Forward converter

Abstract: A forward converter for improving a power factor and suppressing a harmonic noise component appearing at an input current waveform. The forward converter includes: a bridge diode (BD) for full-wave rectifying an AC input voltage to generate a first DC voltage; a first smoothing capacitor (C1) for smoothing the first DC voltage to generate a second DC voltage; a control circuit (2) for generating a switching control signal in response to an input of the second DC voltage; a switching element (Q) connected between the first smoothing capacitor (C1) and a ground terminal and switched on or off in response to the switching control signal; a transformer (T1) having at least a primary winding (Lp) and a secondary winding (Ls), for receiving the second DC voltage through the primary winding (Lp) when the switching element is switched on and inducing the second DC voltage to the secondary winding (Ls); a waveform shaper (CH) connected between the bridge diode (BD) and the first smoothing capacitor (C1), for causing a current flowing into the first smoothing capacitor (C1) to have a sine wave form; a rectifying element (D2) connected forwardly between the secondary winding (Ls) of the transformer (T1) and a load (4), for rectifying a DC voltage induced to the secondary winding (Ls) to generate a third DC voltage; and a second smoothing capacitor (C2) for smoothing the third DC voltage to supply a stable DC voltage to the load.

Description: The present invention relates to a switching mode power supply and, more particularly, to a forward converter for improving a power factor and suppressing a harmonic noise component appearing at an input current waveform. A switching mode power supply (hereinafter, referred to as the "SMPS") represents power supply equipment for converting an input voltage into a stable voltage according to an operation of a switching mode and supplying the stable voltage to a load. Since the SMPS has small size, light weight and high efficiency, it meets the demands of the times such as energy saving etc. The SMPS having such features can be classified into a forward converter and a flyback converter depending on a transformer for transmitting energy to a secondary winding according to the state of a switching element connected to a primary winding. Fig. 1 illustrates the construction of a conventional forward converter, of a type similar to that shown in Figure 29 of the European Patent EP 665632. A bridge diode BD, a capacitor C1, a switching element Q, a diode D1, a control circuit 2, a primary winding Lp of a transformer T1, and a reset winding Lr of the transformer T1 are connected to an input side. A secondary winding Ls of the transformer T1, diodes D2 and D3, a choke coil L1, a capacitor C2 and a load 4 are connected to an output side. The bridge diode BD full-wave rectifies an AC (Alternative Current) input voltage Vinac. The capacitor C1 smooths the voltage full-wave rectified by the bridge diode BD and supplies the smoothed DC (Direct Current) voltage to the control circuit 2 and the primary winding Lp of the transformer T1. The control circuit 2 generates a switching control signal having a prescribed period in response to an input of the DC voltage smoothed by the smoothing capacitor C1. The switching element Q connected between the smoothing capacitor C1 and a ground terminal is switched on or off in response to the switching control signal generated from the control circuit 2. The transformer T1 includes the primary winding Lp connected between the smoothing capacitor C1 and the switching element Q, the reset winding Lr connected between the smoothing capacitor C1 and the diode D1, and the secondary winding Ls connected to the load 4. If the switching element Q is switched on, the DC voltage smoothed by the smoothing capacitor C1 is accumulated in the primary winding Lp and induced to the secondary winding Ls. If the switching element Q is switched off, energy accumulated in the primary winding Lp is transmitted to the ground terminal through the reset winding Lr and the diode D1. Since the winding Lr and the diode D1 perform such a function, they are called a reset winding and a reset diode, respectively, and prevent the switching element Q from being damaged. The diodes D2 connected forwardly between one terminal of the secondary winding Ls of the transformer T1 and the load 4, and the diode D3 connected forwardly between the other terminal of the secondary winding Ls of the transformer T1 and the load 4 are rectifying elements for rectifying the voltage induced to the secondary winding Ls of the transformer T1. The smoothing capacitor C2 connected in parallel to the load 4 smooths the voltage generated through the diode D2 or D3 so that a stable voltage can be