JP5530306B2 - Overcurrent detection device - Google Patents

Description

  The present invention relates to a technique for detecting an overcurrent flowing through a load drive circuit, and more particularly to an overcurrent detection device that detects an overcurrent by setting an overcurrent determination value suitable for a current characteristic of a load.

  For example, a load such as a lamp mounted on a vehicle is provided with an FET (semiconductor switch) between the battery (DC power supply) and each load, and each FET is turned on and off to drive and stop each load. Is controlling. In such a load driving circuit, when an overcurrent flows due to a short circuit accident or the like, it is necessary to quickly shut off the FET and protect the semiconductor switch, the load, and the electric wire connecting them from overheating. is there.

  Therefore, a drain-source voltage Vds (hereinafter referred to as “voltage Vds”), which is a voltage across the FET, is compared with a determination voltage corresponding to a preset overcurrent determination value using a comparator (comparator). An overcurrent detection device is known in which, when the voltage Vds exceeds a determination voltage, it is determined that an overcurrent has occurred and the FET is cut off (see, for example, Patent Document 1).

  FIG. 5 is an explanatory diagram showing a load driving circuit on which a conventional overcurrent detection device is mounted, and an FET (T1) is provided between a battery VB and a load RL. The drain of the FET (T1) is connected to the register 13, and a determination voltage corresponding to the battery output voltage V1 is set in the register 13 and supplied to the plus side input terminal of the comparator CMP1. Further, the source voltage V2 of the FET (T1) is supplied to the negative input terminal of the comparator CMP1.

  When the switch SW1 is turned on and the input voltage of the driver 12 becomes “L”, the driver 12 applies the output voltage of the charge pump 11 to the gate of the FET (T1), and turns on the FET (T1). To do. Thereby, the load RL can be driven.

  When a normal current flows through the load RL, the voltage drop (that is, the voltage Vds) due to the on-resistance Ron of the FET (T1) is small, so the source voltage V2 (= V1−Vds) becomes a large value, and the comparator The output signal of CMP1 is “L”. When an overcurrent flows through the load RL, the voltage Vds increases and the source voltage V2 decreases, so that the output signal of the comparator CMP1 is inverted to “H”, and the overcurrent is detected. When an overcurrent is detected, the driver 12 blocks the FET (T1) and protects the entire load driving circuit.

  In the overcurrent protection device shown in FIG. 5, the determination voltage set in the register 13 is set by dividing the output voltage V1 of the battery VB, so that the voltage V1 fluctuates and the load current changes. However, the determination voltage changes according to the voltage fluctuation, and the occurrence of overcurrent can be detected with high accuracy.

  For example, even when the output voltage V1 of the battery VB is increased and the voltage Vds is increased in the normal time, the determination voltage is increased accordingly, so that the voltage Vds is relatively higher than the determination voltage. Therefore, it is possible to prevent the normal current from being erroneously detected as an overcurrent. On the contrary, even if the output voltage V1 of the battery VB decreases and the voltage Vds at the time of overcurrent occurrence decreases, the determination voltage decreases accordingly, so when an overcurrent occurs The voltage Vds relatively exceeds the determination voltage, and an overcurrent can be detected.

  However, a load having a characteristic that the load current does not increase linearly with an increase in the output voltage V1 of the battery VB, such as an HID lamp (High Intensity Discharge lamp), more specifically, When the output voltage V1 becomes a certain voltage, the current value peaks, and when the output voltage V1 further increases, the load has such a characteristic that current exceeding the peak value does not flow (hereinafter referred to as “special load”). When the output voltage V1 of the battery VB increases, the load current may not increase and the voltage Vds may not increase.

  In such a case, the determination voltage increases even though the voltage Vds has not increased, so the difference between the voltage Vds and the determination voltage becomes large. In other words, since the difference between the normal load current and the cutoff current (overcurrent determination value) becomes large, it takes a long time for the FET to shut off due to the overcurrent flowing, and the FET or load And the electric wire which connects these overheats.

JP 2006-197768 A

  As described above, in the conventional overcurrent detection device, when driving a load having a characteristic that the load current does not fluctuate linearly according to the change in the output voltage of the battery, such as an HID lamp, a load driving circuit is used. Elements such as electric wires and FETs to be configured may be overheated. For this reason, it is necessary to take measures such as increasing the diameter of the electric wires, which leads to a disadvantage that the apparatus is increased in size and cost.

  The present invention has been made to solve such a conventional problem, and an object of the present invention is a drive circuit that drives a load having a characteristic that the load current does not change linearly with respect to the supply voltage. Even if it exists, it is providing the overcurrent detection apparatus which can detect generation | occurrence | production of overcurrent with high precision.

