TECHNICAL FIELD

The present invention relates to a device for converting an alternating voltage to a direct current voltage, said device including a converter and a rectifier bridge across whose input the alternating voltage is applied.

DESCRIPTION OF THE BACKGROUND ART

It is necessary to connect between a chopper-type converter and an alternating voltage source a circuit which will function to rectify and filter the applied alternating voltage. A power supply unit which includes conventional peak rectification, where the rectifying and filtering circuit is comprised of a diode bridge in parallel with a capacitor, delivers a current output of short duration and a power factor which reaches to between 0.5 and 0.7. In this case, the alternating voltage source is loaded over a time interval that extends from slightly before to slightly after the centre of each sinus half-wave.

The conduction time and therewith the power factor (i.e. the broad current consumption) of such power supply units can be increased in many different ways. One method is to connect between the diode bridge and the capacitor a step up convert¬ er which includes a pulse controlled switching element, among other things, said step up converter functioning to provide from the voltage source a current which has the best possible sinus configuration, with the aid of pulse width control. However, this method of procedure is complicated and causes the efficiency of the system to be lowered by several percentage units as a result of losses in the step up converter. Furthermore, it is necessary to provide means which will gently charge the capacitor in the step up

converter when starting up the system.

Another method uses a step down converter upstream of the converter. This step-down converter functions to maintain the voltage to the converter constant, which can result in a simpler and more effective converter. However, the converter input voltage is lowered considerably and all power must pass through the step down converter, which makes it difficult to obtain greater efficiency in this particular case.

Another method known to the art involves the inclusion of a choke in series with the converter. However, in order to achieve any appreciable increase in the power factor, it is necessary to use a choke of an unacceptable size. The size of the choke can be made smaller by including a capacitor with which the choke may be in resonance, although the unit will then be highly frequency-sensitive and load-dependent.

U.S. Patent Specification No. 5,012,161 describes a power factor correction circuit which is used in an electronic ballast device for gas discharge lamps. The electronic ballast device is said to include a diode bridge rectifier whose input is connected to an alternating voltage, and a diode in series with a capacitor is connected between the bridge output and the input of an electronic power circuit. The electronic power circuit supplies one or more gas discharge lamps and keeps the capacitor charged at a given voltage level, by means of a transformer connected in series with a diode. In this way, voltage is supplied to the electronic power circuit by the diode bridge when the bridge output voltage is larger than the voltage level across the capacitor, and by the capacitor when the bridge output voltage is smaller than the voltage level across the capaci¬ tor. Conversion to direct current voltage is effected solely because the electronic ballast device operates on direct current voltage. Its output signal to the lamp, however, is an alternating voltage signal. Similar ballast devices are

described in U.S. Patent Specification Nos. 4,109,307 and 4,902,942.

DE-A1 4,243,943 describes an alternating current/direct current converter which includes, among other things, two series circuits connected in parallel between a diode bridge and a converter. One series circuit includes a capacitor and a first diode and the other series circuit includes a second diode and a controlled switch. A coil is connected between the interconnection point between the capacitor and the first diode in said one series circuit and the interconnection point between the second diode and the controlled switch in said other series circuit respectively. When the absolute value of an alternating voltage applied across the diode bridge exceeds the value of a voltage stored across the capacitor, the device will function to supply the converter with said alternating voltage, while when said absolute value is lower than the voltage on the capacitor, the converter is supplied with the voltage across the capacitor.

US-A 5,146,399 describes a device which includes a converter and a diode bridge. A first diode is coupled across the diode bridge output in parallel with a first controlled switch in series with a first coil which, in turn, is coupled in parallel with a capacitor in series with a second diode. The converter input is connected to the interconnection point between the capacitor and the second diode by means of a second controlled switch. This circuit delivers to the converter a voltage which consists in the absolute value of the voltage applied across the rectifying bridge plus a voltage stored across the capacitor (where the capacitor voltage may be zero) .

However, no circuit is found which can combine the functions of the circuit defined in DE-A1 4,243,943 and US-A 5,146,399 in a simple manner.

