JP2015046710A - Load drive circuit - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a load drive circuit that can detect an overcurrent not with the use of a separate overcurrent detection circuit but with the use of part of the load drive circuit.SOLUTION: A load drive circuit includes: a bridge circuit 100 for controlling on/off a current Im flowing through a motor 110; a drive circuit 200 for driving the bridge circuit 100; a microcomputer 300 for generating a drive instruction signal Vs giving an instruction to drive the motor 110; a current feedback circuit 400 for comparing a current detection signal Vi based on the load current Im with a desired value signal Vref and outputting a comparison output signal Vc; a holding circuit 500 for holding a current detection result on the basis of the drive instruction signal Vs and the comparison output signal Vc; and a gate circuit 600 for driving the drive circuit 200 on the basis of the drive instruction signal Vs and a latch output signal Vr output from the holding circuit 500. The microcomputer 300 performs overcurrent detection by time measurement based on the drive instruction signal Vs and the latch output signal Vr.

Description

  The present invention relates to a load drive circuit, and more particularly to a load drive circuit capable of detecting an overcurrent without separately providing an overcurrent detection unit.

  2. Description of the Related Art Conventionally, there is a load drive circuit that controls an electric current flowing through a load such as a motor, which includes an overcurrent detection circuit (see, for example, Patent Document 1).

  This load driving circuit includes a shunt resistor that detects a current flowing through the load as a voltage value, and a comparator that compares the detected voltage with a threshold voltage. The threshold voltage is generated by a threshold generation circuit, and is set so that the threshold voltage is increased as the power supply voltage value is larger and the threshold voltage is increased as the load temperature is lower. Thereby, desired hysteresis characteristics can be maintained, and an overcurrent detection operation and a return operation from an overcurrent detection state can be stably performed.

JP 2013-62721 A

  However, the conventional load drive circuit does not perform overcurrent detection using a part of the load drive circuit, and it is necessary to separately provide an overcurrent detection circuit in addition to the load drive circuit. Therefore, there is a problem that the circuit space increases and the number of parts increases.

  Accordingly, an object of the present invention is to provide a load drive circuit capable of detecting an overcurrent by using a part of the load drive circuit without separately providing an overcurrent detection circuit.

[1] In order to achieve the above object, the present invention generates a bridge circuit for controlling on / off of a current flowing through a load, a drive circuit for driving the bridge circuit, and a drive instruction signal for instructing driving of the load. A current feedback circuit configured by a control unit, a current detection resistor and a comparator connected in series to the load, and outputting a comparison output signal by comparing a current detection signal of a load current with a target value signal; and the drive A holding circuit that holds a current detection result based on an instruction signal and the comparison output signal; a gate circuit that drives the drive circuit based on the drive instruction signal and a latch output signal output from the holding circuit; And the controller performs overcurrent detection by measuring time based on the drive instruction signal and the latch output signal. To provide the road.

[2] The control unit determines whether or not there is an overcurrent with reference to a reference time set between a time to reach the target current at the normal current and a time to reach the target current at the overcurrent. The load driving circuit according to the above [1] may be characterized.

[3] In addition, in the above [1] or [2], the control unit is a one-chip microcomputer, and the time measurement is performed by a timer unit unit built in the one-chip microcomputer. The load driving circuit may be used.

  According to the present invention, it is possible to provide a load driving circuit capable of detecting an overcurrent using a part of the load driving circuit without separately providing an overcurrent detection circuit.

FIG. 1 is a circuit configuration diagram of a load driving circuit according to an embodiment of the present invention. FIG. 2 is a diagram showing each signal waveform of each part during normal operation of the load driving circuit according to the embodiment of the present invention. FIG. 3 is a diagram showing current detection signal waveforms at the time of normal operation and at the time of occurrence of a short-circuit failure (when an overcurrent occurs). FIG. 4 is a flowchart showing an overcurrent detection operation in the load drive circuit. FIG. 5 is a diagram showing each signal waveform of each part when a short fault occurs (when an overcurrent occurs) in the load driving circuit according to the embodiment of the present invention.

