Abstract:
A local oscillator and logic circuit pulses the open winding of a brushless DC motor at start up and uses the back EMF to generate a voltage to boost the voltage available to the control circuit for optimizing performance when starting with low supply voltage.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    The present invention relates generally to DC motors, and more particularly to low voltage start-up of brushless DC motors.  
           [0002]    As the density and operating speeds of complex integrated circuits increases, the power dissipated thereby needs to be dispersed as heat, typically with the use of fans. Such fans are typically driven from the DC power supply connected to the power dissipating circuit such as to ensure the fan runs when the circuit is powered. DC fan motors such as those used to cool high density or high power integrated circuits, e.g., in a modern PC, are normally designed to operate with the same supply voltages as are used for the integrated circuits themselves for simplicity and cost effectiveness.  
           [0003]    DC fan motors typically include brush-type permanent magnet motors and brushless motors. As is well known, brush-type motors typically include an armature, having windings, attached to a rotor. Brushes press against a commutator attached to the armature. As the armature turns, the brushes come into contact with different segments of the commutator and change the current path through the winding. The interaction between the magnetic field created in the armature and the permanent magnetic field in the stationary part of the motor results in rotation of the armature. Operation of a brushless motor is similar except that the permanent magnets are coupled to the rotor instead of the stationary part, and the windings are on the stationary part instead of the rotor. The winding phases of a brushless motor are switched on and off electronically by means of a control circuit. Hall effect sensors are typically used to detect the (rotational) position of the rotor, which is used by the control circuit as feedback to control the timed switching of the windings. FIG. 1 illustrates an example of a brushless DC motor  10  including permanent magnets  15  coupled to a rotor  20 . Sensors  25  detect the rotational position of magnets  15 . Windings  30 , provided in an armature  35 , are controlled by a drive controller (not shown).  
           [0004]    One trend in integrated circuit design is to reduce the supply voltage and thus reduce the power dissipation as much as possible. However, the integrated circuits and therefore the system cannot be operated without additional cooling since the temperature rise caused by the power density can exceed the safe operating limits of the silicon. The performance of the fan is thus essential for the safe and reliable operation of the system. However, as these voltages drop to low levels, e.g., below 3.5 volts, the control of the fan motors becomes increasingly difficult. Analogue circuitry, which is often needed to handle and process signals from sensor devices, such as Hall effect sensors, used to detect the rotation of the magnet in the DC motor, may not operate effectively or accurately as the supply voltage drops below about 3.5 volts.  
           [0005]    Specialized integrated circuits have been developed, such as the Melexis US79 series of fan drivers, that can handle all the functions required to ensure reliable fan operation. However, the analogue content of these integrated circuits, essential for their correct operation to specification, sets a lower limit to the operating voltage of about 3.5 volts. Below this figure the performance of analogue circuitry cannot be reliably predicted.  
           [0006]    To maintain operation at lower voltages other techniques are typically used. For example, back Electro Motive Force, EMF, generated by the inductive windings of the DC motor when the current through the windings is turned off during normal commutation is typically used to boost the supply voltage available to the analogue circuitry. Digital circuitry can be configured and arranged to operate satisfactorily at voltages down to about 1.5 volts. However, when the motor is stationary, commutation is not taking place so there is no back EMF generated to supply to the analogue circuitry. The smooth and satisfactory performance of the DC motor under these conditions cannot be ensured and indeed the motor may not start if the analogue circuitry cannot resolve the situation sufficiently to enable the selection of, and drive to, the correct winding of the motor.  
           [0007]    Accordingly, it is desirable to provide systems, methods and circuitry to generate sufficient voltage to ensure analogue circuitry of a DC motor performs sufficiently predictably to ensure satisfactory start up of the DC motor.  
         BRIEF SUMMARY OF THE INVENTION  
         [0008]    The present invention provides systems, methods and circuit arrangements for ensuring proper start-up of brushless DC motors including components operating at low voltages compatible with modern IC design voltages.  
           [0009]    According to the invention, a local oscillator and logic circuit pulses the open winding of a brushless DC motor at start up and the back EMF is used to generate a voltage to boost the voltage available to the control circuit for optimizing performance when starting with low supply voltage. As the rotor of a motor rotates and the windings are commutated by the drive electronics there is generated in each winding a voltage caused by the collapse of the current and the inherent inductance of the winding. These voltages exceed the normal operating voltage of the motor. The energy in these voltages is used to generate a regulated power feed to the analogue circuitry of the control circuit at a suitable voltage level.  
