JPH0623190Y2 - DC / DC converter power supply circuit - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial application] The present invention relates to a display device of a fluorescent display tube, and more particularly to a DC / D.
The present invention relates to an overcurrent protection circuit for a C converter power supply circuit.

[Prior Art] Conventionally, a DC / DC converter power supply circuit used in a display device of a fluorescent display tube has a DC power supply connected to one terminal of a primary coil of a pulse transformer 11 as shown in the circuit configuration diagram of FIG. At the other end, the collector terminal 10C of the NPN transistor 10 for converting into a DC current pulse signal
Are connected, and the emitter terminal 10E is the GND terminal 3
Connected to. On the other hand, the base terminal 10B is connected to the DC power supply via the resistor 25 that limits the base current.

The secondary side of the pulse transformer 11 is divided into two circuits,
The first circuit therein is made of a center tap 26 type coil, and both ends of this coil are connected to the filament terminal of the fluorescent display tube, and the center tap 26 is connected to the GND terminal 3 via the resistor 15. ing.

The second circuit is made up of a coil provided with a tap 27 for detecting an output pulse signal on the side of one of the terminals that is slightly closer to the center, and the terminal near the tap 27 is the GND output terminal 22 (GND terminal 3). Is connected to the DC output terminal 21 via a rectifying diode 17 that rectifies the pulse output signal. DC output terminal 2
An electrolytic capacitor 36 is connected between 1 and the GND output terminal 22 in order to further correct the pulsating current signal after rectifying the pulse output signal to direct current.

Between the DC output terminal 21 and the center tap 26 of the first circuit, a cut-off voltage Zener diode 16 of the fluorescent display tube is connected to the diode 16 with the cathode terminal being the DC output terminal 21 side.

Between the output pulse signal detection tap 27 of the second circuit and the base terminal 10B of the NPN transistor 10, a capacitor 12 and a resistor 13 are provided to feed back the output pulse signal to the base terminal 10B of the NPN transistor 10. It is connected in series. Further, since the base potential is set between the DC output terminal 21 and the base terminal 10B so that the oscillation of the NPN transistor 10 operates stably, the Zener diode 14 is placed so that the cathode terminal is on the base terminal 10B side. Are connected.

In the case of the DC / DC converter power supply 23 having the above-described circuit configuration, if the load side of the DC / DC converter power supply 23 or the DC / DC converter power supply circuit has an abnormality, an excessive current flows to the power supply input terminal 2. , The NPN transistor 10 fails, the NPN transistor 10 heats up, the printed circuit board fixing the NPN transistor 10 burns out, and the coil of the pulse transformer 11 suffers a short circuit failure. Sometimes.
Therefore, for the purpose of this overload protection, a film type fuse resistor or an IC protector (IC protector) is connected in series with the power input terminal 2.
P) 24 was used.

[Problems to be solved by the invention]

In the case of the circuit configuration of the conventional DC / DC converter power supply 23 described above, if there is an abnormality in the DC / DC converter power supply 23 or its load side, an overload state occurs and a film-type fuse resistor or ICP24. Is melted down, and the overload protection of the DC / DC converter power supply 23 can be performed. However, since the film type fuse resistor or ICP24 is directly mounted on the printed circuit board and fixed by soldering, once the film type fuse resistor or ICP24 is blown, DC
/ DC converter power supply, or after repairing the abnormal part of its overload circuit, further, film type fuse resistor or ICP2
4 needs to be replaced and repaired by soldering work. Also,
When the film-type fuse resistor is closely mounted on the printed circuit board, the printed circuit board may be burnt and discolored by the heat generated when the film-type fuse resistor melts.

Further, since the DC / DC converter power supply 23 uses the pulse transformer 11, a dash current flows. This peak value of the dash current has a drawback that the film type fuse resistor or the ICP 24 is blown.

DC / used in the above-mentioned conventional fluorescent display device
When an abnormality occurs in the fluorescent display device in the DC converter power supply circuit and the DC / DC converter power supply circuit becomes overloaded, a film-type fuse resistor is provided for the purpose of overload protection of this DC / DC converter power supply circuit. Or IC
P was connected in series to the power input terminal. This conventional D
In contrast to the structure of the C / DC converter power supply circuit, the present invention provides an inexpensive circuit by providing an overload protection circuit composed of a single PNP type transistor on the power supply input terminal side of the DC / DC converter power supply. The DC / DC converter power supply has a configuration capable of performing overload protection.

[Means for solving problems]

The present invention relates to a DC / DC converter power supply circuit for a fluorescent display device, in which an overload protection circuit composed of a single PNP type transistor is provided between a power supply input terminal and a primary side circuit of a pulse transformer. Has become.

