JP4423765B2 - Load drive device - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a load driving device configured to drive a load and to protect the load from an overvoltage when an abnormal voltage of a power supply is detected.
[0002]
[Prior art]
For example, when the vehicle is running, the alternator is driven by the rotation of the engine to generate electricity and charge the battery. In this traveling state, when a so-called load dump occurs in which the connection between the power supply terminal of the battery and the power supply line is disconnected, the load amount of the alternator is drastically reduced, and a positive surge voltage of several tens of volts is generated on the power supply line. End up.
[0003]
Accordingly, if a load dump occurs when the vehicle is running with the headlights turned on at night or the like, an excessive current may flow through the lamp, leading to a break in the filament. Therefore, in order to protect the lamp from the occurrence of load dump, elements such as a protective varistor and a Zener diode are arranged.
[0004]
FIG. 5 shows an example of a circuit configuration in which protective elements are arranged in a conventional lamp driving system. Connected to both ends of the battery 1 are a parallel circuit of lamps 2 a and 2 b and a series circuit of a power MOSFET 3. That is, the drain of the power MOSFET 3 is connected to the lamps 2a and 2b, and the source is connected to the ground.
[0005]
The drive circuit 4 energizes the lamps 2a and 2b in response to an operation signal of a combination switch (not shown), and its output terminal is connected to the gate of the power MOSFET 3. A power Zener diode 5 is connected in parallel to the series circuit of the lamps 2a and 2b and the power MOSFET 3. In such a configuration, when a load dump occurs, the voltage across the lamps 2a and 2b and the power MOSFET 3 is clamped by the Zener voltage of the power Zener diode 5 and the surge voltage is absorbed, so that these elements are protected. Be able to.
[0006]
[Problems to be solved by the invention]
However, the protective elements such as the power Zener diode 5 and the varistor have a problem that the cost is high and the element size is large. Further, turning off the power MOSFET 3 generally protects the lamps 2a and 2b with the withstand voltage of the power MOSFET 3. However, in this case, the vehicle headlight is turned off, which is not preferable. Further, although there are some which drive the lamp with a PWM signal via a switching element and reduce the duty of the PWM signal by reducing the duty of the PWM signal when an abnormal voltage is generated, there is a problem that the configuration becomes complicated. The present invention has been made in view of the above circumstances, and an object thereof is to provide a load driving device that can be easily configured at low cost and can protect a load without increasing the size of the circuit. It is in.
[0007]
[Means for Solving the Problems]
According to the load driving device of the first aspect, in the semiconductor switching element for controlling the driving of the load, the constant voltage generating element is connected between the load side terminal and the control terminal, and the driving means is operated by the abnormal voltage detecting means. When an abnormal voltage of the power supply is detected, the control terminal of the semiconductor switching element is brought into a high impedance state.
[0008]
Then, since the terminal voltage between the load side terminal of the semiconductor switching element and the control terminal is clamped by the constant voltage generated by the constant voltage generating element, the load and the semiconductor switching element can be selected and set appropriately. Withstand voltage distribution ratio can be set. Therefore, when an abnormal voltage is applied to the series circuit of the load and the semiconductor switching element, it can be protected by a relatively simple configuration so that they are not destroyed.
[0009]
Also, according to the above configuration, unlike the conventional configuration, the constant voltage generating element does not need to clamp all of the abnormal voltages, and it is sufficient to clamp a voltage substantially corresponding to the voltage that the semiconductor switching element shares with the load. Therefore, as the constant voltage generating element, it is not necessary to use an expensive element corresponding to high power, such as a power Zener diode, for example, and a normal low power Zener diode can be used. Therefore, an increase in cost can be suppressed and an increase in circuit size can be prevented as much as possible.
