Power supply device and image forming apparatus

Abstract

To improve the power factor of a switching power supply in which a primary side and a secondary side are insulated from each other.SOLUTION: A power supply device comprises: a transformer 206 that has primary windings 206a, 206b and a secondary winding 206c, in which a primary side and a secondary side are insulated from each other; a diode bridge 203 that has output terminals 203c, 203d and performs full-wave rectification of AC voltage; a circuit in which an inductor 204 and a diode 205 are connected in series between the output terminal 203c and a contact point where one end of the primary winding 206a and one end of the primary winding 206b are connected; a field-effect transistor 208 that is connected with the other end of the primary winding 206b at one end and connected with the output terminal 203d at the other end, and is switched to an on state or an off state; and an electrolytic capacitor 207 that is connected with the other end of the primary winding 206a at one end and connected with the output terminal 203d at the other end. The inductance of the inductor 204 is set such that the voltage of the electrolytic capacitor 207 becomes higher than the output voltage of the diode bridge 203.SELECTED DRAWING: Figure 2

Description

[Technical Field]

[0001] The present invention relates to a power supply device and an image forming apparatus equipped with the power supply device. [Background technology]

[0002] Switching power supplies used in general electronic devices often use a capacitor-input type system, in which a smoothing capacitor that smooths the DC voltage is placed after a diode bridge that full-wave rectifies the AC voltage input from the AC power source. Capacitor-input switching power supplies have a characteristic of low power factor because an input current flows to the transformer when the output voltage of the diode bridge exceeds the voltage of the smoothing capacitor. To address this issue, a switching power supply that produces an input current waveform with reduced peaks rather than a sine wave has been devised as a technology for improving the power factor. For example, Patent Document 1 proposes a technology for improving the power factor by connecting an electronic device equipped with this switching power supply and an electronic device equipped with a capacitor-input switching power supply to the same AC power source and making the combined input current waveform closer to a sine wave. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Patent No. 03288367 Summary of the Invention [Problem to be solved by the invention]

[0004] However, the circuit configuration of the above-mentioned switching power supply is based on a general boost-type power factor correction circuit and uses a transformer that outputs a relatively high voltage and whose primary and secondary sides are not insulated. Therefore, this circuit configuration is not suitable for switching power supplies that use a transformer whose primary and secondary sides are insulated and whose output voltage is a low voltage of several volts to several tens of volts.

[0005] The present invention has been made under these circumstances, and has as its object to improve the power factor of a switching power supply in which the primary side and secondary side are insulated. [Means for solving the problem]

[0006] In order to solve the above-mentioned problems, the present invention has the following configuration.

