AC-DC converter with reduced energy loss through a switching element

[0002] The present invention relates to an AC-DC converter in particular of the type comprising a switching element operated with less power loss and improved AC-DC conversion efficiency under a lower voltage of electric power supplied from an AC power source to the AC-DC converter.

[0003] A prior art AC-DC converter comprises a rectifying circuit with input terminals connected to an AC power source; a choke coil and a switching transistor connected in series to the rectifying circuit; a diode connected between the choke coil and one of output terminals; an output capacitor connected between the output terminals; dividing resisters connected in parallel to the output capacitor for presenting an output indicating the output voltage level; a power source for producing a reference voltage; an error amplifier for comparing the reference voltage and the output derived from the dividing resisters to produce an output related to a difference between the reference and divided voltages; and a PWM (Pulse Width Modulation) circuit for receiving the output from the error amplifier to generate drive pulses to a gate of the switching transistor. When the PWM circuit generates drive pulses, the switching transistor is turned ON and OFF alternately. During ON period of the switching transistor, electric current flows from the rectifier circuit through the choke coil and the switching transistor to charge the capacitor and store electric energy in the choke coil. During OFF period of the switching transistor, the choke coil discharges the stored electric energy to produce from the output terminals an output voltage raised by the output from the choke coil superimposed on the input voltage charged in the capacitor. When the output voltage is on a lower level, the error amplifier produces a lower level output to generate drive pulses of longer time width to the switching transistor, thereby turning the transistor ON for a longer period of time. Adversely, when the output voltage is on a higher level, the error amplifier produces a higher level output to generate drive pulses of shorter time width to the switching transistor, thereby turning the transistor ON for a shorter period of time,

[0004] Prior art AC-DC converters are required to keep the output voltage in a substantially constant and elevated level regardless of an input voltage level to the choke coil, and therefore they are driven from high to low step-up ratio of the DC output voltage to the AC input voltage. Specifically, when the input voltage changes between low and high level, they are operated to boost the output voltage to the elevated constant level between high and low step-up ratio in a wide range. When the AC-DC converters are driven to boost the input voltage with high step-up ratio, an excess current flows through the switching transistor, thereby resulting in decrease in AC-DC conversion efficiency and considerable power loss.

[0005] An object of the present invention is to provide an AC-DC converter which can prevent an excessive amount of electric current passing through a switching element for longer service duration of the switching element.

[0006] Another object of the present invention is to provide an AC-DC converter which indicates less power loss in a switching element with improved AC-DC conversion efficiency under supply of lower voltage electric power.

[0007] The AC-DC converter according to the present invention is provided with a booster circuit (9) which comprises a switching element (6) and a reactor (4) connected in series to the switching element (6) so that ON and OFF operation of the switching element (6) accumulates electric energy in the reactor (4) and discharges the electric energy from the reactor (4) to generate a DC voltage (V_OUT) above an AC voltage (V_IN) supplied from an AC power source (1). The converter also is equipped with a control circuit (10) which comprises potential means (11) for providing a reference voltage (V_R); means (30) for comparing the reference voltage (V_R) and DC output voltage (V_OUT) and generating drive signals (V_G) to turn the switching element (6) ON and OFF in response to the difference between the reference voltage (V_R) and DC output voltage (V_OUT); a voltage detector (18) for measuring an input voltage (V_IN) applied on an input terminal of the reactor (4); and a voltage retainer (22) for maintaining the output voltage (V_OUT) above the input voltage (V_IN) measured in the voltage detector (18) by a substantially constant voltage.

[0008] Comparing the reference voltage (V_R) and DC output voltage (V_OUT), the control circuit (10) generates drive pulses (V_G) for turning the switching element (6) ON or OFF to shorten or lengthen ON period of the switching element (6) when the output voltage (V_OUT) is higher or lower respectively to reduce or increase the output voltage (V_OUT). Also, as the voltage retainer (22) keeps the output voltage (V_OUT) above the input voltage (V_IN) measured in the voltage detector (18) by a substantially constant voltage (V_P+V_Ror V_Z), the booster circuit (9) simply raises the input voltage (V_IN) by the substantially constant voltage (V_P+V_Ror V_Z). This means that the step-up ratio (V_OUT/V_IN) is smaller than that of a prior art converter for boosting the input voltage (V_IN) up to a constant high DC output voltage regardless of a level of the input voltage (V_IN). Accordingly, the converter according to the present invention may change the output voltage (V_OUT) in a linear and ramp characteristic, whereas the prior art converter produces the output voltage of a fixed level with the changing step-up ratio (V_OUT/V_IN) in a wide range. The small step-up ratio (V_OUT/V_IN) causes less change in ON period of the switching element (6) and less amount of electric current (I_DS) through the switching element (6) with reduction of power loss in the switching element (6) and improved AC-DC conversion efficiency.

