JP2006287399A - Overcurrent protective circuit - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a circuit facilitating the adjustment of an overcurrent and improving the accuracy of a detection. <P>SOLUTION: The overcurrent protective circuit has an FET 24 having the power-supply interrupting function of a device 10, an analog-digital conversion circuit (A/D) 21 converting a voltage between a source and a drain for the FET 24 into a digital value and a storage circuit 22 storing a potential difference between both ends of the FET 24 at a time when a current value deciding the overcurrent is made to flow through the FET 24 as an overcurrent-voltage threshold. The overcurrent protective circuit further has a comparator 23 comparing the voltage between the source and the drain for the FET 24 output from the A/D 21 and the overcurrent-voltage threshold of the storage circuit 22 and outputting the voltage corresponding to the result of decision deciding whether or not a current is the overcurrent to a control terminal for the FET 24. The overcurrent protective circuit further has a changeover switch 25 disposed between one end of the FET 24 and a load circuit 13, having a function changing over the load circuit 13 and a measuring apparatus on the outside, connecting one end of the FET 24 to the load circuit 13 in the case of a normal operation and connecting one end of the FET 24 to the measuring apparatus on the outside when the threshold of an overcurrent voltage is adjusted and an external test terminal 26. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to an overcurrent protection circuit, and more particularly to an overcurrent protection circuit suitable for adjusting an overcurrent threshold.

  As an overcurrent protection circuit for a mobile radio communication terminal such as a cellular phone, a configuration in which a fuse is used, or a configuration in which the fuse is incorporated in a battery to be attached without being incorporated in the terminal device main body is frequently used.

  By the way, recent mobile phones are equipped with various functions such as videophone and high-speed data communication in addition to the voice call function, and there are operation modes corresponding to the functions. And shows a wide range of values. This is because, apart from the current consumption of the radio circuit (RF circuit, modulation / demodulation circuit, etc.), the current consumption allocated to a control circuit such as a CPU (Central Processing Unit) or DSP (Digital Signal Processor) This is because it increases according to the operating speed.

  The overcurrent protection circuit needs to make the current value of the overcurrent larger than the maximum current consumption of the device. In a mobile phone, in consideration of variations in current consumption of radio circuits and control circuits, the current value as an overcurrent needs to be set with a predetermined margin on the maximum current consumption of the device.

  On the other hand, there are variations in the accuracy with which the overcurrent protection circuit determines that an overcurrent occurs. In the case of a fuse, there is a range from the current value that should not be blown to the current value that blows the fuse, and the overcurrent protection circuit built in the battery also has variations in the current value that is judged as overcurrent. .

  As a result, the current threshold value for determining the overcurrent of the overcurrent protection circuit may be a value considerably larger than the maximum current consumption of the device. In this case, even if an overcurrent occurs due to an abnormality in the apparatus, there is a problem that an apparatus that cannot be determined as an overcurrent and cannot be turned off occurs.

  In Patent Document 1, as a load drive circuit for monitoring a drive current by using a MOSFET (Field Effect Transistor) having a large difference between individual load switching elements and a large temperature difference, the MOSFET itself is used as an alternative to a shunt resistor. The microcomputer detects the potential difference between both ends of the MOSFET as a load switching element, monitors whether the drive current exceeds the overcurrent detection line based on the detection result, and the MOSFET itself stored in the EPROM or the like in advance A configuration is disclosed in which monitoring is performed while correcting the overcurrent detection line based on the individual difference information and the MOSFET temperature information detected by the temperature sensor. However, in the overcurrent protection circuit of Patent Document 1, the on-resistance of the MOSFET is measured at the time of shipping inspection of the MOSFET, grouped into a range of several stages, and the result is stored in the storage circuit on the device. . In this case, there are the following problems.

  The on-resistance of the MOSFET strongly depends on the gate voltage. For this reason, the gate voltage of the MOSFET of the overcurrent protection circuit at the time of shipping inspection must be exactly the same as the gate voltage of the MOSFET of the overcurrent protection circuit when the device is actually used. However, this must be strictly controlled in the design of the device and in the manufacturing process of the device, and is difficult in practice in consideration of variations and the like.

  In addition, regarding the results of grouping the on-resistances of the MOSFETs of the overcurrent protection circuit, when packing each group, increasing the number of stages for grouping to improve accuracy complicates the management of equipment packing operations, etc. There is a problem that it leads to cost increase.

