JP2006313708A - Discharge lamp lighting circuit - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a discharge lamp lighting circuit capable of avoiding occurrence of abnormal high voltage caused by wiring state of the secondary side when the secondary side of a transformer is in non-load state. <P>SOLUTION: The discharge lamp 19 is made to be lighted by gradually changing the oscillation frequency of an inverter circuit 14 from the oscillation frequency of 200 kHz shifted from the resonance frequency of the secondary side of a transformer 15 to the oscillation frequency of 50 kHz at the time of regular lighting of the discharge lamp 19. When a prescribed high voltage can be generated in the secondary side such as no-load or overload, the oscillation circuit 13 is operated at 200 kHz. Thereby, a so-called secondary side resonance state in which the inverter circuit 14 and the secondary side resonance frequency of the transformer 15 coincide is not brought about. As a result, an abnormal high voltage is not generated at the secondary side of the transformer 15. Further, when a prescribed over-voltage exceeding an over-voltage judgement threshold established beforehand is generated, oscillation action of the oscillation circuit 13 is made to be stopped by detecting this. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a discharge lamp lighting circuit for lighting a discharge lamp such as a neon tube and an argon tube.

  In recent years, as a discharge lamp lighting circuit for lighting a discharge lamp such as a neon tube or an argon tube, a high-frequency lighting method using an inverter circuit is becoming widespread. In this discharge lamp lighting circuit, the AC voltage from the AC power supply is full-wave rectified by a full-wave rectifier circuit and converted into a DC voltage, and the DC voltage is smoothed by a smoothing capacitor. The smoothed DC voltage is converted into an AC voltage having a predetermined frequency by the switching operation of the switching elements that constitute the inverter circuit, and is supplied to the primary side of the transformer that also constitutes the inverter circuit. Then, a voltage corresponding to the winding ratio of the winding to the primary side is induced on the secondary side of the transformer, and this induced voltage is applied to the discharge tube connected to the secondary side of the transformer to release the voltage. The light is on.

  Usually, the discharge lamp is made of glass, and a failure such as breakage or a decrease in the degree of vacuum may occur for some reason. Further, the connection between the discharge tube and the transformer may be interrupted for some reason such as disconnection of the wiring, resulting in a non-contact state. In such a case, the secondary side of the transformer is in a so-called no-load state, and a very high voltage (overvoltage) continues to be output from the secondary side of the transformer. The transformer could be damaged by the overvoltage. In order to suppress the damage of the transformer due to such an overvoltage, a discharge lamp lighting circuit provided with an overvoltage protection circuit has been conventionally proposed (for example, see Patent Document 1).

The discharge lamp lighting circuit of the same document can detect the voltage generated on the secondary side of the transformer by the voltage detection winding wound on the secondary side of the transformer, and the detected voltage Based on the above, it is determined whether or not the voltage is normal. That is, the discharge lamp lighting circuit compares the voltage generated on the secondary side of the transformer detected by the voltage detection winding with the preset abnormal voltage determination threshold value, and compares the voltage on the secondary side of the transformer. Is determined to be abnormal when the voltage exceeds the abnormal voltage determination threshold. When it is determined that there is an abnormality, the discharge lamp lighting circuit stops the switching operation of the switching element of the inverter circuit. Thereby, the overvoltage is not continuously output from the secondary side of the transformer, and the transformer is protected.
JP 2003-309023 A

  However, the conventional discharge lamp lighting circuit has the following problems. That is, when the secondary side of the transformer is in a no-load state, the stray capacitance varies depending on the wiring state of the secondary side. When the oscillation frequency of the inverter circuit coincides with the resonance frequency of the secondary circuit of the transformer (the resonance frequency of the C component by the wiring on the secondary side of the transformer and the L component of the transformer), the transformer There was a risk that an abnormally high voltage would occur on the secondary side of the. In this so-called secondary resonance state, the abnormal voltage generated on the secondary side of the transformer has a higher value than the overvoltage generated when the secondary side of the transformer is simply in the no-load state. This abnormal voltage is naturally detected by the detection winding, and the switching operation of the switching element of the inverter circuit is stopped. However, even if it is a short time from when the abnormal voltage is detected to when the switching operation of the switching element is stopped, there is a concern that the transformer is damaged due to the abnormal voltage.

  The present invention has been made to solve the above problems, and its purpose is to avoid the occurrence of abnormal high voltage depending on the wiring state of the secondary side when the secondary side of the transformer is in a no-load state. An object of the present invention is to provide a discharge lamp lighting circuit that can be used.

  According to the first aspect of the present invention, a predetermined AC voltage input from an AC power source is converted into a predetermined DC voltage by a rectifier circuit, and the DC voltage is converted into a predetermined AC voltage by a transformer based on the oscillation operation of the inverter circuit. In the discharge lamp lighting circuit that is converted and applied to the discharge lamp, the oscillation frequency of the inverter circuit is set to a value outside the range that can be taken by the secondary resonance frequency of the transformer assumed in advance. An oscillation frequency and a second oscillation frequency when the discharge lamp is turned on are set, and the oscillation frequency of the inverter circuit is changed from the first oscillation frequency to the second oscillation frequency as the AC power supply is turned on. When control is performed so as to match, and an abnormal state in which abnormal voltage expected in the process can occur is detected, the oscillation frequency of the inverter circuit is set to the first frequency. That it has to provide a protection circuit for setting the oscillation frequency kept as its gist.