[0007] (1) 1. A power supply device that generates a supply voltage to be supplied to a predetermined load based on a first AC voltage input from a first commercial AC power supply when connected to the first commercial AC power supply, a transformer having a first primary winding, a second primary winding, and a secondary winding, the primary side and the secondary side being insulated from each other, and a first output terminal and a second output terminal; The first a rectifier circuit that full-wave rectifies the AC voltage; having a predetermined inductance value a first series circuit in which an inductor and a first rectifying element are connected in series, the first series circuit being connected between the first output terminal and a first connection point at which one end of the first primary winding and one end of the second primary winding are connected; a switching element having one end connected to the other end of the second primary winding and the other end connected to the second output terminal, the switching element being switched between an on state and an off state; and a first capacitor having one end connected to the other end of the first primary winding and the other end connected to the second output terminal, The power supply is connectable to a second commercial AC power supply that outputs a second AC voltage that is smaller than the first AC voltage, and generates the supply voltage to be supplied to the predetermined load based on the second AC voltage, and when the power supply is connected to the first commercial AC power supply and the first AC voltage is input to the rectifier circuit, at a time when the rectifier circuit outputs an output voltage that is a peak value of an output waveform, a value of the voltage of the first capacitor is higher than the peak value. A power supply device characterized by: (2) An image forming unit for forming a toner image on a recording material, and a power supply device that generates a supply voltage to be supplied to the image forming unit based on a first AC voltage input from a first commercial AC power source when connected to the first commercial AC power source, the power supply device including: a transformer having a first primary winding, a second primary winding, and a secondary winding, the primary side and the secondary side being insulated; a rectifier circuit having a first output terminal and a second output terminal, full-wave rectifying the first AC voltage; a first series circuit in which an inductor having a predetermined inductance value and a first rectifier element are connected in series, the first series circuit being connected between the first output terminal and a first connection point at which one end of the first primary winding and one end of the second primary winding are connected; and a first capacitor having one end connected to the other end of the first primary winding and the other end connected to the second output terminal, the power supply device being connectable to a second commercial AC power supply that outputs a second AC voltage that is smaller than the first AC voltage, and generating the supply voltage to be supplied to the predetermined load based on the second AC voltage, wherein when the power supply device is connected to the first commercial AC power supply and the first AC voltage is input to the rectifier circuit, at the time when the rectifier circuit outputs an output voltage that is a peak value of an output waveform, the value of the voltage of the first capacitor is higher than the peak value. (3) An image forming apparatus connected to an optional device for processing a recording material on which an image has been formed, the image forming apparatus comprising: an image forming section for forming a toner image on the recording material; a first power supply device for supplying power to the image forming section; and a second power supply device for generating a supply voltage to be supplied to the optional device based on a first AC voltage input from a first commercial AC power supply when connected to a first commercial AC power supply, the second power supply device including a transformer having a first primary winding, a second primary winding, and a secondary winding, the primary side and the secondary side being insulated; a rectifier circuit having a first output terminal and a second output terminal, full-wave rectifying the first AC voltage; and a first series circuit in which an inductor having a predetermined inductance value and a first rectifier element are connected in series, the first output terminal, one end of the first primary winding, and one end of the second primary winding are connected to the first output terminal. a first series circuit connected between a first connection point and a second connection point connected to the first primary winding; a switching element having one end connected to the other end of the second primary winding and the other end connected to the second output terminal and switchable between an on state and an off state; and a first capacitor having one end connected to the other end of the first primary winding and the other end connected to the second output terminal, wherein the second power supply device is connectable to a second commercial AC power supply that outputs a second AC voltage smaller than the first AC voltage, and generates the supply voltage to be supplied to the predetermined load based on the second AC voltage, and when the second power supply device is connected to the first commercial AC power supply and the first AC voltage is input to the rectifier circuit, the voltage of the first capacitor is higher than the peak value at the time when the rectifier circuit outputs an output voltage that is a peak value of an output waveform. [Effects of the Invention]

[0010] According to the present invention, it is possible to improve the power factor of a switching power supply in which the primary side and secondary side are insulated. [Brief explanation of the drawings]

[0011] [Figure 1] Schematic cross-sectional view illustrating the configuration of an image forming apparatus according to first and second embodiments. [Figure 2] Circuit diagram of the switching power supply of Example 1 [Figure 3] Graph showing current waveforms and voltage waveforms in Example 1 [Figure 4] 1 is a circuit diagram of a capacitor input type switching power supply according to Examples 1 to 3, and a diagram illustrating a current waveform. [Figure 5] Comparison of input current waveforms in Example 1 [Figure 6] FIG. 1 is a diagram illustrating a usage pattern of an electronic device according to a first embodiment. [Figure 7] Circuit diagram of a switching power supply according to a second embodiment [Figure 8] Schematic diagram showing the configuration of an image forming apparatus according to a third embodiment. DETAILED DESCRIPTION OF THE INVENTION

[0012] Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. [Example]

