the current driven toroid - free feedback type ballast of the present invention will be described in further details below with reference to the accompany drawings . please refer to fig2 , which illustrates a current driven toroid - free feedback type ballast according to 1 st embodiment of the present invention comprising a filter and rectifier circuit 10 , a switch and resonant circuit 20 and a lamp load 30 having capacitors , and the same is now described hereunder . the filter and rectifier circuit 10 is coupled to input ends of the switch and resonant circuit 20 with its output ends , and being further coupled to an ac power supply to convert input ac voltage to dc ripple voltage after filtering out the electromagnetic interference thereof . in the embodiment , the filter and rectifier circuit 10 is a full bridge rectifier circuit comprising a bridge rectifier ( d 1 - d 4 ), a filter comprised of an inductor l 0 and a resistor r 0 in shunt connection and an electrolyte capacitor c 1 connected across terminals 1 and 3 of the bridge rectifier ; the filter is coupled with the ac power supply at one end via a fuse fu resistor while coupling with terminal 4 of the bridge rectifier at another end . the switch and resonant circuit 20 is coupled to the lamp load 30 with its output ends and including : a trigger circuit comprised of resistors r 1 , r 2 , a capacitor c 3 , a diode d 5 and a trigger diode db 3 to provide a pulse current for initiation of the operation of the switch and resonant circuit 20 ; a half bridge circuit comprised of transistors q 1 , q 2 and resistors for acting as a power switch ; and a three winding transformer t comprised of a primary winding t 3 and two secondary windings t 1 , t 2 , wherein the primary winding t 3 also possesses a choking effect . preferably , the ratio of winding between the primary winding and the secondary winding of the three winding transformer might range from 30 : 1 to 400 : 1 . in the switch and resonant circuit 20 , the emitter of transistor q 1 is coupled with collector of q 2 via a resistor r 5 ; a junction s is located between the resistor r 5 and the collector of the transistor q 2 ; the resistor r 1 and a capacitor c 2 in shunt connection are provided between collector of the transistor q 1 and the junction s ; the resistor r 1 is coupled to terminal 3 of the rectifier of the filter and rectifier circuit 10 with its one end and coupled to terminal 1 of the rectifier of the filter and rectifier circuit 10 with its another end through the resistor r 2 and the shunted diode d 5 in series connection with the capacitor c 3 ; the diode d 5 is coupled to base of the transistor q 2 with its anode via the bilateral diode db 3 and coupled to terminal 4 of the secondary winding t 1 and terminal 1 of the primary winding t 3 with its cathode through the junction s ; the base of the transistor q 1 is coupled to terminal 3 of the secondary winding t 1 through a resistor r 3 , and the base of the transistor q 2 is coupled to terminal 6 of the secondary winding t 2 through a resistor r 4 ; emitter of the transistor q 2 is coupled to terminal 5 of the secondary winding t 2 through a resistor r 6 , while the terminal 5 of the secondary winding t 2 is coupled with terminal 1 of the rectifier of the filter and rectifier circuit 10 for enabling the secondary windings t 1 , t 2 to provide a drive current for the transistors q 1 , q 2 ; terminal 2 of the primary winding t 3 is coupled to the lamp load 30 and a capacitor c 5 for enabling the primary winding t 3 and the capacitor c 5 to form a series resonance . the lamp load 30 comprises a lamp tube and capacitors c 4 , c 5 wherein the capacitor c 4 is used for dc blocking ; and at both ends of the lamp tube two connection points a , b , a ′, b ′ are respectively provided , the capacitor c 5 in shunt connection with the lamp tube is connected across one connection point b , b ′ at both ends of the lamp tube ; another connection point a ′ at one end of the lamp tube is coupled with the terminal 2 of the primary winding t 3 , while another connection point a at another end of the lamp tube is coupled with the collector of the transistor q 1 via the capacitor c 4 . according to one preferred embodiment , the capacitor c 5 is further in shunt connection with a ptc preheating device . please refer to fig3 , a current driven toroid - free feedback type ballast according to 2 rd embodiment of the present invention is illustrated , which comprises the entire circuit of the first embodiment but the switch and resonant circuit further comprises a resonant capacitor c 6 having its one end coupled with the terminal 2 of the secondary winding t 3 and its another end coupled with the terminal 5 of the secondary winding t 2 . please refer to fig4 , a current driven toroid - free feedback type ballast according to 3 rd embodiment of the present invention is illustrated , wherein the switch and resonant circuit 20 and the lamp load 30 are the same as the first embodiment . however , the rectifier circuit in the filter and rectifier circuit 10 of the present embodiment is a voltage multiplying rectifier circuit formed with two diodes and two capacitors . please refer to fig5 , a current driven toroid - free feedback type ballast according to 4 th embodiment of the present invention is illustrated , which comprises the entire circuit of the third embodiment but the switch and resonant circuit 20 further comprises a resonant capacitor c 6 which is coupled therewith in the same manner as in the second embodiment . please refer to fig6 , a current driven toroid - free feedback type ballast according to 5 th embodiment of the present invention is illustrated , which comprises the entire circuit of the first embodiment and further comprises a power factor correction circuit 40 . it should be noted that the necessity of the arrangement of the optional power factor correction circuit 40 depends on the power to be attained by the current driven toroid - free feedback type ballast . the circuit 40 is coupled to the output end of