JP2016152765A - Surge protection circuit for vehicle meter - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a surge protection circuit for a vehicle meter capable of protecting electronic components, even under a condition of severe fluctuation of a power supply voltage for the vehicle meter.SOLUTION: A surge protection circuit for a vehicle meter includes: switch means 3 consists of a P-channel MOS FET for switching power supply and cutoff from a power supply B to a load 2; a voltage limiting transistor 52 for detecting over-voltage of the power supply B and driving a switch means 3; a current limiting transistor 62 for detecting over-current of the power supply B and driving the switch means 3; a capacitor 4, connected at an output terminal of the switch means 3, for supplementing power to the load 2; and a Zener diode 7 for limiting each voltage between collector and emitter of the voltage limiting transistor 52 and the current limiting transistor 62, and limiting a voltage between gate and source of the switch means 3.SELECTED DRAWING: Figure 1

Description

  The present invention relates to a surge protection circuit for supplying a stable power supply to a load circuit constituting a vehicle instrument.

  Conventionally, as a vehicle instrument for displaying vehicle information such as a vehicle speed and an accumulated travel distance, there is one disclosed in Patent Document 1, for example. These vehicle meters are connected to a battery that can stably supply power, and display various vehicle information using this power. Various sensors mounted on the vehicle also operate based on the power supply from the battery.

  However, in the case of a vehicle with a small battery and a small power supply capacity, or a vehicle with a small power supply capacity due to power consumption such as dark current, the ratio of the supply voltage supplied from the vehicle generator to the vehicle meter Therefore, power supply is interrupted and voltage fluctuations become severe. For this reason, for example, as disclosed in Patent Document 2, the power supply circuit includes an input electrolytic capacitor provided for preventing instantaneous interruption, or a surge protection circuit including a transistor, a Zener diode, and a base resistor. is there.

JP 2002-1104056 A Japanese Patent Laid-Open No. 64-23722

  However, when a surge voltage / power supply noise is applied, or in a vehicle with a large power supply ratio from the generator, a power source with a large power fluctuation is supplied to the vehicle meter, particularly when the engine is started. Therefore, the electrolytic capacitor is repeatedly charged, and an inrush current frequently flows through the transistor. As a result, when this configuration is applied to the vehicle instrument, electronic components such as transistors and electrolytic capacitors may be damaged, which is a problem.

  Accordingly, an object of the present invention is to provide a surge protection circuit for a vehicular instrument that can protect electronic components even when the fluctuation of the power supply voltage supplied to the vehicular instrument becomes severe, focusing on the above-mentioned problems. It is in.

A surge protection circuit for a vehicle instrument according to the present invention,
Switch means comprising a P-channel MOSFET for switching between power supply from the power supply to the load and its stop;
A voltage limiting transistor for detecting an overvoltage of the power source and driving the switch means;
A current limiting transistor for detecting an overcurrent of the power source and driving the switch means;
A capacitor connected to the output terminal of the switch means to supplement the power to the load;
And a Zener diode for limiting the voltage between the collector and the emitter of the voltage limiting transistor and the current limiting transistor and for limiting the voltage between the gate and the source of the switch means.

  The voltage limiting transistor and the current limiting transistor may be PNP transistors, and the voltage between the collector and emitter is limited by the Zener diode.

  The voltage limiting transistor may be configured as a circuit also serving as the current limiting transistor.

  The present invention relates to a surge protection circuit for a vehicular instrument, and can protect electronic components even when power supply voltage fluctuates significantly.

The figure which shows the circuit structure in embodiment of this invention. The figure which shows the meter for vehicles of embodiment same as the above. The figure which shows the circuit structure in another example of embodiment same as the above.

  Embodiments to which the present invention is applied will be described below with reference to the accompanying drawings.

  FIG. 1 is a diagram illustrating a circuit configuration of a surge protection circuit A1 of a vehicle meter A until power from a power source B including a vehicle battery B1 and a generator B2 is supplied to a load in the vehicle meter A. The configuration of is shown.

  As shown in FIG. 2, the vehicle meter A includes a display (display means) C that displays vehicle information, and a surge protection circuit A1 is mounted on a circuit board for driving them. The display C includes a digital display C1 using a liquid crystal display element and a backlight, and a pointer display C2 that indicates a measured value by a pointer that rotates using a motor as a drive source, and is controlled by a control circuit described later. The circuit board is provided in the housing D of the vehicle meter A.

