GB2316817A

GB2316817A - Power supply unit for improving power factor

Info

Publication number: GB2316817A
Application number: GB9718368A
Authority: GB
Inventors: Hiroshi Miyazaki; Yukio Yokomizo
Original assignee: Toshiba Corp
Current assignee: Toshiba Corp
Priority date: 1996-08-30
Filing date: 1997-08-29
Publication date: 1998-03-04
Anticipated expiration: 2017-08-29
Also published as: CN1175813A; JP3578874B2; KR100266034B1; GB9718368D0; KR19980018826A; CN1067500C; JPH1080139A; GB2316817B

Abstract

In a power supply unit which prevents untoward noise from being made by short circuiting of a reactor 2, the zero crossing of an AC voltage of an AC power supply 1 is detected by zero crossing detector 10, after which a power factor improvement pulse (24a, Fig 2) is generated by a drive signal generator 11. The power factor improvement pulse (24a) is applied to short circuit element driver 12 to drive a short circuit element 4 to short circuit the AC power supply 1 via the reactor 2, improving the power factor of the power unit. After the generation of the power factor improvement pulse, a noise reduction pulse (26a) of a very small pulse width is fed from the drive signal generator 11 to short circuit element 4 to short circuit the AC power supply 1 via the reactor 2, preventing noise from being made by the reactor 2. Alternatively (Fig 3) the noise reduction pulse (26a) may be applied before the power factor improvement pulse (24a). The short circuit element 4 and reactor 2 may be connected after the rectifier 5 (Fig 4). The unit may be used for an air conditioner.

Description

TITLE OF THE INVENTION POWER SUPPLY UNIT BACKGROUND OF THE INVENTION Field of the Invention The present invention relates to a power supply unit that rectifies an AC voltage to a DC voltage with an improved high power factor and, more particularly, to a power supply unit capable of reducing noise caused by means intended for enhancing the power factor.

Description of the Prior Art To increase active power available from a power supply unit, it is effective to improve its power factor, and there have been proposed various methods for simply improving the power factor. With improved power factor of the power supply, it is also possible, in many cases, to reduce a harmonic current of the power supply that has become an issue in recent years, and a conventional method for improving the power factor of such a power supply unit is disclosed in Japanese Pat. Laid-Open No. 299470/90, for instance. According to this method, an AC power supply is short-circuited via a reactor by holding ON a switching element only for a proper short period of time after zero crossing of an AC input voltage to extend the conduction period of the power-supply current, thereby improving the power-supply power factor.

Another power factor improving method is disclosed in Japanese Pat. Laid-Open No. 7946/95. This method also provides for improved power-supply power factor by holding ON a switching element for a predetermined period of time after a certain elapsed time subsequent to zero crossing of an AC voltage.

The above-mentioned traditional methods for improving the power factor both attain their object by shortcircuiting the AC power supply via a reactor for a predetermined period of time after a proper elapsed time subsequent to zero crossing of the AC voltage. In this instance, however, short-circuiting of the reactor gives arise to a problem that it makes untoward noises.

SUMMARY OF THE INVENTION Such a reactor-noise problem could be reduced by providing greater rigidity in the reactor or covering it with a soundproofing material, but this will inevitably raise the cost of the power supply system. With these noise reduction measures, however, it is necessary to take into account the reliability or aging of the material used therefor.

It is therefore an object of the present invention to provide a power supply unit which permits reduction of the above-mentioned untoward noises with a simple and hence economical structure.

To attain the above objective, the power supply unit according to the present invention comprises: rectifying means for rectifying an AC voltage from an AC power supply to a DC voltage; a reactor connected in series to the rectifying means; and short-circuit means for shortcircuiting the AC power supply via the reactor. The shortcircuit means performs a first short-circuit operation of short-circuiting the AC power supply via the reactor for a predetermined short period of time after zero crossing of the AC voltage, and a second short-circuit operation of short-circuiting the AC power supply via the reactor for a predetermined shorter period of time after the first shortcircuit operation.

According to another aspect of the present invention, the power supply unit comprises: rectifying means for rectifying an AC voltage from an AC power supply to a DC voltage; a reactor connected in series to the rectifying means; and short-circuit means for short-circuiting the AC power supply via the reactor. The short-circuit means performs a first short-circuit operation of shortcircuiting the AC power supply via the reactor for a predetermined short period of time after zero crossing of the AC voltage, and a second short-circuit operation of short-circuiting the AC power supply via the reactor for a predetermined shorter period of time before the first short-circuit operation.

