Power supply circuit

Abstract: A power supply circuit comprising a cascade arrangement of a full-wave rectifier and a first and a second switched voltage converter. The first switched voltage converter comprises a first inductive element, first switching means, a first rectifier diode and a first storage capacitor. Output terminals of the mains rectifier are coupled to input terminals of the first voltage converter for receiving a rectified mains voltage. The second switched voltage converter comprises a second inductive element, second switching means, a second rectifier diode and a second storage capacitor. Input terminals of the second voltage converter are connected to the first storage capacitor. The second voltage converter is coupled to output terminals of the power supply circuit for supplying an output voltage. The power supply circuit further comprises a control circuit for controlling the period of conductance of the switching means of the switched voltage converters with a switching period which is much shorter than the cycle period of the mains voltage. The first inductive element is connected by means of the first switching means across the output terminals of the mains rectifier during a part of the switching period. The first rectifier diode connected to the inductive element is non-conducting during this part. The first rectifier diode is connected to the first inductive element in such a way that the current through the diode does not flow to the input terminals of the first switched voltage converter during the period when the diode is conducting.

Description: The invention relates to a power supply circuit as defined in the pre-characterizing part of claim 1. Instead of using two cascade-arranged voltage converters, it is common practice to use one voltage converter with a input storage capacitor between the mains rectifier and the voltage converter. A voltage ripple is then produced across this storage capacitor and this voltage ripple is corrected by means of the converter. Moreover, the output voltage is maintained at a constant value, independent of load variations and/or mains voltage variations. The drawback of this construction is that the input storage capacitor must have a large value if the above-mentioned circuit is to operate satisfactorily. Consequently, the current taken from the mains is peak-shaped and discontinuous, resulting in strong disturbances (higher harmonics) of the mains (mains pollution). Mains pollution is prevented by arranging a further switched voltage converter, which is commonly referred to as preconditioner, between the mains rectifier and the said one switched voltage converter. As from about 1996 the requirements for the prevention of mains pollution will become stricter, which will also apply to display devices, computers and the like. In about 1996 the European standard EN 60555 based on IEC-555 will take effect. The first switched voltage converter (preconditioner) is then used for improving a power factor and for reducing harmonics of the current taken from the mains, while the second switched voltage converter is used to correct the mains ripple left on an output storage capacitor of the first switched voltage converter. German Patent Application DE-3328723 describes a prior art power supply circuit. In this solution an up-converter is used as a first converter. The object of arranging an up-converter between the mains rectifier and the existing converter is the wish to take a continuous current from the mains. Although the current taken from the mains is continuous in this solution, the current is peak-shaped so that the mains is still polluted with higher harmonics. Moreover, this circuit also comprises a third switched converter between the second switched converter and the load, making this solution complicated and expensive. Another solution is described in German Patent Application DE-4032199. In this Application a down-converter is used as the first converter. In this solution the current which is taken from the mains is mainly sinusoidal but discontinuous, which again results in higher harmonics of the mains frequency. This is due to the fact that the down-converter can only take a current from the mains when the rectified voltage exceeds the voltage across the storage capacitor of the down-converter. It is known from the Japanese patent application JP-A-58-62714 to have a first and a second switched voltage converter in series. A first output voltage of the first switched voltage converter is stabilized via a first feedback loop arranged between the first output voltage and a switching element of the first switched voltage converter. As the first switched voltage converter is stabilizing one of its own output voltages, the current taken from the mains will not be sinusoidal, so that the mains will again be polluted by higher harmonics. The second voltage converter has an input voltage being a second output voltage of the first switched voltage converter, and the second voltage converter has a second feedback loop, separate from the first feedback loop, for stabilizing one of the output voltages of the second voltage converter. One of the objects of the invention is to fulfil the requirements on mains pollution in a simple and inexpensive manner. To this end, a power supply circuit according to the invention is characterized by the characterizing part of claim 1. Advantageous embodiments are defined in the dependent claims. The invention uses a cascade-arrangement of a first switched voltage converter