Motor control circuit with power factor correction

A motor control circuit with power factor correction capabilities that optimizes the voltage and current load applied to an electric motor for different motor speeds and torque levels. The preferred motor control circuit includes a power factor correction circuit and a step down conversion circuit through which current passes before it reaches the motor. A microprocessor preferably monitors the current supplied to the motor and the motor's speed. If the microprocessor determines that the current supplied to the motor is too high, it can reduce the level of current by either using a pulse width modulation (PWM) digital-to-analog control circuit to instruct the power factor correction circuit to reduce current or it can use a PWM digital control circuit to instruct the step down conversion circuit to reduce current. An output voltage limiter circuit can be used to detect the voltage of current supplied to the motor and turn off current to the motor if the voltage is above a predetermined level.

FIELD OF THE INVENTION

The present invention relates to a motor control circuit that performs power factor correction and helps optimize the electrical output load to the motor, particularly for low speed, low torque motor operation.

BACKGROUND OF THE INVENTION

Recently, governments around the world have encouraged efficient energy use for electronic products and home appliances. In response to this initiative, power factor correction (PFC) circuits have been developed for such electronic products and home appliances. A PFC circuit is a circuit for switching input power and adjusting a phase difference (power factor) between a current and a voltage of the input power to efficiently transfer power to a load.

Integrated circuit chips have been created to allow circuit designers to include PFC functionality in their circuits. One such PFC chip is the “FL7701” Smart LED Lamp Driver IC with PFC Function chip manufactured and sold by On Semiconductor (Linear Technologies). While the FL7701 chip is useful for adding PFC functionality to an electrical circuit, it was designed for continuous resistive load LED lamp applications. Nonetheless, there are many other applications besides LED lamps where PFC functionality is needed, especially for inductive loads such as motors used in electronic products and home appliances.

Electric motors present circuit designers with different requirements and challenges than LED lamps, namely the capacitive and inductive nature of the motor load. For example, electric motors can be operated at different speeds and torque levels. While the FL7701 chip is particularly useful for LED lamps, it is not optimized for operation with the full range of speeds and torques utilized by electric motors. The FL7701 chip is particularly problematical when a motor is operated at low speed and low torque. For low speed, low torque operation, the voltage and current supplied to the motor by the FL7701 chip can be too high. What is needed is an improved PFC circuit that safely enables and optimizes the full range of motor capabilities, particularly for low speed, low torque operation.

BRIEF SUMMARY OF THE INVENTION

The present invention is directed to a motor control circuit with PFC capabilities that optimizes the voltage and current load applied to the electric motor for different motor speeds and torque levels. In the preferred embodiment, the motor control circuit includes a chip providing PFC and current control functionality. The output of the PFC chip is connected to a step down conversion circuit to drive the motor.

To optimize motor performance, additional circuitry is preferably connected to the electric motor, PFC chip and step down conversion circuit. For example, the current load and speed of the motor is preferably monitored by a firmware microprocessor. When appropriate, the microprocessor provides a buffered motor drive signal to a pulse width modulation (PWM) digital-to-analog circuit connected to the PFC chip. This PWM digital-to-analog circuit adjusts the output current of the PFC chip to optimize the current level supplied to the motor. Through use of an optocoupler, the PWM digital-to-analog circuit also preferably isolates the firmware microprocessor from high voltages in the motor drive circuit.

Additionally, current supplied to the motor can be optimized by a PWM digital control circuit which receives a buffered motor drive control signal from the microprocessor. Where the current supplied to the motor is too high, this PWM digital control circuit can intercept and override the output signal from the PFC chip at the step down conversion circuit. The PWM digital control circuit is particularly useful when the output load from the PFC chip is set at its minimum level but the power supplied to the motor is still too high. By adjusting the duty cycle, this PWM digital control circuit can make the output power very low or turn it off completely.

Finally, the output voltage across the motor is monitored by an output voltage limiter circuit which is capable of limiting the speed of the motor. This output voltage limiter circuit preferably includes a Zener diode and an optocoupler. When the output voltage limiter circuit determines that the output voltage is too high, such as higher than 120V, the Zener diode and optocoupler in the voltage limiter circuit directs the PFC chip to pull down the output current. This current restriction, which results in a voltage and corresponding speed reduction to the motor, is accomplished by circuit hardware which is independent of the microprocessor.

DETAILED DESCRIPTION OF THE INVENTION

Referring now toFIG. 1, a preferred form of motor control circuit100of the present invention is shown in functional block diagram form. The motor control circuit100preferably includes a motor200, a PFC circuit300, a step down conversion circuit400, a PWM digital-to-analog circuit500, a PWM digital control circuit600, a firmware microprocessor700and an output voltage limiter circuit800. The motor200is an electric motor, preferably a Permanent Magnet D.C., of the type commonly used in consumer electric products and home appliances. Nonetheless, the principles of this invention are also applicable to other types of electric motors.

Turning now toFIG. 2, the PFC circuit300may correct a power factor by switching rectified power obtained from alternating current (AC) power through a bridge diode12. The PFC circuit300of the present invention preferably includes a PFC chip302, such as the FL7701 chip manufactured and sold by On Semiconductor (Linear Technologies). The preferred FL7701 chip has eight pins, labeled CS, OUT, VCC, RT, HV, NC, GND and ADIM. System reference voltage from the bridge diode12is provided to the PFC chip302through the HV pin. Once the PFC chip302has performed its power factor correction function, a transistor control current leaves the PFC chip302though its OUT pin and controls the power transistor402. The primary flow of current is through the bridge rectifier12and the inductor408before reaching the motor200. The power transistor402is the final piece in the current flow path back to the low side of the main power supply (the AC line input depicted to the left of the bridge rectifier12).

