1. Field of the Invention
The present invention generally relates to a control circuit for a DC motor driving circuit. In particular, the invention is concerned with a control circuit for ensuring safety in operation of an H-bridge circuit employed for driving and controlling a DC (Direct Current) motor, and more particularly a control circuit which is capable of preventing occurrence of a through-current flow upon turning on/off of switching elements constituting the H-bridge circuit.
2. Description of the Related Art
For better understanding of the present invention, description will first be made in some detail of the background technique. FIG. 6 is a block diagram showing a hitherto known DC motor driving circuit which includes an H-bridge switching circuit for driving and controlling a DC motor, FIG. 7 is a circuit diagram showing a predriver circuit serving as a control circuit for driving and controlling the H-bridge switching circuit shown in FIG. 6, and FIG. 8 is a circuit diagram showing in detail a major portion of the predriver circuit shown in FIG. 6.
Referring to FIG. 6, a DC motor 1 is electrically connected in the form of an H-bridge circuit together with four switching elements 3a, 3b, 3c and 3d which serve for controlling the rotation speed of the DC motor 1 by controlling the duty cycle of a pulse current supplied to the DC motor 1 as well as for reversing the direction of rotation of the DC motor 1 by changing the flow direction of the current supplied to the motor 1.
As can be seen, the first and second switching elements 3a and 3b are connected to a power supply source V.sub.B, while the third and fourth switching elements 3c and 3d are connected to the ground potential. The DC motor 1 has one terminal connected to the junction between the first and third switching elements 3a and 3c and the other terminal connected to the junction between the second and fourth switching elements 3b and 3d. Further, the switching elements 3a, 3b, 3c and 3d have electrodes connected to output terminals of driving circuits 5a, 5b, 5c and 5d, respectively, which in turn have input terminals connected to a control unit 9 constituted by a computer or the like via a predriver circuit 7 which also serves as a control circuit for controlling the driving circuits 5a to 5d under the command of the control unit 9.
FIG. 7 shows a circuit configuration of the driving circuit 5a and the first switching element 3a, being understood that the other driving circuits 5b to 5d and the switching elements 3b to 3d are also implemented in substantially equivalent configurations, respectively. As can be seen in the figure, the first switching element 3a is constituted by an N-channel MOSFET (Metal Oxide Semiconductor Field Effect Transistor), while the switch driving circuit 5a is constituted by an emitter-grounded transistor 51 having a base connected to a corresponding output terminal of the predriver circuit 7 and a collector connected to the power supply source V.sub.B via a resistor 52, a transistor 53 having a base connected to the collector of the transistor 51, an emitter connected to the ground potential similarly to the transistor 51 and a collector connected to the power supply source V.sub.B via a resistor 54, and a transistor 55 having a base connected to a junction between a resistor 54 and the collector of the transistor 53, an emitter connected to an input terminal of the first switching element 3a and additionally connected to the collector of the transistor 53 via a diode 56 and a collector connected to the power supply source V.sub.B. The configurations and connection mentioned above apply similarly to the other switching elements 3b, 3c and 3d and the other switch driving circuits 5b, 5c and 5d, as mentioned above.
In operation, when the output of the predriver circuit 7 is at a low level, the transistor 51 is in the non-conducting state (OFF) with the transistor 53 conducting (ON), as a result of which the output level of the output transistor 55 is low. Consequently, the switching element 3a (3b, 3c or 3d) is in the non-conducting or OFF-state. On the other hand, when the output of the predriver circuit 7 becomes high, the transistor 51 is turned on while the transistor 53 is turned off, which results in that the output transistor 55 is turned off. Consequently, a high level output of the transistor 55 is applied to the switching input terminal of the switching element 3a (3b, 3c or 3d) which is thus turned on.
By combining appropriately the ON/OFF operations of the switching elements 3a, 3b, 3c and 3d, each controlled in the manner described above, speed control, reversing of rotational direction of the DC motor 1 as well as braking and stoppage thereof can be realized with the current supply to the DC motor 1 being correspondingly controlled.
More specifically, referring to FIG. 6, when the first and fourth switching elements 3a and 3d are in the OFF-state, a current flows from the power supply source V.sub.B to the ground by way of the second switching element 3b, the DC motor 1 and the third switching element 3c, whereby the DC motor 1 is caused to rotate in a forward direction.
On the other hand, when the first and fourth switching elements 3a and 3d are turned on with the second and third switching elements 3b and 3c being off, a current flows from the power supply source V.sub.B to the ground via the first switching element 3a, the DC motor 1 and the fourth switching element 3d, causing the DC motor 1 to rotate in the backward or reverse direction.
Further, when the first and second switching elements 3a and 3b are turned on with the third and fourth switching element 3c and 3d being turned off, both ends of the DC motor 1 are electrically coupled through the first and second switching elements 3a and 3b, whereby the DC motor 1 is braked. Furthermore, when the first to fourth switching elements 3a to 3d are all turned off, the DC motor 1 is caused to stop. Parenthetically, in practical applications, the second and third switching elements 3b and 3c are mutually interlocked to be turned on and off in combination.