being a preconditioner for minimizing mains pollution, and a second switched voltage converter for generating stabilized voltages. The invention is based on the recognition that the prevention of mains pollution necessitates that a current taken from the mains by the first voltage converter and averaged over a switching period, has the shape of a rectified sine, so that this current thus has the same shape as the voltage supplied by the mains rectifier, and that this can be realised by means of a voltage converter having the characteristic features of claim 1. This is not the case in the circuit known from German Patent Application DE-3328723. For an input voltage having a rectified sine as a waveform, the current taken from the mains has a waveform which is the square of the sine in this known circuit. Moreover, in the power supply circuit described in German Patent Application DE-4032199 the current taken from the mains has the waveform of a sine with discontinuities around the zero-crossings in the case of (again) a rectified sine as a waveform of the input voltage. As has been mentioned hereinbefore, this is due to the fact that the voltage across the first storage capacitor then exceeds the input voltage. By using as a preconditioner, the circuit described in the characterizing part, it is achieved that the voltage built up across the first storage capacitor does not have any influence on the current taken from the mains. The period of conductance of the first switching means should preferably not exhibit any mains frequency modulation. A mains frequency modulation or mains frequency variation is hereinafter understood to mean a modulation or variation which varies with the frequency of the mains and/or with harmonics of this frequency. In an embodiment of a power supply circuit as defined in claim 2 the control signal of the first switching means is therefore substantially independent of mains frequency variations of the rectified mains voltage. A further embodiment of the power supply circuit of claim 2 is defined in claim 3. To prevent the control signal for the first switching means from having a mains frequent modulation (for example 100 Hz), the output signal of the differential amplifier is applied to the first pulse width modulator via the low-pass filter. In accordance with a further aspect of the invention the variations of the period of conductance of the second switching means are generally so small that they are also permissible for the first switching means without aggravating the mains pollution. Based on this recognition, a further embodiment of the power supply circuit is as defined in claim 4. This embodiment has the advantage that the power supply circuit comprises only one pulse width modulator controlling both controlled switches, thus simplifying the circuit and reducing its cost. An embodiment of the power supply circuit as defined in claim 6, has the advantage that the power supply circuit comprises one pulse-width modulator, and only one controlled switch, thus further simplifying the circuit and reducing its cost. In a preferred embodiment as defined in claim 7, the switching means are combined in such a way that the largest of the two currents flows through the controllable switch and the difference of the two currents flows through the diode. This embodiment has the further advantage that the current flowing through the controlled switch is not the sum of the currents through the two controlled switches, but is the largest of the two currents. The difference of the two currents instead of one of the two currents flows through the diode. Since the currents flowing through the switches are small in this preferred embodiment, the power which is dissipated in the switches will be smaller than in the other embodiments allowing the use of less expensive elements. Embodiments of the invention will now be described in greater detail by way of example with reference to the accompanying drawings in which Fig. 1 shows a first embodiment of a power supply circuit according to the invention; Fig. 2 shows a control circuit as used in a power supply circuit according to the invention; Fig. 3 shows a second embodiment of a power supply circuit according to the invention; and Fig. 4 shows a preferred embodiment of a power supply circuit according to the invention. Fig. 1 shows a mains rectifier 1 whose input terminals are connected to the mains. A small capacitor 2 for filtering out high-frequency interferences is connected across the output terminals of the mains rectifier. A series arrangement of a primary winding 5 of a transformer 3 and a first controllable switch 7 is also connected to the output terminals of the mains rectifier. One end of a secondary winding 9 of the transformer 3 is connected to the anode of a diode 11. The cathode of the diode is connected to a first storage capacitor 13, the other terminal of which is connected to the other end of the secondary winding of the transformer 3. A series arrangement of a primary winding 17 of a second transformer 15 and a second controllable switch 19 is arranged parallel to the first storage capacitor 13. An anode of a second diode 23 is connected to a secondary winding of the second transformer. The cathode of the diode 23 is connected to a second storage capacitor 25, the other terminal of which is connected to the other end of the secondary winding 21. A further circuit to be fed with a direct voltage and designated by means of a load Rbel is connected across the storage capacitor 25. A control