JPH09319002A - Power protection circuit - Google Patents

Abstract

(57) [Abstract] [Problem] To prevent the load on the mounting space, a large resistor with a large allowable power loss is not used, and the output impedance of the power supply itself is reduced as much as possible to protect the power supply during an output short circuit or a heavy load. A power supply protection circuit is provided. A power switching element (21) provided between a power source and a load, two resistors (25, 26) provided in series between the power source and the load, and two resistors A voltage detector (22) for detecting the potential of the connection point, and the voltage detector (22) cuts off the power switching element when the potential of the connection point (27) becomes equal to or lower than a threshold value. , Stop the current supply to the load.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a power supply protection circuit for protecting a power supply from a short circuit accident or the like when supplying power to a load such as an external accessory of a camera.

[0002]

2. Description of the Related Art An AF single-lens reflex camera is provided with a contact for power supply between a camera body and an accessory so that the power supply in the camera can be used by an external accessory such as a lens. There are two types of power contacts, one that is used as a normal circuit power source such as a CPU in an accessory and one that supplies large electric power such as a motor.

The former usually outputs a voltage stabilized by a camera to a predetermined voltage, and 5V which is a normal power source of a digital IC is standard. However, recently, the voltage of the digital IC has been lowered, and it is technically possible to set it to 3V or 3.3V. In any case, this power supply is a voltage that is created inside the camera using a DC / DC converter or regulator, and is used for current limiting to prevent an excessive current from flowing even if the output terminal is short-circuited to ground due to an accident. The safety circuit of can be installed to prevent the power supply from being overloaded. With this safety circuit, for example, when using a battery (lithium, alkaline, nicad, etc.) for the power supply, even if this power supply terminal is shorted to the ground,
This prevents the battery from generating heat and causing an explosion in some cases by passing an excessive current.

On the other hand, since the latter is a power source terminal for electric power, in a camera system using a normal battery as a power source,
Depending on the efficiency of use of electric power, the battery output is directly output or is output via a power element such as a power MOS transistor. In the case of directly outputting the battery output, in preparation for a short circuit accident of the output terminal, a mechanical contact is made when the accessory is completely attached to the camera, and at this time the power output is connected to the output terminal . This switch is called a mounting switch. When using a power element, the same mechanical contact is used, and the mechanical contact is turned on or off when the accessory is completely attached to the camera, and the power supply output is connected to the output terminal through the power element. can do. FIG. 1 is an example of this type of circuit.

FIG. 1 shows an N-channel MOS transistor 5
Since this is a circuit example using 1, the gate voltage VG higher than the source voltage is required to turn it on.
It is assumed that the battery output is produced by another booster circuit (not shown). The same applies to subsequent circuit examples using N-channel MOSs. The mounting switch 52 has a polarity that turns off when the lens is properly mounted.

FIG. 2 shows a P-channel MOS transistor 5
This is an example using 3. Recently, a good P-channel power MOS transistor with a small ON resistance has been developed, and a circuit using this is possible. In this case, the transistor can be turned on if the gate voltage VG drops to the ground, which has the advantage of eliminating the need for the high voltage power supply as shown in FIG. At this time, the on / off polarity of the mounting switch 54 is opposite to that in FIG. 1, and it is assumed that the mounting switch 54 is turned on in the normal lens mounting state.

Even if the ON / OFF polarity of the mounting switch 52 or 54 cannot be designed in this way due to the mechanical limitation, the polarity can be electrically judged easily by one transistor. However, whether the mounting switch 52 or 54 eliminates the risk of output short circuit is not perfect. The mounting switch 52 or 54 itself may fail or a short circuit may occur inside the accessory. In such a case, a large current will continue to flow from the power supply, and there is the danger of heat generation and explosion as described above. In order to activate the current limit even in such a case, if the load current is detected and the specified current value is exceeded, the power element is turned off, or the voltage at the output terminal is monitored and the voltage falls below the specified voltage. Similarly, it is necessary to devise to turn off the power element. An example of the former load current detection type is shown in FIG.