[0009] These as well as other objects of the present invention will become apparent during the course of the following description with reference to the accompanying drawings, in which:

[0010] FIG. 1 is an electric circuit diagram of a prior art AC-DC converter.

[0011] FIG. 2 is an electric circuit diagram showing a preferred embodiment of an AC-DC converter according to the present invention.

[0012] FIG. 3 is a graph indicating an output voltage variation of the electric circuit shown in FIG. 2 relative to an input voltage variation.

[0013] FIG. 4 is an electric circuit diagram showing a second embodiment of the present invention.

[0014] FIG. 5 is a graph indicating an output voltage variation of the electric circuit shown in FIG. 4 relative to an input voltage variation.

[0015] FIG. 6 is an electric circuit diagram showing a third embodiment of the present invention.

[0016] FIG. 7 is an electric circuit diagram of a fourth embodiment modified from the first embodiment shown in FIG. 2.

[0017] Referring now to the accompanying drawings, FIG. 1 illustrates a prior art AC-DC converter which comprises a filter circuit 2 connected to an AC power source 1 for producing AC input voltage V_IN; a booster circuit 9 for raising AC input voltage V_INto an elevated output voltage V_OUT; and a control circuit 10 for generating drive pulses V_Gto operate the booster circuit 9. Booster circuit 9 comprises a reactor 4 including primary and secondary windings 4a,4belectro-magnetically connected to each other; a rectifying circuit or diode bridge 3 for full wave rectification connected to filter circuit 2 for supplying rectified DC output to reactor 4; a switching element or MOS-FET (Field Effect Transistor of Metal Oxide Semiconductor) 6 connected between primary winding 4aof reactor 4 and diode bridge 3; a smoothing capacitor 8 connected between a pair of output terminals 31, 32; a rectifier diode 5 connected between primary winding 4aof reactor 4 and smoothing capacitor 8; and a detective resistor 7 connected to MOS-FET 6 and diode bridge 3. Primary winding 4ais connected between a positive terminal of rectifying circuit 3 and an anode of rectifier diode 5, and secondary winding 4bis connected between a negative terminal of diode bridge 3 and control circuit 10. Detective resistor 7 is connected in series to one of two main terminals, e.g. a drain of MOS-FET 6 for detecting a voltage corresponding to a level of electric current I_DSthrough MOS-FET 6.

[0018] Control circuit 10 comprises potential means or a voltage generator 11 for generating a reference voltage V_R; a differential or error amplifier 12 for comparing output voltage V_OUTof smoothing capacitor 8 with reference voltage V_Rof voltage generator 11 to generate an amplified differential voltage; and a drive signal generator 30 for producing drive signals V_Gto a control terminal or gate of MOS-FET 6 based on comparative outputs from comparator 12. Error amplifier 12 has an inverted terminal connected to output terminal 31 and a non-inverted terminal connected to voltage generator 11. Drive signal generator 30 comprises a multiplier 13 for multiplying amplified differential voltage from error amplifier 12 and a rectified voltage V_RCof diode bridge 3 to produce a multiplied output; a comparator 14 for comparing multiplied output from multiplier 13 with a maximal voltage applied on detective resistor 7; and a R-S flip-flop 15 which contains a set terminal S for receiving an induced voltage from second winding 4b, a reset terminal R for receiving output signals from comparator 14 and an output terminal Q for generating drive signals V_Gto gate of MOS-FET 6. Comparator 14 has a non-inverted terminal connected between MOS-FET 6 and resistor 7, and an inverted terminal connected to multiplier 13. Drive signal generator 30 produces drive signals V_Gto gate of MOS-FET 6 in response to differential output voltage from error amplifier 12.

