 , an anode of the diode D 2  is connected to a common terminal of the capacitor and the diode D 1 , and a cathode of the diode D 2  is connected to the terminal {circle around ( 4 )} of the lamp driving circuit  300 . 
     FIG. 2 shows a detailed circuit diagram of the lamp driving circuit  300 . As shown in the drawing, the lamp driving circuit  300  includes a reference current generator  310 , a lamp drive starter  320 , a soft starter  330 , a sawtooth wave oscillator  340 , an adder A 1 , a current source I 1 , a PWM wave generator  350 , and a PWM wave splitter  360 . The reference current generator  310  is connected to the terminal ( 3 ) of the lamp driving circuit  300 , the lamp drive starter  320  is connected to the terminal ( 4 ) of the lamp driving circuit  300 , and the soft starter  330  is connected to both the reference current generator  310  and the lamp drive starter  320 . Further, the sawtooth wave oscillator  340  and the soft starter  330  are connected to the adder A 1 ; the adder A 1  is connected to the terminal ( 2 ) of the lamp driving circuit  300  and the current source I 1  the PWM wave generator  350  is connected to a common terminal of the current source I 1 , the adder A 1 , and the terminal ( 2 ) of the lamp driving circuit  300 ; the PWM wave splitter  360  is connected to the PWM wave generator  350  and the lamp drive starter  320 ; and an output terminal of the PWM wave splitter  360  is connected to the terminals ( 5 ) and ( 6 ) of the lamp driving circuit  300 . 
     The reference current generator  310  includes resistors R 6  and R 7 , a capacitor C 8 , a comparator COM 1 , and a transistor TR 1 . A first end of the resistor R 6  is connected to the terminal ( 3 ) of the lamp driving circuit  300 , the capacitor C 8  is connected between a second end of the resistor R 6  and a ground, a negative terminal of the comparator COM 1  is connected to a common terminal of the capacitor C 8  and the resistor R 6 , and a positive terminal of the comparator COM 1  is connected to a reference voltage Vref. Also, a base of the transistor TR 1  is connected to an output terminal of the comparator COM 1 , an emitter of the transistor is connected to the resistor R 7 , and a collector of the transistor TR 1  is connected to a current mirror  311 . 
     The soft starter  330  includes a current source  12 , switches S 2  and S 3 , a subtractor D 1 , and a multiplier M 1 . The switch S 2  is connected between the lamp drive starter  320  and the terminal ( 1 ) of the lamp driving circuit  300 , the switch S 3  is connected to the terminal ( 1 ) of the lamp driving circuit  300 , the current source  12  is connected between the switch S 3  and a ground, the subtractor D 1  is connected to the terminal ( 1 ) of the lamp driving circuit  300 , and the multiplier M 1  is connected to the subtractor D 1  and the current mirror  311 . 
     The PWM wave generator  350  includes comparators COM 2  and COM 3 , and a latch  351 . A positive terminal of the comparator COM 2  receives an input of 1V, a negative terminal of the comparator COM 2  receives a charge voltage of the capacitor C 3 , a positive terminal of the comparator COM 3  receives the charge voltage of the capacitor C 3 , and a negative terminal of the comparator COM 3  receives an input of 3V. Further, an R terminal of the latch  351  is connected to an output terminal of the comparator COM 2 , and an S terminal of the latch  351  is connected to an output terminal of the comparator COM 3 . 
     An operation of the electric ballast system of the present invention structured as in the above will now be described with reference to FIGS. 1 and 2. 
     The electric ballast system receives power through the input of the voltage source Vin, thereby beginning the operation of the electric ballast system. A current supplied from the voltage source Vin passes through the resistor R 1  to charge the capacitor C 1 . If a charge voltage of the capacitor C 1  exceeds a predetermined level, the lamp driving circuit  300  begins to operate. That is, when a voltage input to the terminal ( 4 ) exceeds a predetermined level, the lamp drive starter  320  begins to operate, which, in turn, controls the switch S 2  from OFF to ON. Further, since the switch S 3  is initially in an ON state, if the switch S 2  is controlled to OFF when a charge voltage Vc 2  of the capacitor C 2  is in a ground voltage state, the charge voltage Vc 2  of the capacitor C 2  increases. At this time, a rate at which the charge voltage of the capacitor C 2  increases is determined by the capacitor C 2 . That is, if a capacity of the capacitor C 2  is small, the charge voltage Vc 2  of the capacitor C 2  is more quickly increased, and if the capacity of the capacitor C 2  is large, the rate at which the charge voltage Vc 2  of the capacitor C 2  increases is decreased. Accordingly, the lamp driving circuit  300  can be started by the presence of the capacitor C 2 . 
