AC-DC converter

Abstract: An AC-DC converter which allows an input current of an input AC power source to have harmonic currents of reduced levels and which supplies a stabilized output DC voltage to a load is realized by a simple circuit configuration. The converter has: a converter circuit consisting of a choke coil which receives a rectified voltage from a rectifying circuit, a switch element, and a diode; a smoothing capacitor which smooths the output voltage of the converter circuit and supplies the smoothed voltage to a load; and a control circuit consisting of a current detection circuit, a voltage detection circuit, and a pulse-width control circuit. In order to stabilize the output voltage, the switch element is turned on and off so that the peak value of the input current of the converter circuit is limited in a DC-like manner, thereby allowing the input current to have a trapezoidal waveform.

Description: Background of the invention: 1. Field of the invention. The invention relates to a AC-DC converter which uses an AC power source, particularly a commercial power source as an input and which supplies a DC voltage, as set forth in the preamble of claim 1. An AC-DC converter of this kind is known from EP-A-O 507 393. 2. Description of the prior art. The afore mentioned document discloses an AC-DC converter which comprises a rectifier, a choke coil, a switch and a capacitor which in the operational state is periodically charged with a charging current in dependence on periodic switching of the switch. The switch is switched to the conducting state periodically during a substantially on-time. The arrangement further comprises a detector for direct detection of the charging current. As a result, the load current which has a sinuzoidal shape by good approximation is drawn from the AC voltage power source by the circuit arrangement. From EP-A-O 387 435, a control arrangement for interfacing converter in a switched mode power supply comprises a switching means fro controlling the input current to the arrangement from an AC mains power source to have the same wave form as input voltage to the arrangement from the mains supply. A control means controls the duty ratio of the switching means in dependence on a predetermined relation between a predetermined time varying signal generated by the control means and a signal derived from the input current. From US-A-5 209 826, a switching mode power supply is known which is characterized by an output in the forward conversion mode and a separate output in the flyback mode, whereby a load having variable power requirements in one aspect of its operation and a stable power requirement in another aspect of its operation maybe powered without interference as between the two sources of power. The flyback power output is monitored for voltage regulation, the regulation being achieved by a constant-off-time variable-on-time switch. Summary of the invention. It is the object of the invention to provide an AC-DC converter which is an improved power factor and reduced level of harmonic currents. This object is attained by the features of claim 1. Preferred embodiments of the invention are subject matter of the independent claims. In the AC-DC converter of the invention, the number of parts is reduced, currents to the choke coil and to the switch elements are limited, thereby the current stress is reduced, and the input AC current which is soothed by an input filter has a trapedoizal wave form which is synchronized with the rectified voltage, resulting in that the power factor is improved and harmonic currents are reduced in level. The invention is now further explained with reference to the accompanying drawings. Brief description of the drawings: Figure 1 is a circuit diagram of the AC-DC converter of the invention. Fig. 2 is a waveform chart showing the operation of the AC-DC converter of the invention. Fig. 3 is a waveform chart showing the operation of the AC-DC converter of the invention. Fig. 4 is a waveform chart showing the operation of the AC-DC converter of the invention. Fig. 5 is a circuit diagram of the AC-DC converter of the invention. Fig. 6 is a waveform chart showing waveforms appearing at main portions in Fig. 5. Fig. 7 is a circuit diagram showing another embodiment of a second voltage detection circuit in Fig. 5. DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, an embodiment of the invention will be described with reference to the drawings. In Fig. 1, 1 designates an input AC power source, 2 designates an input filter, 3 designates a full-wave rectifying circuit, 4 designates a choke coil, 5 designates a switch element, and 6 designates a diode. The choke coil 4, the switch element 5, and the diode 6 constitute a boost converter. The reference numeral 7 designates a smoothing capacitor which smooths the output voltage of the boost converter and supplies the smoothed voltage to a load 8, 9 designates a current detection circuit which detects a current of the switch element 5, 10 designates a voltage detection circuit which detects the voltage of the smoothing