WO2007004429A1

WO2007004429A1 - Brushless dc motor and electric device using the same

Info

Publication number: WO2007004429A1
Application number: PCT/JP2006/312458
Authority: WO
Inventors: Toshiki Tsubouchi; Kenji Sugiura; Hidetaka Noda
Original assignee: Matsushita Electric Industrial Co., Ltd.
Priority date: 2005-07-06
Filing date: 2006-06-15
Publication date: 2007-01-11
Also published as: JP4682985B2; CN1989680B; JP2008538692A; CN1989680A; KR20070032959A; JP2011055703A

Abstract

A brush-less DC motor includes a stator assembly having drive-coils for plural phases, a rotor assembly having a permanent magnet, and a built-in printed wired board (PWB) having a driving circuit for driving the drive coils. The driving circuit includes MOSFETs powering the drive coils, gate drivers controlling the MOSFETs, and a pre-driver supplying a PWM signal to the gate drivers. The PWB includes a power module formed by unifying the MOSFETs and the gate drivers with molding resin. The power module includes a setting section which sets electrical strength of the MOSFETs.

Description

DESCRIPTION

BRUSHLESS DC MOTORAND ELECTRIC DEVICE USING THE SAME

TECHNICAL FIELD

The present invention relates to a brush-less DC motor with a driving circuit built therein, and an electric device using the same motor.

BACKGROUNDART A compact brush-less DC motor, of which shaft output ranges from 20 to

50 watts, is used for driving an air blower used in air-conditioners and various home appliances. In general, this motor includes a driver having various electronic components on its printed wired board.

Fig. 13 shows a sectional view of a conventional brush-less DC motor 101. Stator mold assembly 103 comprises the following elements: stator core 120 formed by layering electromagnetic steel sheets; drive coils 121, for plural phases, wound on stator core 120; and unsaturated polyester resin which houses stator core 120 and drive coils 121 together in one unit. A first end of stator mold assembly 103 is covered with metallic bracket 104. A second end of assembly 103 and a center section of bracket 104 have bearing holders.

Rotor assembly 119 comprises the following elements^: rotor yoke 118 formed by layering electromagnetic steel sheets; multi-pole permanent magnet 117 provided to outer wall of rotor yoke

118 and confronting inner wall of stator core 120 with a given clearance in between; shaft 105 press-fitted at the center of yoke 118; and bearing 114 supporting shaft 115 rotatably.

Printed wired board (PWB) 113 having a driving circuit which drives coils 121 is rigidly mounted to stator mold assembly 103. Ends of coils 121 are coupled to PWB 113 via terminal pin 122. Components such as MOSFET array 110, pre-driver 112 and many other discrete components (not shown) are soldered to PWB 113, and magnetic sensor 125 for sensing a magnetic pole of permanent magnet 117 is also soldered to PWB 113. Insulating board 115 is placed between PWB 113 and bracket 104, so that PWB 113 is insulated from bracket 104.

Fig. 14 shows a plan view of printed wired board assembly (PWB Assy) 107. Components such as MOSFET array 110, pre-driver 112, three gate drivers 111, and many other discrete components 131 are mounted to PWB 113. Lead-wire assembly 102 is provided to PWB 113 for receiving an input and supplying an output of PWB 113. Discrete components 131 include resistors and capacitors.

MOSFET array 110 is formed by unifying six MOSFETs into an array, and drives coils 121. Each one of the MOSFETs has an avalanche resistance, so that it will not be damaged easily if a pulse-like voltage exceeding the withstanding voltage such as a surge voltage of the MOSFET is applied. Use of MOSFET array 110 thus advantageously assures reliability.

On the other hand, in order to boost the electrical strength of the MOSFET for increasing its reliability, it is needed to set various factors strictly such as voltage varying rate "dV/dt" between drain and source, drain current varying rate "dl/dt", turn-on delaying time "td" (on), turn-off delaying time "td" (off). To achieve .this objective, discrete components such as resistors, capacitors, and diodes are needed at least 4 - 6 pieces at respective gates, so that from 4 x 6 = 24 to 6 x 6 = 36 pieces of discrete components are needed. Auxiliary components on top of these numbers will make the total number reach almost 100 pieces, so that the PWB resists being downsized.

As a switching element the MOSFET is replaced with IGBT, so that the driving circuit is formed of a monolithic IC. This structure is disclosed in

Unexamined Japanese Patent Publication No. H03 - 270677. Although this structure allows downsizing the PWB, the reliability improvement due to avalanche resistance proper to MOSFET cannot be expected.

