JP2001210492A - Stroboscope charging circuit - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a stroboscope charging circuit with a short charging period and good charging efficiency. SOLUTION: The stroboscope charging circuit, charging a main capacitor C1 with light emitting energy for stroboscope lighting, comprises a boosting circuit which boosts the voltage inputted to a plurality of primary windings P1, P2 and outputs to the secondary winding S, and FET1, FET2 which control respective currents flowing through a plurality of primary windings. When the output value of the main capacitor C1 is judged to be less than the first prescribed value, a CPU1 drives only one element out of FET1 and FET2, and controls the on-off periods of the respective elements independently.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a flash charging circuit, and more particularly, to a flash charging circuit for a camera.

[0002]

2. Description of the Related Art Conventionally, in a camera equipped with a strobe light emitting device, various circuits have been proposed as charge control circuits for a strobe light emitting main capacitor. For example,
Japanese Unexamined Patent Publication No. Hei 8-203688 discloses a strobe charging circuit for charging by driving a plurality of switching elements by push-pull driving with the aim of shortening the charging time.

In this strobe charging circuit, the primary side of an oscillation transformer is divided into a plurality of primary windings, and a driving switching element is connected to each primary winding. Then, by switching these plurality of switching elements between push-pull drive and single drive and operating them, an efficient charging operation is performed.

[0004] Specifically, when the capacitor voltage of the main capacitor is low, the charging circuit, which is originally capable of push-pull driving, is operated by a single operation and charged. On the other hand, when the capacitor voltage becomes high, the charging circuit is driven by push-pull. By performing such control, an efficient charging operation is realized.

[0005]

SUMMARY OF THE INVENTION However, Japanese Patent Application Laid-Open No.
In a forward charging circuit including a push-pull charging circuit, such as a strobe charging circuit disclosed in JP-A-203688, the energy generated when switching is off is:
It is smaller than the energy created when it is on.

Accordingly, when charging is performed by operating such a push-pull charging circuit in a single operation, setting the switching off time longer than necessary deteriorates charging efficiency. Regarding this point, Japanese Patent Laid-Open No.
In JP 203688, no contrivance is found. In other words, during charging by the single operation, control is not performed to change the switching oscillation frequency or the on / off duty ratio, and the switching off time is unnecessarily spent, which prolongs the charging time. Was a factor.

The present invention has been made in view of the above problems, and has as its object to provide a flash charging circuit having a short charging time and high charging efficiency.

[0008]

In order to achieve the above object, a first strobe charging circuit of the present invention is a strobe charging circuit for charging a strobe light emitting capacitor with light emission energy. A booster circuit that boosts an input voltage and outputs the boosted voltage to a secondary winding; a plurality of switching elements that control respective currents flowing through the plurality of primary windings; and a charging voltage detector that measures a charging voltage of the capacitor. A circuit, a control circuit for driving at least one switching element of the plurality of switching elements,
When the charging voltage detection circuit detects that the charging voltage of the capacitor is equal to or lower than a predetermined voltage, the control circuit drives only one of the plurality of switching elements and ,
The on-time and off-time of the driven switching element are controlled independently.

In order to achieve the above object, a second aspect of the present invention is provided.
In the first flash charging circuit, the control circuit sets the on time of the driven switching element to be longer than the off time.

[0010]

Embodiments of the present invention will be described below with reference to the drawings.

FIG. 1 is an electric circuit diagram showing a configuration of a strobe charging circuit according to an embodiment of the present invention.

As shown in FIG. 1, a strobe charging circuit according to the present embodiment includes a power source E which is a power source for each circuit of the entire camera having the strobe charging circuit, and a power source voltage of the power source E which is boosted to a main power source. An oscillation transformer T1 having a first primary winding P1, a second primary winding P2, and a secondary winding S having different numbers of turns for storing charges in the capacitor C1.
And first and second switching elements (field effect transistors) FET1 and FET2 connected to the first and second primary windings P1 and P2, respectively, and a secondary winding S of the oscillation transistor T1. A bridge diode BDi to be connected, a series connection circuit composed of divided resistors R1 and R2 connected in parallel to an output terminal of the bridge diode BDi, a main capacitor C1 for storing the charge boosted by the oscillation transformer T1, A backflow preventing diode Di for preventing backflow of accumulated charge in the main capacitor C1, a xenon tube Xe for irradiating light to a subject, and a switching element I for controlling the amount of light emitted from the xenon tube Xe
The main parts are a GBT, a trigger circuit having a trigger capacitor C2 for supplying a trigger signal to the xenon tube Xe and a trigger coil T2, and a control circuit (CPU) 1 for performing various controls of the flash charging circuit. It is configured.