In a preferred embodiment of the present invention, the second short-circuit operation is performed once each time the first short-circuit operation is carried out.

In another preferred embodiment of the present invention, the second short-circuit operation is performed a plurality of times each time the first short-circuit operation is carried out.

In another preferred embodiment of the present invention, the first short-circuit operation is performed once for each zero crossing of the AC voltage.

In another preferred embodiment of the present invention, the first short-circuit operation is performed a plurality of times for each zero crossing of the AC voltage.

In another preferred embodiment of the present invention, the short-circuit means performs a third shortcircuit operation of short-circuiting the AC power supply via the reactor again after the second short-circuit operation for a period of time shorter than that of the first short-circuit operation.

In another preferred embodiment of the present invention, the short period of time for short-circuiting the AC power supply by the second or third short-circuit operation and a non-short-circuit period between the first and second or third short-circuit operations are determined by the natural oscillation frequency of the reactor.

In another preferred embodiment of the present invention, first, second and third means for the first, second and third short-circuit operations each comprises: switching means for short-circuiting the AC power supply via the reactor; drive signal generating means for generating a drive signal of a pulse width corresponding to the period of the short-circuit operation by the corresponding short-circuit means; and drive means supplied with the drive signal from the drive signal generating means, for driving the switching means.

In another preferred embodiment of the present invention, the short period of time for short-circuiting the AC power supply by the second or third short-circuit operation and the non-short-circuit period between the first and second or third short-circuit operations are determined by the natural oscillation frequency of the reactor, taking into account delay times of the switching means and the drive means.

According to still another aspect of the present invention, the power supply unit intended for an air conditioner comprises: rectifying means for rectifying an AC voltage from an AC power supply to a DC voltage; a reactor connected to the rectifying means; and switching means for short-circuiting the AC power supply via the reactor. The switching means is held ON for a predetermined short period after zero crossing of the AC voltage to short-circuit the AC power supply via the reactor for a first short-circuit period so as to improve the power factor of the power supply unit, then the switching means is held OFF for a certain quiescent time and turned ON again to short-circuit the AC power supply via the reactor for a second short-circuit period. Letting the time elapsed after first short-circuit period terminates, the quiescent time between the first shortcircuit and the second short-circuit, the second shortcircuit period and the natural oscillation frequency of the reactor be represented by t, x, y and f, respectively, the following condition is substantially satisfied by the use or an angular frequency S = 2of: sin(t) + sin(S(t-x)-S) + sin(S(t-x-y)) = 0 wherein t ' x+y.

BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a block diagram illustrating the circuit configuration of a power supply unit according to a first embodiment of the present invention; Fig. 2 is a diagram showing signal waveforms occurring at respective parts of the power supply unit of Fig. 1; Fig. 3 is a diagram showing signal waveforms occurring at respective parts of a power supply unit according to a second embodiment of the present invention; Fig. 4 is a block diagram illustrating the circuit configuration of a power supply unit according to a third embodiment of the present invention; Fig. 5 is a diagram showing signal waveforms occurring at respective parts of a power supply unit according to a fourth embodiment of the present invention; Fig. 6 is a diagram showing signal waveforms occurring at respective parts of a power supply unit according to a fifth embodiment of the present invention; Fig. 7 is a timing chart for explaining the configuration of drive signal generating means for generating a power factor improvement pulse and a noise reduction pulse; Fig. 8 is a diagram showing a quiescent time between the power factor improvement pulse and the noise reduction pulse and a noise reduction pulse short-circuiting time; Fig. 9 is a diagram showing the quiescent time between the power factor improvement pulse and the noise reduction pulse and the noise reduction pulse short-circuit time in each of the cases of one, two and n noise reduction pulses are generated, respectively; Fig. 10 is a diagram showing how rise and fall times of an i-th noise reduction pulse are defined; Fig. 11 is a diagram showing the relationships of the power factor improvement pulse, the quiescent time and the short-circuit time in the case of two noise reduction pulses; Fig. 12 is a diagram showing how to correct the quiescent time and the short-circuit time in compensation for a delay of short-circuit element drive means and ON and OFF delay times of the short-circuit element; Fig. 13 is a block diagram illustrating an example of the power supply unit of the present invention applied to an air conditioner; and Fig. 14 is a graph showing the noise characteristics of the air conditioner having built therein the power supply unit of the present invention and an air conditioner using a conventional power supply unit with no noise reduction facility.

DESCRIPTION OF THE PREFERRED EMBODIMENTS With reference to the drawings, embodiments of the present invention will hereinbelow be described.