  Moreover, the generator B2 of the power supply B can apply the alternator using rotation of the engine used as a vehicle drive source. The battery B1 has a role of stably supplying the output of the power source B while charging using the AC component from the generator B2. Further, the battery B1 can output electric power even when the generator B2 (engine) is not operating.

  The vehicle meter A is connected to the power source B and supplies power to the load 2 such as the regulator 21 via the diode 1 and the surge protection circuit A1. The surge protection circuit A1 includes a switch means 3, a capacitor 4, an overvoltage detection circuit 5, an overcurrent detection circuit 6, a Zener diode 7, and a resistance element 8. In this case, as a load 2, a regulator 21 And the control circuit 22 are provided on the circuit board.

  The diode 1 is a protection circuit for preventing current from flowing to the opposite side to the normal side, for example, by reversely connecting the terminal of the battery B1 at the input end of the surge protection circuit A1, and from the power source such as the battery B. The wiring and the anode are connected.

  The regulator 21 is a circuit that steps down and supplies the drive power of the control circuit 22 to a predetermined voltage (for example, 5 V) based on the power supply from the power supply B.

  The switch means 3 can apply a P-channel MOSFET (field effect transistor), and switches between power supply from the power source B to the load 2 and its stop. The switch means 3 has a gate connected to the resistance element 31, a source connected to the current detection resistor 61 of the overcurrent detection circuit 6, and a drain connected to the load 2. When the potential difference between the gate and the source is less than the threshold, the switch means 3 is turned off (the current supply from the power source B to the load is stopped), and when the potential difference between the gate and the source is greater than or equal to the threshold To an on state (power supply state from the power source 3 to the load 2).

  The resistance element 31 has one end connected to the gate of the switch means 3 and the other end grounded (GND connection). The resistance element 31 limits the current that flows from the power supply B to GND when the voltage limiting transistor 52 or the current limiting transistor 62 is turned on, and also limits the current that flows to the Zener diode 7 when an overvoltage or static electricity is applied. . Further, when the voltage limiting transistor 52 and the current limiting transistor 62 are normally off, the gate voltage of the switch means 3 is brought close to the GND potential to turn on the switch means 3, and the potential difference between the gate and source of the switch means 3 is set. The switch means 3 is kept on by securing the threshold value or more.

  The capacitor 4 can be an electrolytic capacitor connected to the output end of the surge protection circuit A1. The capacitor 4 has one end branched from the drain of the switch means 3 (the output end of the switch means 3) and the load 2, and is connected to the other end. The capacitor 4 is for holding power when the power is shut off, and can supply power to the load 2 such as the regulator 21 while the power supply from the power source B is stopped by the switch means 3. A capacitor 4 having an optimum capacity can be applied based on the assumed magnitude of the inrush current and the holding time for sufficiently supplying power to the load 2.

  The capacitor 4 can be provided by connecting a plurality of electrolytic capacitors in parallel, or a ceramic capacitor can be applied.

  The overvoltage detection circuit 5 is provided with a Zener diode 51, a transistor (voltage limiting transistor) 52, and a resistance element 53, and can switch and drive the switch means 3.

  The Zener diode 51 has a cathode connected to the cathode side of the diode 1 and the base of the transistor 52 through a resistance element 53, and an anode is grounded. For the Zener diode 51, a Zener voltage is selected in consideration of the voltage characteristics of the switch means 3 and the withstand voltage of each electronic component of the load 2, and when an overvoltage exceeding the Zener voltage is applied, the transistor 52 is connected via the resistance element 53. Apply base current to

  The transistor 52 can be a PNP transistor, and has an emitter connected to the cathode of the diode 1 and a collector connected to the gate of the switch means 3. The transistor 52 is always in an off state (disconnected state between the emitter and collector) in a normal state without overvoltage, but when an overvoltage exceeding the Zener voltage of the Zener diode 51 is applied from the power source, the transistor 52 and the Zener diode 51 and the resistor The base current flowing to GND through 53 is turned on (the emitter and collector are energized), and the switch means 3 is turned off by setting the potential difference between the gate and source of the switch means 3 to be less than the threshold value. ing.

