Discharge lamp igniting apparatus

Abstract: A current flowing through a discharge lamp 4 is detected by a current detecting circuit 5, and the detected current is outputted as a lamp current signal A to a feedback control circuit 7 where a difference between the lamp current signal A and the dimming control signal B is obtained and amplified, and then a amplified signal D is outputted to an adder 12 which adds the amplified signal D to the dimming control signal B to constitute a control signal E. The control signal is fed back to a high frequency power supply 2. When a comparing circuit 15 detects that the light level of the dimming control signal B with respect to the full light becomes higher than or equal to 40% to 60%, the feedback circuit is disconnected by operating a switch 13 provided between an error amplifying circuit 9 and an adder 12 to stop the feedback control. Also, when a light level of a discharge lamp 104 is smaller than or equal to a predetermined value, first and second reference voltage 108a and 108b are switched by a two-stage switching circuit 109. A comparator 107 compares a detection voltage 106 of the discharge lamp 104 with the first reference voltage 108a, or the detection voltage 106 with the second reference voltage 108b. When the voltage of the discharge lamp 104 becomes high, a control circuit 110 supplies to a high frequency power supply 102, a control signal 112 for interrupting, or reducing an output of the high frequency power supply 102.

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an igniting apparatus of a discharge lamp, and more specifically, to a discharge lamp igniting apparatus for performing a feedback control so as to control light of a fluorescent lamp and the like. 2. Description of the Related Art A dimming control apparatus for controlling light of a discharge lamp and the like is known in the art. To prevent either flickers or moving flanges which occur when the light level is deep (dark) and the air temperature is low, the technique related to the feedback control is known from, for example, Japanese Laid-open Patent Application No. 6-302393. Fig. 1 is a structural diagram for showing an outline of this known technique. In this example, the power supplied from the high frequency power supply 2 to the discharge lamp 4 of the load circuit 3 is detected by the current detecting circuit 5. Based upon the lamp current signal "A" detected by this detecting circuit 5 and the externally inputted dimming control signal "B", the control signal E for controlling the high frequency power supply 2 is produced by the feedback control circuit 7. The feedback control circuit 7 subtracts the lamp current signal A from the dimming control signal B, and the error signal "C" is amplified by the error amplifying circuit 9. Furthermore, the amplified signal "D" is added to the dimming control signal B to thereby produce the control signal E. Fig. 2 indicates a relationship between the dimming control signal B and the lamp current signal A under room temperature. In this example, when the value of the lamp current signal A is the same as the dimming control signal B, the error signal C becomes "0", and also the output from the error amplifying circuit 9 becomes "0". Accordingly, the dimming control signal B is directly entered as the control signal E into the high frequency power supply 2. When the value of the lamp current signal A is lower than the dimming control signal B, the error signal C is produced, and then is amplified by the error amplifying circuit 9. Furthermore, the amplified error signal is added to the dimming control signal B, so that the control signal E is formed, and is entered into the higher frequency power supply 2. As described above, the feedback control is carried out in response to the dimming control signal B, so that the lamp current signal A is increased so as to prevent flickers, or moving flanges of the discharge lamp 4. In this sort of discharge lamp igniting apparatus, since the feedback control circuit 7 into which the lamp current signal A is inputted is operated by the low voltage power supply under normal condition, when the detected lamp current signal A becomes high, the capability of the feedback control circuit 7 must be increased. However, as indicated in Fig. 14, in the above-described case, the lamp current signal A is detected which should be controlled with respect to the entire range of the dimming control signal B. Even when the light level is low (shallow) and thus the necessity of the feedback control is lowered, the feedback control becomes active. In other words, most of the cases that the feedback control is required are such cases that the light level is deep (dark). This corresponds to only 1/3 to 1/5 of the range for detecting the lamp current signal A with respect to the overall dimming control range of the dimming control signal B. In other cases, waste feedback controls are carried out. As a consequence, in the above-described example, the lamp current signal A is carried out in a waste manner with respect to the overall dimming control range of the dimming control signal B. As a result, there are problems that the dynamic range of the actually required detecting range is relatively narrowed, and the detection precision would be decreased, and also the noise resistibility would be deteriorated. US-A-4 870 327 discloses a ballast for controlling the light emitted by fluorescent lamps. DC power is switched on and off at a high frequency rate by a switch to control the flow of current through the primary winding of the transformer of a flyback converter. The switched output is rectified and applied by the flyback converter to the filaments of the fluorescent lamps to be controlled. The rectified, switched output is also applied by a π filter to a commutator circuit that controls the starting of the fluorescent lamps. The commutator circuit is switched at a lower frequency rate. The bridge input voltage and current are sensed and fed back to a control circuit, which includes a regulating pulse width modulator and a bridge switching circuit. The control circuit also receives a pulse current feedback signal from the switch and an external dimming control signal. The control circuit controls the application of high frequency