This is a typical current limiting circuit,
Is an N-channel power MOS transistor, 2 is a load current detection resistor, and 3 and 4 are transistors. Since the load current ILOAD flows through the resistor 2, the voltage drop is ILOAD × r, where r is the resistance value of the resistor 2. When this voltage exceeds about 0.6 V which is the on-voltage of the transistor 3, the transistor 3 is turned on and the transistor 4 is turned on to pull down the gate voltage of the power MOS transistor 1 to near the ground potential. Cut off the MOS transistor 1. The load current decreases and I
When LOAD × r returns to the threshold value (0.6 V) or less, the power MOS transistor 1 is turned on again, so that this circuit has no hysteresis and the output load current characteristic is as shown in FIG. Although a battery normally uses a battery as a power source in a camera, even a new battery has an internal resistance of about 0.2 to 0.5Ω. Considering this, the load current characteristic is as shown in FIG.

Next, FIG. 6 shows an example of a circuit using a P-channel power MOS transistor. Similar to FIG. 2, if the gate potential is dropped to the ground, the P-channel MOS transistor 11 turns on. Therefore, in FIG. 6, only one PNP transistor 13 is used in the overcurrent detection unit, which is a simpler configuration than that in FIG. The load characteristic is similar to that of the circuit of FIG. The maximum current that should be allowed varies depending on what kind of battery is used, but is about 3 A for a lithium battery or an AA alkaline battery. With this value, if the voltage is 6 V, the coil resistance can be up to 2Ω, and a motor that is quite powerful for a camera can be driven.

[0010]

In each of the power supply circuits having the current limiting function shown in FIGS. 1, 2, 3, and 6, the load current is detected by the voltage for detecting the load current. The detection resistance for converting into was put in series with the power supply output line. However, the resistance value r of this detection resistor is
Assuming that VBE for turning on the transistor is 0.6 V and the allowable current is 3 A, the power loss is 200 mΩ and the power loss is 1.8 W. It is not preferable in terms of space to mount such a large resistance with large heat loss in the camera, and it is also a loss of power. From the side of the accessory that is powered, too (mostly batteries)
Since a resistance is inserted in series with the output impedance, the output impedance increases by that amount, which is not preferable.

The present invention has been made in view of the above problems, and does not use a large resistor with a large allowable power loss so as not to burden the mounting space, and also lowers the output impedance of the power source itself as much as possible. It is an object of the present invention to provide a power supply protection circuit that protects a power supply when an output short circuit or a heavy load occurs.

[0012]

To achieve this object, a power supply protection circuit of the present invention comprises a power switching element (21) provided between a power supply and a load and a power supply and a load. 2 resistors in series (25, 26)
And a voltage detector (22) for detecting the potential of the connection point of the two resistors, the voltage detector (22) is for power when the potential of the connection point (27) is below a threshold value. The switching element is cut off to stop the current supply to the load.

[0013]

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

In the present invention, it is assumed that a battery is used as a power source. The purpose of the power protection circuit is not to limit the maximum value of the current supplied to the accessories, but to prevent the current from being supplied to the accessories through the power supply terminals so that the voltage of the power supply battery drops significantly. It will be. That is, since the protection circuit does not work while the current supply capability is present (= the equivalent internal resistance of the battery is small), it is possible to protect the power supply according to the ability of the power supply battery.

In fact, since accessories are designed in consideration of available power supplies, a large amount of current that normally causes a failure in the power supply system does not flow. It can be said that the protection circuit has a first practical purpose to suppress the load current in order to protect the power supply when the power supply terminal is erroneously short-circuited to the ground.

When the current drawn from the power supply terminal is increased, the battery voltage and the terminal voltage of the power supply terminal drop due to the voltage drop due to the internal resistance of the battery. Therefore, the present invention detects the drop of the power supply terminal voltage. Then, this is fed back to limit the output current. Also, until the power supply terminal is completely short-circuited to the ground, two series resistors are connected between the battery and the power supply terminal so that the power supply drop detection circuit works, and the potential at the intermediate connection point 27 is connected to the voltage detection device. Configure as input.