The gist of the invention according to claim 2 is that, in the discharge lamp lighting circuit according to claim 1, the abnormal state is a no-load state on the secondary side of the transformer.
According to a third aspect of the present invention, in the discharge lamp lighting circuit according to the first or second aspect of the present invention, when the AC voltage exceeds a preset overvoltage determination threshold, the protection circuit is configured to An overvoltage protection circuit for stopping the oscillation operation, and determining that the secondary side of the transformer is in a no-load state as an abnormal state assumed in advance when the AC voltage exceeds a preset no-load determination threshold The gist of the invention is that it includes a no-load detection circuit that sets and maintains the oscillation frequency of the inverter circuit at the first oscillation frequency.

  According to a fourth aspect of the present invention, in the discharge lamp lighting circuit according to any one of the first to third aspects, the first oscillating frequency is larger than the second oscillating frequency. This is the gist.

(Function)
According to the first aspect of the present invention, from the first oscillation frequency set to a value outside the range that can be taken by the secondary side resonance frequency during normal load to the second oscillation frequency during steady lighting of the discharge lamp. The discharge lamp is turned on gradually. In this process, when an abnormal state where an abnormal voltage can be generated on the secondary side of the transformer that is assumed in advance is detected, the oscillation frequency of the inverter circuit is set to the first oscillation frequency and maintained in that state. For this reason, a so-called secondary resonance state in which the oscillation frequency of the inverter circuit and the secondary resonance frequency of the transformer coincide with each other is not reached, and an abnormal high voltage may be generated on the secondary side of the transformer. Avoided.

  This abnormal high voltage becomes particularly large when the secondary side of the transformer is in an unloaded state. For this reason, as described in claim 2, when the no-load state on the secondary side of the transformer is detected, if the oscillation frequency of the inverter circuit is set and maintained at the first oscillation frequency, It is avoided that the secondary side resonance state is reached when the secondary side of the transformer is in a no-load state. Therefore, effective circuit protection is achieved.

  According to the third aspect of the present invention, when a predetermined overvoltage is detected, the oscillation operation of the inverter circuit is stopped by the overvoltage protection circuit. As a result, the above-described overvoltage is not induced on the secondary side of the transformer, and damage to the transformer is avoided. Further, when the no-load detection circuit detects the secondary no-load state of the transformer, the oscillation frequency of the inverter circuit is set and maintained at the first oscillation frequency. Damage to the transformer is avoided. In the no-load state, the AC voltage induced on the secondary side of the transformer can reach a predetermined overvoltage level, even if not as much as the abnormal voltage in the secondary side resonance state. In that case, circuit protection is achieved by the overvoltage protection circuit.

  According to the fourth aspect of the invention, the oscillation frequency of the inverter circuit changes from the first oscillation frequency having a higher value to the second oscillation frequency having a lower value. In general, transformers are designed for the lowest oscillation frequency. For this reason, compared with the case where the oscillation frequency of the inverter circuit is changed from a lower value to a higher value, the transformer can be downsized. That is, in the present invention, the transformer is designed in accordance with the second oscillation frequency which is the lowest oscillation frequency. On the other hand, when the oscillation frequency of the inverter circuit is changed from a lower value to a higher value, in the present invention, an oscillation having a value lower than the second oscillation frequency, which is the lowest oscillation frequency. The transformer is designed according to the frequency. This increases the size of the transformer.

  ADVANTAGE OF THE INVENTION According to this invention, generation | occurrence | production of the abnormal high voltage depending on the wiring state of the said secondary side can be avoided in the secondary side of a transformer in a no-load state.

Hereinafter, an embodiment in which the present invention is embodied in a discharge lamp lighting circuit for lighting a discharge lamp such as a neon tube will be described with reference to FIGS.
<Outline of discharge lamp lighting circuit>
As shown in FIG. 1, the discharge lamp lighting circuit 11 includes a rectifier circuit 12, an oscillation circuit 13, an inverter circuit 14, and a transformer 15. The rectifier circuit 12 full-wave rectifies the AC voltage from the AC power supply 16 to obtain a DC voltage. The oscillation circuit 13 drives and controls the inverter circuit 14 at a predetermined oscillation frequency. The inverter circuit 14 employs, for example, a half-bridge type in which two switching elements are alternately turned on, and the switching element performs a switching operation based on the oscillation frequency of the oscillation circuit 13. The transformer 15 outputs a predetermined AC voltage based on the switching operation of the switching element, that is, the oscillation operation of the inverter circuit 14. When the AC voltage from the transformer 15 is applied to both ends of the discharge lamp 19, the discharge lamp 19 is turned on.

