JPH06294988A - Electronic flash device - Google Patents

Abstract

PURPOSE:To prevent an accident, which is caused by the heating of an oscilla tion transformer etc., due to a short circuit by a defect in insulation between primary and secondary windings of the oscillation transformer, with simple constitution as to an electronic flash circuit which includes a DC/DC converter. CONSTITUTION:A constant voltage element 30 which has a higher constant voltage level than the voltage of a power source battery 1 is connected between the base of an oscillation transistor 10 and the secondary winding (S winding) of the oscillation transformer 12 and then the constant voltage element 30 cuts off a current when the primary winding (P winding) and S winding of the oscillation transformer 10 are short-circuit owing to a defect in insulation, so that a loop which returns to the battery 1 through the base-emitter part of the oscillation transistor 10, constant voltage element 30, and the S winding and P winding of the oscillation transformer 12 is cut off.

Description

Detailed Description of the Invention

[0001]

FIELD OF THE INVENTION The present invention relates to an electronic flash device for a camera.

[0002]

2. Description of the Related Art Conventionally, cameras have been downsized due to the requirement of portability. Therefore, there is an indispensable demand for downsizing of electronic flash devices used externally and electronic flash devices incorporated in cameras.

FIG. 3 shows an example of a circuit diagram of a conventional electronic flash device of this type. This electronic flash device is a known DC / DC
A converter is included in the circuit configuration, and the operation of the DC / DC converter is performed by a control terminal c, a voltage is applied to the c terminal by a control circuit (not shown), the transistor 15 is turned on via the resistor 16, and the oscillation transformer 12 A starting current is applied to the PNP-type oscillation transistor 10 via the feedback winding (F winding). A known operation is performed by this starting current, and charges for causing the xenon tube 24 to emit light are accumulated in the main capacitor 25.

When the voltage of the main capacitor 25 exceeds the Zener voltage of the Zener diode 11, a voltage is generated at the control terminal a and is given to the charge completion detection circuit of the control circuit as a charge completion signal, and this signal is generated. As a result, the control terminal b changes the initial non-connection state (open state) to the negative potential of the battery 1, so that the transistor 7 is turned on, the base and the emitter of the oscillation transistor 10 are short-circuited, and the oscillation transistor 10 is turned off. State. At the same time, the terminal c changes to the low state, the transistor 15 changes from the on state to the off state, the oscillation of the DC / DC converter is completely stopped, and the initial state is restored.

The terminal d is a terminal for applying a trigger voltage to the xenon tube 24 to cause it to emit light. By applying a voltage to the d terminal from an open state, the thyristor 20 is turned on and a high voltage pulse is applied to the xenon tube 24 by the trigger transformer 23. Is applied to bring the xenon tube 24 into an excited state, and the xenon tube 24 is caused to emit light by the charging energy of the main capacitor 25.

As described above, when the DC / DC converter operates, a voltage corresponding to the charging voltage of the positive main capacitor 25 and the oscillation transformer are provided between the secondary winding (S winding) of the oscillation transformer 12 and the diode 13. Twelve backspike voltages occur at negative voltages. This voltage is a high voltage of about 1 KV.

[0007]

However, in an electronic flash device using a xenon lamp, it is general that the xenon lamp has a light emission voltage of more than 200V.
/ DC converter is used to boost the voltage and the emission energy is stored in the main capacitor. Therefore, the boosted voltage and the high back spike voltage of the oscillation transformer of the DC / DC converter generated when boosting the voltage cause From the problem, it has become a big problem for downsizing. In addition, as described above, the secondary winding of the oscillation transformer 12 and the diode 13
The oscillating transformer of the DC / DC converter is designed in consideration of the high voltage generated between the two. However, if the oscillating transformer is downsized, the distance between the windings inside the transformer will be reduced and the margin of the withstand voltage will be sufficient. It's gone. As a countermeasure against these, there is a method of reducing the spike voltage to several hundreds V or less by using a capacitor (not shown) for absorbing the spike voltage at both ends of the secondary winding of the oscillation transformer 12. If the value is increased, the charging current loss at the time of oscillation and the absorption charge of the spikes will cause oscillation transformer 10
It has the drawback of delaying the off time and lengthening the charging time.