(Overall configuration of load drive circuit 1)
FIG. 1 is a circuit configuration diagram of a load driving circuit according to an embodiment of the present invention.

  The load drive circuit 1 according to the embodiment of the present invention performs on / off control of a current Im flowing through the motor 110 as a load, a drive circuit 200 that drives the bridge circuit 100, and a drive instruction for the motor 110. A microcomputer 300 serving as a control unit that generates the drive instruction signal Vs, a shunt resistor 410 that is a current detection resistor connected in series to the motor 110, and a comparator 420 are configured. The current detection signal Vi of the load current Im and the target A current feedback circuit 400 that outputs a comparison output signal Vc by comparison with the value signal Vref, a holding circuit 500 that holds a current detection result based on the drive instruction signal Vs and the comparison output signal Vc, and a drive instruction signal Vs. The drive circuit 200 is driven based on the latch output signal Vr output from the circuit 500. It has the bets circuit 600, the microcomputer 300 is schematically configured to perform overcurrent detection by the time measurement based on a drive command signal Vs and the latch output signal Vr.

  The bridge circuit 100 includes four MOSFETs, and has a bridge configuration in which the motor 110 is connected between the FET1 and the FET3 and between the FET2 and the FET4. When FET1 and FET4 are turned on and FET2 and FET3 are turned off, a motor current flows in the Im direction shown in FIG. 1, and the motor 110 rotates forward. Conversely, when FET1 and FET4 are OFF and FET2 and FET3 are ON, the motor current flows in the reverse direction, and the motor 110 rotates in the reverse direction. The rotation of the motor 110 is controlled by the combination and timing of these MOSFETs. Note that the ON / OFF combination and timing of the MOSFETs are controlled by inputting predetermined on / off signals from the drive circuit 200 and the gate circuit 600.

  The drive circuit 200 has an input side connected to the gate circuit 600 and an output side connected to the bridge circuit 100. Each FET is subjected to switching control based on the drive signal Vd output from the gate circuit 600, whereby a current is supplied from the power supply voltage 12V to the bridge circuit 100 and the motor 110.

  The microcomputer 300 includes a drive instruction signal generator 310 that generates a drive instruction signal Vs for instructing driving of the motor 110, a motor drive direction controller 320, an NchFET control signal generator 330, a feedback input unit 340, and a target value signal generator 350. A one-chip microcomputer that functions as a control unit including a timer unit unit 360 and the like.

  The drive instruction signal generator 310 generates a drive instruction signal Vs for instructing driving of the motor 110 as a PWM (Pulse Width Modulation) signal. For example, by using the PWM function of the microcomputer 300 or the like, it is possible to generate the drive instruction signal Vs that inverts and outputs the output from the Hi level to the Lo level at regular intervals. The output drive instruction signal Vs is connected to the holding circuit 500 and the gate circuit 600.

  The motor drive direction control unit 320 outputs a motor drive direction control signal Vm that is a signal for controlling the rotation direction of the motor 110. The motor drive direction control signal Vm is input to the AND circuit 610 of the gate circuit 600.

NchFET control signal generator 330 outputs a NchFET control signal _{V F} which is a signal for controlling the rotation direction of the motor 110. The NchFET control signal _{V F} at a predetermined timing Hi, Lo signal is input to the FET 3, FET 4, ON the MOSFET, to OFF control. The bridge circuit 100 is controlled together with the motor drive direction control signal Vm described above to control the rotation direction of the motor 110.

  The feedback input unit 340 receives the latch output signal Vr output from the holding circuit 500. The input latch output signal Vr (= current feedback signal) is input to the timer unit 360 in the microcomputer.

  The target value signal generator 350 is a DC voltage signal that indicates a target current value to the current feedback circuit 400, and inputs a target value signal Vref serving as a reference voltage (threshold value) of the comparator 420 to the comparator 420. By adjusting the target value signal Vref, the motor current Im can be controlled to adjust the steady rotational speed of the motor.