[0010]
According to the load drive device of the second aspect, the drive means pulls down the control terminal of the semiconductor switching element when the abnormal voltage of the power source is detected by the abnormal voltage detection means. Then, as in the first aspect, the terminal voltage between the load side terminal of the semiconductor switching element and the control terminal is clamped by the constant voltage generated by the constant voltage generating element. Accordingly, the withstand voltage distribution ratio between the load and the semiconductor switching element can be set by appropriately selecting and setting the constant voltage generated by the constant voltage generating element and the resistance value of the pull-down resistor. As a result, the load and the voltage burden ratio of the constant voltage generating element can be reduced accordingly.
[0011]
According to the load driving device of the third aspect, the load is the vehicle lamp. That is, when an abnormal voltage is generated, the semiconductor switching element is turned on by a voltage applied to the control terminal of the semiconductor switching element via the constant voltage generating element, so that the vehicle lamp as a load is energized. Therefore, even when a load dump or the like occurs in the battery unit as the power source, the vehicle lamp is hardly turned off, and safety is not lowered even when the vehicle is traveling at night.
[0012]
According to the load driving device of the fourth aspect, by using the power MOSFET as the semiconductor switching element, the withstand voltage can be improved and the on-resistance during conduction can be reduced.
[0013]
DETAILED DESCRIPTION OF THE INVENTION
(First embodiment)
A first embodiment in which the present invention is applied to an apparatus for driving a vehicle headlamp as a load will be described below with reference to FIGS. The same parts as those in FIG. 5 are denoted by the same reference numerals, and the description thereof is omitted. Only different parts will be described below.
[0014]
A series circuit of resistors 11 and 12 is connected between the battery (power source) 1 and the ground, and a common connection point of these resistors 11 and 12 is connected to a non-inverting input terminal of the comparator 13. . A reference power supply 14 is connected to the inverting input terminal of the comparator 13. These constitute an overvoltage detection circuit (abnormal voltage detection means) 15.
[0015]
When the overvoltage detection circuit 15 detects a surge voltage applied to the power supply line L by a load dump or the like, it outputs an overvoltage detection signal AB to a drive circuit (drive means) 16 instead of the drive circuit 4 in FIG. It has become. The detection signal AB has a resistance value of the resistors 11 and 12 and the reference power supply 14 so that the detection signal AB becomes a high level when the voltage level of the power supply line L reaches a level corresponding to a Zener voltage VZ of a Zener diode 23 described later. The voltage is set.
[0016]
The drive circuit 16 includes two AND gates 17 and 18, two NPN transistors 19 and 20, and a current source 21. A light switch ON / OFF signal SW (not shown) is supplied to one input terminal of the AND gate 17 and one negative logic input terminal of the AND gate 18. An overvoltage detection signal AB is supplied to the other negative logic input terminal of the AND gate 17 and the other negative logic input terminal of the AND gate 18. The output terminals of the AND gates 17 and 18 are connected to the bases of the transistors 19 and 20, respectively.
[0017]
The transistors 19 and 20 are totem-pole connected, the collector of the transistor 19 is connected to the battery 1 via the current source 21, and the emitter of the transistor 20 is connected to the ground. The emitter of the transistor 19 and the collector of the transistor 20 are connected to the gate (control terminal) of the power MOSFET (semiconductor switching element) 3.
[0018]
The anode of a diode 22 is connected to the drain (load side terminal) of the power MOSFET 3, and the anode of a Zener diode (constant voltage generating element) 23 is connected to the gate. The cathodes of these diodes 22 and 23 are connected in common, and both are connected in opposite directions.
The power Zener diode 5 shown in FIG. 5 is removed.
[0019]
Here, if the Zener voltage of the Zener diode 23 is set too high, the voltage applied between the drain and gate of the power MOSFET 3 becomes large, and the power MOSFET 3 may be destroyed. On the contrary, if the setting of the Zener voltage is too low, the voltage borne by the lamps (loads) 2a and 2b becomes large, and the filament may be broken. Therefore, the Zener voltage needs to be set appropriately in consideration of the balance of protection between the two.
[0020]
Next, the operation of this embodiment will be described with reference to FIGS. When the headlight is turned off and the output signal SW of the light switch is low level, the output signal levels of the AND gates 17 and 18 are low and high, respectively, and the transistors 19 and 20 are off and on, respectively. Accordingly, since the FET 3 is turned off when the gate potential is low, the lamps 2a and 2b are not energized and the headlight is not turned on.