  Further, as shown in the figure, the discharge lamp lighting circuit 11 includes a protection circuit 21 for protecting the transformer 15. The protection circuit 21 has an overvoltage protection circuit 22 and a no-load detection circuit 23. The overvoltage protection circuit 22 detects the AC voltage output from the transformer 15 and determines whether or not the AC voltage is at an overvoltage level. When determining that the AC voltage is at the overvoltage level, the overvoltage protection circuit 22 outputs an oscillation stop signal S1 to the oscillation circuit 13. . In response to the oscillation stop signal S1, the oscillation circuit 13 stops the oscillation operation of the inverter circuit. As a result, no overvoltage is output from the transformer 15, and damage to the transformer 15 due to the overvoltage is suppressed. The overvoltage protection circuit 22 will be described in detail later.

  The no-load detection circuit 23 determines whether or not the output side of the transformer 15 is in a presumed no-load state based on the AC voltage output from the transformer 15, and outputs an oscillation frequency control signal S2 in advance to output an abnormal voltage. If it is in the state, the oscillation frequency control signal S2 is continuously output. When this oscillation frequency control signal S2 is input, the oscillation circuit 13 is configured to perform the oscillation operation of the inverter circuit 14 at a preset oscillation frequency so as not to coincide with the secondary side resonance frequency of the transformer 15. ing. Thereby, it is avoided that the oscillation frequency of the oscillation circuit 13 and the secondary resonance frequency of the transformer 15 coincide.

  Here, the no-load state means that the discharge lamp 19 is broken for some reason or a failure such as a decrease in the degree of vacuum occurs, or the connection between the transformer 15 and the discharge lamp 19 is caused by a wiring disconnection or the like. It is a state in which a very high voltage (overvoltage) can be output from the transformer 15 by being cut off and in a non-contact state. When the AC voltage output from the transformer 15 reaches an overvoltage level due to the fact that the output side of the transformer 15 is in an unloaded state, it is detected by the overvoltage protection circuit 22, and the overvoltage is detected in the same manner as described above. An oscillation stop signal S 1 is output from the protection circuit 22 to the oscillation circuit 13. As a result, the oscillation operation of the inverter circuit 14 is stopped, and output of an overvoltage level voltage from the transformer 15 is avoided. The no-load detection circuit 23 will be described in detail later.

<Overvoltage protection circuit>
Next, the overvoltage protection circuit 22 will be described in detail. That is, as shown in FIG. 2, the overvoltage protection circuit 22 includes a detection winding 31, a smoothing circuit 32, a Zener diode 33, an operational amplifier 34, a diode 35, and a series circuit of two resistors 36 and 37, an electrolytic capacitor 38, a resistor 39 and the switching element 40 are provided.

  The detection winding 31 is wound around the secondary side of the transformer 15 having the primary side winding 15a and the secondary side winding 15b separately from the secondary side winding 15b. Both ends of the detection winding 31 are connected to the smoothing circuit 32 via wirings 31a and 31b. The smoothing circuit 32 is configured by appropriately combining a resistor, a diode, a capacitor, and the like, and smoothes a predetermined AC voltage detected by the detection winding 31 to a state close to a DC voltage. The output side of the smoothing circuit 32 is connected to the plus side input terminal of the operational amplifier 34.

  The Zener diode 33 is connected between the wiring 31 a drawn from one end side of the detection winding 31 and the negative input terminal of the operational amplifier 34. The anode side of the Zener diode 33 is connected to the detection winding 31, and the cathode side is also connected to the negative side input terminal of the operational amplifier 34.

  The series circuit of the diode 35 and the resistors 36 and 37 is connected to the output terminal of the operational amplifier 34 in this order. The anode side of the diode 35 is connected to the output terminal of the operational amplifier 34, and the cathode side is connected to the resistor 36. An electrolytic capacitor 38 is connected between the connection point of the resistors 36 and 37 and the wiring 31 a drawn from one end of the detection winding 31.

  A field effect transistor is used as the switching element 40, the drain terminal of the switching element 40 is the oscillation circuit 13, the source terminal is the wiring 31 a drawn from one end of the detection winding 31, and the gate terminal is the same. Is connected to a resistor 37. A resistor 39 is connected between a connection point between the resistor 37 and the gate terminal of the switching element 40 and a wiring 31 a drawn from one end of the detection winding 31.

  The operational amplifier 34 outputs a predetermined voltage based on the potential difference between the plus side input terminal and the minus side input terminal. In the present embodiment, a DC voltage from the smoothing circuit 32 is input to the positive input terminal of the operational amplifier 34, and a constant voltage (that is, a reference voltage) generated by the Zener diode 33 is input to the negative input terminal. . The operational amplifier 34 compares the voltage input to the positive input terminal with the reference voltage input to the negative input terminal, and the voltage input to the positive input terminal exceeds the reference voltage of the negative input terminal. When it is, a predetermined voltage, that is, an oscillation stop signal S1 is output. This oscillation stop signal S1 is supplied to the gate terminal of the switching element 40 through a series circuit of the diode 35 and the resistors 36 and 37, whereby the switching element 40 is turned on. Then, when the oscillation circuit 13 is short-circuited, the drive of the oscillation circuit 13 is stopped, and the oscillation operation of the inverter circuit 14 is also stopped.

