JP5136144B2 - Load current supply circuit - Google Patents

Description

  The present invention relates to a load current supply circuit in which a current detection transistor is connected in parallel to a power transistor that supplies current to a load.

  For a power transistor that supplies a large current to a load, a current detection circuit and an overcurrent protection circuit are usually provided in order to prevent destruction due to a current exceeding a specified value flowing through the power transistor. If a current detection resistor is directly connected to the power transistor as the current detection circuit, the voltage that can be applied to the load is reduced by the voltage drop caused by the resistance. For this reason, a current detection transistor that supplies a detection current in which the load current flowing through the power transistor is reduced by a predetermined ratio is connected in parallel to the power transistor, and the load current of the power transistor is calculated from the detection current of the current detection transistor. It is common to detect.

For example, in the circuit described in Patent Document 1, the gates of the power transistor and the current detection transistor are connected to a common driver. Further, the drain of the power transistor is connected to the non-inverting input terminal of the operational amplifier, and the drain of the current detection transistor is connected to the inverting input terminal of the operational amplifier. By adopting such a circuit configuration, the operating state of the power transistor and the current detection transistor is almost the same, and the load current flowing through the power transistor can be accurately measured without loss from the detection current flowing through the current detection transistor. It can be detected well.
Japanese Patent Laid-Open No. 7-113826

  However, when the circuit configuration of the prior art is adopted, there is a problem that the accuracy of current detection varies as the temperature changes. That is, an element such as a wiring resistance or a resistance inserted in a circuit has a characteristic that a resistance value changes with temperature. Therefore, for example, at room temperature, even if the ratio of the load current flowing through the power transistor to the detection current flowing through the current detection transistor is set to a predetermined ratio, if the temperature changes, the load current and the detection current This ratio deviates from the predetermined ratio, and as a result, the accuracy of current detection varies.

  The present invention has been made in view of the above-described points, and an object of the present invention is to provide a load current supply circuit capable of suppressing deterioration of current detection accuracy as much as possible even when a temperature change occurs.

In order to achieve the above object, a load current supply circuit according to claim 1 comprises:
A power transistor for supplying a load current to the load;
A current detection transistor that is connected in parallel with the power transistor and flows a detection current in which the load current supplied to the load by the power transistor is reduced by a predetermined ratio;
A current detection circuit for detecting a current value of a detection current flowing through the current detection transistor;
A first compensation resistor connected in series with the power transistor;
A second compensation resistor connected in series with the current detection transistor,
The current detection circuit has a negative feedback circuit including an operational amplifier for equalizing the operating environment of the power transistor and the current detection transistor,
The resistance value of the first compensation resistor is adjusted so as to reduce the offset voltage of the operational amplifier, and the second compensation resistor reduces fluctuation of the load current and the detection current from a predetermined ratio due to temperature change. Thus, the resistance value is adjusted .

Although it is inevitable that the resistance value of the wiring resistance changes according to the temperature, if the current change due to such a resistance value change occurs at the same rate in the load current and the detection current, the load The ratio between the current and the detection current can be kept substantially constant. Therefore, the load current supply circuit according to claim 1, the current detection transistor and the series resistance value is connected to the second compensation resistor to be adjusted. This second compensation resistor also has a characteristic that the resistance value changes according to the temperature. Therefore, by adjusting the resistance value of the second compensation resistor, the ratio of the resistance value change due to the temperature characteristic of the path through which the load current flows and the ratio of the resistance value change due to the temperature characteristic of the path through which the detection current flows It becomes possible to make it almost coincide. This makes it possible to maintain the ratio between the load current and the detection current at an almost predetermined ratio even when the temperature changes. As a result, the load current can be detected with high accuracy based on the detection current. can do.

  In the load current supply circuit, the factor that deteriorates the detection accuracy of the load current is not only the change in the ratio between the load current and the detection current accompanying the temperature change. For example, there is some variation in the offset voltage of the operational amplifier that constitutes the current detection circuit. For this reason, even if the circuit is designed and configured so that the ratio between the load current and the detection current is a predetermined ratio, the ratio may deviate from the target predetermined ratio due to the offset voltage. .