[0013] [Configuration of image forming device] In the first embodiment, a case where a power supply device of the present invention is applied to an image forming apparatus will be described with reference to FIGS. 1 to 6. FIG. 1 is a cross-sectional view showing a schematic configuration of a laser beam printer as an example of an image forming apparatus. The laser beam printer 100 (hereinafter referred to as printer 100) includes a photosensitive drum 101 on which an electrostatic latent image is formed, a charging unit 102 that uniformly charges the photosensitive drum 101, and a developing unit 103 that develops the electrostatic latent image formed on the photosensitive drum 101 to form a toner image. The printer 100 also includes an exposure device 110 that irradiates the photosensitive drum 101 with laser light to form an electrostatic latent image on the surface of the photosensitive drum 101. In the printer 100, the toner image formed on the photosensitive drum 101 is transferred by a transfer unit 105 to a sheet P as a recording material fed from a cassette 104 by a roller 111 or the like. The sheet P onto which the toner image has been transferred is transported to a fixing device 106, where the toner image is fixed to the sheet P, and the sheet P onto which the toner image has been fixed is discharged onto a tray 107. The photosensitive drum 101, charging unit 102, developing unit 103, and transfer unit 105 constitute an image forming unit. The printer 100 also includes a low-voltage power supply unit 108, which is a power supply device, and the low-voltage power supply unit 108 supplies power to a control unit (not shown) that controls the image forming operation by the image forming unit, the driving units such as motors, and the sheet P transport operation.

[0014] [Configuration of switching power supply] FIG. 2 is a circuit diagram showing the circuit configuration of a switching power supply 200 of this embodiment, which is provided in the printer 100 of FIG. 1 as the low-voltage power supply device 108. In FIG. 2, when an AC plug 201 is connected to an outlet, AC voltage is input to the switching power supply 200 from a commercial AC power supply (not shown). The input AC voltage is input to a diode bridge 203 via a filter circuit 202. The diode bridge 203, which is a rectifier circuit, has input terminals 203a and 203b and output terminals 203c (first output terminal) and 203d (second output terminal). The diode bridge 203 full-wave rectifies the AC voltage input from the input terminals 203a and 203b, and outputs the full-wave rectified AC voltage to the output terminals 203c and 203d. On the other hand, when the external load to which power is supplied from the switching power supply 200 is substantially constant and the output voltage Vout is stable, the charging voltage Vc charged in the electrolytic capacitor 207 serving as smoothing means contains some ripple voltage depending on the capacitance of the electrolytic capacitor, but is a substantially constant voltage. Note that, since the switching power supply 200 has a power factor correction circuit, the electrolytic capacitor 207 is arranged downstream of the primary winding of the transformer 206.

[0015] 2, output terminal 203c of diode bridge 203 is connected to one end of inductor 204. The other end of inductor 204 is connected to the anode terminal of diode 205 (first rectifying element), and the cathode terminal of diode 205 is connected to primary winding 206a (first primary winding) and primary winding 206b (second primary winding) of transformer 206. In this way, inductor 204 and diode 205 are connected in series to form a series circuit.

[0016] Transformer 206 is an isolation transformer for converting primary-side energy to secondary-side energy, and includes primary windings 206a, 206b, and secondary winding 206c. In transformer 206, primary windings 206a, 206b and secondary winding 206c have opposite polarities. Primary winding 206a and primary winding 206b of transformer 206 are connected in series. One end of primary winding 206a is connected to the positive side of electrolytic capacitor 207 (first capacitor), and the other end of primary winding 206a is connected to one end of primary winding 206b and the cathode terminal of diode 205. The other end of primary winding 206b is connected to the drain terminal of field-effect transistor (hereinafter referred to as FET) 208, which is a switching element. Meanwhile, the source terminal of FET 208 is connected to the negative side of electrolytic capacitor 207 and terminal 203d on the output side of diode bridge 203. That is, the FET 208 is connected in series to the primary winding 206b of the transformer 206. The gate terminal of the FET 208 is connected to a control IC (not shown), and the FET 208 is set to an ON state or an OFF state in response to a signal input from the control IC to the gate terminal. With the above-mentioned connection configuration, the electrolytic capacitor 207 is connected in parallel to the primary windings 206a and 206b of the transformer 206, which are connected in series.

