JP2013031273A - Overvoltage protection circuit - Google Patents

Abstract

An overvoltage protection circuit capable of effectively limiting an overvoltage with an accurate voltage and capable of continuously operating an internal circuit.
A protection circuit section 13 in which a switching element (FET 13a) and a Zener diode (Zener diode 13b) are connected in series is inserted between a power line 15 and a ground 16. When the voltage detection circuit unit 11 detects that the voltage between the power supply line 15 and the ground 16 has risen beyond a predetermined potential, the control circuit unit 12 causes the switching element to conduct, The overvoltage is limited by a short circuit path formed between the protection circuit unit 13 and the ground. At this time, the Zener diode operates in a variable resistance manner following the reverse current.
[Selection] Figure 1

Description

  The present invention relates to an overvoltage protection circuit that protects an internal circuit connected between a power supply line and a ground from an overvoltage.

  Conventionally, when supplying power to a circuit, it is known that an overvoltage protection circuit is provided in order to protect an internal circuit from an excessive voltage generated due to fluctuations in the power supply voltage or an induced excessive surge voltage. . In particular, in-vehicle electrical components are often used in harsh environments and may be exposed to momentary high voltages, and thus protection measures against surge voltages are indispensable.

  For this reason, for example, in Patent Document 1, in order to reliably prevent a load from being destroyed at an overvoltage with a simple configuration, when an overvoltage is detected, the power source and the ground are short-circuited by an FET (Field Effect Transistor), A technique for cutting off a power supply by cutting a fuse is disclosed. Patent Document 2 discloses a load resistor connected in series between a power supply terminal and a ground in order to remove a surge voltage applied from the input port to the power supply terminal without causing an increase in dark current and an increase in cost. And a transistor are inserted, and when an overvoltage is detected, a technique for turning on the transistor and limiting the overcurrent with a resistor to limit the voltage is introduced.

JP 2009-177865 A JP 2007-31460 A

  Conventionally, as an overvoltage protection circuit, a method of inserting a Zener diode between a power supply line and the ground is generally used. However, as is well known, Zener diodes have variations in the operation start voltage and also have temperature dependence, so that overvoltage cannot be suppressed with an accurate voltage. Therefore, it is necessary to provide voltage tolerance margins for components used such as internal circuits, resulting in high equipment costs.

  On the other hand, according to the technique disclosed in Patent Document 1, since the FET is grounded when an overvoltage is applied, the supply voltage cannot be maintained, and the device cannot be operated continuously. Moreover, according to the technique disclosed in Patent Document 2, since the resistance is connected in series to the short circuit, there is a risk that the voltage limitation may be insufficient when a large current flows due to a ground fault. .

  The present invention has been made to solve the above-described problems, and an object thereof is to provide an overvoltage protection circuit capable of effectively limiting an overvoltage with an accurate voltage and capable of continuously operating an internal circuit.

  In order to solve the above-described problem, an overvoltage protection circuit according to the present invention includes a protection circuit unit in which a switching element and a Zener diode are connected in series between a power supply line and the ground, and a voltage between the power supply line and the ground. A voltage detection circuit unit to be monitored; and when the voltage detection circuit unit detects that the voltage has risen beyond a predetermined potential, the switching element is made conductive, the power supply line, a front protection circuit unit, And a control circuit unit that limits the voltage by a short-circuit path formed between the ground and the ground.

  According to the present invention, when the voltage detection circuit unit detects that the voltage between the power supply line and the ground has risen beyond a predetermined potential, the control circuit unit causes the switching element to conduct, The overvoltage is limited by a short circuit path formed between the protection circuit unit and the ground. At this time, the Zener diode can effectively limit the overvoltage with an accurate voltage and enable continuous operation of the internal circuit.

  In the present invention, the Zener diode has a characteristic that a clamping voltage increases when a reverse current exceeding a predetermined value flows due to conduction of the switching element. According to the present invention, the Zener diode has a clamping voltage that rises when a reverse current exceeding a predetermined value flows when the switching element is turned on. As a result, the source-drain voltage of the switching element decreases. The current is limited, so that the switching element is immune from destruction. In addition, since the clamping voltage of the Zener diode does not increase until a certain amount of reverse current, the current is hardly limited like a resistor, so that a steep surge can be effectively limited.

