Protection circuit

Abstract

Provides an easily adjustable protection circuit. [Solution] The protection circuit 10 according to the first aspect includes a switch F1 which is an FET connected between the battery BTT and the load 30, and the protection circuit 10 turns off (shuts off) the switch F1 when an overvoltage is applied from the battery BTT, and comprises a voltage detection unit 11 for detecting the overvoltage, a resistor R5 which is the gate-source resistance of the switch F1, a capacitor C1 which is the gate-source capacitor of the switch F1, and a diode D1 connected between the gate of the switch F1 and ground.

Description

Technical Field

[0001] The present invention relates to a protection circuit.

Background Art

[0002] As a conventional protection circuit, for example, a surge protection circuit included in a vehicle instrument disclosed in Patent Document 1 is known. In Patent Document 1, the surge protection circuit detects an overvoltage that may be applied from a power source (B) and performs a protection operation.

Prior Art Documents

Patent Documents

[0003] <P

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0004] When this protection circuit drops below the voltage required for the FET to turn on due to a voltage drop, the FET is turned off. Such a voltage drop is likely to occur particularly in circuits mounted on a vehicle, and this voltage drop is sometimes called cranking. When the FET is turned off due to a voltage drop, the subsequent circuit (such as a display device, a load device) may shut down, enter sleep, or stop functioning as part of a safety operation.

[0005] Therefore, the object of the present invention is to provide a protection circuit that can easily adjust the operation of the FET due to a voltage drop, paying attention to the above problems.

Means for Solving the Problems

[0006] To achieve the above object, the protection circuit according to the present disclosure includes an FET connected between a power source and a load, and is a protection circuit that turns off the FET when an overvoltage is applied from the power source, an overvoltage detection unit that detects the overvoltage, The gate-source resistance of the FET and The gate-source capacitor of the aforementioned FET, A diode connected between the gate and ground of the FET, It is equipped with. [Brief explanation of the drawing]

[0007] [Figure 1] This is a circuit diagram showing the protection circuit 10 of this disclosure. [Figure 2] This is the voltage waveform shown by the protection circuit 10 of this disclosure. [Figure 3] This is the voltage waveform shown by the comparative example protection circuit 101. [Figure 4] This is the voltage waveform shown by the comparative example protection circuit 102. [Modes for carrying out the invention]

[0008] Each embodiment will be described in the following order with reference to the attached drawings. Note that in some drawings, reference numerals may be omitted for parts with the same attribute that exist in multiple locations, for the sake of clarity. [First Embodiment] 1-1. Explanation of the structure 1-2. Explanation of Behavior 1-3. Comparison with Comparative Examples [Example of effect] [Differentiation]

[0009] [First Embodiment] <1-1. Explanation of the structure> Figure 1 is a circuit diagram showing a part of the circuit configuration of an electrical component (including the protection circuit 10 according to this disclosure) installed in vehicle C. Vehicle C is a vehicle such as a four-wheeled automobile, a motorcycle, or a work vehicle (including agricultural vehicles and construction vehicles), but it may also be a mobile entity such as a ship or an aircraft. Vehicle C includes a battery BTT, a protection circuit 10 (shown by the roughly dashed line in Figure 1), and a load 30.

[0010] The vehicle battery BTT consists of a lead-acid battery or the like, and supplies a predetermined amount of power to electrical components with a rated power supply voltage of 12.5V. The electrical components include a load 30 connected via a protection circuit 10. The battery BTT is connected to the protection circuit 10 by wiring such as a harness.

[0011] Load 30 can be a variety of in-vehicle electronic components, such as a display device. The display device displays various information to the occupants of vehicle C or to people outside of vehicle C. Examples of display devices include meters, center information displays, head-up display devices, and touch panel display devices.

[0012] The protection circuit 10 is configured as a power supply protection circuit (board) that, when the applied power supply voltage is abnormal, switches the power supply to the load 30 on and off (continuity interruption switching) based on the behavior of the circuit elements mounted on it. The protection circuit 10 is configured as a circuit board on which various electrical and electronic elements are mounted on the mounting surface of a blank substrate formed of a substrate and wiring. The blank substrate consists of a substrate, for example, a glass epoxy substrate (FR4), and a printed wiring pattern, which is copper foil printed on the substrate. The blank board only needs to have a base material and wiring pattern that can be used to construct a circuit board. The base material can be FR-1, FR-2, FR-3, or an aluminum substrate (including an insulating layer), and the wiring pattern can be made of something other than copper foil.

