JP2005142099A - Lamp open detection circuit for discharge tube - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To detect, with a simple structure, a lamp open state where a discharge tube is broken and a current does not flow through it. <P>SOLUTION: The discharge tubes 4 are connected to secondary output sides of output transformers driven by an inverter. A shunt resistor RS is connected to a current-carrying path of each discharge tube 4. A rectifier diode 8 is connected to each shunt resistor RS. Each rectifier diode 8 outputs a direct current signal varying according to the change of a current flowing through the discharge tube 4. A threshold value is inputted to one input end of each comparator, and the direct current signal of the rectifying diode is inputted to the other input end thereof by being converted into a voltage signal. When the voltage signal is set to zero, the comparator outputs a lamp open signal. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to a lamp open detection circuit for detecting a state in which a discharge tube such as a plurality of cold cathode fluorescent lamps driven on the secondary side of an output transformer is cut off, that is, a lamp open state.

2. Description of the Related Art Conventionally, a discharge tube dimming control circuit that detects a tube current of a discharge tube using a shunt resistor and controls the tube current of the discharge tube using this detection signal is known (see, for example, Patent Document 1).
Conventionally, when a cold cathode fluorescent lamp is being driven, when the lamp does not light or breaks and the lamp is open, this is detected and the secondary voltage of the output transformer is isolated from the secondary winding. As shown in FIG. 2, a circuit for stopping the output before the withstand voltage is exceeded has capacitors C1 and C2 connected in series to both ends of the winding L2 on the secondary side S of the output transformer T. The generated divided voltage is detected from the lead wire W, and the lamp open state is detected by the increase of the divided voltage. In FIG. 2, RS is a shunt resistor for detecting a tube current flowing in the cold cathode fluorescent lamp LP, CB is a ballast capacitor, and CT is a resonance capacitor.
Japanese Patent Laid-Open No. 2003-1000049 (first page, FIG. 6)

As described above, the method of dividing the secondary side of the output transformer by the capacitors C1 and C2 uses a capacitor with a high withstand voltage, for example, for 3KV, and is large and expensive. In this case, the number of lead wires for high-voltage wires increases. Further, when a winding transformer is used as the output transformer and a series resonance circuit is formed on the primary side P of the output transformer T by L of the primary winding L1 and C of the resonance capacitor CT, the resonance frequency of the primary side P is determined. The resonance frequency of the secondary side S is determined by C1 + C2 and L of the secondary winding L2 with respect to the capacitor CT and L of the primary winding L1. Without the capacitors C1 and C2, the secondary side resonance frequency C is determined only by the secondary side stray capacitance, but the capacitors C1 and C2 set a strong secondary side resonance frequency. There is a problem that this affects the primary side and leads to a decrease in lamp driving efficiency due to primary resonance self-excited oscillation.
The present invention aims to solve the above problems.

  In order to achieve the above object, the present invention provides a lamp for detecting when a discharge tube is connected to the secondary output side of an output transformer driven by an inverter and the discharge tube is in a lamp open state and no current flows. An open detection circuit, which is connected to a shunt resistor connected to a current path of a discharge tube and outputs a direct current signal that changes in accordance with increase / decrease of a current flowing through the discharge tube, and a threshold at one input terminal A signal processing circuit in which a value is inputted and a DC signal of the rectifying diode is converted into a voltage signal and inputted to the other input terminal, and a predetermined signal is outputted when the voltage signal changes exceeding the threshold value It is equipped with.

According to the present invention, a plurality of discharge tubes are connected to a plurality of secondary output sides of one or a plurality of output transformers driven by an inverter, and at least one of the plurality of discharge tubes is connected. A lamp open detection circuit for detecting when a discharge tube is in an open state and no current flows, wherein a shunt resistor connected to each energization path of each discharge tube, and one end of which is connected to the shunt resistor A rectifying diode provided for each of the plurality of shunt resistors so as to output a DC current signal that changes in accordance with a change in increase / decrease in the AC current flowing through the discharge tube and provided for each of the rectifying diodes. A threshold value is input to one input terminal, a DC current signal of the rectifying diode is converted to a voltage signal and input to the other input terminal, and output when the voltage signal decreases. A signal processing unit configured to output a lamp open signal, and at least one of the signal processing units outputs a lamp open signal from its output terminal to the output end of each signal processing unit. A lamp open signal is output from the lead wire to be connected.
According to the present invention, the signal processing circuit is connected to a malfunction prevention circuit, and the malfunction prevention circuit includes a semiconductor switching element that is turned on when the power is turned on and turned off when a predetermined time elapses. The output side is grounded via the semiconductor switching element.