JP 2002-290222 A

  In a conventional overcurrent protection circuit, the threshold that can be determined as overcurrent (overcurrent threshold) is large, and if the lower limit is set larger than the maximum current consumption of the device, the overcurrent that can be detected becomes very high. May end up. In this case, even if an overcurrent flows due to an abnormality in the apparatus, it cannot be determined as an overcurrent, and there is a problem that a protection function for turning off the apparatus does not work.

  Further, in the case of the configuration of Patent Document 1, the on-resistance of the MOSFET of the overcurrent protection circuit is measured at the time of shipping inspection of the MOSFET, grouped into several stages, and the result is stored in the storage circuit on the device. However, it must be strictly controlled in the device design and in the device manufacturing process, and it is difficult to realize.In addition, when packing for each group, if the number of grouping steps is increased to improve accuracy, the device There is a problem that the management of the packaging of the container becomes complicated and leads to an increase in cost.

  Therefore, an object of the present invention is to provide an apparatus that improves the detection accuracy of overcurrent.

  Another object of the present invention is to provide an apparatus that simplifies the adjustment of the overcurrent threshold at the time of inspection or the like while achieving the above object.

  Another object of the present invention is to provide a device that facilitates detection of overcurrent according to the operation of the device.

  The invention disclosed in the present application is generally configured as follows in order to achieve the above object.

  In the present invention, in a state where the semiconductor switching element having the power-off function of the device is mounted, for example, a current is passed through the semiconductor switching element up to a current value determined to be an overcurrent, and the potential difference between both ends of the semiconductor switching element at that time Is stored as an overcurrent voltage threshold value, thereby enabling accurate determination of overcurrent.

  An overcurrent protection circuit according to one aspect of the present invention includes a semiconductor switching element having one end connected to a power supply terminal, and a changeover switch that connects the other end of the semiconductor switching element to one of a load circuit and an external terminal. And receiving a voltage across the one end and the other end of the semiconductor switching element, comparing it with an overcurrent threshold stored in a memory circuit, and controlling the signal of the level corresponding to the comparison result of the semiconductor switching element And comparison means for supplying to the terminal.

  In the present invention, the comparison means receives the voltage across the semiconductor switching element and converts it into a digital signal, an output of the analog-digital conversion circuit, and an overcurrent stored in the storage circuit A comparison circuit for comparing the threshold value and supplying the comparison result as a binary signal to the control terminal of the semiconductor switching element.

  In the present invention, in a state where the semiconductor switching element is mounted, a current is passed through the semiconductor switching element from a measuring device connected to the external terminal to a current value determined to be an overcurrent, and the semiconductor switching element at that time Is stored in the storage circuit as an overcurrent voltage threshold.

  In the present invention, the measuring device connected to the external terminal includes a DC power source and an ammeter connected in series between the external terminal and the power feeding terminal.

  In the present invention, a plurality of overcurrent thresholds respectively corresponding to a plurality of operation modes of the device having the overcurrent protection circuit are stored in the storage circuit, and the operation mode of the device is supported from the storage circuit. Means for reading out an overcurrent threshold to be supplied to the comparison circuit.

  According to the present invention, in a state where a semiconductor switching element having a power-off function of the device is mounted, a current is passed through the semiconductor switching element up to a current value determined to be an overcurrent, and the voltage across the semiconductor switching element at that time is , And stored as an overcurrent voltage threshold value, enabling accurate determination of overcurrent.

  In addition, according to the present invention, it is possible to set an overcurrent voltage threshold value corresponding to the actual use at the time of inspection or the like, and the adjustment process is simplified and easy.

  Furthermore, according to the present invention, overcurrent detection is facilitated by comparing the overcurrent voltage threshold corresponding to the operation mode of the apparatus with the voltage across the semiconductor switching element.

  The above-described present invention will be described below with reference to the accompanying drawings in order to explain in more detail. One embodiment of the present invention includes an FET 24 having a power-off function of the device 10 and forming a semiconductor switching element, an analog-digital conversion circuit (A / D) 21 that converts a potential difference between both ends of the semiconductor switching element into a digital value, A memory circuit 22 that stores a potential difference between both ends of the semiconductor switching element when a current value determined to be an overcurrent flows through the semiconductor switching element as an overcurrent voltage threshold, and between both ends of the semiconductor switching element output from the A / D 21 Is compared with the overcurrent voltage threshold value of the memory circuit 22 to determine whether the current is an overcurrent and to output a voltage corresponding to the determination result to the control terminal of the semiconductor switching element; It is disposed between one end and the load circuit 13 and has a function of switching between the load circuit 13 and an external measuring device, and during normal operation When one end of the semiconductor switching element is connected to the load circuit 13 and the threshold value of the overcurrent voltage is adjusted, the changeover switch 25 that connects one end of the semiconductor switching element to the external measurement device and a terminal for connecting to the external measurement device An external test terminal 26 is provided.