The turns ratio of the two primary windings P1 and P2 in the oscillation transformer T1 is set such that the first primary winding P1 has a larger number of turns than the second primary winding P2. I have. Further, the positive terminal of the power source E is connected to the midpoint between the first primary winding P1 and the second primary winding P2, and the power supply control terminal E of the CPU 1 is connected.
Under the control of 1, the battery voltage is applied.

A first switching element FET is connected between both ends of the two primary windings P1 and P2 and the ground.
1. The second switching element FET2 is connected. The gate terminals of the first switching element FET1 and the second switching element FET2 are respectively
It is connected to the CHG1 and CHG2 terminals of U1. Then, under the control of the CPU 1, the first switching element FET
The first and second switching elements FET2 are turned on and off, whereby the currents flowing through the first primary winding P1 and the second primary winding P2 are respectively controlled.

The middle point between the above-mentioned divided resistors R1 and R2 is the CPU1.
CPU1 measures the charging voltage of the main capacitor C1 based on the input voltage of the VST terminal. Note that, in the flash charging circuit of the present embodiment, during charging of the main capacitor C1, the voltage generated at the resistor R2 is
The values of the resistors R1 and R2 monitored in the A / D circuit (not shown) provided in the PU1 are adjusted in advance, and the resistance ratio of the voltage applied to the resistor R2 (R1 + R2 /
R2) times the voltage is applied to the main capacitor C1. Thereby, the CPU 1 can know the voltage of the main capacitor C1.

The diode Di is provided to prevent the charge stored in the main capacitor C1 from being released through a series circuit of the resistors R1 and R2.

The gate terminal of the switching element IGBT is connected to the STON terminal of the CPU 1, and the xenon tube X
In e, the light emission amount is controlled under the control of the CPU 1.

Next, the operation of the flash charging circuit according to this embodiment having the above-described configuration will be described with reference to a timing chart shown in FIG. 2 and a diagram shown in FIG.

FIG. 2 is a timing chart showing output signals of the CHG1 terminal and CHG2 terminal of the CPU 1 in the flash charging circuit of the present embodiment. FIG. 3 is a timing chart showing the main capacitor C1 in the flash charging circuit of the present embodiment.
FIG. 3 is a diagram showing the charging characteristics of FIG.

In FIG. 3, a capacitor voltage Va is a first predetermined voltage, a voltage Vb is a second predetermined voltage, a light emission enabling voltage, and a voltage Vc is a charging completion voltage. A is a period from the start of charging until the first predetermined voltage is reached, and is a single operation section using only the first switching element FET1, and section B is a period from when the first predetermined voltage is reached to when the light emission enabling voltage is reached. During the push-pull operation period between the first switching element FET1 and the second switching element FET2, section C, the second period from when the light emission enabling voltage is reached to when the charge completion voltage is reached.
Respectively shows a single operation section by the switching element FET2.

Now, as an initial state, the main capacitor C
It is assumed that the charging voltage of 1 is 0 [V]. From this initial state, the strobe charging circuit of this embodiment first outputs an ON / OFF signal alternately from the CHG1 terminal of the CPU 1, that is, the first switching element FET by duty driving.
1 to operate the oscillation transformer T1 (FIG. 2,
(See section A in FIG. 3).

Here, the state of the oscillation transformer T1 when the ON signal is output from the CHG1 terminal and the action of charging the main capacitor C1 will be briefly described.

When an ON signal is output from the CHG1 terminal of the CPU 1, the first switching element FET1 is turned on, and a current flows through the first primary winding P1 of the oscillation transformer T1. When a current flows through the first primary winding P1, an electromotive force corresponding to a current change (di / dt) is generated in the oscillation transformer T1. Then, the generated electromotive force is transmitted to the secondary winding S.