capacitor 7 and compares the detected voltage with a reference voltage to detect a voltage error, and 14 designates a pulse-width control circuit which controls the switch element 5 to be turned on and off at a high frequency. The pulse-width control circuit 14 receives an output signal of the current detection circuit and also that of the voltage detection circuit, and, when the detection current reaches a predetermined level, controls the switch element 5 to be turned off. The current detection circuit 9, the voltage detection circuit 10, and the pulse-width control circuit 14 constitute a control circuit 15. The operation of the thus configured AC-DC converter will be described with reference to Fig. 2. An input AC voltage is subjected to the full-wave rectification by the full-wave rectifying circuit 3. When the switch element 5 is on, the rectified voltage Ei (Fig. 2(a)) is applied to the choke coil 4. A current flows along the path elongating from the full-wave rectifying circuit 3 to the switch element 5 through the choke coil 4, and energizes the choke coil 4. When the switch element 5 is off, a current flows from the full-wave rectifying circuit 3 to the smoothing capacitor 7 through the diode 6 so that the energization energy of the choke coil 4 is released. The voltage detection circuit 10 has a response characteristic which is lower than the frequency of the input AC voltage, and detects the voltage of the smoothing capacitor 7. The voltage detection circuit 10 compares the detected voltage with a reference voltage and amplifies the difference between the voltages. The amplified voltage difference is output as a voltage error signal Ve which is a DC voltage such as shown in Fig. 2(b). When the output voltage is raised, the level of the signal is lowered, and, when the output voltage is lowered, the level is raised. The current detection circuit 9 outputs a current signal Vi (see Fig. 2(b)) which corresponds to the waveform of the current of the switch element 5. When the rectified voltage Ei is low and the current signal Vi has a low level, the pulse-width control circuit 12 drives the switch element 5 at a predetermined on/off ratio so as to energize the choke coil 4. When the level of the current signal Vi reaches the voltage error signal Ve, in contrast, the pulse-width control circuit 12 restricts the on period of the switch element 5 so that the level of the current signal Vi does not exceed the voltage error signal Ve. Fig. 2(b) shows the operation described above. Therefore, the current Il flowing through the choke coil 4 has a waveform shown in Fig. 2(c). An input AC current Ii which is obtained by smoothing the current I1 by means of the input filter has a trapezoidal waveform shown in Fig. 2(d). As described above, according to the embodiment, the control circuit 15 can be constituted by a conventional control IC for a current-mode switching regulator, and without using a multiplying circuit, etc. Furthermore, while the output voltage is substantially stabilized, the input AC current Ii has a trapezoidal waveform, and the converter has characteristics of a high power factor and harmonic currents of reduced levels. Since a trapezoidal waveform which is lower in peak level than a sinusoidal waveform is obtained, moreover, the converter has a feature that the current stress imposed on the choke coil 4 and the switch element 5 can be relieved. In the embodiment, the switching frequency of the switch element 5 is set so as to be sufficiently higher than the frequency of the input AC voltage. As compared with the case where the switching frequency is fixed, the levels of harmonic currents can be reduced by a larger degree in the case where the off period is fixed and the on period is controlled. The reason of this phenomenon will be described with reference to Figs. 3 and 4. Fig. 3 shows the operation which is conducted when the level of the current signal Vi reaches that of the voltage error signal Ve in the case of a fixed frequency. When the level of the current signal Vi reaches for the first time that of the voltage error signal Ve at time T1, the on period of the switch element 5 which has operated in the maximum on period up to that time is suddenly shortened, and the off period is extended by the period corresponding to the shortened period. This causes the choke coil 4 to be deenergized to a level lower than required, and several switching periods are passed before the level of the current signal Vi again reaches that of the voltage error signal Ve. The above-mentioned operation is repeated until the level of the current signal Vi can reach that of the voltage error signal Ve within each switching period (see Fig. 3(a)). In the waveform of the input current Ii in the period corresponding to this portion, therefore, there appears an oscillation waveform so that harmonic currents are increased in level. By contrast, in the case where the off period is fixed, when the level of the current signal Vi reaches for the first time that of the voltage error signal Ve at time T1, the on period of the switch