Abstract:
A switching control signal generator generates a switching control signal which results in a soft switching operation in a small precision motor such as a brushless DC motor, without use of a separate switching noise reduction circuit, includes a circuit for supplying a linear slope value of the current supplied during a switching period of the switch using a back electromotive force for each phase and a neutral point voltage of the motor, and a circuit for adjusting a switching period by the thus supplied slope value.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a circuit for generating a switching control signal for a switch connected to each phase of a small precision motor, and more particularly, to a switching control signal generator for generating an on off switching control signal for a switch connected to each phase of a three phase brushless DC motor (herein called a BLDC motor). 
     2. Description of the Prior Art 
     Most small precision motors currently used in the electronic industry are BLDC motors. In order to start such motors, a magnetic pole detector has been commonly employed for detecting the position of a rotor. However, the current trend is to rely on a back electromotive force generated during the rotation of a motor to start the BLDC motor, without use of a magnetic pole detector. In particular, when a small BLDC motor is driven using a motor driving integrated circuit (IC), a current-type driving method is typically adopted in which a square wave current is applied to the motor. 
     In such a case of driving the motor by applying a square wave current, in terms of the characteristics of the square wave, since a switch connected to each phase of the motor is instantaneously turned on or off, the back electromotive force caused by the internal inductance of each phase increases substantially, thereby generating a significant amount of switching noise. This in turn can result in erroneous operation of the motor driving IC and the motor. 
     In an effort to overcome such a problem, a method for controlling an on/off state of the switch of each motor phase has been proposed in which a filter is formed in the exterior of the motor using a RC element and a point is searched at which the back electromotive force of each phase is electrically delayed by 30° after the back electromotive force of each phase passes by a zero point. However, poor frequency response characteristics cause an angle of delay from initialization to normal operation to vary with the speed of the rotating motor, and thus this method cannot exert an optimum performance. Another solution is to use a digital circuit to search for the current timing. However, the method requires the separate provision of a circuit for detecting a zero point and a circuit for generating an angle of delay. 
     SUMMARY OF THE INVENTION 
     Accordingly, the present invention has been made with a view toward overcoming the drawbacks occurring in the prior art, and an object of the present invention is to provide a switching control signal generator for generating a signal which results in a soft switching operation so as to reduce a switching noise, without use of a separate circuit, in a small precision motor such as a BLDC motor. 
     To accomplish the above object, there is provided a circuit for generating a switching control signal for a switch of each phase of a small precision motor, comprising: means for supplying a linear slope value of the current supplied during a switching period of the switch using a back electromotive force for each phase and a neutral point voltage of the motor; and means for adjusting a switching period of a switching control signal in accordance with the slope value supplied from the slope value supplying means. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The above objects, and other features and advantages of the present invention will become more apparent by describing in detail a preferred embodiment thereof with reference to the attached drawings, in which: 
     FIGS. 1A through 1D are detailed diagrams of a switching control signal generator for a small precision motor according to the present invention; 
     FIG. 2 shows the relationship between the back electromotive force of the motor and twelve operational modes; and 
     FIG. 3 shows the relationship between the back electromotive force of the motor and various voltages output from the switching control signal generator according to the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     FIGS. 1A through 1C show slope value supply circuits 101, 102 and 103 which receive the back electromotive forces Uemf, Vemf and Wemf and neutral point voltages for each respective phase of a brushless DC motor and which supply a slope value causing the current supplied during the switching of each phase to have linear characteristics. FIG. 1A shows the slope value supply circuit 101 for the back electromotive force Uemf, FIG. 1B shows the slope value supply circuit 102 for the back electromotive force Vemf, and FIG. 1C shows the slope value supply circuit 103 for the back electromotive force Wemf. FIG. 1D shows a switching period adjusting circuit 110 for adjusting a switching period according to the value output from the slope value supplying circuits 101, 102 and 103. 
     The slope value supply circuits shown in FIGS. 1A through 1C are constituted by three input amplifiers OP1, OP2 and OP3 for receiving the electromotive forces Uemf, Vemf and Wemf and neutral point voltages for each respective phase of the motor, three diodes D1, D2 and D3 for outputting respective voltages Uoh, Voh and Woh indicative of the slope during the switching of each phase, and output from the input amplifiers OP1, OP2 and OP3 as voltages Uol, Vol and Wol each reduced by 0.7V, and resistors R1 through R12, connected as shown to the input amplifiers OP1, OP2 and OP3, for adjusting the respective amplification factors of the differences between the applied back electromotive forces Uemf, Vemf and Wemf and neutral point voltages of each phase of the motor. 
     The switching period adjusting circuit 110 shown in FIG. 1D is constituted by six comparators COM1 through COM6 each having an emitter coupled pair for receiving the voltages Uoh, Uol, Voh, Vol, Woh and Wol output from the respective input amplifiers OP1, OP2 and OP3 of the slope value supply circuits 101, 102 and 103, three comparators COM7 through COM9 each having an emitter coupled pair for receiving and comparing the voltages Uol, Vol and Wol output via the diodes D1, D2 and D3, and an output voltage level determining circuit 111 for determining the level of the output voltages supplied to the switch of each respective phase, as adjusted and output by the comparators COM 1 through COM9. 