the filter and rectifier circuit 10 with its input end and coupled to the input end of the switch and resonant circuit 20 with its output end . the power factor correction circuit 40 comprises a mosfet vt 1 , a booster inductor l , a booster diode vd , an output capacitor c 0 and an apfc controller integrated circuit ; wherein the booster inductor l is coupled to terminal 3 of bridge rectifier with one end and coupled to the collector of the transistor q 1 with another end through the booster diode vd ; the booster diode vd is coupled with terminal 1 of the bridge rectifier at its cathode via the output capacitor c 0 and coupled with the terminal 1 of the bridge rectifier via the mosfet vt 1 , while the gate of the mosfet vt 1 is coupled to the apfc . please refer to fig7 , a current driven toroid - free feedback type ballast according to 6 th embodiment of the present invention is illustrated , which comprises the entire circuit of the fifth embodiment but the switch and resonant circuit 20 further comprises a resonant capacitor c 6 being coupled therewith in the same manner as in the second embodiment . please refer to fig8 , a current driven toroid - free feedback type ballast according to 7 th embodiment of the present invention is illustrated , which comprises the entire circuit of the third embodiment but it further comprises a power factor correction circuit 40 being coupled therewith in the same manner as in the fifth embodiment . please refer to fig9 , a current driven toroid - free feedback type ballast according to 8 th embodiment of the present invention is illustrated , which comprises the entire circuit of the seventh embodiment but the switch and resonant circuit 20 further comprises a resonant capacitor c 6 being coupled therewith in the same manner as in the second embodiment . the working principle of the present invention is as follows . after connecting with the power supply , the current from the dc voltage charge up the integrating capacitor c 3 after passing through the resistors r 1 , r 2 of the trigger circuit , once the voltage thereof reaches or exceeds the breakdown voltage of the trigger diode db 3 ( about 30 - 40v ), the trigger diode will conduct in reverse such that a current flows into the base of the transistor q 2 and turns on q 2 . as the collector current of the transistor q 2 is increasing , induced electrodynamic potentials are generated on the primary winding t 3 and also the secondary windings t 1 , t 2 of the transformer , ( wherein the ends denoted with • represent a positive polarity ) such that the base potential of q 2 rises whereby the base current and collector current thereof is further increased which in turns boost further the base potential . in this regard , chain reaction takes place in the circuit and such chained positive feedback makes q 2 conducting and saturating . the resistor r 6 at emitter of q 2 is provided for effecting negative current feedback , and in the process of the chain reaction , the increase of base current also makes the voltage drop on r 6 increasing and the increased portion of the voltage drop will be looped back to the base - emitter loop of the q 2 whereby the voltage applied from the outside of the base - emitter of the q 2 is decreased and the base current will be also automatically reduced , which in turns restrain the increase of the collector current . by increasing the resistance of the resistor r 6 at the emitter , the negative feedback action thereof will be enhanced such that the working frequency can be increased . after the transistor q 2 is turned on , along with the increase of the collector current passing through the transistor q 2 , there are circumstances that the voltage on the secondary winding t 2 of the transformer will possibly drop below the base voltage of the transistor q 2 such that the base current is reversed whereby the transistor q 2 quits from its saturated mode and enters into amplifying mode . once it enters into the amplifying mode , the decrease in the current flowing through the collector of the transistor q 2 will reduce the base current with the positive feedback of the transformer such that the collector current is further decreased and q 2 will be soon entering cutoff mode while the polarity of the voltage on the secondary winding t 1 of the transformer is changed ( now terminal 3 is positive and terminal 4 is negative ). after delaying for a period of time , a current occurs in the transistor q 1 , the transformer will generate an induced electrodynamic potential being opposite to the same generated from the increase of the collector current of q 2 such that the base and collector current of q 1 is further increased and q 1 will quickly be changed from cutoff state to turnon state . the resistor r 5 at emitter of q 1 is also provided for effecting negative current feedback and the working principle thereof is the same as r 6 , the resistors r 5 , r 6 are equal in their resistance . the above process moves in cycles , wherein q 1 and q 2 are alternatively turned on and off . in the middle point between two half bridge an alternating square wave voltage is formed . after passing through the capacitor c 5 and being affected by the action of the series resonant of the primary winding t 3 of the transformer , the waveform of such alternating voltage will be changed to resemble a sine wave , and a very high voltage is generated on c 5 and then applied to the lamp tube for the ignition of the same . it should be appreciated that the above are only provided for illustrating but not limiting the technical solutions of the present invention . while the present invention has been described in details with references to above embodiments , it will be understood by those skilled in the art that various changes , additions or deletions may be made and equivalents may be substituted for elements thereof without departing from the spirit and scope of the invention and its claims , and all such alteration and / or modification shall fall into the scope of the present invention .