When an electromotive force is generated in the oscillation transformer T1, the oscillation transformer T1 passes through the bridge diode BDi.
A current boosted to a turn ratio of 1 flows through the main capacitor C1, and charges are stored. At this time, in the oscillation transformer T1, I1 = N * I2 (N is the transformer winding ratio), so that the primary current and the secondary current have similar shapes.

However, when all the generated electromotive force is released, the secondary current disappears but the primary side continues to flow as a resistor. That is, the relationship of the above equation I1 = N * I2 does not hold.

In consideration of this point, the flash charging circuit of this embodiment is set so that the primary current is forcibly cut off before the secondary current becomes zero. That is, a time before an on signal is output from the CHG1 terminal and the secondary current becomes zero is stored in advance in a storage circuit such as an EEPROM in the CPU 1, and when the time reaches the time, an off signal is output from the CHG1 terminal. It has become.

When the current in the winding stops, a back electromotive force corresponding to the reverse current change (-di / dt) of the first primary winding P1 is generated in the oscillation transformer T1, and the generated back electromotive force is generated. It is transmitted to the secondary winding S. As described above, the secondary side of the oscillation transformer T1 is bridge-connected to a diode, so that a current in the opposite direction also flows to the main capacitor C1 and can store an electric charge.

Note that the charging by the back electromotive force is performed only for a short time because the generated energy is smaller than that at the time of ON.
Therefore, the off time may be shorter than the on time, and the duty ratio is determined to be longer in the present embodiment.

Further, in the strobe charging circuit of the present embodiment, the time before the secondary current becomes zero is set to the EE in the CPU 1 in advance similarly to the setting of the ON time from the CHG1 terminal.
The data is stored in a storage device such as a PROM, and returns to the initial state.

The charging method described above is based on the CPU 1
ON / OFF signals are output only from the CHG terminal, that is, only the first switching element FET1 is duty-driven, and in this specification, this operation is referred to as a single operation of the push-pull charging circuit.

As described above, in the flash charging circuit of this embodiment, immediately after the start of charging, first, the ON / OFF signal is repeatedly output only from the CHG1 terminal to drive only the first switching element FET1 (single operation). Thus, the main capacitor C1 is charged (section A shown in FIGS. 2 and 3).

In this section A, the current consumption I1t [A · S] in charging can be expressed as I1t = NI2t = NCV. Here, I1t: Transformer primary current amount of the oscillation transformer T1 I2t: Transformer secondary current amount of the oscillation transformer T1 N: Transformer winding ratio C: Capacity of the main capacitor C1 V: Charging voltage Charging with a small transformer is more efficient than charging with a transformer with a large winding ratio.
It can be seen that the amount of electric current (the amount of charge) taken out of the device is small.

However, the voltage value that can be boosted is lower than V1N = V2.

In the strobe charging circuit of the present embodiment, the first primary winding P1 has a larger number of turns than the second primary winding P2, and thus has a small turn ratio. Therefore, charging can be performed with lower current consumption.

By the way, when the charging voltage of the main capacitor C1 is low, the current flowing to the primary side of the oscillation transformer T1 is large, and the current decreases as the voltage increases. Here, if the primary current is large, the power supply voltage is greatly reduced and the CPU
1 drive voltage and may adversely affect the camera system.

In consideration of this point, the strobe charging circuit of this embodiment provides a period in which the primary current is turned off by performing the above-described single control immediately after the start of charging, thereby alleviating the drop in the power supply voltage. , To ensure a stable voltage. The secondary winding flowing during the period in which the output of the CHG1 terminal is off is charged by the back electromotive force, so that efficient charging is achieved.

Returning to FIGS. 2 and 3, in charging the first switching element FET1 only by the single operation described above, the CPU 1 sets the voltage of the main capacitor C1 to a predetermined first predetermined value based on the value input to the VST terminal. Voltage value Va
[V], the charging by the single control is temporarily terminated, and the first switching element FET1
And the second switching element FET2 is turned on and off alternately to shift to push-pull control for charging (FIG. 2,
Section B in FIG. 3).

