JP3332797B2 - Positive and negative voltage power supply circuit - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a positive / negative voltage power supply circuit, and more particularly to a camera using a CCD imager and an L-type power supply.
A positive / negative voltage power supply circuit for supplying a positive power supply voltage and a negative power supply voltage to a display device using a CD panel.

[0002]

2. Description of the Related Art For example, in a moving image and / or still image camera using a CCD imager, a 3.3 V, 5 V
To require four power supply voltages, such as V, 15V and -7.5V, a positive / negative voltage circuit is used. As described above, in the positive / negative voltage power supply circuit, when the power switch of the camera is turned on or off, each terminal of the CCD imager needs to satisfy the absolute maximum rating of the CCD. In particular, 15 V when the power switch is turned off and -7.
Discharge time of relatively high voltage such as 5V (voltage gradual decrease time)
May be too long to meet the above requirements.

[0003]

On the other hand, the discharge time when the power is turned off is determined by the capacity of the smoothing capacitor and the input impedance of the load (CCD imager). Therefore, the discharge time can be set optimally by adjusting the discharge resistance and the smoothing capacitor capacity connected in parallel to the load. That is, the discharge time can be shortened by reducing the capacitance of the smoothing capacitor and / or reducing the discharge resistance.

However, there is a limit to reducing the capacitance of the smoothing capacitor because it causes inconvenience in securing the performance as a power supply circuit, such as an increase in noise. In addition, reducing the discharge resistance increases steady-state current consumption (load current), causing another problem such as a reduction in efficiency of the power supply circuit. Therefore, a main object of the present invention is to provide a positive / negative voltage power supply circuit capable of shortening a discharge time without reducing a smoothing capacitor and a discharge resistance.

[0005]

According to the present invention, there is provided a first circuit for generating a positive voltage including a rectifier circuit and a capacitor.
A positive voltage output terminal for outputting a positive voltage from the circuit ; a second circuit including a rectifier circuit and a capacitor for generating a negative voltage; a negative value for outputting a negative voltage from the second circuit Voltage output terminal and an electrode for applying a reference potential between a positive voltage and a negative voltage.
A positive-negative voltage power supply circuit having a source terminal, further comprising a short-circuit that substantially short-circuits the positive voltage output terminal and the negative voltage output terminal in response to a power-off signal; The positive / negative voltage power supply circuit is characterized in that the residual charges pass through a short circuit.

[0006]

For example, when the power switch of the camera is turned off, a power off signal is output from the microcomputer of the camera.
In response to the power-off signal, for example, the PWM switching controller is disabled, and the first terminal and the second terminal are substantially short-circuited by the short-circuit.

When the short circuit includes a series circuit of a switching element and a current limiting element, the switching element is turned on by the power-off signal, and therefore the first terminal and the second terminal are turned off.
A current flows through the current limiting element due to residual charges of the respective smoothing capacitors connected to the terminals, and the current is consumed as heat energy in the current limiting element. Therefore, the residual charges are quickly discharged, and the discharge time of the positive voltage and the negative voltage is reduced.

[0008] Even when the short circuit does not include a current limiting element,
Since the current due to the residual charge is consumed by the internal resistance (on-resistance) of the switching element, the residual charge is similarly rapidly discharged, and the discharge time of the positive voltage and the negative voltage is reduced.

[0009]

According to the present invention, the discharge time when the current is turned off can be reduced only by connecting a short circuit between the first terminal and the second terminal without reducing the capacitance of the smoothing capacitor and the discharge resistance. Can be. The above object of the present invention,
Other objects, features and advantages will become more apparent from the following detailed description of embodiments, which proceeds with reference to the accompanying drawings.

[0010]

Referring to FIG. 1, a positive / negative voltage power supply circuit 10 of this embodiment is used as a power supply circuit of a camera 12.
However, the positive / negative voltage power supply circuit of the present invention has a
It should be pointed out in advance that it can be used as a power source of a display device using a CD panel.

The camera 12 includes a CCD imager 14 and a timing generator 16 for applying various driving voltages to the CCD imager 14. In this embodiment, the CCD imager 14 and the timing generator 16 are supplied with 15 V from the power supply circuit 10 and -7.
Two power supply voltages of 5V are provided. The camera 12 further includes a microcomputer 18 for controlling the entire operation of the camera 10, and the microcomputer 18 receives an operation signal of a power switch 20. The microcomputer 18 operates at a power supply voltage of, for example, 5 V or 3.3 V (not shown). When the power switch 20 is turned on, the microcomputer 18 outputs a power on signal, and when the power switch 20 is turned off, the microcomputer 18 outputs a power off signal. The power-on signal and the power-off signal are used as power control signals in FIG.
Is shown in FIG.