Also in the structure of the oscillation transformer, it is impossible to prevent the insulation deterioration due to the pinholes of each winding caused by manufacturing problems by the flux of the impregnated member, and the insulation such as epoxy is used for insulation. The current situation is to take measures such as strengthening. However, the short-circuit accident due to the insulation failure between the secondary winding and the primary winding (P winding) of the transformer 12 causes the base-emitter of the oscillation transistor 10, the secondary winding, the primary winding, and the battery 1 from the battery 1. The current flows in the negative loop of the transistor 10, the transistor 10 operates by the bias of the base, and the battery 1 is short-circuited, which may cause heat generation of the oscillation transformer 12 or thermal destruction of the oscillation transistor 10.

In order to prevent such a situation, a method of interrupting the current with a thermal fuse in a transistor or a transformer or a switching element is provided in the current loop of the DC / DC converter from the battery, and the current is switched when the thermal is detected. There is also a method of cutting off the current by turning it off, but for the former, it is necessary to anticipate the problem that the fuse will break during soldering and which parts the heat will concentrate, and it is necessary to install it in advance. There is a drawback that the variation of the cutoff temperature becomes large.

It is an object of the present invention to provide a miniaturized electronic flash device which eliminates the above-mentioned drawbacks of the prior art and can prevent an accident due to heat generation of an oscillation transformer with a simple structure.

[0011]

In order to achieve the above-mentioned object, an electronic flash device of the present invention is an electronic flash circuit including a DC / DC converter, wherein a base of an oscillating transistor and a secondary winding of an oscillating transformer are provided. Is connected to a constant voltage element having a constant voltage level higher than the power supply voltage.

[0012]

In the electronic flash device having the above-described structure, when the secondary winding and the primary winding of the oscillating transformer are short-circuited due to insulation failure during operation of the DC / DC converter, the power supply battery and the oscillation are thereby caused. Since the constant voltage element is present in the loop formed during the run, the current is blocked and the loop is broken, so that the temperature of the battery or the transformer is prevented from rapidly increasing.

[0013]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS A first embodiment of the present invention will be described below with reference to FIG. For simplification of description, the same parts as those of the conventional example shown in FIG. 3 are designated by the same reference numerals and only different points will be described. In this embodiment, the constant voltage element 30 having a constant voltage level higher than the power supply voltage is provided between the base of the oscillation transistor 10 and the feedback winding (F winding) of the oscillation transformer 12.
And a resistor 26 and a transistor 27 between the base of the oscillation transistor 10 and the negative electrode of the power supply battery 1.
3 is connected to a resistor 28 connected to the base of the transistor 27, and a signal current limiting resistor 29 connected to the base of the transistor 27 is connected to the control terminal e, and the control terminal c in FIG. Since the same operation as the control terminal e can be performed, the transistor 15 and the resistor 16 are deleted together with the control terminal c, and the resistor 14 is connected to the negative electrode of the power supply battery 1. Other configurations are similar to those of the conventional example shown in FIG.

In this embodiment having the above-mentioned structure, DC / D
The C converter is started up at the control terminal e, and when a voltage is applied to the control terminal e, the transistor 27 passes through the resistor 29.
A current flows through the base and emitter of the transistor 27 and the transistor 27 is turned on. When the transistor 27 is turned on, the base current of the oscillation transistor 10 flows through the current limiting resistor 26. This current becomes the starting current of the DC / DC converter, and the DC / DC converter starts oscillation. Since the voltage generated in the S winding and F winding of the oscillation transformer 12 is higher than the voltage of the constant voltage element 30, the induced current of the oscillation transformer 12 is the diode 13, It flows through the main capacitor 25, the battery 1, the base-emitter of the oscillation transistor 10, and the constant voltage element 30.

