Protective devices

Abstract

To suppress cost and to protect a wire from a temperature rise of the wire.SOLUTION: A protection device is a protection circuit which cuts off current flowing from a switch installed between a power supply and a load to the load via the wire, and includes: a current measurement part for measuring current flowing in a shunt resistor connected in series with the wire; and a determination part for turning the switch on or off on the basis of current measured by the current measurement part. The shunt resistor has the same temperature at energization as the wire when temperature characteristics are approximately the same as the wire and an environment temperature is equal to the environment temperature of the wire. The determination part turns off the switch when a current value measured by the current measurement part is equal to or higher than a threshold which is previously determined from an environment temperature upper limit of the wire, a wire limit temperature of the wire and the temperature characteristics of the shunt resistor.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a protection device. [Background technology]

[0002] An example of an invention for protecting the wires connecting a power source and a load is the device disclosed in Patent Document 1. This device detects the current flowing through the load, uses the detected current to calculate the heat generation and heat dissipation of the wire, and uses the calculation results to estimate the temperature of the wire. In this calculation, the thermal resistance used in the calculation formula is set to a pseudo thermal resistance so that it is between the minimum and maximum current / breaking time characteristics of the fuse, and if the estimated temperature reaches a pseudo allowable temperature that is lower than the allowable temperature, it is determined to be an overcurrent and the current flowing to the load is cut off. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Patent No. 5768176 Summary of the Invention [Problem to be solved by the invention]

[0004] The device disclosed in Patent Document 1 needs to calculate heat generation and heat radiation to estimate the temperature of the electric wire, and therefore requires a calculation circuit, which increases costs.

[0005] The present invention has been made in view of the above, and has an object to reduce costs and protect electric wires from temperature rise. [Means for solving the problem]

[0006] In order to solve the above-mentioned problems and achieve the object, a protection device according to one embodiment of the present invention is a protection circuit that cuts off a current flowing from a switch provided between a power source and a load through an electric wire to the load, and includes a current measurement unit that measures the current flowing through a shunt resistor connected in series to the electric wire, and a judgment unit that turns the switch on or off based on the current measured by the current measurement unit, wherein the shunt resistor has temperature characteristics that are approximately the same as those of the electric wire, and when the ambient temperature is equal to the ambient temperature of the electric wire, its temperature during current flow is equal to that of the electric wire, and the judgment unit turns off the switch when the current value measured by the current measurement unit is equal to or greater than a threshold that is predetermined based on the upper ambient temperature limit of the electric wire, the electric wire limit temperature of the electric wire, and the temperature characteristics of the shunt resistor.

[0007] In the protection device according to one aspect of the present invention, the product of the resistance value of the electric wire and the thermal resistance of the electric wire may be approximately equal to the product of the resistance value of the shunt resistor and the thermal resistance of the shunt resistor.

[0008] In addition, in a protection device according to one aspect of the present invention, the product of the resistance value of the electric wire and the transient thermal resistance of the electric wire may be approximately equal to the product of the resistance value of the shunt resistor and the transient thermal resistance of the shunt resistor.

[0009] In the protection device according to an aspect of the present invention, the shunt resistor may be a conductor formed on a substrate. [Effects of the Invention]

[0010] The present invention has the effect of reducing costs and protecting the electric wire from a rise in temperature of the electric wire. [Brief explanation of the drawings]

[0011] [Figure 1] FIG. 1 is a block diagram showing the configuration of a protection device according to an embodiment. [Figure 2] FIG. 2 is a graph showing the relationship between the electric wire environmental temperature and the electric wire limit overcurrent, and the relationship between the current flowing through the electric wire and the voltage of the current detection signal. DETAILED DESCRIPTION OF THE INVENTION

[0012] Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. Note that the present invention is not limited to the embodiments described below. In addition, in the description of the drawings, the same or corresponding elements are appropriately designated by the same reference numerals.

[0013] [Embodiment] Fig. 1 is a block diagram showing the configuration of a protection device according to an embodiment of the present invention. A power source 2 is, for example, a storage battery mounted on a vehicle. Power supplied from the power source 2 is supplied to a load 3 via a semiconductor switch 4, a shunt resistor 6, and an electric wire 7. The semiconductor switch 4 is, for example, a switch including a field effect transistor (FET), and is connected to the power source 2. The semiconductor switch 4 is turned on or off by a signal output from a drive circuit 5, and outputs or cuts off the power supplied from the power source 2.