switching signals to the switch that controls the application of power by the flyback converter based on the voltage, current and pulse current feedback signals. Which one of these three signals actually controls is determined by which signal first reaches a reference level. If the magnitude of the feedback voltage first reaches the magnitude set by an open circuit voltage reference, the feedback voltage signal controls. If the magnitude of the feedback pulse current signal first exceeds a predetermined magnitude, the feedback pulse current signal controls. If the feedback current first exceeds a reference magnitude controlled by the external dimming control, the feedback current signal controls. The present invention has been made to solve the above described problem, and an object of the invention is to obtain a discharge lamp igniting apparatus capable of further widening a dynamic range of an actually necessary detection range, capable of increasing detection precision, and also capable of improving noise resistibility in an apparatus for preventing a discharge lamp and the like from disappearing by executing a feedback control. This object according to the invention is solved by a discharge lamp igniting apparatus including the features of claim 1. Preferred embodiments of the discharge lamp igniting apparatus are defined in the subclaims. A discharge lamp igniting apparatus comprising a load circuit containing a discharge lamp; a high frequency power supply for supplying high frequency power whose frequency is higher than of a commercial power supply to the load circuit; a current detecting circuit for detecting a lamp current flowing through the discharge lamp to thereby output a lamp current signal; a dimming control circuit for controlling the high frequency power supply to thereby output a dimming control signal used to control the light of the discharge lamp; an error amplifying circuit for amplifying an error between the lamp current signal and the dimming control signal; and an adder for adding the signal amplified by the error amplifying circuit to the dimming control signal to thereby output the added signal as a control signal to the high frequency power supply; according to the invention is characterized in that, when the dimming control signal reaches a preset value, entering of the signal amplified by the error amplifying circuit into the high frequency power supply is blocked. Also, a discharge lamp igniting apparatus according a preferred embodiment is featured by that when a light level of the dimming control signal is set within a range of the light level between approximately 40% and approximately 100%, or between approximately 60% and approximately 100% for full light, entering of the signal amplified by the error amplifying circuit into the high frequency power supply is blocked. A further preferred embodiment of this apparatus is featured by that plural pieces of the load circuits are provided with the high frequency power supply, and plural pieces of the current detecting circuits are connected with the high frequency power supply; and the lamp current signals outputted from the respective current detecting circuits are compared with each other, whereby a small one of the compared lamp signals is outputted to a subtracter. A still further preferred embodiment of this apparatus is featured by that plural pieces of the load circuits are connected in parallel to the high frequency power supply, and plural pieces of the current detecting circuits are connected in parallel to the high frequency power supply; and each of the load circuits and each of the current detecting circuits are provided via a balancer transformer. A still further preferred embodiment of this apparatus is featured by that a temperature detecting circuit for detecting an ambient temperature is connected; and when the temperature detected by the temperature detecting circuit is lower than, or equal to a predetermined value, the value of the lamp current signal is reduced and then the reduced lamp current signal is outputted to a subtracter. A still further preferred embodiment of this apparatus is featured by that the discharge lamp igniting apparatus includes a sequence control circuit for sequentially controlling at least turn-on/turn-off operations of the discharge lamp; and when the discharge lamp is under turn-off state, the sequence control circuit blocks entering of the control signal into the high frequency power supply. A still further preferred embodiment of this apparatus is featured by that when the discharge lamp is under turn-off state, the sequence control circuit replaces the control signal by a sequence control signal corresponding to the turn-off state and inputs the sequence control signal into the high frequency power supply. A still further preferred embodiment of this apparatus is featured by that when the dimming control signal reaches a preset value, the error amplifying circuit sets the output signal thereof to 0. A still further preferred embodiment of this apparatus is featured by that the error amplifying circuit is such a variable error amplifying circuit that when the light level of the dimming control signal becomes lower (darker), a feedback control gain is increased, whereas when the light level of the dimming control signal becomes higher (lighter), the feedback control gain is decreased. A still further preferred embodiment of this apparatus is featured by that the discharge lamp igniting apparatus includes a limiting circuit for limiting that the magnitude of the feedback control gain becomes larger than, or equal to a predetermined value. The invention will be more apparent from the following description taken in conjunction with the accompanying drawings. BRIEF DESCRIPTION OF THE DAWINGS Fig. 1 is a structural diagram of a conventional apparatus. Fig. 2 is a diagram for explaining operation of the conventional apparatus. Fig. 3 is a structural diagram according to a first embodiment of the present invention. Fig. 4 is a diagram for explaining operation/non-operation of a feedback control according to the first embodiment of the present invention. Fig. 5 is a diagram for explaining operation/non-operation of a feedback control according to the first embodiment of the present invention. Fig. 6 