supplied to the load 4. The diode D3 is typically called a flywheel diode. If the AC input voltage Vinac is applied, the bridge diode BD and the smoothing capacitor C1 rectify and smooth the AC input voltage Vinac and generate a corresponding DC voltage. This DC voltage is supplied to the control circuit 2 and the primary winding Lp of the transformer T1. The control circuit 2 is operated according to the DC voltage and generates the switching control signal. The switching element Q is repeatedly switched off and on in response to the switching control signal. The transformer T1 induces the DC voltage supplied to the primary winding Lp to the secondary winding Ls as the switching element Q is switched on or off. The DC voltage induced to the secondary winding Ls of the transformer T1 is rectified by the output diode D2 or the flywheel diode D3 and smoothed by the capacitor C2 via the choke coil L1. The smoothed DC voltage is supplied to the load 4. As indicated in Fig. 1, the capacitor C1 for smoothing the voltage rectified by the bridge diode BD is connected to the bridge diode BD. Therefore, a charge current flowing into the capacitor C1 approximates to a pulse form as shown in Fig. 2d. That is, a waveform of the voltage (ripple voltage) rectified by the bridge diode BD and a waveform of the voltage smoothed by the capacitor C1 continue to be repeated with a half period between 0 and a maximum value. In this case, the voltage smoothed by the capacitor C1 has a longer high voltage interval than the voltage rectified by the bridge diode BD. During this interval, there is no current flowing into the capacitor C1, and only during a very short interval around the maximum value of the ripple voltage, a current flows into the capacitor C1. Therefore, the current flowing into the capacitor C1 approximates to the pulse form. This form of the current may deteriorate a power factor and generate harmonic noise, thereby leading to a malfunction in the forward converter and increasing a reactive power. The abovementioned European Patent EP 665632 describes a switching power source apparatus that comprises a full-wave rectifying circuit for rectifying an input alternating-current power source, a transformer possessing a primary winding having one end connected to a positive electrode output terminal of the full-wave rectifying circuit, and a secondary winding, a switch element connected to the other end of the primary winding, a smoothing capacitor connected parallel to a series circuit of the primary winding and switch element, first and second diodes having anode terminals connected to a pair of input terminals of the full-wave rectifying circuit, and cathode terminals connected to the connection point of the primary winding and switch element, a choke coil connected between the connection point of the switch element and the smoothing capacitor, and the negative electrode output terminal of the full-wave rectifying circuit, a rectifying and smoothing circuit for rectifying and smoothing the output of the secondary winding, and supplying a direct-current output voltage to a load, and a control circuit for controlling the on/off ration of the switch element so that the direct-current output voltage may be stabilized. This document also describes a number of other embodiments of the switching of power source apparatus. The European Patent EP 725475 also describes a number of embodiments of a switching converter in which the on and off operation of a transistor switch causes a current to flow intermittently through a primary winding of a choke coil to induce voltage in a secondary winding of the choke coil, a smoothing capacitor that can be charged during a wider period of time in a cycle of alternating current frequency due to a current flowing in to the smoothing capacitor by the sum of a rectified output voltage of a diode bridge rectifier and the induced voltage in the secondary winding of the choke coil. This improves the input power factor for the alternating current input. The United States Patent 5 442 539 describes a switching converter with input current shaping for unity power factor operation based on operating a Cuk converter in a discontinuous inductor current mode at a constant switching frequency and constant duty ratio without input current feedback control. It is an aim of embodiments of the invention to provide a forward converter for improving its power factor. It is another aim of embodiments of the invention to provide a forward converter for suppressing harmonic noise. It is still another aim of embodiments of the invention to provide a forward converter for preventing a malfunction. It is still yet another aim to provide a forward converter for improving a reactive power. In forward converters in accordance with