JPH0638364A - High voltage power supply circuit - Google Patents

Abstract

PURPOSE:To contrive miniaturization and cost reduction by providing a current detecting means for generating a signal when the load current of a transformer is increased to a fixed value or more and an oscillation stopping means for stopping oscillation of an oscillating circuit in accordance with the signal from the current detecting means. CONSTITUTION:When a load current flows in a direction of arrow mark 58 to generate a potential difference in a resistor 60 of a load current detecting circuit 56, negative voltage is applied to a base of a transistor 62. Now when the load current exceeds a certain fixed value, the transistor 62 is turned on, and a current flows in a base of a transistor 66 through the transistor 62 from a DC power supply 64. Accordingly, the transistor 66 is turned on, and since an output of an operational amplifier 50 is drawn into the earth, a series regulator 14 is turned off. As a result, input voltage is not applied to an oscillating circuit 16 to stop oscillating the oscillating circuit 16, and a secondary side output of a transformer 18 is stopped to suppress an excessive load current. In this way, an overcurrent detection value can be accurately set, to eliminate necessity for using a large sized part.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION The present invention relates to a high voltage power supply circuit,
In particular, it relates to a high-voltage power supply circuit for applying a high voltage to a corona discharger such as a copying machine.

[0002]

2. Description of the Related Art Referring to FIG. 2, in a conventional high-voltage power supply circuit 1 of this type, when a load current becomes large, a base winding 3 of a push-pull type self-excited collector tuning oscillation circuit 2 to a base of a transistor 4 are introduced. The current flowing into the transistor 6 increases and the output voltage of the voltage dividing circuit 5 increases, and the transistor 6
Is turned on, the output of the operational amplifier 7 is drawn to the ground through the transistor 6 and the series regulator 8 is turned off, thereby stopping the oscillation when an excessive current flows to the load and ensuring safety. Was there.

[0003]

In such a conventional high voltage power supply circuit 1, since the overcurrent state is detected based on the magnitude of the base current from the detection winding 3, for example, the amplification degree of the transistor or the like. If the characteristics of the element change, it is necessary to redesign each time, and it is difficult to perform the optimum design. In order to avoid such difficulties, it is necessary to set the overcurrent detection value with a margin in consideration of variations in element characteristics, but this requires a design that greatly exceeds the required rating. There are problems that the circuit parts are large and the cost is high.

Therefore, a main object of the present invention is to provide a high-voltage power supply circuit which is small in size and low in cost.

[0005]

The present invention provides a high voltage power supply circuit including a transformer in which an output from an oscillation circuit is provided as a primary side input and a secondary side output is provided to a load.
Characterized by further comprising current detection means for generating a signal when the load current of the transformer exceeds a certain value, and oscillation stop means for stopping the oscillation of the oscillation circuit in response to the signal from the current detection means. It is an overcurrent suppression circuit.

[0006]

The load current of the transformer flows in the current path coupled to the output of the transformer, and the voltage corresponding to this current is compared with the reference value by the comparator. Therefore, when the load current exceeds a certain value, a signal is output from this comparator, that is, the current detecting means. This signal, for example, turns on the transistor, thereby turning off the series regulator and stopping oscillation.

[0007]

According to the present invention, since the overcurrent is suppressed by directly detecting the load current, the overcurrent detection value can be set more accurately. Therefore, it is not necessary to use an unnecessarily large circuit component, so that a small-sized and inexpensive high-voltage power supply circuit can be obtained. The above-mentioned objects, other objects, features and advantages of the present invention will become more apparent from the following detailed description of the embodiments with reference to the drawings.

[0008]

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring to FIG. 1, a high voltage power supply circuit 10 of this embodiment includes a DC power supply 12. The direct current from the direct current power supply 12 is stabilized by a series regulator 14 including two transistors 14a and 14b, and is included in a transformer 18 included in a push-pull type self-excited collector tuning oscillation circuit (hereinafter, simply "oscillation circuit") 16. Is input to the center tap of the main winding 20 of the. The stabilized DC output from the series regulator 14 also passes through the resistor 22 and the Zener diode 24, and then the base winding 26a and the resistor 28a of the oscillator circuit 16 and the base winding 26b and the resistor 28b.
Through the base of the transistor 30a and the base of the transistor 30b. This oscillator circuit 16
As is well known in the art, since the electromotive force generated in the base windings 26a and 26b changes according to the change in the magnetic flux generated in the main winding 20, the transistors 30a and 30b.
Are alternately turned on, so that a high voltage corresponding to the transformation ratio is output to the secondary winding 21 of the transformer 18. This high voltage output is supplied to the load 38, which is simulated by the resistor 34 and the capacitor 36 for convenience, via the resistor 32.

Further, an electromotive force is generated in the detection winding 40 by the magnetic flux generated in the main winding 20. The detection voltage from the detection winding 40 is rectified by the diode 42, smoothed by the smoothing capacitor 44, and then divided by the voltage dividing circuit 46. Then, this divided voltage is applied to the operational amplifier 50 via the resistor 48.
Is input to the (-) terminal and is compared with the reference voltage input to the (+) terminal via the resistor 52. This operational amplifier 5
The output of 0 is input to the base of the series regulator 14.

Further, a capacitor 54 is connected in parallel to the secondary winding 21 of the transformer 18, and a load current detection circuit 56 is connected in series with the capacitor 54.
The load current detection circuit 56 operates when a load current of a certain value or more flows in the direction of arrow 58 through the ground. That is, when a load current flows in the direction of arrow 58 and a potential difference occurs in the resistor 60, a negative potential is applied to the base of the transistor 62. Here, when the load current exceeds a certain value and the base potential of the transistor 62 exceeds the base-emitter voltage V _BE , the transistor 62 is turned on,
A current flows from the DC power source 64 to the base of the transistor 66 via the transistor 62. Therefore, the transistor 66 is turned on and the output of the operational amplifier 50 is pulled to the ground, so that the series regulator 14 is turned off. As a result, the input voltage to the oscillation circuit 16 disappears, the oscillation of the oscillation circuit 16 stops, and the secondary side output of the transformer 18 disappears, so that an excessive load current is suppressed.

In the above embodiment, the self-excited collector tuning oscillation circuit 16 is used, but a separately excited oscillation circuit may be used, and it is not a push-pull circuit but a one-stone type oscillation circuit. Of course, it is also good.

[Brief description of drawings]

FIG. 1 is a circuit diagram showing an embodiment of the present invention.

FIG. 2 is a circuit diagram showing a conventional technique.

[Explanation of symbols]

 10 ... High-voltage power supply circuit 14 ... Series regulator 16 ... Oscillation circuit 40 ... Detection winding 50 ... Operational amplifier 56 ... Load current detection circuit 62, 66 ... Transistor

Claims (1)

[Claims] 1. A high-voltage power supply circuit including a transformer in which an output from an oscillation circuit is given as a primary side input and a secondary side output is given to a load, wherein a load current of the transformer exceeds a certain value. An overcurrent suppressing circuit, further comprising: current detecting means for generating a signal when the current is detected; and oscillation stopping means for stopping the oscillation of the oscillation circuit in response to the signal from the current detecting means.