In the load current supply circuit according to the first aspect, the first compensation resistor is connected in series with the power transistor, and the second compensation resistor is connected in series with the current detection transistor. For this reason, not only can the fluctuation of the ratio between the load current and the detection current due to the temperature change be suppressed by the second compensation resistor, but also the deviation from the predetermined ratio due to the offset of the operational amplifier is caused by the first compensation resistor. It becomes possible to reduce.

According to a second aspect of the present invention, in order to detect the current value of the load current, the current detection circuit has a resistor that converts the detection current into a voltage, and the resistance value of the resistor can be adjusted. preferable. If the resistance value of the second compensation resistor is adjusted so as to cancel the influence of the temperature characteristics, the magnitude of the current value of the detection current may change. Then, when the detection current is converted into a voltage, the relationship between the current and the voltage that was originally planned is not established. In such a case, such a problem can be easily solved if the resistance value of the resistor for converting the detection current into a voltage can be adjusted.

As described in claim 3 , it is preferable that the resistance values of the first and second compensation resistors are adjusted by trimming. As a result, the resistance value of the compensation resistor can be adjusted with high accuracy.

As described in claim 4, the load current supply circuit, it is preferable to provide a terminal for measuring the resistance value of the second compensation resistors. Accordingly, the resistance value of the second compensation resistor can be accurately measured independently of the characteristic value of each element, and the resistance value can be adjusted to the target resistance value by trimming.

As described in claim 5 , the first and second compensation resistors are preferably provided on the circuit board. For example, by using a thick film resistor formed on a ceramic substrate as the first and second compensation resistors, a low resistance compensation resistor can be realized and the resistance value can be easily adjusted.

As described in claim 6, adjustment of the first and second resistance values of the compensating resistors are preferably elements for constituting the load current supply circuit is performed after it has been configured. It is also possible to measure the characteristics of each element in advance and adjust the resistance value of the compensation resistor before the circuit configuration based on the measured characteristics. Variations in characteristics cannot be considered. If the compensation resistor is adjusted after the load current supply circuit is formed, it is possible to cope with such variations in individual elements.

  Hereinafter, a load current supply circuit according to an embodiment of the present invention will be described with reference to the drawings. FIG. 1 is a circuit diagram showing the configuration of the entire load current supply circuit according to the present embodiment.

  As shown in FIG. 1, the load current supply circuit 40 includes, as an example, a power transistor Tr1 that supplies a load current I1 to a motor R7 that is a load. Furthermore, the load current supply circuit 40 includes a current detection transistor Tr2 that is connected in parallel with the power transistor Tr1 and that flows the detection current I2. The power transistor Tr1 and the current detection transistor Tr2 are each composed of a MOSFET.

  The power transistor Tr1 is configured by connecting in parallel several thousands of MOSFET cells formed on a semiconductor substrate (not shown). Therefore, the power transistor Tr1 can supply a large load current I1 to the motor R7 through the thousands of MOSFET cells. On the other hand, the current detection transistor Tr2 is composed of one or several MOSFET cells connected in parallel. The current capacity of each transistor Tr1 and Tr2 is determined by the number of MOSFET cells constituting each transistor Tr1 and Tr2, and when the same voltage is applied to each transistor Tr1 and Tr2 gate, The load current I1 and the detection current I2 having current values corresponding to the respective current capacities flow. In the present embodiment, for example, the current value ratio between the load current I1 flowing through the transistors Tr1 and Tr2 and the detection current I2 is the load current I1 of the power transistor Tr1: the detection current I2 of the current detection transistor 2 = 10000: 10. The current capacities of the transistors Tr1 and Tr2 are set so that

  The drain of the power transistor Tr1 is connected to the motor R7 via a thick film conductor resistance R5 which is a resistance component of a thick film conductor formed on a ceramic substrate, for example. The resistor R1 indicates the on-resistance when the power transistor Tr1 is turned on, and the resistor R3 indicates the wiring resistance of the power transistor Tr1. Further, the resistance value of the thick film conductor resistance R5 can be adjusted by laser trimming, for example.

