Sensorless motor circuit and operating method thereof

Abstract

A sensorless motor circuit includes a brushless DC motor, a drive circuit, a voltage regulator circuit, a virtual neutral point circuit, a comparator and a control circuit. The brushless DC motor includes first to third coils. The drive circuit outputs a pulse width modulation voltage to the first coil, couples the second coil to a ground terminal, and floats the third coil. The voltage adjusting circuit generates partial voltage according to terminal voltage of the third coil. The virtual neutral point circuit generates a virtual neutral point voltage. The comparator compares the partial voltage and the virtual neutral voltage to generate a comparison signal. The control circuit determines a sampling time point according to the working period of the pulse width modulation voltage, and samples the comparison signal at the sampling time point to detect a back electromotive force zero crossing point, thereby performing phase commutation.

Description

Technical Field

[0001] The present invention relates to a motor circuit, and more particularly to a sensorless motor circuit and an operation method thereof. Background Art

[0002] Brushless direct current (BLDC) motors are widely used in applications such as robots, computer devices, medical devices, automotive drives, pumps, and household appliances, etc. Compared with traditional brushed motors, BLDC motors can improve efficiency, require less maintenance (no carbon brush wear), and can achieve a flat torque curve over all rated speed ranges.

[0003] Common BLDC motors provide a three-phase configuration. Since the rotor of a BLDC motor rotates under the influence of the magnetic field of the stator coil, it is necessary to track the position of the rotor in order to effectively drive the rotor. The motor system can track the rotor position based on the zero-crossing point of the back electromotive force voltage of the BLDC motor. In related technologies, a filter is used to filter out the noise in the back electromotive force voltage. However, the filter also affects the phase of the back electromotive force voltage, so the zero-crossing point of the back electromotive force voltage cannot be accurately detected. Summary of the Invention

[0004] An embodiment of the present invention provides a sensorless motor circuit, including a brushless direct current motor, a drive circuit, a voltage adjustment circuit, a virtual neutral point circuit, a comparator, and a control circuit. The brushless direct current motor includes a first coil, a second coil, and a third coil. Each of the first coil, the second coil, and the third coil includes a first end having a terminal voltage and a second end. The drive circuit is coupled to the brushless direct current motor for outputting a pulse width modulation voltage to the first end of the first coil, coupling the first end of the second coil to the ground terminal, and floating the first end of the third coil. The voltage adjustment circuit is coupled to the first end of the third coil for generating a voltage division based on the terminal voltage of the third coil. The virtual neutral point circuit is coupled to the second ends of the first coil, the second coil, and the third coil for generating a virtual neutral point voltage. Two input terminals of the comparator are respectively coupled to the output terminal of the voltage adjustment circuit and the output terminal of the virtual neutral point circuit for comparing the voltage division and the virtual neutral point voltage to generate a comparison signal. The control circuit is directly coupled to the comparator. The control circuit determines the sampling time point of the comparison signal according to the working cycle of the pulse width modulation voltage, and samples the comparison signal at the sampling time point to detect the back electromotive force zero-crossing point.

[0005] Another embodiment of the present invention provides an operation method for a sensorless motor circuit. The sensorless motor circuit includes a brushless DC motor, a drive circuit, a voltage adjustment circuit, a virtual neutral point circuit, a comparator, and a control circuit. The brushless DC motor includes a first coil, a second coil, and a third coil. Each of the coils includes a first end having a terminal voltage and a second end. The drive circuit is coupled to the brushless DC motor, the voltage adjustment circuit is coupled to the first end of the third coil, the virtual neutral point circuit is coupled to the second ends of the first coil, the second coil, and the third coil, the comparator is coupled to the voltage adjustment circuit and the virtual neutral point circuit, and the control circuit is directly coupled to the comparator. The operation method may include the drive circuit outputting a pulse width modulation voltage to the first end of the first coil, coupling the first end of the second coil to the ground terminal, and floating the first end of the third coil. The voltage adjustment circuit generates a voltage division based on the terminal voltage of the third coil, and the virtual neutral point circuit generates a virtual neutral point voltage. The operation method may further include the comparator comparing the voltage division and the virtual neutral point voltage to generate a comparison signal, the control circuit determining a sampling time point of the comparison signal based on the duty cycle of the pulse width modulation voltage, and the control circuit sampling the comparison signal at the sampling time point to detect the back electromotive force zero crossing point. BRIEF DESCRIPTION OF THE DRAWINGS

[0006] Figure 1 FIG. is a circuit diagram of a sensorless motor circuit according to an embodiment of the present invention.