DISCLOSURE OF INVENTION

The brush-less DC motor of the present invention comprises the following elements^ a stator assembly having drive coils for plural phases; a rotor assembly having a permanent magnet; and a printed wired board (PWB) built in the motor and having a driving circuit mounted thereon for driving the drive coils.

The driving circuit includes MOSFETs for powering the drive coils, gate drivers for controlling the MOSFETs, and a pre-driver for supplying a PWM (pulse width modulation) signal to the gate drivers. The PWB includes a power module formed by unifying the MOSFETs and the gate drivers with molding resin. This power module features inclusion of a setting section which sets the electrical strength of the MOSFETs.

The structure discussed above allows providing a reliable brush-less DC motor featuring compact, light-weight, and easy to be wired and mounted. The present invention thus can provide also electric devices using the same brush-less DC motor. , BRIEF DESCRIPTION OF THE DRAWINGS

Fig. 1 shows an external appearance of a brush-less DC motor in accordance with a first embodiment of the present invention.

Fig. 2 shows a sectional view of the motor shown in Fig. 1. Fig. 3 shows a circuit diagram of the motor shown in Fig. 1.

Fig. 4 shows a sectional view of a power module of the brush-less DC motor shown in Fig. 1.

Fig. 5 shows a plan view of a printed wired board assembly of the motor shown in Fig. 1. Fig. 6 shows a circuit diagram detailing one phase of the phases of the motor shown in Fig. 1.

Fig. 7 shows a timing chart illustrating a relation between an input signal and an output signal of a gate driver of the motor shown in Fig. 1.

Fig. 8 shows a timing chart detailing an enlarged section around time "tl" shown in Fig. 7.

Fig. 9 shows a timing chart illustrating a relation similar to that shown in Fig. 7,^" however, this timing chart has a shorter time span between times "t2" and "t3" than those shown in Fig. 7.

Fig. 10 shows a timing chart detailing a case when a threshold voltage of a MOSFET is boosted.

Fig. 11 shows an external appearance of an electric device in accordance with a second embodiment of the present invention.

Fig. 12 shows a circuit diagram of the electric device shown in Fig. 11.

Fig. 13 shows a sectional view of a conventional brush-less DC motor. Fig. 14 shows a plan view of a printed wired board assembly of the conventional brush-legs DC motor shown in Fig. 13. DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Exemplary embodiments of the present invention are demonstrated hereinafter with reference to the accompanying drawings. Embodiment 1 Fig. 1 shows an external appearance of a brush-less DC motor in accordance with the first embodiment of the present invention. Spindle motor assembly 3 is covered with lid-like bracket 4, and shaft 5 extends through bracket 4. Lead-wire assembly 2 extends from a lateral face of stator mold assembly 3. Fig. 2 shows a sectional view of the brush-less DC motor in accordance with the first embodiment. Stator mold assembly 3 comprises the following elements^ stator core 20 formed by layering electromagnetic steel sheets! drive coils 21 for plural phases wound on stator core 20! and unsaturated polyester resin which encloses stator core 20 and drive coils 21 together in one unit.

A first end of stator mold assembly 3 is covered with metallic bracket 4. A second end of assembly 3 and a center section of bracket 4 have bearing holders.

Rotor assembly 19 comprises the following elements^ rotor yoke 18 formed by layering electromagnetic steel sheets; multi-pole permanent magnet 17 provided to outer wall of rotor yoke 18 and confronting inner wall of stator core 20 with a given clearance in between! shaft 5 press-fitted at the center of yoke 18! and bearing 14 supporting shaft 15 rotatably.

Printed wired .board (PWB) 13 having a driving circuit which drives coils 21 is rigidly mounted to stator mold assembly 3. Ends of coils 21 are coupled to PWB 13 via terminal pin 22. Components such as MOSFET array 10, pre-driver 12 and many other discrete components (not shown) are soldered to PWB 13, and magnetic sensor 25 for sensing a magnetic pole of permanent magnet 17 is also soldered to PWB 13. Insulating board 15 is placed between PWB 13 and bracket 4, so that PWB 13 is insulated from bracket 4.