[0013] The protection circuit 10 includes a voltage detection unit 11 and a switching unit 12. When the voltage detection unit 11 detects an abnormality in the power supply voltage, the protection circuit 10 shuts off the power supplied from the input line HL to the load 30 using the switching unit 12.

[0014] The voltage detection unit 11 has its input terminals connected to two locations on the input line HL and its output terminal connected to the switching unit 12 (in particular, the gate of the switch F1, which is an FET). When it detects an abnormality in the power supply voltage (especially an overvoltage), it turns off the switch F1 and cuts off the power. The voltage detection unit 11 exhibits the above-described behavior by being configured with Zener diodes Z1 and Z2, resistors R1, R2, R3, and R4, and transistors TR1 and TR2 as shown in FIG. 1.

[0015] The opening / closing unit 12 allows the power supplied via the input line HL to pass during normal times, but cuts off the power supply according to the behavior of the voltage detection unit 11. Therefore, the opening / closing unit 12 is connected between the input line HL and the load 30. The opening / closing unit 12 includes a switch F1, a Zener diode Z3, a capacitor C1, resistors R5 and R6, a diode D1, a transistor TR3, and a control unit 20.

[0016] The switch F1 is a switch connected between the input line HL and the load 30, and is composed of, for example, a P-channel MOSFET among field-effect transistors (FETs). The switch F1 supplies the power supply voltage to the subsequent-stage circuit connected to the drain by conducting between the source and the drain during normal times. On the other hand, the switch F1 cuts off the power supply by the power supply voltage by cutting off between the source and the drain when the power supply voltage is excessive.

[0017] The Zener diode Z3 is connected between the gate and the source of the switch F1, and functions to protect the voltage between the gate and the source of the switch F1 from becoming excessive due to an excessive power supply voltage.

[0018] The resistor R5 and the capacitor C1 are connected between the gate and the source of the switch F1, and are installed to suppress the unintentional behavior of the switch F1 due to various noises (such as EMI noise and external noise). The resistor R5 particularly functions to stabilize the gate voltage. The capacitor C1 particularly functions to absorb noise.

[0019] The resistor R6 is connected between the gate of the switch F1 and the ground GND, and functions to protect the Zener diode Z3 from an overcurrent flowing through it. Further, the resistor R6 is set to an appropriate resistance value according to the voltage division ratio with the resistor R5 so as to ensure a sufficient voltage between the gate and the source of the switch F1.

[0020] Diode D1 is connected with its anode to the gate of switch F1 and its cathode to ground (GND). Diode D1 is a reverse-blocking diode that prevents reverse current from ground (GND). Diode D1, as a reverse-blocking diode, may be connected to any other point between the gate of switch F1 and ground (GND).

[0021] Transistor TR3 performs a cutoff operation to prevent current from flowing throughout the circuit when vehicle C is stopped or when load 30 does not need to be operated. Transistor TR3 is, for example, an NPN type transistor, and control unit 20 is connected to its base. Control unit 20 can be any element or IC that can switch transistor TR3 on and off. Transistor TR3 is switched on and off according to the on / off operation of control unit 20, and the current to the entire circuit is also switched on and off.

[0022] <1-2. Explanation of Behavior> In the protection circuit of this disclosure, a CR circuit between the gate and source (a resistor R5 and a capacitor C1 connected in parallel between the gate and source) suppresses the off-turning of switch F1 due to momentary voltage drops mainly caused by cranking. Specifically, by exhibiting the voltage waveform behavior shown in Figure 2, the gate-source voltage is maintained at or above the voltage V1 required to turn on switch F1, and the supplied power does not decrease.

[0023] Therefore, in a protection circuit 10 having such a CR circuit, power interruption can be easily suppressed by adjusting the constants of the CR circuit.

[0024] In particular, in the protection circuit 10 of this disclosure, the time constant of the CR circuit is the product of the resistance value of resistor R5 and the capacitance value of capacitor C1. The most preferred time constant is 0.07 or greater. For example, in a configuration exhibiting a time constant of 0.0726, which is within that range, the resistance value of resistor R5 is 330kΩ and the capacitance value of capacitor C1 is 0.22μF.