<No-load detection circuit>
Next, the no-load detection circuit 23 will be described in detail. That is, as shown in FIG. 2, the no-load detection circuit 23 includes an operational amplifier 41. The negative input terminal of the operational amplifier 41 is connected to a connection point between the Zener diode 33 and the negative input terminal of the operational amplifier 34 via a resistor 42. Similarly, the positive input terminal is a series circuit of a diode 43 and a resistor 44. Is connected to a wiring 31b drawn from the other end of the detection winding 31. The anode of the diode 43 is connected to the resistor 44 side, and the cathode is connected to the operational amplifier 41 side.

  A series circuit of a resistor 45 and a diode 46 is connected between a connection point between the resistor 42 and the negative input terminal of the operational amplifier 34 and a connection point between the diode 43 and the positive input terminal of the operational amplifier 41. Has been. The anode of the diode 46 is connected to the resistor 45 side, and the cathode is connected to the operational amplifier 41 side. A parallel circuit of a capacitor 47 and a resistor 48 is connected between a connection point between the diode 43 and the positive input terminal of the operational amplifier 41 and a wiring 31 a drawn from one end side of the detection winding 31.

  In addition, a parallel circuit of a switching element 49 and an electrolytic capacitor 50 is connected between a connection point between the resistor 42 and the negative terminal of the operational amplifier 41 and a wiring 31 a drawn from one end side of the detection winding 31. Yes. The switching element 49 is a field effect transistor. The drain terminal of the switching element 49 is drawn to the connection point between the resistor 42 and the negative terminal of the operational amplifier 41, and the source terminal is also drawn from one end of the detection winding 31. Similarly, the gate terminal of the wiring 31a is connected to the connection point of the resistors 36 and 37, respectively.

  A series circuit of a diode 51, a resistor 52, and a diode 53 is connected between the output terminal of the operational amplifier 41 and the oscillation circuit 13. Both diodes 51 and 53 are connected so as to be bidirectional with the resistor 52 interposed therebetween. That is, the anode of the diode 51 is connected to the output terminal of the operational amplifier 41 and the cathode is connected to the resistor 52. The anode of the diode 53 is connected to the oscillation circuit 13, and the cathode is connected to the resistor 52.

  Further, a parallel circuit of a resistor 54 and a capacitor 55 is connected between a connection point between the resistor 52 and the diode 53 and a wiring 31 a drawn from one end side of the detection winding 31. A diode 56 is connected between the connection point between the diode 53 and the oscillation circuit 13 and the detection winding 31. The diode 56 has an anode connected to the detection winding 31 and a cathode connected to the oscillation circuit 13.

  The operational amplifier 41 outputs a predetermined voltage based on the potential difference between the plus side input terminal and the minus side input terminal. In the present embodiment, the voltage detected by the detection winding 31 is input to the plus side input terminal of the operational amplifier 41 via the series circuit of the resistor 44 and the diode 43. Further, the voltage divided by the resistor 45 and the resistor 48 is input to the plus side input terminal of the operational amplifier 41 in a state where it is smoothed by the capacitor 47 so as to have a value lower than the Zener voltage of the Zener diode 33. .

  A constant voltage generated by the Zener diode 33 is input to the negative input terminal of the operational amplifier 41 via the resistor 42, and the input voltage becomes the reference voltage of the operational amplifier 41. Therefore, when the AC power is on, the input voltage of the positive input terminal of the operational amplifier 41 is preferably increased and becomes higher than the input voltage of the negative input terminal, that is, the reference voltage. That is, when the AC power supply is on, the operational amplifier 41 starts up in the output state. When the operational amplifier 41 is in the output state, that is, in the output state of the oscillation frequency control signal S2, the oscillation circuit 13 has a predetermined oscillation frequency determined by the diodes 51, 53, 56, the resistors 52, 54, the capacitor 55, and the operational amplifier 41. The inverter circuit 14 is configured to perform a switching operation.

  Specifically, the oscillation frequency of the oscillation circuit 13 when the oscillation frequency control signal S2 is output is set to a value outside the range that can be taken by the secondary resonance frequency of the transformer 15 that is assumed in advance. The resonance point varies depending on the state of the load (here, the discharge lamp 19) connected to the transformer 15, the wiring state of the secondary side of the transformer 15, and the like. The range of the oscillation frequency of the oscillation circuit 13 in which resonance can occur is determined by experiments using a predetermined lighting circuit model. In the present embodiment, 40 to 150 kHz (transformer 15; 4 kVo-p) is assumed as the range of the oscillation frequency of the oscillation circuit 13 that can generate resonance, and 200 kHz outside the range is assumed to be the oscillation frequency control signal. The oscillation frequency of the oscillation circuit 13 when S2 is input is used. The resistance values and capacitances of the diodes 51, 53, 56, the resistors 52, 54, and the capacitor 55 are set so that the oscillation frequency of the oscillation circuit 13 when the oscillation frequency control signal S2 is input is 200 kHz. Yes.