     Here, the output voltage level determining circuit 111 is constituted by resistances Rs connected between a supply power Vcc and collectors of the respective comparators COM1, COM2 and COM3, and current sources IS connected between emitters of the respective comparators COM7, COM8 and COM9 and a ground port. The output voltages Uout, Vout and Wout are generated via the supply power Vcc and resistances Rs. 
     The operation of the circuit configured as above will now be described. 
     First, the electromotive forces Uemf, Vemf and Wemf and the neutral point voltages are supplied from each respective phase of the motor. The electromotive forces Uemf, Vemf and Wemf are supplied to one input terminal of the respective input amplifiers OP1, OP2 and OP3, and the neutral point voltages are at the same time supplied to the other input terminal of the respective input amplifiers OP1, OP2 and OP3. The respective input amplifiers OP1, OP2 and OP3 amplify the differences between the corresponding applied back electromotive forces and neutral point voltages according to amplification factors set by the resistances R1 through R12 connected to the respective input terminals and feedback paths of the amplifiers OP1, OP2 and OP3. At this time, if the amplification factor is large, a switching control signal which is akin to a square wave is supplied which remains for a short time in a linear region of the on/off switching period. If the amplification factor is small, the switching control signal remains a longer time in a linear region of the on/off switching period. In other words, if the amplification factor is large, the on/off switching period is sharp. However, if the amplification factor is small, the on/off switching period varies a blunt manner. Therefore, as is intended by the present invention, in order to attain a soft switching operation, the values of the resistances R1 through R12 connected to the respective input amplifiers OP1, OP2 and OP3 are set so as to result in a small amplification factor. By accordingly setting the values of the resistances R1 through R12, a soft switching operation is achieved during a high speed rotation of the motor. 
     As described above, the difference values Uoh, Voh and Woh between the back electromotive forces and neutral point voltages of each respective phase, amplified and output from the respective input amplifiers OP1, OP2 and OP3, are input to bases of transistors Q1 through Q12 provided in the comparators COM1 through COM6 of the switching period adjusting circuit 110. The difference values Uol, Vol and Wol output via the diodes D1, D2 and D3 are transmitted to one input terminal of the respective comparators COM7, COM8 and COM9 of the switching period adjusting circuit 110. 
     In the comparator COM1, configured by an emitter coupled pair, an on/off state of the transistors Q1 and Q2 is determined by the respective magnitudes of the difference values Uoh and Voh applied to the bases of the respective transistors Q1 and Q2. 
     In the comparator COM2, an on/off state of the transistors Q3 and Q4 is determined by the respective magnitudes of the difference values Voh and Woh applied to the bases of the respective transistors Q3 and Q4. 
     In the comparator COM3, an on/off state of the transistors Q5 and Q6 is determined by the respective magnitudes of the difference values Woh and Uoh applied to the bases of the respective transistors Q5 and Q6. 
     In the comparator COM4, an on off state of the transistors Q7 and Q8 is determined by the respective magnitudes of the difference values Uoh and Voh applied to the bases of the respective transistors Q7 and Q8. 
     In the comparator COM5, an on/off state of the transistors Q9 and Q10 is determined by the respective magnitudes of the difference values Voh and Woh applied to the bases of the respective transistors Q9 and Q10. 
     In the comparator COM6, an on/off state of the transistors Q11 and Q12 is determined by the respective magnitudes of the difference values Woh and Uoh applied to the bases of the respective transistors Q11 and Q12. 
     In the comparator COM7, an on off state of the transistors Q13 and Q14 is determined by the respective magnitudes of the output Uol of the input amplifier OP1 and the neutral point voltage, and a collector of the transistors Q13 is coupled to an emitter of the comparator COM1 and a collector of the transistors Q14 is coupled to an emitter of the comparator COM4. 
     In the comparator COM8, an on/off state of the transistors Q15 and Q16 is determined by the respective magnitudes of the output Vol of the input amplifier OP2 and the neutral point voltage applied to bases of the respective transistors Q15 and Q16, and a collector of the transistor Q15 is coupled to an emitter of the comparator COM2 and a collector of the transistor Q16 is coupled to an emitter of the comparator COM5. 
     In the comparator COM9, an on/off state of the transistors Q17 and Q18 is determined by the respective magnitudes of the output Wol of the input amplifier OP3 and the neutral point voltage applied to bases of the respective transistor Q17 and Q18, and a collector of the transistors Q17 is coupled to an emitter of the comparator COM3 and a collector of the transistor Q18 is coupled to an emitter of the comparator COM6. 
     When a period of the back electromotive force induced during the rotation of the three-phase BLDC motor is divided into 12 modes, as shown in FIG. 2, the switched states of the transistors Q1 through Q18 of the comparators COM 1 through COM8 are as indicated in Table 1 below. The output voltages Uout, Vout and Wout are as indicated in Table 2 below. 