[0019] In operation, AC electric power from AC power source 1 is converted through filter circuit 2 and diode bridge 3 into DC electric power supplied to booster circuit 9. Electric current flows through diode bridge 3, a primary winding 4aof reactor 4 and diode 5 to capacitor 8 to charge capacitor 8. When R-S flip-flop 15 is switched to “SET” condition by a trigger circuit not shown, it produces an initial drive signal V_Gfrom output terminal Q to MOS-FET 6 to turn it ON, and electric current flows from diode bridge 3 through primary winding 4a, MOS-FET 6 and detective resistor 7 to accumulate electric energy in reactor 4 and simultaneously to charge smoothing capacitor 8 to rectified voltage V_RCof diode bridge 3. When drive pulse V_Gfrom R-S flip-flop 15 is terminated, MOS-FET 6 is turned OFF and reactor 4 discharges accumulated energy to cause electric current to flow from primary winding 4aof reactor 4 through output diode 5 to an external electric machine connected to output terminals 31 and 32. Comparing reference voltage V_Rand DC output voltage V_OUT, control circuit 10 produces drive pulses V_Gto turn MOS-FET 6 ON and OFF, shortening or lengthening ON period of MOS-FET 6 when output voltage V_OUTis higher or lower respectively to reduce or increase output voltage V_OUT, and therefore, booster circuit 9 produces output voltage V_OUTfrom output terminals 31 and 32 above input voltage V_INsupplied from AC power source 1 because primary winding 4adischarges electric energy to generate electric current which provides an output voltage derived from primary winding 4aand superimposed on rectified voltage V_RCcharged in capacitor 8 to produce elevated output voltage V_OUT1. When reactor 4 finishes discharging energy, second winding 4binduces a voltage which is then applied to set terminal S of R-S flip-flop 15, and it is turned into “SET” condition so that output terminal Q produces drive signals V_Gof high level to gate of MOS-FET 6 to turn it ON.

[0020] During operation, error amplifier 12 compares output voltage V_OUTthrough output terminals 31, 32 with reference voltage V_Rof voltage generator 11 to produce amplified output from error amplifier 12 relative to difference between reference voltage V_Rand DC output voltage V_OUT. Multiplier 13 performs multiplication of amplified differential voltage from error amplifier 12 and rectified voltage V_RCof diode bridge 3 to produce a multiplied output as a reference voltage to comparator 14 relative to AC input voltage. Then, comparator 14 compares maximal voltage of detective resistor 7 with multiplied output from multiplier 18, and produces output signals when an over-current flows through detective resistor 7 causing maximal voltage of detective resistor 7 higher than multiplied output. When output signal from comparator 14 is applied to reset terminal R, R-S flip-flop 15 is turned to “RESET” condition to produce drive signals V_Gof low level from output terminal Q to gate of MOS-FET 6 which is then turned OFF. Comparator 14 can serve to increase input power factor to approximately 1.0 because instantaneous change of AC input current I_Nfollows instantaneous change of AC input voltage V_IN. In this way, as set terminal S and reset terminal R of R-S flip-flop 15 respectively and alternately receive induced voltage from second winding 4bof reactor 4, and output signal from comparator 14, output terminal Q forwards drive signals V_Gof high and low levels to gate of MOS-PET 6 which therefore is alternately turned ON and OFF.

[0021] In a usual worldwide practice, AC-DC converter is designed to generally produce a constant output voltage V_OUTof about 380 volts by boosting an AC input voltage V_INof 85 to 265 volts through AC-DC converter. For that reason, control circuit 10 is provided with a voltage generator 11 of reference voltage V_Rkept on about 380 volts so that a step-up ratio V_OUT/V_INof DC output voltage V_OUTto AC input voltage V_INvaries from 4.47 (=380/85) to 1.43 (=380/265) between lowest and highest levels of about 85 volts and about 265 volts of AC input voltage V_IN. This means that the converter is required to change the step-up ratio V_OUT/V_INover three times upon the lowest level, compared with the highest level of AC input voltage V_IN. When AC input voltage V_INis lower, AC-DC converter is required to boost AC input voltage V_INwith extremely high step-up ratio V_OUT/V_INof above 4 so that excess electric current flows through MOS-FET 6, thereby resulting in deterioration of conversion efficiency and increase of power loss in MOS-FET 6.

[0022] FIGS. 2 to 7 illustrate preferred embodiments of AC-DC converter according to the present invention wherein same reference symbols are used in FIGS. 2, 4, 6 and 7 to indicate substantially similar components as those shown in FIG. 1, and to avoid the repeated description.