〔Example〕

FIG. 1 is a circuit diagram of a DC / DC converter power supply showing an embodiment of the present invention. Since the same components as those in FIG. 2 are designated by the same reference numerals, the description thereof will be omitted. DC / DC
The voltage input to the converter power supply is supplied to the power supply input terminal 2 and GN.
It is applied between the D terminals 3 and the applied voltage is divided to
DC depending on the current flowing into the / DC converter power supply 1
The voltage dividing resistors 5 and 6 are provided between the power supply input terminal 2 and the GND terminal 3 in order to set the operation base potential of the PNP transistor 8 that stops the oscillation of the / DC converter power supply. The contacts of the voltage dividing resistors 5 and 6 are connected to the base terminal 8B of the PNP type transistor 8.

Next, in order to prevent the PNP transistor 8 from being in the OFF operation state and stopping the oscillation of the DC / DC converter power supply by detecting an excessive current that instantaneously flows when the power is turned on, a short time is required when the power is turned on. The electrolytic capacitor 7 is inserted in parallel with the voltage dividing resistor 6 between the base terminal 8B and the GND terminal 3 so that the base potential of the PNP type transistor 8 is lowered and the PNP type transistor 8 is turned on. The resistor 5 and the electrolytic capacitor 7 form an integrating circuit.

Next, in order to detect the current flowing into the DC / DC converter power supply and control the oscillation and stop of the DC / DC converter power supply, a current is detected between the power supply input terminal 2 and the upper terminal 46 of the primary coil of the pulse transformer 1. The resistor 4 is connected, and the upper terminal 46 of the primary coil and the emitter terminal 8E of the PNP transistor 8 are connected.

Further, the base terminal 10B of the NPN transistor 10
A resistor 9 for limiting the base current of the NPN transistor 10 is connected between the collector terminal 8C of the PNP transistor 8 and the collector terminal 8C of the PNP transistor 8.

[Effect of device]

As described above, a DC / D for a fluorescent display device
In the C converter power source, by providing an overload protection circuit on the power source input side of the primary side circuit, which controls the oscillation and stoppage of an NPN type transistor that converts a DC current into a pulsed signal according to the load current, When an abnormality occurs in the display tube display device and an overload occurs, the input current can be cut off promptly, so that the NPN transistor is not heated and the printed circuit board can be prevented from burning.

Moreover, the base potential of the PNP transistor is momentarily lowered when the power is turned on because of the integration function of the electrolytic capacitor inserted between the base terminal and the GND terminal of the PNP transistor that constitutes the overload protection circuit. Therefore, the overload cutoff function of the PNP transistor is momentarily released, and the malfunction of the overload protection circuit due to the dash current when the power is turned on can be prevented.

[Brief description of drawings]

FIG. 1 is a DC / DC converter power supply circuit diagram with an overload protection circuit for a fluorescent display device according to an embodiment of the present invention,
FIG. 2 is a conventional DC / DC converter power supply circuit diagram. 1 ... DC / DC converter power supply, 2 ... power supply input terminal, 3 ... GND terminal, 4 ... current detection resistor, 5, 6
...... Resistance, 7 ...... Electrolytic capacitor, 8 ...... PNP type transistor, 8B ...... PNP type transistor base terminal, 8C ...... PNP type transistor collector terminal, 8E ...... PNP type transistor emitter terminal, 9 ...... Current limiting resistor.

Claims (1)

[Scope of utility model registration request] 1. A transformer (11) in which one end of a primary coil is connected to a DC power supply (2), a base is connected to the DC power supply (2), an emitter is connected to ground (3), and a collector is the above. Connected to the NPN transistor (10) connected to the other end of the primary coil, the first circuit connected to the filament of the fluorescent display tube on the secondary side of the transformer, and the DC output terminal (21) (12, 13) for feeding back the output pulse signal in the second circuit to the base of the NPN transistor (10).
In a DC / DC converter power supply circuit for use in a fluorescent display device comprising: a PNP transistor (8) connected between the base of the NPN transistor (10) and the DC power supply (2). The collector of the PNP transistor (8) is connected to the base of the NPN transistor through a resistor (9) that limits the base current of the NPN transistor, and the base of the PNP transistor (5, 5) divides the input voltage. 6), a capacitor (7) is connected in parallel with the ground side resistor (6) of the dividing resistor, and the PNP transistor (8)
A DC / DC converter power supply circuit, wherein a resistor (4) for detecting an input current of the DC / DC converter power supply circuit is provided between the emitter and the base of the device to prevent an excessive input current when overloaded.