aspects of the invention, a phase of an input current approximates to a phase of an input voltage and the input current has a sine wave form. In accordance with a first aspect of the invention, a forward converter comprising: a bridge diode for full-wave rectifying an AC (alternative current) input voltage to generate a first DC (direct current) voltage; a first smoothing capacitor for smoothing said first DC voltage to generate a second DC voltage; a control circuit for generating a switching control signal having a prescribed period in response to an input of said second DC voltage; a switching element connected between said first smoothing capacitor and a ground terminal and switched on or off in response to said switching control signal; a transformer having at least a primary winding and a second winding, for receiving said second DC voltage through said primary winding when said switching element is switched on and inducing said second DC voltage to said secondary winding; a waveform shaper connected between said bridge diode and said first smoothing capacitor, for causing a current flowing into said first smoothing capacitor to have a sine wave form; a first rectifying element connected forwardly between said secondary winding of said transformer and a load, for rectifying a DC voltage induced to said secondary winding to generate a third DC voltage; a second smoothing capacitor for smoothing said third DC voltage to supply a stable DC voltage to said load; and characterised in that said waveform shaper is a transformer having a primary winding connected between said first rectifying element and said load and having a secondary winding connected between said bridge diode and said first smoothing capacitor. The forward converter in accordance with the first aspect of the invention may further comprise: a choke coil connected between said waveform shaper and said first smoothing capacitor, for eliminating a harmonic component of a current flowing into said first smoothing capacitor. The forward converter in accordance with the first aspect of the invention may still further comprise: a second rectifying element connected forwardly between the other side of said secondary winding of said main transformer and said load, for rectifying a voltage induced to said secondary winding of said main transformer; a second smoothing capacitor connected in parallel to said load, for smoothing a voltage generated through said first or second rectifying element to supply a stable DC voltage to said load; and the transformer may have a primary winding connected between a reverse terminal of each of said first and second rectifying elements and said second smoothing capacitor and a secondary winding connected between said bridge diode and said first smoothing capacitor. In accordance with a second aspect of the invention, a forward converter comprising: a bridge diode for full-wave rectifying an AC (alternative current) input voltage to generate a first DC (direct current) voltage; a first smoothing capacitor for smoothing said first DC voltage to generate a second DC voltage; a control circuit for generating a switching control signal having a prescribed period in response to an input of said second DC voltage; a switching element connected between said first smoothing capacitor and a ground terminal and switched on or off in response to said switching control signal; a first diode connected forwardly between said bridge diode and said first smoothing capacitor; a choke coil connected between said first diode and said first smoothing capacitor; a main transformer having a primary winding connected between said first smoothing capacitor and said switching element and having a secondary winding connected to a load, for accumulating said second DC voltage in said primary winding so as to be induced to said secondary winding when said switching element is switched on; a first rectifying element connected forwardly between said secondary winding of said main transformer and said load, for rectifying a voltage induced to said secondary winding of said main transformer; a second smoothing capacitor connected in parallel to said load, for smoothing a voltage generated through said first rectifying element to supply a stable DC voltage to said load; and current control means connected in parallel to said first diode, for increasing a current flowing into said choke coil when said switching element is switched on and providing a reverse current to said choke coil when said switching element is switched off. Preferably, said current control means comprises: a resonant circuit having a secondary winding of a transformer installed correspondingly to a primary winding connected between said rectifying element and said secondary smoothing capacitor and