US 5668449 A
A circuit for operating a polyphase DC motor, such as the type having a plurality of "Y" connected stator coils, has circuitry for charging the coils at a rate which will reduce EMI and other noise, while maintaining an acceptable charge rate. The gate of a selected high side driving transistor is charged at a relatively high rate during a ramping phase. During the ramping phase, the gates of the selected transistor is charged to a voltage near the voltage needed to form a channel in the transistor for conduction. After the ramping phase, the gates are charged at a lesser rate in order to control the rate of charging of the stator coils to prevent noise.
1. A method of operating a polyphase DC motor having a plurality of coils, comprising the steps of:
selecting one of a plurality of power driving transistors, each of said power driving transistors coupled to a respective coil;
receiving a control signal which identifies either a first or a second mode of operation; and
in said first mode, providing a voltage signal to a gate of the selected driving transistor such that said voltage signal varies at a first rate until the voltage on the gate of the selected power driving transistor is near a point at which the selected driving transistor will conduct and varies at a second rate, which is less than said first rate, while the transistor is conducting current to the respective coil;
in said second mode, providing said voltage signal to the gate of the selected driving transistor such that said voltage signal varies at said first rate.
2. The method of claim 1, wherein said control signal identifies said first mode of operation when noise reduction is desired and said second mode of operation when noise reduction is not desired.
3. A control circuit for operating a polyphase DC motor having a plurality of coils, comprising:
a plurality of power driving transistors for driving respective coils, each said power driving transistors driving said respective coils at rate responsive to a voltage applied to a control node of the transistors;
a control signal which indicates either a first or a second mode of operation;
circuitry for providing a voltage signal to a selected control node such that in said first mode of operation, said voltage signal varies at a first rate during a first phase and at a second rate, which is less than said first rate, during a second phase, and in said second mode of operation, said voltage signal varies at said first rate.
4. The control circuit of claim 3, wherein said power driving transistors are FET transistors.
5. The control circuit of claim 3, wherein said first phase lasts until the voltage on a gate of the selected transistor is near a point at which the selected transistor will conduct and said second phase occurs while the transistor is conducting current to the respective coil.
6. A control circuit for operating a polyphase DC motor having a plurality of coils, comprising:
a plurality of power FET transistors each having respective source and drain regions coupled between a first voltage supply and a respective coil;
circuitry for selecting one of said power FET transistors to apply current to its respective coil;
a control signal which indicates either a first or a second mode of operation;
drive control circuitry for controlling a voltage on a gate of the selected transistor, such that
in said first mode of operation, the gate is charged to a first predetermined voltage at a first rate and to a second predetermined voltage at a second rate, which is less than said first rate, and
in said second mode of operation, the gate is charged to said second predetermined voltage at said first rate.
7. The control circuit of claim 6 wherein said power FET transistors comprises n-channel FET transistors.
This invention relates in general to motors, and more particularly to a driving circuit using a controlled high side driver for improved noise conditions.
The present invention pertains to polyphase DC motors, and particularly to three-phase DC motors. Brushless, sensorless-type DC motors are used for rotating data media, such as found in computer-related applications, including hard disk drives, CD-ROM drives, floppy disks, VCRs (video cassette recorders), and the like. In computer applications, three-phase, brushless, sensorless DC motors are becoming more popular, due to their reliability, low weight and accuracy.
Motors of this type can typically be thought of as having a stator with three coils connected in a "Y" configuration, although a larger number of stator coils are usually employed with multiple motor poles. Typically, in such applications, 8-pole motors are used, which have four electrical cycles per revolution of the rotor. The stator coils, however, can be analyzed in terms of three "Y" connected coils. In operation, the coils are energized in sequences in which a current path is established through two coils of the "Y" with the third coil left floating in bipolar applications. The sequences are arranged so that as the current paths are changed, or "commutated", one of the coils of the current path is switched to float and the previously floating coil is switched into the current path. The sequence is defined such that when the floating coil is switched into the current path, current will flow in the same direction in the coil which was included in the prior current path. In this manner, six commutation sequences are defined for each electrical cycle in a three-phase motor.