is a structural diagram according to a second embodiment of the present invention. Fig. 7 is a structural diagram according to a third embodiment of the present invention. Fig. 8 is a diagram for explaining operation of a temperature detecting circuit according to the third embodiment of the present invention. Fig. 9 is a partial structural diagram according to a fourth embodiment of the present invention. Fig. 10 is a diagram for explaining a sequence of a sequence control circuit according to the fourth embodiment of the present invention. Fig. 11 is a structural diagram according to a fifth embodiment of the present invention. Fig. 12 is a diagram for explaining operation/non-operation of a feedback control according to the fifth embodiment of the present invention. Fig. 13 is a structural diagram according to a sixth embodiment of the present invention. Fig. 14 is a diagram for explaining an upper limit of a feedback control gain according to the sixth embodiment of the present invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Now, a description will be given in more detail of preferred embodiments of the present invention with reference to the accompanying drawings. It should be noted that the same reference numerals are employed as those for indicating the same or similar circuit elements. (First Embodiment) Fig. 3 is a structural diagram of an igniting apparatus of a display lamp according to a first embodiment. Reference numeral 1 indicates a commercial power supply, and reference numeral 2 shows a high frequency power supply connected to the commercial power supply 1, for supplying high frequency power to a load circuit 3 containing a discharge lamp 4. The high frequency power supply 2 is composed of a general VCO (voltage-controlled oscillator) and a general switching element and the like. Reference numeral 5 shows a current detecting circuit for detecting a lamp current flowing through the discharge lamp 4, and symbol "A" indicates a lamp current signal detected/outputted by the current detecting circuit 5. Reference numeral 7 is a feedback control circuit including a subtracter 8 and an error amplifying circuit 9, known in the field. The subtracter 8 subtracts the lamp current signal A from the dimming control signal B to obtain an error signal C which will be outputted to the error amplifying circuit 9. The dimming control signal B is outputted as a predetermined voltage from a dimming control circuit 11 containing light intensity varying means such as a control dial for controlling a light level. The error amplifying circuit 9 is composed of an operational amplifier which receives the error signal C, and also amplifies this error signal C which will be outputted. Reference numeral 13 denotes a switch having connection/disconnection functions, and connected between the feedback control circuit 7 and the high frequency power supply 2. Precisely speaking, this switch 13 is connected between the adder 12 and the error amplifying circuit 9. The connection/disconnection operations of the switch 13 are carried out by a comparing circuit 15 such as a comparator for comparing the dimming control signal B with a value of an operation switching light level signal F outputted from a reference signal generating unit 14. The operation switching light level signal F outputted to the comparing circuit 15 corresponds to a signal for constituting a reference value used to determine whether the feedback control is started, or stopped in a certain light level. The comparator 15 is actuated by judging as to whether or not the value of this operation switching light level signal F is larger than the dimming control signal B. Fig. 4 in this embodiment represents such a case that when the light level of the discharge lamp 4 is present in a range between 0% (namely, light of discharge lamp is controlled to be the darkest condition) and 50% (namely, medium light condition), the feedback control is started. In this graphic representation, the feedback control is set in such a manner that a cross point between an "L1" indicative of the dimming control signal B and an "L2" indicative of the operation switching light level signal F is set at 50% of the light level. It is preferable that the value of this operation switching light level signal F is set within a range between approximately 40% and approximately 60% of the light level of the dimming control signal B. An upper portion of Fig. 5 indicates operation/non-operation of the feedback control for the dimming control signal B, and a lower portion of Fig. 5 represents a diagram for explaining characteristic matters related to the dimming control operation. As shown in Fig. 5, when the light level is below approximately 40%, there are some problems that the discharge lamp disappears and/or is flickered under low temperature conditions. When the light level is below approximately 60%, it could be understood that there are product differences (for instance, product differences of constructive components) occurred when the discharge lamp igniting apparatuses are manufactured, and when the light level is about 50%, necessities of the feedback control become low. As a consequence, the highest effects can be achieved by stopping the feedback control when the light level is set between approximately 40% and approximately 60%. This is because it is preferable to widen the range where the unnecessary feedback control is omitted, and further to sufficiently prevent flickers of the discharge lamp. In accordance with the first embodiment, when the value of the dimming control signal 11 reaches a pre-set value (when dimming control signal becomes higher than, or equal to 50% for full light in this embodiment), the switch 13 is brought into the disconnection condition, so that the feedback control is stopped. Accordingly, the lamp current signal A may be detected only within the limited range (light levels from 0% to 50%) from the current detecting circuit 5, so that there is no need to detect also such a lamp current signal A unnecessary to the feedback control. Furthermore, the current detection range is narrowed so as to relatively widen the dynamic range. Therefore, the detection precision of the lamp current signal can be increased, and also the noise resistibility can be improved. In