Abstract:
A discharge lamp driver circuit is provided which features a field canceller. The driver circuit includes a power supply circuit which turns on and off a switching element to step up a dc voltage and provide it for turning on a discharge lamp. The power supply circuit includes an electrical path through which an interrupted current arising from the on-off operation of the switching element flows. The field canceller includes an electric line through which the interrupted current having passed through the electrical path flows in an opposite direction, thereby producing a field canceling a field caused by flow of the interrupted current through the electrical path.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Technical Field of the Invention  
           [0002]    The present invention relates generally to a discharge lamp driver circuit working to turn on a discharge lamp, and more particularly to a noise canceller structure of such a discharge lamp driver circuit which is designed to minimize radiation of noises arising from a switching operation of the driver circuit.  
           [0003]    2. Background Art  
           [0004]    [0004]FIG. 7 shows a typical discharge lamp driver circuit  100  for automotive vehicles which includes a filter circuit  110 , a DC/DC converter  120 , an inverter  130 , and a control circuit  150 . The discharge lamp driver circuit  100  works to step up a dc voltage supplied from a storage battery  10  through the DC/DC converter  120  when a lighting switch  20  is turned on and converts it into an ac voltage through the inverter  130  to initiate a discharge in a lamp  30 .  
           [0005]    The lamp  30  is a discharge lamp such as a metal halide lamp typically used as a headlamp of the vehicle. Starting the lamp  30  is achieved by inducing a dielectric breakdown through a transformer (not shown) of a starter circuit to develop a high voltage between electrodes of the lamp  30 . After the dielectric breakdown, the status of the lamp  30  is shifted from a glow discharge to an arc discharge to keep the lamp  30  lightened stably.  
           [0006]    The filter circuit  110  consists of a coil  111 , a capacitor  112 , and a capacitor  113  and works as a noise filter.  
           [0007]    The DC/DC converter  120  consists of a transformer  121  made up of a primary winding  121   a  connected to the battery  10  and a secondary winding  121   b  connected to the lamp  30 , a MOS transistor (field-effect transistor)  122  connected to the primary winding  121   a,  rectifier diode  123 , and a smoothing capacitor  124  and works to step up and output the voltage from the battery  10 . Specifically, when the MOS transistor  122  is turned on, it will cause a primary current to flow through the primary winding  121   a  so that energy is accumulated in the primary winding  121   a.  When the MOS transistor  122  is turned off, it will cause the energy in the primary winding  121   a  to be supplied to the secondary winding  121   b.  Such turning on and off the MOS transistor  122  is repeated, thereby causing a high voltage to be outputted from a junction of the diode  123  and the smoothing capacitor  124 . The transformer  121  may alternatively be so constructed that the primary and secondary windings  121   a  and  121   b  are electrically connected to each other.  
           [0008]    The inverter  130  includes MOS transistors (not shown) arrayed in the form of an H-bridge which work to provide the ac current for turning on the lamp  30 .  
           [0009]    The control circuit  150  is responsive to a signal (lamp power signal) provided by a power detector (not shown) as functions of a lamp current and a lamp voltage to control the MOS transistor  122  in a PWM mode so as to bring the lamp power into agreement with a maximum (e.g., 65 W) when turning on the lamp  30  and with a constant power (e.g., 35 W) subsequently.  
           [0010]    The control circuit  150  consists of a gate control circuit  150   a  controlling the on-off operation of the MOS transistor  122  in the PWM mode, the power detector detecting the lamp voltage, and a lamp power control circuit (not shown) controlling the lamp power to bring it into agreement with a target one based on the detected lamp current and voltage.  
           [0011]    In operation, when the lighting switch  20  has been turned on, and the control circuit  150  has started to control the MOS transistor  122  in the PWM mode, the DC/DC converter  120  outputs the voltage produced by stepping up the voltage of the battery  10  through the transformer  121 . The high-voltage produced by the DC/DC converter  120  (300V to 500V in the course of preparation for turning on the lamp  30 , and about 100V after turning on the lamp  30 ) is further stepped up to, for example, 25 kV through the inverter  130  so that the dielectric breakdown may occur in the transformer of the starter circuit and applied to the lamp  30 . This causes the lamp  30  to be turned on. After turning on the lamp  30 , the polarity of the voltage to be outputted by the inverter  130  is reversed cyclically to provide the ac voltage to the lamp  30 .  