[0007] Figure 2 is Figure 1 a flowchart of an operation method of the sensorless motor circuit in

[0008] Figure 3 is Figure 1 a schematic diagram of the motor circuit sampling during the off period of the pulse width modulation voltage in

[0009] Figure 4 is Figure 1 a schematic diagram of the motor circuit sampling during the on period of the pulse width modulation voltage in

[0010] Figure 5 FIG. is a circuit diagram of another sensorless motor circuit according to an embodiment of the present invention.

[0011] Figure 6 is Figure 1 a waveform diagram of the motor circuit in

[0012] Figure 7 is Figure 6 an enlarged waveform diagram of the motor circuit in

[0013] SYMBOL DESCRIPTION

[0014] 1, 5: Motor circuit

[0015] 10: Driving circuit

[0016] 12: Virtual neutral point circuit

[0017] 14: Brushless DC motor

[0018] 16: Voltage regulation circuit

[0019] 18: Control circuit

[0020] 200: Operating method

[0021] S202 to S214: Steps

[0022] 50: Multiplexer

[0023] CMPU, CMPV, CMPW and CMP: Comparators

[0024] D1 to D6: Parasitic diodes

[0025] GND: Ground voltage

[0026] N1: Neutral point

[0027] N2: Virtual neutral point

[0028] VN: Virtual neutral voltage

[0029] T1 to T6: Switches

[0030] Ru, Rv and Rw: Virtual neutral point resistors

[0031] Ru1, Ru2, Rv1, Rv2, Rw1, Rw2: Resistors

[0032] Scu, Scv, Scw and Sc: Comparison signals

[0033] Sel: Selection signal

[0034] t, t1 to t4: Time

[0035] Vsel: Selected voltage

[0036] Vrf: Reference voltage

[0037] VCC: Supply voltage

[0038] VDC: DC voltage

[0039] Vdu, Vdv and Vdw: Voltage divisions

[0040] Vu, Vv and Vw: Terminal voltages

[0041] Wu, Wv and Ww: Coils Detailed implementation manner

[0042] Figure 1 This is a circuit diagram of a sensorless motor circuit 1 in an embodiment of the present invention. The motor circuit 1 may include a drive circuit 10, a virtual neutral point circuit 12, a brushless DC motor 14, a voltage regulation circuit 16, comparators CMPU, CMPV, and CMPW, and a control circuit 18. The drive circuit 10 may be coupled to the virtual neutral point circuit 12 and the brushless DC motor 14. The brushless DC motor 14 may be directly coupled to the voltage regulation circuit 16. The voltage regulation circuit 16 and the virtual neutral point circuit 12 may be directly coupled to the comparators CMPU, CMPV, and CMPW. And the comparators CMPU, CMPV, and CMPW and the drive circuit 10 may be coupled to the control circuit 18. The voltage regulation circuit 16 does not need to use a filter and samples the back electromotive force (back EMF) voltage of the brushless DC motor 14 only after the back EMF voltage is stabilized to detect the zero crossing point of the back EMF, and then performs commutation, avoiding the phase delay of the sensed back EMF voltage signal caused by using a filter, correctly detecting the zero crossing point of the back EMF and reducing the manufacturing cost at the same time.

[0043] The brushless DC motor 14 may include a rotor and a stator and operates with three-phase (U-phase, V-phase, and W-phase) power supply. The rotor may be installed with permanent magnets and may rotate or translate with a gap from the stator. The stator may include a yoke and coils Wu, Wv, and Ww. The yoke may be cylindrical and arranged to surround the rotor. The coils Wu, Wv, and Ww may be distributed on the circumference of the yoke, and the coils Wu, Wv, and Ww may be mutually coupled to the neutral point N1. Specifically, the coil Wu may include a first end with a terminal voltage Vu and a second end coupled to the neutral point N1 with a neutral point voltage, and the neutral point N1 may have a neutral point voltage. Similarly, the coil Wv may include a first end with a terminal voltage Vv and a second end coupled to the neutral point N1 with a neutral point voltage. The coil Ww may include a first end with a terminal voltage Vw and a second end coupled to the neutral point N1 with a neutral point voltage.