such a case that the dimming control signal B becomes higher than, or equal to approximately 40% to approximately 60% for full light, the feedback control is stopped. As a result, it is possible to prevent the flickers of the discharge lamp when the light level is low (dark) under low temperature. While preventing the adverse influences caused by the product differences among the discharge lamp igniting apparatuses, the range for detecting the lamp current signal A is narrowed, so that the dynamic range is relatively widened, and the detection of the lamp current signal can be more precise. (Second Embodiment) In a second embodiment, the discharge lamp 4 is connected in parallel with the high frequency power supply 2 in the first embodiment. That is, Fig. 6 is a structural diagram for showing this embodiment. A series circuit defined by the load circuit 3 - the discharge lamp 4 - the current detecting circuit 5, and another series circuit defined by a load circuit 3a - a discharge lamp 4a - a current detecting circuit 5a are connected through a balancer transformer 30 in parallel to the high frequency power supply 2. Other remaining circuit constructions of the second embodiment are similar to those of the first embodiment. The balancer transformer 30 is a two-winding transformer having the same magnetic path and the same coil turns. When the same currents will flow the respective windings, no magnetic flux is produced and thus the balancer transformer itself is not actuated. To the contrary, when there is a difference between the currents flowing through the respective windings, a magnetic flux corresponding to the current difference is produced, and thus a voltage is generated from the respective windings along one direction. As a consequence, the balancer transformer 30 is operated in such a manner that the currents flowing through the respective windings become identical to each other, and the currents derived from the high frequency power supply 2 are equally distributed to the respective load circuits 3 and 3a. As a consequence, the equal currents may flow through the respective series circuits. A switching circuit 31 is connected between the respective current detecting circuits 5, 5a and the subtracter 8. This switching circuit 31 selectively switches a connection (contact 33) between the current detecting circuit 5 and the subtracter 8, and another connection (contact 34) between the current detecting circuit 5a and the subtracter 8 in response to the output derived from the comparator 32. The comparator 32 compares a lamp current signal A1 flowing through the current detecting circuit 5 with another lamp current signal A2 flowing through the current detecting circuit 5a in order that a small one of these lamp current signals A1 and A2 is entered into the subtracter 8. For instance, in the case that the lamp current signal A1 is smaller than the lamp current signal A2, the comparator 32 outputs a positive logic signal, whereas in the case that the lamp current signal A1 is larger than the lamp current signal A2, the comparator 32 outputs a negative logic signal. When the comparator circuit 32 outputs the positive logic signal, the switching circuit 31 is switched to the current detecting circuit 5 (contact 33), so that the lamp current signal A1 is entered into the subtracter 8. When the comparator circuit 32 outputs the negative logic signal, the switching circuit 31 is switched to the current detecting circuit 5a (contact 34), so that the lamp current signal A2 is inputted into the subtracter 8. According to this embodiment 2, the current detecting circuit is connected to the side of the small lamp current signal by the comparing circuit 32 and the switching circuit 31 so as to select the discharge lamp side which easily disappears, or is readily flickered. Then, the logic signal is outputted to the feedback control circuit 7. Accordingly, the discharge lamp side which easily disappears is selected as the base for the feedback control. Therefore, the embodiment 2 can effectively prevent more than two discharge lamps from disappearing, or being flickered. Also, since the discharge lamps 4 and 4a are connected through the balancer transformer 30 in parallel to the high frequency power supply 2, not only the brightness of the respective discharge lamps can be made uniform, but also the brightness of the respective discharge lamps can be made substantially uniform even when the feedback control is carried out by detecting the current flowing through one discharge lamp. In particular, when the light level is low (dark), the brightness is easily fluctuated due to the product differences in the discharge lamps. However, this embodiment can prevent this brightness fluctuation. (Third Embodiment) A third embodiment is arranged by that a dimming control apparatus additionally with an arrangement capable of accepting variations in temperature conditions is employed in combination with those of the first embodiment mode. Fig. 7 is a structural diagram for representing a discharge lamp dimming control apparatus according to this embodiment 3. This arrangement owns a different point from that of Fig. 3, namely a formation of a lamp current signal. That is, reference numeral 35 shows a general-purpose temperature detecting circuit. For instance, when an ambient temperature (room temperature) becomes higher than, or equal to 10 C°, this temperature detecting circuit outputs 0, and when the ambient temperature becomes below 10 C°, the temperature detecting circuit produces a negative temperature detection signal "H" to be outputted to a second adder 36 (see Fig. 8). The second adder 36 is connected between the current detecting circuit 5 and the subtracter 8, and adds the lamp current signal A outputted from the current detecting circuit 5 to the temperature detection signal H, and then outputs the added signal to the adder 8. Other circuit arrangements of this embodiment 3 are the same as those of the embodiment 1. In the case that the ambient temperature becomes below 10 C°, since the temperature detecting signal H outputted from the temperature detecting circuit 35 is negative, the signal which has been added by the second adder 36 to be inputted into the feedback control circuit 7 has a smaller value than the actually