[0014] The drive circuit 5 is a circuit that drives the FET included in the semiconductor switch 4. The drive circuit 5 outputs a signal to turn the semiconductor switch 4 on or off based on a control signal supplied from an ECU (Electronic Control Unit) 8 that controls the power supply to the load 3 and a determination signal output from a determination unit 10 (described later). When the drive circuit 5 receives a control signal to turn the semiconductor switch 4 off from the ECU 8, the drive circuit 5 outputs a signal to turn the semiconductor switch 4 off. When the drive circuit 5 receives a control signal to turn the semiconductor switch 4 on from the ECU 8, the drive circuit 5 outputs a signal to turn the semiconductor switch 4 off if a low-level determination signal is supplied from the determination unit 10, and outputs a signal to turn the semiconductor switch 4 on if a high-level determination signal is supplied from the determination unit 10.

[0015] The shunt resistor 6 has one end connected to the semiconductor switch 4 and the other end connected to the electric wire 7, and is connected in series with the semiconductor switch 4 and the electric wire 7. The shunt resistor 6 is a so-called shunt resistor used to measure the current flowing from the semiconductor switch 4 to the load 3 via the electric wire 7. The temperature characteristics of the shunt resistor 6 are matched to the temperature characteristics of the electric wire 7, and have the same temperature coefficient. In order to match the temperature characteristics of the shunt resistor 6 with those of the electric wire 7, the shunt resistor 6 may be made of the same material as that of the electric wire 7, for example. The shunt resistor 6 may also be a conductor formed on a substrate.

[0016] The electric wire 7 is an electric wire laid in a vehicle and connected to a load 3. The load 3 is an electrical component in the vehicle that is driven by power supplied from a power source 2. The load 3 is connected to a ground GND. The protection device 1 is a device that prevents the electric wire 7 from emitting smoke or catching fire, and is mounted on the vehicle.

[0017] The protection device 1 has a determination unit 10 and a current measurement unit 11. The current measurement unit 11 has a well-known current measurement circuit equipped with an operational amplifier. The current measurement unit 11 outputs a current detection signal whose voltage is proportional to the current value of the current flowing through the shunt resistor 6. This current detection signal reflects the ambient temperature of the electric wire 7 and the temperature characteristics of the electric wire 7.

[0018] The determination unit 10 has a comparator circuit, and when the voltage of the current detection signal supplied from the current measurement unit 11 is equal to or greater than a predetermined threshold, outputs a low-level determination signal to the drive circuit 5, and when the voltage of the signal supplied from the current measurement unit 11 is less than the predetermined threshold, outputs a high-level determination signal to the drive circuit 5. As a result, when the voltage value of the voltage of the current detection signal is equal to or greater than the threshold, the semiconductor switch 4 is turned off, and the power supply from the semiconductor switch 4 to the load 3 is cut off.

[0019] Here, we will explain the current detection signal that reflects the environmental temperature and temperature characteristics of the electric wire 7, and the threshold value used for judgment by the judgment unit 10. First, according to the automobile standard JASO D 609 "Automotive parts - Current capacity of low voltage lines," the calculation of the limit overcurrent of an automotive electric wire is shown in equation (1).

[0020]

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[0021] In equation (1), I o is the limiting overcurrent of the wire, T3 is the limiting temperature of the wire, r T3 is the conductor resistance of the wire at its limiting temperature, T2 is the ambient temperature of the wire, R is the thermal resistance of the wire, α2 is the reciprocal of the thermal time constant τ of the wire, and t is time. From equation (1), the limiting overcurrent Iwm for wire 7 at ambient temperature Ta can be expressed by equation (2) because the time constant term can be ignored.

[0022]

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[0023] In equation (2), Twm is the wire limit temperature of the wire 7, Rwo is the resistance value of the wire 7 at the reference temperature, Ctw is the resistance temperature coefficient of the wire 7, To is the reference temperature, and Rhw is the thermal resistance of the wire 7. Furthermore, when the environmental temperature Ta of the wire 7 is the upper environmental temperature limit Tamax, the limit overcurrent Iwm is expressed by equation (3).