     
                                           TABLE 1__________________________________________________________________________1     2  3  4  5  6  7  8  9  10 11 12__________________________________________________________________________Q1 ON ON ON ON OFF             OFF                OFF                   OFF                      OFF                         OFF                            ON ONQ2 OFF OFF    OFF       OFF          ON ON ON ON ON ON OFF                               OFFQ3 OFF OFF    ON ON ON ON ON ON OFF                         OFF                            OFF                               OFFQ4 ON ON OFF       OFF          OFF             OFF                OFF                   OFF                      ON ON ON ONQ5 OFF OFF    OFF       OFF          OFF             OFF                ON ON ON ON ON ONQ6 ON ON ON ON ON ON OFF                   OFF                      OFF                         OFF                            OFF                               OFFQ7 ON ON ON ON OFF             OFF                OFF                   OFF                      OFF                         OFF                            ON ONQ8 OFF OFF    OFF       OFF          ON ON ON ON ON ON OFF                               OFFQ9 OFF OFF    ON ON ON ON ON ON OFF                         OFF                            OFF                               OFFQ10   ON ON OFF       OFF          OFF             OFF                OFF                   OFF                      ON ON ON ONQ11   OFF OFF    OFF       OFF          OFF             OFF                ON ON ON ON ON ONQ12   ON OFF    ON ON ON ON OFF                   OFF                      OFF                         OFF                            OFF                               OFFQ13   ON ON ON ON ON OFF                OFF                   OFF                      OFF                         OFF                            OFF                               ONQ14   OFF ON OFF       OFF          OFF             ON ON ON ON ON ON OFFQ15   OFF OFF    OFF       ON ON ON ON ON ON OFF                            OFF                               OFFQ16   ON ON ON OFF          OFF             OFF                OFF                   FF OFF                         ON ON ONQ17   ON OFF    OFF       OFF          OFF             OFF                OFF                   ON ON ON ON ONQ18   OFF ON ON ON ON ON ON OFF                      OFF                         OFF                            OFF                               OFF__________________________________________________________________________ 
    
     
                       TABLE 2______________________________________Mode/Output   Uout        Vout         Wout______________________________________1       Vcc-(2 × Rs × Is)               Vcc          Vcc-(Rs × Is)2       Vcc-(2 × Rs × Is)               Vcc          Vcc-(Rs × Is)3       Vcc-(2 × Rs × Is)               Vcc-(Rs × Is)                            Vcc4       Vcc-(2 × Rs × Is)               Vcc-(Rs × Is)                            Vcc5       Vcc-(Rs × Is)               Vcc-(2 × Rs × Is)                            Vcc6       Vcc-(Rs × Is)               Vcc-(2 × Rs × Is)                            Vcc7       Vcc         Vcc-(2 × Rs × Is)                            Vcc-(Rs × Is)8       Vcc         Vcc-(2 × Rs × Is)                            Vcc-(Rs × Is)9       Vcc         Vcc-(Rs × Is)                            Vcc-(2 × Rs × Is)10      Vcc         Vcc-(Rs × Is)                            Vcc-(2 × Rs × Is)11      Vcc-(Rs × Is)               Vcc          Vcc-(2 × Rs × Is)12      Vcc-(Rs × Is)               Vcc          Vcc-(2 × Rs × Is)______________________________________ 
    
     As is apparent from Table 2, the values of the output voltages Uout, Vout and Wout for the respective modes are determined by the resistances Rs coupled between the supply voltage Vcc and the collectors of the respective transistors Q1, Q3 and Q5 of the comparators COM1, COM2 and COM3, and by the current sources IS coupled to the emitters of the respective comparators COM7, COM8 and COM9. 
     In the case two of the outputs of the input amplifiers OP1, OP2 and OP3 are compared using the emitter coupled pair, since the ratio of the current flowing in the two switches of the emitter coupled pair, e.g., transistors Q1 and Q2, is changed exponentially, the output voltages are changed exponentially at the moment of switching. 
     In other words, while the output difference of the input amplifiers is in the linear region of the emitter coupled pair, i.e., the region where the difference of the electromotive forces ranges from -2×Vt to 2×Vt (here, Vt represents a thermal voltage), the output voltages have rhombic waveforms, as shown in FIG. 3. If the output difference of the input amplifiers exceeds 4×Vt, the current flow is substantially in one switch only. 
     The output voltages Uout, Vout and Wout generated in the circuit according to the present invention are input to a transconductance amplifier (not shown), and a current waveform which is the same as the voltage waveform is transmitted to the switch of each phase of the motor. 
     As described above, in the three phase BLDC motor according to the present invention, by using input amplifiers for amplifying the difference between an electromotive force and neutral point voltage of each respective phase and by using comparators each configured by an emitter coupled pair, the switching period is increased so that the current supplied during the switching of each phase exhibits a linear characteristic. Thus, the switch coupled to each phase of the motor may be slowly turned on or off, and motor malfunctions caused by switching noise can thus be prevented. 
     While the present invention has been described and illustrated with reference to a preferred embodiment thereof, it is to be readily understood that the present invention is not limited to the embodiment, and various changes and modifications can be made therein without departing from the spirit and scope of the invention defined in the appended claims.