     The reference current generator  310  generates a reference current in the following manner. The reference voltage Vref is supplied to the positive terminal of the comparator COM 1 , and because a voltage of the negative terminal of the comparator COM 1  also becomes the reference voltage and a current flowing to the resistor R 6  becomes almost zero, a voltage of the resistor R 5  becomes the reference voltage Vref. As a result, a current flowing to the resistor R 5  becomes Vref/R 5 , and since the current flowing to the resistor R 6  becomes almost zero, a current Is flowing to the resistor R 7  also becomes Vref/R 5 . The current mirror  311  receives the input of the current Is, then outputs a reference current Ik, the reference current Ik being proportional to the current Is. The reference current Ik output by the reference current generator  310  is determined by a size of the resistor R 5 , which is connected to the terminal ( 3 ) of the lamp driving circuit  300 . 
     The subtractor D 1  outputs a difference between the reference voltage Vref and the charge voltage Vc 2  of the capacitor C 2 , and the multiplier M 1  multiplies the reference current Ik output from the reference current generator  310  by the difference between the reference voltage Vref and the charge voltage of the capacitor C 2  output by the subtractor D 1 , after which a resulting value is output to the adder A 1 . The resulting value, or an output current Ih, therefore, is derived by the following calculation: Ik×(Vref−Vc 2 )/Vref. The output current Ih can be varied as needed, as is understood by those in the art to which the present invention pertains. 
     The output current Ih of the multiplier M 1  and an output sawtooth wave current Ic of the sawtooth wave oscillator  340  are received by the adder A 1 , after which the adder A 1  adds these two values and outputs a resulting current value (i.e., an output current Ia) to the capacitor C 3 . 
     The charge voltage of the capacitor C 3  is shown in FIG.  3 . 
     Since the output current Ia of the adder A 1  is the sum of the sawtooth wave current Ic and the output current Ih of the multiplier M 1 , the output current Ia results in a waveform as shown in (a) of FIG. 3 such that the charge voltage of the capacitor C 3  is also depicted by (a) of FIG.  3 . The charge voltage of the capacitor C 3  varies between the 3V input voltage of the negative terminal of the comparator COM 3  and the 1V input voltage of the positive terminal of the comparator COM 2 . Here, the input voltage of the negative terminal of the comparator COM 3  and the input voltage of the positive terminal of the comparator COM 2  can be varied as needed. 
     Since the charge voltage of the capacitor C 3  takes on a sawtooth waveform between 1V and 3V as shown in FIG. 3, an output waveform of the latch  351  results in a waveform as shown by (b) of FIG.  3 . The reason for this is as follows. When the charge voltage of the capacitor C 3  is at 1V, since an output value of the comparator COM 2  (the input value of the R terminal of the latch  351 ) is 1 and an output value of the comparator COM 3  (the input value of the S terminal of the latch  351 ) is 0, an output value Q of the latch  351  becomes 1. Further, in an interval where the charge voltage of the capacitor C 3  increases from 1V to 3V, since the output values of the comparator COM 2  and the comparator COM  3  are both 0, the output value Q of the latch  351  is maintained at the previous value of 1. However, when the charge voltage of the capacitor becomes 3V, since the output value of the comparator becomes 1 and the output value of the comparator COM 2  becomes 0, the output value Q of the latch  351  becomes 0. In the case where the output value Q of the latch  351  is 1, the switch S 1  and the switch S 3  are controlled to ON. If the switch S 1  is controlled to ON, since the current charged in the capacitor C 3  is minimized by as much as a current value of the current source I 1 , the charge voltage of the capacitor C 3  is reduced. At this time, if the current value of the current source  11  is set to be larger than the output current Ia of the adder A 1 , the voltage of the capacitor C 3  is discharged more quickly than when charged such that the charge voltage of the capacitor results in a waveform as shown by (a) of FIG.  3 . 
     In addition, in an interval where the charge voltage of the capacitor C 3  reduces from 3V to 1V by the ON operation of the switch S 1 , since the output values of the comparators COM 2  and COM 3  become 0, the output value Q of the latch  351  is maintained at the previous value of 0. Accordingly, the output waveform of the latch  351  results in a waveform as shown by (b) of FIG.  3 . 