circuit 27 receives a reference voltage Vref as a first input signal and a signal V1 as a second input signal which is dependent on the output voltage of the power supply circuit and, with reference thereto, determines the control signals for the two switches (7 and 19). The transformer 3, the switch 7, the diode 11 and the storage capacitor 13 constitute a first switched voltage converter. The transformer 15, the switch 19, the diode 23 and the storage capacitor 25 constitute a second switched voltage converter. The power supply circuit operates as follows. The mains rectifier 1 full-wave rectifies the mains voltage. Due to the absence of a large storage capacitor across the output terminals of the rectifier (the capacitor 2 has only a relatively small value of, for example 1 microF) a voltage approximately having the shape of a rectified sine is produced across these terminals. This voltage is applied to the series circuit of the primary winding 5 and the switch 7. During the period when the switch 7 is conducting, magnetic energy is built up in the transformer 3. During the period when the switch 7 is non-conducting, this magnetic energy is stored in the storage capacitor 13 via the secondary winding 9 and the diode 11. In this way a direct voltage with a ripple at the double mains frequency is stored in the storage capacitor 13. This voltage is present across the series circuit of the second primary winding 17 and the second switch 19. During the period when the switch 19 is conducting, magnetic energy is stored in the second transformer 15. During the period when the switch 19 is non-conducting, this magnetic energy is stored in the second storage capacitor 25 via the secondary winding 21 and the diode 23. The voltage across this storage capacitor is stabilized by varying the duty cycle of the switch 19. The control circuit 27 (shown in greater detail in Fig. 2) determines the duty cycle for the switches with reference to the output voltage and a reference voltage Vref. The switches are high-frequency switched (for example, at a switching frequency of 31,25 kHz). The two transformers 3 and 15 have discontinuous transformer currents, while during each period of the switching frequency the magnetic energy stored in the relevant transformer during the first part of the period is supplied again during the second part of the period. The current through the primary winding of the transformer increases linearly from zero in each period. The larger the voltage across the transformer and the switch, the faster the current will increase. Fig. 2 shows the control circuit 27 in greater detail. A differential amplifier 271 receives the reference voltage Vref at a first input and the voltage V1 at a second input and an output supplies the differences between these two voltages in an amplified form. This difference voltage is applied to a low-pass filter 273. An output of the low-pass filter is connected to an input of a first pulse width modulator 275. Dependent on the voltage at the input, the pulse width modulator supplies a control signal to the switch 7 (not shown) under the control of a clock generator 277. Since the pulse width modulator 275 receives the difference voltage from the differential amplifier via the low-pass filter, mains frequency variations are filtered out. The output of the differential amplifier 271 is also connected to a second pulse width modulator 279 which is also controlled by the clock generator 277. An output of the pulse width modulator 279 applies a control signal to the switch 19 (not shown). The periods of conductance of the switch 19 are therefore modulated by possible mains frequency variations of the voltage V1 so that these variations are inhibited in the output voltage across the capacitor 25. In contrast thereto, the periods of conductance of the switch 7 are not modulated by said mains frequency variations. It has been found that this leads to an optimum reduction of mains pollution. However, a circuit of considerably lower cost can be obtained if the two switches are simultaneously opened and closed, with the periods of conductance for the switch 7 being mains frequency modulated in the same way as those for the switch 19. This is shown in Fig. 3 in which identical elements have the same reference numerals. If the two switches 7 and 19 (of Fig. 1) have the same duty cycle, the two switches 7 and 19 can be replaced by a single controlled switch 19 and a diode 29 (to be considered as a non-controlled switch). This diode creates a conducting path between the primary winding 5 and the junction point of the primary winding 17 and the switch 19. If instead of the diode 29 a conducting connection were made, unwanted short-circuit currents would flow in the circuit 9, 11, 17, 5, 1. Otherwise, the operation of the circuit will be mainly equal. During the period when the switch 19 is conducting a linearly increasing current will flow through the primary winding 5 of transformer 3 and a linearly increasing current will also flow through the primary winding 17 of transformer 15. The sum of the two currents through the two switches in the circuit of Fig. 1 flows through the switch 19 during this part of the period. The switch 19 is also high-frequency switched in this circuit and the duty cycle is also adapted to the output voltage (across a load Rbel.). In Fig. 4 elements identical to those in Fig. 1 and/or Fig. 3 have the same reference numerals. In this embodiment the primary winding 5 and the secondary winding 9 of the transformer 3 are coupled