DISCLOSURE OF THE INVENTION

The object of the present invention is to provide a power supply unit for converting alternating voltage to DC-voltage, which will enable two mutually different voltage conversion functions to be combined readily and cheaply, therewith enabling the device to be used in a number of different electric networks on a worldwide basis.

This object is achieved with a device that includes a rectifier bridge on whose input an alternating voltage is applied and in which two series circuits are coupled in parallel across the input of the converter. One series circuit includes a first capacitor and a first diode, and the other series circuit includes a second diode and a controlled switch. A coil is coupled between the interconnection point between the first capacitor and the first diode in said one series circuit and between the interconnection point between the second diode and the controlled switch in said other series circuit. One output terminal of the rectifier bridge is connected to the interconnection point between the first diode and the controlled switch, while the other output terminal of said rectifier bridge is connected to a switch for selective connection of said bridge to the intercon- nection point between the first capacitor and the second diode or, alternatively, to the interconnection point between the first capacitor and the first diode. --

When operating according to one of the voltage conversion functions, the present invention will provide a high power factor and satisfy the requirements placed on input current time variation and harmonic content for classification as a class A device in accordance with European Standard EN60555- 2.

BRIEF DESCRIPTION OF THE DRAWINGS

A preferred embodiment of the invention will now be described with reference to the accompanying drawings, in which

Fig. 1 is a circuit diagram of a power supply unit according to one preferred embodiment of the invention;

Fig. 2a shows a positive half period of the input voltage and the input current of the power supply unit shown in Fig. 1 when its switch is in a position A, and also shows corre- sponding current input to a conventional power supply unit and the class areas according to European Standard EN555-2;

Fig. 2b shows a positive half period of the input voltage and the input current of the power supply unit shown in Fig. 1 when its switch is in a position B;

Fig. 3 shows two voltages as a function of the time measured in the power supply unit shown in Fig. 1 when the switch is in position A; and

Fig. 4 illustrates voltages as a function of the time measured at the same locations as those in Fig. 3, but with the switch in position B.

BEST MODES OF CARRYING OUT THE INVENTION

The power supply unit according to a preferred embodiment of the present invention converts alternating voltage to DC- voltage with an improved power factor which satisfies the requirement of power supply unit input current time variation for the unit to be classified as a class A-type apparatus in accordance with European Standard E555-2, and in one case maintains a high efficiency. The preferred embodiment of the invention also enables switching between two different voltage states, for instance between 230 and 115 V, where one converter (for instance a fly back converter) in the power supply unit receives in both cases essentially the same DC-

voltage when there is applied across the power supply unit in one voltage state an alternating voltage which is essen¬ tially twice as large as an alternating voltage that is applied in the second voltage state.

Fig. 1 illustrates a power supply unit according to the preferred embodiment of the invention. The power supply unit includes a converter P2, which is preferably a fly back converter. A load R _L is connected across the output of the converter P2. The power supply unit also includes a diode rectifying bridge Dl whose input is connected to an alternat¬ ing voltage source (mains voltage) V and a signal V _in is obtained on the output of the bridge. A series circuit and an auxiliary converter circuit PI (shown in broken lines) are connected between the output of the bridge Dl and the input of the converter P2. The series circuit includes a first capacitor Cl and a first diode D2, while the auxiliary converter circuit PI includes a second diode D3 in series with a controlled switch T and a coil LI. The coil LI is coupled between the connecting point or node 10 between the first capacitor Cl and the first diode D2 in the series circuit, and between the connecting node between the second diode D3 and the control switch T in the auxiliary converter circuit PI. The series circuit and a second capacitor C2, the second diode D3 and the switch T are all coupled in parallel across the converter input, where an input voltage U _C2 to the converter P2 is also shown. One output terminal of the rectifier bridge Dl is connected to a connection node between the first diode D2 and the switch T, while the second output terminal is connected to a switch SWl which functions to connect the terminal selectively either to the node 10 or to the connecting node 12 between the first capacitor Cl and the second diode D3. The nodes 10 and 12 correspond respectively to the two positions B and A of the switch SWl. The switch SWl is shown in position A in the Figure. The switch T is preferably a MOSFET-tranεistor and is controlled convention¬ ally by a control circuit 14, normally by a PWM control