[0021]
On the other hand, when the output signal SW of the light switch is high, the output signal levels of the AND gates 17 and 18 are high and low, respectively, and the transistors 19 and 20 are on and off, respectively. Therefore, the FET 3 is turned on when the gate potential is high, and the lamps 2a and 2b are energized to turn on the headlight. At this time, the potential of the gate of the FET 3 is substantially equal to the power supply voltage of the battery 1, and immediately after the FET 3 is turned on, the potential of the drain slightly decreases. As a result, temporarily (gate potential)> (drain potential), but the current that flows from the gate to the drain side is blocked by the diode 22 in the reverse direction.
[0022]
Here, FIG. 2 is a timing chart showing waveforms of respective parts when a surge voltage is applied to the battery 1 side of the lamps 2a and 2b due to the occurrence of a load dump or the like. Assume that a surge voltage is applied as shown in FIG. 2 (a) while the lamps 2a and 2b are turned on as shown in FIGS. 2 (c) and 2 (d). In that case, when the surge voltage rises and reaches about the Zener voltage VZ, the comparator 13 of the abnormality detection circuit 15 outputs the detection signal AB (high level) (see FIG. 2B). Then, the output signal levels of the AND gates 17 and 18 are both low, the transistors 19 and 20 are both turned off (see FIGS. 2C and 2D), and the gate of the FET 3 is in a high impedance state. Therefore, FET3 is cut off.
[0023]
At this time, when a slight current flows through the FET 3 drain → zener diode 23 → gate → source path and the surge voltage VS exceeds the Zener voltage VZ, the drain-gate voltage of the FET 3 is clamped by the Zener diode 23 to 22V. (See FIG. 2E). Then, the voltage VL corresponding to the resistance is applied to both ends of the lamps 2a and 2b, and the gate-source voltage VGS of the FET 3 is determined as follows.
VGS = VS -VL -VZ
[0024]
As a result, when the voltage VGS is fixed at a high level, the FET 3 becomes conductive, a current flows between the drain and the source, and the surge voltage is absorbed. At this time, when the FET 3 is turned on, the drain-source voltage VDS of the FET 3 becomes substantially equal to the Zener voltage VZ of the Zener diode 23 as a result.
[0025]
Here, FIG. 2 (e) shows the waveform of the voltage VDS. The time tf from when the overvoltage detection circuit 15 outputs the detection signal AB to when the gate of the FET 3 becomes in the high impedance state is the surge voltage. The lamps 2a and 2b can be protected by setting so as to be shorter than the time t0 until the level 2a and 2b rise to the level for disconnection.
[0026]
When the FET 3 conducts and the surge voltage is absorbed and the voltage drops below the Zener voltage VZ, the output of the detection signal AB by the overvoltage detection circuit 15 stops, so the transistor 19 of the drive circuit 16 is turned on, The gate potential of the FET 3 becomes high level and the headlight is continuously turned on.
[0027]
Note that even if the drive circuit 16 sets the gate of the FET 3 in a high impedance state when an overvoltage is detected, the above process proceeds within a very short period and the FET 3 is turned on, so that the vehicle headlight is turned off. There is almost no.
[0028]
Here, in FIG. 3, the result of the experiment which the inventors of this invention measured is shown. For example, the power MOSFET is MTP75N06HD (TO-220) manufactured by Motorola, the lamp is 55 W / 60 W (double filament configuration for low beam / high beam), and a decay time constant τ = 0 with a peak value of 80 V simulating a surge voltage A voltage of 188 s (test standard value) was applied. The Zener voltage VZ of the Zener diode is set to 22V. The measured values were the drain-source voltage VDS, the gate-source voltage VGS, and the drain current I of the FET. In this experiment, the gate of the FET is measured in an open state, and the equivalent of the diode 23 is not connected between the drain and the gate.