           [0012]    The above structure of the discharge lamp driver circuit  100  has a drawback in that interrupted currents arising from the on and off operations of the MOS transistor  122  of the DC/DC converter  120  to step up the voltage of the battery  10  result in radiation of noises.  
           [0013]    The interrupted currents flow through three electrical loops: a first electrical path Lp 1  extending from the capacitor  113  through a power source positive line to the primary winding  121   a  of the transformer  121  to a drain and a source of the MOS transistor  122  and back to the capacitor  113  through a ground line, a second electrical path Lp 2  extending from the rectifier diode  123  to the smoothing capacitor  124  to the ground line to the secondary winding  121   b  and back to the rectifier diode  123 , and a third electrical path Lp 3  extending from the gate control circuit  150   a  to the gate of the MOS transistor  122  to the ground line and back to the gate control circuit  150   a.  The first, second, and third electrical paths Lp 1 , Lp 2 , and Lp 3  carry currents i 1 , i 2 , and i 2  arising from the on and off operations of the MOS transistor  122  by the gate control circuit  150   a.    
           [0014]    Particularly, in a case where the above structure of the discharge lamp driver circuit  100  is installed in an automotive vehicle for lighting headlamps, when a traffic light has changed to red, and the vehicle has stopped close to an antenna installed in the rear of a preceding vehicle, it may cause electric noises to be radiated forwardly, which raise a radio disturbance in the preceding vehicle.  
         SUMMARY OF THE INVENTION  
         [0015]    It is therefore a principal object of the invention to avoid the disadvantages of the prior art.  
           [0016]    It is another object of the invention to provide a discharge lamp driver circuit designed to minimize adverse effects caused by interrupted currents produced in the driver circuit.  
           [0017]    According to one aspect of the invention, there is provided a discharge lamp driver circuit which may be employed in turning on a discharge lamp as used as a headlamp of automotive vehicles. The discharge lamp driver circuit comprises: (a) a power supply circuit connected to a de power supply; and (b) a field canceller. The power supply circuit includes a switching element and performs an on-off operation on the switching element to step up a dc voltage from the dc power supply and provide the stepped up dc voltage for turning on a discharge lamp. The power supply circuit includes an electrical path through which an interrupted current arising from the on-off operation of the switching element flows. The field canceller includes an electrical line through which the same interrupted current as that flowing through the electrical path of the power supply circuit flows, thereby producing a field canceling a field caused by flow of the interrupted current through the electrical path. This causes electrical noises radiated outside from the electrical path of the power supply circuit to be eliminated.  
           [0018]    In the preferred mode of the invention, the power supply circuit includes a DC/DC converter. The DC/DC converter consists of a transformer made up of a primary winding connected to the dc power supply and a secondary winding connected to the discharge lamp and the switching element and works to turning on and off the switching element to provide the stepped up dc voltage to the discharge lamp through the transformer.  
           [0019]    The electrical line of the field canceller is connected in series with the electrical path of the power supply circuit and extends so as to have the interrupted current bypass the electrical path in an orientation opposite flow of the interrupted current through the electrical path. This causes the field to be produced by the field canceller which is identical in strength and 180° out of phase with the field arising from the interrupted current flowing through the electrical path.  
           [0020]    The electrical line of the field canceller may be so geometrically shaped as to have an area surrounded by the electrical line which is substantially identical with an area surrounded by the electrical path of the power supply circuit. The field provided by the field canceller will, thus, be identical in strength with that produced around the electric path, thereby resulting in complete cancellation of noises arising from the field produced around the electrical path.  
           [0021]    The electrical line of the field canceller may be laid over the electrical path of the power supply circuit so that a magnetic flux produced by the electric line of the field canceller overlaps a magnetic flux produced by the electrical path of the power supply circuit, thereby promoting cancellation of noises arising from the field produced around the electric path.  
           [0022]    The field canceller includes an insert molded body within which the electrical line is disposed.  