  Further, the drain of the power transistor Tr1 is also connected to one input terminal (inverting input terminal) of the operational amplifier 20 constituting the current detection circuit 30 via the thick film conductor resistance R5. The other input terminal (non-inverting input terminal) of the operational amplifier 20 is connected to the drain of the current detection transistor Tr2 through an adjustable thick film resistor R6 in the same manner as the thick film conductor resistor R5. The resistor R4 indicates the wiring resistance of the current detection transistor Tr2. When the ratio of the current capacity between the power transistor Tr1 and the current detection transistor Tr2 is n: 1, the on-resistance of the current detection transistor Tr2 is nR1.

  In addition to the operational amplifier 20 described above, the current detection circuit 30 includes a diode D1, an NPN transistor Tr3, and a current detection resistor R2. The collector of the NPN transistor Tr3 is connected to the source of the current detection transistor Tr2 via the diode D1, and the emitter of the NPN transistor Tr3 is connected to the current detection resistor R2. The base of the NPN transistor Tr3 is connected to the output terminal of the operational amplifier 20.

  Connected to the gates of the power transistor Tr1 and the current detection transistor Tr2 is a drive circuit (not shown) that outputs a common drive signal (voltage signal) for controlling the operation state of the transistors Tr1 and Tr2. Has been. A control signal corresponding to the current to be supplied to the load is given to the drive circuit from a control circuit (not shown). The drive circuit generates and outputs the common drive signal described above based on the control signal.

  When the drive circuit outputs a drive signal, the power transistor Tr1 is turned on and the load current I1 is supplied to the motor R7. Then, the voltage applied to the motor R7 is input to the input terminal of the operational amplifier 20. Therefore, a voltage signal for turning on the NPN transistor Tr3 is output from the output terminal of the operational amplifier 20. When the NPN transistor Tr3 is turned on by the voltage signal from the operational amplifier 20, a detection current I2 flows through the current detection transistor Tr2.

  The operational amplifier 20 and the NPN transistor Tr3 have the connection configuration described above, so that the terminal voltage (drain-source voltage) of the power transistor Tr1 and the terminal voltage (drain-source voltage) of the power transistor Tr2 for current detection are substantially equal. In other words, the negative feedback circuit is configured so that the operating environments of the transistors Tr1 and Tr2 are equal. As a result, when the power transistor Tr1 is turned on and the load current I1 is supplied to the motor R7, at the same time, the current detection transistor Tr2 is turned on and the detection current I2 flows in the same operating environment. Is controlled to be conductive. As described above, since the ratio of the current values of the power transistor Tr1 and the current detection transistor Tr2 is determined in advance, the current value of the detection current I2 flowing through the current detection transistor Tr2 Thus, the current value of the load current I1 flowing through the power transistor Tr1 can be detected.

  For example, the detection current I2 flowing through the current detection transistor Tr2 is converted into a voltage signal V by the current detection resistor R2 and input to an A / D converter (not shown). Then, after being converted into a digital value by the A / D converter, the digital value is input to the control circuit so that it can be used when the control circuit generates a control signal. Although not shown, it is determined whether or not the voltage signal V exceeds a reference value corresponding to the maximum allowable current value of the power transistor Tr1. If the voltage signal V exceeds the reference value, the load of the power transistor Tr1 is exceeded. An overcurrent protection circuit that controls the current I1 to be reduced or to shut off the power transistor 1 may be provided.

  Next, adjustment of the resistance values of the thick film conductor resistance R5 and the thick film resistance R6, which is a feature of the present embodiment, will be described.