Power module 10 mounted on PWB 13 dissipates its heat to bracket 4 via highly heat-conductive resin 16, which employs silicone -based resin excellent in heat conductivity. Since the silicone -based resin is elastic, it can absorb dispersion in a space between power module 10 and bracket 4. Fig. 3 shows a circuit diagram of the brush-less DC motor in accordance with the first embodiment. Driving circuit 6 receives the following voltages: high DC voltage Vdc supplied from high voltage DC power supply 9, control voltage Vcc from control power supply 23, and control signal voltage Vsp of speed control signal 24. Driving circuit 6 outputs motor rotating signal FG to be used for speed control.

Driving circuit 6 comprises the following elements • three magnetic sensors 25 for sensing a magnetic pole position of the rotor? pre-driver 12 for receiving a signal from magnetic sensor 25 and generating a PWM signal; three gate drivers 11 for receiving a signal from pre-driver 12 and generating a control signal for MOSFETs 8; and six MOSFETs 8 connected in three-phase bridge.

Magnetic sensor 25 usually employs a Hall element or a Hall IC. An output current is sensed by current sensing resistor 26, and fed back to pre-driver 12.

The three gate, drivers 11 and six MOSFETs 8 are unified into pne unit, thereby forming power module 10. Fig. 4 shows a sectional view of this power module 10. Gate drivers 11 and MOSFETs 8 are rigidly bonded to frame 28, and bonding wires 30 connect gate drivers 11 to MOSFETs 8 as well as lead-electrodes 27 to those two elements. All of these elements are molded with epoxy resin 29 into the one unit. Fig. 5 shows a plan view of printed wired board (PWB) assembly 7.

Power module 10, pre-driver 12 and discrete components 31 are mounted on PWB 13, which also includes lead-wire assembly 2 for receiving an input and supplying an output of PWB 13. Discrete components 31 include resistors, capacitors, and so on. Fig. 6 shows a part of the circuit diagram of the brush-less DC motor in accordance with the first embodiment, and this diagram details one phase of the entire circuit diagram shown in Fig. 3. MOSFET 8 shown in Fig. 3 is actually formed of MOSFETQl connected to MOSFETQ2 in series, and receives high DC voltage Vdc. MOSFETQl includes flywheel diode Dl and gate capacitor Cl as parasitic elements. MOSFETQ2 also includes flywheel diode D2 and gate capacitor C2 as parasitic elements.

Gate driver 11 includes hysteresis comparator HSl, HS2, level shifting circuit LSI, resistors Rl, R2, R3, R4, and output electronic switches SWl, SW2, SW3, and SW4. Gate driver 11 receives PWM signals generated in pre-driver 12 as input signals HIN, LIN, and supplies output signals HO, LO to the respective gates of MOSFETQl and MOSFETQ2.

Fig. 7 shows a timing chart illustrating the relations between input signals HIN, LIN and output signals HO, LO. In the period of time "t" < tl, both of input signals HIN and LIN stay at a low level, so that output electronic switches SWl, SW3 on the upper side are turned off and switches SW2, SW4 on the lower side are turned on. Both of output signals HO and LO are thus kept at a low level, and MOSFETQl, MOSFETQ2 stay in OFF status. Next, at the time of "t" = tl, input signal HIN rises to a high level, and input signal LIN remains at the low level. Switch SWl is thus turned on, and switch SW2 is turned off. Rise of output signal HO to a high level turns on MOSFETQl. MOSFETQ2 remains in OFF status. At this time, stored electric charge in boot capacitor C3 is supplied to the gate of MOSFETQl via resistor Rl, and output signal HO is to rise with a certain time constant. MOSFETQl stays in OFF status until output signal HO reaches threshold voltage Vth of MOSFETQl, and at the time of "t" = tlO, i.e. when signal HO reaches voltage Vth, MOSFETQl is turned ON, which prompts supplying high DC voltage Vdc to drive coils 21.

Next, at the time of "t" = t2, input signal HIN falls to the low level, and input signal LIN remains at the low level. Switch SWl is thus turned off, and switch SW2 is turned on, and output signal HO falls with a time constant determined by resistor R2 and gate capacitor Cl of MOSFETQl. MOSFETQl stays in ON status until output signal HO becomes lower than threshold voltage Vth of MOSFETQl, and it becomes OFF status when output signal HO becomes lower than threshold voltage Vth, i.e. at the time oft" = t20.