Each coil is connected to a high side driver and a low side driver. When the high side driver is enabled, that coil will be operatively connected to a voltage supply for providing current through the coils. The low side driver selectively couples a coil to ground. Hence, the two coils can be chosen by appropriately enabling and disabling the high side and low side drivers. It is advantageous to enable the high side driver to quickly turn-on once its respective coil has been selected. However, a fast transition at the output of the driver will result in EMI (electromagnetic interference) and noise, which may create errors in the signals from the reading head of a hard drive. In the past, a significant amount of noise could be tolerated in the reading circuitry. However, with higher density hard drives and other devices, the circuitry is much more sensitive to electromagnetic interference and noise. Consequently, the use of prior art motors, and particularly the circuitry to drive the coils of a DC motor, can negatively effect performance of the hard drive or similarly sensitive device.
Therefore, a need has arisen in the industry for a motor with driving circuitry with reduced EMI and noise, while maintaining satisfactory speed performance.
In the present invention, stator coils are selectively driven through an associated high side drive transistor coupled between a voltage source and the coil. The gate of a selected high side drive transistor is charged at a relatively high rate during a ramping phase. During the ramping phase, the gates of the selected transistor is charged to a voltage near the voltage needed to form a channel in the transistor for conduction. After the ramping phase, the gates are charged at a lesser rate in order to control the slew rate of the output driving the stator coils to prevent noise.
The present invention provides significant advantages over the prior art. First, noise is reduced for superior performance in sensitive devices, such as high density hard drives. Second, delays are minimized by the ramping phase. Third, the controller can be operated in either linear mode or PWM mode (where the slew function is disabled) to conserve energy when desired.
For a more complete understanding of the present invention, and the advantages thereof, reference is now made to the following descriptions taken in conjunction with the accompanying drawings, in which:
FIG. 1 is an electrical schematic block diagram of a motor driver system incorporating apparatus in accordance with a preferred embodiment of the invention;
FIG. 2 is an electrical schematic diagram of the power stage used in the motor driver system of FIG. 1 for providing motor driving signals to the "Y" connected stator coils of the motor with which the driver system might be associated;
FIG. 3 is an electrical schematic diagram of a typically "Y" connected rotor coil arrangement, illustrating the current flow direction for six phases of a three-phase motor, in a bipolar driving configuration;
FIG. 4a is an electrical schematic diagram of a sequencer circuit used in the motor driver system of FIG. 1;
FIGS. 4b and 4c are graphs of a desired voltage profile for enabling the high side drivers 44, 44' and 44" of FIG. 2 during linear and PWM modes, respectively; and
FIG. 4d is a electrical schematic diagram of the controlled driver circuits of FIG. 4a.
The preferred embodiment of the present invention and its advantages are best understood by referring to FIGS. 1-4 of the drawings, like numerals being used for like and corresponding parts of the various drawings.
A portion of an electrical schematic block diagram of a motor controller 10 in which the apparatus and method in accordance with a preferred embodiment of the invention may be incorporated is shown in FIG. 1. The diagram of FIG. 1 is simplified to demonstrate underlying principles of motor operation and control with the understanding that one skilled in the art could make various additions or substitutions while still benefitting from the present invention. Additional description of the structure and operation of polyphase DC motors is provided in U.S. Pat. No. 5,306,988, entitled "Method and Apparatus For Operating Polyphase DC Motors," issued Apr. 26, 1994, assigned to SGS-Thomson Microelectronics, Inc. and incorporated herein by reference.
The driving voltage is provided to the output terminals OUT A, OUT B, and OUT C by a power stage 11, which may be configured as described below with reference to FIG. 2. The power stage 11 is sequenced to provide sequential control output signals to the output terminals OUT A, OUT B, and OUT C by a sequencer circuit 13, described below with reference to FIG. 4a, and a signal interface circuit 12 supplies the output signals from the sequencer circuit 13 to the power stage 11, as well as enabling other functions, such as brake and output enable functions. The sequencer 13 also provides drive signals to other circuits of the controller circuitry to control the various aspects of rotation of the motor driven by the circuit 10.
The output terminals OUT A, OUT B, and OUT C are also switchably connected to a back-emf sense amplifier 14. The back-emf sense amplifier 14 in turn delivers signals to a reference voltage crossing detector circuit 16, which provides input signals to a digital delay circuit 17. The output of the digital delay circuit 17 controls the operation of the sequencer 13, in a manner below described in detail. The motor controller circuitry 10 includes a system clock circuitry 23, and phase lock loop (PLL) frequency/phase detector circuitry 24, and may include various other circuitry, not shown, such as circuitry to support pulse width modulation operation of the motor, "align and go" start up circuitry to facilitate start up of the motor from a stopped condition, port control logic and associated shift register circuitry to facilitate control of the motor controller circuitry by an external microprocessor (not shown), and so forth. Details of the operation of particular blocks of FIG. 1 are described in detail in U.S. Pat. No. 5,306,988, referenced above.