           [0010]    During steady state conditions, when the fan is running, the commutation of the windings is continual and there is ample energy available to power the analogue electronics, and, if required, the associated digital electronics as well. At start up, however, when the motor is stationary, there is no commutation and thus no additional voltage pulses from which to generate a supply for the analogue circuitry. Accordingly, additional circuitry is included to drive one of the motor windings with short voltage pulses such as to create inductive voltages that can be used to create the desired regulated power feed for the analogue circuitry. This feed, once established, enables the analogue circuitry to accurately determine the state and position of the rotor and cause the correct winding to be driven. The motor will then start and usual steady state conditions will become established.  
           [0011]    According to an aspect of the present invention, a circuit arrangement for driving at low voltage a brushless dc motor having a rotor and at least two windings for driving one or more permanent magnets on the rotor is provided. The arrangement typically includes drive circuits configured to provide drive signals to the windings, one or more sensors arranged to determine the rotational position of the rotor, and an analogue processing circuit configured to process signals received from the one or more sensors so as to provide a feedback signal. The circuit arrangement also typically includes a regulation circuit configured to extract energy from inductive voltages produced by the windings and to generate a voltage power source for the processing circuit, an oscillator circuit configured to provide a pulse signal, and a control circuit configured to receive the feedback signal and the pulse signal. The control circuit also configured to control the drive circuits such that in a first mode of operation, when the rotor is turning, the drive circuits are selectively enabled based on the feedback signal, and in a second mode of operation, when the rotor is not initially turning, the drive circuits are pulsed based on the pulse signal so as to generate inductive voltages in the windings.  
           [0012]    According to another aspect of the present invention, a system for driving at low voltage a brushless dc motor having a rotor and at least two windings is provided. The system typically includes driving means for driving the individual windings on the dc motor so as to rotate the rotor, sensor means for determining the rotational position of the rotor, and analogue interface circuitry for interfacing to the sensor circuits and providing a feedback signal based on signals received from the sensor means. The system also typically includes regulation circuitry configured to extract energy from the inductive voltages produced by the windings and to generate a high voltage power source for the analogue interface circuitry, means for providing a pulse signal, and control means coupled to the analogue interface circuitry for controlling the driving means such that when the motor is turning the drive circuits are selectively enabled by timing determined from the feedback signal, and when the motor is not turning the drive means are pulsed in time based on the pulse signal such as to generate inductive voltages on the windings when they would otherwise be not present.  
           [0013]    According to yet another aspect of the present invention, a method is provided for driving a brushless dc motor having a rotor and at least two windings configured to drive one or more permanent magnets coupled to the rotor. The method typically includes generating pulsed drive signals using drive circuits connected to the windings so as to rotate the rotor and thereby generate inductive voltage signals in the windings, and generating a high voltage power source using the inductive voltage signals generated by the windings. The method also typically includes sensing the rotational position of the rotor using one or more sensors, processing signals provided by the one or more sensors using an analogue processing circuit so as to produce a feedback signal, the analogue processing circuit being powered by the generated high voltage power source, and selectively powering the drive circuits based on the feedback signal so as to rotate the rotor in a continuous manner.  
           [0014]    Reference to the remaining portions of the specification, including the drawings and claims, will realize other features and advantages of the present invention. Further features and advantages of the present invention, as well as the structure and operation of various embodiments of the present invention, are described in detail below with respect to the accompanying drawings. In the drawings, like reference numbers indicate identical or functionally similar elements. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0015]    [0015]FIG. 1 illustrates an example of a brushless DC motor including permanent magnets coupled to a rotor.  
         [0016]    [0016]FIG. 2 is a schematic diagram of DC motor drive circuitry according to an embodiment of the invention showing the basic elements and their connections. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0017]    [0017]FIG. 2 illustrates an embodiment of a permanent magnet DC motor including two windings  107  and  108  which are driven alternately by two driver circuits  105  and  106 . The driver circuits are controlled by drive signals applied thereto by control circuit  103 .  