[0017] Furthermore, one end of the secondary winding 206c of the transformer 206 is connected to the anode terminal of the diode 209, and the cathode terminal of the diode 209 is connected to the + side of the electrolytic capacitor 210. The + side of the electrolytic capacitor 210 is connected to the cathode terminal of the diode 209, and the - side is connected to the other end of the secondary winding 206c of the transformer 206. The charging voltage of the electrolytic capacitor 210 is output as the output voltage Vout of the switching power supply 200 to an external load connected to the switching power supply 200.

[0018] When a gate voltage is applied to the gate terminal of FET 208 from a control IC (not shown) and FET 208 is turned on, the charging voltage of electrolytic capacitor 207 is divided by primary windings 206a and 206b of transformer 206. When the output voltage of diode bridge 203 is higher than this divided voltage, the input current to transformer 206 flows through inductor 204 and diode 205.

[0019] Here, by making the number of turns of primary winding 206a greater than the number of turns of primary winding 206b, the voltage value divided by primary winding 206a and primary winding 206b becomes lower, and the input current flows from a voltage lower than the output voltage of diode bridge 203. Furthermore, while the voltage value divided by primary windings 206a and 206b is a substantially constant voltage, the output voltage of diode bridge 203 varies sinusoidally over time, so the waveform of the input current also varies approximately sinusoidally. Therefore, switching power supply 200 can obtain power supply characteristics with a high power factor.

[0020] On the other hand, during the period when a gate voltage is not supplied from a control IC (not shown) to the gate terminal of FET 208, the power (energy) stored on the primary side of transformer 206 is transferred to the secondary side. The operation of switching power supply 200 is similar to that of a flyback power supply, since power is supplied to the secondary side of transformer 206 when FET 208 is off. The output voltage Vout can be set arbitrarily by adjusting the turns ratio between the primary and secondary windings of transformer 206, and can be output from several volts (V).

[0021] [Relationship between the diode bridge output voltage and the transformer input current] Figure 3[A] illustrates the waveforms of input current Iin and output voltage Vin in a high power factor state, where Iin is the input current to primary windings 206a and 206b of transformer 206, and Vin is the output voltage at terminal 203c of diode bridge 203. The waveform diagram in Figure 3[A](a) shows the current waveform of input current Iin, with the vertical axis representing the current value and the horizontal axis representing time t. Meanwhile, the waveform diagram in Figure 3[A](b) shows the voltage waveform of output voltage Vin, with the vertical axis representing the voltage value and the horizontal axis representing time t. Note that t1 to t6 indicate timing. In Figure 3[A](b), voltage Vc is the voltage waveform of electrolytic capacitor 207, and the dashed line represents the voltage obtained by dividing voltage Vc of electrolytic capacitor 207 by the turns ratio between primary winding 206a and primary winding 206b. The voltage indicated by the broken line is also the voltage at the connection point (first connection point) between the primary winding 206a and the primary winding 206b.

[0022] As shown in FIG. 3[A], the voltage of the electrolytic capacitor 207 is divided between the primary winding 206a and the primary winding 206b, and the input current Iin starts to flow when the output voltage Vin of the diode bridge 203 exceeds the divided voltage (voltage division value) (time t1 (t3, t5, . . .)). The input current Iin continues to flow until the output voltage Vin becomes equal to or less than the voltage division value (time t2 (t4, t6, . . .)). Paradoxically, the input current Iin does not flow (Iin = 0) until the output voltage Vin reaches the voltage division value (for example, from time t2 to time t3). In this way, the input current Iin starts to flow when the output voltage Vin of the diode bridge 203 exceeds the voltage obtained by dividing the charging voltage of the electrolytic capacitor 207 between the primary winding 206a and the primary winding 206b, but the input current Iin does not flow for a predetermined time.

