JP2007336620A - Power supply input circuit - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide various types of protection functions and a power supply input circuit with low loss. <P>SOLUTION: Two drains of MOSFET5 and 12 are connected with each other in series. The MOSFET5 is functioned as a switch circuit and the MOSFET12 is functioned as an ideal diode. A counter-current prevention circuit 16 turns off the MOSFET12 at V1<V2 and turns on the MOSFET12 at V1≥V2. An over-voltage protection circuit 13 turns off the MOSFET5 when a voltage of the power supply input terminal 2 exceeds a specified voltage. An over-current protection circuit 6 turns off the MOSFET5 when a current passing through a power supply input line 3 exceeds a specified value. The MOSFET5 is turned on and off by an on/off command signal from CPU. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to a power input circuit provided on a power input line extending from a power input terminal to an internal circuit.

Power supply circuits used in automotive electronic devices must have a function to protect against reverse battery connection. Generally, a configuration is adopted in which a diode is inserted into the power input line as shown in Patent Documents 1 and 2. Has been. In addition, since the battery supplies power to a large number of in-vehicle electronic devices, each power supply circuit also has an overcurrent protection function so that a load short circuit or the like caused by one in-vehicle electronic device does not lower the battery voltage. I have. Furthermore, in order to suppress battery consumption, it also has a function of interrupting dark current flowing in the power supply circuit when the key switch of the vehicle is turned off.
JP 2003-150255 A JP 2002-223558 A

  In a vehicle-mounted electronic device, the above-described functions may be provided in the power supply circuit itself, but the power supply input circuit is interposed between a power supply input terminal for battery connection and an internal circuit such as a power supply circuit. A configuration in which each of the above functions is provided is also used. FIG. 2 shows a configuration of a power supply input circuit conventionally used.

  In the power input circuit 1, a power input line 3 extending from a power input terminal 2 to an internal circuit such as a power circuit includes a reverse connection protection diode D1, an overvoltage / surge voltage protection Zener diode D2, a reactor L1, and a capacitor. A filter circuit 4 composed of C1, a current detection resistor R1, and a MOSFET 5 functioning as a switch circuit are provided. In the MOSFET 5, a parasitic diode 5d is formed.

  The overcurrent protection circuit 6 outputs an off command signal when the voltage across the resistor R1 exceeds a specified value, and the drive circuit 7 receives an on / off command signal and an overcurrent from a CPU (not shown). The MOSFET 5 is driven based on the off command signal given from the protection circuit 6. Further, a measure against instantaneous interruption of the battery voltage is taken by the diode D1 and the capacitor C1 of the filter circuit 4.

  In the power supply circuits described in Patent Documents 1 and 2 and the power supply input circuit 1 shown in FIG. 2, the diode D1 is inserted into the power supply input line 3, so that all the consumption current of the in-vehicle electronic device flows to the diode D1. A large power loss occurs. In-vehicle electronic devices such as navigation devices have become highly functional and high-speed, and accordingly, the current consumption of the in-vehicle electronic devices and the power loss in the diode D1 tend to increase.

  The present invention has been made in view of the above circumstances, and an object thereof is to provide a low-loss power input circuit having various protection functions.

  According to the means described in claim 1, the first and second FETs having the same conductivity type are connected in series to the power input line in the reverse polarity state in which the drains or the sources are connected to each other. . The reason why they are connected in the reverse polarity is that the parasitic diodes formed in the first and second FETs are oriented in different directions so that no current path is formed via both parasitic diodes.

  For example, when the FET is a P-channel type, the first FET whose source is on the power input terminal side functions as a switch circuit, and the overvoltage protection circuit has the first voltage when the voltage at the power input terminal exceeds a predetermined specified voltage. The FET is controlled to be turned off. Thereby, the conventionally used Zener diode for overvoltage protection becomes unnecessary.

  On the other hand, the second FET whose source is on the internal circuit side functions as an ideal diode, and the backflow prevention circuit has a terminal voltage on the power input terminal side among the two terminals of the series circuit consisting of the first and second FETs. When the voltage drops below the terminal voltage on the side, the second FET is controlled to be turned off. Thereby, in the power supply input line, a current flows through the second FET that is in the on state, and the power loss is very small as compared with the case where the current flows through the conventional diode. The second transistor blocks the current from the internal circuit side to the power input terminal side. Therefore, the protection when the power source is reversely connected to the power input terminal and the internal circuit side voltage when the power source is momentarily interrupted. Maintenance is planned.