  Further, when the AC power is turned on, the input voltage (that is, the reference voltage) of the negative input terminal of the operational amplifier 41 is gradually based on a time constant determined by a time constant circuit composed of the resistor 42 and the electrolytic capacitor 50. Has come to rise. The operational amplifier 41 compares the input voltage at the plus side input terminal with the reference voltage at the minus side input terminal, and stops the output of the oscillation frequency control signal S2 when the reference voltage exceeds the input voltage. It has become.

<Operation of Embodiment>
Next, the operation of the discharge lamp lighting circuit configured as described above will be described in order for the normal load and the no load. In the present embodiment, the oscillation frequency of the oscillation circuit 13 during steady lighting of the discharge lamp 19 is 50 kHz. The description will be made assuming that the secondary side resonance frequency of the transformer 15 is 50 kHz.

<Normal load>
First, a description will be given of a normal load when the discharge lamp 19 as a load is normally connected to the secondary side of the transformer 15. In this case, when the AC power supply is turned on, the oscillation circuit 13 performs the switching operation of the inverter circuit 14 at a predetermined oscillation frequency (200 kHz in this case) determined by the diodes 51, 53, 56, the resistors 52, 54, the capacitor 55, and the operational amplifier 41. Let The oscillation frequency of the oscillation circuit 13 when the AC power is on is set to a value outside the range that can be taken by the secondary resonance frequency of the transformer 15 assumed in advance as shown in FIG. Sometimes, the oscillation frequency of the oscillation circuit 13 and the secondary resonance frequency of the transformer 15 do not match.

  After the AC power is turned on, the input voltage at the negative input terminal of the operational amplifier 41 gradually increases based on a time constant determined by a time constant circuit composed of the electrolytic capacitor 50 and the resistor 42. That is, the constant voltage generated by the Zener diode 33 is input to the negative input terminal of the operational amplifier 41 through the resistor 42. At that time, the electrolytic capacitor 50 is gradually charged, and accordingly, the input voltage (reference voltage) of the negative input terminal of the operational amplifier 41 gradually increases. The input voltage at the negative input terminal of the operational amplifier 41 increases until the electrolytic capacitor 50 reaches a predetermined charging voltage.

  The input voltage to the positive input terminal of the operational amplifier 41 also gradually increases based on a time constant determined by a time constant circuit composed of the resistor 45, the diode 46, the capacitor 47, and the resistor 48. In addition to the voltage detected by the detection winding 31, the following voltage is also input to the positive input terminal of the operational amplifier 41. More specifically, the constant voltage generated by the Zener diode 33 is divided by the resistor 45 and the resistor 48 to a value lower than the Zener voltage of the Zener diode 33, and the divided voltage is smoothed by the capacitor 47. The smoothed voltage is input to the plus side input terminal of the operational amplifier 41.

  Therefore, as shown in FIG. 4, at the normal load, the input voltage V1 at the plus side input terminal rises earlier than the input voltage V2 (reference voltage) at the minus side input terminal. On the way, the input voltage V2 at the negative input terminal of the operational amplifier 41 becomes higher than the input voltage V1 at the positive input terminal. As a result, the output of the oscillation frequency control signal S2 from the operational amplifier 41 is stopped. Thereafter, the oscillation frequency of the oscillation circuit 13 attenuates and gradually decreases toward 50 kHz, which is the oscillation frequency at the time of steady fall of the discharge lamp 19.

  Then, the oscillation frequency of the oscillation circuit 13 is gradually attenuated to reach 50 kHz, which is the oscillation frequency when the discharge lamp 19 is steadily lit. When a predetermined starting voltage is applied across the discharge lamp 19, the discharge lamp 19 is Light.

  As described above in detail, the discharge lamp 19 is preset from a predetermined oscillation frequency (200 kHz) obtained by shifting the oscillation frequency of the oscillation circuit 13 from the secondary resonance frequency (50 kHz) of the transformer 15 during normal load. The frequency was gradually changed to the oscillation frequency (50 kHz) during steady lighting. Thereby, it is possible to avoid the occurrence of immediate resonance when the AC power is turned on. As a result, when the AC power supply is turned on, a so-called secondary resonance state does not occur, and the abnormal high voltage Vmax shown in FIG. 3 is induced on the secondary side of the transformer 15 due to this. The In this way, the transformer 15 is protected. The abnormal high voltage Vmax due to the secondary resonance state is a value much higher than the overvoltage (voltage exceeding the overvoltage determination threshold Vh) to be protected by the overvoltage protection circuit 22. Incidentally, when the overvoltage determination threshold Vh is 6 kV, the abnormal high voltage Vmax is about 24 kV, for example.

  In the present embodiment, the predetermined oscillation frequency 200 kHz shifted from the secondary resonance frequency of the transformer 15 corresponds to the first oscillation frequency in the present invention, and the oscillation frequency 50 kHz during steady lighting of the discharge lamp 19 is This corresponds to the second oscillation frequency in the present invention.