At the time of "t" = t3, input signal HIN remains at the low level, and input signal LIN rises to the high level, so that switch SW3 is turned on and switch SW4 is turned off. Output signal LO rises with a time constant determined by capacitor C2 and resistor 3. MOSFETQ2 stays in OFF status until output signal LO reaches threshold voltage Vth of MOSFETQ2, and at the time oft" = t30, i.e. when signal LO reaches voltage Vth, MOSFETQ2 is turned ON. At the time of "t" = t4, input signal HIN remains at the low level, and input signal LIN falls to the low level. Switch SW3 is thus turned, off, and switch SW4 is turned on. Output signal LO falls with a time constant determined by gate capacitor C2 of MOSFETQ2 and resistor 4. MOSFETQ2 stays in ON status until output signal LO becomes lower than threshold voltage Vth of MOSFETQ2, and at the time of "t" = t40, i.e. when signal LO becomes lower than voltage Vth, MOSFETQ2 is turned OFF. Then at the time of "t" = t5, the state turns to the same state appearing at "t" = tl, and the sequence of the states discussed above is repeated.

Fig. 8 enlarges a portion of tl shown in Fig. 7, and additionally shows variation of terminal voltage VU of the drive coil. At the time oft" = tl, when input signal HIN changes from the low level to the high level, output signal HO starts rising at "t" = til after the transfer time of hysteresis comparator HSl, level shifting circuit LSI and output electronic switch SWl (Fig. 7 omits this transfer time).

Then output signal HO reaches threshold voltage Vth, and MOSFETQl is turned on; however, in actual, signal HO scarcely changes at a voltage around threshold voltage Vth during the transition period of MOSFETQl changing from OFF to ON status. Because the status shift of MOSFETQl from OFF to ON changes a voltage between its drain and source, so that gate capacitance Cl of Ql becomes apparently much greater (mirror effect). To be more specific, at "t" = tl2, output signal HO reaches threshold voltage Vth, and MOSFETQl starts turning on and terminal voltage of the drive coil starts rising. During the rise of voltage VU, signal HO scarcely changes due to the mirror effect. At "t"= tl3, voltage VU reaches approx. high DC voltage Vdc, and signal HO starts further rising because the mirror effect does not work anymore. At this time, the rate of change "dV/dt" in voltage VU becomes greater at a shorter time between tl2 and tl3. .

The rate of change "dV/dt" in voltage VU is determined by a time constant depending on resistor Rl and the feedback capacitance between the gate and drain of MOSFETQl, so that the rate of change "dV/dt" can be set by adjusting resistor Rl or the feedback capacitance. Usually, the rate of change "dV/dt" is set with a value of resistor Rl easily adjustable. For instance, in the case of equipping home appliances with a motor, resistor Rl is adjusted such that the rate of change "dV/dt" becomes approx. 2kV/μsec.

The foregoing description refers to a case where the status of MOSFETQl shifts from OFF to ON; however, the rate of change "dV/dt" in terminal voltage VU of the drive coil can be set when the status of MOSFETQl shifts from ON to OFF, namely, the rate of change "dV/dt" in this case can be set by adjusting resistor R2.

A similar setting to what is discussed above can be done to MOSFETQ2, i.e. a "dV/dt" of Q2 shifting from OFF to ON can be set by adjusting resistor R3, and a "dV/dt" of Q2 shifting from ON to OFF can be set by adjusting resistor R4. As discussed above, resistors Rl, R2, R3, and R4 work as a setting section for setting the rate of change in voltage between the drain and source of MOSFETQl and Q2. Next, a rate of change in electric current "dl/dt" is described hereinafter.

The rate of change in drain current "dl/dt" at the status shift from OFF to ON of MOSFETQl is observed while voltage VU is varying in Fig. 8. During the variation of voltage VU, signal HO scarcely changes around threshold voltage Vth of MOSFETQl because the mirror effect works as discussed previously! however, it slightly changes in actual. This slight change in the rate of change around threshold voltage Vth and a mutual conductance of MOSFETQl determine the rate of change in drain current "dl/dt" of MOSFETQl. The slight change in the rate of change of signal HQ around threshold voltage Vth can be determined by resistor Rl or capacitor Cl between the gate and source of MOSFETQl. In other words, adjustment of resistor Rl or the mutual conductance of MOSFETQl can set the rate of change in drain current "dl/dt" of MOSFETQl. However, since resistor Rl is used for setting the rate of change in voltage "dV/dt", capacitor Cl or the mutual conductance is used for setting the rate of change in drain current "dl/dt".