Further, since the induction current of the F winding sets the winding ratio of the S winding and the F winding larger than that of the conventional circuit, when a voltage higher than the voltage of the constant voltage element 30 is induced, the resistance is increased. 14, a current flows between the battery 1, the base-emitter of the oscillation transistor 10 and the constant voltage element 30. Then, during the operation of the DC / DC converter, by applying a voltage to the terminal e, it is possible to perform oscillation equivalent to that of a known DC / DC converter.

Assuming that the above trouble should occur during the operation of the DC / DC converter as described above, the explanation will be made. 1, a loop returning to the battery 1 through the base-emitter of the oscillation transistor 10, the constant voltage element 30, the S winding, and the P winding. However, since the constant voltage element 30 uses a constant voltage element having a voltage higher than the power supply voltage, in such an abnormal mode where normal oscillation is not performed due to a short circuit, the constant voltage element is in a current blocking state and the loop Will be refused. Further, even when a voltage is applied to the terminal e, a large current does not flow because the base current of the transistor is limited by the current limiting resistor 26, and the temperature of the battery and the temperature of the transformer rise rapidly. Prevent that.

Further, by providing a timer circuit in the control circuit which operates from the start of charging and stopping the charging after a predetermined time, the current flowing through the limiting resistor 26 can be cut off, and the voltage of the battery 1 is reduced by the constant voltage element 30. Since no current flows, the operation of the DC / DC converter can be surely stopped. Therefore, in this embodiment, even if the charge completion signal is not generated at the control terminal a, a large current does not flow in the oscillation transistor 10 and thermal destruction does not occur.

FIG. 2 shows a second embodiment of the present invention. In order to simplify the description, the same parts as those in the first embodiment will be designated by the same reference numerals. In this embodiment, the oscillation transistor 10 is changed from the PNP type to the NPN type, and the connection direction is changed along with the other components accordingly. Reference numeral 40 denotes a flash circuit block including a flash discharge tube and a light emitting circuit.

In this embodiment having the above-mentioned structure, the oscillation transformer 1
When a high voltage causes insulation failure between the secondary winding (S winding) and feedback winding (P winding) of No. 2, a short circuit occurs from the P winding of the battery 1 and the oscillation transformer 12. The line, the constant voltage element 30, the base-emitter of the oscillation transistor 10 and the negative loop of the battery 1 are formed,
When the normal oscillation is not performed, the current of this loop is cut off by the constant voltage element 30, so that the same effect as that of the first embodiment can be obtained.

[0020]

As described above, the present invention provides DC / D.
DC / DC in an electronic flash circuit including a C converter
By connecting a constant voltage element having a constant voltage level higher than the power supply voltage between the base of the oscillation transistor in the converter startup current loop and the secondary winding of the oscillation transformer,
It is possible to prevent heat generation of the oscillation transformer and thermal destruction of the oscillation transistor with a simple configuration, and it is possible to reduce the size of the device.

[Brief description of drawings]

FIG. 1 is a circuit diagram of an electronic flash device according to a first embodiment of the present invention.

FIG. 2 is a circuit diagram of an electronic flash device according to a second embodiment of the present invention.

FIG. 3 is a circuit diagram of a conventional electronic flash device.

[Explanation of symbols]

1 ... Battery, 10 ... Oscillation transistor, 11 ... Zener diode, 12 ... Oscillation transformer, 24 ... Xenon tube, 25 ... Main capacitor, 30 ... Constant voltage element

Claims (1)

[Claims] 1. An electronic flash circuit including a DC / DC converter, wherein a constant voltage element having a constant voltage level higher than a power supply voltage is connected between a base of an oscillation transistor and a secondary winding of an oscillation transformer. Electronic flash device.