  According to the means described in claim 2, the overcurrent protection circuit controls the first FET to be turned off when the current detected by the current detection circuit exceeds the specified current. Since the parasitic diode of the first FET is connected in the direction opposite to the direction of the load current in the power input line, the current flowing through the power input line can be cut off by controlling the first FET to the off state. It is possible to protect the power supply and the internal circuit from overcurrent.

  According to the means described in claim 3, the first FET is configured to perform an on / off operation in response to a power supply command signal to the internal circuit. Therefore, when the power supply to the internal circuit is unnecessary, the current flowing through the power input line can be cut off, and so-called dark current can be cut.

Hereinafter, an embodiment of the present invention will be described with reference to FIG. 1 showing a configuration of a power input circuit. In addition, the same code | symbol is attached | subjected to FIG. 2 which shows a prior art.
In a vehicle-mounted electronic device such as a navigation device, the power input circuit 11 is provided between a power input terminal 2 and an internal circuit of the vehicle-mounted electronic device, for example, a power circuit (not shown) that generates a control voltage. . A battery voltage VB is applied to the power input terminal 2 from an in-vehicle battery (not shown).

  The power input line 3 includes a current detection resistor R1 (corresponding to a current detection circuit), P-channel MOSFETs 5 and 12 (corresponding to first and second FETs), and a filter circuit including a reactor L1 and a capacitor C1. 4 are provided in order. The MOSFETs 5 and 12 are connected in series with their drains being main terminals connected to each other in opposite polarities, and parasitic diodes 5d and 12d each having a source side as a cathode are formed in parallel.

  The overvoltage protection circuit 13 controls the MOSFET 5 to be turned off when the voltage at the power input terminal 2 exceeds a specified voltage that is an overvoltage determination level. Between the power supply input terminal 2 and the ground, a diode D11, a Zener diode D12, and a resistor R11 are connected in series. The cathode of the diode D11 (zener diode D12) is connected to the gate of the MOSFET 5 via the emitter and collector of the transistor 14, and the base of the transistor 14 is connected to the ground via the collector and emitter of the transistor 15. ing. The base of the transistor 15 is connected to the anode of the Zener diode D12.

  The overcurrent protection circuit 6 outputs an off command signal to the drive circuit 7 when the voltage across the resistor R1 exceeds a specified value that is an overcurrent determination level. In the reverse current blocking circuit 16, the voltage V1 of the terminal on the power input terminal side (source of the MOSFET 5) among both terminals of the series circuit composed of the MOSFETs 5 and 12 is lower than the voltage V2 of the terminal on the internal circuit side (source of the MOSFET 12). Sometimes an OFF command signal is output to the drive circuit 17, and an ON command signal is output to the drive circuit 17 when the voltage V1 is higher than V2.

  The drive circuit 7 drives the MOSFET 5 based on an on / off command signal provided from a CPU or the like (not shown) provided in the internal circuit and an off command signal provided from the overcurrent protection circuit 6. The drive circuit 17 drives the MOSFET 12 based on the on / off command signal given from the backflow prevention circuit 16.

Next, the operation of this embodiment will be described.
A battery voltage VB is constantly applied from the battery to the power input terminal 2 of the in-vehicle electronic device. When a vehicle key switch (not shown) is in the off position, an off command signal is given to the drive circuit 7 from a CPU or the like provided in the internal circuit, and the MOSFET 5 is turned off. As a result, dark current flowing in the internal circuit (power circuit) of the in-vehicle electronic device can be cut off, and battery consumption can be suppressed.

  When the key switch is set to the Acc position or the on position, an on command signal is given to the drive circuit 7 from the CPU or the like, and the MOSFET 5 is turned on. The source voltage V1 of the MOSFET 5 is substantially equal to the battery voltage VB, and the source voltage V2 of the MOSFET 12 is a voltage corresponding to the charge state of the capacitor C1 and the input capacitor (not shown) of the power supply circuit. At the start of energization of the internal circuit and in a steady state thereafter, the relationship of V1> V2 is established, the backflow prevention circuit 16 outputs an on command signal to the drive circuit 17, and the MOSFET 12 is turned on.

  As a result, a current flows from the power input terminal 2 to the internal circuit through the source and drain of the MOSFET 5 and between the drain and source of the MOSFET 12. Since the source-drain voltage of the MOSFET 5 in the on state and the drain-source voltage of the MOSFET 12 are very small compared to the forward voltage of the diode, the power loss in the power supply input circuit 11 becomes very small.