Fig. 1 illustrates in block form the circuit configuration of a power supply unit according to an embodiment of the present invention and Fig. 2 shows waveforms of signals occurring at respective parts of the power supply unit of Fig. 1. The power supply unit of this embodiment has a reactor 2 connected at one end to one of two output ends of an AC power supply 1 and at the other end to one input terminal of each of a first diode bridge 3 composed of four diodes all connected in forward direction and a second diode bridge 5 forming a full-wave rectifier.

The other input ends of the first and second diode bridges 3 and 5 are connected to the other output end of the AC power supply 1.

The first diode bridge 3 has its both ends connected between the collector and emitter of a short-circuit element that functions as a switching element formed by a bipolar transistor, IGBT, MOSFET or the like. When the short-circuit element 4 is turned ON, the AC power supply 1 is short-circuited via the first diode bridge 3 and the reactor 2, thereby improve the power factor of the power supply unit. The short-circuit element 4 has its gate connected to short-circuit element drive means 12 so that the former is turned ON by the latter.

Connected across the AC power supply 1 is zero crossing detect means 10 formed of a photocoupler, current transformer or similar circuit. The zero crossing detect means 10 detects the point in time when the AC voltage of the AC power supply 1, which has such a sinusoidal waveform as indicated by reference numeral 21 in Fig. 2, crosses the zero point or passes through the zero crossing point, and the detect means 10 applied the detecting signal to drive signal generating means 11.

The drive signal generating means 11 is formed by a micro computer or dedicated circuit, for instance, which generates a power factor improvement pulse indicated by 24a in Fig. 2 and a noise reduction pulse 26a of a width far smaller than that of the power factor improvement pulse 24a and provides these pulses to the short-circuit element drive means 12. The short-circuit element drive means 12 responds to the power factor improvement pulse 24a and the noise reduction pulse 26a to turn ON the short-circuit element 4, short-circuiting the AC power supply 1 via the first diode bridge 3 and the reactor 2. As a result, when the short-circuit element 4 is turned ON in response to the power factor improvement pulse 24a, the power-supply power factor can be improved by short-circuiting the AC power supply 1 via the first diode bridge 3 and the reactor 2.

When the short-circuit element 4 is turned ON in response to the noise reduction pulse 26a, it is possible to abate noise that is caused by the reactor 2 short-circuited for improving the power factor.

The output ends of the second diode bridge are connected via smoothing electrolytic capacitors 6, 7 and 8 to a load 9 so that the AC voltage from the AC power supply 1 is voltage-double rectified by the second diode bridge 5 and the smoothing electrolytic capacitors 6, 7 and 8 and fed as a DC voltage to the load 9.

In the power supply unit of the above construction, when such an AC voltage as indicated by 21 in Fig. 2 is provided from the AC power supply 1, it is fed via the reactor 2 to a voltage doubler rectifier circuit composed of the second diode bridge 5 and the smoothing electrolytic capacitors 6, 7 and 8, from which it is applied as a DC voltage to the load 9. When the AC voltage passes through the zero point, it is detected by the zero crossing detect means 10 and the detecting signal is applied to the drive signal generating means 11 to drive it.

When driven by the zero crossing detecting signal from the zero crossing detect means 10, the drive signal generating means 11 generates the power factor improvement pulse 24a and the noise reduction pulse 26b after the zero crossing of the AC voltage. The power factor improvement pulse 24a and the noise reduction pulse 26a are fed via the short-circuit element drive means 12 to the short-circuit element 4 to turn it ON, by which the AC power supply 1 is short-circuited via the first diode bridge 3 and the reactor 2. As the result of this, when the short-circuit element 4 responds to the power factor improvement pulse 24a to short-circuit the AC power supply 1 via the reactor 2, the power-supply current conduction period is extended and the power factor of the power supply unit is improved.

When the short-circuit element 4 responds to the noise reduction pulse 26a to operate, it is possible to reduce noise that is made by turning-OFF of a short-circuit current when the reactor 2 is released from its shortcircuited state by the power factor improvement pulse 24a.

This will be described below more specifically with reference to Fig. 2.

By processing the power-supply current waveform 22 with the power factor improvement pulse 24a, the powersupply current conduction period is extended as indicated by 23 in Fig. 2, from which it is seen that the powersupply power factor is improved. And, by driving the short-circuit element 4 with the noise reduction pulse 26a of a very small pulse width subsequent to the power factor improvement pulse 24a, the noise by the reactor, which is made by the turning-OFF of the short-circuit current applied to the reactor 2 in response to the power factor improvement pulse 24a, can be reduced with no substantial changes in the waveform of the power-supply current and the power-supply power factor.