[0023] Therefore, when the frequency of the pulse signal input to the gate terminal of the FET 208 to switch the FET 208 between the on state and the off state is set to a fixed frequency, the following pulse signal control is performed to maintain the output voltage Vout of the switching power supply 200 constant. That is, when the output voltage Vin from the diode bridge 203 is low, the width (on-state time) of the pulse signal input to the gate terminal is widened (lengthened) to lengthen the on-state time of the FET 208. On the other hand, when the output voltage Vin from the diode bridge 203 is high, the width (on-state time) of the pulse signal input to the gate terminal is narrowed (shortened) to shorten the on-state time of the FET 208. Therefore, as shown in FIG. 3[A](a), the current waveform of the input current Iin has a waveform in which the height of the peak portion of a sine wave is suppressed, including a period when no current flows (a period when the input current Iin = 0). Note that during the period when no input current Iin flows, power is supplied to the load by flowing current from the electrolytic capacitor 207 to the primary winding of the transformer 206. Furthermore, an important condition for the current waveform of the input current Iin to have the waveform shape shown in FIG. 3[A](a) is that the voltage Vc of the electrolytic capacitor 207 is always higher than the output voltage Vin of the diode bridge 203, as shown in FIG. 3[A](b).

[0024] On the other hand, Figure 3[B] is a diagram illustrating the current waveform of the input current Iin and the voltage waveform of the output voltage Vin when the charging voltage Vc of the electrolytic capacitor 207 momentarily falls below the output voltage Vin of the diode bridge 203. The waveform diagram shown in Figure 3[B](a) shows the current waveform of the input current Iin, with the vertical axis representing the current value and the horizontal axis representing time t, while the waveform diagram shown in Figure 3[B](b) shows the voltage waveform of the output voltage Vin, with the vertical axis representing the voltage value and the horizontal axis representing time t. Note that t11 ​​to t22 indicate timing.

[0025] The waveform diagram shown in FIG. 3[B] differs from the waveform diagram shown in FIG. 3[A] in that in FIG. 3[B], the voltage near the peak of the output voltage Vin of the diode bridge 203 is higher than the charging voltage Vc of the electrolytic capacitor 207. When the output voltage Vin becomes higher than the charging voltage Vc of the electrolytic capacitor 207 (periods from t12 to t13, t16 to t17, and t20 to t21 in the diagram), the input current Iin momentarily becomes a large current. As a result, as shown in FIG. 3[B](a), the waveform of the input current Iin has a peak-like shape compared to the current waveform shown in FIG. 3[A](a). In this embodiment, to prevent the peak-like shape from forming in the input current waveform, the charging voltage Vc of the electrolytic capacitor 207 is set to be always higher than the output voltage Vin of the diode bridge 203. This allows the current waveform of the input current Iin to have a waveform that suppresses the peaks of a sine wave that includes periods when no current flows.

[0026] [Capacitor input type switching power supply] Fig. 4(a) is a circuit diagram showing the circuit configuration of an example of a capacitor-input type switching power supply in which a smoothing capacitor for smoothing the input DC voltage is arranged after a diode bridge for full-wave rectifying the AC voltage input from an AC power supply. The switching power supply 400 shown in Fig. 4(a) is a flyback converter in which the winding directions of the primary winding 405a and the secondary winding 405b of the transformer 405 are opposite to each other.

[0027] In FIG. 4(a), when AC plug 401 is connected to an outlet, AC voltage is input from an AC power supply (not shown) to switching power supply 400. The input AC voltage is input to diode bridge 403 via filter circuit 402. Diode bridge 403, which is a rectifier circuit, full-wave rectifies the AC voltage input from input terminals 403a and 403b and outputs it to output terminals 403c and 403d. Having full-wave rectified the AC voltage, diode bridge 403 outputs a DC voltage, which is smoothed to a substantially constant voltage by electrolytic capacitor 404 and charged.