[0023] As shown in FIG. 2, the control circuit 10 of AC-DC converter according to the present invention, comprises a voltage detector 18 for measuring input voltage V_RCor rectified voltage V_RCapplied on an input terminal of reactor 4; and a voltage retainer 22 for keeping output voltage V_OUTabove input voltage V_INby a substantially constant voltage V_R. For example, the voltage detector 18 discerns a peak V_Pof input voltage V_INwherein peak input voltage V_IN, peak rectified voltage V_RCand peak voltage V_Pare approximately equivalent to each other (V_IN=V_RC=V_P). Connected between two terminals of diode bridge rectifier 3 is the voltage detector 18 that comprises a rectifier diode 16 for preventing backflow and a capacitor 17 connected in series to rectifier diode 16 to charge the capacitor 17 to a peak level of input voltage V_IN. Voltage detector 18 has an output terminal formed by an intermediate point between rectifier diode 16 and capacitor 17, and voltage retainer 22 connects output terminal of voltage detector 18 in series to a negative terminal of voltage generator 11 to superimpose output V_Pfrom voltage detector 18 on reference voltage V_Rand thereby to provide a total reference voltage V_Sindicative of a sum of input voltage V_INof voltage detector 18 and reference voltage V_Rof voltage generator 11. In other words, voltage retainer 22 adds output voltage V_Pfrom voltage detector 18 to reference voltage V_Rto maintain output voltage V_OUTon a voltage level elevated by a constant level V_Rabove input voltage V_IN. In this embodiment, AC power source 1 produces AC input voltage V_INto diode bridge 3 for fall wave rectification to generate input voltage V_INwhich brings forth electric current flowing through rectifier diode 16 and capacitor 17 of voltage detector 18 to charge capacitor 17 to the peak voltage V_Pof rectified voltage V_RC. Total reference voltage V_Sfor error amplifier 12 is given by a sum of reference voltage V_Rand detected peak voltage V_Pto produce output voltage V_OUTin reduced step-up ratio V_OUT/V_INeven when AC input voltage V_INis on the lower level. Accordingly, when AC input voltage V_INis on the lower level, the AC-DC converter can reduce the power loss generated in MOS-FET 6 with improved conversion with reduction of electric current I_DSthrough MOS-FET 6. Capacitor 17 can be charged up to peak voltage V_Pof AC input voltage V_INthrough diode bridge 3 and rectifier diode 16 of voltage detector 18 to add peak voltage V_Pto reference voltage V_Rby a simple circuit variation, thereby resulting in the summed total reference voltage V_S.

[0024] A solid line in FIG. 3 shows a ramp property of output voltage V_OUTvarying with change of input voltage V_INin an area above the start-up voltage while a prior art converter produces a constant output voltage as shown by a dotted line in FIG. 3. Accordingly, even if input voltage V_INis on a lower level, booster circuit 9 simply raises input voltage V_INby substantially constant level V_Rso that small step-up ratio V_OUT/V_INcauses less change in ON period of time for MOS-FET 6 and less amount of electric current I_DSthrough MOS-FET 6 to reduce power loss in MOS-FET 6 and improve AC-DC conversion efficiency by means of a simple construction for connecting output terminal of voltage detector 18 in series to voltage generator 11.

[0025] For example, when voltage generator 11 provides a reference voltage V_Rof 50 volts, and AC power source 1 supplies an effective value V_INof AC 100 volts, voltage detector 18 perceives a peak voltage V_Pof 141 volts in input voltage V_INto control and maintain output voltage V_OUTon total voltage V_Sof 191 volts (V_S=V_P+V_R) with step-up ratio V_OUT/V_INof 1.91. On the other hand, with AC input voltage V_INof effective value 265 volts and detected voltage V_Pof 374 volts, output voltage V_OUTis controlled to total 424 volts in step-up ratio V_OUT/V_INof 1.60. In this way, when AC input voltage V_INis lower, booster circuit 9 simply produces output voltage V_OUTof lower level than that of conventional AC-DC converter shown in FIG. 1 so that step-up ratio V_OUT/V_INin the instant invention can be reduced. Further description on FIG. 1 is omitted because the AC-DC converter of this embodiment has its basic operation fully similar to that of AC-DC converter shown in FIG. 1. Also, total voltage V_Smay be calculated without forward voltage drop, for example, 0.5 volt of rectifier diode 16 because it is negligibly low, compared with total voltage V_S.