function, to either open or close the current circuit in which it is connected in the auxiliary converter circuit PI, this making or breaking of the current path being effected in response to the voltages detected in the nodes 10 and 12. The control circuit 14 is also connected to the connecting node between the first diode D2 and the switch T. The difference between the voltages in the nodes 10 and 12, which constitute the voltage across the first capacitor Cl, is also shown in the Figure as the voltage u _Ci .

When the switch SWl is in position A, the auxiliary converter circuit PI forms a step down converter whose input is located at the output of the rectifier bridge Dl and whose output is located across the first capacitor Cl. When the switch SWl is in position B, the auxiliary converter circuit PI forms a step up converter whose input is located at the output of the rectifier bridge Dl and whose output is located across the second capacitor C2. When the switch SWl is in position A, the unit according to the preferred embodiment is opera- tive at 230 V (European mains voltage), and in position B is operative at 115 V (American mains voltage).

Fig. 2a is a diagram which illustrates voltage/current as a function of time. The diagram shows a half period of the input voltage V and the input current or the current I _A taken from the mains to the inventive power supply unit .when the switch SWl in Fig. 1 is in position A, and also shows the time variation of a corresponding input current I _τ to a conventional power supply unit for the same input voltage and the same load R _L . The conventional power supply unit is comprised of a rectifier bridge and a converter, between which a large capacitor is coupled. Fig. 2a also shows lines which are used in the division of the classification areas for classification of units in accordance with European Standard EN555-2.

According to this European Standard EN555-2, there is formed

one such classification area for a unit in relation to the maximum value of the unit current output during the input voltage half-period interval. The classification area is formed as follows: A straight line which is tangential to the maximum current output is drawn in a first interval of size π/3 around the maximum value of the input current. The remainder of the half-period is divided into a second and a third interval in which there are drawn straight lines that are parallel with the tangential line and which are located at a level which is 0.35 times the maximum current output. The lines at the different current levels are mutually connected at their common interval limits by means of straight lines extending at right angles thereto. Finally, the current levels in the second and the third interval are connected to the time axis at the beginning and the end of the half-period. The area that lies within these mutually connected lines is the classification area. When the input current lies within the classification area during at least 95% of the duration of the half-period, the unit falls outside the A classification.

It will be seen from Fig. 2a in this regard that the current output I _τ of the conventional power supply unit falls completely within its classification area whose limits are referenced EN555-2 and are shown in full lines, and cannot therefore be classified as a class A unit, whereas when the switch SWl of the inventive power supply unit is in position A, the current output I _x from the inventive power supply unit will lie within its classification area under less than 95% of the length of the half period, the limits of this area also being referenced EN555-2 but being shown in broken lines. Fig. 2a also shows the conducting times of the two power supply units, and the input voltage levels 1, 2, 3 and 4 which control these conducting times. The inventive unit conducts between the levels 1 and 2 on the input voltage curve, whereas the conventional unit conducts between the levels 3 and 4. It will be seen from the Figure that the

inventive unit has a markedly longer conduction time (and therewith also a higher power factor) than the conventional unit. The voltage levels 1 and 2 that control the conduction time of the inventive power supply unit will be explained in more detail below.

Fig. 3 shows the voltage U _C2 as a function of the time when the switch SWl is in position A, together with the voltage V _ln , which is a voltage of 230 V which coincides partly with the voltage U _C2 and which is shown in broken lines when no such coincidence occurs. Fig. 3 also shows a part of the voltage U _C1 over this time period.