  According to the present invention, it is possible to provide an overvoltage protection circuit capable of effectively limiting overvoltage with an accurate voltage and capable of continuously operating an internal circuit.

It is a block diagram which shows the structure of the overvoltage protection circuit which concerns on embodiment of this invention. It is a figure which shows an example of the surge waveform generated on a trial basis. It is a figure which shows the surge clamping characteristic and cathode side voltage waveform by the Zener diode used with the overvoltage protection circuit which concerns on embodiment of this invention. It is a figure which shows an example of the reverse current-clamping voltage characteristic of the Zener diode used with the overvoltage protection circuit which concerns on embodiment of this invention.

  DESCRIPTION OF EMBODIMENTS Hereinafter, embodiments for carrying out the present invention (hereinafter referred to as embodiments) will be described in detail with reference to the accompanying drawings. In addition, the same number is attached | subjected to the same component through the whole description of embodiment.

(Configuration of the embodiment)
FIG. 1 is a block diagram showing an internal configuration of the overvoltage protection circuit according to the present embodiment. As shown in FIG. 1, the overvoltage protection circuit 10 according to the present embodiment includes a voltage detection circuit unit 11 between a power supply line and a ground (hereinafter referred to as GND 16) in order from the side closer to the power supply source Vcc. The control circuit unit 12 and a protection circuit unit 13 in which an FET 13a (Tr1), which is a switching element, and a Zener diode 13b (D1) are connected in series are connected in parallel. In addition to this overvoltage protection circuit 10, an internal circuit 17 is connected between the power supply line 15 and the grant.

  The voltage detection circuit unit 11 constantly monitors the power supply voltage applied between the power supply line 15 and the GND 16 and activates the control circuit unit 12 when the power supply voltage exceeds a predetermined value. The control circuit unit 12 makes the FET 13a conductive (ON), a power supply voltage is applied to the Zener diode 13b, and the power supply voltage is clamped to a constant voltage. The voltage detection circuit unit 11 includes a circuit (not shown) including a built-in resistor and a comparator that compares a voltage divided by the resistor with a reference voltage (reference).

  The overvoltage protection circuit 10 according to the present embodiment uses the characteristic of the Zener diode 13b in which the Zener voltage increases as the current flowing through the Zener diode increases. That is, the Zener diode 13b has a characteristic that the clamping voltage increases when the FET 13a is turned on and a reverse current exceeding a predetermined value (zener current) flows. Further, the Zener diode 13b does not increase the Zener voltage with respect to a certain amount of current, and therefore hardly restricts the current as in the resistance shown in Patent Document 2. That is, the Zener diode 13b performs a variable resistance operation following the reverse current, and thus effectively limits a steep surge.

  The internal circuit 17 is, for example, an in-vehicle electrical component.

(Operation of the embodiment)
Hereinafter, the operation of the overvoltage protection circuit 10 according to the present embodiment shown in FIG. 1 will be described in detail with reference to FIGS.

  FIG. 2 is a diagram showing a surge waveform generated on a trial basis. The surge waveform generated here is applied to the power supply line 15. When the voltage detection circuit unit 11 detects this overvoltage, the control circuit unit 12 turns on the FET 13 a of the protection circuit unit 13. As a result, for example, the overvoltage can be limited as shown in FIG.

  Specifically, FIG. 3 shows the voltage waveform of the power supply line 15 when the surge waveform having a peak voltage of about 120 V shown in FIG. 2 is input to the power supply line 15, and the voltage waveform on the cathode side of the Zener diode 13b indicated by symbol b. It is a graph to show.

  The surge clamping waveform indicated by symbol a in FIG. 3 is the voltage of the power supply line when the surge waveform shown in FIG. 2 is applied to the power supply line. At this time, the power supply voltage Vcc is maintained at 40 V or less, and the internal circuit 17 can continue to operate. The clamping voltage of the Zener diode 13b used here is 30V, but according to the waveform measured on the cathode side of the Zener diode shown in this graph, the clamping voltage rises to around 35V. As a result, the source-drain voltage of the FET 13a is reduced, and the drain current is limited to protect the FET 13a from overcurrent.