FIG. 7 is a block connection diagram showing an example of a power supply protection circuit before the present invention. This circuit is a basic circuit that detects a voltage drop at an external power supply terminal for an accessory by using a voltage detector. When the voltage of the external power supply terminal drops due to a heavy load or a short circuit and falls below the threshold value of the voltage detector 22, the open drain output Vout of the voltage detector 22 turns on, and the gate potential of the N-channel MOS transistor 21 is brought to near ground. Lower. Therefore, the N-channel MOS transistor 21 is cut off to cut off the load current. At present, such a voltage detector 22 that can operate even at about 1 V is easily available, and the current consumption is very small, such as a few μA, so it is advantageous to construct a circuit using it. However, in the configuration of FIG. 7, when the external power supply terminal is completely short-circuited to the ground, the power supply VIN of the voltage detector 22 becomes 0V.
Therefore, there is a drawback that the voltage detector 22 does not operate.

FIG. 8 shows a first power protection circuit according to the present invention.
It is a block connection diagram showing an example. In this circuit, two resistors 25 (resistance value R3) and 26 (resistance value R4) are inserted in series between the source and drain of the power MOS transistor 21, and the connection point 27 of these resistors is connected to the voltage detector 22. Connect to VIN for detection voltage. Since the current consumption of the voltage detector 22 is small, the potential drop due to the resistors 25 and 26 can be ignored. Even if the output is completely short-circuited to the ground by this device, VBAT × R4 / (R3 + R4) becomes the detection voltage VIN of the voltage detector 22, and the voltage detector 22 does not operate as in the first embodiment of FIG. It never happens.

Next, the circuit operation will be described. When the power MOS transistor 21 is on, there is almost no potential drop between the drain and source, so the voltage detector 22
The detected voltage is almost the same as the allowable voltage of the external power supply terminal. When the load is heavy, that is, when the impedance of the accessory attached to the external power supply terminal is reduced, the output current of the battery increases and the battery terminal voltage VBAT drops due to the internal resistance of the battery. Since the voltage detector 22 is still off, the voltage VBAT has almost the same potential as the external power supply terminal. When the load current is further increased, the external power supply terminal voltage finally falls below the threshold value of the voltage detector 22, and the voltage detector 22 pulls down the gate potential of the MOS transistor 21 to near the ground to cut off the MOS transistor 21. Such a heavy load is of the order of several Ω, and the resistance 2
Since it is much smaller than the resistance values (R3 and R4) of 5 and 26, VIN of the voltage detector 22 becomes VBAT * R4 / (R3 + R4) after this. Therefore, the ratio of the resistance values R3 and R4 must be selected so that the minimum operable voltage VINmin of the voltage detector 22 becomes larger than this value. That is, assuming that the minimum value of the voltage VBAT is VBATmin,

The ratio of the resistance values R3 and R4 must be selected so that VBATmin × R4 / (R3 + R4)> VINmin.

Another condition for selecting the resistance values R3 and R4 is to set the maximum value of the voltage VBAT to VBATmax so that the voltage detector 22 operates even when the output terminal is short-circuited at the maximum voltage of the battery voltage VBAT. When the threshold of the detector 22 is Vth,

The condition of Vth> VBATmax × R4 / (R3 + R4) is added.

The resistance values (R3 and R4) of the resistors 25 and 26 may be selected so as to meet the above two conditions.
These are constraint conditions regarding the ratio of resistance values R3 and R4, and absolute values are not restricted. It was stated earlier that once the protection circuit is activated, the load current cannot be supplied again unless the load is practically removed. However, the resistance values R3 and R4 are different from the load resistance (several Ω) when the protection circuit works. By selecting a small value up to several times, the voltage detector 22 can be turned off without completely interrupting the load resistance. However, in that case, a large resistor is built in as shown in FIGS. 3 and 6, and the advantage of this circuit is halved. Therefore, in FIG. 8, such an automatic return function is not provided. The capacitor 28 that is connected in parallel with VIN of the voltage detector 22 is a smoothing capacitor that slightly stabilizes the voltage VIN by filtering it. The characteristic of this filter depends on the time constant which is the product of the parallel combined resistance (R3 × R4) / (R3 + R4) of the resistors 25 and 26 and the capacitance C of the capacitor 28. Therefore, the absolute value of the resistors R3 and R4 is Just decide. Actually a few k
Determine so that the resistance of about Ω will satisfy the above two restrictions.
The terminal current when the output is short-circuited is VBAT / (R3 + R4).