     The PWM wave splitter  360  splits output PWM waves of the PWM wave generator  350  through the terminals ( 5 ) and ( 6 ) of the lamp driving circuit  300 . That is, the output PWM waves as shown in (b) of FIG. 3 are output alternatingly through terminal ( 5 ) then through terminal ( 6 ) of the lamp driving circuit  300 . By this operation, the lamp driving circuit  300  generates PWM waves, which are input to both ends of the primary coil of the half bridge converter  100 . 
     When the PWM waves are output through the terminal ( 5 ) of the lamp driving circuit  300 , a current of a counterclockwise direction is induced to the upper secondary coil of the transformer T 1 , while a current of a clockwise direction is induced to the lower secondary coil of the transformer T 1 . As a result, the transistor Q 1  is controlled to ON and the transistor Q 2  is controlled to OFF. In this case, current flows through a path of the transistor Q 1 , the inductor L 1 , the lamp Lamp 1 , and the capacitor C 7 ; as well as a path of the transistor Q 1 , the inductor L 1 , the lamp Lamp 1 , and the capacitor C 6 . A frequency of this current is a resonance frequency between the inductor L 1  and the capacitor C 7 . 
     When the PWM waves are output through the terminal ( 6 ) of the lamp driving circuit  300 , a current in the clockwise direction is induced to the upper secondary coil of the transformer T 1 , while a current of a counterclockwise direction is induced to the lower secondary coil of the transformer T 1 . As a result, the transistor Q 1  is controlled to OFF and the transistor Q 2  is controlled to ON. In this case, current flows through a path of the capacitor C 6 , the lamp Lamp 1 , the inductor L 1 , and the transistor Q 2 ; as well as a path of the capacitor C 7 , the lamp Lamp 1 , the inductor L 1 , and the transistor Q 2 . A frequency of this current is a resonance frequency between the inductor L 1  and the capacitor C 6 . 
     The lamp is operated by the lamp driving circuit  300  using the operational principles as outlined above. In the preferred embodiment of the present invention, the operation of the lamp driving circuit  300  is discontinued by the lamp protector  400  when there is no bulb installed in the lamp. 
     An operation of the lamp protector  400  will now be described. 
     The lamp protector  400 , with reference to FIG.  1  and as described above, includes the capacitor C 4  and the diodes D 1  and D 2 . The capacitor C 4  is connected to a common terminal of the capacitor C 6 , the capacitor C 7  and the lamp Lamp 1  such that a part of the current applied to the lamp Lamp 1  is supplied through the terminal ( 4 ) of the lamp driving circuit  300 . If a bulb is installed in the lamp Lamp 1 , the current is supplied to the lamp driving circuit  300 , but when there is no bulb, current does not flow through the lamp Lamp 1 . As a result, current does not flow to the lamp driving circuit  300  through the lamp protector  400 . The current supplied through the lamp protector  400  is supplied to the lamp drive starter  320  through the terminal ( 4 ) of the lamp driving circuit  300 . That is, the current supplied to the lamp drive starter  320  is supplied through the voltage source Vin and the lamp protector  400 , and if the current is not supplied through the lamp protector  400 , the lamp drive starter  320  does not operate. This is because the lamp drive starter  320  operates only when a current of above a predetermined level is supplied thereto. 
     As a result, the operation of the lamp drive circuit  300  can be discontinued when there is no bulb installed in the lamp Lamp 1 , thereby preventing the continuous flow of current to the lamp driving circuit  300  when there is no bulb. Therefore, the burning out of the internal elements of the lamp driving circuit  300  is prevented. Further, a complicated lamp protecting circuit as used in the prior art is not needed. 
     In addition, since no element is provided at a point where the transistor Q 1  and the transistor Q 2  meet, the transistors Q 1  and Q 2  can perform zero voltage switching. Accordingly, an increase in the operational temperature of the transistors Q 1  and Q 2 , which causes the lamp driving circuit  300  to malfunction, is prevented. 
     Although preferred embodiments of the present invention have been described in detail hereinabove, it should be clearly understood that many variations and/or modifications of the basic inventive concepts herein taught which may appear to those skilled in the present art will still fall within the spirit and scope of the present invention, as defined in the appended claims.