element 5 which has operated in the maximum on period up to that time is suddenly shortened, but the off period is not extended. Consequently, the degree of deenergizing the choke coil 4 is not changed. As shown in Fig. 4(a), the state of the maximum on period is transferred to the pulse-width control state in a relatively smooth manner. Also in the waveform of the input AC current Ii, as shown in Fig. 4(b), an oscillation waveform does not appear or appears in a suppressed level, thereby reducing the levels of harmonic currents. Furthermore, it is preferable to control the off period of the switch element 5 in such a manner that, when the rectified voltage Ei is low, the off period is short, and, when the voltage Ei is high, the off period is long. For example, the case where the rectified voltage Ei is 360 | sin  | [V], the output voltage Eo is 370 [V], and the current flowing through the choke coil 4 in the period from  = π/6 to 5 π/6 is continuous will be considered. When the off period Toff of the switch element 5 is fixed to be 10 [µsec], the on period Ton varies in the range of 0.28 and 10.56 [µsec]. By contrast, when the off period Toff of the switch element 5 is made proportional to the rectified voltage Ei so that Toff = 10 | sin  | [ µ sec] (  = π /6 to 5π/6), Toff varies in the range of 5 to 10 [µsec], and Ton varies in the range of 0.28 to 5.28 [µsec]. Namely, the variation range of Ton can be reduced to one half of that of the above case under the same conditions. In other words, a wider control ability is attained. Fig. 5 is a circuit diagram of the AC-DC converter of the invention which shows in more detail the control circuit 14 for realizing the above-mentioned control operation. Fig. 6 is a waveform chart showing waveforms appearing at various portions. The current detection circuit 9 detects the current of the switch element 5 with using a resistor, and outputs the signal Vi. In the first voltage detection circuit 10, the output voltage is divided by detection resistors 101 and 102. An error amplifier 104 compares a divided voltage with a reference voltage 103 and outputs the voltage error signal Ve. The signals Vi and Ve are shown in Fig. 6(a). A second voltage detection circuit 11 divides the rectified voltage Ei by means of resistors 111 and 112, and outputs a signal V(Ei) which corresponds to the rectified voltage Ei. In order to cause the signal V(Ei) to have the minimum value when the rectified voltage Ei is low, a Zener diode 113 is connected to the circuit. The pulse-width control circuit 12 comprises comparators 121 and 122, a constant current source 123, a capacitor 124, a flip-flop 125, a resistor 126, and a transistor 127. The comparator 121 compares the signals Vi and Ve with each other, and outputs a turn-off pulse signal Vf. Fig. 6(b) shows the signal Vf. The capacitor 124 is charged by the constant current source 123 during the switch element 5 is off, and the comparator 122 compares the voltage Vc of the capacitor with the signal V(Ei). Fig. 6(c) shows the voltage Vc of the capacitor 124 and the signal V(Ei). The comparator 122 outputs a turn-on pulse signal Vr. Fig. 6(d) shows the signal Vr. The flip-flop 125 receives the signals Vr and Vf and outputs a driving pulse signal Vg to the switch element 5. The driving pulse signal Vg is shown in Fig. 6(e). The resistor 126 and the transistor 127 cooperate to cause the capacitor 124 to be discharged during the on period of the switch element 5. The symbol Vcc indicates a bias voltage by which the control circuit 14 is operated, and which is stabilized. In this configuration, the on period of the switch element 5 is determined so as to stabilize the output voltage, and the off period is determined so as to be proportional to the charge period of the capacitor 124 or to the rectified voltage Ei. Even when the voltage Ei is in the vicinity of 0 [V], the signal V(Ei) has the minimum value. Therefore, the off period can be ensured to have the minimum off period. Although not described in the description of the configuration of Fig. 1, it is apparent that, if the resistor 111 is not disposed, the operation of fixing the off period can be conducted. In the second voltage detection circuit 11 shown in Fig. 5, the rectified voltage Ei is divided by the resistors 111 and 112, and the resulting signal V(Ei) is output. Alternatively, a voltage which is generated across an additional winding 41 wound on the choke coil 4 may be used as shown in Fig. 7. In the alternative shown in Fig. 7, the voltage which is generated across the additional winding 41 is subjected to the voltage-double rectification by a combination of diodes 113 and 114 and capacitors 115 and 116, and the bias voltage which is stable and proportional to the output voltage is supplied to the control circuit 14. Since the forward voltage generated across the additional winding 41 is rectified and smoothed, a voltage which is proportional to the rectified voltage Ei appears at a capacitor 116, and the voltage is divided by the resistors 111 and 112, thereby outputting the signal V(Ei).