The foregoing description refers to a case where the status of MOSFETQl shifts from OFF to ON; however, the rate of change "dl/dt" in drain current can be set when the status of MOSFETQl shifts from ON to OFF, namely, the rate of change "dl/dt" can be set by adjusting capacitor Cl or the mutual conductance.

A similar setting to what is discussed above can be done to MOSFETQ2, i.e. a "dl/dt" of Q2 can be set by adjusting capacitor C2 or the mutual conductance of MOSFETQ2.

As discussed above, capacitors Cl, C2, or mutual conductance of MOSFETQl or Q2 can work as a setting section for setting the rate of change "dl/dt" in drain current of MOSFETQl or Q2.

Fig. 9 illustrates how the motor operates when t2 is close to t3 in the circuit shown in Fig. 6. To be more specific, input signal HIN shifts from the high level to the low level at "t" = t2, and input signal LIN shifts from the low level to the high level at "t" = t3. Fig. 9 thus shows the operation when t2 is close to t3, namely, output signal HO starts falling at "t" = t2, and still stays higher than threshold voltage Vth at "t" = t3, when output voltage LO starts rising. In this case, if MOSFETQ2 is turned ON before MOSFETQl is turned OFF, a pass current runs from Ql to Q2, thereby damaging these MOSFETs. To prevent such > a pass current from running, the relation between resistors Rl and R2 working as the setting section should be adjusted ,to be Rl » R2 in advance so that at least "t" = t20 can appear before "t" = t30. The time span between "t" = t2 at which input signal HIN changes from the high level to the low level and "t" = t3 at which input signal LIN changes from the low level to the high level is generally called a dead time. This dead time is prepared to be long enough with respect to a delajr time occurring usually in gate driver 11, MOSFET 8 and the components nearby. However, since the drive coils are not powered if the dead time lasts long, the motor used in a fan of home appliances, of which shaft output ranges from 20 to 50 watts, sometimes invites inconveniences such as noises and vibrations. The dead time should be thus minimized, so that the values of resistors Rl, R2 and the capacitance of the gate capacitor should be carefully studied in advance.

Fig. 10 illustrates an operation when threshold voltage Vth of MOSFET per se is boosted from Vthl to Vth2. In this case, a delay time from ON to OFF of MOSFETQl becomes shorter, and a delay time from OFF to ON of MOSFETQ2 becomes longer. This preparation allows providing a brush-less DC motor which has no fear that MOSFETQ2 is turned on before MOSFETQl is turned off even when the dead time is extremely short in the circuit diagram shown in Fig. 6, i.e. when "t" = t2, at which input signal HIN changes from the high level to the low level, is close to "t" = t3, at which input signal LIN changes from the low level to the high level. As discussed above, the brush-less DC motor of the present invention comprises a power module formed by unifying at least MOSFETs and gate drivers into one unit with molding resin, and a driving circuit which includes the power module. A setting section, built in one of the power module, the gate driver, or the MOSFETs, can set electrical strength of the MOSFETs. The foregoing structure allows obtaining a reliable brush-less DC motor featuring compact, light-weight, and easy to be wired and mounted. Embodiment 2

Electric devices of the present invention having brush-less DC motor 1 are demonstrated hereinafter with reference to Figs. 11 and 12. Fig. 11 shows a structure of an electric device (outdoor unit of air-conditioner) in accordance with the second embodiment of the present invention.

In Fig. 11, outdoor unit 51 in accordance with the second embodiment is divided by partition plate 54 standing on bottom plate 52 into compressor room 56 and heat exchanger room 59. Compressor 55 is placed in room 56. Heat exchanger 57 and blower fan motor 58 are placed in room 59. Box 60 carrying electrical components is placed over partition plate 54.

Fan motor 58 is formed of brush-less DC motor 1 demonstrated in Embodiment 1 and a blower-fan mounted on motor's rotary shaft, and is supplied high DC voltage Vdc and control voltage Vcc from power supply 53 housed in box 60. The rotation of fan motor 58 entails the blower fan to spin, which generates wind for cooling heat exchanger room 59. As discussed in Embodiment 1, since brush-less DC motor 1 includes a driving circuit built therein, motor 1 is compact in size and easy to be wired and mounted, so that motor 1 is useful for sophistication and cost reduction of electric devices such as air-conditioners. Fig. 12 shows a schematic circuit diagram of an electric device (outdoor unit of air-conditioner) in accordance with the second embodiment of the present invention. In Fig. 12, commercial power source 62 supplies power to power supply 53, which includes a rectifier, a smoothing capacitor, a switching power supply and others. Power supply 53 outputs high DC voltage Vdc and control voltage Vcc to fan motor 58. High DC voltage Vdc is also supplied to inverter 61 for driving compressor 55.