having a capacitor, said resonant circuit being connected in parallel to said first diode. The forward converter in accordance with the second aspect of the invention may further comprise: a second rectifying element connected forwardly between the other side of said secondary winding of said main transformer to said first rectifying element and said load, for rectifying a voltage in use to said secondary winding of said main transformer; and said current control means may comprise: a transformer having a primary winding connected between a reverse terminal of each of said first and second rectifying elements and said second smoothing capacitor and having a secondary winding connected in parallel to said first diode; a choke coil connected between said first diode and said first smoothing capacitor; and a third capacitor connected between said secondary winding of said auxiliary transformer and said choke coil. For a better understanding of the invention, and to show how embodiments of the same may be carried into effect, reference will now be made, by way of example, to the accompanying diagrammatic drawings, in which: Fig. 1 illustrates the construction of a conventional forward converter; Figs. 2a to 2d are operational waveform charts of the forward converter of Fig. 1; Figs. 3a to 3d are waveform charts conceptually illustrating an operation of a forward converter according to an embodiment of the present invention; Fig. 4 illustrates the construction of a forward converter according to a preferred embodiment of the present invention; Figs. 5a to 5l are operational waveform charts of the forward converter of Fig. 4; Fig. 6 illustrates the construction of a forward converter according to another preferred embodiment of the present invention; and Figs. 7a to 7l are operational waveform charts of the forward converter of Fig. 6. In the following description, well known features or constructions which may obscure the present invention in unnecessary detail are not described in detail. Figs. 3a to 3d are waveform charts conceptually illustrating an operation of a forward converter according to embodiments of the present invention. In the conventional forward converter, the current flowing into the smoothing capacitor C1 has the pulse form as shown in Fig. 2d. However, in the forward converter according to the present invention, a phase of an input current approximates to a phase of an input voltage. A waveform of the input current flowing into the smoothing capacitor C1 has a sine wave form as indicated in Fig. 3d. Fig. 4 illustrates the construction of a forward converter according to a preferred embodiment of the present invention. Like the conventional forward converter, the bridge diode BD, smoothing capacitor C1, control circuit 2, switching element Q, reset diode D1, primary winding Lp of the transformer T1, and reset winding Lr of the transformer T1 are connected to an input side. The secondary winding Ls of the transformer T1, diodes D2 and D3, smoothing capacitor C2 and load 4 are connected to an output side. However, unlike the conventional forward converter, there is provided a transformer T2 including a primary winding L1 connected between the rectifying elements D2 and D3 and the smoothing capacitor C2 and including a secondary winding Lms connected between the bridge diode BD and the smoothing capacitor C1. There is also provided a choke coil CH connected between the secondary winding Lms of the transformer T2 and the smoothing capacitor C1. The transformer T2 causes the current flowing into the smoothing capacitor C1, that is, the current flowing into the choke coil CH to have the sine wave form as shown in Fig. 3d. The choke coil CH eliminates a harmonic signal component contained in the current flowing into the smoothing capacitor C1. Figs. 5a to 51 are operational waveform charts of the forward converter of Fig. 4. Fig. 5a indicates an AC input voltage Vinac, Fig. 5b an AC input current Iin, Fig. 5c a gate-to-source voltage Vgs of the switching element Q (field effect transistor), Fig. 5d a drain-source voltage Vds of the switching element Q, Fig. 5e a voltage of the reset diode D1, Fig. 5f a current flowing into the drain of the switching element Q, Fig. 5g a current flowing into the diode D1, Fig. 5h a voltage of the choke coil CH, Fig. 5i a current flowing into the choke coil CH, Fig. 5j a voltage of the secondary winding Ls of the transformer T1, Fig. 5k a voltage of the primary winding L1 of the transformer T2, and Fig. 5l a current flowing into the primary winding L1 of the transformer T2. Referring back to Fig. 4, if the AC input voltage Vinac is applied, the voltage full-wave rectified by the bridge diode BD is smoothed by the smoothing capacitor C1 via the secondary winding Lms of the transformer