  The timer unit unit 360 is a timer unit unit built in the microcomputer 300. The timer unit 360 receives the drive instruction signal Vs and the latch output signal Vr (= current feedback signal), and measures the time from when the drive instruction signal Vs rises to when the latch output signal Vr rises.

  The current feedback circuit 400 includes a shunt resistor 410 and a comparator (comparator) 420 connected in series to the motor 110, and outputs a comparison output signal Vc by comparing the current detection signal Vi of the motor current Im with the target value signal Vref. To do. The output of the current feedback circuit 400 is connected to the input side of the holding circuit 500 as a comparison output signal Vc. For example, when the bridge circuit 100 is driven by turning on the drive circuit 200 and a current flows through the motor 110, the same current (Im) as the current flowing through the motor 110 also flows through the shunt resistor 410 connected to the motor 110. Due to the motor current Im flowing through the resistor Ra of the shunt resistor 410, a current detection signal Vi = Ra × Im is generated at both ends of the shunt resistor 410. The comparator 420 compares the current detection signal Vi and the target value signal Vref generated by the target value signal generator 700. When the target value signal Vref <the current detection signal Vi, the comparison output signal Vc becomes Lo level. The comparison output signal Vc is input to the holding circuit 500.

  The holding circuit 500 receives the drive instruction signal Vs from the microcomputer 300 and the comparison output signal Vc from the current feedback circuit 400, and latches the current detection result based on the drive instruction signal Vs and the comparison output signal Vc ( Hold. The latch output signal Vr is input to the feedback input unit 340 and also input to the gate circuit 600 via the drive stop Tr 620.

  The gate circuit 600 outputs a drive signal Vd based on the drive instruction signal Vs and the latch output signal Vr output from the holding circuit 500. In this embodiment, since the direction of motor rotation is also controlled, the AND circuit 610 outputs the drive signal Vd as the logical product of the drive instruction signal Vs and the latch output signal Vr or the motor drive direction control signal Vm. Driving the drive circuit 200 with the drive signal Vd energizes the motor 110. In addition to the gate control using the motor drive direction control signal Vm, the NchFET control signal generator 330 controls the rotation direction of the motor.

(Normal operation of the load drive circuit 1)
FIG. 2 is a diagram showing signal waveforms at various parts during normal operation of the load driving circuit according to the embodiment of the present invention. The normal operation (constant current control operation) of the load drive circuit 1 will be described in the order of the following numbers (1) to (9) along the waveform diagram of main points during normal operation shown in FIG.

(1) The drive instruction signal Vs becomes a Hi level output (ON signal Von).
(2) The drive signal Vd is turned on from the gate circuit 600, the FETs 1 and 4 are turned on, and the FETs 2 and 3 are turned off.
(3) The current Im flows through the motor 110 and the current detection signal Vi is generated by the shunt resistor 410.
(4) When the current detection signal Vi exceeds the target value signal Vref, the latch output signal Vr becomes Hi level.
(5) As a result, the FETs 1 and 4 are turned OFF, the motor 110 is stopped, and the current detection signal Vi becomes 0.
(6) After a predetermined period, the drive instruction signal Vs switches to the Lo level. (The ON signal Von changes to the latch clear signal Vclr.)
(7) The holding state of the holding circuit 500 is released at the switching edge (latch clear signal Vclr) from the Hi level to the Lo level of the drive instruction signal Vs.
(8) During the Lo level period of the drive instruction signal Vs (latch clear signal Vclr period), the Lo level of the drive signal Vd is continued.
(9) After a predetermined period, the drive instruction signal Vs is switched to the Hi level (becomes the ON signal Von).
Thereafter, the operations of (1) to (9) are repeated, and the current feedback circuit 400 increases and decreases while the actual motor current increases and decreases as shown in FIG. 2 in the respective periods (3) and (5). The motor 110 is driven at a constant current with a current value corresponding to the target value signal Vref specified in.