[0028] The transformer 405 is a transformer whose primary and secondary sides are insulated from each other, and includes a primary winding 405a and a secondary winding 405b wound in the opposite direction to the primary winding 405a. One end of the primary winding 405a is connected to the positive side of the electrolytic capacitor 404 and a terminal 403c on the output side of the diode bridge 403, and the other end is connected to the drain terminal of a field-effect transistor (hereinafter referred to as FET) 406. The source terminal of the FET 406 is connected to the negative side of the electrolytic capacitor 404 and a terminal 403d on the output side of the diode bridge 403. The gate terminal of the FET 406 is connected to a control IC (not shown), and the FET 406 is set to an on or off state in response to a signal input from the control IC to the gate terminal. The secondary winding 405b includes a diode 407 and an electrolytic capacitor 408, which are rectifying and smoothing means for rectifying and smoothing the voltage induced in the secondary winding 405b. One end of the secondary winding 405b is connected to the anode terminal of the diode 407, and the other end of the secondary winding 405b is connected to the negative side of the electrolytic capacitor 408. In addition, the cathode terminal of the diode 407 is connected to the positive side of the electrolytic capacitor 408, and the negative side of the electrolytic capacitor 408 is connected to the other end of the secondary winding 405b and is also grounded.

[0029] When a gate voltage is applied to the gate terminal of FET 406 from a control IC (not shown) and FET 406 enters a conductive state (ON state), current is supplied from electrolytic capacitor 404 and power (energy) is stored in primary winding 405a. When the supply of gate voltage from the control IC to the gate terminal of FET 406 is cut off and FET 406 enters a non-conductive state (OFF state), the power (energy) stored in primary winding 405a is induced in secondary winding 405b. The induced voltage is rectified and smoothed by diode 407 and electrolytic capacitor 408, and output voltage Vout is output. Note that a similar function can be achieved by using a field effect transistor (FET) instead of diode 407, and power loss can be further reduced.

[0030] FIG. 4(b) shows the current waveform of the input current of the switching power supply 400, which is the flyback converter of FIG. 4(a). In the circuit diagram of FIG. 4(a), the input current from the diode bridge 403 flows only when the charging voltage of the electrolytic capacitor 404 falls below the output voltage of the diode bridge 403. Therefore, as shown in FIG. 4(b), the waveform of the input current from the diode bridge 403 has a very narrow conduction angle, which is the phase angle through which the input current Iin flows. When the conduction angle is very narrow, the power factor is very low and the reactive current is large. Note that a flyback converter has been used as an example of a capacitor-input switching power supply. For example, other types of power supplies, such as forward converters or LLC power supplies, in which the primary and secondary windings are wound in the same direction, will also produce an input current waveform similar to that shown in FIG. 4(b).

[0031] [Composite current waveform of two switching power supplies] FIG. 5 shows a current waveform of a combined current obtained by combining the current waveforms of the switching power supply 200 of FIG. 2 (shown in FIGS. 3A(a) and 3B(a)) and the current waveform of the capacitor-input type switching power supply 400 of FIG. 4(a) (shown in FIG. 4(b)). FIG. 5(a) shows a current waveform of a combined current obtained by combining the input current waveform of FIG. 3(A)(a) with the input current waveform of the capacitor-input type switching power supply 400 of FIG. 4(b). The current waveform shown in FIG. 5(a) is obtained by adding (combining) the input current waveform of the capacitor-input type switching power supply 400 of FIG. 4(a) in a manner that compensates for the suppression of the peaks of the sinusoidal waveform shown in FIG. 3(A)(a). As a result, the waveform has a shape similar to a sinusoidal waveform, enabling a high power factor to be achieved.