  In order to achieve the above object, according to the first aspect of the present invention, a semiconductor switch (eg, FET) is provided in a circuit connecting a DC power supply (eg, battery VB) and a load, and the semiconductor switch is turned on / off. In the overcurrent detection device for detecting overcurrent of the load drive circuit that controls driving and stopping of the load by switching, a first resistor (for example, R1) and a second resistor (for example, R1) connected in series with each other , R2), and a Zener diode (for example, ZD1), one end of the first resistor is connected to a connection point between the semiconductor switch and the load, and one end of the Zener diode is grounded to ground Based on the voltage (V1) generated at the connection point between the superposition circuit (for example, the voltage superposition circuit 14) and the semiconductor switch and the DC power supply, the overcurrent determination voltage is determined. A determination voltage setting means (for example, a register 13) to be determined, the determination voltage is compared with a measurement voltage generated at a connection point between the first resistor and the second resistor, and the measurement voltage determines the determination voltage. In the case of exceeding, the comparison means (for example, the comparator CMP1) that outputs an overcurrent determination signal and the on / off of the semiconductor switch are controlled according to the drive command, and the overcurrent determination signal is output And a control means (for example, driver 12) for turning off the semiconductor switch.

  According to a second aspect of the present invention, the DC power source is a battery mounted on a vehicle, and the load is a lamp load whose load current does not change linearly with respect to a change in applied voltage (for example, HID). Lamp).

  In the overcurrent detection device according to the present invention, a voltage superimposing circuit is provided between the connection point of the semiconductor switch and the load and the ground, and the voltage across the Zener diode included in the voltage superimposing circuit exceeds the Zener voltage. A current flows through the voltage superimposing circuit, and a voltage drop occurs in the first resistor. Then, this voltage drop becomes a superimposed voltage and added to the voltage across the semiconductor switch, and an overcurrent is detected by comparing the voltage after the addition with the determination voltage. Therefore, even when driving a load (for example, an HID lamp) having a characteristic that the load current does not increase linearly with an increase in the output voltage of the DC power supply, the occurrence of overcurrent is detected with high accuracy. be able to.

It is a circuit diagram which shows the structure of the overcurrent detection apparatus which concerns on 1st Embodiment of this invention. It is a characteristic view which shows the relationship between the battery output voltage V1 and interruption | blocking current Ith of the overcurrent detection apparatus which concerns on 1st Embodiment of this invention. It is explanatory drawing which shows typically the relationship between the determination voltage and interruption | blocking current when not providing a voltage superimposition circuit. It is explanatory drawing which shows typically the relationship between the determination voltage and interruption | blocking current at the time of providing a voltage superimposition circuit. It is a circuit diagram which shows the structure of the conventional overcurrent detection apparatus.

  Hereinafter, embodiments of the present invention will be described with reference to the drawings. FIG. 1 is a circuit diagram showing a configuration of a load drive circuit equipped with an overcurrent detection device according to the first embodiment of the present invention. As shown in FIG. 1, the load drive circuit includes a battery (DC power supply) VB, an FET (T1; semiconductor switch), and a load RL. By switching on and off of the FET (T1), the load drive circuit The driving and stopping of the RL can be controlled. That is, the positive terminal of the battery VB is connected to the drain of the FET (T1), the source of the FET (T1) is connected to one end of the load RL, and the other end of the load RL is grounded. The load RL is, for example, a load having a characteristic that the load current does not increase linearly with an increase in the output voltage V1 of the battery VB, such as an HID lamp (hereinafter referred to as “special load”). is there.

  Further, the overcurrent detection device includes a driver 12, a charge pump 11, a comparator (comparing means) CMP1, a register (determination voltage setting means) 13, and a voltage superimposing circuit 14. Further, resistors R3 and R4 and a switch SW1 are provided.

  The driver (control means) 12 is connected to the charge pump 11, the switch SW1 is turned on, and when the input signal is switched from “H” to “L”, the gate of the FET (T1) via the resistor R4. A drive signal is applied to. Thereby, the FET (T1) can be turned on.

  The register 13 is connected to the drain of the FET (T1), divides the voltage V1 (output voltage of the battery VB) generated at the drain at a predetermined ratio, and uses the divided voltage as the determination voltage VM. Supply to the side input terminal.

  The voltage superimposing circuit 14 has resistors R1 and R2 and a Zener diode ZD1, one end of the resistor R1 is connected to the source of the FET (T1), the other end is branched into two systems, and the first branch The line is grounded via a resistor R2 and a Zener diode ZD1. The second branch line is connected to the negative input terminal of the comparator CMP1. That is, the voltage (measurement voltage) generated at the connection point between the resistors R1 and R2 is supplied to the negative input terminal of the comparator CMP1. In the present embodiment, the ratio between the resistors R1 and R2 is set to be {R1 / (R1 + R2)} = 0.004.