[0044] The virtual neutral point circuit 12 can generate a virtual neutral point voltage VN approximating the neutral point voltage. The virtual neutral point circuit 12 can include virtual neutral point resistors Ru, Rv, and Rw. The virtual neutral point resistors Ru, Rv, and Rw can be high-value resistors, and values greater than the voltage division resistor values in the voltage regulation circuit can be selected to avoid the load effect from affecting the voltage regulation circuit. The virtual neutral point resistor Ru includes a first end coupled to the first end of the coil Wu and a second end for generating the virtual neutral point voltage VN. The second end of the virtual neutral point resistor Ru can also be referred to as the virtual neutral point N2. The virtual neutral point resistor Rv includes a first end coupled to the first end of the coil Wv and a second end coupled to the second end of the virtual neutral point resistor Ru. The virtual neutral point resistor Rw includes a first end coupled to the first end of the coil Wv and a second end coupled to the second end of the virtual neutral point resistor Ru.

[0045] The drive circuit 10 can receive a pulse width modulation (PWM) signal output by the control circuit 18 to generate a pulse width modulation voltage to drive the brushless DC motor 14. In addition, the drive circuit 10 can alternately excite two of the coils Wu, Wv, and Ww (hereinafter referred to as the first coil and the second coil) and float the remaining one of the coils Wu, Wv, and Ww (hereinafter referred to as the third coil) in a predetermined order to form a rotating magnetic field, thereby driving the rotor to rotate. Switching the current direction in the winding in a predetermined order to maintain a constant magnetic field direction and keep the rotor rotating or translating smoothly is called commutation. Specifically, the drive circuit 10 can output a pulse width modulation voltage to the second end of the first coil, couple the second end of the second coil to the ground terminal, and float the second end of the third coil. The ground terminal can provide a ground voltage GND (e.g., 0V). For example, if the first coil is the coil Wu, the second coil is the coil Wv, and the third coil is the coil Ww, the drive circuit 10 can output a pulse width modulation voltage to the first end of the coil Wu, couple the second end of the coil Wv to the ground terminal, and float the first end of the coil Ww. Thus, the terminal voltage Vu can be the pulse width modulation voltage, the terminal voltage Vv can be the ground voltage GND, and the terminal voltage Vw can be the back electromotive force voltage signal. When the rotor rotates, the coils Wu, Wv, and Ww can respectively generate back electromotive force voltage signals, and the back electromotive force voltage signals will gradually increase from a negative value to a positive value or gradually decrease from a positive value to a negative value as the rotor rotates. Since only the third coil is floating and not affected by external signals, only the voltage difference between the first end and the second end of the third coil can directly reflect the back electromotive force voltage signal of the third coil. When the voltage difference between the first end and the second end of the third coil is 0V, it can be called the back electromotive force zero crossing point.

[0046] The driving circuit 10 may include switches T1 to T6. The switches T1 to T6 may be N-type metal oxide semiconductor field-effect transistors (MOSFETs). In some embodiments, the switches T1 to T6 may also be P-type transistors or other types of N-type transistors, such as N-type bipolar junction transistors (BJTs). The switch T1 may include a control terminal coupled to the control circuit 18, a first terminal coupled to the power supply terminal, and a second terminal coupled to the first terminal of the coil Wu. The switch T2 may include a control terminal coupled to the control circuit 18, a first terminal coupled to the second terminal of the switch T1, and a second terminal coupled to the ground terminal. The switch T3 may include a control terminal coupled to the control circuit 18, a first terminal coupled to the power supply terminal, and a second terminal coupled to the first terminal of the coil Wv. The switch T4 may include a control terminal coupled to the control circuit 18, a first terminal coupled to the second terminal of the switch T3, and a second terminal coupled to the ground terminal. The switch T5 may include a control terminal coupled to the control circuit 18, a first terminal coupled to the power supply terminal, and a second terminal coupled to the first terminal of the coil Ww. The switch T6 may include a control terminal coupled to the control circuit 18, a first terminal coupled to the second terminal of the switch T5, and a second terminal coupled to the ground terminal. The power supply terminal may provide a DC voltage VDC, such as 24V, and the ground terminal may provide a ground voltage GND, such as 0V. The switches T1 to T6 may respectively have parasitic diodes D1 to D6.

[0047] The voltage regulation circuit 16 may generate a voltage division of the terminal voltage according to the terminal voltage of the third coil. In some embodiments, the voltage regulation circuit 16 may generate voltage divisions Vdu, Vdv, and Vdw of the terminal voltages Vu, Vv, and Vw respectively. The voltage regulation circuit 16 may include resistors Ru1, Ru2, Rv1, Rv2, Rw1, and Rw2. The resistor Ru1 includes a first terminal for receiving the terminal voltage Vu and a second terminal for outputting the voltage division Vdu. The resistor Ru2 includes a first terminal coupled to the second terminal of the resistor Ru1 and a second terminal coupled to the ground terminal. The resistor Rv1 includes a first terminal for receiving the terminal voltage Vv and a second terminal for outputting the voltage division Vdv. The resistor Rv2 includes a first terminal coupled to the second terminal of the resistor Rv1 and a second terminal coupled to the ground terminal. The resistor Rw1 includes a first terminal for receiving the terminal voltage Vw and a second terminal for outputting the voltage division Vdw. The resistor Rw2 includes a first terminal coupled to the second terminal of the resistor Rw1 and a second terminal coupled to the ground terminal.

