Abstract:
A safety ballast for a discharge lamp capable of interrupting the supply of the electric power upon failure of starting the lamp. The ballast includes a converter providing a DC power voltage from a DC voltage source, a booster providing a boosted DC voltage to a starter which generates, based upon the boosted DC voltage, a starting voltage for starting the lamp, and an inverter receiving the DC power from the converter to provide an AC power for operating the lamp. The booster includes a capacitor which is charged by the DC power and accumulate the boosted voltage. A controller monitors a load condition of the lamp and issues a disable signal when a no-load condition continues over a predetermined starting period. The inverter includes a plurality of switching elements one of which is regulated to be turned on in response to the no-load signal for establishing a supply path of supplying the boosted voltage from the booster&#39;s capacitor through the switching element to the starter. The one switching element is also controlled to be turned off in response to the disable signal for interrupting the supply path. Thus, when the no-load condition continues over the predetermined starting period as indicative of the that the lamp has been removed or the lamp reaching its end of life, the booster&#39;s capacitor bearing the boosted voltage is disconnected from the starter, applying no voltage to the starter and therefore preventing the starter from being activated to generate unnecessary starting voltage.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention is directed to a ballast for a discharge lamp, more particularly a high density discharge lamp such as a metal halide lamp and a mercury lamp. 
     2. Description of the Prior Art 
     As disclosed in Japanese Laid-Open Patent Publication No. 7-142182, a prior ballast for a discharge lamp is generally known to have a DC-to-DC converter supplying a DC voltage from a DC voltage source, a DC-to-AC inverter providing an AC voltage from the DC voltage source for operating the discharge lamp, a booster generating a boosted DC voltage, and a starter receiving the boosted voltage and providing a starting voltage of sufficiently high level for starting the lamp. The booster includes a capacitor which accumulates the boosted DC voltage to be supplied to the starter for developing the starting voltage. A problem remains in the ballast that, even if the ballast is deactivated as a consequence of that the discharge lamp fails to start due to absence of the lamp or the lamp reaching its end of operation life, the residual capacitance in the capacitor of the booster may cause the starter to develop the starting voltage or at least gives the boosted voltage to the starter, thereby giving unnecessary high voltage which may give undue stress to the components of the starter and create a possible electrical shock hazard to a personnel who accidentally touch the connection between the booster and the starter. 
     SUMMARY OF THE INVENTION 
     In view of the above problem, the present invention has been achieved to provide a safety ballast for a discharge lamp which is capable of disconnecting a supply of high voltage upon failure of starting the lamp, thereby avoiding undue occurrence of the high voltage and protecting a personnel from the electrical shock hazard. The ballast in accordance with the present invention includes a DC-to-DC converter providing a DC power voltage of a predetermined level from a DC voltage source, and a booster including a booster&#39;s capacitor which is charged through the DC-to-DC converter by the DC voltage source to accumulate a boosted voltage to be supplied to a starter so that the starter generates a starting voltage for starting the discharge lamp. Also included in the ballast is a DC-to-AC inverter which receives the DC power from the DC-to-DC converter to provide an AC power to operate the discharge lamp. The DC-to-AC inverter has a plurality of switching elements controlled to turn on and off for providing the AC power. Further, the ballast includes a controller which provides a no-load signal when the lamp is not started. The inverter is controlled by the controller to establish a supply path of supplying the boosted voltage from the booster&#39;s capacitor through the inverter to the starter. The one switching element is also controlled by the controller so as to be turned off in response to a condition where the discharge lamp fails to start within a predetermined time period, thereby interrupting the supply path. Thus, when the no-load condition continues over a predetermined starting period as indicative of the that the lamp has been removed or the lamp reaching its end of life, the booster&#39;s capacitor bearing the boosted voltage is disconnected from the starter, applying no voltage to the starter and therefore preventing the starter from being activated to generate unnecessary starting voltage. Further, since the boosted voltage is not applied to the connection between the starter and the booster, it is possible to protect personnel accidentally touching the connection from an electrical shock hazard which would be otherwise presented. 
     The controller may provides a disable signal when the no-load signal lasts over the predetermined starting period so as to turn off the one switching element, thereby interrupting the supply path. In this connection, the load detector may includes a timer which provides the disable signal when the discharge lamp fails to start within the predetermined time period as indicative of that the discharge lamp is disconnected. 