In the switching circuit composed of the switching elements 3a, 3b, 3c and 3d connected in the H-bridge form as described above, a short-circuit fault may take place between the power supply source V.sub.B and the ground if the first and third switching elements 3a and 3c or the second and fourth switching elements 3b and 3d should simultaneously be turned on, which may occur in dependence on erroneous timing at which operations of these switching elements are changed over. Such being the circumstances, the predriver circuit 7 is provided and dedicated to the control of the switching elements 3a, 3b, 3c and 3d with a view to preventing the occurrence of such short-circuit fault.
Description will now turn to the predriver circuit 7. FIG. 8 is a circuit diagram showing an exemplary configuration of a predriver circuit known heretofore. As can be seen in the figure, the predriver circuit 7 includes first to third input terminals 70a, 70b and 70c and four NOR circuits 71a, 71b, 71c and 71d provided at the output side and connected to the switch driving circuits 5a, 5b, 5c and 5d, respectively, wherein the input sides of the four NOR circuits 71a, 71b, 71c and 71d are connected in such a manner as illustrated in FIG. 8.
More specifically, the first input terminal 70a is connected to one input terminal of the first and fourth NOR circuits 71a and 71d, respectively, via an amplifier 72a and at the same time connected to one input terminal of the second NOR circuit 71b via an inverter 73a. On the other hand, the second input terminal 70b is connected to another one of the input terminals of the second NOR circuit 71b and one of the input terminals of the third NOR circuit 71c, respectively, and additionally connected to another input terminal of the first NOR circuit 71a via an inverter 73b.
Further, the first to third terminals 70a, 70b and 70c are connected to input terminals of a flip-flop circuit 74 via amplifiers 72a, 72b and 72c and inverters 73a, 73b and 73c, respectively, wherein output terminals of the flip-flop circuits 74 are connected to other input terminals of the third and fourth NOR circuits 71c and 71d, respectively.
Additionally, the first and second input terminals 70a and 70b are connected to input terminals of a delay circuit 75 via the amplifiers 72a and 72b and the inverters 73a and 73b, respectively, wherein output terminals of the delay circuit 75 are connected to the other input terminals of the first to fourth NOR circuits 71a, 71b, 71c and 71d via inverters 76a, 76b, 76c and 76d, respectively,
The delay circuit 75 is comprised of four series connections, each of which includes a transistor 10, a constant current circuitry 11 and a capacitor 12 connected in parallel to the transistor 10, wherein the four series connections are connected in parallel with one another. In response to the state change (i.e., from ON-state to OFF-state or from OFF-state to ON-state) of the input signal supplied from the control unit 9 to the first and second input terminals 70a and 70b of the predriver circuit 7, the output levels of the NOR circuits 71a, 71b, 71c and 71d constituting the output stage of the predriver circuit 7 are caused to change, whereby the switching elements 3a, 3b, 3c and 3d are correspondingly turned on and off. In that case, the delay circuit 75 serves to delay the operations of the switching elements 3a, 3b, 3c and 3d such that the first and third switching elements 3a and 3c or the second and fourth switching elements 3b and 3d are positively prevented from being simultaneously turned on for the purpose of excluding the short-circuit fault mentioned hereinbefore. As a consequence, when the operating state of the DC motor 1 is to be changed over, the power supply to the DC motor 1 is temporarily or transiently cut off.
FIG. 9 shows signal waveforms at various circuit points in the predriver circuit 7 shown in FIG. 8 for illustrating operation thereof on the assumption that operation state of the DC motor 1 is changed over in the sequence of stop, forward rotation, braking, backward (reverse) rotation, braking, forward rotation and backward rotation in this order. More specifically, in FIG. 9, waveforms S1 to S3 illustrate changes in the output signals (control signals) of the control unit 9 which are applied to the first to third input terminals 70b, 70a and 70c, respectively, of the predriver circuit 7, while A to H represent signal waveforms at circuit points A to H shown in FIG. 8, respectively. Further, waveforms S5a to S5d represent output signals supplied from the first to fourth NOR circuits 71a, 71b, 71c and 71d to the first to fourth switch driving circuits 5a, 5b, 5c and 5d respectively.
The period during which the electric power supply to the DC motor 1 is cut off is referred to as the dead time. Usually, this dead time is optimally selected in dependence on the type of the DC motor 1, the switching elements 3a, 3b, 3c and 3d and other elements employed actually. With the circuit arrangement shown in FIG. 8, the dead time may be selectively determined by varying the capacity of the capacitor 12.
In the predriver circuit 7 for the switching elements 3a, 3b, 3c and 3d which constitute the H-bridge switching circuit known heretofore, at least two capacitors 12 (four capacitors in the case of the illustrated example) are required to be incorporated in the delay circuit 75 for preventing the short-circuit fault from occurring in the H-bridge switching circuit. In this conjunction, it is noted that when the predriver circuit 7 is to be implemented in the form of an integrated circuit, the capacitors 12 will have to be provided externally so that the individual capacitances can be adjusted by exchanging the capacitors in conformance with the switching elements used in the H-bridge circuit as well as the DC motor. In that case, the number of terminals of the integrated circuit for electrical connection of the capacitors 12 will necessarily be increased in dependence on the number of the capacitors to be externally attached, which provides a great obstacle to miniaturization of the integrated predriver circuit. Needless to say, increase in the number of circuit elements involves a problem that high operation reliability of the whole circuit can not be ensured.