detected lamp current signal A. Since the same feedback control as in the embodiment 1 is carried out with respect to this smaller value, as compared with the actually detected lamp current signal A, a control signal E compensated by this feedback control becomes large. As a result, when the feedback control is performed under low temperature, lowering of the light output of the discharge lamp 4 can be greatly compensated. Therefore, since such a large light output compensation is made under low temperatures when the discharge lamp easily disappears, or is readily flickered, it is possible to obtain the light output similar to that obtained under room temperature. It should be understood that the temperature under which the temperature detecting circuit 35 outputs the negative temperature detection signal may be properly determined, and therefore, is not limited to 10 C°. Also, the magnitude of the negative temperature detection signal outputted from the temperature detecting circuit 35 may be properly set by considering the power consumption of the discharge lamp. (Fourth Embodiment) A fourth embodiment relates to a discharge lamp igniting apparatus capable of preventing an erroneous operation of a feedback control in the first embodiment. Fig. 9 shows a partial circuit arrangement connected between the high frequency power supply 2 and the second adder 12. Since other circuit arrangements are identical to those of Fig. 3, they are omitted. In this drawing, reference numeral 40 is a sequence control circuit for switching sequential operations of preheating, starting, igniting, and fail-protecting of the discharge lamp 4. For instance, the sequence control circuit 40 outputs a positive logic signal as a sequence control signal "I" to a switching circuit 41 during a time period while the discharge lamp 4 is ignited (turned ON) and outputs a negative logic signal as a preheat/start/fail-protect signal "J" to the switching circuit 41 during a preheating period, a starting period, and a fail-protecting period while the discharge lamp 4 is turned OFF. The switching circuit 41 switches a contact 42 and a contact 43 in response to a change in the logic signals. When the positive logic signal is received, the switching circuit 41 is connected to the contact 43, whereas when the negative logic signal is received, the switching circuit is connected to the contact 42. This sequence operation is shown in Fig. 10. As a result, in the case that the sequence control circuit 40 turns ON the discharge lamp 4 and outputs the sequence control signal I to the switching circuit 41, the control signal E is outputted to the high frequency power supply 2. In such a case that the discharge circuit 4 is not turned ON as during the filament preheating period, the starting period, and the fail-protecting period, the sequence control circuit 40 controls the switching circuit 41 to be connected to the contact 42, and also outputs the preheat/start/fail-protection signal J as the control signal E to the high frequency power supply 2. As a consequence, when the discharge lamp 4 is not turned ON, the control signal used to execute the feedback control is not entered into the high frequency power supply 2, so that it is possible to prevent the erroneous operation by receiving this control signal E. When the discharge lamp 4 is turned OFF, since the lamp current signal A is not inputted into the feedback control circuit 7, there is a large value difference in the control signal E under stable turn-ON condition and the control signal E under turn-OFF condition. Under turn-OFF state, the preheat/start /fail-protection signal J is outputted as the sequence control signal corresponding to the turn-OFF state to the high frequency power supply 2 instead of the control signal E. As a consequence, the value change between the signals inputted into the high frequency power supply 2 under both turn-ON state and turn-OFF state is essentially made small. It is possible to prevent the discharge lamp from being rapidly emitted during turn-ON operation. (Fifth Embodiment) It should be noted that although the operation/non-operation of the feedback control is controlled by the switch is controlled by the switch 13 in the first embodiment, the operation/non-operation of the feedback control is controlled by employing another means in this embodiment. Fig. 11 indicates a structural diagram of a discharge lamp igniting apparatus according to this embodiment. In this drawing, reference numeral 16 is a variable error amplifying circuit whose amplification is variable with respect to an error signal C, and is arranged by, for example, the trans-conductance amplifier (known as "NJM 13600/13700" marketed by SHINNIHON MUSEN company). The variable error amplifying circuit 16 varies the amplification for the error signal C by detecting the voltage of the dimming control signal B. An upper side of Fig. 12 is a diagram for representing the amplification and the time instant when the variable error amplifying circuit 16 commences the feedback control. A lower portion of Fig. 12 is a diagram for explaining a characteristic operation related to the dimming control. Similar to the first embodiment, when the light level of the dimming control signal B becomes 50% for full light, the feedback control is started. When the light level of the dimming control signal B becomes lower (darker), the feedback control gain of the variable error amplifying circuit 16 is increased, whereas when the light level becomes higher (lighter), the feedback control gain is decreased. In other words, the variable error amplifying circuit 16 gradually increases the feedback control gain in response to the dimming control circuit B at the time when the feedback control is started. When the light level of the dimming control signal becomes higher (light), the feedback control gain is gradually decreased in response to the dimming control signal B. It should be noted that the variable error amplifying circuit 16 outputs a signal "D" produced by amplifying the error signal C to the second adder 12. When the subtraction result obtained by subtracting the lamp current signal A from the dimming control signal B by the subtracter 8 becomes 0, since the amplified signal "D" also becomes 0, the control signal outputted from