     In a preferred embodiment where the DC-to-DC converter, the booster, and the controller are integrated into a single driver module, the inverter has first and second output terminals through which the inverter is connected to the starter, and the booster has a third output terminal such that the booster is connected to the starter through the third and the second terminals. The booster&#39;s capacitor has its one end connected to the third terminal and has the other end connected to the second terminal through the switching element of the inverter so as to supply the boosted voltage to the starter. The switching element of the inverter is connected to the controller and is turned off in response to the disable signal, thereby interrupting a discharge loop starting from the booster&#39;s capacitor through the third and second output terminals. 
     Thus, the booster&#39;s capacitor responsible for supplying the boosted voltage is disconnected from the second and the third terminal by the switching element when the lamp is removed, thereby preventing an electrical shock hazard due to the boosted voltage, even if the user touches the connection between the third and second terminals. 
     Preferably, the booster&#39;s capacitor has its one end connected to the third output terminal and has the other end connected to the first terminal through another switching element of the inverter which is turned off in response to the disable signal. Thus, even if there is a short-circuit between the third and first output terminals, the booster&#39;s capacitor can be prevented from discharging the current through these terminals, therefore avoiding another possible electrical shock hazard. 
     It is preferred that the DC-to-AC inverter includes two pairs of switching transistors arranged in a full-bridge with each pair of the transistors disposed on opposed sides of the full-bridge. One pair of the transistors are controlled to turn on and off alternately with the other pair of the switching transistors to provide said AC power. The full-bridge has inputs connected to receive the DC power from the DC-to-DC converter and having outputs defining the first and second output ends, respectively. One of the switching transistors defines the switching element connected to the second terminal, while another transistor defines the switching element connected to the first terminal. Thus, the prevention of undue activation of the starter and the electrical shock hazard can be made by utilization of the switching transistors of the inverter responsible for providing the AC power to the lamp. 
     In a preferred embodiment, the DC-to-DC converter, the booster, the DC-to-AC inverter, and the load detector are accommodated in a single housing to form a driver module with the first, second and third output terminals for detachable connection with a lamp module integrating the discharge lamp and the starter. Thus, the ballast can be easily installed for detachable connection with the lamp module. 
     These and still other objects and advantageous features of the present invention will become more apparent from the following description of the preferred embodiments when taken in conjunction with the attached drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a circuit diagram of a ballast in accordance with a first embodiment of the present invention; 
     FIGS. 2 to  4  are waveform charts illustrating the operation of the ballast, respectively; 
     FIGS. 5 to  7  are circuit diagrams of a ballast in accordance with modifications of the fist embodiment, respectively; 
     FIG. 8 is a circuit diagram of a ballast in accordance with a second embodiment of the present invention; 
     FIG. 9 is a circuit diagram of a ballast in accordance with a modification of the second embodiment; 
     FIG. 10 is a circuit diagram of a ballast in accordance with a third embodiment of the present invention; 
     FIG. 11 is a circuit diagram of a ballast in accordance with a modification of the third embodiment; 
     FIG. 12 is a circuit diagram of a ballast in accordance with a fourth embodiment of the present invention; 
     FIG. 13 is a circuit diagram of a ballast in accordance with a fifth embodiment of the present invention; and 
     FIGS. 14 and 15 are circuit diagrams respectively illustrating modifications of DC-to-DC converter which may be utilized in the ballast of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
     First Embodiment 
     Referring now to FIG. 1, there is shown a ballast for a discharge lamp in accordance with a first embodiment of the present invention. The ballast is suitable for operating a high intensity discharge lamp for use as a headlamp of an automobile and a light source of LCD projector. The ballast comprises a DC-to-DC converter  10  adapted to be connected to a DC power source  1 , such as a car battery or the like fixed voltage source, for providing a DC power, a booster  20  connected to the DC voltage source  1  through the converter  10  to generate a boosted voltage, and an inverter  40  receiving the DC power from the converter  10  and proving an AC power for operating the discharge lamp L. The booster  20  is connected to provide the boosted DC voltage to a starter  60  which responds to generate a starting voltage of sufficiently high level for starting the lamp L. Also included in the ballast is a power controller  50  which is responsible for controlling the converter  10  and the inverter  40  to start and operate the lamp based upon a monitored condition of the discharge lamp. The converter  10 , the booster  20 , the inverter  40 , and the power controller  50  are accommodated in a single housing to form a driver module, while the lamp L is accommodated together with the starter  60  to form a lamp module detachable to the driver module. 