Actual width of the noise reduction pulse is largely depending upon the design of the actual system and the environmental condition. In the case of a commercial power supply having a voltage of lOOV at 50 Hz or 60 Hz, the natural oscillation frequency of the reactor is usually from 7 KHz to 8 KHz so that the power factor improvement pulse can be followed by a single quiescent time of from 20 a sec to 25 tt sec and a single noise reduction pulse whose width is also from 20 tt sec to 25 M sec. For example, if the natural oscillation frequency of the reactor is 7.8 KHz, the noise reduction pulse of 22.2 ti sec may be applied for one time after a single quiescent time of 22.2 y sec.

Fig. 3 shows signal waveforms occurring at respective parts of a power supply unit according to a second embodiment of the present invention. The second embodiment is identical in construction and operation to the first embodiment except that the order of generation of the power factor improvement pulse 24a and the noise reduction pulse 26a is reversed.

The circuit configuration of the second embodiment is basically identical to the circuit configuration of the Fig. 1 embodiment, but the power factor improvement pulse 24a and the noise reduction pulse 26a are generated by the drive signal generating means 11 in reverse order, that is, the noise reduction pulse 26a of a very short period is generated first and then the power factor improvement pulse 24a as shown in Fig. 3. In Fig. 3, reference numerals 21 and 22 denote the waveform of the AC voltage of the AC power supply 1 and its current waveform prior to the power factor improvement and 25 a current waveform obtained by the processing for power factor improvement and noise reduction.

In the second embodiment, the noise reduction pulse 26a is generated prior to the generation of the power factor improvement pulse 24a and is used to instantaneously short-circuit the reactor 2 prior to short-circuiting the AC power supply 1 by the power factor improvement pulse 24a via the reactor 2. This allows reduction of the noise that is made by the turning-ON of the short-circuit current of the reactor 2. Thereafter, the AC power supply 1 is shortcircuited by the power factor improvement pulse 24a via the reactor 2, by which the power-supply current conduction period is extended as indicated by 25 in Fig. 3; hence, the power-supply power factor can be enhanced.

While in the above the noise reduction pulse 26a is generated before or after the power factor improvement pulse 24a and is used to short-circuit the AC power supply via the reactor 2 to thereby reduce the noise by turning-ON or OFF of the reactor short-circuit current, it is a matter of course that the generation of the noise reduction pulse 26a both before and after the power factor improvement pulse 24a will further ensure the reduction of noises at the times of turning ON and OFF the reactor short-circuit current.

Fig. 4 is a block diagram illustrating the circuit configuration of the power supply unit according to a third embodiment of the present invention. In the illustrated power supply unit, one end of the reactor 2 is connected to the one output end of the second diode bridge 5 forming a full-wave rectifier and the collector and emitter of the short-circuit element formed by a switching transistor are connected to the other end of the reactor 2 and the other output end of the second diode bridge 5 so that when the short-circuit element 4 is turned ON, the AC current 1 is short-circuited via the reactor 2 and the second diode bridge 5. Further, the other end of the reactor 2 is connected via a reverse-current preventive diode 13 to the smoothing electrolytic capacitor 8 and the load 9. The diode 13 functions to prevent the smoothing electrolytic capacitor 8 from being short-circuited by the short-circuit element 4 when the latter is turned ON to short-circuit the AC power supply 1 via the reactor 2 and the second diode bridge 5.

The zero crossing detect means 10 connected across the AC power supply 1, the drive signal generating means 11 and the short-circuit element drive means 12 are identical in construction and operation to those shown in Fig. 1. The drive signal generating means 11 generates the power factor improvement pulse 24a first and then the noise reduction pulse 26a as depicted in Fig. 2. These pulses are applied to the short-circuit element drive means 12, by which the short-circuit element 4 is turned ON with the power factor improvement pulse 24a first and then with the noise reduction pulse 26a. As is the case with the first embodiment, the power factor improvement pulse 24a improves the power-supply power factor and the noise reduction pulse 26a reduces the noise that is made when the reactor shortcircuit current is turned OFF. Also in this embodiment, the order of generation of the power factor improvement pulse 24a and the noise reduction pulse 26a may be reversed, in which case the same results as described above in respect of the second embodiment of Fig. 3 can be obtained.