[0025] Here, if we let V1 be the on-voltage of switch F1 (the minimum voltage required for switch F1 to be turned on), V0 be the gate-source voltage, τ be the time constant, R be the resistance of resistor R5, C be the capacitance of capacitor C1, and ln be the natural logarithm, then the time t during cranking in which switch F1 remains on is expressed by the following equation (1). t = -C·R·ln(V1 / V0) ···(1)

[0026] Furthermore, as a general rule, the ISO 16750-2 standard describes typical vehicle cranking scenarios where the power supply voltage drops for approximately 60 milliseconds in a system with a 12V power supply, and for approximately 100 milliseconds in a system with a 24V power supply.

[0027] Considering these factors, if the time constant is 0.07 or greater, switch F1 can be kept on during cranking in any typical power supply voltage system.

[0028] First, the on-time t1 for a 24V system can be calculated as follows. t1 = -C·R·ln(V1 / V0) = -0.07·ln(4V / 18V) =105 [msec]

[0029] Next, the on-time t2 for a 12V system can be calculated as follows. t2 = -C·R·ln(V1 / V0) = -0.07·ln(4V / 10V) =64 [msec]

[0030] Here, V1 is assumed to be 4V, which is the standard FET voltage V1. In a 24V system, V0 is assumed to be 18V due to the protection voltage of the Zener diode Z3 for FET protection. In a 12V system, V0 is assumed to be 10V, assuming a voltage drop of approximately 2V from the power supply voltage of 12V due to the influence of electronic components.

[0031] As described above, within this time constant range, even when typical cranking occurs in a system with a typical power supply voltage, the protection circuit avoids unnecessary power supply interruptions and maintains more stable load operation.

[0032] As mentioned above, the time constant τ does not need to be large as long as at least its lower limit is met. However, an excessively large time constant can cause deterioration of the overvoltage protection circuit, so it is desirable to avoid making it unnecessarily large. For example, if C is large, the time required to turn off switch F1 in the event of overvoltage will be longer, increasing the risk of excessive voltage being applied to load 30. On the other hand, if R is large, the current flowing from resistor R6 to ground GND will be extremely small, increasing the impedance of the circuit and degrading performance in terms of EMI noise. Therefore, depending on the characteristics of load 30, it is desirable that the time constant be between 0.07 and 0.5, more preferably between 0.07 and 0.3, and more preferably between 0.07 and 0.1.

[0033] Figure 2 shows the voltage waveform exhibited by the protection circuit 10 of this disclosure when cranking occurs. The horizontal axis represents time t, with the moment immediately before cranking occurs set to 0 seconds. The vertical axis represents various voltages, with the ground voltage (GND) set to 0. In Figure 2, the solid line waveform represents the gate voltage of switch F1, the dashed-dotted line waveform represents the drain voltage (output voltage) of switch F1, and the double-dotted-dotted line waveform represents the source voltage (i.e., power supply voltage) of switch F1. Note that, because the source voltage and drain voltage are plotted over a long period of time when they remain at similar values, the single-dotted line will represent the source voltage during the time periods when the double-dotted line does not appear. From the above, the gate-source voltage V0 corresponds to the difference between the solid line and the dashed line. That is, if the dashed line is lower than the solid line, the gate-source voltage V0 takes a negative value.

[0034] As shown in Figure 2, in the protection circuit 10 of this disclosure, even when cranking occurs, the solid line and the dashed line maintain a sufficient potential difference from each other, and the gate-source voltage V0 is always above the voltage V1.

[0035] More specifically, at 0 milliseconds just before cranking, the source and drain voltages are approximately 24V, the gate voltage is approximately 6V, and the gate-source voltage is approximately 18V. The source and drain voltages remain roughly equal to each other throughout the cranking process. Strictly speaking, the drain voltage is slightly lower than the source voltage due to the internal on-resistance of switch F1. Approximately 5 milliseconds after cranking occurs, the source voltage is approximately 4V, the gate voltage is approximately -12V, and the gate-source voltage is approximately 16V. Approximately 60 milliseconds after cranking begins, the cranking process ends, the voltage starts to rise, the source voltage is approximately 4.5V, the gate voltage is approximately -3.5V, and the gate-source voltage is approximately 7V. The gate-source voltage is sufficient to turn on switch F1. As a result, even if cranking occurs, the source voltage (power supply voltage) and drain voltage (output voltage) can be maintained at nearly equal voltages.

[0036] <1-3. Comparison with Comparative Examples> Here, we will compare the protection circuit 10 of this disclosure with two comparative examples shown in Figures 3 and 4. Note that elements whose configuration is not mentioned are the same as those in Figure 1, and therefore detailed explanations are omitted.