     The converter  10  includes a transformer with a primary winding  11  and a secondary winding  12 . The primary winding  11  is connected in series with a switching transistor  14  across the DC power source  1 . The switching transistor  14  is controlled by the power controller  50  to repetitively turn on and off at a frequency of about several tens to several hundreds kHz, inducing a voltage across the secondary winding  12 . The induced voltage is fed through a diode  15  to charge a capacitor  16  which outputs the resulting DC voltage to the inverter  40 . 
     The booster  20  includes a winding  22  which is magnetically coupled to the primary winding  11  of the converter  10  to induce a corresponding voltage across the winding  22 . The winding  22  is formed integrally with the secondary winding  12  and is functionally separated therefrom by a center tap. The voltage is fed through a diode  24  to charge a capacitor  25  which provides the boosted voltage to the starter  60  through a resistor  26 . 
     The inverter  40  includes two pairs of switching transistors  41 ,  42 , and  43 ,  44  which are arranged in a full-bridge configuration and are controlled by the power controller  50  so that each pair of the transistors  41 ,  42  and  43 ,  44  on opposed sides of the full-bridge turn on and off simultaneously. Normally, the pair of the transistors  41  and  42  are controlled to turn on and off alternately with the other pair of the transistors  43  and  44  at a frequency of several hundreds kHz, thereby providing the AC power for operating the lamp L. The inverter  40  has its output ends terminating at first and second output terminals X 1  and X 2  through which the inverter  40  is detachably connected to the starter  60  including the lamp L. 
     The capacitor  25  of the booster  20  has its one end connected through resistor  26  to a third output terminal X 3  and has the other end connected to the second output terminal X 2  through the switching transistor  42 , and to the first output terminal X 1  through the switching transistor  44 . It is through the second and third output terminals X 2  and X 3  that the booster  20  is detachably connected to starter  60  for providing the boosted voltage to the starter. 
     The starter  60  comprises a capacitor  63  connected across the first and second terminals X 1  and X 2 , a capacitor  64  connected across the second and third output terminals X 2  and X 3 , and a transformer with a primary winding  61  and a secondary winding  62 . The primary winding  61  is connected in series with a spark gap element  65  across capacitor  64 , while the secondary winding  62  is connected in series with the discharge lamp L across capacitor  63 . Capacitor  64  is connected to receive the boosted voltage from capacitor  25  of the booster  20  through the switching transistor  42  of the inverter  40  so as to be charged by capacitor  25 . When capacitor  64  is charged up to a discharge starting voltage of the spark gap element  65 , the element  65  becomes conductive to induce across the secondary winding  62  the starting voltage which is applied to start the discharge lamp L. 
     A voltage divider of resistors  5  and  6  is connected across capacitor  16  to provide to the power controller  50  a divided voltage indicative of a lamp voltage being applied to the lamp. Based upon the lamp voltage, the power controller  50  gives the functions of: 
     1) activating the booster  20  to cause the starter  60  to generate the starting voltage for starting the lamp and subsequently activating the inverter  40  to apply the AC power for operating the lamp; and 
     2) detecting a no-load condition as indicative of the lamp having not being operated or extinguished when the lamp voltage does not decrease to a predetermined level, and subsequently starting the lamp; and 
     3) detecting a failure of starting the lamp as indicative of the lamp being disconnected or the lamp reaching its end of operation life when within a predetermined starting time period, i.e., when the no-load signal lasts over the predetermined period. For this purpose, the power controller  50  includes a timer that counts a predetermined starting period for determination of the failure of starting the lamp. 