Fig. 5 shows signal waveforms occurring at respective parts of a power supply unit according to a fourth embodiment of the present invention. The fourth embodiment is identical in construction and operation to the first embodiment except that the generation of the power factor improvement pulse 24a is followed by the generation of a plurality (two in this example) of noise reduction pulses 26a and 26b. In Fig. 5, reference numerals 21 and 22 denote the waveform of the AC voltage of the AC power supply 1 and its current waveform prior to the power factor improvement, 25 a current waveform obtained by the processing for power factor improvement and noise reduction, 24a the power factor improvement pulse, and 26a and 26b the noise reduction pulses.

The circuit configuration of the power supply unit according to the fourth embodiment is basically identical to the circuit configuration shown in Fig. 1 or 4, except that the drive signal generating means 11 generates the power factor improvement pulse 24a first and then the noise reduction pulses 26a and 26b.

By generating the plurality of noise reduction pulses 26a and 26b after the power factor improvement pulse 24a as in the fourth embodiment, it is possible to surelycurtail the noise which is made by the turning-OFF of the reactor short-circuit current, after improving the power factor with the power factor improvement pulse 24a. Incidentally, the plurality of noise reduction pulses 26a and 26b may be generated prior to the generation of the power factor improvement pulse 24a as shown in Fig. 3--this permits reduction of the noise that is made when the reactor shortcircuit current is turned ON.

The generation of the plurality of noise reduction pulses 26a and 26b as in the fourth embodiment causes substantially no change in the current waveform of the power supply unit nor does it affect the power factor because their pulse widths are far smaller than that of the power factor improvement pulse 24a.

Fig. 6 is a diagram showing signal waveforms occurring at respective parts of a power supply unit according to a fifth embodiment of the present invention. This embodiment is a modified form of the first embodiment, in which a plurality of power factor improvement pulses 24a and 24b are generated in succession and the noise reduction pulses 26a and 26b are each generated after one of the power factor improvement pulses 24a and 24b. In Fig. 6 the power factor improvement pulses 24 and the noise reduction pulses 26 are shown to be generated in pairs.

By generating the power factor improvement pulses 24a and 24b in succession and each of the noise reduction pulses 26a and 26b after one of the pulses 24a and 24b as mentioned above, it is possible not only to enhance the power factor of the power supply unit but also to surely reduce the noise at the time of turning OFF the reactor short-circuit current. While in the above one noise reduction pulse 26 has been described to be generated after the power factor improvement pulse 24, a plurality of noise reduction pulses 26 may be generated for each power factor improvement pulse. In this instance, the noise reduction pulses may be produced before or after or before and after the generation of the power factor improvement pulse 24.

The generation of the noise reduction pulses 26 before and after the power factor improvement pulse 24 ensures reduction of noises at the times of turning ON and OFF the reactor short-circuit current.

Next, a description will be given, with reference to Fig. 7, of the drive signal generating means 11 that generates the power factor improvement pulse 24a and the noise reduction pulse 26a. The noise reduction pulse 26a, which is used to short-circuit the reactor 2 via the shortcircuit element 4 to curtail noise by the oscillation of the reactor 2, has a pulse width far smaller than that of the power factor improvement pulse 24a that is used to short-circuit the reactor 2 via the short-circuit element 4 so as to enhance the power factor of the power supply unit.

In practice, the noise reduction pulse 26a has so small a time width that it does not causes any substantial change in the current waveform of the power supply nor does it affect the power factor of the power supply unit.

The drive signal generating means 11 can be formed using, for example, a timer facility built in a micro computer or by a dedicated waveform generator circuit.

Fig. 7 shows, however, an operation in the case of using a counter, a comparator and four registers A, B, C and D. In each of the four registers there are prestored each pulse interval and each pulse width. More specifically, the period from the detection of zero crossing of the AC powersupply voltage by the zero crossing detect means 10 to the generation of the power factor improvement pulse 24a is stored as D1 in the register A; the sum of the value D1 and the pulse width of the power factor improvement pulse 24a is stored as D2 in the register B; the sum of the value D2 and the quiescent time between the power factor improvement pulse 24a and the noise reduction pulse 26a is stored as D3 in the register C; and the sum of the value D3 and the pulse width of the noise reduction pulse 26a is stored as D4 in the register D.

Upon detection of zero crossing of the AC power-supply voltage by the zero crossing detect means 10, a zerocrossing interrupt is generated, then the counter is started, and the count value of the counter is compared by the comparator with the value of each register. When the count value of the counter reaches the value D1 stored in the register A, the power factor improvement pulse 24a is generated; when the count value reaches the value D2 stored in the register B, the power factor improvement pulse 24a is discontinued; when the count value reaches the value C stored in the register C, the noise reduction pulse 26a is generated; and when the count value reaches the value D4 stored in the register D, the noise reduction pulse 26a is discontinued. By suitable setting of the value of each register, the power factor improvement pulse 24a and the noise reduction pulse 26a each having a desired pulse width and a desired quiescent time can produced with accuracy and with ease.