[0048] Since the voltage used in the brushless DC motor 14 is relatively high, the voltage adjustment circuit 16 can reduce the terminal voltage of the third coil for subsequent use. In some embodiments, the resistance values of the resistors Ru1, Rv1, and Rw1 can be equal, the resistance values of the resistors Ru2, Rv2, and Rw2 can be equal, and the resistance value of the resistor Ru2 can be less than that of the resistor Ru1. For example, the resistance values of the resistors Ru1, Rv1, and Rw1 can be 6.8 K ohms, the resistance values of the resistors Ru2, Rv2, and Rw2 can be 1 K ohm. If the terminal voltage of the third coil is 24 V, then the voltage adjustment circuit 16 can generate 3.08 V (= 24 * 1 K / 7.8 K) as the divided voltage of the terminal voltage. In some embodiments, if the subsequent circuit can use a high voltage, the voltage adjustment circuit 16 can be omitted from the motor circuit 1.

[0049] Comparator CMPU can compare the divided voltage Vdu and the virtual neutral point voltage VN to generate a comparison signal Scu. Comparator CMPV can compare the divided voltage Vdv and the virtual neutral point voltage VN to generate a comparison signal Scv. And comparator CMPW can compare the divided voltage Vdw and the virtual neutral point voltage VN to generate a comparison signal Scw. If the divided voltage Vdu exceeds the virtual neutral point voltage VN, comparator CMPU can set the comparison signal Scu to the output logic high level (such as the supply voltage VCC); if the divided voltage Vdu does not exceed the virtual neutral point voltage VN, then comparator CMPU can set the comparison signal Scu to the output logic low level (such as the ground voltage GND). Similarly, if the divided voltage Vdv exceeds the virtual neutral point voltage VN, comparator CMPV can set the comparison signal Scv to the output logic high level; if the divided voltage Vdv does not exceed the virtual neutral point voltage VN, then comparator CMPV can set the comparison signal Scv to the output logic low level. If the divided voltage Vdw exceeds the virtual neutral point voltage VN, comparator CMPW can set the comparison signal Scw to the output logic high level; if the divided voltage Vdw does not exceed the virtual neutral point voltage VN, then comparator CMPW can set the comparison signal Scw to the output logic low level. Comparator CMPU includes a supply terminal for receiving the supply voltage VCC (such as 3.3V); a ground terminal for receiving the ground voltage GND; a positive input terminal coupled to the second terminal of resistor Ru1 for receiving the divided voltage Vdu; a negative input terminal coupled to the virtual neutral point N2 for receiving the virtual neutral point voltage VN; and an output terminal for outputting the comparison signal Scu. Similarly, comparator CMPV includes a supply terminal for receiving the supply voltage VCC; a ground terminal for receiving the ground voltage GND; a positive input terminal coupled to the second terminal of resistor Rv1 for receiving the divided voltage Vdv; a negative input terminal coupled to the virtual neutral point N2 for receiving the virtual neutral point voltage VN; and an output terminal for outputting the comparison signal Scv. Comparator CMPW includes a supply terminal for receiving the supply voltage VCC; a ground terminal for receiving the ground voltage GND; a positive input terminal coupled to the second terminal of resistor Rw1 for receiving the divided voltage Vdw; a negative input terminal coupled to the virtual neutral point N2 for receiving the virtual neutral point voltage VN; and an output terminal for outputting the comparison signal Scw.

[0050] The control circuit 18 may include a microcontroller, a microprocessor, a field programmable gate array (FPGA), or other types of control circuits. The control circuit 18 may generate control signals to control switches T1 to T6 respectively, thereby controlling the commutation of the DC motor 14. The control circuit 18 may determine the sampling time point of the comparison signal corresponding to the third coil according to the duty cycle of the pulse width modulation voltage of the first coil. For example, if the third coil is coil Ww, the control circuit 18 may determine the sampling time point of the comparison signal Scw. The sampling time point may occur during the OFF time or the ON time of the pulse width modulation signal of the first coil.