In the push-pull control in the section B, the CPU 1 alternately outputs ON and OFF signals from the CHG1 terminal and the CHG2 terminal, and connects the first switching element FET1 and the second switching element FET2 to a so-called push-pull signal. Pull drive. Hereinafter, the push-pull drive will be specifically described.

First, the CPU 1 outputs an ON signal from the CHG1 terminal to turn on the first switching element FET1. As a result, the current flows to the first primary winding P1. When the predetermined time stored in the storage circuit in the CPU 1 has been reached, an off signal is output from the CHG1 terminal,
At the same time, an ON signal is output from the CHG2 terminal. As a result, the second switching element FET2 is turned on next, a current flows through the second primary winding P2, and the charging is performed in the same state as the charging by the back electromotive force described above.

In this embodiment, the ON time of the first switching element FET1 and the ON time of the second switching element FET2 are set to be the same.

The push-pull drive described above ends when the charging voltage of the main capacitor C1 reaches the second predetermined voltage value Vb (light emission enabling voltage).

In the push-pull operation, each primary winding P1, P
Since the charging is performed by alternately turning on 2, the charging time can be reduced.

In the push-pull operation section, the windings of the respective primary windings of the oscillation transformer T1 are different, but since each winding is set to have a winding ratio capable of reaching the charging voltage Vb, the charging is performed. Has no problem.

The above-described first switching element FET
In charging by the push-pull operation by both the first and second switching elements FET2, the CPU 1 determines that the voltage of the main capacitor C1 has reached a predetermined second predetermined voltage value Vb [V] from the value input to the VST terminal. Then
The charging by the push-pull control is temporarily terminated, and the second
The operation shifts to the single control in which only the switching element FET2 is turned on and off to perform charging (section C in FIGS. 2 and 3).

In the single control in this section C, C
PU1 outputs ON / OFF signals only from the CHG2 terminal, and drives only the second switching element FET2. The charging by the single control is performed by the light emission enabling voltage Vb.
The operation is performed from [V] to the charge completion voltage Vc [V] (section C in FIGS. 2 and 3).

By the way, with regard to the charging speed after reaching the light emission enabling voltage Vb, it is considered that fast charging is not required much because strobe light emission is already possible.
However, there is a difference between the amount of light emitted near the voltage at which light emission is possible and the amount of light emitted near the voltage at which charging is completed, and the higher charging voltage is still preferable.

On the other hand, as described above, when the voltage reaches the light emission enabling voltage, a circuit other than flash charging such as auto focus must be operated in preparation for light emission. Therefore, it is necessary to avoid lowering the power supply voltage E. No.

Here, in the strobe charging circuit of this embodiment, FIG. 5 shows a general example of the voltage change of the power supply E1 when the control is performed by the single control, and the voltage change of the power supply E when the control is performed by the push-pull control. 6 is shown in FIG.

As shown in FIGS. 5 and 6, it can be understood that the power supply voltage drop in the single control is smaller than that in the push-pull control. The strobe charging circuit of the present embodiment has been made in consideration of such circumstances, and the above-described single charging is performed after the charging voltage of the main capacitor C1 reaches the light emission enabling voltage Vb and reaches the charging completion voltage. Since control (duty drive of only the second switching element FET2 in this embodiment) is performed, the efficiency of flyback charging can be improved.

When the charging of the main capacitor C1 is completed, the voltage of the trigger capacitor C2 is changed to the resistances R1 and R1.
2 is discharged. Therefore, when light emission is required, the trigger capacitor C2 must be charged again and a light emission signal must be output.

In consideration of this point, the flash charging circuit according to the present embodiment turns on the CHG1 terminal of the CPU 1 first.
An off signal is output to perform single control of the charging circuit to charge the trigger capacitor C2 (see FIG. 4). Note that charging before light emission is referred to as pre-charging.

By the way, generally, the trigger capacitor C2
Is much smaller than the main capacitor C1. Therefore, the charging time of the trigger capacitor C2 may be extremely short.