While in Fig. 7 the noise reduction pulse 26a is shown to be generated after the power factor improvement pulse 24a, the order of generation of these pulses can easily be reversed by changing the values of the respective registers. With additional registers, it is also possible to generate pluralities of power factor improvement pulses and noise reduction pulses or generate a plurality of noise reduction pulses for each power factor improvement pulse.

By adapting each or one of the registers so that its value can be re-written or updated, the above processing can be done without increasing the number of registers used.

Next, a detailed description will be given, with reference to Figs. 8 through 11, of the quiescent time between the power factor improvement pulse and the noise reduction pulse and the pulse width of the latter, i.e. the short-circuit time of the reactor 2.

The reason for which the reactor 2 makes noise is that when the short-circuit current is abruptly or rapidly turned ON or OFF, the reactor 2 is vibrated and oscillate at its natural oscillation frequency. When the current rises and falls, forces are produced in opposite directions. The oscillation of the reactor 2 lasts for a period T of about 1 ms.

Referring first to Figs. 8 and 9, the oscillation of the reactor will be described in connection with the case of only one noise reduction pulse being generated. Letting the point of termination of the power factor improvement pulse be represented by t=O, the quiescent time until the generation of the noise reduction pulse by x, the shortcircuit time by the noise reduction pulse by y, the natural oscillation frequency of the reactor by f and its angular frequency by =2of, the oscillation of the reactor at the times of turning ON and OFF the current is given by the following equation: Oscillation at time t=O: sin(t) Oscillation at time t=x: sin(S(t-x)-X) Oscillation at time t=x+y: sin(o(t-x-y)) The effective value of reactor oscillation from time t=O to t=T is as follows:

where F(t) = sin(t)+sin(X(t-x)-v)+sin(X(t-x-y)) The minimum effective value of reactor oscillation is obtained from V.ff of the above equation, but it needs only to satisfy a condition F(t)=O. Calculating the minimum effective value of oscillation from the equation, the quiescent time x between the power factor improvement pulse and the noise reduction pulse and the short-circuit time y of the noise reduction pulse bear the following relationship as shown in Fig. 9 time and the short-circuit time are displaced 2also (a=1,2,3,...) and 2bR/o (b=0,1,2,3,...) apart in phase, and the quiescent time x and the short-circuit time y needs only to be chosen as given by the following equations.

x = +w/(3X) + 2arno y = +ir/(3) + In this instance, x > O and y > O and the values a and b need not be equal. When the sign of the first term of the equation of the quiescent time x is made +, the sign of the first term of the equation of the short-circuit time y is +, whereas when the sign of the first term of the former is made -, the sign of the first term of the latter is also -.

Next, a description will be given of the case where n (n=1,2,3,...) noise reduction pulses are generated. To represent a general equation in the same manner as the effective value F(t) of the rector oscillation described above, the time of generation of each noise reduction pulse is indicated as shown in Fig. 10. Letting the times of rise and fall of an i-th noise reduction pulse be represented by as and ss, respectively, using the time of termination of the power factor improvement pulse as a reference time (t=O), the effective value of the reactor oscillation from time 0 to T is given as follows:

where

The minimum effective value of the reactor oscillation needs only to be G(t)=O. There are a lot of relationships of ai and ss that satisfy the condition G(t). For example, when the quiescent time x and the short-circuit time y are equal to each other and the number n of noise reduction pulses is two as shown in Fig. 9, they bear such a relationship given by the following equation: xi = yi = o/(5S) Similarly, when the number of the same noise reduction pulses is n (n=1,2,3,...), their relationship is as follows: xi = yi = n/((2n+l)o) Further, they are displaced 2air/ (a=0,1,2,3,...) apart in phase, the following relationship of obtained because of the property of the periodic function.

xi = yi = When n=2, such noise reduction pulses as depicted in Fig. 11 can readily be imagined, but they are also among the relationships that satisfy the above-mentioned condition G(t)=O. There are other relationships that meet this condition, but no description will be given of them.

As described above, the reactor noise can effectively be reduced by determining the quiescent time x and the short-circuit time of the noise reduction pulse on the basis of the natural oscillation frequency of the reactor.

With this method, the optimum condition does not change with the power-supply frequency or the state of a load.