  Furthermore, in this embodiment, the Zener voltage (breakdown voltage) of the Zener diode ZD1 is set to 10V. In addition, when the applied voltage is about 12V, the load RL has a peak current of 10A, and when the voltage becomes lower than 12V, the load current decreases linearly. Further, when the voltage becomes higher than 12V, the load current decreases linearly by a certain amount. That is, when an HID lamp is used as the load RL, the load current I1 is I1 = 4A (load resistance 2Ω) when V1 = 8V and V1 = 12V, as shown by the curve S1 in FIG. And I1 = 10A (load resistance 1.2Ω), and when V1 = 16V, I1 = 8 (load resistance 2Ω).

  Next, the operation of the overcurrent detection device according to this embodiment will be described. FIG. 2 is a characteristic diagram showing the relationship between the output voltage V1 of the battery VB, the load current I1 (S1 in the figure) flowing through the load RL, and the cutoff current Ith (S2 in the figure), and the curve S2a is a voltage superposition circuit. When 14 is provided, S2b indicates characteristics when the voltage superimposing circuit 14 is not provided. FIG. 3 is an explanatory diagram schematically showing the relationship between the determination voltage VM and the cutoff current Ith when V1 = 8V, 12V, and 16V when the voltage superimposing circuit 14 is not provided (conventional case). FIG. 4 is an explanatory diagram schematically showing the relationship between the determination voltage VM and the cutoff current Ith when V1 = 8V, 12V, and 16V when the voltage superimposing circuit 14 is provided (in the case of the present embodiment). is there.

  When the switch SW1 shown in FIG. 1 is turned on, the signal supplied to the driver 12 is switched from “H” to “L”. Therefore, the driver 12 sends the output voltage of the charge pump 11 to the FET ( To the gate of T1). As a result, the FET (T1) is turned on and the load RL is driven.

  Here, it is assumed that the ON resistance Ron of the FET (T1) is 5 mΩ, and the determination voltage VM output from the register 13 is set at a ratio of “0.00518 / 1” with respect to the voltage V1. Therefore, for example, when the load current I1 is 4 A, the drain-source voltage Vds of the FET (T1) is 20 mV. When the voltage V1 is 8V, the determination voltage VM is 41.5 mV.

  Hereinafter, the load RL is a resistance load, that is, a load in which the load current increases linearly as the voltage increases; hereinafter referred to as “normal load”), and a special load such as an HID lamp. The relationship between the output voltage V1 and the cutoff current Ith will be described.

[For normal load]
When the load RL shown in FIG. 1 is a normal load (resistance value 1Ω), the voltage superimposing circuit 14 is not provided. Therefore, as shown by the curves S2 and S2b in FIG. 2, the cutoff current Ith changes linearly with respect to the change in the output voltage V1. That is, as shown in FIG. 3A, when V1 is 8V, the determination voltage VM = 41.5 mV and the load current I1 is 8A, so the voltage Vds is 40 mV. The cutoff current Ith is 8.3 A (41.5 / 5 = 8.3).

  As shown in FIG. 3B, when V1 is 12 V, the determination voltage VM is 62 mV, and the load current I1 is 12 A, so the voltage Vds is 60 mV. The cutoff current Ith is 12.4 A (62/5 = 12.4).

  Further, as shown in FIG. 3C, when V1 is 16V, the determination voltage VM is 83 mV and the load current I1 is 16A, so the voltage Vds is 80 mV. The cut-off current Ith is 16.6 A (83/5 = 16.6).

  As described above, when the load RL is a normal load, the cut-off current Ith corresponding to the fluctuation of the output voltage V1 of the battery VB is set without providing the voltage superposition circuit 14, and an overcurrent can be detected. .

[For special loads]
Next, the relationship between the output voltage V1 and the cutoff current Ith when the load RL, which is a feature of the present embodiment, is a special load such as an HID lamp will be described. When a special load is used, a voltage superimposing circuit 14 is provided as shown in FIG.