  In the load current supply circuit 40 having the configuration shown in FIG. 1, when the thick film conductor resistance R5 and the thick film resistance R6 are not provided, or the adjustment of the thick film conductor resistance R5 and the thick film resistance R6 is not performed, As the temperature changes, the current detection accuracy varies. That is, the resistances R3 and R4, which are wiring resistances, and the resistance R1, which is an on-resistance of the power transistor Tr1, have a characteristic that the resistance value changes with temperature. For this reason, as shown in FIG. 2, for example, at room temperature, even if the ratio of the load current I1 flowing through the power transistor Tr1 and the detection current I2 flowing through the current detection transistor Tr2 is set to be a predetermined ratio, Changes, the ratio between the load current I1 and the detection current I2 (current mirror ratio) deviates from a predetermined ratio, and as a result, the accuracy of current detection varies. In FIG. 2, the magnitude of the difference in the ratio between the load current I1 and the detection current I2 is shown as a percentage.

  Therefore, in this embodiment, the resistance value of the thick film resistor R6 provided in series with the current detection transistor Tr2 is trimmed and adjusted, and the deviation in the ratio between the load current I1 and the detection current I2 due to the temperature change described above. It was decided to reduce.

  That is, it is inevitable that the resistances R3, R4, etc., which are wiring resistances, change the resistance value according to the temperature, but the current change due to such a resistance value change causes the load current I1 and the detection current I2 to change. , The ratio between the load current I1 and the detection current I2 can be kept substantially constant.

  Here, the thick film resistor R6 connected in series with the current detection transistor Tr2 also has a characteristic that the resistance value changes according to the temperature. Therefore, by adjusting the resistance value of the thick film resistor R6, the ratio of the resistance value change due to the temperature characteristic of the path through which the load current I1 flows and the ratio of the resistance value change due to the temperature characteristic of the path through which the detection current I2 flows. Can be substantially matched.

  FIG. 3 shows an example of resistance values of the resistors R1, R3 to R6 of the load current detection circuit 40 and their temperature characteristic coefficients (resistance change rate per unit temperature). As shown in FIG. 3, the thick film resistor R6 has a larger resistance value than the other resistors R1, R3 to R5. In the present embodiment, the thick film resistor R6 is connected in series to the current detection transistor Tr2 instead of the power transistor Tr1. Therefore, a large current mirror ratio adjustment margin can be ensured without reducing the voltage that can be applied to the motor R7 as the load.

  Hereinafter, the fact that the deviation of the ratio between the load current I1 and the detection current I2 due to the temperature change can be reduced by adjusting the resistance value of the thick film resistor R6 will be described using equations.

  First, the temperature characteristic coefficients of the resistors R1, R3 to R6, that is, the resistance change rates per unit temperature are α, β, γ, δ and ε, respectively, and n is the power transistor Tr1 with respect to the number of cells of the power transistor Tr2. When the number of cells is assumed and the operational amplifier is an ideal operational amplifier (that is, offset voltage Vo = 0), the detection current I2 can be expressed as Equation 1 below.

  When the above Equation 1 is differentiated by temperature, the following Equation 2 is established.

When the right side of Formula 2 is zero, the detection current I2 is a value that does not depend on temperature. In order for the right side of Formula 2 to be zero, the following Formula 3, that is, Formula 4, may be established.

  A thick film resistor R6 for which Equation 4 is satisfied is expressed as Equation 5 below.

  Therefore, by trimming the thick film resistor R6 and adjusting the resistance value so as to satisfy the relationship shown in Formula 5, a detection current I2 having no temperature dependency (reduced) can be obtained.

  The detection current I2 with reduced temperature dependency is also obtained by setting the resistance value in accordance with the expected characteristic value of each element and forming the thick film resistor R6 so as to have the resistance value. I can. However, depending on the manufacturing process, there may be some variation in the characteristic value of each element. However, if the resistance value of the thick film resistor R6 is set in advance, such variation cannot be compensated.