The power stage 11 of the motor control circuitry 10 may be a conventional H-bridge 30, commonly referred to as a triple-1/2-H-bridge, as shown in FIG. 2. Also, the Y connected stator windings 32, 33, and 34 are shown in FIG. 2 connected to switchably receive the driving current delivered by the power stage 11 of the motor control circuitry 10. Three series current paths 37, 38, and 39 are provided between a supply voltage applied to a line 41, through the lower driver transistors and a reference potential (ground) on line 42. Each current path includes, respectively, two switching transistors 44 and 45, 44' and 45' and 44" and 45". Transistors 44, 44' and 44" are known as the "high side drivers" while transistors 45, 45' and 45" are known as the "low side drivers". The transistors 44, 44', 44", 45, 45' and 45" can be of known power switch FETS, or other switch devices, as desired. In the illustrated embodiment, N-channel transistors 44, 44' and 44" and 45, 45' and 45" are used. The line 42 may be connected to an external sense resistor 49 (externally connected to terminals R SENSE, as shown in FIG. 1), which in turn is connected to ground. Other configurations for sensing output current including SENSEFETS, current mirrors, and the like are also possible.
One end of each of the stator coils 32, 33 and 34 is connected together at a common center tap 36, the other ends being connected to respective nodes OUT A, OUT B, and OUT C between the respective pairs of switching transistors 44-45, 44'-45' and 44"-45". Each of the switching transistors 44, 45, 44', 45', 44" and 45" has a flyback diode 47, 48 47', 48' and 47", 48" which maybe connected in parallel with its current path, as shown.
In operation, during an energized phase, one node (for example, node OUT A) is driven high by one of the upper switches 44. One node (for example node OUT B) is driven low by one of the lower switches 45' and the other node (for example, node C) is left floating with both switches 44" and 45" off. This may be referred to as the "AB phase." The coils are then switched in a commutation sequence determined by the sequence circuit 13 in a manner such that in each commutation phase current always flows in two of the three coils, with the third coil floating, and that after switching current will continue to flow, and in the same direction, in one of the two coils in which current was flowing in the previous phase. More particularly, as shown in the diagram of FIG. 3, in a three-phase motor, actually six operational phases exist. The current flow in each of these six phases as shown in the following Table A:
TABLE A______________________________________ Current Flows FloatingPhase From: To: Coil:______________________________________1 A B C2 A C B3 B C A4 B A C5 C A B6 C B A______________________________________
The switching of the driver transistors of the power stage 11 to effect the switching currents for each phase is accomplished by a sequencer circuit 13 and an associated interface circuit 12, as shown in FIG. 4a. The sequencer circuit 13 provides signals through the interface circuit 12 to the upper driver output 52 and the lower driver outputs 53 to accomplish the switching sequence outlined above in Table A. The upper and lower driver outputs 52 and 53 are connected to the upper and lower driver input lines, shown in FIG. 2.
More specifically, a shift register 55 (FIG. 4a) determines the specific upper and lower output lines which are activated at any particular instant. To accomplish the proper switching sequence, an appropriate sequence may be initially loaded into the shift register 55, and sequentially shifted through the various data positions of the shift register 55.
The other circuitry of the sequence circuit 13 includes a initialize line 59 to initialize or reset the shift register 55, reestablishing the commutation sequence code above described in an initial starting position within the shift register 55. The outputs Q1 -Q6 of the shift register 55 are also connected via lines 66 to the reference voltage crossing logic control circuit. If desired, a brake signal developed elsewhere, perhaps externally to the motor control circuit 10, may be applied on line 67 to logic circuit 63 to prevent output to the motor on the upper and lower driver output lines 52 and 53 when such brake signal exists. The brake signal, which may be a software generated or external signal, is applied in the embodiment illustrated to turn on all the low side drivers, and to turn off all of the high side drivers. At the same time all of the coils 32, 33 and 34 may be shorted together so that the currents generated by the back emf in the coils 32, 33, and 34 brake the motor. Outputs of the logic circuit 63 are coupled to respective controlled drive circuits 64A, 64B and 64C, which generate outputs UA, UB and UC. The controlled drive circuits, referred to generally by reference numeral 64, are identical in structure.