T2 and the input coil CH, and supplied to the primary winding Lp of the transformer T1 and the control circuit 2. The control circuit 2 starts to oscillate as the voltage passing through the choke coil CH is supplied and generates a switching control (on/off) signal. The switching element Q is repeatedly switched on or off according to the switching control signal generated from the control circuit 2. When the switching element Q is switched on, the operation of the forward converter of Fig. 4 will now be described. If the switching-on signal of a logic "HIGH" level generated from the control circuit 2 is applied to the gate of the switching element Q, the switching element Q is switched on. If the switching element Q is switched on, energy accumulated in the primary winding Lp of the transformer T1 is transmitted to the secondary winding Ls of the transformer T1. In this case, a reverse voltage is applied to the choke coil CH through the secondary winding Lms of the transformer T2. Therefore, the current flowing into the choke coil CH, that is, the current flowing into the smoothing capacitor C1 decreases. Referring to Figs. 5a to 5l, a forward voltage is applied to the primary winding L1 of the transformer T2 at an interval T1 during which the switching element Q is switched on, as shown in Fig. 5k. Since the reverse voltage is then applied to the secondary winding Lms of the transformer T2, the reverse voltage is applied to the choke coil CH as indicated in Fig. 5h. The current flowing into the choke coil CH deceases as shown in Fig. 5i. When the switching element Q is switched off, the operation of the forward converter of Fig. 4 will now be described. If the switching-off signal of a logic "LOW" level generated from the control circuit 2 is applied to the gate of the switching element Q (field effect transistor), the switching element Q is switched off. If the switching element Q is switched off, energy transmitted to the secondary winding Ls of the transformer T1 decreases. In this case, the forward voltage is applied to the choke coil CH through the secondary winding Lms of the transformer T2. Therefore, the current flowing into the choke coil CH, that is, the current flowing into the smoothing capacitor C1 increases. Referring to Figs. 5a to 5l, there is no voltage applied to the primary winding L1 of the transformer T2 at an interval T2 during which the switching element Q is switched off, as shown in Fig. 5k. Since the forward voltage is then applied to the secondary winding Lms of the transformer T2, the forward voltage is applied to the choke coil CH as indicated in Fig. 5h. Hence, the current flowing into the choke coil CH increases as shown in Fig. 5i. Consequently, when the switching element Q is switched on, the forward converter of Fig. 4 reduces the amount of the input current by supplying the reverse voltage to the capacitor C1 for supplying the smoothed DC voltage to the primary winding Lp of the transformer T1. When the switching element Q is switched off, the forward converter of Fig. 4 increases the amount of the input current by supplying the forward voltage to the capacitor C1 for supplying the smoothed DC voltage to the primary winding Lp of the transformer T1. Therefore, the current flowing into the capacitor C1 has the sine wave form as shown in Fig. 5i. This indicates that the amount of an active power increases and that the harmonic noise component is eliminated. Further, a power factor showing about 0.6 in a typical SMPS is improved to about 0.8 in this embodiment of the present invention. Fig. 6 illustrates the construction of a forward converter according to another preferred embodiment of the present invention. Referring to Fig. 6, like the conventional forward converter, the bridge diode BD, smoothing capacitor C1, control circuit 2, switching element Q, primary winding Lp of the transformer T1 are connected to an input side. The secondary winding Ls of the transformer T1, diodes D2 and D3, capacitor C2 and load 4 are connected to an output side. However, there are not provided the reset diode D1 and the reset winding Lr of the transformer T1 for preventing the switching element Q from being damaged when the switching element Q is switched off. On the other hand, unlike the conventional forward converter, there are provided a choke coil CH and a diode D4 which is forwardly connected between the bridge diode BD and the smoothing capacitor C1. There is also provided a transformer T2 including a primary winding L1 connected between the diodes D2 and D3 and the smoothing capacitor C2 and including a secondary winding Lms connected in parallel to the diode D4 together with the capacitor C3. The diode D4 and the capacitor C3 which are used instead of the diode D1 and the reset winding Lr of