[0051] In some embodiments, if the duty cycle of the pulse width modulation voltage of the first coil is less than a preset duty cycle, the control circuit 18 may determine that the sampling time point is during the OFF time of the pulse width modulation voltage and after the pulse width modulation voltage is stable. The preset duty cycle may be 50%. Since noise and / or jitter will occur after the pulse width modulation voltage of the first coil switches states (for example, the control circuit 18 generates a pulse width modulation signal that switches from a logic high level to a logic low level), and the signal path may include parasitic elements, resulting in noise and / or jitter and instability in the back electromotive force voltage signal, virtual neutral voltage VN, and comparison signal of the subsequent third coil. Therefore, the control circuit 18 may sample after the comparison signal is stable to obtain a correct sampling value, and then detect the zero crossing of the back electromotive force based on the sampling value. In addition, since the comparison signal takes a certain period of time to reach stability, when the duty cycle of the pulse width modulation voltage of the first coil is less than 50% of the duty cycle, the OFF time of the pulse width modulation voltage will be greater than the ON time of the pulse width modulation voltage. Therefore, the control circuit 18 may determine that the sampling time point is during the OFF time of the pulse width modulation voltage, allowing the comparison signal to have sufficient time to reach stability. In some embodiments, the sampling time point may be the end time point of the OFF time of the pulse width modulation voltage, allowing the comparison signal to have more sufficient time to reach stability, as Figure 3 shown. Figure 3 It is a schematic diagram of the motor circuit 1 sampling during the OFF time of the pulse width modulation voltage, where the first coil is coil Wu, the second coil is coil Wv, the third coil is coil Ww, and the duty cycle of the pulse width modulation voltage of coil Wv is 30%. Figure 3Also shown is a schematic diagram of the comparison signal Scw. Since the motor circuit 1 does not employ a filter, the comparison signal Scw will carry unfiltered noise. Since the duty cycle of the pulse-width modulation voltage is less than the preset duty cycle (30% < 50%), the control circuit 18 can determine that the sampling time point Ts of the comparison signal Scw is the end time point of the cut-off period of the pulse-width modulation voltage. That is, the time point before the switch T1 switches from the cut-off state to the conducting state is the sampling time point Ts, so that the comparison signal Scw has sufficient time to reach stability.

[0052] In some other embodiments, if the duty cycle of the pulse-width modulation voltage of the first coil is not less than the preset duty cycle, the control circuit 18 determines the sampling time point during the conducting period of the pulse-width modulation voltage and after the pulse-width modulation voltage has stabilized. The preset duty cycle can be 50%. Since the comparison signal takes a certain period of time to reach stability, when the duty cycle of the pulse-width modulation voltage of the first coil is not less than 50% of the duty cycle, the conducting period of the pulse-width modulation voltage will be greater than or equal to the cut-off period of the pulse-width modulation voltage. Therefore, the control circuit 18 can determine the sampling time point during the conducting period of the pulse-width modulation voltage, so that the comparison signal has sufficient time to reach stability. In some embodiments, the sampling time point can be the end time point of the conducting period of the pulse-width modulation voltage, so that the comparison signal has more sufficient time to reach stability, as Figure 4 shown. Figure 4 It is a schematic diagram of the motor circuit 1 sampling during the conducting period of the pulse-width modulation voltage, where the first coil is the coil Wu, the second coil is the coil Wv, the third coil is the coil Ww, and the duty cycle of the pulse-width modulation voltage is 70%. Since the duty cycle of the pulse-width modulation voltage is greater than the preset duty cycle (70% > 50%), the control circuit 18 can determine that the sampling time point Ts of the comparison signal Scw is the end time point of the conducting period of the pulse-width modulation voltage. That is, the time point before the switch T1 switches from the conducting state to the cut-off state is the sampling time point Ts, so that the comparison signal Scw has sufficient time to reach stability.

[0053] Next, the control circuit 18 can sample the comparison signal corresponding to the third coil at the sampling time point Ts to detect the zero-crossing point of the back electromotive force. In some embodiments, the control circuit 18 can sample the comparison signal at the sampling time point Ts to generate the current sampling data. If the states of the current sampling data and the previous sampling data are different, it is determined that the zero-crossing point of the back electromotive force is detected. For example, if the previous sampling data is at a logic low level and the current sampling data is at a logic high level, the control circuit 18 determines that the zero-crossing point of the back electromotive force is detected. Once the zero-crossing point of the back electromotive force is detected, the control circuit 18 can then control the drive circuit 10 to perform commutation of the brushless DC motor 14 after a delay of 30 electrical degrees.