the adder 12 to the high frequency power supply 2 becomes the dimming control signal B. In accordance with this embodiment, the variable error amplifying circuit 16 turns ON/OFF the feedback control in response to the dimming control signal B, and does not perform the feedback control in the light level under which the feedback control is essentially required. As a result, the range of the lamp current to be detected can be narrowed, so that the dynamic range can be relatively widened and the detection precision of the lamp current can be increased, and also the noise resistibility can be improved. Furthermore, since the variable error amplifying circuit 16 gradually amplifying the feedback control gain in response to the magnitude of the dimming control signal B, the rapid change in the control signal E can be suppressed. Therefore, it is possible to avoid that the light output from the discharge lamp 4 is rapidly varied. Also, since the variable error amplifying circuit 16 commences the feedback control when the light level of the dimming control signal is present in the range between approximately 40% and approximately 60% for the full light, even when the light level is made low under low temperature state, it is sufficiently possible to prevent the discharge lamp from disappearing, and also accept the product differences of the discharge lamp igniting apparatus. The discharge lamp igniting apparatus according to the embodiment 5 can be constituted by such a simple structure, as compared with that shown in Fig. 3. (Sixth Embodiment) Fig. 13 is a structural diagram for showing a discharge lamp igniting apparatus according to a sixth embodiment. Since the circuit arrangements other than a limiting circuit 17 are the same as those of the fifth embodiment, explanations thereof are omitted. The limiting circuit 17 controls the dimming control signal B inputted into the variable error amplifying circuit 16 in order to avoid that the discharge lamp 4 is oscillated by executing an excessive feedback control. In this drawing, reference numeral 18 is a limit value generating circuit for outputting a limit signal "G". This limit signal G is such a signal for preventing the feedback control gain of the variable error amplifying circuit 16 from being higher than, or equal to a constant value. Reference numeral 19 indicates a comparator constructed of a comparator. The comparator 19 compares the limit signal G with the dimming control signal B to output a comparison signal by which an inverter 20 constructed of an inverter is inverted. Then, the inverter 20 causes any one of the switch 21 and the switch 22 to be selectively brought into the connection condition, so that any one of the dimming control signal B and the limit signal G is entered into the variable error amplifying circuit 16 of the feedback control circuit 7. It should be noted that Fig. 13 represents the following conditions. That is, the switch 21 side is brought into the connection condition by the inverter 20, so that the limit signal G is inputted into the variable error amplifying circuit 16, and the switch 22 side is brought into the non-connection condition. As a result, in the case that the value of the dimming control signal B is smaller than the limit signal G, the inverter 20 brings the switch 22 into the connection condition, so that the dimming control signal B is entered into the variable error amplifying circuit 16. In the case that the value of the dimming control signal B is larger than the limit signal G, the inverter 20 is inverted by the output derived from the comparator 19, so that the switch 21 is brought into the connection condition, and the limit signal G is outputted to the variable error amplifying circuit 16. At the same time, the switch 22 is brought into the non-connection condition, so that the dimming control signal is not outputted to the variable error amplifying circuit 16. In this case, the limit signal G may have such a value that the unstable operations of the entire system such as oscillations can be prevented, because the feedback control gain of the variable error amplifying circuit 16 is excessively increased higher than, or equal to a preselected value. Also, when the light level becomes low (dark) and thus the value of the dimming control signal B becomes smaller than the limit signal G, the inverter 20 is again inverted by receiving the comparison output from the comparator 19. Then, the switch 22 is brought into the connection condition, and the dimming control signal B is entered into the variable error amplifying circuit 16. Fig. 14 shows a relationship diagram between the feedback control gain of the variable error amplifying circuit 16 and the dimming control signal B. The limit signal G is set with respect to the dimming control signal B, so that the upper limit of the feedback control gain is set. In accordance with this embodiment, when the value of the dimming control signal B becomes larger than the limit signal G by comparing the dimming control signal B with the limit signal G, the limit circuit 17 limits that the feedback control gain of the variable error amplifying circuit 16 is continuously increased in response to the magnitude of the dimming control signal B. As a result, the limit circuit 17 can prevent the unstable operations of the entire system such as oscillations. Also, since the upper limit value is provided in the feedback control gain, the change rate of the feedback control gain with respect to the dimming control signal B is relatively increased. Even when the change width of the dimming control signal B is narrowed, it is possible to avoid the extraordinary oscillations caused when the feedback gain is increased. It should be understood that the limit circuit 17 can prevent the feedback control gain of the variable error amplifying circuit 16 from being excessively increased in response to the limit signal G outputted from the limit value generating circuit 18. Alternatively, the limit value may be set to the variable error amplifying circuit 16 itself in order to avoid that the feedback control gain is extraordinarily increased. As was described above, the present invention can obtain the following advantages. In accordance with the first aspect of the present invention, in the discharge