When the pre-charging is completed, the CPU 1
An ON signal is output from the STON terminal. The charge stored in the trigger capacitor C2 is
2 → IGBT → primary winding of the trigger coil T2.

When a current flows through the primary side of the trigger coil T2, energy is transmitted to the secondary side, and a high voltage is applied to the surface of the xenon tube Xe. When a trigger voltage is applied to the xenon tube Xe, the resistance value inside the xenon tube Xe decreases, the electric charge stored in the main capacitor C1 is released, and the xenon tube Xe emits light.

When the ON time of the STON terminal reaches a predetermined time by a timer circuit in the CPU 1, the CPU 1 sets the STON terminal to the ON state.
An off signal is output from the ON terminal to turn off the IGBT and stop light emission.

Here, the voltage change of the power supply E when the pre-charging is performed by the single control and the voltage change of the power supply E when the pre-charging is performed by the push-pull control are shown in FIGS.
Following the change similar to the change shown in FIG. As shown in these figures, similarly to the above, it can be seen that the decrease in the power supply voltage in the single control is smaller than that in the push-pull control.

When emitting a flash, a CPU or AF
If there is a portion such as a circuit that is driven at the same time as the strobe light emission, a necessary voltage may not be secured due to a power supply voltage drop due to pre-charging. That is, in the case of the push-pull control, there is a possibility that a voltage required for the CPU cannot be secured. In consideration of such circumstances, the strobe charging circuit of the present embodiment uses the CHG
1 and CHG2 are set to be controlled.

As described above, in the strobe charging circuit of the present embodiment, when the voltage of the main capacitor C1 is low, or for a short time before light emission,
When the voltage of is between the voltage at which light emission is possible and the voltage at which charging is completed,
By performing the control by the single control, it is possible to perform charging with a small decrease in the power supply voltage and a small current consumption.

Also, when the control signal is turned on during single control,
By providing the OFF time, a charging circuit with a short charging time and high charging efficiency can be provided.

Further, the number of turns of the two primary windings of the oscillation transformer T1 is set, and when the voltage of the capacitor C1 is low, the single winding is controlled using the primary winding having a large number of turns, so that charging with low current consumption can be performed. It becomes possible.

[0061]

As described above, according to the present invention, a strobe charging circuit having a short charging time and high charging efficiency can be provided.

[Brief description of the drawings]

FIG. 1 is an electric circuit diagram showing a configuration of a strobe charging circuit according to an embodiment of the present invention.

FIG. 2 shows a CP in the strobe charging circuit of the embodiment.
5 is a timing chart showing output signals of a CHG1 terminal and a CHG2 terminal of U.

FIG. 3 is a diagram showing charging characteristics of a main capacitor in the flash charging circuit of the embodiment.

FIG. 4 is a timing chart showing an operation related to pre-charging in the flash charging circuit of the embodiment.

FIG. 5 is an explanatory diagram showing an example of a power supply voltage change when charging is performed by single control in the strobe charging circuit of the embodiment.

FIG. 6 is an explanatory diagram showing an example of a power supply voltage change when charging is performed by push-pull control in the flash charging circuit of the embodiment.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 ... CPU1 E ... Power supply T1 ... Oscillation transformer P1 ... 1st primary winding P2 ... 2nd primary winding S ... Secondary winding FET1 ... 1st switching element FET2 ... 2nd switching element C1 ... Main capacitor Xe: Xenon tube

Claims (2)

[Claims] 1. A strobe charging circuit for charging a strobe light emitting capacitor with luminous energy, comprising: a boosting circuit for boosting a voltage input to a plurality of primary windings and outputting the boosted voltage to a secondary winding; A plurality of switching elements for controlling respective currents flowing through the winding, a charging voltage detection circuit for measuring a charging voltage of the capacitor, and a control circuit for driving at least one switching element of the plurality of switching elements. When the charging voltage detection circuit detects that the charging voltage of the capacitor is equal to or lower than a predetermined voltage, the control circuit drives only one of the plurality of switching elements, A flash device characterized in that the on time and the off time of the driven switching element are controlled independently. Electrical circuit. 2. The strobe charging circuit according to claim 1, wherein the control circuit sets an on time of the driven switching element to be longer than an off time.