The power supply circuit 10 includes a switching controller 22 to which the above-described power supply control signal is applied. Therefore, the switching controller 22 is activated by the power-on signal and deactivated by the power-off signal. Switching controller 22 receives power supply voltage VB from, for example, a battery (not shown), and outputs a switching signal to the base of pnp transistor Q1 according to, for example, a PWM method when activated. The emitter-collector of the transistor Q1 is connected between the power supply voltage from the battery and one terminal of the primary winding of the transformer T.
The other terminal of the next winding is grounded.

Two circuits 24 are provided on the secondary winding side of the transformer T.
And 26 are formed, and circuit 24 functions as a first circuit, and generates a positive voltage, for example, 15V. Circuit 26
Functions as a second circuit, and has a negative voltage, for example, -7.
Generates 5V. 15V generated by the first circuit 24 is
The signal is output from the first terminal 24a and supplied to the CCD imager 14 and the timing generator 16 of the camera 12. The −7.5 V generated by the second circuit 26 is output from the second terminal 26 a and supplied to the CCD imager 14 and the timing generator 16.

The first circuit 24 is a so-called flyback circuit, and includes a rectifier diode D1 connected to a first terminal of a secondary winding of the transformer T, a smoothing capacitor C11 receiving a DC voltage from the diode D1, and a smoothing capacitor C11. A filter including a choke coil L1 connected to the capacitor C11 and a capacitor C12 is included. The second circuit 26 is also a flyback circuit, and the second circuit 26
Rectifier diode D2, smoothing capacitor C21, and choke coil L2 and capacitor C2
2 is included.

In this embodiment, an FET Q2 functioning as a switching element and a resistor R connected in series to the FET Q2 are provided between the first terminal 24a and the second terminal 26a.
1 is connected. That is, FE
The source of TQ2 is connected to the first terminal 24a and the FET Q
2 is connected to one end of the resistor R1. Resistance R1
Is connected to the second terminal 26a. A pull-up resistor R2 is connected between the source and the gate of the FET Q2.

The gate of FET Q2 forming short circuit 28 is connected to the collector of npn transistor Q3, the emitter of transistor Q3 is grounded, and the bases are connected to bias resistors R3 and R4. A speed-up capacitor C1 is connected in parallel to the resistor R4. The base of transistor Q3 is thus connected via a parallel circuit of resistor R4 and capacitor C1 to npn transistor Q3.
4 collector. The collector of transistor Q4 is connected to power supply voltage VB from a battery (not shown) via collector resistor R5. The base of the transistor Q4 receives the aforementioned power control signal output from the microcomputer 18 of the camera 12.

In response to turning on the power switch 20, the microcomputer 18 outputs a power on signal of, for example, 5V. In response, the switching controller 22 is activated and the first terminal 24a is connected to the first terminal 24a according to the well-known PWM method.
V is output, and −7.5 V is output to the second terminal 26a. On the other hand, since the power-on signal is applied to the base of the transistor Q4, the transistor Q4 is turned on, the base of the transistor Q3 becomes substantially 0 V, and the transistor Q3 is turned off. At this time, since the gate of the FET Q2 is pulled up to 15V by the resistor R2,
Q2 is also off. Therefore, the short circuit 28 does not operate, and the first circuit 24 and the second circuit 26 continue normal operation.

In response to the turning off of the power switch 20, the microcomputer 18 outputs a power off signal of, for example, 0V. In response, switching controller 22 is deactivated, so that first circuit 24 and second circuit 26 stop operating. Therefore, both 15V of the first terminal 24a and -7.5V of the second terminal 26a are shut off. However, the smoothing capacitors C11 and C21
15V and -7.5V do not immediately become 0V because charges are stored in the filter capacitors C12 and C22.