  In the state where one end of the semiconductor switching element is mounted and the same voltage as the actual use state is applied to the control terminal of the semiconductor switching element, the connection of the changeover switch 25 is set to the external test terminal 26 side, Is supplied to the semiconductor switching element, and a potential difference between both ends of the semiconductor switching element output by the A / D 21 at that time is determined as an overcurrent voltage threshold value and stored in the storage circuit 22. Hereinafter, description will be made with reference to examples.

  FIG. 1 is a diagram showing the configuration of an overcurrent protection circuit as an embodiment of the present invention. Although not particularly limited, in the present embodiment, the device 10 is a mobile radio communication terminal such as a mobile phone driven by a battery. That is, the battery 1 is provided as a power source, and power is supplied from the battery 1 to the load circuit 13. In connection between the battery 1 and the device 10, the plus terminal 2 of the battery 1 and the plus terminal 11 of the device 10 are connected, and the minus terminal 3 of the battery 1 and the minus terminal 12 of the device 10 are connected.

  The load circuit 13 is a circuit that consumes power to perform a predetermined function of the portable wireless terminal. The FET (Field Effect Transistor) 24 is an element that functions as a power switch that forcibly stops the current of the device 10 when an overcurrent flows due to an abnormality of the device 10.

  The FET 24 is composed of an N-type FET (N-channel MOSFET), and is arranged between the minus terminal 12 of the device 10 and the changeover switch 25, the source is connected to the minus terminal 12, and the drain is the changeover switch 25. Connected to one end (fixed end).

  The changeover switch 25 switches the connection destination of the drain of the FET 24 to either the load circuit 13 or the external test terminal 26. In other words, the changeover switch 25 is disposed between the drain of the FET 24 and the load circuit 13, and when the device 10 is normally used, the drain of the FET 24 is connected to the load circuit 13 so that it can be used during a test (excessive). When adjusting the current-voltage threshold), the connection destination of the drain of the FET 24 is switched to the external test terminal 26. Note that the changeover of the changeover switch 25 may be set manually, or a changeover control signal may be given to the changeover switch 25 from a CPU (not shown).

  The analog-to-digital conversion circuit (A / D) 21 connects an input terminal (analog input) to the drain and source of the FET 24 and inputs a drain-source voltage to convert it to a digital value. With this configuration, the voltage drop (drain-source voltage) due to the on-resistance of the FET 24 can be measured.

  An overcurrent voltage threshold for determining an overcurrent is stored and set in the storage circuit 22 in advance.

  The comparator 23 compares the drain-source voltage value (digital value) of the FET 24 detected by the A / D 21 with the overcurrent voltage threshold value read from the storage circuit 22, and compares the value from the A / D 21. When the voltage value exceeds the overcurrent voltage threshold value of the memory circuit 22, it is determined as an overcurrent, and a logic L (LOW level) is output as an output signal to the gate of the FET 24 to turn off the FET 24.

  When the value (digital value) of the drain-source voltage of the FET 24 detected by the A / D 21 is lower than the overcurrent voltage threshold of the memory circuit 22, it is determined that there is no overcurrent, and the comparator 23 Then, a fixed voltage of logic H (HIGH level) is output as an output signal, and the FET 24 is kept on.

  The on-resistance of the FET 24 varies even in the same product, and changes depending on the gate voltage applied to the FET 24.

  The fixed voltage output from the comparator 23 to the gate of the FET 24 may also vary depending on the individual comparator 23. For example, the HIGH level / LOW level fixed voltage output from the comparator 23 to the gate of the FET 24 also varies depending on the operating environment such as the power supply voltage and temperature, the manufacturing process, and the like.

  In consideration of variations in the on-resistance and gate voltage of the FET 24, it is necessary to determine the overcurrent voltage threshold of each device 10 after combining the FET 24 and the comparator 23.

  Therefore, in this embodiment, the changeover switch 25 and the external test terminal 26 are provided to enable adjustment by an external measuring device.

  In the present embodiment, the external test terminal 26 is preferably arranged on the surface of a substrate (not shown) of the apparatus 10 so that it can be easily connected to an external measuring device when adjusting the overcurrent voltage threshold.