Signals which might be generated in response to an internal error or other abnormal condition, or to an external enable signal that would not require the motor to be braked, but which may require the removal of drive signals to the coils of the motor, may be applied on a line 68 to the final driver logic gates 63 as shown. Thus, in the presence of a signal on the line 68 (that is, a change to a low state from a normally high state), the drive is removed or inhibited from both the upper drivers 52 and the lower drivers 53 allowing the rotor of the motor to coast.
FIGS. 4b and 4c illustrate electrical diagrams for the desired gate voltage to the driving transistors 44, 44' and 44" for linear mode and pulse width modulated (PWM) mode, respectively. The gate voltage, VGATE is provided by the controlled drive circuit 64 to the appropriate driving transistor 44, 44' or 44". In FIG. 4b, the desired gate voltage is shown for applying to the driving transistors while the motor controller 10 is in linear mode, the mode used while data is being written to or read from the hard drive. The gate voltage signal in this mode is structured in two phases: a initial ramping phase shown from t0 to t1 and a slew phase shown from t1 to t2. During the ramping phase, the gate voltage transitions from approximately zero volts to a predetermined voltage, VREF at a steep slope. The fast transition during the ramping phase allows the gate voltage of the driving transistor 44 to reach a voltage slightly above the voltage of the source as soon as possible. A channel will not be formed in the high side drive transistor 44 until the gate voltage exceeds the voltage of the source of drive transistor 44 by one threshold voltage, Vt (typically on the order of one volt). It should be noted that the initial voltage of the associated stator coil VLOAD (and hence the voltage of the high side driver's source) is very close to VPOWER, the voltage of the power supply, because that is the optimal switching point. Once the ramping phase is complete, the slew phase begins, wherein VGATE increases at a lesser slope, in order to avoid an abrupt change in voltage applied to one of the coils which would result in unacceptable EMI. Consequently,during the slew phase, the voltage of the associated coil, will transition relatively slowly between its initial voltage and VPOWER while VGATE transitions between VREF and VHIGH, the output of a charge pump.
FIG. 4c illustrates an electrical diagram for the output of the controlled drive circuit 64 for PWM mode. PWM mode is used during periods where reading and writing to the hard disk will not occur. PWM mode reduces the currents from the supply significantly, and therefore is used during periods when noise is not a factor, such as during start-up or when the disk is idle. In PWM, since noise is not a factor, VGATE transitions from zero volts to VHIGH at a steep slope. In the preferred embodiment, the slope is the same as the ramping phase slope of VGATE in linear mode. Accordingly, VLOAD has a steep transition from its initial value to VPOWER.
FIG. 4d illustrates circuitry for providing the gate signal to the driving transistors 44, 44' and 44", as described in connection with FIGS. 4b and 4c. For purposes of illustration, the circuitry will be shown for connection to driving transistor 44, it being understood that the same circuitry can be used for each of the drive control circuits 64A, 64B and 64C, with only the connections between the logic 33 and the high side drive transistors 44, 44' and 44" being different.
As shown in FIG. 4d, a current mirror 200 is comprised of FET transistors 202 and 204. The first source/drain of each transistor 202 and 204 is coupled to VHIGH. The second source/drain of transistor 204 is coupled to current source 206 and to the gates of transistors 202 and 204. The second source/drain of transistor 202 is coupled to a first node 208 of switch 210, the cathode of diode 212 and the base of npn transistor 214. Current source 206 is also coupled to second node 216 of switch 210. Switch 210 selectively switches between nodes 208 and 216 responsive to signal UA. The emitter of transistor 214 and the anode of diode 212 are coupled to the gate of driving transistor 44. The collector of transistor 214 is coupled to the cathode of diode 218 and the emitter of npn transistor 220. The anode of diode 218 is coupled to VREF power source 219. The collector of transistor 220 is coupled to VHIGH. Current source 222 is coupled between the base of transistor 220 and VHIGH. N-channel transistor 224 has a first source/drain coupled to ground a second source/drain coupled to the base of transistor 220 and a gate coupled to control signal LIN/#PWM. When LIN/#PWM=1, the motor controller 10 is in linear mode; when LIN/#PWM=0, the motor controller 10 is in PWM mode.