The manner in which the inventive device operates when the switch SWl is in position A will now be explained with reference to Figs. 1 and 3. The function of the auxiliary converter circuit PI is to keep the capacitor Cl charged at a given voltage level U _C1 . The inventive device then functions to apply the voltage output V _ln from the rectifier bridge Dl across the input of the converter P2 when this voltage V _in is higher than the voltage U _C1 across the capacitor Cl, and the voltage U^ is applied across the input of the converter P2 when the voltage V _ln is lower than the voltage U _Ci . Conse¬ quently, the voltage U _C2 is comprised of two different contributions, partly the contribution consisting of sinus wave peaks from the voltage V _ln which extends from point 1 to point 2 on the voltage curve U _C2 , and partly by the essential¬ ly linear contribution from the voltage U _C1 extending from point 2 to point 1 on the voltage curve U _C2 . The points 1 and 2 in Fig. 3 correspond to the points 1 and 2 in Fig. 2a. As will be seen, the voltage U _C2 slowly falls when comprised of the voltage U _C1 . This is because the capacitor Cl is dis¬ charged to supply the converter P2 with voltage. When the auxiliary converter circuit PI is coupled as a step down converter, the circuit will have a much smaller power output than the converter P2 and therewith causes a very small reduction in unit efficiency. The circuit also charges the

capacitor Cl gently when starting up the system, which normally requires the provision of separate circuits. The voltage level U _C1 to which the capacitor Cl is charged by the auxiliary converter circuit PI lies in a range of 1/4-1, preferably about 2/3 of the peak value of the output voltage V _in of the rectifier bridge Dl. The higher the voltage level U _cl across the capacitor Cl, the more the inventive power supply unit will operate as a conventional unit, as described with reference to Fig. 2a. The input current I _A can be made more uniform with less distortion, by allowing the auxiliary converter circuit PI to charge the capacitor Cl for a shorter time than the time over which the converter P2 operates. When the capacitor Cl is charged to about 2/3 of the maximum output voltage of the rectifier bridge Dl, there is obtained a unit power factor of about 0.92-0.95.

Fig. 2b shows a diagram of input voltage V and input current I _A to the inventive power supply unit from the mains supply when the switch SWl is in position B. The form of this current output I _A also satisfies European Standard EN555-2, even though this classification area is not shown in the Figure. As will be seen, when the switch SWl is in position B the unit will operate for the whole of the half-period. This is because the auxiliary converter circuit PI operates continuously and because the voltage U _C1 is in series with the mains voltage V. The time variation of the input current is even more sinusoidal than when the switch SWl was in position A.

Fig. 4 shows voltage curves that correspond to the voltage curves shown in Fig. 3 for the inventive device, but with the switch SWl in position B. The Figure shows the voltage U _C2 as a function of time, together with the voltage V _ln , which is a rectified voltage of 115 V. The Figure also shows the voltage U _C1 as the difference between V _IN and U _C2 .

The manner in which the device operates when the switch SWl

is in position B will now be described with reference to Fig. 1 and Fig. 4. The auxiliary converter circuit PI, which now functions as a step up converter, charges the first capacitor Cl to the voltage level U _cl , this voltage lying in the range of 75-300 V and being preferably about 200 V. The voltage U _Ci is preferably the same for both switching positions A and B. The auxiliary converter PI charges the capacitor Cl over the full period and delivers, at the same time, the output voltage U _C2 which is applied across the input of the converter P2. The voltage U _C1 across the capacitor Cl falls in series with the output voltage V _in from the rectifier bridge Dl, meaning that the converter input voltage U _c2 will be the sum of the output voltage V _ln of the rectifier bridge Dl and the voltage U _cl across the capacitor Cl. The auxiliary converter circuit PI is more active when the switch SWl is in position B than when the switch was in position A, and the power output of the auxiliary converter circuit PI is about 2/3 of the converter power output.

When the unit input voltage is 230 V with the switch SWl in the position A and the unit input voltage is 115 V when the switch SWl is in position B, essentially the same voltage input to the converter P2 is obtained in both cases.

The main function of the second capacitor C2 is to compensate for ripple currents and to filter out frequencies caused by the switch T. The capacitor Cl has a much higher capacitance than the capacitor C2, the capacitance of capacitor Cl being at least 50 μF, preferably 200 μF, and at least 10 times, preferably 100 times, greater than the capacitance of the capacitor C2.