  When adjusting the overcurrent voltage threshold, the changeover switch 25 is set so as to connect the drain of the FET 24 to the external test terminal 26. A DC power source 31 and an ammeter 32 are connected in series between the negative terminal 12 and the external test terminal 26 as an external measuring device. The positive terminal and the negative terminal of the DC power supply 31 are connected to the external test terminal 26 (the drain side of the FET 24) and the negative terminal 12 (the source side of the FET 24), respectively, and current flows from the drain to the source of the FET 24. A logic H fixed voltage output from the comparator 23 during normal operation is applied to the gate of the FET 24, and the output value of the A / D 21 when the drain current of the FET 24 becomes a value indicating an overcurrent is stored in the memory circuit 22. Store and set as an overcurrent voltage threshold.

  Thereby, FET24 can be adjusted in the same state as the overcurrent flowed at the time of actual operation. Although not particularly limited, when the value of the ammeter 32 becomes an overcurrent value at the time of adjusting the overcurrent voltage threshold, the person in charge of the inspection inputs a command from the operation unit (not shown) of the apparatus 10 or the like. Under the control of the CPU shown in the figure, the digital output value of the A / D 21 when the drain current of the FET 24 becomes a value indicating an overcurrent may be stored in the storage circuit 22. Note that the memory circuit 22 may use a programmable (rewritable) nonvolatile memory (EEPROM) that retains data even when the power of the apparatus 10 is turned off (for example, when the battery is removed).

  Next, the operation of the overcurrent protection circuit of this embodiment will be described.

  In FIG. 1, the overcurrent voltage threshold Vth set in the memory circuit 22 is determined by the following equation.

Vth = Ron × Ith
Ron is an on-resistance of the FET 24, and Ith is a current threshold at which the drain current of the FET 24 is determined to be an overcurrent.

  FIG. 2 is a graph showing the relationship (VI characteristics) of the drain current Id with respect to the gate-source voltage Vgs of the FET 24. It can be seen that the drain current of the FET 24 varies with the gate voltage, that is, the on-resistance Ron varies with the gate voltage.

  When the overcurrent voltage threshold is adjusted, the FET 24 is given the same gate voltage from the comparator 23 as in actual use, so the on-resistance Ron can be uniquely determined. The drain current Ith is set at the time of adjustment so as to have a predetermined overcurrent value. Therefore, in the overcurrent voltage threshold adjustment step, the overcurrent voltage threshold for determining an overcurrent can be accurately set in the storage circuit 22 when the device 10 is actually used. Therefore, when the device 10 is actually used, the current value when the drain current of the FET 24 reaches the overcurrent voltage threshold stored in the storage circuit 22 becomes equal to the overcurrent value. That is, the overcurrent value can be set accurately.

  Although the overcurrent voltage threshold needs to be larger than the maximum current consumption of the device 10, in this embodiment, the overcurrent protection circuit itself has a configuration in which there is no error in the current value determined as overcurrent. The problem that the overcurrent voltage threshold is set to a value larger than necessary and cannot be determined as an overcurrent is solved. Even when an overcurrent occurs due to an abnormal phenomenon such as a failure (failure) of the device 10, it is possible to appropriately control the power-off of the device 10.

  As described above, in this embodiment, in adjusting the overcurrent voltage threshold, the drain current of the FET 24 is set to the overcurrent under the condition that the FET 24 is mounted and the same gate voltage as that in actual use is applied. By flowing to the determination value and storing the drain-source voltage of the FET 24 at that time in the storage circuit 22 as the overcurrent voltage threshold value, the overcurrent can be accurately determined. As a result, the overcurrent voltage threshold value can be set from the maximum current consumption of the apparatus without having a margin, and the probability of powering off the apparatus when an overcurrent occurs can be increased.

  Next, another embodiment of the present invention will be described. FIG. 3 is a diagram showing the configuration of the second exemplary embodiment of the present invention. In this embodiment, the CPU 14 of the load circuit 13 is further connected to the storage circuit 22 in addition to the configuration of the embodiment shown in FIG. The CPU 14 recognizes the operation mode state of the apparatus 10 and notifies the storage circuit 22 of the operation mode.

  The storage circuit 22 stores an overcurrent voltage threshold value corresponding to each operation mode, and an overcurrent voltage threshold value corresponding to the operation mode is read by a notification from the CPU 14.