  The resistance element 53 is provided with one end connected between the cathode of the diode 1 for reverse connection protection and the cathode of the Zener diode 51. In a normal state where there is no overvoltage, the transistor 53 is caused by leakage current, electrical noise, or the like. The resistor 52 is provided as a countermeasure against malfunction so that 52 is not accidentally turned on. In addition, the resistance element 53 is also provided between the cathode of the Zener diode 51 and the base of the transistor 52 to limit the base current to the transistor 52.

  The overcurrent detection circuit 6 includes a current detection resistor 61 and a transistor (current limiting transistor) 62.

  The current detection resistor 61 has one end connected to the cathode of the diode 1 and the other end connected to the source of the switch means 3. The current detection resistor 61 increases the voltage at both ends based on the increase in the current flowing through the switch means 3.

  As the transistor 62, a PNP transistor can be applied, the emitter is connected to the cathode of the diode 1 for reverse connection protection, the collector is connected to the gate of the switch means 3, and the base is connected to the source of the switch means 3. The transistor 62 is turned on when the potential difference between both ends of the current detection resistor 61 exceeds the base-emitter on-voltage (about 0.4 V) due to overcurrent, and the potential difference between the gate and source of the switch means 3 is less than the threshold value. In this way, the switch means 3 is turned off. For example, when the on-voltage of the transistor is 0.4 V and the current detection resistor 61 is 0.25Ω, the transistor 62 is turned on around 0.4 V / 0.25Ω = 1.6 A, and the switch means 3 can be turned off. .

  The control circuit 22 can be applied to a microcomputer driven by the electric power from the regulator 21, and based on vehicle information input via a cable (not shown), the liquid crystal display element of the display C mounted on the vehicle meter A, Controls electronic components such as backlights and motors.

  The zener diode 7 has an anode connected to the gate of the switch means 3 and a cathode connected to the source of the switch means 3. The Zener diode 7 plays a role of clamping with a voltage at which the gate-source voltage (Vgs) of the switch means 3 which is a P-channel MOSFET does not exceed the absolute maximum rating when an overvoltage or static electricity is applied. The Zener diode 7 is configured to limit the collector-emitter voltage of the voltage limiting transistor 52 and the current limiting transistor 62.

  The resistance element 8 has one end connected to the gate of the switch unit 3 and the other end connected to the source of the switch unit 3. The resistance element 8 is a resistor provided as a countermeasure against malfunction so that the switch means 3 is not accidentally turned off due to leakage current or electrical noise in a normal time without overvoltage.

  It should be noted that the resistance elements 8 and 31 can sufficiently maintain the drive capability of the switch means 3 with respect to the load 2 even when the power supply voltage is lowered, and the dark current is as small as possible when the vehicle is stopped (when not operating). The constant is set to be small. For this reason, the vehicle meter A can be driven for a long time even when there is no power generation by the generator B2.

  The control circuit 22 can detect the state of the ignition switch, and can operate the control circuit 22 in the power saving mode when the ignition switch shifts from on to off. At this time, as a power saving mode, the control circuit 22 can stop processing related to display control by the display C, periodically perform limited processing such as timing processing, and switch to a state where current consumption is small. In particular, a two-wheeled vehicle is suitable because it is necessary to keep the dark current as low as possible so as not to waste the power of the battery B1, assuming a period in which the engine is often not started, such as during snowfall.

  With the above configuration, for example, even when an intermittent overvoltage mainly generated from the generator B2 is applied for a long time, a large inrush current is applied to the capacitor 4 one after another because it can protect against the overcurrent. Therefore, a P-channel MOSFET having a small current resistance can be applied to the switch means 3.

  The surge protection circuit A1 of the vehicle meter A detects the overvoltage of the power supply B by driving the switch means 3 by detecting the overvoltage of the power supply B and the switch means 3 composed of a P-channel MOSFET for switching the power supply from the power supply B to the load 2 and stopping the power supply. A voltage limiting transistor 52; a current limiting transistor 62 for detecting the overcurrent of the power supply B and driving the switch means 3; a capacitor 4 connected to the output terminal of the switch means 3 to supplement the power to the load 2; And a Zener diode 7 for limiting the voltage between the collector and emitter of the transistor 52 and the current limiting transistor 53 and for limiting the gate-source voltage of the switch means 3.