     Operation of the ballast will be now discussed with reference to FIGS. 2 to  4 . Upon energization of the ballast at time T 0 , the inverter  40  is controlled to turn on the transistors  41  and  42  while keeping the other two transistors  43  and  44  turned off, as shown in FIG.  2 . During this starting time period, the DC output voltage V 16  of capacitor  16  of the converter  10  is applied through terminals X 1  and X 2  to the lamp L, and at the same time, the DC voltage from capacitor  25  of the booster  20  is applied through the terminals X 2  and X 3  to charge capacitor  64  of the starter  60 . As soon as capacitor  64  is charged up to the discharge starting voltage of the spark gap element  65 , the element  65  becomes conductive so that a pulse voltage V p is generated across the secondary winding  62  and is additive to the voltage V 16  to give the starting voltage. The starting voltage is applied to the lamp at time T 1  and T 2 , attempting to start the lamp. When the lamp is started successfully, the lamp voltage VL is lowered so that the power controller  50  can acknowledge the starting of the lamp. In this instance, the lamp is detected to start at time T 2  in FIGS. 2 and 3. Subsequently, the power controller  50  responds to turn on and off the transistors  43  and  44  alternately with transistors  41  and  42  so that the inverter  40  provides the AC power for continuing to operate the lamp. 
     Even if the lamp is disconnected, i.e., the lamp module is disconnected from the driver module at terminals X 1 , X 2 , and X 3 , the power controller  50  still activates the starter  60  to generate the starting voltage, attempting to start the absent lamp. That is, as shown in FIG. 4, voltage V 64  across the capacitor  64  repeats to increase up to the discharge starting voltage or the spark gap voltage VSG and drop to zero, failing to start the lamp. Thus, the lamp voltage VL is kept high, which is acknowledged by the controller  50 . If the lamp voltage is not lowered to such a level indicative of the lamp being started within the starting period TPX defined by the timer, the controller  50  determines that the lamp module is disconnected or the lamp reaches its end of operation life, and issues a disable signal at the end Tx of the starting time period TPX. The starting time period TPX is selected to be longer than a given period TP 1  within which the power controller  50  gives two or more chances of generating the starting voltage for starting the lamp. In response to the disable signal, the power controller  50  causes the transistors  41  and  42  to turn off, while keeping the transistors  43  and  44  turned on, thereby interrupting a closed loop of supplying the voltage from capacitor  25  to capacitor  64  through the terminals X 3  and X 2  and through the transistor  42 . Therefore, no current is fed to the starter to prevent the starter from generating the unnecessary starting voltage which would give undue stress to the components of the starter. This is also advantageous in protecting the personnel from a possible electrical hazard when the personnel touches the components of the starter  60  with the lamp itself disconnected and with the starter being kept connected to the ballast. Further, even if personnel should accidentally touch the terminals X 3  and X 2  simultaneously, no current path is formed through the personnel from capacitor  25  bearing the increased voltage, protecting the personnel from electrical shock hazard which would otherwise occur. 
     It is noted in this connection that the converter  10  may be deenergized in response to the disable signal. In such case, there still remains a danger of causing the above-mentioned electrical shock hazard due to the residual electrical charge in capacitor  25 . Therefore, the interruption of the discharge path from capacitor  25  is essential in preventing the above undue generation of the starting voltage as well as the shock hazard. 
     FIG. 5 shows a modification of the first embodiment which is identical to the first embodiment except that a winding  22 A of the booster  20 A is separately formed from the second winding  12 A of the converter  10 A. Like parts are designated by like numerals with a suffix letter of “A”. 
     FIG. 6 shows another modification of the first embodiment which is identical to the first embodiment except that the booster  20 B provides a voltage doubler  21  for developing the increased voltage across capacitor  25 B. Like parts are designated by like reference numerals with a suffix letter of “B”. The voltage doubler  21  comprises the winding  22 B, a diode  27  connected across the winding  22 B, and diode  24 B connected between the capacitor  25 B and the winding  22 B. The winding  22 B has a center tap which is connected in circuit such that capacitor  25 B is connected across each half segment of the winding through each of diodes  24 B and  27 , thereby accumulating the increased DC voltage across capacitor  25 B. 