the transformer T1 of the conventional forward converter prevent the switching element Q from being damaged when the switching element Q is switched off. The choke coil CH suppresses a noise component of an input signal and suppresses a variation in an instantaneous current by increasing an impedance of an input line. If the AC input voltage Vinac is applied to the bridge diode BD, the AC voltage Vinac is full-wave rectified and applied to the secondary winding Lms of the transformer T2 and the diode D4. Thereafter, the voltage is smoothed by the smoothing capacitor C1 via the choke coil CH and applied to the control circuit 2 and the winding Lp of the transformer T1. The control circuit 2 generates the switching control signal (switching-on/switching-off signal). The switching element Q is repeatedly switched on or off according to the switching control signal. When the switching element Q is switched on, the operation of the forward converter of Fig. 6 will now be described. If the switching element Q is switched on, the DC voltage accumulated in the primary winding Lp of the transformer T1 is induced to the secondary winding Ls. This induced DC voltage is rectified by the diode D2 and transmitted to the primary winding L1 of the transformer T2. The DC voltage transmitted to the primary winding L1 of the transformer T2 is smoothed by the smoothing capacitor C2 and supplied to the load 4 and the secondary winding Lms of the transformer T2. In this case, the voltage transmitted to the secondary winding Lms of the transformer T2 is determined by a voltage decided by a turn ratio of the primary windings L1 and the secondary winding Lms of the transformer T2 and a voltage rectified through the diode D2. The voltage transmitted to the secondary winding Lms of the transformer T2 is applied to the choke coil CH via the capacitor C3. A voltage determined by the voltage of the capacitor C3 and the voltage of the smoothing capacitor C1 among the sum voltage of the voltage passing through the diode D2 and the voltage transmitted to the secondary winding Lms of the transformer T2 passing through the capacitor C3 is applied to the choke coil CH. Therefore, the current flowing into the choke coil CH increases. The capacitor C3 is charged by the current flowing into the choke coil CH. The voltage applied to the choke coil CH is reduced by a slop (the current flowing into the choke coil CH/a capacitance of the capacitor C3). When the switching element Q is switched off, the operation of the forward converter of Fig. 6 will now be described. If the switching element Q is switched off, a serial resonant circuit consisting of the capacitor C3 and an exiting inductance by the secondary winding Lms of the transformer T2 starts to resonate. Then the diode D4 is turned on. Therefore, the current flows into the choke coil CH via the diode D4. A positive (+) voltage appears at the capacitor C3 connected to the secondary winding Lms of the transformer T2, and a forward voltage appears at the secondary winding Lms of the transformer T2. The secondary winding Lms of the transformer T2 induces power to the primary winding L1 and causes the DC voltage to be supplied to the load 4. While this DC voltage is applied, if the voltage of the capacitor C3 becomes 0 volts, there is no voltage supplied to the load 4 via the secondary winding Lms of the transform T2. The resonant current caused by the secondary winding Lms of the transformer T2 and the capacitor C3 gradually decreases and a reverse current starts to flow. The reverse resonant current offsets the current flowing into the choke coil CH and increases until the current flowing into the bridge diode BD becomes 0. If the current flowing into the bridge diode BD becomes 0, the current flowing into the choke coil CH flows through the winding Lms of the transformer T2 and the capacitor C3 and the voltage is not accumulated in the winding Lms of the transformer T2. The supply of the voltage to the load 4 is performed through the flywheel diode D3. Since there is no voltage in the winding Lms of the transformer T2, the voltage supplied to the choke coil CH increases and the capacitor C3 is charged by the current flowing into the choke coil CH. Figs. 7a to 7l show operational waveforms of the forward converter of Fig. 6. A reference symbol T1 indicates an interval during which the switching element Q is switched on, and T2 designates an interval during which the switching element Q is switched off. Fig. 7a indicates the AC input voltage Vinac, Fig. 7b an AC input current Iin, Fig. 7c a base-to-emitter voltage Vbe of the switching element Q (transistor), Fig. 5d a collector-emitter voltage Vce of the switching element Q, Fig. 7e a current flowing into the collector of the switching element Q, Fig. 7f a voltage