In linear mode, transistor 224 is in a low impedance mode, thereby passing current from the current source 222 to ground. Thus, transistor 220 is effectively disabled. When switch 210 is enabled by UA to couple node 216 to ground, current source 206 causes a current (IB) to pass into the base of transistor 214. While transistor 214 is not saturated (the ramping phase in FIG. 4b), a current βIB passes to the gate of high side driver 44 through the emitter of transistor 214. When VGATE is charged to within approximately one volt of VREF (i.e., VREF -1 volt), transistor 214 will be in saturation and the emitter current will equal IB, rather than βIB. Consequently, the rate of change in VGATE will diminish when VGATE >VREF -1. This saturation condition results in the slew phase shown in FIG. 4b. Since the high side driver 44 will not pass current to its associated coil 32 until VGATE >VLOAD +Vt, the charging of the coil will occur during the slew phase, provided VLOAD +Vt >VREF -1. During the slew phase, the amount of current passed from VPOWER to the coil 32 will depend upon the difference between VGATE and VLOAD (until transistor 44 is fully on). Therefore, by controlling the gate voltage of the drive transistor 44, the rate of change in the coil can be controlled to suppress noise. The rate of change of VGATE during the slew phase can be controlled by the magnitude of IB.
If switch 210 is set by signal UA to couple node 208 to ground, any current through transistor 202 passes to ground; therefore IB =0, and the gate of transistor 44 is discharged. Also, since node 216 is floating, there is not current through current source 206 and therefore the current mirror 200 will not produce any current to the base of transistor 214.
In PWM mode (LIN/#PWM=0), transistor 224 is in a high impedance mode, thus the current from current source drives the base of transistor 220, effectively coupling VHIGH to the collector of transistor 214. Consequently, transistor 214 will not saturate until VGATE >VHIGH -1. Therefore, VGATE will be charged by βIB until VGATE is much greater than VPOWER. Consequently, the transition of VGATE will be at the ramping phase rate, thereby charging the coil 32 faster than in the linear mode discussed above.
This aspect of the invention provides significant advantages over the prior art. Noise is reduced by charging gates of the high side drivers 44, 44' and 44" at a controlled slew rate while the drivers are conducting; the slew rate can be set to meet a desired noise requirement. However, during an initial ramping phase prior completed to enabling the high side driver, VGATE is charged at a faster rate. Thus, delays in charging the coils are minimized.
The present invention provides significant advantages over the prior art. Importantly, the EMI and noise is reduced by charging gates of the high side drivers 44, 44' and 44" at a controlled slew rate while the drivers are conducting; the slew rate can be adjusted as desired to meet a desired noise requirement. Delays in charging the coils are minimized by a ramping phase which charges the gate of the high side driver at a faster rate up until the point where the high side driver will conduct.
It should be noted that the circuit of FIGS. 4b-4d could be adapted to any circuit in which it is desirable to turn on a transistor at a fast rate while controlling the output of the transistor once the gate voltage reaches the point of conduction.
According to a disclosed class of innovative embodiments, there is provided: A control circuit for operating a polyphase DC motor having a plurality of coils, comprising: a plurality of driving transistors for driving respective coils, each said driving transistors driving said respective coils at rate responsive to a voltage applied to a control node of the transistor; circuitry for providing a voltage signal to a selected control node such that voltage signal varies at a first rate during a first phase and a second rate during a second phase.
According to another disclosed class of innovative embodiments, there is provided: A method of operating a polyphase DC motor having a plurality of coils, comprising the steps of: selecting one of a plurality of driving transistors, each of said driving transistors coupled to a respective coil; and providing a voltage signal to a control node of the selected driving transistor such that voltage signal varies at a first rate during a first phase and a second rate during a second phase.
According to another disclosed class of innovative embodiments, there is provided: A control circuit for operating a polyphase DC motor having a plurality of coils, comprising: a plurality of FET transistors each having first and second source/drain regions coupled between a first voltage supply and a respective coil; circuitry for selecting one of said FET transistors to apply current to its respective coil; drive control circuitry for controlling the voltage on a gate of the selected transistor, such that the gate is charged to a first predetermined voltage at a first rate and to a second predetermined voltage at a second rate. Within this class of embodiments, the control circuit may further comprise circuitry to disable said drive control circuitry such that the gate is charged to said second predetermined voltage at said first rate.
Although the invention has been described and illustrated with a certain degree of particularity, it is understood that the present disclosure has been made by way of example only, and that numerous changes in the combination and arrangements of parts and features can be made by those skilled in the art without departing from the spirit and scope of the invention, as hereinafter claimed.