     FIG. 7 shows a further modification of the first embodiment which is identical to the first embodiment except for the configuration of a booster  20 C. Like parts are designated by like numerals with a suffix letter of “C”. The booster  20 C is connected to receive the DC voltage from capacitor  16 C of the converter, and comprises a series combination of a resistor  31  and a capacitor  32  connected across capacitor  16 C, and a transformer with a primary winding  33  and a secondary winding  34 . The primary winding  33  is connected in series with a bi-directional thyristor  35  across capacitor  32 , while the secondary winding  34  is connected in series with a diode  36  across the capacitor  25 C. As the capacitor  32  is charged up to a break-over voltage of thyristor  33 , thyristor  33  becomes conductive to initiate an oscillation in a closed circuit of capacitor  32 , thyristor  33 , and primary winding  33 , thereby inducing across the secondary winding  34  a voltage which is rectified by diode  36  to accumulate the resulting DC voltage in capacitor  25 C connected between terminals X 2  and X 3 . Thus, capacitor  25 C provides the boosted DC voltage to the starter (not shown) through terminals X 2  and X 3 . The break-over voltage of thyristor  33  is determined to be higher than the output voltage from the converter  10 C when the inverter is active to operate the lamp such that thyristor  33  becomes conductive only during the starting period in which transistors  41 C and  42 C are turned on with the other transistors  43 C and  44 C being kept turned off. Therefore, once the lamp is started, thyristor  33  no longer becomes conductive to thereby lower the voltage of capacitor  25 C, making the starter inactive and therefore eliminating a possibility of generating unnecessary starting voltage thereat. 
     Second Embodiment 
     FIG. 8 shows a ballast in accordance with a second embodiment of the present invention which is identical to the first embodiment except that a booster  20 D is integrated in the inverter  40 D. Like parts are designated by like numerals with a suffix letter of “D”. The booster  20 D includes a series combination of a diode  37 , a resistor  31 D, and a capacitor  32 D connected across transistor  44 D, and a transformer with a primary winding  33 D and a secondary winding  34 D. The primary winding  33 D is connected in series with a bi-directional thyristor  35 D across capacitor  32 D, while the secondary winding  34 D is connected in series with a diode  36 D across the capacitor  25 D. While the transistor  41 D is on, capacitor  32 D is charged through diode  37  and resistor  31 D by the DC voltage from capacitor  16 D of the converter  10 D. As the capacitor  32 D is charged up to a break-over voltage of thyristor  33 D, thyristor  33 D becomes conductive to initiate an oscillation in a closed circuit of capacitor  32 D, thyristor  33 D, and primary winding  33 D, thereby inducing across the secondary winding  34 D a voltage which is rectified by diode  36 D to accumulate the resulting voltage in capacitor  25 D connected between terminals X 2  and X 3 . Thus, capacitor  25 D provides the boosted DC voltage to the starter (not shown) through terminals X 2  and X 3  during the starting period given by the power controller  50  where the transistors  41 D and  42 D are turned on with the other transistors  43 D and  44 D being kept turned off. 
     Since the capacitor  32 D is charged sufficiently to initiate the oscillation and give the increased DC voltage to capacitor  25 D only during the starting period in which the transistor  41 D is kept turned on for a long while, capacitor  32 D is not charged sufficiently after the inverter  40 D begins operating the lamp by turning on and off the transistors  41 D and  42 D alternately with transistors  43 D and  44 D. Thus, the booster  20 D can be inactivated once the lamp is started, and such inactivation of the booster can be made without requiring any additional circuit component. 
     It is noted in this connection that thyristor  33 D may be selected to have a break-over voltage higher than the output voltage from the converter  10 C when the inverter  40 D is active to operate the lamp, as made in the circuit of FIG.  7 . 
     Also, in this embodiment, in response to the disable signal from the power controller, transistors  41 D and  42 D are turned off with transistors  43 D and  44 D being kept turned off, whereby interrupting a discharge path of capacitor  25 D between terminals X 3  and X 2 , as well as another discharge path of capacitor  25 D between terminals X 3  and X 1 . 