[0037] (1) When the time constant is 0.03 First, the comparative example protection circuit 101 has a CR time constant of 0.03 compared to protection circuit 10. The voltage waveform exhibited by protection circuit 101 is shown in Figure 3. Note that if the time constant of 0.03 is satisfied, the magnitude of the resistance and capacitance values ​​does not significantly affect the voltage waveform.

[0038] Approximately 50-70 milliseconds after cranking begins, the gate-source voltage drops to approximately 4V (V1) (i.e., switch F1 is not fully turned on), causing a significant decrease in the drain voltage (output voltage). This is due to the time constant being smaller than necessary, resulting in rapid charge loss from capacitor C1 during cranking. In this case, load 30 may perform safety actions such as a reset due to the drop in input voltage (i.e., the drain voltage of switch F1), and may cease operation.

[0039] (2) When the time constant is 0.07 but diode D1 is not available The comparative example, protection circuit 102, has a CR time constant of 0.07 compared to protection circuit 10, but does not have diode D1 (i.e., the wiring location where diode D1 is mounted is short-circuited). The voltage waveform exhibited by protection circuit 102 is shown in Figure 4. Note that if the time constant of 0.07 is satisfied, the magnitude of the resistance and capacitance values ​​does not significantly affect the voltage waveform.

[0040] Approximately 30 to 70 milliseconds after cranking occurs, the drain voltage (output voltage) drops significantly because the gate-source voltage becomes approximately 4V (V1) (i.e., switch F1 is not fully turned on). This is because, as the gate voltage temporarily becomes negative, and there is no diode D1 to prevent reverse current flow, a reverse current is generated through the path from ground GND, transistor TR1, resistor R6, and capacitor C1. The effect of this reverse current attempting to return the negative voltage of capacitor C1 to a positive voltage causes a relatively significant drop in the gate-source voltage. Even in this case, load 30 may perform safety operations such as a reset operation due to the drop in input voltage (i.e., the drain voltage of switch F1), and stop operating. In this respect, the protection circuit 10 of this disclosure has a diode D1, which can suppress a significant drop in gate-source voltage.

[0041] [Example of effect] The protection circuit 10 relating to the first aspect is, A protection circuit 10 includes a switch F1 which is an FET connected between the battery BTT and the load 30, and which turns off (shuts off) the switch F1 when an overvoltage is applied from the battery BTT, A voltage detection unit 11 for detecting overvoltage, The resistor R5 is the gate-source resistance of switch F1, Capacitor C1 is the gate-source capacitor of switch F1, Diode D1 is connected between the gate and ground of switch F1, It is equipped with.

[0042] This configuration allows for easy adjustment of the protection operation by adjusting the time constants of capacitor C1 and resistor R5. Furthermore, diode D1 prevents reverse current during cranking, suppressing a rapid drop in gate-source voltage and preventing switch F1 from being easily turned off.

[0043] The protection circuit 10 relating to the second aspect is, The time constant τ, which is the product of the resistance value of resistor R5 (gate-source resistance) and the capacitance value of capacitor C1 (gate-source capacitor), is 0.07 or greater.

[0044] This configuration prevents an unnecessary drop in output voltage even when normal cranking occurs without any particular abnormality, resulting in a protection circuit 10 that maintains high marketability for the load 30.

[0045] [Differentiation] Although the protection circuit of the present invention has been described using the configuration of the above-described embodiment as an example, the present invention is not limited thereto, and various improvements and changes to the display are possible in other configurations without departing from the spirit of the present invention.

[0046] For example, the load may have a configuration other than those exemplified, and the mobile object on which it is mounted may also have a configuration other than those exemplified. [Explanation of symbols]

[0047] C Vehicle BTT Battery GND (Ground) 10 Circuit device HL input line 11 Voltage detection unit Z1, Z2 Zener diodes R1,R2,R3,R4 resistance TR1, TR2 Transistors 12 Opening / Closing Section Z3 Zener diode C1 Capacitor R5,R6 resistance D1 diode F1 switch TR3 Transistor 20 Control Unit 30 load

Claims

1. A protection circuit that includes an FET connected between a power supply and a load, and turns off the FET when an overvoltage is applied from the power supply, The overvoltage detection unit detects the overvoltage, The gate-source resistance of the aforementioned FET, The gate-source capacitor of the aforementioned FET, A diode connected between the gate and ground of the aforementioned FET, Equipped with A protective circuit characterized by the following features.

2. The time constant, which is the product of the resistance value of the gate-source resistor and the capacitance value of the gate-source capacitor, is 0.07 or greater. The protective circuit according to claim 1, characterized in that

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