  On the other hand, if the resistance value is adjusted by trimming the thick film resistor R6, it is possible to deal with the variation in the characteristics of each element depending on the manufacturing process. The adjustment of the resistance value by trimming the thick film resistor R6 is, for example, measuring the resistance value (R1 + R3 + R5) by the resistors R1, R3, R5 using the connection terminal 41 to the load R7 in a high temperature environment. At the same time, the resistance value (nR1 + R4 + R6) by the resistors nR1, R4, and R6 is obtained from the current value of the detection current I2, and the resistance is obtained by using the connection terminal 42 provided between the resistors R4 and R6. The resistance value of R6 alone is obtained. Thereafter, the resistance value at the same location is obtained under a room temperature environment, and the resistance value of the thick film resistor R6 for obtaining the detection current I2 having no temperature dependency (reduced) is obtained from each resistance value. Then, the resistance value of the thick film resistor R6 is adjusted by trimming so that the obtained resistance value is obtained. Thus, even if the magnitude of the change in the ratio (current mirror ratio) between the load current I1 and the detection current I2 due to the temperature change is not constant with respect to the temperature, the thick film resistor R6 can be easily changed. The resistance value can be adjusted.

  By adjusting the resistance value of the thick film resistor R6 as described above, the ratio between the load current I1 and the detection current I2 can be maintained at a substantially predetermined ratio even if a temperature change occurs. Based on the detection current I2, the load current I1 can be detected with high accuracy.

  In this embodiment, in addition to the thick film resistor R6, the resistance value of the thick film conductor resistor R5 is also adjusted by trimming. One of the reasons is to reduce the offset voltage of the operational amplifier 20 by adjusting the resistance value of the thick film conductor resistance R5.

  In the load current supply circuit 40, the factor that deteriorates the detection accuracy of the load current I1 is not only the change in the ratio between the load current I1 and the detection current I2 due to the temperature change. For example, there is some variation in the offset voltage of the operational amplifier 20. For this reason, even if the circuit 40 is designed and configured so that the ratio between the load current I1 and the detection current I2 is a predetermined ratio, the ratio is determined from the target predetermined ratio due to the offset voltage of the operational amplifier 20. It may shift.

  Therefore, in this embodiment, the offset voltage of the operational amplifier 20 is also reduced by adjusting the resistance value of the thick film conductor resistor R5 connected to the power transistor Tr1.

  When the temperature characteristics of the power transistor Tr1 and each resistor are ignored, the detection current I2 can be expressed as Equation 6 below. Vo is an offset voltage of the operational amplifier 20.

  As is apparent from Equation 6, the variation in the offset voltage Vo can be compensated by adjusting the resistance value of the thick film conductor resistance R5 to be (R3 + R5) I1 = Vo.

  The preferred embodiments of the present invention have been described above. However, the present invention is not limited to the above-described embodiments, and various modifications can be made without departing from the spirit of the present invention.

  For example, in the above-described embodiment, the offset voltage Vo of the operational amplifier 20 is reduced by adjusting the resistance value of the thick film conductor resistance R5, and the load current I1 and the detection current accompanying the temperature change are adjusted by adjusting the resistance value of the thick film resistance R6. Fluctuation from a predetermined ratio with I2 was reduced.

  However, even by adjusting the resistance value of the thick film conductor resistance R5, it is possible to reduce the fluctuation from the predetermined ratio between the load current I1 and the detection current I2 due to the temperature change. Only one of the resistor R6 may be provided, and only a change from a predetermined ratio between the load current I1 and the detection current I2 due to a temperature change may be reduced. Further, while providing both the thick film conductor resistance R5 and the thick film resistance R6, the combination of these two resistances reduces the variation from the predetermined ratio between the load current I1 and the detection current I2 due to the temperature change. Anyway. This is because the fluctuation of the current ratio accompanying the temperature change has a great influence on the current detection accuracy.

  Further, if the resistance value of the thick film resistor R6 or the like is adjusted so as to cancel the influence due to the temperature characteristic, the magnitude of the current value of the detection current I2 may change. Then, when the detection current I2 is converted into a voltage, the initially planned relationship between the current and the voltage is not established. In such a case, if the resistance value of the current detection resistor R2 for converting the detection current I2 into a voltage can be adjusted, such a problem can be easily solved.