[0054] Since the voltage adjustment circuit 16 does not need to use a filter and samples after the comparison signal is stable, it avoids the phase delay of the sensed back electromotive force voltage signal caused by using a filter, correctly detects the zero crossing of the back electromotive force, and reduces the manufacturing cost at the same time.

[0055] In addition, the present invention is not limited to sampling only once in the same pulse width modulation period, and can perform multiple samplings after the comparison signal is stable.

[0056] Figure 2 It is a flowchart of the operation method 200 of the sensorless motor circuit 1. The operation method 200 includes steps S202 to S214 for determining the sampling time point of the comparison signal, so as to correctly detect the zero crossing of the back electromotive force and reduce the manufacturing cost at the same time. Any reasonable technical change or step adjustment belongs to the scope disclosed by the present invention. The details of steps S202 to S214 are described as follows:

[0057] Step S202: The drive circuit 10 outputs a pulse width modulation voltage to the second end of the first coil, couples the second end of the second coil to the ground terminal, and floats the second end of the third coil;

[0058] Step S204: The voltage adjustment circuit 16 generates a voltage division based on the terminal voltage of the third coil;

[0059] Step S206: The virtual neutral point circuit 12 generates a virtual neutral point voltage VN;

[0060] Step S210: The comparator compares the voltage division and the virtual neutral point voltage VN to generate a comparison signal;

[0061] Step S212: The control circuit 18 determines the sampling time point of the comparison signal according to the working cycle of the pulse width modulation voltage;

[0062] Step S214: The control circuit 18 samples the comparison signal at the sampling time point to detect the zero crossing of the back electromotive force;

[0063] The descriptions of steps S202 to S214 can be found in the previous paragraphs and will not be repeated here.

[0064] Since the operation method 200 samples after the comparison signal is stable, the voltage adjustment circuit 16 does not need to use a filter, avoiding the phase delay of the sensed back electromotive force voltage signal, correctly detecting the zero crossing of the back electromotive force, and reducing the manufacturing cost at the same time.

[0065] Figure 5This is the circuit diagram of another sensorless motor circuit 5 in the embodiments of the present invention. The difference between the motor circuit 5 and the motor circuit 1 is that the comparators CMPU, CMPV, and CMPW in the motor circuit 1 are replaced by the multiplexer 50 and the comparator CMP in the motor circuit 5. The multiplexer 50 can be directly coupled to the voltage regulation circuit 16. The other components of the motor circuit 5 are similar to those of the motor circuit 1, and their descriptions can be found in the previous paragraphs and will not be repeated here. The multiplexer 50 and the comparator CMP in the motor circuit 5 are described below.

[0066] The multiplexer 50 includes a first input terminal directly coupled to the second terminal of the resistor Ru1 for receiving the divided voltage Vdu; a second input terminal directly coupled to the second terminal of the resistor Rv1 for receiving the divided voltage Vdv; a third input terminal directly coupled to the second terminal of the resistor Rw1 for receiving the divided voltage Vdw; a selection terminal for receiving the selection signal Sel; and an output terminal for outputting the selected voltage Vsel. The comparator CMP includes a power supply terminal for receiving the supply voltage VCC; a ground terminal for receiving the ground voltage GND; a positive input terminal coupled to the output terminal of the multiplexer 50 for receiving the selected voltage Vsel; a negative input terminal coupled to the virtual neutral point N2 for receiving the virtual neutral point voltage VN; and an output terminal for outputting the comparison signal Sc.

[0067] The multiplexer 50 can select one of the divided voltages Vdu, Vdv, and Vdw as the selected voltage Vsel according to the selection signal Sel and transmit it to the comparator CMP. The selected voltage Vsel can be generated by the control circuit 18 or other circuits and can correspond to the third coil. For example, if the third coil is the coil Ww, the control circuit 18 can generate the selection signal Sel to select the divided voltage Vdw as the selected voltage Vsel. The comparator CMP can compare the selected voltage Vsel and the virtual neutral point voltage VN to generate the comparison signal Sc. If the selected voltage Vsel exceeds the virtual neutral point voltage VN, the comparator CMP can set the comparison signal Sc to the output logic high level (such as the supply voltage VCC); if the selected voltage Vsel does not exceed the virtual neutral point voltage VN, the comparator CMP can set the comparison signal Sc to the output logic low level (such as the ground voltage GND).

[0068] The voltage regulation circuit 16 does not need to use a filter and samples after the comparison signal is stable, avoiding the phase delay of the sensed back electromotive force voltage signal caused by using a filter, correctly detecting the zero crossing of the back electromotive force, and reducing the manufacturing cost.