[0026] A second embodiment of the present invention shown in FIG. 4 includes a voltage retainer 22 which comprises a regulating circuit 19 for maintaining output voltage V_OUTon a level elevated by at least a constant voltage V_Zabove input voltage V_INgenerated by a voltage detector 18 with a discharge resistor 27 connected in parallel to capacitor 17. Regulating circuit 19 comprises a regulating element or Zener diode 26 connected to output terminal of voltage detector 18; a series resistor 23 and voltage dividing resistors 20, 21 connected between an output terminal of booster circuit 9 and earth; and an adjusting element or PNP type transistor 24 connected in parallel to resistor 23 to supply outputs of Zener diode 26 to a control terminal or base of transistor 24. Transistor 24 has a collector and an emitter connected to both ends of series resistor 28, and base connected between rectifier diode 16 and capacitor 17 through Zener diode 28 to supply outputs of voltage detector 18 to base of transistor 24. A start-up capacitor 25 is connected between emitter and base of transistor 24 to adjust starting voltage.

[0027] Neglecting a small forward voltage drop across rectifier diode 16, peak voltage V_Pis generally equal to peak voltage of AC input voltage V_IN, and is applied through Zener diode 26 to base of transistor 24. With Zener voltage of 50 volts across diode 26 and detected voltage V_Plower than 50 volts, no electric current flows through base of transistor 24 which accordingly is kept in the OFF condition. When detected voltage V_Pis increased above 50 volts, electric current flows through base of transistor 24 which is turned ON so that impedance between emitter and collector of transistor 24 varies relative to level of detected voltage V_Pto thereby adjust picked voltage V_Capplied on lower resistor 21 due to change in impedance of transistor 24. Error amplifier 12 detects picked voltage V_Cindicative of output voltage V_OUTto output terminals 31, 32 and compares it with reference voltage V_Rof voltage generator 11 to produce a differential output like the conventional AC-DC converter shown in FIG. 1. Control circuit 10 produces drive signals V_Gto turn MOS-FET 6 ON and OFF in order to allow level of divided voltage V_Cto generally equal to reference voltage V_R. Also, at the time of starting AC-DC converter, start-up capacitor 25 is charged to charged voltage in smoothing capacitor 8 to turn transistor 24 ON immediately when detected voltage V_Pis increased above 50 volts thereby maintaining divided voltage V_Con a lower level. This attains widening of ON pulse width of drive signals V_Gfrom control circuit 10 to extend ON period of MOS-FET 6 for the elongated on-duty. Thus, voltage retainer 22 serves to maintain output voltage V_OUTon summed voltage V_Sof input voltage V_INor detected voltage V_Pof voltage detector 18 plus reference voltage V_Rof voltage generator 11 with a smaller step-up ratio V_OUT/V_INthan that of a prior art converter for boosting input voltage V_INup to a constant high DC output voltage regardless of a level of input voltage V_IN.

[0028] In operation, electric current flows through transistor 24 and Zener diode 26 under output voltage V_OUT, and amount of the electric current increases with drop of input voltage V_INso that impedance in transistor 24 is reduced, narrowing the potential difference between two main terminals, namely emitter and collector of transistor 24. Adversely, amount of the electric current flowing through transistor 24 and Zener diode 26 decreases with rise of input voltage V_INso that impedance in transistor 24 is increased, widening the potential difference between emitter and collector of transistor 24 to maintain electric current through voltage dividing resistors 20, 21 constant. Thus, output voltage V_OUTcan be retained on a level above input voltage V_INby substantially constant voltage V_Zplus voltage drop V_BEbetween base and emitter of transistor 24, and accordingly regulating circuit 19 maintains output voltage V_OUTon summed level V_Sof input voltage V_IN, Zener voltage V_Zof Zener diode 26 and voltage drop V_BEacross base and emitter of transistor 26 as shown in FIG. 5. In this case, as voltage drop V_BEacross base and emitter of transistor 26 is a negligibly small amount of about 0.7 volt, output voltage V_OUTis maintained on summed level of input voltage V_INplus Zener voltage V_Zof Zener diode 26. Consequently, in the second embodiment shown in FIG. 4, booster circuit 9 simply raises DC output voltage V_OUTon an elevated level by substantially constant voltage V_Zwith a smaller step-up ratio V_OUT/V_INthan that for prior art voltage increase up to a constant high DC output voltage regardless of a level of input voltage V_IN. When output voltage V_OUTis higher than reference voltage V_R, control circuit 10 produces drive signals V_Gfor shortening ON period of time for MOS-FET 6 to reduce output voltage V_OUT. Specifically, resistor 23 connected in parallel to transistor 24 provides an aligning circuit for maintaining output voltage V_OUTon a predetermined value shown by the following formula:

[0029] V_OUT={(R₂₀+R₂₁+R₂₈)/R₂₁}·V_X

[0030] wherein R₂₀, R₂₁and R₂₃indicate resistance values of resistors 20, 21 and 23, and V_Xindicates a value of reference voltage V_Rof voltage generator 11 or picked voltage V_Cof regulating circuit 19 so that aligning circuit or resistor 23 in operation serves to prevent output voltage V_OUTfrom unnecessarily rising beyond an upper voltage limit. In this way, picked voltage V_Cin regulating circuit 19 of the embodiment shown in FIG. 4 can be used to adjust output voltage V_OUTin response to peak voltage V_Pof voltage detector 18 in order to maintain output voltage V_OUTon lower level when AC input voltage V_INis on lower level.