     FIG. 9 shows a modification of the second embodiment which is identical to the second embodiment except for connections of the booster  20 E with the inverter  40 E. Like parts are designated by like reference numerals with a suffix letter of “E”. The booster  20 E includes a series combination of a diode  37 E, a resistor  31 E, and a capacitor  32 E connected across a series combination of transistors  44 E and  42 E, and a transformer with a primary winding  33 E and a secondary winding  34 E. The primary winding  33 E is connected in series with a bi-directional thyristor  35 E across capacitor  32 E, while the secondary winding  34 E is connected in series with a diode  36 E across the capacitor  25 E. While the transistors  41 D and  42 E are on, capacitor  32 E is charged through diode  37 E and resistor  31 E by the DC voltage from capacitor  16 E of the converter  10 E. As capacitor  32 E is charged up to a break-over voltage of thyristor  33 E, thyristor  33 E becomes conductive to initiate an oscillation in a closed circuit of capacitor  32 E, thyristor  33 E, and primary winding  33 E, thereby inducing across the secondary winding  34 E a voltage which is rectified by diode  36 E to accumulate the resulting voltage in capacitor  25 E connected between terminals X 2  and X 3 . Thus, capacitor  25 E provides the boosted DC voltage to the starter (not shown) through terminals X 2  and X 3  during the starting period given by the power controller where the transistors  41 E and  42 E are turned on with the other transistors  43 E and  44 E being kept turned off. 
     Third Embodiment 
     FIG. 10 shows a ballast in accordance with a third embodiment which is identical to the second embodiment except that a booster  20 F is connected differently with the inverter  40 F. Like parts are designated by like numerals with a suffix letter of “F”. The booster  20 F comprises a series combination of a diode  37 F, a resistor  31 F, and a capacitor  32 F connected across a transistors  43 F, and a transformer with a primary winding  33 F and a secondary winding  34 F. The primary winding  33 F is connected in series with a bi-directional thyristor  35 F across capacitor  32 F, while the secondary winding  34 F is connected in series with a diode  36 F across the capacitor  25 F. While the transistor  42 F are on, capacitor  32 F is charged through diode  37 F and resistor  31 F by the DC voltage from capacitor  16 F of the converter  10 F. As capacitor  32 F is charged up to a break-over voltage of thyristor  33 F, thyristor  33 F becomes conductive to initiate an oscillation in a closed circuit of capacitor  32 F, thyristor  33 F, and primary winding  33 F, thereby inducing across the secondary winding  34 F a voltage which is rectified by diode  36 F to accumulate the resulting voltage in capacitor  25 F connected between terminals X 2  and X 3 . Thus, capacitor  25 F provides the boosted DC voltage to the starter (not shown) through terminals X 2  and X 3  during the starting period given by the power controller where the transistors  41 F and  42 F are turned on with the other transistors  43 F and  44 F being kept turned off. Upon occurrence of the disable signal, transistors  41 F and  42 F are turned off with transistors  43 F and  44 F being kept turned off, whereby interrupting a discharge path of capacitor  25 F between terminals X 3  and X 2 , as well as another discharge path of capacitor  25 F between terminals X 3  and X 1 . 
     FIG. 11 shows a modification of the third embodiment which is identical to the third embodiment except for removal of diode  37 F from the circuit of FIG.  10 . Like parts are designated by like reference numerals with a suffix letter of “G”. Due to the removal of diode, once the lamp is started and operated by turning on and off transistors  41 G and  42 G alternately with transistors  43 G and  44 G, capacitor  32 G repeats being charged and discharged so as not to reach the break-over voltage of thyristor  35 G, thereby prohibiting capacitor  25 G from accumulating the boosted voltage. Thus, the booster  20 G is substantially inactivated after the lamp is started. 
     Fourth Embodiment 
     FIG. 12 shows a ballast in accordance with a fourth embodiment of the present invention which is basically identical to the first embodiment except that capacitor  25 H of the booster  20 H is connected in series with capacitor  16 H of the converter  10 H between terminals X 3  and X 2  through transistor  42 H and also between terminals X 3  and X 1  through transistor  44 H. The converter  10 H includes a transformer with a primary winding  11 H and a secondary winding  12 H which is electrically isolated from the primary winding but is magnetically coupled thereto. The primary winding  11 H is connected in series with a switch  14 H across the DC source  1 , while the capacitor  16 H is connected in series with a diode  15 H across the secondary winding  12 H. The switch  14 H is controlled by the power controller  50 H to repetitively turn on and off for inducing across the secondary winding a corresponding voltage which is rectified by diode  15 H to charge capacitor  16 H. The booster  10 H includes an auxiliary winding  22 H which is magnetically coupled to the primary winding  11 H to induce thereacross a voltage which is rectified by diode  24 H to charge capacitor  25 H. Thus, the capacitor  25 H is additive to capacitor  16 H to provide the boosted voltage to the starter  60 H for charging capacitor  64 H. This means that capacitor  25 H can be selected to be smaller to accumulate less voltage than the like capacitor utilized in the previous embodiments. 