<Unloaded>
Next, an operation in the case where the secondary side of the transformer 15 is in a no-load state due to breakage of the discharge lamp 19 and disconnection of the secondary side wiring of the transformer 15 will be described.

  In the case of no load, overload, etc., the voltage detected by the detection winding 31 is higher than that in the normal load described above. Along with this, the input voltage V3 input to the plus side input terminal of the operational amplifier 41 also becomes higher than that at the normal load as shown by a dotted line in FIG. The rise of the input voltage V3 is also earlier than the rise of the input voltages V1 and V2 of the operational amplifier 41 at the normal load. This is because the resistor 44, the diode 43, and the resistor 48 cause a suitable difference in the input voltages V1 and V3 of the plus side input terminal of the operational amplifier 41 between normal load and no load. That is, the resistor 44, the diode 43, and the resistor 48 distinguish between normal load and no load, and the input voltage V1, V3 of the positive input terminal of the operational amplifier 41 has a predetermined potential difference between the normal load and no load. This is a configuration for securing ΔV (see FIG. 4).

  The constant voltage generated by the Zener diode 33 is divided by the resistors 45 and 48. When the voltage detected by the detection winding 31 becomes higher than the divided voltage, the series circuit of the resistor 44 and the diode 43 is changed. Then, charging of the capacitor 47 is started. As a result, the input voltage of the positive input terminal of the operational amplifier 41 becomes higher than the input voltage (reference voltage) of the negative input terminal. Then, the operational amplifier 41 outputs a predetermined voltage, that is, an oscillation frequency control signal S2 to the oscillation circuit 13, assuming that the secondary side of the transformer 15 is in a no-load state. When the oscillation frequency control signal S2 is input, the oscillation circuit 13 operates at an oscillation frequency of 200 kHz. The input voltage at the positive input terminal of the operational amplifier 41 corresponds to the no-load determination threshold in the present invention.

  Here, the input voltage of the positive input terminal of the operational amplifier 41 is always larger than the reference voltage of the negative input terminal. Therefore, the operational amplifier 41 is in a state where the oscillation frequency control signal S2 is continuously output from the time when the AC power supply is turned on, and the oscillation frequency of the oscillation circuit 13 is maintained at 200 kHz. For this reason, when the AC power supply is on, the oscillation frequency of the oscillation circuit 13 does not coincide with the secondary resonance frequency of the transformer 15, and an abnormal high voltage Vmax is generated on the secondary side of the transformer 15 due to the resonance. Occurrence is avoided.

  On the other hand, in the case of no load, overload, and the like, depending on the wiring state of the secondary side of the transformer 15 and the like, unlike the above, the no load state may not be detected immediately when the AC power supply is on. That is, in the process of gradually changing from the frequency (200 kHz) shifted from the secondary resonance frequency of the transformer 15 to the oscillation frequency (50 kHz) at the time of steady lighting of the discharge lamp 19, an overvoltage determination is made on the secondary side of the transformer 15. A predetermined high voltage exceeding the threshold value Vh may occur. The high voltage is detected by the overvoltage protection circuit 22, and the oscillation operation of the inverter circuit 14 by the oscillation circuit 13 is temporarily stopped by the operation of the overvoltage protection circuit 22. Thereby, generation | occurrence | production of the abnormal high voltage in the secondary side of the transformer 15 is avoided.

  More specifically, the operational amplifier 34 compares the input voltage detected by the detection winding 31 and input to the plus side input terminal with the input voltage (reference voltage) input to the minus side input terminal. When the input voltage exceeds the reference voltage, the oscillation stop signal S1 is output from the operational amplifier 34 to the switching element 40.

  The electrolytic capacitor 38 is charged by the voltage (oscillation stop signal S1) output from the operational amplifier 34. When the electrolytic capacitor 38 is saturated, the electric charge of the electrolytic capacitor 38 is supplied to the switching elements 40 and 49, respectively. As described above, the time from when the oscillation stop signal S1 is output from the operational amplifier 34 to when the switching elements 40 and 49 are turned on is determined based on the time constant of the time constant circuit including the resistor 36 and the electrolytic capacitor 38. When the switching element 40 is turned on, an oscillation control terminal (not shown) of the oscillation circuit 13 is short-circuited, and the oscillation operation of the inverter circuit 14 is stopped. As a result, the occurrence of an abnormal high voltage on the secondary side of the transformer 15 is avoided. Further, when the switching element 49 is turned on, the electric charge stored in the electrolytic capacitor 50 is discharged and returns to the initial state.

  Since the reference voltage of the operational amplifier 34 is set to an overvoltage determination threshold Vh (6 kV) that is lower than an abnormal high voltage Vmax (24 kV) that can occur in the secondary resonance state, the secondary side of the transformer 15 The operation of the oscillation circuit 13 and thus the oscillation operation of the inverter circuit 14 are stopped before the abnormal high voltage Vmax caused by the resonance state is induced. In other words, the switching operation of the inverter circuit 14 is stopped before the oscillation frequency of the oscillation circuit 13 reaches the secondary resonance frequency and resonates. For this reason, unlike the case where the switching operation of the inverter circuit 14 is stopped after detecting a predetermined overvoltage, the abnormal high voltage Vmax is induced on the secondary side of the transformer 15 even for a short time. Never happen. Therefore, the transformer 15 is more reliably protected.