FIG. 9 shows a second power protection circuit according to the present invention.
It is a block connection diagram showing an example. This circuit is a circuit which can be turned on again when the load returns to normal after the voltage detector 22 once detects a decrease in the output voltage and turns off the power MOS transistor 21. In this circuit, in order to output the external power again, the CPU 31 is placed at the center of control of the system so that a sufficient voltage can be applied to VIN of the voltage detector 22 from one of the general-purpose ports P of the CPU 31 via the diode D1. I am doing it. Since the CPU 31 controls the entire control, it is often known when it is necessary to supply power to the accessory. At this time,
If the port P is set to high output for an extremely short time immediately before power supply is required, the power MOS transistor 21 can be turned on if the load is normal.

At this time, if the output terminal is short-circuited to the ground or an abnormal heavy load is still present, h
While the high pulse is being output, a large current is supplied from the output terminal, but as soon as the pulse goes low, the voltage detector 22 again detects a low voltage and cuts off the power MOS transistor 21. Therefore, it is necessary to set the pulse time to such an extent that the power MOS transistor 21 is not thermally destroyed by such a transient current.
On the other hand, the above filter time constant C × {R3 × R4) / (R3 +
R4)} is required twice as much. Since it is not preferable to measure a very long time in the control software, in practical use, by giving a high pulse of about several tens of microseconds as the voltage VIN as a numerical example, the power MOS transistor 21
Can be turned on again.

The CPU 31 of FIG. 9 is supplied with power from both the output of the DC / DC converter 32 and the battery 33 (voltage VBAT). Either higher potential is provided by diode D2 or D3. The DC / DC converter 32 produces a stabilized operating voltage Vdd of the CPU 31, and also produces a gate voltage VG for turning on the N-channel MOS transistor 21. Normally, Vdd =
5V, VG> VBAT + 5V or so.

When powering off the system, DC
The / DC converter 32 is turned off, the CPU 31 stops the oscillation of the high-speed oscillator, reduces the current consumption to about 10 μA or less, and reduces the battery consumption to the maximum. At this time, VG
The output also turns off, and the gate potential drops to the ground by the resistor 24, so that the power MOS transistor 21 is cut off. Therefore, when the power is off, the battery voltage is output to the external power supply terminal, but since the resistors 25 and 26 are connected in series, even if an output short circuit occurs, a slight current of VBAT / (R3 + R4) does not flow, which is dangerous. There is no.

[0028]

As described above, according to the power supply protection circuit of the present invention, when an excessive current flows due to a short-circuit accident or the like to cause a drop in the power supply voltage from the power supply terminal for the accessory attached to the camera, the power supply voltage is instantaneously reduced. Since the supply of electric current is shut off and the power source such as the battery is not overloaded, the danger of abnormal heat generation or explosion of the battery can be avoided. In addition, the circuit inside the camera is not destroyed by the abnormal drop of the power supply voltage (due to reset or the like).

[Brief description of drawings]

FIG. 1 is a block connection diagram showing an example of a conventional power supply protection circuit.

FIG. 2 is a block connection diagram showing an example of a conventional power supply protection circuit.

FIG. 3 is a block connection diagram showing an example of a conventional power supply protection circuit.

FIG. 4 is a block connection diagram showing an example of a conventional power supply protection circuit.

FIG. 5 is a block connection diagram showing an example of a conventional power supply protection circuit.

FIG. 6 is a block connection diagram showing an example of a conventional power supply protection circuit.

FIG. 7 is a block connection diagram showing an example of a conventional power supply protection circuit.

FIG. 8 is a block connection diagram showing an embodiment of a power supply protection circuit according to the present invention.

FIG. 9 is a block connection diagram showing an embodiment of a power supply protection circuit according to the present invention.

[Explanation of symbols]

 21 N-Channel Power MOS Transistor 22 Voltage Detector 23 Resistor 24 Resistor 25 Resistor 26 Resistor 27 Intermediate Connection Point 28 Capacitor 31 CPU

Claims (2)

[Claims] 1. A power switching element provided between a power source and a load, two resistors provided in series between the power source and the load, and a potential at a connection point of the two resistors is detected. Characterized in that the voltage detector cuts off the power switching element when the potential at the connection point becomes equal to or lower than a threshold value, and stops the current supply to the load. And a power protection circuit. 2. When the load is not installed normally,
The power supply protection circuit according to claim 1, further comprising a mechanical switch that turns off the power switching element.