     The connection between capacitors  25 H and  16 H are grounded so that only a portion of the boosted voltage, i.e., voltage only from capacitor  25 H may be responsible for flowing a short-circuit current if a grounded personnel should touch a single terminal X 3 . Thus, an electrical shock hazard in this condition can be weakened as compared to the case where capacitor  16 H would be grounded at the opposite end to flow a corresponding short-circuit current from the combination of capacitors  25 H and  16 H. 
     In this embodiment, a current sensing resistor  7  is provided between the converter  10 H and the inverter  40 H instead of the voltage divider as utilized in the previous embodiments. The resistor  7  is connected to give the lamp current to a power controller  50 H such that the power controller  50 H can responds to start and operate the lamp based upon the level of the monitored lamp current, and that the controller  50 H can determine the failure of starting the lamp and issue the disable signal indicative of that the lamp is disconnected or damaged. A filter  70  is provided between the inverter  40 H and the terminals X 1  and X 2 . 
     Fifth Embodiment 
     FIG. 13 shows a ballast in accordance with a fifth embodiment of the present invention which is identical to the first embodiment except that a capacitor  16 J of the converter  10 J is grounded at its positive side. Like parts are designated by like numerals with a suffix letter of “J”. The converter  10 J includes a transformer with a primary winding  11 J and a secondary winding  12 J which is electrically isolated from the primary winding but is magnetically coupled thereto. The primary winding  11 J is connected in series with a switch  14 J across the DC source  1 , while the capacitor  16 J is connected in series with a diode  15 J across the secondary winding  12 J. The switch  14 J is controlled by the power controller  50 J to repetitively turn on and off for inducing across the secondary winding a corresponding voltage which is rectified by diode  15 J to charge capacitor  16 J. The booster  10 J includes an auxiliary winding  22 J which is magnetically coupled to the primary winding  11 J to induce thereacross a voltage which is rectified by diode  24 J to charge capacitor  25 J. The connection between diode  15 J and capacitor  16 J is grounded such that capacitor  25 J may be connected in anti-series with capacitor  16 J. Thus, if a grounded personnel should touch terminal X 3 , only capacitance from capacitor  26 J minus capacitor  16 J would responsible for flowing a current through the personnel, reducing the corresponding electrical shock hazard. 
     The ballast of the present embodiment further includes an auxiliary starter  80  between the converter  10 J and the inverter  40 J. The auxiliary starter  80  includes a series combination of a resistor  81 , a diode  82 , and a capacitor  83  connected across capacitor  16 J, and a resistor connected across the series combination of resistor  81  and diode  82 . Thus configured auxiliary starter  80  functions to release capacitance from capacitor  83  to the lamp upon discharge of the lamp, thereby facilitating to start the lamp. 
     Although in the above embodiments and modifications, switching transistors  41  and  42  are kept turned on while transistors  43  and  44  are kept turned off during the starting period, it is possible that the transistors  41  and  42  are turned on and off alternately with transistors  43  and  44  provided that transistor  42  is turned off in response to the disable signal, i.e., a detection of the lamp failing to start during the period. 
     Further, although the DC-to-DC converter of fly-back type is utilized in the above embodiments and modification, other types of DC-to-DC converter may be equally utilized as shown in FIGS. 14 and 15. The converter of FIG. 14 includes a series combination of an inductor  91  and a switching transistor  92  connected across the DC source  1 , and a capacitor  94  connected in series with a diode  93  across the transistor  92 . Capacitor  94  provides an output voltage to the inverter  40 . The converter of FIG. 15 includes a series combination of a switching transistor  95  and a diode connected across the DC source  1 , and a capacitor  98  connected in series with an inductor  97  across diode  96 . Capacitor  98  provides an output voltage to the inverter  40 .