[0029]
From this measurement result, the FET conducts substantially simultaneously with the application of the surge voltage, and the drain current I flows, and the drain-source voltage VDS is immediately clamped at the Zener voltage VZ. The gate-source voltage VGS is about 5V. As a result, the burden voltage of the lamp is a little over 50V, but the filament could be protected without breaking.
[0030]
In this measurement, the time from the application of the surge voltage to the operation of the overvoltage detection circuit 15 and the drive circuit 16 is not taken into consideration, but the comparator 13, the AND gates 17 and 18, and the transistors 19 and 20 constituting these are not considered. Since the propagation delay time is about several ns to several tens ns, it is sufficiently possible to configure so that tf <to.
[0031]
As described above, according to the present embodiment, the Zener diode 23 is connected between the drain and gate of the power MOSFET 3, and the drive circuit 16 detects that the surge voltage is applied to the power supply line L by the overvoltage detection circuit 15. Then, the gate of the FET 3 is set in a high impedance state, and the drain-gate is clamped by the Zener voltage VZ.
[0032]
Therefore, by selectively setting the value of the Zener voltage VZ, the withstand voltage distribution ratio between the lamps 2a and 2b and the FET 3 at the time of applying the surge voltage can be set as appropriate, so that neither of them is destroyed. Can be protected by a simple configuration.
[0033]
The Zener diode 23 does not have to clamp all of the surge voltage, and it is sufficient to clamp a voltage substantially equivalent to the voltage shared by the FET 3 with the lamps 2a and 2b. Therefore, the Zener diode 23 is expensive for high power like a power Zener diode. Therefore, it is possible to use a normal low-power zener diode. Therefore, an increase in cost can be suppressed and an increase in circuit size can be prevented as much as possible.
[0034]
Further, even if the drive circuit 16 detects that an overvoltage is detected and the gate of the FET 3 is in a high impedance state, the FET 3 is immediately turned on and there is almost no period in which the vehicle headlight is turned off. Even if a load dump occurs, the safety is not lowered. In addition, by using the power MOSFET 3 for the semiconductor switching element, the withstand voltage can be improved and the on-resistance during conduction can be reduced.
[0035]
(Second embodiment)
FIG. 4 shows a second embodiment of the present invention. The same parts as those of the first embodiment are denoted by the same reference numerals and the description thereof is omitted. Only different parts will be described below. In the second embodiment, a resistor (pull-down resistor) 24 is interposed between the common connection point of the transistors 19 and 20 and the anode of the Zener diode 23. One of the AND gates 18 of the drive circuit 16 is replaced with an OR gate 25 having a negative logic input, and the output signal SW of the light switch is given to the negative logic input terminal. ) 26 is configured. Other configurations are the same as those of the first embodiment.
[0036]
Next, the operation of the second embodiment will be described. When the headlight is turned off and the output signal SW of the light switch is at low level, the output signal level of the AND gate 17 is low, the output signal level of the OR gate 25 is high, and the transistors 19 and 20 are in the first embodiment. As well as off and on respectively. Accordingly, the FET 3 is turned off when the gate potential is low, and the headlight is not turned on.
[0037]
On the other hand, when the output signal SW of the light switch is at a high level, the output signal level of the AND gate 17 is high, the output signal level of the OR gate 25 is low, and the transistors 19 and 20 are respectively the same as in the first embodiment. Turns on and off. Therefore, the FET 3 is turned on when the gate potential is high, and the lamps 2a and 2b are energized to turn on the headlight.
[0038]
As in the first embodiment, it is assumed that a surge voltage is applied to the power supply line L with the lamps 2a and 2b lit. In this case, when the comparator 13 of the abnormality detection circuit 15 outputs the detection signal AB (high level), the output signal level of the AND gate 17 becomes low, the output signal level of the OR gate 25 becomes high, and the lamps 2a and 2b are turned off. Similarly, the transistor 19 is turned off, the transistor 20 is turned on, and the gate of the FET 3 is pulled down via the resistor 24.