[0024]

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[0025] In Figure 2, the relationship between the environmental temperature Ta of the wire 7 and the limiting overcurrent Iwm of the wire 7 is shown by line L1. In calculating the limiting overcurrent Iwm, the reference temperature To is set to 25°C and the wire limiting temperature Twm is set to 165°C. As shown by line L1, as the environmental temperature Ta of the wire 7 decreases, the limiting overcurrent Iwm of the wire 7 increases. For example, if the environmental temperature Ta of the wire 7 is set to 80°C, which is the upper environmental temperature limit Tamax, the limiting overcurrent Iwm is 23.42 A.

[0026] Next, the temperature Ts of the shunt resistor 6 is given by equation (4), and the temperature Tw of the electric wire 7 is given by equation (5).

[0027]

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[0028]

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[0029] In equation (4), Tb is the temperature of the substrate on which the shunt resistor 6 is provided, Rs is the resistance value of the shunt resistor 6, and Rths is the thermal resistance of the shunt resistor 6. In equation (5), Ta is the ambient temperature Ta of the electric wire 7, Rw is the resistance value of the electric wire 7, and Rthw is the thermal resistance of the electric wire 7. In addition, in equations (4) and (5), Iw is the current value of the current flowing through the shunt resistor 6 and the electric wire 7.

[0030] For equations (4) and (5), if the shunt resistor 6 and the wire 7 are selected such that the resistance value Rw of the wire 7 × the thermal resistance Rthw of the wire 7 = the resistance value Rs of the shunt resistor 6 × the thermal resistance Rths of the shunt resistor 6, and the environment is set such that the ambient temperature Ta of the wire 7 = the substrate temperature Tb of the substrate on which the shunt resistor 6 is mounted, then the temperature Tw of the wire 7 = the temperature Ts of the shunt resistor 6. For example, if the wire 7 has a resistance Rwo = 0.01 Ω and a thermal resistance Rhw = 10°C / W at the reference temperature To, the shunt resistor 6 has a resistance Rso = 0.003 Ω and a thermal resistance Rths = 33.33333°C / W at the reference temperature To. Note that the resistance value Rw of the wire 7 × the thermal resistance Rthw of the wire 7 may be approximately equal to the resistance value Rs of the shunt resistor 6 × the thermal resistance Rths of the shunt resistor 6. When the electric wire 7 is at the electric wire limit temperature Twm, the temperature Tw of the electric wire 7 = the temperature Ts of the shunt resistor 6 = the electric wire limit temperature Twm of the electric wire 7, and the temperature of the shunt resistor 6 also becomes equal to the electric wire limit temperature Twm. In other words, the shunt resistor 6 reflects the temperature change of the electric wire 7.

[0031] Next, if the voltage value of the current detection signal output from the current measurement unit 11 is Viw, the voltage value Viw is expressed by equation (6). In equation (6), Ga is the gain of the operational amplifier of the current measurement unit 11, and Vsofst is the offset voltage of the operational amplifier of the current measurement unit 11.

[0032]

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[0033] The resistance value Rs of the shunt resistor 6 included in equation (6) changes with temperature, and this resistance value is expressed by equation (7). In equation (7), Rso is the resistance value of the shunt resistor 6 at the reference temperature, and Cts is the resistance temperature coefficient of the shunt resistor 6.

[0034]

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[0035] When the temperature Tw of the electric wire 7 = the temperature Ts of the shunt resistor 6 = the electric wire limit temperature Twm of the electric wire 7, the resistance value Rs of the shunt resistor 6 is given by equation (8).

[0036]

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[0037] If the resistance value Rs of the shunt resistor 6 at the wire limit temperature Twm is the resistance value Rsm and the voltage value Viw of the current detection signal at the wire limit temperature Twm is the voltage value Viwm, the voltage value Viwm is expressed by equation (9). The determination unit 10 uses Viwm obtained from equation (9) as a threshold value.

[0038]

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[0039] In addition, the wire limit temperature Twm, the upper limit environmental temperature Tamax, the resistance value Rwo of the wire 7 at the reference temperature, the resistance temperature coefficient Cts of the shunt resistor 6, the reference temperature To, and the thermal resistance Rhw of the wire 7 in equation (3) for obtaining the limit overcurrent Iwm are known. In addition, the resistance value Rsm for obtaining the voltage value Viwm, the gain Ga of the current measuring unit 11, and the offset voltage Vsofst of the current measuring unit 11 are also known, so the voltage value Viwm can be calculated in advance from known parameters and set as the threshold value of the determining unit 10.