           [0023]    The field canceller may be implemented by a flexible substrate. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0024]    The present invention will be understood more fully from the detailed description given hereinbelow and from the accompanying drawings of the preferred embodiments of the invention, which, however, should not be taken to limit the invention to the specific embodiments but are for the purpose of explanation and understanding only.  
         [0025]    In the drawings:  
         [0026]    [0026]FIG. 1 is a circuit diagram which shows a discharge lamp driver circuit according to the first embodiment of the invention;  
         [0027]    [0027]FIG. 2 is a partially cutaway view which shows the discharge lamp driver circuit of FIG. 1 installed in a casing;  
         [0028]    [0028]FIG. 3 is a partially cutaway view which shows a discharge lamp driver circuit according to the second embodiment of the invention is installed in a casing;  
         [0029]    [0029]FIG. 4 is an illustration which shows a positional relation between a magnetic flux arising from an on-off operation of a switching element and a magnetic flux working to cancel the former;  
         [0030]    [0030]FIG. 5 is a circuit diagram which shows a discharge lamp driver circuit in a first modification of the invention;  
         [0031]    [0031]FIG. 6 is a circuit diagram which shows a discharge lamp driver circuit in a second modification of the invention;  
         [0032]    [0032]FIG. 7 is a circuit diagram which shows a typical discharge lamp driver circuit; and  
         [0033]    [0033]FIG. 8 is a partially cutaway view which shows the discharge lamp driver circuit of FIG. 7 installed in a casing. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0034]    Referring to the drawings, wherein like reference numbers refer to like parts in several views, particularly to FIG. 1, there is shown a discharge lamp driver circuit  100  according to the first embodiment of the invention which may be employed in turning on headlamps of an automotive vehicle. The same reference numbers as employed in FIG. 7 will refer to the same parts, and explanation thereof in detail will be omitted here.  
         [0035]    The discharge lamp driver circuit  100  of this embodiment is different from the one shown in FIG. 7 in structure of the DC/DC converter  120 . Specifically, the DC/DC converter  120  includes, as shown in FIG. 2, a field canceller  127  which works to cancel a magnetic field produced by the interrupted current i 1  which arises, as already described in the introductory part of this application, from on and off operations of the MOS transistor  122  and flows through the first electrical path Lp 1 .  
         [0036]    The field canceller  127  is, as clearly shown in FIG. 1, made of a circuit line  127   a  which is connected in series with the first electrical path Lp 1  and has the interrupted current i 1  bypass the first electrical path Lp 1 . In the conventional structure as illustrated in FIG. 7, the first electrical path Lp 1 , as discussed above, extends from the capacitor  113  to the primary winding  121   a  of the transformer  121  to the drain and source of the MOS transistor  122  and back to the capacitor  113 , however, the first electrical path Lp 1  of this embodiment extends from the capacitor  113  to the primary winding  121   a  of the transformer  121  to the drain and source of the MOS transistor  122  and to a junction  210 . The circuit line  127   a  extends from the junction  210  to the capacitor  113  along the first electrical path Lp 1  and is so laid that a magnetic flux produced by the circuit line  127   a  may overlap with that produced by the first electrical path Lp 1  vertically.  
         [0037]    The circuit line  127   a  works to carry a current which is identical in scale with the interrupted current i 1  and oriented, as indicated by a dashed line in FIG. 1, in a direction opposite the flow of the interrupted current i 1  to produce a field which is identical in strength and 180° out of phase with the field arising from the interrupted current i 1 , thereby canceling noises radiated by the on and off operations of the MOS transistor  122  controlled by the gate control circuit  150   a.    
         [0038]    The circuit line  127   a  of the field canceller  127  may be provided flush with the first electrical path Lp 1  or on a plane which is in the proximity of the first electrical path Lp 1  and different in level from a plane containing the flow of the interrupted current i 1  through the first electrical path Lp 1 .  
         [0039]    [0039]FIG. 2 is a partially cutaway view which shows an internal structure of the DC/DC converter  120 . Similarly, FIG. 8 is a partially cutaway view which shows the DC/DC converter  120  in the typical discharge lamp driver circuit  100  of FIG. 7. In FIG. 8, a hatched portion is the first electrical path Lp 1 .  