[0069] Figure 6 This is the waveform diagram of the motor circuit 1, where the horizontal axis represents time t and the vertical axis represents voltage. Figure 6Display terminal voltage Vw, divided voltage Vdw, virtual neutral voltage VN, and comparison signal Scw.

[0070] At time t1, the terminal voltage Vw starts to be floating, the divided voltage Vdw is less than the virtual neutral voltage VN, so the sampled data of the comparison signal Scw is at a logic low level. Between time t1 and t2, the terminal voltage Vw remains floating, the divided voltage Vdw gradually rises but is still less than the virtual neutral voltage VN, so the sampled data of the comparison signal Scw remains at a logic low level. At time t2, the divided voltage Vdw exceeds the virtual neutral voltage VN, so the sampled data of the comparison signal Scw is at a logic high level. Since the state of the sampled data changes from a logic low level to a logic high level, the control circuit 18 determines that the back electromotive force zero crossing is detected, and based on the back electromotive force zero crossing, the drive circuit 10 can be controlled to commutate the brushless DC motor 14 after a 30-degree electrical angle delay. The terminal voltage Vw is a pulse width modulation voltage.

[0071] At time t3, the terminal voltage Vw starts to be floating, the divided voltage Vdw exceeds the virtual neutral voltage VN, so the sampled data of the comparison signal Scw is at a logic high level. Between time t3 and t4, the terminal voltage Vw remains floating, the divided voltage Vdw gradually decreases but is still greater than the virtual neutral voltage VN, so the sampled data of the comparison signal Scw remains at a logic high level. At time t4, the divided voltage Vdw is less than the virtual neutral voltage VN, so the sampled data of the comparison signal Scw is at a logic low level. Since the state of the sampled data changes from a logic high level to a logic low level, the control circuit 18 determines that the back electromotive force zero crossing is detected, and the drive circuit 10 is controlled to commutate the brushless DC motor 14 based on the back electromotive force zero crossing. The terminal voltage Vw is the ground voltage GND.

[0072] Figure 7 Is Figure 6 The amplified waveform diagram of the motor circuit 1, where the horizontal axis represents time t and the vertical axis represents voltage. Figure 7 Display the terminal voltage Vw, divided voltage Vdw, virtual neutral voltage VN, and comparison signal Scw. Since the working cycle of the pulse width modulation voltage is greater than the preset working cycle, the control circuit 18 determines that the sampling time point of the comparison signal Scw is the end time point of the conduction period of the pulse width modulation voltage (i.e., sampling time points t1, t2, and t3).

[0073] At the sampling time point t1, both the partial pressure Vdw and the virtual neutral voltage VN are stable. The control circuit 18 samples the comparison signal Scw and generates a logic low level as the sampled data. At the sampling time point t2, both the partial pressure Vdw and the virtual neutral voltage VN are stable. The control circuit 18 samples the comparison signal Scw and generates a logic low level as the sampled data. At the sampling time point t3, both the partial pressure Vdw and the virtual neutral voltage VN are stable. The control circuit 18 samples the comparison signal Scw and generates a logic high level as the sampled data. Since the state of the sampled data changes from a logic low level to a logic high level, the control circuit 18 determines that the back electromotive force zero crossing is detected. Based on the back electromotive force zero crossing, the driving circuit 10 can be controlled to commutate the brushless DC motor 14 after a delay of 30 electrical degrees.

[0074] The above are only the preferred embodiments of the present invention. All equivalent changes and modifications made according to the claims of the present invention shall fall within the scope of the present invention.

Claims

1. A sensorless motor circuit, comprising: A brushless DC motor, including a first coil, a second coil, and a third coil, each of the first coil, the second coil, and the third coil including a first end having an end voltage and a second end; A drive circuit, coupled to the brushless DC motor, for outputting a pulse-width modulation signal to a first end of the first coil, coupling a first end of the second coil to a ground terminal, and floating a first end of the third coil; A voltage adjustment circuit, coupled to the first end of the third coil, for generating a voltage division based on an end voltage of the third coil; A virtual neutral point circuit, coupled to the second ends of the first coil, the second coil, and the third coil, for generating a virtual neutral point voltage: A comparator, coupled to the voltage adjustment circuit and directly coupled to the virtual neutral point circuit, for comparing the voltage division and the virtual neutral point voltage to generate a comparison signal; and A control circuit, directly coupled to the comparator, for determining a sampling time of the comparison signal according to a duty cycle of the pulse-width modulation signal, and sampling the comparison signal at the sampling time to detect a back electromotive force zero crossing.