lamp igniting apparatus for detecting the lamp current signal flowing through the discharge lamp to perform the feedback control, when said dimming control signal reaches a preset value, entering of the signal amplified by said error amplifying circuit into said high frequency power supply is blocked. Accordingly, the lamp current signal is detected only in the more limited range, and the dynamic range of the actually required detecting range can be relatively widened. There are effects that the detected precision can be increased and the noise resistibility can be improved. In accordance with the second aspect of the present invention, when a light level of said dimming control signal is located within a range of the light level between approximately 40% and approximately 100%, or between approximately 60% and approximately 100% for full light, entering of the signal amplified by the error amplifying circuit into the high frequency power supply is blocked. In other words, when the light level reaches 40% to 60%, entering of the signal amplified by the error amplifying circuit into the high frequency power supply is started to be blocked. Since the range where the useless feedback control is eliminated can be increased, there are effects that the dynamic range of the lamp current signal within the necessary detecting range can be further widened, and moreover, the product differences in the discharge lamp igniting apparatus and the flickers of the discharge lamps caused by this product difference can be prevented. In accordance with the third aspect of the present invention, plural pieces of said load circuits are connected to the high frequency power supply, and plural pieces of said current detecting circuits are connected to the high frequency power supply; and the lamp current signals outputted from the respective current detecting circuits are compared with each other, whereby a small one of the compared lamp signals is outputted to a subtracter. Thus, the feedback control is carried out on the basis of the lamp current signal of the discharge lamp which may easily disappear. There is such an effect that the dimming control operations of the plural discharge lamps can be performed under stable conditions. In accordance with the fourth aspect of the present invention, plural pieces of said load circuits are connected in parallel to said high frequency power supply, and plural pieces of said current detecting circuits are connected in parallel to said high frequency power supply; and each of said load circuits and each of said current detecting circuits are connected through a balancer transformer. There is an advantage that the dimming control operation of the plural discharge lamps can be performed under stable condition. In accordance with the fifth aspect of the present invention, a temperature detecting circuit for detecting an ambient temperature is provided; and when the temperature detected by said temperature detecting circuit is lower than, or equal to a predetermined value, the value of the lamp current signal is reduced and then the reduced lamp current signal is outputted to a subtracter. Accordingly, the feedback control is carried out on the basis of the value lower than the actually detected value, and the compensation for the light level can be made large only under low temperature. Therefore, there is an effect that the stability of the dimming control under low temperature can be increased. In accordance with the sixth aspect of the present invention, said discharge lamp igniting apparatus includes a sequence control circuit for sequentially controlling at least turn-on/turn-off operations of the discharge lamp; and when said discharge lamp is under turn-off state, said sequence control circuit blocks entering of the control signal into the high frequency power supply. As a result, when the discharge lamp is not turned ON, the control signal for the feedback control is not inputted into the high frequency power supply. There is an effect that the erroneous operation when the discharge lamp is turned OFF can be avoided. In accordance with the seventh aspect of the present invention, when the discharge lamp is under turn-off state, said sequence control circuit replaces the control signal by a sequence control signal corresponding to the turn-off state and inputs said sequence control signal into the high frequency power supply. As a result, the changes in the currents flowing through the discharge lamps when the sequence is changed from the turn-ON to the turn-OFF can be made small, while the erroneous operation during the turn-OFF operation can be prevented. Thus, there is an effect that the rapid light emission can be avoided. In accordance with the eighth aspect of the present invention, when said dimming control signal reaches a preset value, said error amplifying circuit sets the output signal thereof to 0. As a consequence, there is an advantage that when the feedback control is stopped based on the light level, it is possible to realize stopping of this feedback control in a simple structure. In accordance with the ninth aspect of the present invention, said error amplifying circuit is such a variable error amplifying circuit that when the light level of the dimming control signal becomes lower and darker, a feedback control gain is increased, whereas when the light level becomes higher and lighter, the feedback control gain is decreased. Therefore, since the variable error amplifying circuit gradually amplifies the feedback control gain in response to the magnitude of the dimming control signal, the rapid change of the control signal can be suppressed. As a result, such an effect can be achieved while preventing the rapid change in the light output of the discharge lamp. In accordance with the tenth aspect of the present invention, said discharge lamp igniting apparatus includes a limiting circuit for limiting that the magnitude of the feedback control gain becomes larger than, or equal to a predetermined value. Accordingly, the limit circuit limits such operations that the dimming control signal is increased, and thus the feedback control gate is continuously increased. Therefore, there is an effect that the unstable operation of the entire system such as the oscillation can be prevented.