On/Off of inverter 61 entails superimposing a surge voltage (not shown) on high DC voltage Vdc, so that the voltage possibly exceeds a withstanding voltage of the MOSFET built in fan motor 58. However, the MOSFET per se has an avalanche resistance large enough to withstand an over-voltage in a short time, so that the MOSFET is not broken. Thus a reliable fan motor is obtainable.

In this second embodiment, an outdoor unit of air-conditioner is taken as an example of electric devices! however, the present invention can be applied to blower fans used in indoor units of air-conditioners and the indoor units per se with an advantage similar to that of the outdoor unit.

INDUSTRIAL APPLICABILITY

A brush-less DC motor of the present invention comprises a stator assembly including drive-coils for plural phases, a rotor assembly having a permanent magnet, and a built-in printed wired board (PWB) having a driving circuit mounted thereon for driving the drive-coils. The driving circuit includes MOSFETs which power the drive-coils, gate drivers which control the MOSFETs, and a pre-driver which supplies a PWM signal to the gate drivers. The PWB has a power module mounted thereon, and the power module is formed by unifying at least the MOSFETs and the gate drivers with molding resin. The power module includes a setting section therein or in the gate drivers or in the MOSFETs for setting electrical strength of the MOSFETs. The present invention further includes electric devices including this brush-less DC motor. The structure discussed above allows the motor to be reliable, compact in size and light in weight, and makes the motor easj^ be mounted and wired. This structure > can also provide electric devices using this brush-less DC motor.

Claims

1. A brush-less DC motor comprising: a stator assembly having drive coils for plural phases! a rotor assembly having a permanent magnet; and a printed wired board (PWB) built in the motor and having a driving circuit for driving the drive coils, wherein the driving circuit includes:

(a) a MOSFET for powering the drive coils! (b) a gate driver for controlling the MOSFET; and

(c) a pre-driver for supplying a PWM signal to the gate driver, wherein the PWB includes a power module formed by unifying the MOSFET and the gate driver with molding resin, and the power module includes a setting section within one of the power module, the gate driver, and the MOSFET, for setting electrical strength of the MOSFET.

2. The brush-less DC motor of claim 1, wherein the PWB further includes the pre-driver.

3. The brush-less DC motor of claim 1, wherein the setting section includes at least one of a resistor and a capacitor.

4. The brush-less DC motor of claim 1, wherein the driving circuit further includes a magnetic sensor for sensing a magnetic pole position of the permanent magnet, and the magnetic sensor is provided to the PWB.

5. The brush-less DC motor of claim 1 further comprises a metallic bracket, wherein the power module dissipates heat to the bracket via highly heat-conductive resin.

6. The brush-less DC motor of claim 5, wherein the highly heat-conductive resin is silicone -based resin.

7. The brush-less DC motor of claim 1, wherein the power module is formed by coupling the MOSFET to the gate driver with a bonding wire and then being sealed with epoxy resin.

8. The brush-less DC motor of claim 1, wherein the setting section sets a change of rate in voltage between drain and source of the MOSFET to be a predetermined value.

9. The brush-less DC motor of claim 1, wherein the setting section sets a change of rate in drain current of the MOSFET to be a predetermined value.

10. The brush-less DC motor of claim 1, wherein the setting section sets an on/off delay time of the MOSFET to be a predetermined value.

11. The brush-less DC motor of claim 10, wherein the setting section further sets a threshold voltage of a gate of the MOSFET to be a predetermined value.

12. The brush-less DC motor of claim 1, wherein the MOSFET has a given avalanche resistance .

13. An electric device including the brush-less DC motor as defined in claim 1.

14. The electric device of claim 13 is an air-conditioner, and the brush-less DC motor is a fan motor.

15. The electric device of claim 13 further including a power supply and another device than the brush-less DC motor, wherein both of the motor and the another device are coupled to the power supply.

16. The electric device of claim 15, wherein the electric device is an outdoor unit of an air-conditioner, the brush-less DC motor is a fan motor, and the another device is a compressor.