of the diode D4, Fig. 7g a current flowing into the diode D4, Fig. 7h a voltage of the capacitor C3, Fig. 7i a current flowing into the capacitor C3, Fig. 7j a voltage of the secondary winding Lms of the transformer T2, Fig. 7k a current flowing into the choke coil CH, and Fig. 7l an output current flowing into the load 4. Referring to Figs. 7a to 7l, during the switching-on operation of the forward converter of Fig. 6, since the switching control signal of a logic "HIGH" level is applied to the base of the switching element Q, the switching element Q is switched on. Then the DC voltage accumulated in the primary winding Lp of the transformer T1 is induced to the secondary winding Ls and this induced voltage is rectified by the diode D2. The rectified DC voltage is smoothed by the capacitor C2 and supplied to the load 4. At the same time, since the rectified DC voltage is transmitted to the secondary winding Lms of the transformer T2, the current flowing into the choke coil CH increases as shown in Fig. 7k. During the switching-off operation of the forward converter of Fig. 6, since the switching control signal of a logic "LOW" level is applied to the base of the switching element Q, the switching element Q is switched off. The secondary winding Lms of the transformer T2 and the capacitor C3 start to resonate. Then the resonant circuit consisting of the winding Lms of the transformer T2 and the capacitor C3 supplies the reverse current to the choke coil CH as shown in Fig. 7i. This reverse current increases until the current flowing into the choke coil CH becomes 0. Consequently, the forward converter of Fig. 6 has the diode D4 connected forwardly between the bridge diode BD and the smoothing capacitor C1. The diode D4 is connected in parallel to the secondary winding Lms of the transformer T2 and the capacitor C3 which are serially connected to each other. The winding Lms of the transformer T2 is installed correspondingly to the winding L1 connected between the rectifying elements D2 and D3 and the smoothing capacitor C2. The windings L1 and Lms constitute the primary winding and the secondary winding of the transformer T2, respectively. Moreover, there is also provided the choke coil CH connected between a reverse terminal of the diode D4 and the smoothing capacitor C1. During the switching-on operation of the forward converter, the voltage is transmitted to the winding Lms of the transformer T2 and the capacitor C3 and thus the current flowing into the smoothing capacitor C1, that is, the current flowing into the choke coil CH increases. During the switching-off operation of the forward converter, the resonant circuit consisting of the winding Lms of the transformer T2 and the capacitor C3 starts to resonate and the current flowing into the choke coil CH flows into the diode D4. The resonant circuit generates the reverse resonant current to offset the current flowing into the choke coil CH and increases the resonant current until the current flowing into the choke coil CH becomes 0. As noted above, forward converters in accordance with embodiments of the invention cause the input current flowing into the smoothing capacitor to approximate to the sine wave form. Therefore, the power factor is improved by reducing the inactive power of the SMPS and a harmonic noise component is suppressed. As a result, a malfunction in the SMPS is prevented.

Claims: A forward converter comprising: a bridge diode (BD) for full-wave rectifying an AC (alternative current) input voltage to generate a first DC (direct current) voltage; a first smoothing (C1) capacitor for smoothing said first DC voltage to generate a second DC voltage; a control circuit (2) for generating a switching control signal having a prescribed period in response to an input of said second DC voltage; a switching element (Q) connected between said first smoothing capacitor (C1) and a ground terminal and switched on or off in response to said switching control signal; a transformer (T1) having at least a primary winding (Lp) and a second winding (Ls), for receiving said second DC voltage through said primary winding (Lp) when said switching element (Q) is switched on and inducing said second DC voltage to said secondary winding (Ls); a waveform shaper (T2) connected between said bridge diode (BD) and said first smoothing capacitor (C1), for causing a current flowing into said first smoothing capacitor (C1) to have a sine wave form; a first rectifying element (D2) connected forwardly between said secondary winding (Ls) of said transformer (T1) and a load (4), for rectifying a DC voltage induced to said secondary winding (Ls) to generate a third DC voltage; a second smoothing capacitor (C2) for smoothing said third DC voltage to supply a stable DC voltage to said