2. The sensorless motor circuit as described in claim 1, wherein, If the duty cycle is less than a preset duty cycle, the control circuit determines that the sampling time is in a cut-off period of the pulse-width modulation signal and after the pulse-width modulation signal is stable.

3. The sensorless motor circuit as described in claim 2, wherein, The sampling time is an end time point of the cut-off period of the pulse-width modulation signal.

4. The sensorless motor circuit as claimed in claim 1, wherein, If the duty cycle is not less than a preset duty cycle, the control circuit determines that the sampling time is in a conduction period of the pulse-width modulation signal and after the pulse-width modulation signal is stable.

5. The sensorless motor circuit as described in claim 4, wherein, The sampling time is an end time point of the conduction period of the pulse-width modulation signal.

6. The sensorless motor circuit as described in claim 1, wherein, The control circuit samples the comparison signal at the sampling time to generate current sampling data, and if the states of the current sampling data and the previous sampling data are different, it is determined that the back electromotive force zero crossing is detected.

7. The sensorless motor circuit as claimed in claim 1, wherein, The comparator is directly coupled to the voltage adjustment circuit.

8. The sensorless motor circuit according to claim 1, further comprising a multiplexer, directly coupled to the voltage adjustment circuit and the comparator, for transmitting the voltage division to the comparator according to a selection signal.

9. The sensorless motor circuit according to claim 1, wherein, The virtual neutral point circuit includes: A first virtual neutral point resistor, including a first end coupled to a first end of the first coil and a second end for generating the virtual neutral point voltage; A second virtual neutral point resistor, including a first end coupled to a first end of the second coil and a second end coupled to the second end of the first virtual neutral point resistor; and A third virtual neutral point resistor, including a first end coupled to a first end of the third coil and a second end coupled to the second end of the first virtual neutral point resistor.

10. A method for operating a sensorless motor circuit, the sensorless motor circuit comprising a brushless DC motor, a drive circuit, a voltage regulation circuit, a virtual neutral point circuit, a comparator, and a control circuit. The brushless DC motor includes a first coil, a second coil, and a third coil. Each of the second coil and the third coil includes a first end having an end voltage and a second end. The drive circuit is coupled to the brushless DC motor. The voltage regulation circuit is coupled to a first end of the third coil. The virtual neutral point circuit is coupled to a second end of the first coil, a second end of the second coil, and the second end of the third coil. The comparator is coupled to the voltage regulation circuit and directly coupled to the virtual neutral point circuit. The control circuit is directly coupled to the comparator. The method includes: The drive circuit outputs a pulse width modulation signal to a first end of the first coil, couples a first end of the second coil to a ground terminal, and floats the first end of the third coil; The voltage regulation circuit generates a voltage division based on an end voltage of the third coil; The virtual neutral point circuit generates a virtual neutral point voltage; The comparator compares the voltage division and the virtual neutral point voltage to generate a comparison signal; The control circuit determines a sampling time of the comparison signal based on a duty cycle of the pulse width modulation signal; and The control circuit samples the comparison signal at the sampling time to detect a back electromotive force zero crossing to detect a back electromotive force zero crossing.

11. The method according to claim 10, wherein, The control circuit determining the sampling time of the comparison signal based on the duty cycle of the pulse width modulation signal includes: If the duty cycle is less than a preset duty cycle, the control circuit determines that the sampling time is in a cut-off period of the pulse width modulation signal and after the pulse width modulation signal is stable.

12. The method according to claim 11, wherein, The sampling time is at an end time point of the cut-off period of the pulse width modulation signal.

13. The method as claimed in claim 10, wherein, The control circuit determining the sampling time of the comparison signal based on the duty cycle of the pulse width modulation signal includes: If the duty cycle is not less than a preset duty cycle, the control circuit determines that the control circuit determines that the sampling time is in a conduction period of the pulse width modulation signal and after the pulse width modulation signal is stable.

14. The method according to claim 13, wherein, The sampling time is at an end time point of the conduction period of the pulse width modulation signal.

15. The method according to claim 10, wherein, The control circuit sampling the comparison signal at the sampling time to detect a back electromotive force zero crossing to detect a back electromotive force zero crossing includes: The control circuit samples the comparison signal at the sampling time to generate current sampling data; and If the states of the current sampling data and the previous sampling data are different, it is determined that the back electromotive force zero crossing is detected.

16. The method according to claim 10, wherein, The sensorless motor circuit further includes a multiplexer directly coupled to the voltage regulation circuit and the comparator. The method further includes the multiplexer transmitting the voltage division to the comparator according to a selection signal.

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