(Operation when load drive circuit 1 short-circuit failure occurs (overcurrent occurs))
FIG. 3 is a diagram showing current detection signal waveforms at the time of normal operation and at the time of occurrence of a short-circuit failure (when an overcurrent occurs).

The current detection signal Vi during normal operation in FIG. 3 is the same as the current detection signal in FIG. During normal operation, current flows through the motor 110, so that the current gradually increases due to resistance and coil components. When τe is the ratio of the inductance and the armature resistance, the motor current Im is expressed by the following equation.
Motor current Im = (power supply voltage 12V / armature resistance) × (1−e ^{−t / τe} )
From the above equation, the time T1 required to reach the target current set by the target value signal Vref during normal operation can be obtained.

  On the other hand, when a short circuit failure occurs, no current flows through the motor 110, so there is no influence of the coil component and the current flows sharply. As shown in FIG. 3, the current detection signal Vi reaches the target value signal Vref in the time T2 <T1.

  From the above, as shown in FIG. 3, the reference time Ta satisfying T2 <Ta <T1 is set as the overcurrent detection determination time.

  As shown in FIG. 2, the time until the current detection signal Vi reaches the target value signal Vref from 0 (zero) is from the rising edge of the drive instruction signal Vs to the rising edge of the latch output signal Vr (= current feedback signal). Equivalent to time to time. Therefore, the microcomputer 300 counts the time from the rising edge of the drive instruction signal Vs to the rising edge of the latch output signal Vr (= current feedback signal), so that a normal current is flowing in the motor 110 or a short circuit failure occurs. It can be determined whether or not an overcurrent flows.

  FIG. 4 is a flowchart showing an overcurrent detection operation in the load drive circuit. Hereinafter, a method of overcurrent detection by the microcomputer 300 will be described based on this flowchart.

  First, the microcomputer 300 determines whether or not the rising of the drive instruction signal Vs has been detected (Step 1). When the rising edge of the drive instruction signal Vs is detected, the process proceeds to Step 2, and when not detected, Step 1 is repeatedly executed.

  In Step 2, counting by the timer unit unit 360 is started. Counting is started from the state where the internal counter value TCNT is reset.

  The microcomputer 300 determines whether or not the rising edge of the latch output signal Vr is detected. If the rising edge of the latch output signal Vr is detected, the process proceeds to Step 4. If not detected, 1 is added to the counter value TCNT and Step 3 is repeatedly executed (Step 3).

  In Step 4, the counting by the timer unit unit 360 is stopped.

  The microcomputer 300 determines whether the time T corresponding to the counter value TCNT is equal to or less than the reference time Ta. If the time T is less than or equal to the reference time Ta, the process proceeds to Step 6, and if the time T exceeds the reference time Ta, the determination operation is terminated (Step 5).

  The microcomputer 300 controls the drive instruction signal Vs to Low to stop the motor 110. That is, the microcomputer 300 determines that an overcurrent has occurred, stops the drive instruction signal Vs, which is a PWM signal having a predetermined period, and changes it to the Low level (Step 6). Thereby, the motor 110 stops and the safety at the time of a short circuit failure is ensured.

  FIG. 5 is a diagram showing signal waveforms at various parts when a short-circuit failure occurs (when an overcurrent occurs) in the load drive circuit according to the embodiment of the present invention. The operation of the load drive circuit 1 will be described in the order of the following numbers (1) to (6) along the waveform diagram of main points at the time of occurrence of a short fault shown in FIG.

(1) The drive instruction signal Vs becomes a Hi level output (ON signal Von).
(2) The drive signal Vd is turned on from the gate circuit 600, the FETs 1 and 4 are turned on, and the FETs 2 and 3 are turned off.
(3) A short circuit failure occurs. Since no current flows through the motor 110, the actual motor current is zero.
(4) However, when a short current flows through the shunt resistor 410, the current detection signal Vi rapidly increases.
(5) The microcomputer 300 determines that the time T when the current detection signal Vi reaches the target value signal Vref is equal to or less than the reference time Ta.
(6) The microcomputer 300 stops the motor 110 by controlling the drive instruction signal Vs to Low. As a result, the FETs 1 and 4 are turned off, and the motor 110 is stopped.
By the operations (1) to (6), the driving of the motor 110 is stopped when a short circuit failure occurs (overcurrent occurs).