The power loss of the power input circuit 11 (FIG. 1) of the present embodiment is compared with the power loss of the conventional power input circuit 1 (FIG. 2) under the following conditions.
Current flowing through the power input line 3 = 2A
On-resistance of MOSFETs 5 and 12 = 50 mΩ
Forward voltage of the diode D1 = 0.6V
Power loss of power input circuit 11 = 2 × 2A × 2A × 50 mΩ = 0.4 W
Power loss of power input circuit 1 = 2A × 0.6V + 2A × 2A × 50mΩ = 1.4W

  When a ripple is superimposed on the battery voltage VB or when an instantaneous interruption occurs, if the source voltage V1 of the MOSFET 5 becomes smaller than the source voltage V2 of the MOSFET 12, the backflow prevention circuit 16 is in a period when V1 <V2. Outputs an off command signal to the drive circuit 17, and the MOSFET 12 is turned off. As a result, the flow of current from the internal circuit side to the power input terminal side can be blocked, and the loss of the electric charge of the capacitor C1 on the internal circuit side and the input capacitor (not shown) of the power circuit can be prevented by backflow. it can.

  The battery voltage VB is likely to fluctuate and may become an excessive voltage or a surge voltage may be superimposed. When the voltage at the power input terminal 2 exceeds a specified voltage that is an overvoltage determination level, the Zener diode D12 of the overvoltage protection circuit 13 is turned on, and the transistor 15 and thus the transistor 14 are turned on. At this time, since the gate-source voltage of the MOSFET 5 is lowered to the forward voltage VF of the diode D11, the MOSFET 5 having a threshold voltage higher than VF is turned off. As a result, the subsequent circuit (internal circuit) can be protected from excessive power supply voltage and surge voltage. According to this configuration, the Zener diode D2 having a large current capacity used in the conventional circuit (see FIG. 2) is not necessary.

  Further, when the voltage across the resistor R1 exceeds a specified value that is an overcurrent determination level, the overcurrent protection circuit 6 outputs an off command signal to the drive circuit 7, and the MOSFET 5 is turned off. As a result, even if a failure such as a short circuit occurs in the internal circuit, it is possible to prevent an excessive current from flowing from the battery to the in-vehicle electronic device, thereby protecting the battery and the internal circuit.

  As described above, in the power supply input circuit 11 of the present embodiment, two MOSFETs 5 and 12 of the same conductivity type (P channel type) are connected in series in the power supply input line 3 in the reverse polarity state where the drains are connected to each other. Since they are connected, current paths that flow through the parasitic diodes 5d and 12d formed in the MOSFETs 5 and 12 respectively are not formed. Since the backflow prevention circuit 16 is provided and on / off control of the MOSFET 12 is performed based on the magnitude relationship between the voltages V1 and V2, the MOSFET 12 can function as an ideal diode with low loss and unidirectional characteristics.

  Since the MOSFET 5 functioning as a switch circuit is turned on / off in accordance with an on / off command signal given from the CPU or the like according to the rotation position of the overvoltage protection circuit 13, the overcurrent protection circuit 6, and the key switch, it is protected from overvoltage and overcurrent. The dark current can be cut when it is not necessary to operate the internal circuit.

The present invention is not limited to the embodiment described above and shown in the drawings. For example, the present invention can be modified or expanded as follows.
The sources of the MOSFETs 5 and 12 may be connected to each other. Further, N-channel MOSFETs may be used as the first and second FETs.
The MOSFET 5 may be used as a current detection circuit, and the current flowing through the power input line 3 may be detected based on the source-drain voltage.

The block diagram of the power input circuit which shows one Embodiment of this invention 1 equivalent diagram showing the prior art

Explanation of symbols

  2 is a power supply input terminal, 3 is a power supply input line, 5 and 12 are MOSFETs (first and second FETs), 6 is an overcurrent protection circuit, 11 is a power supply input circuit, 13 is an overvoltage protection circuit, and 16 is a backflow prevention circuit. A circuit, R1, is a resistor (current detection circuit).

Claims (3)

First and second FETs having the same conductivity type, the same main terminals being connected to each other in a power input line extending from the power input terminal to the internal circuit, and connected in series with opposite polarities;
Of the two terminals of the series circuit composed of the first and second FETs, when the terminal voltage on the power input terminal side is lower than the terminal voltage on the internal circuit side, the second FET is controlled to be turned off. A backflow prevention circuit;
An overvoltage protection circuit that controls the first FET to be turned off when the voltage of the power input terminal exceeds a predetermined specified voltage. A current detection circuit for detecting a current flowing in the power input line;
2. The power input circuit according to claim 1, further comprising an overcurrent protection circuit configured to control the first FET to an off state when a current detected by the current detection circuit exceeds a specified current.   3. The power input circuit according to claim 1, wherein the first FET is configured to perform an on / off operation in response to a power supply command signal to the internal circuit.

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