  Next, the operation of this embodiment will be described. In the case of a mobile radio terminal such as a mobile phone, the operation mode is a mode in which only reception is performed during standby, a voice call mode, a videophone mode, or the like. The current consumption of the device 10 varies depending on each mode. For example, since the current consumption is small during standby, the overcurrent value can be set small. By setting the overcurrent value to be small, it is possible to easily detect an excess current generated when the apparatus is abnormal. That is, even when the overcurrent at the time of abnormality is less than the maximum current consumption of the apparatus, it can be detected as an overcurrent.

  The current consumption in each operation mode can be known from a design value or a statistical value of a plurality of devices, and an overcurrent value in each operation mode is determined from the value. Then, at the time of adjusting the overcurrent voltage threshold value, adjustment is made by the overcurrent value corresponding to each operation mode, and each value is stored in the storage circuit 22 as the overcurrent voltage threshold value for each operation mode.

  As described above, according to the present embodiment, the overcurrent voltage threshold can be changed according to the operation mode of the apparatus 10, so that the overcurrent can be detected more easily. Further, for the same reason, it is possible to detect even when the overcurrent is equal to or less than the maximum current consumption of the device.

  Although the present invention has been described with reference to the above embodiment, the present invention is not limited to the configuration of the above embodiment, and various modifications that can be made by those skilled in the art within the scope of the present invention. Of course, modifications are included.

It is a figure which shows the structure of one Example of this invention. It is a figure which shows the VI characteristic of the gate-source voltage and drain current of FET. It is a figure which shows the structure of the other Example of this invention.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Battery 2 + terminal 3-terminal 10 Device 11 + terminal 12-terminal 13 Load circuit 14 CPU
21 A / D conversion circuit 22 Memory circuit 23 Comparator 24 FET
25 switch 26 external test terminal 31 DC power supply 32 ammeter

Claims (9)

A semiconductor switching element having one end connected to the power supply terminal;
A changeover switch for connecting the other end of the semiconductor switching element to one of a load circuit and an external terminal;
A voltage across the one end and the other end of the semiconductor switching element is received, compared with an overcurrent threshold stored in a storage circuit, and a signal corresponding to the comparison result is applied to the control terminal of the semiconductor switching element. Comparison means to supply;
An overcurrent protection circuit comprising: An analog-to-digital conversion circuit that receives the voltage across the semiconductor switching element and converts the voltage into a digital signal;
A comparison circuit that compares the output of the analog-to-digital conversion circuit with the overcurrent threshold stored in the storage circuit and supplies the comparison result to the control terminal of the semiconductor switching element as a binary signal;
The overcurrent protection circuit according to claim 1, further comprising:   In a state where the semiconductor switching element is mounted, a current is passed through the semiconductor switching element from a measuring device connected to the external terminal to a current value determined to be an overcurrent, and a voltage across the semiconductor switching element at that time The overcurrent protection circuit according to claim 1, wherein the overcurrent protection circuit is stored as the overcurrent threshold in the storage circuit.   The overcurrent according to claim 3, wherein the measuring device connected to the external terminal includes a power source connected between the external terminal and the power feeding terminal and supplying a direct current to the semiconductor switching element. Protection circuit.   4. The overcurrent protection circuit according to claim 3, wherein the measuring device connected to the external terminal includes a direct current power source and an ammeter connected in series between the external terminal and the power feeding terminal. The semiconductor switching element is an FET (field effect transistor),
One end of the semiconductor switching element is one of the source and drain of the FET,
The other end of the semiconductor switching element is the other of the source and drain of the FET,
The control terminal of the semiconductor switching element consists of the gate of the FET,
The analog-to-digital conversion circuit receives a drain-source voltage of the FET, compares it with an overcurrent threshold stored in the storage circuit, and supplies a voltage corresponding to the comparison result to the gate of the FET. The overcurrent protection circuit according to claim 2, wherein: A plurality of overcurrent threshold values respectively corresponding to a plurality of operation modes of the device including the overcurrent protection circuit are stored in the storage circuit,
The overcurrent according to any one of claims 1 to 6, further comprising means for reading an overcurrent threshold value corresponding to an operation mode of the device from the storage circuit and supplying the threshold value to the comparison means. Protection circuit. A device provided with the overcurrent protection circuit is a device driven by a battery,
The overcurrent protection circuit according to claim 1, wherein the power supply terminal to which one end of the semiconductor switching element is connected is a terminal connected to one end of the battery.   A mobile radio communication terminal comprising the overcurrent protection circuit according to claim 1.

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