  Therefore, even when the power supply voltage supplied from the power source B to the vehicle meter A becomes severe due to an increase in the ratio of power supply by the generator B2, the transistors 52, 62, 3 and the switch means 3, the surge protection circuit A1 of the vehicle meter A that can protect the electronic components mounted on the capacitor 4 and the load 2.

  Further, by making the voltage limiting transistor 52 and the current limiting transistor 62 into PNP type transistors, the Zener diode 7 can set the voltage (Vce) between the collector emitters of the voltage limiting transistor 52 and the current limiting transistor 62 to a voltage. It is configured to protect against damage due to ascent.

  Here, the cathode side of the Zener diode 7 is protected via the current detection resistor 61, but the potential difference between both ends of the current detection resistor 61 is the voltage between the base and emitter of the current limiting transistor 62 (about 1 at the maximum). .0V) and does not affect the protection of the collector-emitter voltage (Vce) of the PNP transistor by the Zener diode 7. Therefore, the Zener diode 7 originally intended to protect the switch means 3 can also serve as a voltage protection for the voltage limiting transistor 52 and the current limiting transistor 62, and the voltage limiting transistor 52 and the current limiting transistor 62 are resistant to each other. An inexpensive transistor with a low voltage can be used.

  Although the present invention has been described by way of example in the configuration of the above-described embodiment, the present invention is not limited to this, and various other configurations can be used without departing from the spirit of the present invention. Of course, it is possible to improve the design and change the design. For example, the control circuit 22 that is driven via the power supply circuit 21 as the load 2 to be supplied with the power of the surge protection circuit A1 has been described as an example. However, the liquid crystal display element, the backlight light source, and the pointer driving needle described above have been described. An electronic component such as a motor can also be connected as a load.

  The surge protection circuit A10 shown in FIG. 3 can also be configured by a voltage / current detection circuit 560 provided as both a voltage limiting transistor and a current limiting transistor. The same reference numerals are given to the same configurations as those in the above embodiment, and detailed description thereof is omitted. However, the voltage / current detection circuit 560 of the surge protection circuit A10 includes a Zener diode 561, a voltage / current limiting transistor 562, A detection resistor 563 and a resistance element 564 are used. Even when the voltage detection and the current detection are driven by a single PNP transistor as in the circuit configuration shown in FIG. 3, the switch means 3 is protected and the Zener diode 7 is provided with the voltage / current limiting transistor 562. The voltage / current limiting transistor 562 can be an inexpensive transistor with a low withstand voltage. Further, the circuit configuration is simpler than the embodiment shown in FIG.

  The present invention can be applied, for example, as a surge protection circuit for a vehicle instrument mounted on a moving body including an automobile, a motorcycle, an agricultural machine, or a construction machine.

DESCRIPTION OF SYMBOLS 1 Diode 2 Load 21 Regulator 22 Control circuit 3 Switch means 31 Resistance element 4 Capacitor 5 Overvoltage detection circuit 51 Zener diode 52 Transistor (voltage limiting transistor)
53 Resistance Element 6 Overcurrent Detection Circuit 61 Current Detection Resistor 62 Transistor (Current Limiting Transistor)
A Vehicle instrument A1 Surge protection circuit B Power supply B1 Battery B2 Generator C Display (display means)

Claims (3)

Switch means comprising a P-channel MOSFET for switching between power supply from the power supply to the load and its stop;
A voltage limiting transistor for detecting an overvoltage of the power source and driving the switch means;
A current limiting transistor for detecting an overcurrent of the power source and driving the switch means;
A capacitor connected to the output terminal of the switch means to supplement the power to the load;
And a Zener diode for limiting a voltage between the collector and the emitter of the voltage limiting transistor and the current limiting transistor and for limiting a voltage between a gate and a source of the switch means. Surge protection circuit.   2. The vehicle instrument surge protection according to claim 1, wherein a PNP transistor is applied to the voltage limiting transistor and the current limiting transistor, and the voltage between the collector and the emitter is limited by the Zener diode. circuit.   The surge protection circuit for a vehicle instrument according to claim 1 or 2, wherein the voltage limiting transistor also serves as a circuit to serve as the current limiting transistor.

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