[0039]
At this time, a current flows along the path of the drain of the FET 3 → the Zener diode 23 → the resistor 24 → the ground, and the drain-gate voltage of the FET 3 is clamped by the Zener diode 23. A voltage VL obtained by dividing a voltage obtained by subtracting the zener voltage VZ from the surge voltage VS by a resistance component such as a filament and a resistor 24 is applied to both ends of the lamps 2a and 2b.
[0040]
Further, when the gate-source voltage VGS of the FET 3 is determined to be at a high level by the terminal voltage of the resistor 24, the FET 3 becomes conductive, a current flows between the drain and the source, and the surge voltage is absorbed.
[0041]
As described above, according to the second embodiment, when the overvoltage detection circuit 15 detects that a surge voltage has been applied to the power supply line L, the drive circuit 26 pulls down the gate of the FET 3 so that the drain-gate The gap is clamped by the zener voltage VZ.
[0042]
Accordingly, the withstand voltage distribution ratio between the lamps 2a and 2b and the FET 3 can be set by appropriately selecting and setting the Zener voltage VZ by the Zener diode 23 and the resistance value of the resistor 24, so that the voltage is also applied to the resistor 24. By doing so, the voltage burden ratio of the lamps 2a and 2b and the Zener diode 23 can be reduced accordingly.
[0043]
The present invention is not limited to the embodiments described above and illustrated in the drawings, and the following modifications or expansions are possible.
The semiconductor switching element is not limited to the power MOSFET 3 and may be a voltage driven element.
The load is not limited to the lamps 2a and 2b, and any load may be used as long as it requires a countermeasure against overvoltage.
[Brief description of the drawings]
FIG. 1 is a first embodiment when the present invention is applied to a vehicle headlamp driving device, and shows an electrical configuration. FIG. 2 is a timing chart of each part when a surge voltage is applied. FIG. 3 is a diagram showing a waveform of an actual measurement result performed by the inventors of the present invention. FIG. 4 is a diagram corresponding to FIG. 1 illustrating a second embodiment of the present invention. Explanation of]
1 is a battery (power supply), 2a and 2b are lamps (loads), 3 is a power MOSFET (semiconductor switching element), 15 is an overvoltage detection circuit (abnormal voltage detection means), 16 is a drive circuit (drive means), and 23 is a zener A diode (constant voltage generating element), 24 is a resistor (pull-down resistor), and 26 is a drive circuit (drive means).

Claims (4)

A voltage-driven semiconductor switching element that is connected between a load that requires protection against overvoltage and a ground, and that controls driving of the load;
A constant voltage generating element connected between the load side terminal and the control terminal of the semiconductor switching element;
An abnormal voltage detecting means for detecting an abnormal voltage of the power supply;
A drive voltage is applied to the control terminal of the semiconductor switching element, and the control terminal of the semiconductor switching element is set to a high impedance state when the abnormal voltage of the power source is detected by the abnormal voltage detecting means. A load drive device comprising: drive means. A voltage-driven semiconductor switching element that is connected between a load that requires protection against overvoltage and a ground, and that controls driving of the load;
A constant voltage generating element connected between the load side terminal and the control terminal of the semiconductor switching element;
An abnormal voltage detecting means for detecting an abnormal voltage of the power supply;
Driving means configured to apply a driving voltage to the control terminal of the semiconductor switching element and to pull down the control terminal of the semiconductor switching element when the abnormal voltage of the power source is detected by the abnormal voltage detecting means; equipped with a,
The driving means includes
A current source connected to the power source;
Two transistors connected in a totem pole between the current source and ground;
A pull-down resistor connected between a common connection point of the two transistors and a control terminal of the semiconductor switching element;
Of the two transistors, the drive voltage is applied to the control terminal by turning on the power supply side transistor, and when the abnormal voltage detection means detects the power supply abnormal voltage, the ground side transistor is turned on. By doing so, the control terminal is pulled down . The load driving device according to claim 1, wherein the load is a vehicle lamp. The load driving device according to claim 1, wherein the semiconductor switching element is a power MOSFET.

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