<Retry>
If the voltage exceeding the predetermined overvoltage determination threshold value Vh is detected by mistake for some reason, the operational amplifier 34 does not continuously output the oscillation stop signal S1. That is, the output of the oscillation stop signal S1 from the operational amplifier 34 is stopped before the electrolytic capacitor 38 reaches a predetermined charging voltage. Then, the electrolytic capacitor 38 starts discharging, and the discharged electric charge is supplied to the switching element 40 while being consumed by the resistor 37 and the resistor 39. Further, the switching element 49 is turned on by the electric charge of the electrolytic capacitor 38, and the electrolytic capacitor 50 is discharged to return to the initial state. When the discharge of the electrolytic capacitor 38 is finished, the switching element 40 is turned off, and the oscillation circuit 13 starts the oscillation operation of the inverter circuit 14 again at the oscillation frequency (200 kHz in this case) when the AC power supply is on. As described above, even when a predetermined overvoltage is erroneously detected, the oscillation operation of the inverter circuit 14 is automatically restored by the time constant circuit of the resistor 36 and the electrolytic capacitor 38. For this reason, the stop of the transmission operation of the inverter circuit 14 due to erroneous detection of a predetermined overvoltage is avoided as much as possible.

  As described in detail above, when the voltage detected by the detection winding 31 rises to a predetermined value and the input voltage at the positive input terminal of the operational amplifier 41 is also higher than the reference voltage, the transformer 15 Since the oscillation operation of the inverter circuit 14 is stopped before the secondary side enters the no-load state and reaches the secondary resonance state, the transformer 15 is reliably protected.

  Here, when the oscillation circuit 13 is operated only at a constant oscillation frequency (for example, 50 kHz), when a load having a resonance point of 50 kHz is connected to the secondary side of the transformer 15, the AC power supply Since it becomes a resonance point from the moment when it is turned on, a high voltage may be generated even in a short time. On the other hand, in the present embodiment, when the AC power is turned on, the oscillation circuit 13 is caused to oscillate at a starting oscillation frequency of 200 kHz set to a value outside the range that can be assumed by the secondary resonance frequency of the transformer 15 assumed in advance. I have to. That is, it is checked whether or not the secondary side of the transformer 15 is in a no-load state when the AC power is turned on. If there is no problem, the oscillation frequency of the oscillation circuit 13 is gradually reduced from 200 kHz to a predetermined oscillation frequency (50 kHz). I am doing so. If the secondary side of the transformer 15 is in a no-load state, a voltage exceeding a predetermined overvoltage determination threshold Vh (6 kV) is generated on the secondary side of the transformer 15 until the oscillation frequency reaches 50 kHz. For this reason, in the no-load state, the oscillation of the oscillation circuit 13 is stopped before reaching the resonance point (here, 50 kHz) at which the oscillation frequency of the oscillation circuit 13 matches the predetermined secondary resonance frequency.

<Effect of embodiment>
Therefore, according to the present embodiment, the following effects can be obtained.
(1) During normal load, the discharge frequency is gradually changed from the oscillation frequency 200 kHz shifted from the secondary resonance frequency of the transformer 15 to the oscillation frequency 50 kHz when the discharge lamp 19 is steadily lit. 19 was turned on. When a predetermined high voltage can be generated on the secondary side such as no load and overload, the oscillation circuit 13 is operated at 200 kHz. For this reason, a so-called secondary resonance state in which the oscillation frequency of the inverter circuit 14 and the secondary resonance frequency of the transformer 15 coincide with each other is not reached. As a result, an abnormal high voltage Vmax ( 24 kV) does not occur. This is because the transformer 15 is protected before the abnormal high voltage Vmax due to resonance occurs.

  (2) When a predetermined overvoltage (overvoltage determination threshold Vh) occurs, the oscillation operation of the inverter circuit 14 is stopped by the overvoltage protection circuit 22, thereby generating an abnormal high voltage Vmax on the secondary side of the transformer 15. As a result, damage to the transformer 15 is avoided. Further, when the no-load detection circuit 23 detects the secondary-side no-load state of the transformer 15 when the AC power is on, the no-load detection circuit 23 cannot resonate the oscillation frequency of the oscillation circuit 13. The frequency is set, and the overvoltage protection circuit 22 stops the operation of the oscillation circuit 13 and thus the oscillation operation of the inverter circuit 14. Thereby, damage to the transformer 15 due to the secondary resonance state is avoided. In the no-load state, the AC voltage output from the transformer 15 becomes very high, so that it is eventually detected and protected by the overvoltage protection circuit 22.