Once the reactor to be used is determined, its natural oscillation frequency can be known, and hence the optimum noise reduction pulse can be generated.

Next, a description will be given, with reference to Fig. 12, of a method for correcting the quiescent time and the short-circuit time determined as described above, in accordance with a delay of the short-circuit element drive means 12 and delays in turning ON and OFF of the short circuit element 4. In Fig. 12 the short-circuit element drive means 12 and the short-circuit element 4 are shown to make the correction on the assumption that the turning-ON operation is faster than the turning-OFF operation.

To determine the current ON/OFF timing of the shortcircuit element 4 so that the noise is minimized, the quiescent time of the output signal from the drive signal generating means 11 is set to be short by the delay times of the short-circuit element drive means 12 and the shortcircuit element 4 and the short-circuit time of the noise reduction pulse is set to be long accordingly. By this, the short-circuit element 4 can be operated at the ON/OFF timing calculated as described above. In Fig. 12 the current of the short-circuit element 4 is shown to have a rectangular waveform for the sake of brevity. In this example, the power factor improvement pulse is followed by the noise reduction pulse, but also when the order of generation of these pulses is reversed, the reactor noise can be reduced by making the same correction as mentioned above. In the case of two or more noise reduction pulses being generated, too, the reactor noise can similarly be prevented.

Turning next to Fig. 13, the power supply unit of the present invention will be described as being applied to an air conditioner. In Fig. 13i the AC power supply 1, the reactor 2, the first diode bridge 3, the short-circuit element 4, the second diode bridge 5, the smoothing electrolytic capacitors 6, 7 and 8, the zero crossing detect means 10 and the short-circuit element drive means 12 are the same as those in the power supply unit of Fig.

1. Reference numeral 15 denotes a micro computer, 16 an inverter circuit and 17 a compressor motor.

The micro computer 15 controls the inverter circuit 16 to operate the motor 17 to exert control over the air conditioner. At the same time, upon reception of the zero crossing detection signal from the zero crossing detect means 10, the micro computer 15 performs the same operation as that of the drive signal generating means 11 to generate the power factor improvement pulse and the noise reduction pulse. These pulses are applied to the short-circuit element drive means 12 to drive therethrough the shortcircuit element 4, by which the AC power supply 1 is shortcircuited via the reactor 2 and the first diode bridge 3, thus improving the power factor of the power supply unit and reducing the reactor noise.

Fig. 14 is a graph showing noise characteristics of the air conditioner with the power supply unit of the present invention equipped with the power factor improvement function and the noise reduction function as described above and a conventional air conditioner with a power supply unit having no noise reduction function. The curve (A) indicates noise of the air conditioner with the conventional power supply unit built therein, from which it is apparent that the noise level is high at 7 through 8 KHz. On the other hand, the curve (B) indicates noise of the air conditioner with the power supply unit of the present invention incorporated therein, from which it is seen that the noise level at 7 through 8 KHz is low.

As described above, the AC power supply is shortcircuited via the reactor for a predetermined short period of time after zero crossing of the AC voltage so as to improve the power factor of the power supply unit, after which the AC power supply is short-circuited again for a short time. By this, it is possible with a simple structure not only to improve the power factor but also to economically and surely reduce noise that is made by the short-circuiting of the reactor. The configuration therefor can be obtained only by adding second shortcircuit means to first one to generate the noise reduction pulse in addition to the power factor improvement pulse; hence, this can be done by a simple modification of the circuit configuration or an addition of a simple circuit.

Since no vibrationproofing or soundproofing material is needed, the power supply unit of the present invention is inexpensive, highly reliable and free from aging.

It will be apparent that many modifications and variations may be effected without departing from the scope of the novel concepts of the present invention.

Claims

WHAT IS CLAIMED IS:

1. A power supply unit comprising: rectifying means for rectifying an AC voltage from an AC power supply to a DC voltage; a reactor connected in series to said rectifying means; and short-circuit means for short-circuiting said AC power supply via said reactor; wherein said short-circuit means performs a first short-circuit operation of short-circuiting said AC power supply via said reactor for a predetermined short period of time after zero crossing of said AC voltage, and a second short-circuit operation of short-circuiting said AC power supply via said reactor for a predetermined shorter period of time after said first short-circuit operation.

2. A power supply unit as claimed in claim 1 wherein said second short-circuit operation is performed once each time said first short-circuit operation is carried out.

3. A power supply unit as claimed in claim 1 wherein said second short-circuit operation is performed a plurality of times each time said first short-circuit operation is carried out.

4. A power supply unit as claimed in claim 1 wherein said first short-circuit operation is performed once for each zero crossing of said AC voltage.

