in fig4 ( a )- 4 ( b ), schematic circuit diagrams of a dc / ac inverter and a controller circuit of a ballast circuit according to the first preferred embodiment of the present invention are shown respectively . in fig4 ( a ), only the dc / ac inverter of the second stage of the ballast circuit , the discharge lamp and the controller circuit are shown . the dc / ac inverter is a half - bridge circuit acting as a double down - converter . the double down - converter circuit includes a first mosfet s 1 , a second mosfet s 2 , a first and second body diodes d 1 and d 2 , an inductor l 2 connected in series with the discharge lamp , a capacitor c 2 connected in parallel with the discharge lamp , and two electrolytic bridge capacitors ch 1 and ch 2 connected in series . the double down - converter circuit operates in the critical continuous mode with the controller , e . g ., l6562 . as shown in fig4 ( b ), the controller circuit includes an rc filter having a resistor r 3 and a capacitor c 3 wherein the resistor r 3 having a first terminal to receiving a first driving signal ( in the first preferred embodiment , it is a square wave driving signal ) and a second terminal connected to a first terminal of the capacitor c 3 , a mcu having an analog - to - digital input terminal a / d and a digital - to - analog output terminal d / a , a first control circuit dcmb ( discontinuous conduction mode boundary ) controller generating the first driving signal , and a driver receiving the first driving signal and generating a second and a third driving signals ( in the first preferred embodiment , they are switch driving signals of s 1 and s 2 ). the duty ratio of the square wave driving signal generated by the dcmb controller reflects the value of the discharge lamp voltage v lamp . under the dcmb condition , based on the voltage - second balance theory , the following relation can be obtained : ( v dc / 2 − v lamp )* duty ratio =( v dc / 2 + v lamp )*( 1 − duty ratio ) ( 2 ) therefore , the relationship between the discharge lamp voltage and the duty ratio is rewritten in equation ( 3 ). after the aforementioned rc filter ( r 3 / c 3 ) receives and filters the square wave driving signal the discharge lamp voltage can be sensed indirectly . since the voltage magnitude of the driving signal is usually a low voltage e . g . 15v , so the rc filter is very simple and low cost . with this indirectly sensing method , the cost reduction , high reliability and small sizes of the voltage sensing apparatus can all be achieved . in the above - mentioned first preferred embodiment , the double down - converter operates in the dcmb mode . this indirect sampling method can also be applied on the other converter operates in other mode only if the definite relationship between the switch duty cycle and the discharge lamp voltage exists . for example , when the double down - converter operates in the continuous conduction mode ( ccm ) mode , the relationship between the switch duty cycle and the lamp voltage can also be expressed by equation ( 3 ), thus the indirect sampling method can also be used . fig5 ( a )- 5 ( b ) show schematic circuit diagrams of a dc / ac inverter and a controller circuit of a ballast circuit for the hid lamp according to the second preferred embodiment of the present invention respectively . in fig5 ( a ), the two electrolytic bridge capacitors ch 1 and ch 2 as shown in fig4 ( a ) are replaced by two mosfets q 1 and q 2 and their body diodes d 3 and d 4 . the remaining portions of fig5 ( a )- 5 ( b ) are the same as those of fig4 ( a )- 4 ( b ). and the two mosfets q 1 and q 2 operate at lower frequency , e . g ., the commutation frequency , while the two mosfets s 1 and s 2 operate at higher frequency . in the above - mentioned first and second preferred embodiments , the second stage of the ballast converter circuit operates under the analog control method . in fact , some ballast circuits adopt the totally digital control method as shown in fig6 ( a )- 6 ( b ). fig6 ( a )- 6 ( b ) show schematic circuit diagrams of a dc / ac inverter and a controller circuit of a ballast according to the third preferred embodiment of the present invention respectively . the schematic circuit diagram of the dc / ac inverter as shown in fig6 ( a ) is the same as that of fig4 ( a ). the mcu and the dcmb controller shown in fig4 ( b ) and 5 ( b ) are replaced by a digital controller in fig6 ( b ), wherein the digital controller having an analog - to - digital input terminal a / d and a pulse - width modulation output terminal pwm . in fig6 ( b ), the digital controller calculates and outputs the square driving signal to control s 1 and s 2 according to the typical control strategy for the discharge lamps shown in fig1 . surely , the digital controller as shown in fig6 ( b ) can also directly attain the signal reflecting the discharge lamp voltage v lamp via calculating the duty ratio of the switch driving signals of s 1 and s 2 instead of getting the discharge lamp voltage v lamp by sensing the square wave signal via the voltage sensing apparatus r 3 and c 3 . thus , the proposed indirect lamp voltage sensing method can also be used under digital control . fig7 ( a )- 7 ( b ) show schematic circuit diagrams of a ballast converter circuit and a controller circuit according to the fourth preferred embodiment of the present invention respectively . in fig7 ( a ), the ballast converter circuit is a three - stage converter , which includes a pfc circuit , a buck converter and a full - bridge inverter . the pfc circuit includes an inductor l 1 , a switch s 1 , a diode d 1 and a capacitor c 1 . the buck converter includes an inductor l 2 , a switch s 2 , a diode d 2 and a capacitor c 2 . the full - bridge inverter includes switches s 3 - s 6 and an igniter . the basic configuration of fig7 ( b ) is the same as those of fig4 ( b ) and 5 ( b ), and the only difference is that a dcmb controller , e . g . l6562 , controls the buck converter instead of the full - bridge inverter . in the prior art , the discharge lamp voltage is attained by sensing the voltage across capacitor c 2 in fig7 ( a ). and since the duty ratio of the driver of the switch s 2 has a certain relationship with the voltage across c 2 , the discharge lamp voltage can be indirectly sensed through the duty ratio of s 2 . surely , the configuration of fig7 ( b ) can also be the same as that of fig6 ( b ). in the above - mentioned first to third preferred embodiments , the dc / ac inverter accepts the constant output voltage of the pfc circuit as its input . in fact , the output voltage of the pfc circuit can also be varied , e . g ., having a higher output voltage during the ignition state for easier ignition and a lower output voltage for high efficiency of the dc / ac inverter in the normal operation mode or causing the output voltage of the pfc circuit to vary in proportional to the input voltage of the pfc circuit . and since the relationship between the duty cycle of switches and the discharge lamp voltage can still be expressed , this indirect sampling method can still be adopted . according to the aforementioned descriptions , the present invention provides a ballast converter having a voltage sensing apparatus and a controlling method thereof , which employs an rc filter to obtain a duty ratio of a switch driving signal of the ballast circuit to sense a discharge lamp voltage indirectly so as to achieve the power regulation and the over - voltage protection of the discharge lamp . and the provided voltage sensing apparatus possesses the advantages of lower cost , higher reliability and smaller sizes since the rated voltage of the switch driving signal received by the rc filter is relatively lower . thus , the present invention indeed possesses the non - obviousness and the novelty . 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 limited 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 present invention which is defined by the appended claims .