  Therefore, when using one or both of the thick film conductor resistance R5 and the thick film resistance R6 to reduce the fluctuation from the predetermined ratio between the load current I1 and the detection current I2 due to the temperature change, this current detection It is preferable to adjust the resistance value of the resistor R2.

  In the above-described embodiment, the example in which the thick film conductor resistance R5 and the thick film resistance R6 are provided on the ceramic substrate has been described. In this case, the load current supply circuit is configured as a hybrid IC including a normal semiconductor substrate such as silicon and a ceramic substrate. However, the present invention can also be applied to a monolithic IC.

  In the embodiment described above, the thick film conductor resistance R5 and the thick film resistance R6 are used in order to cancel the temperature change of the wiring resistance, which is a low resistance, and the on-resistance of the power transistor. Therefore, the thick film conductor resistance R5 and the thick film resistance R6 need to have low resistance, but by using the thick film conductor or thick film resistor formed on the ceramic substrate, the low resistance can be easily reduced. realizable. On the other hand, in the case of a monolithic IC, there is a demerit that a relatively large area is required to achieve low resistance.

  In the embodiment described above, P-channel MOS transistors are used as the power transistor Tr1 and the current detection transistor Tr2. However, it goes without saying that an N-channel MOS transistor may be used. Further, not only a MOS transistor but also a high power semiconductor element such as an IGBT can be used. Furthermore, the load of the load current supply circuit 40 is not limited to the motor, and can be applied to various loads.

  In the above-described embodiment, the high-side drive configuration in which the power transistor Tr1 and the current detection transistor Tr2 are connected to the power supply side of the motor R7, which is a load, is employed, but a low-side drive configuration can also be employed. It is. Further, a bridge circuit configuration in which power transistors are arranged on the high side and the low side of the load may be employed.

It is a circuit diagram which shows the structure of the whole load current supply circuit by embodiment. It is a graph which shows a mode that the ratio (current mirror ratio) of load current I1 and the electric current for detection I2 which deviates from a predetermined ratio accompanying a temperature change. It is a figure which shows an example of resistance value of each resistance R1, R3-R6 of a load current detection circuit, and its temperature characteristic coefficient (resistance change rate per unit temperature).

Explanation of symbols

Tr1 Power transistor Tr2 Current detection transistor R1 Power transistor on-resistance R3, R4 Wiring resistance R5 Thick film conductor resistance R6 Thick film resistance 30 Current detection circuit 40 Load current supply circuit

Claims (6)

A power transistor for supplying a load current to the load;
A current detection transistor that is connected in parallel with the power transistor and that flows a detection current that is reduced by a predetermined ratio of a load current that the power transistor supplies to a load;
A current detection circuit for detecting a current value of a detection current flowing through the current detection transistor;
A first compensation resistor connected in series with the power transistor;
A second compensation resistor connected in series to the current detection transistor,
The current detection circuit has a negative feedback circuit including an operational amplifier for equalizing the operating environment of the power transistor and the current detection transistor,
The resistance value of the first compensation resistor is adjusted so as to reduce the offset voltage of the operational amplifier, and the second compensation resistor has a predetermined ratio between the load current and the detection current accompanying a temperature change. A load current supply circuit, wherein a resistance value thereof is adjusted so as to reduce fluctuations from the load. The current detection circuit, for detecting a current value of the load current, has a resistance which converts the detection current to a voltage, claim 1, the resistance value of the resistance, characterized in that adjustable The load current supply circuit described in 1. 3. The load current supply circuit according to claim 1, wherein resistance values of the first and second compensation resistors are adjusted by trimming. 4. 4. The load current supply circuit according to claim 3 , further comprising a terminal for measuring a resistance value of the second compensation resistor. Load current supply circuit according to any one of claims 1 to 4, characterized in that provided in the first and second compensating resistor circuit board. The adjustment of the first and second resistance value of the compensation resistance, as claimed in any one of claims 1 to 5 elements for constituting the load current supply circuit is characterized in that it is executed after being configured Load current supply circuit.

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