  The present invention can detect the lamp open state of the discharge tube with a simple configuration. Further, since it is not necessary to provide a voltage dividing capacitor on the secondary side of the output transformer, the cost can be reduced, and the influence on the resonance frequency on the secondary side of the output transformer can be reduced. it can.

Embodiments of the present invention will be described below in detail with reference to the accompanying drawings.
The figure shows a lamp open detection circuit and a tube current detection circuit of the discharge tube, and in order to avoid complication of the drawing, self-excited oscillation for supplying an AC drive signal to the primary side of the output transformers T1, T2, T3 and the primary side or The separately excited inverter circuit is omitted. This inverter circuit includes a PWM (pulse width modulation) control circuit, and controls the AC power supplied to the primary side of the output transformers T1, T2, T3 according to the tube current detection output supplied from the lead wire 2, Control is performed so that the discharge tube 4 including a plurality of cold cathode fluorescent lamps (CCFLs) has a predetermined brightness.

  In the present embodiment, the output transformers T1, T2, and T3 are each composed of three winding transformers each having one input and two outputs. The primary sides of the output transformers are connected in parallel and are output from an inverter circuit. It is configured to be driven by an AC drive signal. The pair of discharge tubes 4 and 4 connected to the output transformer T1 are connected in series with each other, and the electrode at one end of the pair of discharge tubes 4 and 4 is connected to the secondary winding S1 of the output transformer T1 via the ballast capacitor CB. The other end of the electrode is connected to the high voltage terminal of the secondary winding S2 of the output transformer T1 via the ballast capacitor CB.

  Similarly, the other pair of discharge tubes 4, 4, 4, 4 connected in series with each other are connected to the secondary electrodes of the output transformers T 2, T 3 via the ballast capacitor CB, as shown in FIG. Connected to the high voltage terminal of the winding. One end of the shunt resistor RS is connected to each ground terminal of the secondary winding S1 of the output transformers T1, T2, T3, and the other end of the shunt resistor RS is grounded through a common lead wire 6. Yes. The shunt resistor on the ground terminal side of the secondary winding S2 of each output transformer T1, T2, T3 and the circuit connected thereto are not shown. Each one end of the shunt resistor RS is connected in parallel to the common output lead 2 via a DC blocking capacitor C1 and rectifying diodes 8, 10, and 12.

  Rectifying diodes 14, 16, and 18 for full-wave rectification are connected to the other end sides of the shunt resistor RS, and the rectifying diodes 14, 16, and 18 are connected to the corresponding rectifying diodes 8, 10, and 12, respectively. They are connected as shown. The output lead wire 2 is grounded via a capacitor C2 and a resistor R1 constituting a smoothing circuit, and the output lead wire 2 is connected to a tube current detection circuit (not shown) of an inverter circuit. Further, one end of each shunt resistor RS is connected through a capacitor C1 and rectifying diodes 20, 22, and 24, and through corresponding lead wires 32, 34, and 36, and comparators 26, 28, and 30 having corresponding open collector output terminals, respectively. Is connected to the inverting input terminal.

  A smoothing circuit comprising a capacitor C3 and a resistor R2 is connected to each of the lead wires 32, 34, and 36. The non-inverting input terminal of each comparator 26, 28, 30 is applied with a threshold voltage divided by resistors R3, R4 from a positive power source. The comparators 26, 28, and 30 having the open collector type output terminal are turned off when the potential of the inverting input terminal is higher than the threshold value of the non-inverting input terminal. When the impedance becomes a very large value and the potential of the inverting input terminal is lower than the threshold value of the non-inverting input terminal, the output terminal is set to an on state.

  As shown in the figure, one of the output terminals of the comparators 26, 28, 30 is connected in parallel to an output lead wire 38 connected to a positive DC power supply line via a resistor R5. The other of the output lead wires 38 is grounded via a malfunction prevention circuit 40 including a CR time constant circuit of a resistor R6 and a capacitor C4 and a semiconductor switching element Q1. A lead wire 42 connected to a lamp open detection circuit (not shown) of the inverter circuit is connected to the output lead wire 38. In the above embodiment, the circuit for detecting the tube current on the secondary winding S2 side is not shown, but a circuit for detecting the tube current is also provided for these as in the case of the secondary winding S1. Can do. In this embodiment, a plurality of one-input two-output winding transformers are used. However, a configuration in which a single multi-output winding transformer is provided, and a piezoelectric transformer or the like may be used as the output transformer. An output transformer can be used.