Claims: An AC-DC converter comprising: a rectifying circuit (3) which receives an input AC current having an AC waveform and outputs a rectified voltage (Ei); a converter circuit (4, 5, 6) comprising a choke coil (4) which receives the rectified voltage (Ei), a switch element (5), and a diode (6); a smoothing capacitor (7) which smoothes the output voltage of said converter circuit (4, 5, 6) and supplies the smoothed voltage to a load circuit (8); and a control circuit (15) which controls said switch element (5) to be turned on and off characterised in that said control circuit (15) comprises: a current detection circuit (9) which detects the current of said choke coil (4) or said switch element (5); a voltage detection circuit (10) which detects the output voltage of said converter circuit (4, 5, 6), compares the detected output voltage with a reference voltage, and detects a voltage error, and a pulse-width control circuit (14) which compares the output (Vi) of said current detection circuit (9) with the output (Ve) of said voltage detection circuit (10), to control the pulse width of a driving pulse signal by which said switch element (5) is made turn on and off to restrict the peak value of the input current of said converter circuit (4, 5, 6). An AC-DC converter according to claim 1, wherein said control circuit (15) controls the on period of said switch element (5) while maintaining the off period thereof constant: An AC-DC converter according to claim 1, wherein said control circuit (15) further comprises: a second voltage detection circuit (11) which detects the rectified voltage (Ei), and controls the pulse-width control circuit (14) so that as the rectified voltage (Ei) is higher, the off period of said switch element (5) is made longer. An AC-DC converter according to claim 3, wherein said second voltage detection circuit (11) also detects the rectified voltage (Ei) from a voltage which appears in an additional winding (41) formed in said choke coil (4). An AC-DC converter according to any one of claims 1 to 4, wherein the control circuit (15) switches said switch element (5) on and off to linearly restrict the peak value of the input current of said converter circuit (4, 5, 6). An AC-DC converter according to claim 3, wherein the pulse-width control circuit switches said switch element (5) on and off to linearly restrict the peak current of said choke coil (4) and said switch element (5).

Inventors: IKEDA SATOSHI, ISHII TAKUYA, KOBAYASHI YOSHINORI, MIZUTANI YOSHIO, SEKINE YUTAKA, WATANABE HARUO, IKEDA, SATOSHI, ISHII, TAKUYA, KOBAYASHI, YOSHINORI, MIZUTANI, YOSHIO, SEKINE, YUTAKA, WATANABE, HARUO
Applicants: MATSUSHITA ELECTRIC IND CO LTD, SHINDENGEN ELECTRIC MFG, MATSUSHITA ELECTRIC INDUSTRIAL CO., LTD., SHINDENGEN ELECTRIC MANUFACTURING COMPANY, LIMITED
Application year: 1995
Main classification: H02M3
IPC codes: H02M3, H02M1