Claims: A discharge lamp igniting apparatus, comprising: a load circuit (3) containing a discharge lamp (4) ; a high frequency power supply (2) for supplying high frequency power whose frequency is higher than that of a commercial power supply (1) to the load circuit (3); a current detecting circuit (5) for detecting a lamp current flowing through said discharge lamp (4) to thereby output a lamp current signal; a dimming control circuit (11) for controlling said high frequency power supply (2) to thereby output a dimming control signal used to control the light of said discharge lamp (4); an error amplifying circuit (9) for amplifying an error between said lamp current signal and said dimming control signal; and an adder (12) for adding the signal amplified by said error amplifying circuit (9) to said dimming control signal to thereby output the added signal as a control signal to said high frequency power supply (2); characterized in that, when said dimming control signal reaches a preset value, entering of the signal amplified by said error amplifying circuit (9) into said high frequency power supply (2) is blocked. A discharge lamp igniting apparatus as claimed in claim 1 wherein: when a light level of said dimming control signal is set within a range of the light level between approximately 40 % and approximately 100 %, or between approximately 60 % and approximately 100 % for full light, entering of the signal amplified by the error amplifying circuit (9) into the high frequency power supply (2) is blocked. A discharge lamp igniting apparatus as claimed in claim 1 wherein: plural pieces of said load circuits (3) are connected with the high frequency power supply (2), and plural pieces of said current detecting circuits (5) are connected with the high frequency power supply (2); and the lamp current signals outputted from the respective current detecting circuits (5) are compared with each other, whereby a small one of the compared lamp signals is outputted to a subtracter (8). A discharge lamp igniting apparatus as claimed in claim 1 wherein: plural pieces of said load circuits (3) are connected in parallel to said high frequency power supply (2), and plural pieces of said current detecting circuits (5) are connected in parallel to said high frequency power supply (2); and each of said load circuits (3) and each of said current detecting circuits (5) are connected through a balancer transformer (30). A discharge lamp igniting apparatus as claimed in claim 1 wherein: a temperature detecting circuit (35) for detecting an ambient temperature is connected; and when the temperature detected by said temperature detecting circuit (35) is lower than, or equal to a predetermined value, the value of the lamp current signal is reduced and then, the reduced lamp current signal is outputted to a subtracter (8). A discharge lamp igniting apparatus as claimed in claim 1 wherein: said discharge lamp igniting apparatus includes a sequence control circuit (40) for sequentially controlling at least turn-on/turn-off operations of the discharge lamp (4); and when said discharge lamp (4) is under turn-off state, said sequence control circuit (40) blocks entering of the control signal into the high frequency power supply (2). A discharge lamp igniting apparatus as claimed in claim 6 wherein: when the discharge lamp (4) is under turn-off state, said sequence control circuit (40) replaces the control signal by a sequence control signal corresponding to the turn-off state and inputs said sequence control signal into the high frequency power supply (2). A discharge lamp igniting apparatus as claimed in claim 1 wherein: when said dimming control signal reaches a preset value, said error amplifying circuit (9) sets the output signal thereof to 0. A discharge lamp igniting apparatus as claimed in claim 8 wherein: said error amplifying circuit is such a variable error amplifying circuit (16) that when the light level becomes lower (darker), a feedback control gain is increased, and when the light level becomes higher (lighter), the feedback control gain is decreased. A discharge lamp igniting apparatus as claimed in claim 8, wherein: said discharge lamp igniting apparatus includes a limiting circuit (17) for limiting that the magnitude of the feedback control gain becomes larger than, or equal to a predetermined value.

Inventors: ARAI TAKESHI, BUNYA JUN, EGUCHI KENTARO, NAGAI SATOSHI, NISHI KENICHIRO, OGAWA ISAMU, SHIBATA KOJI, SHIMIZU KAZUTAKA, ARAI, TAKESHI, BUNYA, JUN, EGUCHI, KENTARO, NAGAI,SATOSHI, NISHI, KENICHIRO, OGAWA, ISAMU, SHIBATA, KOJI, SHIMIZU, KAZUTAKA, Nagai,Satoshi, c/o Mitsubishi Denki K.K.
Applicants: MITSUBISHI ELECTRIC CORP, MITSUBISHI LIGHTING FIXTURE CO, MITSUBISHI DENKI KABUSHIKI KAISHA, MITSUBISHI LIGHTING FIXTURE CO., LTD.
Application year: 1997
Main classification: H05B41
IPC codes: H05B41