  (3) The oscillation frequency of the inverter circuit 14 (more precisely, the oscillation circuit 13) is gradually decreased from a higher value to a lower value. Specifically, the oscillation frequency was decreased from 200 kHz to 50 kHz. In general, the transformer 15 is designed for the lowest oscillation frequency. For this reason, the transformer 15 can be reduced in size as compared with the case where the oscillation frequency of the inverter circuit 14 (more precisely, the oscillation circuit 13) is changed from a lower value to a higher value. That is, in the present embodiment, the transformer 15 is designed in accordance with the lowest oscillation frequency of 50 kHz. On the other hand, when the oscillation frequency of the inverter circuit 14 is changed from a lower value to a higher value, in this embodiment, the oscillation frequency is set to a value lower than 50 kHz which is the lowest oscillation frequency. At the same time, the transformer 15 is designed. For this reason, the transformer 15 is enlarged.

  (4) Even if a voltage exceeding the overvoltage determination threshold Vh is erroneously detected, the oscillation operation of the inverter circuit 14 is automatically restored by the time constant circuit of the resistor 36 and the electrolytic capacitor 38. I made it. For this reason, the stop of the transmission operation of the inverter circuit 14 due to erroneous detection of a predetermined overvoltage can be avoided as much as possible.

(5) The load state on the secondary side of the transformer 15 is detected by the detection winding 31 provided on the secondary side of the transformer 15. For this reason, simplification of a structure is achieved.
<Another embodiment>
In addition, you may implement the said embodiment as follows.

  In the present embodiment, the oscillation frequency of the oscillation circuit 13 when the AC power supply is turned on is set to 200 kHz, for example, but may be set to a value exceeding 200 kHz, for example. Even if it does in this way, the effect similar to (1)-(5) of the said embodiment can be acquired.

  In the present embodiment, for example, a half-bridge type is adopted as the inverter circuit 14, but other types such as a push-pull type and a full-bridge type may be adopted.

  In this embodiment, in order to set the oscillation frequency of the oscillation circuit 13 to 200 kHz when the oscillation frequency control signal S2 is input, the diodes 51, 53, 56, the resistors 52, 54, and the capacitor 55 are connected to the output terminal of the operational amplifier 41. Although the circuit which combined these was connected, you may make it as follows. That is, the configuration of the circuit is appropriately changed in accordance with the characteristics of an IC (integrated circuit) (not shown) that constitutes the oscillation circuit.

<Another technical idea>
Next, a technical idea that can be grasped from the embodiment and the other embodiment will be added.
A detection winding is provided on the secondary side of the transformer, and the load state on the secondary side of the transformer is detected based on the voltage detected by the detection winding.

The block diagram of the discharge lamp lighting circuit in this embodiment. The circuit diagram which similarly shows the structure of a protection circuit. The characteristic view which similarly shows the relationship between the oscillation frequency and voltage of an inverter circuit. The characteristic view which shows the time-dependent change of the positive side voltage of the operational amplifier which comprises the no-load detection circuit in the same no-load state, the positive side voltage of the operational amplifier in the loaded state, and the negative side voltage of the operational amplifier.

Explanation of symbols

DESCRIPTION OF SYMBOLS 11 ... Discharge lamp lighting circuit, 12 ... Rectifier circuit, 14 ... Inverter circuit, 15 ... Transformer,
16 ... AC power supply, 19 ... Discharge lamp, 21 ... Protection circuit, 22 ... Overvoltage protection circuit,
23: No-load detection circuit, Vh: Overvoltage determination threshold.

Claims (4)

A predetermined AC voltage input from an AC power source is converted into a predetermined DC voltage by a rectifier circuit, and the DC voltage is converted into a predetermined AC voltage by a transformer based on the oscillation operation of the inverter circuit and applied to the discharge lamp. In the discharge lamp lighting circuit
As for the oscillation frequency of the inverter circuit, a first oscillation frequency set to a value outside the range that can be taken by the secondary resonance frequency of the transformer assumed in advance, and a second oscillation frequency when the discharge lamp is lit Set each
When the AC power supply is turned on, the inverter circuit is controlled so that the oscillation frequency of the inverter circuit is matched from the first oscillation frequency to the second oscillation frequency, and an abnormal state in which an abnormal voltage assumed in the process can occur is generated. A discharge lamp lighting circuit provided with a protection circuit for setting and maintaining the oscillation frequency of the inverter circuit at the first oscillation frequency when detected. In the discharge lamp lighting circuit according to claim 1,
The discharge lamp lighting circuit in which the abnormal state is a no-load state on the secondary side of the transformer. In the discharge lamp lighting circuit according to claim 1 or 2,
The protection circuit is
An overvoltage protection circuit that stops the oscillation operation of the inverter circuit when the AC voltage exceeds a preset overvoltage determination threshold;
When the AC voltage exceeds a preset no-load determination threshold, it is determined that the secondary side of the transformer is in the no-load state as an abnormal state assumed in advance, and the oscillation frequency of the inverter circuit is set to the first frequency. A discharge lamp lighting circuit comprising: a no-load detection circuit that sets and maintains the oscillation frequency. In the discharge lamp lighting circuit according to any one of claims 1 to 3,
A discharge lamp lighting circuit in which the first oscillation frequency is larger than the second oscillation frequency.

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