The power transistor402in step down conversion circuit400is the primary control element responsible for controlling current flow from its input (supply) to its output (load). In the preferred embodiment, the step down conversion circuit400preferably includes a power transistor402(which is preferably an insulated-gate bipolar transistor (IGBT)), a diode404, a capacitor406and an SMC compliant inductor408. The power transistor402is a three-terminal power semiconductor device used as an electronic switch. Since it is designed to turn on and off rapidly, the power transistor402can synthesize complex waveforms with pulse width modulation (PWM).

PWM is a method of reducing the average power delivered by an electrical signal by effectively chopping it up into discrete parts. The average value of voltage (and current) fed to the load is controlled by turning the switch on and off at a fast rate between supply and load. The longer the switch is on compared to the off periods, the higher the total power supplied to the load. PWM is particularly suited for running inertial loads such as electric motors, which are not easily affected by discrete switching because they have the inertia to react slowly. Nonetheless, the PWM switching frequency needs to be high enough not to affect the load, which is to say that the resultant waveform perceived by the load must be as smooth as possible.

PWM digital-to-analog circuit500is one of the preferred ways the present invention varies the current load applied to the motor200so that it is optimized for different motor speeds and torque. The PWM digital-to-analog circuit500is particularly useful when the motor200needs to be operated at a low-speed, low-torque level where the voltage and/or current supplied to the motor200by the PFC chip302, even at its minimum level, is too high. To operate the PWM digital-to-analog circuit, signals202,204are received from the motor200by the microprocessor700indicative of the operating speed202of the motor200and current204being supplied to the motor (FIG. 1). Based upon these signals202,204, the microprocessor700determines whether the current needs to be adjusted to achieve optimum motor performance. If adjustment is needed, the microprocessor700can send a buffered control signal702to the PWM digital-to-analog control circuit500to adjust the output of the PFC chip302.

The buffered control signal702passes through resistor502until it reaches optocoupler504which, in the preferred embodiment, is a combination of LED-photodiode506and phototransistor508. The optocoupler504transfers the buffered control signal702using light. Voltage to reconstitute the buffered control signal702is provided by the VCC pin of PFC chip302and passes through capacitors510,512and514. The reconstituted control signal706passes through resistors516,518until it reaches the ADIM input pin of PFC chip302. If the microprocessor700determines that the current being supplied to the motor200is too high, the reconstituted control signal706will direct the PFC chip302to intermittently turn off current to the motor200using pulse width modulation. Conversely, if the microprocessor700determines that the current supplied to the motor200is too low, the reconstituted control signal706can direct the PFC chip302to increase the current provided to the motor200.

A second preferred way in the present invention to vary current load applied to the motor200so that it is optimized for different motor speeds and torque is through the use of PWM digital control circuit600. If the microprocessor700determines that current needs to be adjusted to achieve optimum motor performance, particularly if the motor frequency is determined to be too high, the microprocessor700can send a buffered control signal704to PWM digital control circuit600. The buffered control signal704passes through resistor602until it reaches optocoupler604which, in the preferred embodiment, is a combination of LED-photodiode606and phototransistor608. Again, the optocoupler610uses light to transfer the buffered control signal704and protect control circuitry from high voltages. Voltage to reconstitute the buffered control signal704is provided by VCC pin of PFC chip302and passes through capacitor514. The reconstituted control signal604is then fed to the power transistor402of step down conversion circuit400. When activated, the PWM digital control circuit600can intercept and override the output signal from the PFC chip302at the step down conversion circuit400and, in the process, intermittently turn off the power transistor402if the motor frequency is too high. The PWM digital control circuit600is particularly useful when the output load from the PFC chip302is set at its minimum level but the power supplied to the motor200is still too high. By adjusting the duty cycle, this PWM digital control circuit600can make the output power very low or turn it off completely.

Output voltage limiter circuit800provides a third way of optimizing motor performance. The output voltage limiter circuit800allows current to the motor200to be turned off if voltage across the motor200is higher than a predetermined level, such as 120V. The output voltage limiter circuit800preferably includes diode806, Zener diode808, resistor810and optocoupler804.

A Zener diode is a diode that normally allows current to flow in the conventional manner from its anode to its cathode. Nonetheless, when the voltage across the Zener diode reaches a predetermined level, referred to as the “Zener voltage,” the junction will break down and current will flow in the reverse direction.

For the output limiter circuit800of the present invention, current normally flows from bridge diode12through SMC compliant inductor408and into motor200. The Zener diode808normally blocks current from flowing through the output voltage limiter circuit800. Nonetheless, when the voltage reaches a predetermined threshold level, such as 120V, the Zener diode junction will break down so that current flows through the Zener diode808of the output limiter circuit800. The predetermined threshold voltage for the Zener diode808is preferably set at a level where the current and voltage impairs operation of the motor200. When that predetermined threshold level is reached, current flows through resistor810, Zener diode808and diode806. In this mode of operation, Zener diode808works with optocoupler804to send a signal to the ADIM pin of PFC chip302to temporarily turn off current to the motor200until the voltage across the Zener diode808has dropped below the predetermined threshold level. In this way, the output limiter circuit800protects the motor200from potentially damaging high levels of voltage and current.

In the foregoing specification, the invention has been described with reference to specific preferred embodiments and methods. It will, however, be evident to those of skill in the art that various modifications and changes may be made without departing from the broader scope of the invention as set forth in the appended claims. The specification and drawings are, accordingly, to be regarded in an illustrative, rather than restrictive sense; the invention being limited only by the appended claims.