  In the above configuration, when an AC drive signal PWM-controlled from the inverter circuit is supplied to the primary side of the output transformers T1, T2, and T3, a high-voltage AC voltage is applied to the secondary side of each of the output transformers T1, T2, and T3. Induced, each discharge tube 4 is lit. The rectifier diodes 8, 10, 12 for detecting the magnitude of the tube current of each discharge tube 4 output a direct current signal corresponding to the current flowing through the discharge tube 4, and the output current of each rectifier diode 8, 10, 12 is output. The added current flowing through the lead wire 2 is smoothed by the smoothing circuits R1 and C2, and appears on the downstream side of the lead wire 2 as a voltage signal representing the magnitude of the tube current.

  The output signals of the rectifier diodes 8, 10, and 12 are usually substantially the same, and the voltage signal appearing on the lead wire 2 is substantially constant. If even one of the rectifier diodes 8, 10, 12 increases the output signal, this increase appears on the lead wire 2 and is supplied from the lead wire 2 to the tube current detection circuit of the inverter circuit. That is, the rectifier diodes 8, 10, and 12 constitute a wired OR circuit in the analog circuit. On the other hand, while the discharge tube 4 is lit, the output terminals of the comparators 26, 28, and 30 are in an OFF state, and the lead wire 42 is in a Hi state due to the voltage supplied from the positive power supply terminal. During driving of the inverter circuit, for example, when one of the discharge tubes 4 is cut and no current flows through the shunt resistor RS, the voltage across the shunt resistor RS becomes zero, and the output current of the corresponding rectifier diode, for example 20, becomes zero. It becomes.

  As a result, the potential of the inverting input terminal of the comparator 26 becomes lower than the threshold value, the output terminal of the comparator 26 is turned on, and the internal impedance of the comparator 26 is lowered. When the output terminal of the comparator 26 is turned on, the lead wire 42 is grounded through the comparator 26, the voltage is lowered, and the lead wire 42 is in a low state. This Low signal is supplied to the lamp open detection circuit of the inverter circuit. When the tube current is reduced to zero by the dimming operation of the inverter circuit, a malfunction prevention circuit (not shown) provided separately prevents the lamp open detection circuit of the inverter circuit from receiving a low signal. It is set not to detect lamp open.

In the malfunction prevention circuit 40, the switching element Q1 is normally in an off state. When the power is turned on, the switching element Q1 is instantaneously turned on and turned off after a predetermined time has elapsed. This prevents a malfunction signal from being input to the inverter circuit from the lead wire 42 when the power is turned on.
In this embodiment, the signal of the rectifying diode is processed using an open collector type comparator. However, the present invention is not particularly limited to a circuit using an open collector type comparator. A semiconductor having a signal processing function in which a threshold value is input to an input terminal and a DC signal of the rectifying diode is converted to a voltage signal and input to the other input terminal, and a predetermined signal is output when the voltage signal changes. As long as it is an element or circuit, it is not particularly limited to an element or circuit using a comparator.

It is an electronic circuit diagram showing an embodiment of the present invention. It is an electronic circuit diagram of a prior art.

Explanation of symbols

2 Lead wire
4 discharge tube 6 lead wire 8 rectifier diode 10 rectifier diode 12 rectifier diode 14 rectifier diode 16 rectifier diode 18 rectifier diode 20 rectifier diode 24 rectifier diode 26 comparator 28 comparator 30 comparator 32 lead wire 34 lead wire 36 lead wire 38 lead Wire 40 Malfunction prevention circuit 42 Lead wire

Claims (2)

  A lamp open detection circuit for detecting when a discharge tube is connected to a secondary output side of an output transformer driven by an inverter and the discharge tube is in a lamp open state and no current flows, A rectifier diode that is connected to a shunt resistor connected to a current path and outputs a DC current signal that changes in accordance with an increase or decrease in the current flowing through the discharge tube, and a threshold value is input to one input terminal, and the other input terminal A lamp comprising: a signal processing circuit configured to input a DC signal of a rectifying diode converted into a voltage signal and to output a predetermined signal when the voltage signal changes beyond the threshold value Open detection circuit. A plurality of discharge tubes are connected to a plurality of secondary output sides of one or a plurality of output transformers driven by an inverter, and at least one of the plurality of discharge tubes is open. A lamp open detection circuit for detecting when the current stops flowing and a shunt resistor connected to each energization path of each discharge tube, and one end connected to the shunt resistor and the discharge tube A rectifying diode provided for each of the plurality of shunt resistors so as to output a direct current signal that changes in accordance with a change in increase / decrease of the alternating current flowing through the rectifier, and one input terminal provided for each of the rectifying diodes. When a threshold value is input and the DC current signal of the rectifying diode is converted to a voltage signal and input to the other input terminal and the voltage signal decreases, the output terminal is switched off. A signal processing unit configured to output a pun signal, and when one of the signal processing units outputs a lamp open signal from its output terminal, the lead connected to the output terminal of each signal processing unit A lamp open detection circuit characterized in that a lamp open signal is output from a wire.

Images