(Effect of the embodiment of the present invention)
The load driving circuit 1 configured as described above has the following effects.
(1) Since the load driving circuit 1 according to the embodiment of the present invention counts and detects the time when the current detection signal Vi reaches the target value signal Vref by the microcomputer 300, the overcurrent at the time of occurrence of a short circuit failure is detected. Can be detected. Thus, it is possible to provide a load driving circuit capable of detecting an overcurrent using a part of the load driving circuit without separately providing an overcurrent detection circuit.
(2) The time for the current detection signal Vi to reach the target value signal Vref by the microcomputer 300 is counted by measuring the time from the rise of the drive instruction signal Vs to the rise of the latch output signal Vr (current feedback signal). That is, since overcurrent detection is performed by a microcomputer using a part of the load driving circuit 1, it is not necessary to provide an overcurrent detection circuit separately.
(3) Since the reference time Ta satisfying T2 <Ta <T1 is set as the overcurrent detection determination time, the overcurrent detection can be reliably performed.
(4) Since it is not necessary to provide an overcurrent detection circuit separately, the circuit space can be reduced, and the board can be downsized and the board space can be effectively used.
(5) By reducing the circuit space, reducing the size of the board, and effectively using the board space, it is effective in improving the product cost.

  Although the embodiment of the present invention has been described above, the above embodiment is merely an example and does not limit the invention according to the claims. These novel embodiments and modifications thereof can be implemented in various other forms, and various omissions, replacements, changes, and the like can be made without departing from the scope of the present invention. . Moreover, not all the combinations of features described in the above embodiments are essential to the means for solving the problems of the invention. Further, the above-described embodiment and its modifications are included in the scope and gist of the invention, and are included in the invention described in the claims and the equivalents thereof.

DESCRIPTION OF SYMBOLS 1 ... Load drive circuit 100 ... Bridge circuit 110 ... Motor 200 ... Drive circuit 300 ... Microcomputer 310 ... Drive instruction signal generation part 320 ... Motor drive direction control part 330 ... NchFET control signal generation part 340 ... Feedback input part 350 ... Target value signal Generator 360 ... Timer unit 400 ... Current feedback circuit 410 ... Shunt resistor 420 ... Comparator 500 ... Holding circuit 600 ... Gate circuit 610 ... AND circuit 620 ... Tr for stopping driving
Im: Motor current Vs ... Drive instruction signal Vref ... Target value signal Vc ... Comparison output signal Vd ... Drive signal Vr ... Latch output signal Vi ... Current detection signal Vm ... Motor drive direction control signal Von ... ON signal Vclr ... Latch clear signal

Claims (3)

A bridge circuit that controls on / off of the current flowing through the load;
A drive circuit for driving the bridge circuit;
A control unit for generating a drive instruction signal for instructing driving of the load;
A current feedback circuit configured by a current detection resistor and a comparator connected in series to the load, and outputting a comparison output signal by comparing a current detection signal of the load current with a target value signal;
A holding circuit for holding a current detection result based on the drive instruction signal and the comparison output signal;
A gate circuit that drives the drive circuit based on the drive instruction signal and a latch output signal output from the holding circuit;
The load drive circuit, wherein the control unit performs overcurrent detection by measuring time based on the drive instruction signal and the latch output signal.   The control unit determines whether or not there is an overcurrent with reference to a reference time set between a time to reach the target current at a normal current and a time to reach the target current at an overcurrent. The load drive circuit according to claim 1.   3. The load driving circuit according to claim 1, wherein the control unit is a one-chip microcomputer, and the time measurement is performed by a timer unit unit built in the one-chip microcomputer.

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