         [0018]    In a first control mode, e.g., in steady state operation, the DC motor rotates and the rotation is sensed by one or more sensors, preferably hall effect sensors,  101 . Hall IC latches, such as devices identified as model number US1881, US2881, and US3881, and provided by Melexis (41 Locke Road, Concord, N.H. 03301), are useful sensors for such brushless DC motor applications. The output of the sensor(s) is connected to an analogue interface circuit  102  where the signal is amplified and passed to the control circuit  103 . As the motor rotates and control circuit  103  turns the driver circuits alternately on and off, the combination of the changes in current and the inductance of the windings  107  and  108  results in voltage pulses (e.g., narrow voltage spikes) being generated. The voltage pulses are provided to a regulator circuit  104  over line(s)  110 . These voltage pulses will typically be at a voltage level that is higher than the normal operating voltage of the DC motor. In one embodiment, regulator circuit  104  is configured and arranged to rectify the narrow voltage pulses and to generate a DC supply voltage, e.g., at a voltage level higher than the operating voltage. For example, in one embodiment, the normal operating voltage is about 1.5 V or higher and the regulation circuit  104  is configured to supply a DC voltage of 3.5V or higher. Regulator circuit  104  preferably includes, in one embodiment, a voltage regulator and/or a step-up circuit. In one embodiment, for example, the regulator includes a diode to rectify, a zener to limit the voltage and some capacitance. The DC supply voltage generated by regulator circuit  104  is then used to supply the analogue interface circuit  102  over line  112 .  
         [0019]    During the period of time when the DC motor is rotating and the driver circuits are switching, regulator  104  is able to generate the higher level voltage supply and the system is self-maintaining. When, however, the motor is stationary, as for example when power is first applied, there is no switching of the drive circuits and hence no inductively generated voltage pulses and no energy for regulator  104  to use for the high voltage supply. Analogue interface circuit  102  will therefore be operating at a supply voltage below its optimum, and the performance may well be degraded to such an extent that control circuit  103  may not switch the drive circuits and the DC motor will not start.  
         [0020]    According to one embodiment of the present invention, an oscillator circuit  109  is provided to supply a periodic signal to the control circuit  103 . In this embodiment, control circuit  103  is configured to operate in a second control mode wherein the control circuit uses the signal provided by oscillator  109  to control one (or both) of the drive circuits  105  and  106  to turn on and off one (or both) of the DC motor windings such as to create inductively induced voltage pulses. Regulator  104  uses the inductively induced voltage pulses to generate a high voltage supply such that the analogue interface  102  can operate within design parameters and the DC motor will be properly started.  
         [0021]    The design of oscillator  108  can be any well-known oscillator circuit arrangement configured to start and continue to provide an oscillation signal at low voltages, e.g., voltages that are high enough to turn the DC motor but too low for the analogue interface  102  to operate sufficiently and generate useful signals such as to enable the control circuit  103  to switch the drive circuits  105  and  106  and so maintain steady state rotation of the DC motor. The choice of oscillator will generally depend on the technology chosen for the main integrated circuitry. For example, the oscillator can include a ring containing an odd number of inverters which will oscillate. It should be appreciated that any oscillator as is well known may be used, so long as the chosen oscillator starts and keeps running on a low voltage.  
         [0022]    In another embodiment, the control circuit  103  is further configured and arranged to receive the high voltage supply from regulator  104  as an input on line  114 . In this embodiment, control circuit  103  is also configured to switch between the second control mode and the first control mode once the high voltage supply of regulator circuit  104  has reached or risen above a predetermined threshold level indicating that the high voltage supply is at a sufficient level for the analogue interface  102  to operate within specification. A trigger circuit or threshold circuit is preferably included in circuit  103  to facilitate mode switching.  
         [0023]    In a further embodiment of the invention, the high voltage supply is also provided to the sensor circuit(s) over line  116  to enable the use of sensor(s) circuits that require a higher operating voltage for optimum performance.  
         [0024]    The drive control circuitry, e.g., control circuit  103 , regulator  104 , analog interface circuitry  102 , oscillator  109 , and drive circuits  105  and  106 , are preferably implemented on a single Integrated Circuit (IC) structure, e.g., a controller IC. However, it should be understood that each or a combination of such circuit elements may be implemented on separate ICs.  
         [0025]    While the invention has been described by way of example and in terms of the specific embodiments, it is to be understood that the invention is not limited to the disclosed embodiments. To the contrary, it is intended to cover various modifications and similar arrangements as would be apparent to those skilled in the art. For example, it should be appreciated that more than two windings (and corresponding drive circuits) may be used. Therefore, the scope of the appended claims should be accorded the broadest interpretation so as to encompass all such modifications and similar arrangements.