  When a special load is used, as shown by a curve S1 in FIG. 2, when the output voltage V1 of the battery VB is 8V, the resistance of the load RL is 2Ω and the load current I1 is 4A. . As shown in FIG. 4A, since the output voltage V1 is 8V, the determination voltage VM is 41.5 mV, and the cutoff current Ith is 8.3 A (= 41.5 / 5). Further, as indicated by the curve S2, the cutoff current Ith increases linearly as the voltage V1 increases. Further, since the voltage superimposing circuit 14 is provided, when the output voltage V1 reaches 10V, the source voltage V2 of the FET (T1) shown in FIG. 1 becomes approximately 10V, and the reverse voltage of the Zener diode ZD1 reaches the Zener voltage. A reverse current flows through the Zener diode ZD1. That is, the current I2 flows through the path of the source T1, R1, R2, ZD1, and ground. Therefore, a voltage drop indicated by “I2 * R1” occurs in the resistor R1, and this voltage (hereinafter referred to as a superimposed voltage Vg) is superimposed on the voltage Vds. Therefore, the cutoff current Ith changes as shown by the curve S2a in FIG. Then, an overcurrent is detected by comparing the determination voltage VM with Vds + Vg.

  When the output voltage V1 becomes 12V, the difference from the Zener voltage 10V is 2V, and since {R1 / (R1 + R2)} = 0.004 as described above, both ends of the resistor R1. The resulting voltage is 8 mV. Therefore, as shown in FIG. 4B, the superimposed voltage Vg is added to the voltage Vds, which is equivalent to the determination voltage VM being reduced from 62 mV to (62−8) = 54 mV. Therefore, the breaking current Ith is 54/5 = 10.8A.

  Further, if the voltage superimposing circuit 14 is not provided, there is no voltage drop 8 mV due to the resistor R1, so the cutoff current Ith is 62/5 = 12.4A. That is, by providing the voltage superimposing circuit 14, the cutoff current Ith is reduced from 12.4A to 10.8A. This means that when an overcurrent occurs, the FET (T1) is cut off more quickly to protect the entire load drive circuit from overheating.

  Next, when the output voltage V1 is 16V, the difference from the Zener voltage 10V is 6V, so the voltage generated across the resistor R1 is 6 * 0.004 = 24mV. Accordingly, as shown in FIG. 4C, the determination voltage VM is equal to that reduced from 83 mV to (83-24) = 59 mV, and the cut-off current Ith is 59/5 = 11.8 A.

  If the voltage superimposing circuit 14 is not provided, since there is no voltage drop 24 mV due to the resistor R1, the cutoff current Ith is 83/5 = 16.6A. That is, by providing the voltage superimposing circuit 14, the cutoff current Ith is reduced from 16.6A to 11.8A. Therefore, when an overcurrent occurs, the FET (T1) can be cut off faster and the entire load drive circuit can be protected from overheating.

  In this way, in the overcurrent protection device according to the present embodiment, when the reverse voltage applied to the Zener diode ZD1 reaches the Zener voltage by providing the voltage superimposing circuit 14, a constant current is supplied to the Zener diode ZD1. A voltage drop (superimposed voltage Vg) is generated in the resistor R1. Therefore, in the comparator CMP1, the voltage obtained by adding the superimposed voltage Vg to the voltage Vds is compared with the determination voltage VM, which is equivalent to the determination voltage VM being reduced by the superimposed voltage Vg. As a result, an appropriate cutoff current Ith can be set for a special load such as an HID lamp.

  Therefore, it is not necessary to take measures such as increasing the wire diameter or using a large FET as in the conventional case, and the device can be reduced in size and cost.

  The overcurrent detection device of the present invention has been described based on the illustrated embodiment. However, the present invention is not limited to this, and the configuration of each part is replaced with an arbitrary configuration having the same function. be able to.

  The present invention can be used to protect a load having current characteristics such as an HID lamp from overcurrent.

11 Charge pump 12 Driver 13 Register CMP1 Comparator (comparison means)
VB battery (DC power supply)
RL load T1 FET (semiconductor switch)
R1 resistance (first resistance)
R2 resistance (second resistance)
ZD1 Zener diode

Claims (2)

In an overcurrent detection device that detects an overcurrent of a load drive circuit that controls driving and stopping of the load by providing a semiconductor switch in a circuit that connects a DC power supply and a load, and switching the semiconductor switch on and off ,
A first resistor, a second resistor, and a Zener diode connected in series with each other. One end of the first resistor is connected to a connection point between the semiconductor switch and the load, and one end of the Zener diode is A voltage superposition circuit grounded to ground,
Determination voltage setting means for setting an overcurrent determination voltage based on a voltage generated at a connection point between the semiconductor switch and the DC power supply;
Comparing means for comparing the determination voltage with a measurement voltage generated at a connection point between the first resistor and the second resistor, and outputting an overcurrent determination signal when the measurement voltage exceeds the determination voltage When,
Control means for turning on and off the semiconductor switch according to a drive command, and turning off the semiconductor switch when the overcurrent determination signal is output;
An overcurrent detection device comprising: The DC power source is a battery mounted on a vehicle,
2. The overcurrent detection device according to claim 1, wherein the load is a lamp load in which a load current does not change linearly with respect to a change in applied voltage.

Images