[0031] FIG. 6 demonstrates a third embodiment according to the present invention utilizing a booster circuit 9 which comprises a pair of output diodes 5a,5bconnected in parallel to each other and to a positive output terminal 31; a pair of two MOS-FETs 6a,6bconnected in parallel to each other and between negative output terminal 32 and output diodes 5a,5brespectively. A reactor 4 is connected between a positive output of filter circuit 2 and output diode 5a,and a negative output of filter circuit 2 is connected to MOS-FET 6b.Provided on a line between filter circuit 2 and reactor 4 is a current detector 28 which discerns a value of electric current through reactor 4 and forwards the value to comparator 14 whose output is supplied to a PWM circuit 29 for generating drive signals V_G1and V_G2to each gate of MOS-FETs 6a,6b.

[0032] As shown in FIG. 7, AC-DC converter may comprises a biasing circuit 33 for increasing reference voltage V_Ron which peak voltage V_Pof voltage detector 18 is superimposed to extend ON period in an ON-OFF cycle of MOS-FET(s) 6 for elevation of output voltage V_OUTat the time of starting AC-DC converter, thereby eliminating ripple components or fluctuation involved in output voltage V_OUT.

[0033] Voltage dividing resistors 20, 21 are operated to share output voltage V_OUTwith upper and lower resistors 20, 21 so that an inverted terminal of error amplifier 12 picks out a divided voltage V_Capplied on lower resistor 21. Voltage regulator 22 adjusts divided voltage V_Capplied on lower resistor 21 in response to peak voltage V_Pthrough voltage detector 18.

[0034] Accordingly, AC-DC converter shown in FIG. 4 can reduce power loss in MOS-FET 6 with the improved conversion efficiency because reduced step-up ratio V_OUT/V_INdecreases electric current I_DSthrough MOS-PET 6. At the time of starting AC-DO converter, picked voltage V_Ccan be kept on lower level by voltage charged in starting capacitor 25 to expand ON pulse width of drive signals V_Gto MOS-FET 6 so that level of output voltage V_OUTcan be immediately increased, restricting occurrence of ripple components in output voltage V_OUTfrom booster circuit 9 to an external DC power supply such as DC-DC converter connected to output terminals 31, 32 in starting AC-DC converter. A similar operation can be obtained with series resistor 23 of voltage regulator 22 connected between voltage dividing resistor 21 and earth as a variation of the embodiment shown in FIG. 4.

[0035] The present invention is not limited to the foregoing embodiments, and more various kinds of variations in the embodiments can be made. For example, the AC-DC converter according to the present invention may perceive effective or average values of AC input voltage V_INin lieu of peak voltage V_Pof AC input voltage V_INdetected by voltage detector 18.

[0036] Also, the AC-DC converter may include voltage limiting means such as a limiter circuit provided in control circuit 10 for restricting output voltage V_OUTacross booster circuit 9 up to a predetermined value to advantageously prevent considerable increase of output voltage V_OUTwhen AC input voltage V_INsupplied from AC power source 1 reaches a maximum rating value. The converter does not necessarily maintain a constant step-up ratio V_OUT/V_IN, and step-up ratio V_OUT/V_INmay be varied depending on level of AC input voltage V_INof AC power source 1. In lieu of MOS-FET 6, a switching semiconductor of another type may be selected from an IGBT (insulated gate type bipolar transistor), a J-FET (junction type field effect transistor), a junction type bipolar transistor and a thyristor. Also, it is apparent that the present invention can be applied to AC-DC converters of three-phase or polyphase alternating current type as well as single-phase alternating current type.

[0037] As mentioned above, the AC-DC converter according to the present invention can control step-up ratio to the lower level to boost the output voltage to a higher level than the AC voltage of the AC power source by a substantially constant height. Therefore, the converter can achieve decrease in electric current through the switching element with reduction of the power loss and improvement of the conversion efficiency for the voltage of worldwide AC power source when the voltage supplied from the AC power source is lower.