         [0040]    The circuit line  127   a  of the field canceller  127  is, as shown in FIG. 2, implemented by a terminal made of, for example, copper wire which is installed in a resinous insulator in the insert molding and joined in series with the first electrical path Lp 1 . The circuit line  127   a  extends over the first electrical path Lp 1  on a plane defined at a level different from the first electrical path Lp 1 . The use of the resinous insulator results in fixing of a geometric pattern of the circuit line  127   a,  thus keeping the strength of the field produced by the interrupted current flowing through the circuit line  127   a  constant.  
         [0041]    It is advisable that the circuit line  127   a  be arranged along at least a portion of the first electrical path Lp 1  in order to produce the field which is exactly identical and 180° out of phase with the field arising from the interrupted current i 1  flowing through the first electrical path Lp 1 .  
         [0042]    The discharge lamp driver circuit  100  may be, as shown in FIG. 2, disposed in a metal casing  170 . In the illustrated case, the control circuit  150 , the MOS transistor  122 , etc. are fabricated in a hybrid IC. The circuit line  127   a  is connected in series with the first electrical path Lp 1  through a terminal  171   a  installed in a resinous inner casing  171 .  
         [0043]    [0043]FIG. 3 shows a discharge lamp driver circuit  100  according to the second embodiment of the invention.  
         [0044]    The circuit line  127   a  of the field canceller  127  of this embodiment is, unlike the first embodiment, shifted horizontally from the first electrical path Lp 1 . An area S 127 , as shown in FIG. 4, surrounded by the circuit line  127   a  is set substantially equal to an area SLp surrounded by the first electrical path Lp 1  so that the field φ 127  produced from the circuit line  127   a  may be identical in strength with the field φLp produced from the first electrical path Lp 1 . Therefore, to the extent that the interval Ld between the circuit line  127   a  and the first electrical path Lp 1  is much smaller than both distance L 1  between the first electrical path Lp 1  and a field-applied point B (e.g., an antennal installed on the rear of a vehicle traveling ahead of a vehicle equipped with the discharge lamp driver circuit  100 ) and distance L 2  between the circuit line  127   a  and the field-applied point B, the field φ 127  cancels the field φLp sufficiently at the field-applied point B, thus eliminating an electric disturbance arising from the field φLp.  
         [0045]    The circuit line  127   a  of the field canceller  127  may be made of a flexible substrate, thereby facilitating setting of the area S 127  surrounded by the circuit line  127   a.  The use of such a flexible substrate also allows the discharge lamp driver circuit  100 , especially circuit elements around the DC/DC converter  120  to be reduced in size.  
         [0046]    The field canceller  127  may also be installed, as shown in FIG. 5, in the discharge lamp driver circuit  100  in order to cancel the field arising from the interrupted current i 2  flowing through the second electrical path Lp 2  extending from the junction  310  to the secondary winding  121   b  to the rectifier diode  123  and to the smoothing capacitor  124 . The field canceller  127  is implemented by the circuit line  127   a  extending from an end of the second electrical path Lp 2  extending downward, as viewed in the drawing, from the smoothing capacitor  124  to the junction  410  along the second electrical path Lp 2 .  
         [0047]    Further, the field canceller  127  may be installed, as shown in FIG. 6, in the discharge lamp driver circuit  100  in order to cancel the field arising from the interrupted current i 3  flowing through the third electrical path Lp 3  extending from the junction  510  to the MOS transistor  122  to the gate control circuit  150   a.  The field canceller  127  is implemented by the circuit line  127   a  extending from an end of the third electrical path Lp 3  (i.e., the collector of the transistor of the gate control circuit  150   a ) to the junction  410  along the third electrical path Lp 3 .  
         [0048]    While the present invention has been disclosed in terms of the preferred embodiments in order to facilitate better understanding thereof, it should be appreciated that the invention can be embodied in various ways without departing from the principle of the invention. Therefore, the invention should be understood to include all possible embodiments and modifications to the shown embodiments witch can be embodied without departing from the principle of the invention as set forth in the appended claims.  
         [0049]    The invention is not limited to use with the DC/DC converter  122 . For instance, the field canceller  127  may be installed in a power supply unit including a semiconductor switching element such as a MOS transistor.