together at the end to which diode 29 is connected. The junction point of the diode 29 and the two windings 5 and 9 is connected, via a diode 31, to the terminal of the switch 19 which is not connected to the primary winding 17. During the part of the period when the switch 19 is conducting a current i1 flows through the primary winding 5, while a current i2 flows through the primary winding 17 during this part of the period. To prevent the sum of the two currents (i1+i2) from flowing through the switch 19 (as in Fig. 4), a different conducting path must be found for one of the currents. If the current i1 is larger than the current i2, the current i1 can flow through the diode 29 from the primary winding to the switch 19. The current i2 can then also flow through the diode 29 from the primary winding 17 to the secondary winding 9 (because i1 > i2, and the connection of the primary winding 5 and the secondary winding 9 of the first transformer). If the current i2 is larger than the current i1, there must be a conducting path from the low end of the switch 19 to the secondary winding 9. The diode 31 may be used for this purpose. The current i1 can then also flow through the diode 31 from the primary winding 5 to the low end of the capacitor 2 (because i2 > i1). If it is known in advance which of the two currents is the largest, either the diode 29 (at i2 > i1) or the diode 31 (at i1 > i2) can be omitted from the circuit. Due to this solution the switches (31 and 19) dissipate much less energy than the switches 29 and 19 in the circuit according to Fig. 3. It is therefore sufficient to use les expensive elements. A number of embodiments of power supply circuits according to the invention have been described hereinbefore. The second switched voltage converter is always a flyback converter in the embodiments described hereinbefore. This is not essential and this converter can be replaced without any difficulty by any known type of converter. The second switched voltage converter may also be connected to a plurality of rectifier circuits such as flyback rectifiers, which similarly as the diode 23, conduct during the period when the switch 19 is non-conducting, and forward interval rectifiers, which conduct simultaneously with the switch 19. The first switched voltage converter may also be connected to a plurality of flyback rectifiers, but to prevent extra mains pollution, it is preferably not connected to forward interval rectifiers. Moreover, in embodiments which do not require any mains separation, an autotransformer or a wound coil or ordinary coil can be used instead of the transformer 5.

Claims: A power supply circuit comprising: a cascade arrangement of a full-wave rectifier (1), and first (3,11,13/7;29;31) and second (15,23,25,19) switched voltage converters, the first switched voltage converter (3,11,13/7;29;31) comprising a first inductive element (3), a first switching element (7;29;31), a first rectifier diode (11), and a first storage capacitor (13), and the second switched voltage converter (15,23,25,19) comprising a second inductive element (15), a second switching element (19), a second rectifier diode (23), and a second storage capacitor (25), the second switched voltage converter (15,23,25) being coupled to output terminals of the power supply circuit for supplying an output voltage, wherein a series arrangement of the first inductive element (3) and the first switching element (7;29;31) is coupled across output terminals of the full wave rectifier (1) for receiving a rectified mains voltage, a series arrangement of the second inductive element (15) and the second switching element (19) is coupled across terminals of the first storage capacitor (13), and wherein the first rectifier diode (11) is coupled to the first inductive element (3) such that the first rectifier diode (11) is non-conducting during a conducting period of the first switching element (7;29;31), and such that a current through the first rectifier diode (11) does not flow to the output terminals of the full wave rectifier (1), and the power supply circuit further comprises a control circuit (27) coupled to receive a feedback voltage (V1) related to the output voltage of the second switched voltage converter (15,23,25,19), for controlling a period of conductance of the second switching element (19) in dependence on the feedback voltage (V1) with a switching period which is much shorter than a cycle period of the rectified mains voltage, characterized in that the first switched voltage converter (3,11,13/7;29;31) is regulated by controlling a period of conductance of the first switching element (7;29;31) depending on the feedback voltage (V1) related to the output voltage of the second switched voltage converter (15,23,25,19), so as to cause the first switched voltage converter (3,11,13) to operate as a preconditioner for improving the power factor. A power supply circuit as claimed in Claim 1, characterized in that the first switching element (7;29;31) comprises a controllable switch (7), and in that the control circuit (27) is adapted to provide the controllable switch (7) with a control signal which is substantially independent on mains frequency variations of the rectified mains voltage. A power supply circuit as claimed in Claim 2, characterized in that the second switching element (19) comprises a second controllable switch (19), and the control circuit (27) comprises a differential amplifier (271) for supplying an output signal dependent on a difference between a reference voltage (Vref) received at a first input and the feedback voltage (V1) received at a second input, a first (275) and a second (279) pulse width modulator for controlling the first-mentioned (7) and the second (19) controllable switch, respectively, and a low-pass filter (273), wherein the first pulse-width modulator (275) is coupled to receive the output signal from the differential amplifier (271) via the low-pass filter (273), and the second pulse width modulator (279) is coupled to receive the output signal directly from the differential amplifier (271). A power supply circuit as claimed in Claim 1, characterized in that the first (7;29;31) and the second (19) switching elements comprise a first (7) and a second (19) controllable switch respectively, and the control circuit (27) is adapted to provide both the first (7) and the second (19) controllable switch with a same control signal. A power supply circuit as claimed in Claim 1, 2, 3 or 4, characterized in that the first inductive element (3) includes a first transformer (3) having a primary winding (5) coupled in series with the first switching element (7;29;31), and having a secondary winding (9) across which a series arrangement of the first rectifier diode (11) and the first storage capacitor (13) is coupled, the second inductive element (15) includes a second transformer (15) having a primary winding (17) coupled in series with the second switching element (19), and having a secondary winding (21) across which a series arrangement of the second rectifier diode (23) and the second storage capacitor (25) is coupled, and the second storage capacitor (25) is coupled to the output terminals of the power supply circuit. A power supply circuit as claimed in Claim 1, characterized in that the series arrangement of the second inductive element (15) and the second switching element (19) is coupled across a first and a second terminal of the first storage capacitor (13), the first terminal of the first storage capacitor (13) being coupled with the second inductive element (15), the second terminal of the first storage capacitor (13) being further coupled to a second output terminal of the full-wave rectifier (1), the second switching element (19) comprises a controllable switch (19) being controlled by the control circuit (27), the first switching element (7;29;31) comprises a diode (29), and the series arrangement of the first inductive element (3) and the diode (29) is coupled across a first and the second output terminal of the full wave rectifier (1) via the second switching element (19), said first output terminal being coupled to the first inductive element (3), an anode of the diode (29) is coupled to the first inductive element (3), and a cathode of the diode (29) is coupled to a junction of the second inductive element (15) and the controllable switch (19). A power supply circuit as claimed in Claim 1, characterized in that the first switching element (7;29;31) comprises a diode (31), the series arrangement of the first inductive element (3) and the diode (31) is coupled across a first and a second output terminal of the full wave rectifier (1), said first output terminal being coupled with the first inductive element (3), said second output terminal being coupled with an anode of the diode (31), the series arrangement of the second inductive element (15) and the second switching element (19) is coupled across a first and a second terminal of the first storage capacitor (13) via the first switching element (31), the first terminal of the first storage capacitor (13) being coupled with the second inductive element (15), and the second terminal of the first storage capacitor (13) being coupled to a junction of a cathode of the diode (31) and the first inductive element (3), and the second switching element (19) comprises a controllable switch (19) being controlled by the control circuit (27) and having an end coupled to the anode of the diode (31). A power supply circuit as claimed in Claim 7, characterized in that an anode of a second diode (29) is coupled with the junction of the first inductive element (3) and the cathode of the first mentioned diode (31), and a cathode of the second diode (29) is coupled with a junction of the second inductive element (15) and the controllable switch (19). A power supply circuit as claimed in Claim 7 or 8, characterized in that the first inductive element (3) includes a first transformer (3) having a primary winding (5) and a secondary winding (9), a series arrangement of the primary winding (5) and the first switching element (7;29;31) is coupled across the first and the second output terminal of the full wave rectifier (1), a series arrangement of the first rectifier diode (11) and the first storage capacitor (13) is coupled across the secondary winding (9), an end of the primary winding (5) not connected to the full-wave rectifier (1) and an end of the secondary winding (9) connected to the first storage capacitor (13) being coupled together, and the second inductive element (15) is a second transformer (15) having a primary winding (17) arranged in series with the controllable switch (19), and having a secondary winding (21) coupled to a series arrangement of the second rectifier diode (23) and the second storage capacitor (25).

Inventors: BUETHKER HENRICUS CORNELIS JOH, BUETHKER, HENRICUS CORNELIS JOHANNES, Büthker, Henricus Cornelis Johannes
Applicants: PHILIPS ELECTRONICS NV, PHILIPS ELECTRONICS N.V.
Application year: 1992
Main classification: H02M3
IPC codes: H02M7, H02M1, H02M3