  FIG. 4 shows an example of surge reverse current-clamping voltage characteristics of the Zener diode 13b. This graph shows the general characteristics of a Zener diode. FIG. 4 shows a surge reverse current that can flow in the reverse direction on the vertical axis, and a clamping voltage and a characteristic diagram on the horizontal axis.

  Here, it is assumed that the protection circuit unit 13 is composed of only the FET 13a. In this case, the FET 13a is instantaneously destroyed by a ground fault. In order to prevent this, for example, as introduced in Patent Document 2, it is sufficient to insert a resistor between the source terminal of the FET 13a and the GND 16, but the current cannot be sufficiently passed and the surge voltage cannot be limited. There is a fear. For this reason, in the overvoltage protection circuit 10 according to the present embodiment, a Zener diode 13b that performs a variable resistance operation following the reverse current is inserted instead of the resistor.

  Specifically, the maximum voltage on the cathode side of the Zener diode 13b is around 35V (FIG. 3). If this is applied to the characteristic diagram shown in FIG. 4, it can be seen that the current flowing through the drain of the FET 13a is limited to about 50 A, as indicated by reference numeral c. Furthermore, if a current flows due to an excessive surge, the clamping voltage further increases, and the source-drain voltage of the FET 13a decreases, so that the drain current is limited and the destruction of the FET 13a due to the overcurrent is prevented. Can do.

  Thus, when a large current flows in the reverse direction of the Zener diode 13b, the Zener voltage rises and the voltage between the source and drain of the FET 13a decreases, so that the drain current is limited and the FET 13a can be prevented from being destroyed. In addition, the Zener diode 13b does not increase the Zener voltage even if a current up to a certain level is supplied, and therefore hardly limits the current like a resistor. For this reason, a steep surge can be effectively limited. In other words, it can be said that the Zener diode 13b works like a variable resistor that follows the current.

(Effect of embodiment)
As described above, according to the overvoltage protection circuit 10 according to the present embodiment, the control circuit unit 12 has the voltage detection circuit unit 11 and the power supply voltage applied between the Vcc 15 and the GND 16 has risen beyond a predetermined potential. When this is detected, the FET 13a is turned on, and the overvoltage is limited by a short-circuit path formed between the Vcc 15, the protection circuit unit 13 (FET 13a, Zener diode 13b), and the GND 16. At this time, since the Zener diode 13b clamps the power supply voltage to a constant voltage, the internal circuit 17 can be continuously operated.

Further, the Zener diode 13b has a clamping voltage that rises when a reverse current exceeding a predetermined value flows due to a surge voltage applied when the FET 13a is conducted, and as a result, a source-drain voltage of the FET 13a decreases. The drain current is limited and the FET 13a is prevented from being destroyed. In addition, the clamping voltage of the Zener diode 13b does not increase until a certain amount of reverse current, so that the current is hardly limited like a resistor, so that a steep surge can be effectively limited.

  As mentioned above, although preferable embodiment of this invention was described, this invention is not limited to embodiment mentioned above, In the range of the summary of this invention described in the claim, various deformation | transformation and change Is possible.

DESCRIPTION OF SYMBOLS 10 Overvoltage protection circuit 11 Voltage detection circuit part 12 Control circuit part 13 Protection circuit part 13a Switching element (FET)
13b Zener diode (Zener diode)
15 Power line 16 Ground (GND)
17 Internal circuit

Claims (2)

A protection circuit unit in which a switching element and a Zener diode are connected in series between a power supply line and a ground;
A voltage detection circuit unit for monitoring a voltage between the power line and the ground;
When a predetermined voltage is detected by the voltage detection circuit unit, a control circuit unit for conducting the switching element;
An overvoltage protection circuit comprising: The Zener diode is
2. The overvoltage protection circuit according to claim 1, wherein the overvoltage protection circuit has a characteristic that a clamping voltage increases when a reverse current of a predetermined level or more flows.

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