5. A power supply unit as claimed in claim 1 wherein said first short-circuit operation is performed a plurality of times for each zero crossing of said AC voltage.

6. A power supply unit as claimed in claim 1 wherein said short-circuit means performs a third short-circuit operation of short-circuiting said AC power supply via said reactor again after said second short-circuit operation for a period of time shorter than that of said first shortcircuit operation.

7. A power supply unit as claimed in claim 6 wherein said short period of time for short-circuiting said AC power supply by said second or third short-circuit operation and a non-short-circuit period between said first and second or third short-circuit operations are determined by the natural oscillation frequency of said reactor.

8. A power supply unit as claimed in claim 6 which further comprises first, second and third short-circuit means for said first, second and third short-circuit operations, said first, second and third short-circuit means each comprising: switching means for short-circuiting said AC power supply via said reactor; drive signal generating means for generating a drive signal of a pulse width corresponding to the period of the short-circuit operation by the corresponding short-circuit means; and drive means supplied with said drive signal from said drive signal generating means, for driving said switching means.

9. A power supply unit as claimed in claim 8 wherein said short period of time for short-circuiting said AC power supply by said second or third short-circuit operation and the non-short-circuit period between said first and second or third short-circuit operations are determined by the natural oscillation frequency of said reactor, taking into account delay times of said switching means and said drive means.

10. A power supply unit comprising: rectifying means for rectifying an AC voltage from an AC power supply to a DC voltage; a reactor connected in series to said rectifying means; and short-circuit means for short-circuiting said AC power supply via said reactor; wherein said short-circuit means performs a first short-circuit operation of short-circuiting said AC power supply via said reactor for a predetermined short period of time after zero crossing of said AC voltage, and a second short-circuit operation of short-circuiting said AC power supply via said reactor for a predetermined shorter period of time before said first short-circuit operation.

11. A power supply unit as claimed in claim 10 wherein said second short-circuit operation is performed once each time said first short-circuit operation is carried out.

12. A power supply unit as claimed in claim 10 wherein said second short-circuit operation is performed a plurality of times each time said first short-circuit operation is carried out.

13. A power supply unit as claimed in claim 10 wherein said first short-circuit operation is performed once for each zero crossing of said AC voltage.

14. A power supply unit as claimed in claim 10 wherein said first short-circuit operation is performed a plurality of times for each zero crossing of said AC voltage.

15. A power supply unit as claimed in claim 10 wherein said short-circuit means performs a third short-circuit operation of short-circuiting said AC power supply via said reactor again after said second short-circuit operation for a period of time shorter than that of said first short circuit operation.

16. A power supply unit as claimed in claim 15 wherein said short period of time for short-circuiting said AC power supply by said second or third short-circuit operation and a non-short-circuit period between said first and second or third short-circuit operations are determined by the natural oscillation frequency of said reactor.

17. A power supply unit as claimed in claim 15 which further comprises first, second and third short-circuit means for said first, second and third short-circuit operations, said first, second and third short-circuit means each comprising: switching means for short-circuiting said AC power supply via said reactor; drive signal generating means for generating a drive signal of a pulse width corresponding to the period of the short-circuit operation by the corresponding short-circuit means; and drive means supplied with said drive signal from said drive signal generating means, for driving said switching means.

18. A power supply unit as claimed in claim 17 wherein said short period of time for short-circuiting said AC power supply by said second or third short-circuit operation and the non-short-circuit period between said first and second or third short-circuit operations are determined by the natural oscillation frequency of said reactor, taking into account delay times of said switching means and said drive means.

19. A power supply unit for an air conditioner, comprising: rectifying means for rectifying an AC voltage from an AC power supply to a DC voltage; a reactor connected to said rectifying means; and switching means for short-circuiting said AC power supply via said reactor; wherein said switching means is held ON for a predetermined short period after zero crossing of said AC voltage to short-circuit said AC power supply via said reactor for a first short-circuit period so as to improve the power factor of said power supply unit, then said switching means is held OFF for a certain quiescent time and turned ON again to short-circuit said AC power supply via said reactor for a second short-circuit period; wherein, letting the time elapsed after first shortcircuit period terminates, said quiescent time between said first short-circuit and said second short-circuit, said second short-circuit period and the natural oscillation frequency of said reactor be represented by t, x, y and f, respectively, the following condition is substantially satisfied by the use of an angular frequency X = 2of: sin(t) + sin(S(t-x)-U) + sin(X(t-x-y)) = 0 wherein t 2 x+y.