[0040] In Figure 2, lines L2 and L3 show the relationship between the current value Iw of the current flowing through the shunt resistor 6 and the wire 7 and the voltage value Viw of the current detection signal. Line L2 in Figure 2 shows the relationship between the current value Iw of the current flowing through the shunt resistor 6 and the wire 7 and the voltage value Viw of the voltage of the current detection signal when the temperature coefficient of resistance Cts of the shunt resistor 6 is 0.00393 / Ω and the temperature Ts of the shunt resistor 6 = the temperature Tw of the wire 7 = the reference temperature To. In this embodiment, if the upper limit ambient temperature Tamax is 80°C and the wire limit temperature Twm of the wire 7 is 165°C, the limit overcurrent Iwm is 23.42 A as measured by line L1. When temperatures Ts and Tw are equal to the reference temperature To, and this 23.42 A flows, the voltage value Viw is 2.6 V as measured by line L2.

[0041] 2 is a line showing the relationship between the limit overcurrent Iwm of the wire 7 and the voltage value Viw when the temperature Ts of the shunt resistor 6 and the temperature Tw of the wire 7 are equal to the wire limit temperature Twm of the wire 7. If the limit overcurrent Iwm of 23.42 A flows when the ambient temperature upper limit Tamax is 80°C and the temperature Ts and temperature Tw are equal to the wire limit temperature Twm, the voltage value Viwm is 3.77 V according to the line L3. Therefore, the judgment unit 10 sets the threshold value to 3.77 V, which is the voltage value corresponding to the limit overcurrent Iwm of 23.42 A when the ambient temperature upper limit Tamax is 80°C and the wire limit temperature Twm is 165°C.

[0042] Viwm=3.77V is a voltage value corresponding to the limit overcurrent Iwm when the environmental temperature Ta of the electric wire 7 is the upper limit environmental temperature Tamax=80°C and the temperature Tw of the electric wire 7 is the electric wire limit temperature Twm=165°C. Therefore, when the environmental temperature Ta of the electric wire 7 is lower than the upper limit environmental temperature Tamax, a current larger than the limit overcurrent Iwm can be allowed to flow to the electric wire 7.

[0043] According to this embodiment, by matching the ambient temperature and temperature characteristics of the shunt resistor 6 and the electric wire 7, the shunt resistor 6 changes the voltage value Viw to reflect the temperature change of the electric wire 7. Therefore, by determining the voltage value that reflects the temperature change, it is possible to cut off the current and protect the electric wire 7 without performing a calculation to estimate the temperature of the electric wire.

[0044] In the present invention, the shunt resistor 6 may be selected so that the resistance value of the electric wire 7 × the transient thermal resistance of the electric wire 7 = the resistance value of the shunt resistor 6 × the transient thermal resistance of the shunt resistor 6 . [Explanation of symbols]

[0045] 1 Protective device 2 power supply 3. Load 4. Solid-state switches 5. Drive circuit 6 shunt resistors 7 Electric wire 8 ECU 10 Judgment section 11 Current measurement section

Claims

1. A protection circuit that cuts off current flowing from a switch provided between a power source and a load through an electric wire to the load, a current measuring unit that measures a current flowing through a shunt resistor connected in series to the electric wire; a determination unit that turns on or off the switch based on the current measured by the current measurement unit; Equipped with The shunt resistor has a temperature characteristic, that is, a resistance temperature coefficient, which is approximately equal to a resistance temperature coefficient of the electric wire, the shunt resistor is selected so that, in an environment in which the temperature of a substrate on which the shunt resistor is provided is equal to the environmental temperature of the electric wire, the product of the resistance value of the electric wire and the thermal resistance of the electric wire is approximately equal to the product of the resistance value of the shunt resistor and the thermal resistance of the shunt resistor, and the temperature of the shunt resistor when current is applied is equal to that of the electric wire; The determination unit turns off the switch when a voltage of a current detection signal based on a current value measured by the current measurement unit is equal to or greater than a threshold value that is predetermined based on an upper limit of an environmental temperature of the electric wire, a wire limit temperature of the electric wire, and a resistance temperature coefficient of the shunt resistor. Protective device.

2. The shunt resistor is selected so that the product of the resistance value of the electric wire and the transient thermal resistance of the electric wire is approximately equal to the product of the resistance value of the shunt resistor and the transient thermal resistance of the shunt resistor. The protection device of claim 1 .

3. The shunt resistor is a conductor formed on a substrate.

3. The protection device according to claim 1 or 2.

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