On the other hand, since the power-off signal is applied to the base of the transistor Q4, the transistor Q4 is turned off, and the base of the transistor Q3 is connected to the voltage VB and the resistor R5.
It receives a bias voltage determined by R4 and R3. Therefore, the transistor Q3 is turned on, the gate of the FET Q2 becomes substantially 0 V, and the FET Q2 is also turned on. Therefore, the short circuit 28 operates, and the first terminal 24a and the second terminal 2
6a is short-circuited via the resistor R1. Therefore, a current flows through the resistor R1 due to the positive charge stored in the smoothing capacitor C11 and the filter capacitor C12 and the negative charge stored in the smoothing capacitor C21 and the filter capacitor C22. At this time,
The current value is limited by the resistor R1 so that an excessive rush current does not flow through the FET Q2. That is, the resistance R1
Functions as a current limiting resistor.

When the short circuit 28 operates and a current flows through the current limiting resistor R1, the current causes the current limiting resistor R1 to generate heat. In other words, the above-mentioned capacitors C11 and C1
2, the residual charges of C21 and C22 are consumed as heat energy by the resistor R1. Therefore, the residual charge eventually becomes zero. In this manner, according to the embodiment of FIG. 1, as shown in FIG. 2, the discharge time at 15 V and -7.5 V is reduced to, for example, about 7.5 milliseconds. Incidentally, in the prior art in which the short circuit 28 and its related circuits are not provided in the embodiment of FIG. 1, as shown in FIG.
The discharge time of V is about 200 milliseconds, and the discharge time of -7.5V is longer, about 20 seconds. Therefore,
It can be seen from FIG. 1 that the discharge time can be greatly reduced without changing the smoothing capacitor and the discharge resistance (not shown).

In the embodiment of FIG. 1, 15 V and -7.
When the first circuit 24 and the second circuit 26 for 5 V are provided independently of each other, it is possible to individually adjust the discharge time so as to satisfy the above-described rating. However, even if the first circuit 24 and the second circuit 26 separately provide the transformer T, as long as the switching controller 22 is used in common, the problem that the discharge time of -7.5 V becomes too long similarly to the related art. It cannot be resolved.

It should be noted that the short circuit 28 in the above-described embodiment is used.
Needless to say, the polarity of the FET Q2 and the other transistors Q3-Q4, etc., or the polarity of the power supply control signal can be arbitrarily selected and set. And the short circuit 28 is F
It may be composed only of a switching element such as ET. In this case, since the current limiting resistance is eliminated, the residual charge is discharged (consumed as heat energy) only by the ON resistance of the switching element.

If the current limiting resistor is not provided in the short circuit 28, the rush current due to the residual charge is expected to increase. In this case, it is desirable to use a switching element having a higher current rating. As the switching element, a bipolar transistor, a GTO, or the like may be used in addition to the FET of the embodiment.

[Brief description of the drawings]

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

FIG. 2 is a graph showing the discharge time in the embodiment of FIG. 1, in which the horizontal axis represents time and the vertical axis represents voltage.

FIG. 3 is a graph showing a discharge time in the prior art without a short circuit of the embodiment of FIG. 1, wherein the horizontal axis represents time and the vertical axis represents voltage.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 10 ... Positive and negative voltage power supply circuit 12 ... Camera 14 ... CCD imager 18 ... Microcomputer 20 ... Power switch 22 ... Switching controller 24 ... First circuit 26 ... Second circuit 24a ... First terminal 26a ... Second terminal 28 ... Short circuit Q2 ... FET R1 ... current limiting resistance

Claims (3)

(57) [Claims] A first circuit for generating a positive voltage including a rectifier circuit and a capacitor; a positive voltage terminal for outputting the positive voltage from the first circuit; a negative value including a rectifier circuit and a capacitor; second circuit for generating a voltage, negative value voltage input terminal and outputs a pre-Symbol the negative value voltage from the second circuit, and the positive value
In a positive / negative voltage power supply circuit having a ground terminal for providing a reference potential between a voltage and the negative voltage, a short circuit for substantially short-circuiting the positive voltage output terminal and the negative voltage output terminal in response to a power-off signal Wherein the residual charge of the capacitor passes through the short circuit when power is turned off. 2. The positive / negative voltage power supply circuit according to claim 1, wherein said short circuit includes a series circuit of a switching element and a current limiting element connected between said positive voltage output terminal and said negative voltage output terminal. . 3. The positive / negative voltage power supply circuit according to claim 1, wherein said short circuit includes a switching element connected between said positive voltage output terminal and said negative voltage output terminal.