load; and characterised in that said waveform shaper (T2) is a transformer having a primary winding (L1) connected between said first rectifying element (D2) and said load (4) and having a secondary winding (Lms) connected between said bridge diode (BD) and said first smoothing capacitor (C1). The forward converter according to claim 1, further comprising: a choke coil connected between said waveform shaper (T2) and said first smoothing capacitor (C1), for eliminating a harmonic component of a current flowing into said first smoothing capacitor. The forward converter according to claim 1 or 2, further comprising: a second rectifying element (D3) connected forwardly between the other side of said secondary winding (Ls) of said main transformer (T1) and said load (4), for rectifying a voltage induced to said secondary winding (Ls) of said main transformer (T1); a second smoothing capacitor (C2) connected in parallel to said load (4), for smoothing a voltage generated through said first or second rectifying element (D2, D3) to supply a stable DC voltage to said load (4); and the transformer (T2) having a primary winding (L1) connected between a reverse terminal of each of said first and second rectifying elements (D2, D3) and said second smoothing capacitor (C2) and having a secondary winding (Lms) connected between said bridge diode (BD) and said first smoothing capacitor (C1). A forward converter comprising: a bridge diode (BD) for full-wave rectifying an AC (alternative current) input voltage to generate a first DC (direct current) voltage; a first smoothing capacitor (C1) for smoothing said first DC voltage to generate a second DC voltage; a control circuit (2) for generating a switching control signal having a prescribed period in response to an input of said second DC voltage; a switching element (Q) connected between said first smoothing capacitor (C1) and a ground terminal and switched on or off in response to said switching control signal; a first diode (D4) connected forwardly between said bridge diode (BD) and said first smoothing capacitor (C1) ; a choke coil (CH) connected between said first diode (D4) and said first smoothing capacitor; a main transformer (T1) having a primary winding (Lp) connected between said first smoothing capacitor (C1) and said switching element (Q) and having a secondary winding (Ls) connected to a load (4), for accumulating said second DC voltage in said primary winding (Lp) so as to be induced to said secondary winding (Ls) when said switching element (Q) is switched on; a first rectifying element (D2) connected forwardly between said secondary winding (Ls) of said main transformer (T1) and said load (4), for rectifying a voltage induced to said secondary winding (Ls) of said main transformer (T1); a second smoothing capacitor (C2) connected in parallel to said load (4), for smoothing a voltage generated through said first rectifying element (D2) to supply a stable DC voltage to said load (4); and current control means (T2) connected in parallel to said first diode (D4), for increasing a current flowing into said choke coil (CH) when said switching element (Q) is switched on and providing a reverse current to said choke coil when said switching element (Q) is switched off. The forward converter according to claim 4, wherein said current control means comprises: a resonant circuit having a secondary winding (Lms) of a transformer (T2) installed correspondingly to a primary winding (L1) connected between said rectifying element (D2) and said secondary smoothing capacitor (C2) and having a capacitor (C3), said resonant circuit being connected in parallel to said first diode (D4). A forward converter according to claim 9 or 10 further comprising: a second rectifying element (D3) connected forwardly between the other side of said secondary winding (Ls) of said main transformer (T1) to said first rectifying element (D2) and said load (4), for rectifying a voltage in use to said secondary winding (Ls) of said main transformer; wherein said current control means comprises: a transformer (T2) having a primary winding (L1) connected between a reverse terminal of each of said first and second rectifying elements (D2, D3) and said second smoothing capacitor (C2) and having a secondary winding (Lms) connected in parallel to said first diode (D4); a choke coil (CH) connected between said first diode (D4) and said first smoothing capacitor (C1); and a third capacitor (C3) connected between said secondary winding (Lms) of said auxiliary transformer (T2) and said choke coil (CH).

Inventors: LEE JONG-WOON, LEE, JONG-WOON, LEE, JONG-WOON, SAMSUNG ELECTRONICS CO. LTD
Applicants: SAMSUNG ELECTRONICS CO LTD, SAMSUNG ELECTRONICS CO., LTD.
Application year: 1997
Main classification: H02M3
IPC codes: H02M3, H02M1