[0032] On the other hand, Figure 5(b) shows the current waveform of a composite current obtained by combining the input current waveform of Figure 3[B](a) with the input current waveform of the capacitor-input type switching power supply 400 of Figure 4(b). The current waveform shown in Figure 5(b) has a protrusion formed on the sinusoidal waveform shown in Figure 3[B](a), and the input current waveform of the capacitor-input type switching power supply 400 of Figure 4(b) is further added. As a result, as shown in Figure 5(b), the waveform becomes nearly triangular. This makes it difficult to achieve a high power factor.

[0033] In the circuit configuration of the switching power supply 200 shown in FIG. 2, the inductor 204 adjusts the charging voltage Vc of the electrolytic capacitor 207. For example, decreasing the inductance value of the inductor 204 increases the input current from the diode bridge 203. As a result, the amount of charge in the electrolytic capacitor 207 increases, making the charging voltage Vc more likely to rise and resulting in an input current waveform similar to that shown in FIG. 3[A](a). On the other hand, increasing the inductance value of the inductor 204 decreases the input current from the diode bridge 203. As a result, the amount of discharge from the electrolytic capacitor 207 increases, making the charging voltage Vc more likely to drop and resulting in an input current waveform similar to that shown in FIG. 3[B](a). As such, the inductance value of the inductor 204 is significantly affected by the input voltage range of the switching power supply 200 and significantly affects the withstand voltage of the electrolytic capacitor 207. Therefore, it is necessary to adjust the inductance value taking into account the power supply specifications of the switching power supply 200 and the ratings of the components used.

[0034] [Examples of product usage] FIG. 6 is a diagram illustrating the use of a product equipped with the above-described switching power supply. In FIG. 6, printer 100, as described in FIG. 1, is equipped with low-voltage power supply device 108 having switching power supply 200 of the present embodiment shown in FIG. 2. Meanwhile, electronic device 600 is an electronic device different from printer 100, and is equipped with capacitor-input type switching power supply 400 shown in FIG. 4 as a switching power supply. AC plug 201 of switching power supply 200 installed in printer 100 and AC plug 401 of switching power supply 400 installed in electronic device 600 are connected to wall outlet 601. As shown in FIG. 6, switching power supplies 200 and 200 are connected in parallel to the same AC power source (same AC power source outlet 601). In this case, as described above, the combined current waveform of the input current to switching power supply 200 of printer 100 and the input current to switching power supply 400 of electronic device 600 approaches the sine wave shown in FIG. 5(a), thereby improving the power factor.

[0035] As described above, the switching power supply 200 of this embodiment has the circuit configuration shown in FIG. 2, and is set so that the voltage of the electrolytic capacitor 207 is higher than the output voltage Vin of the diode bridge 203. This causes the current waveform of the input current Iin to become a sinusoidal waveform with the height of the peak portion of the sine wave suppressed. As a result, when the printer 100 equipped with the switching power supply 200 and the electronic device 600 equipped with the capacitor-input type switching power supply 400 are connected to a common wall outlet 601, the power factor of the combined current waveform of both devices can be increased. Note that this embodiment has been described using an example in which the printer 100 and the electronic device 600 are connected to a common wall outlet 601, but the same effect can be obtained even when, for example, the same power strip is used.

[0036] The switching power supply 200 shown in FIG. 2 may also be designed as a universal power supply to accommodate a wide input voltage range (e.g., from 85 V to 264 V). In this case, if it is difficult to achieve a peak-free input current waveform, such as that shown in FIG. 3A(a), across the entire input voltage and load current ranges, the following may be adopted. Specifically, only the input current near the maximum load at low input voltages may be configured to have a peak-shaped current waveform, as shown in FIG. 5(b). In this case, since the input current near the maximum load flows instantaneously, a deterioration in the power factor during this period is not considered to cause any problems. Specifically, when the voltage input to the diode bridge 203 is a first voltage, the inductance value of the inductor 204 is set so that the voltage of the electrolytic capacitor 207 is always higher than the output voltage Vin of the diode bridge 203. In this case, when the input voltage is a second voltage lower than the first voltage, the voltage of the electrolytic capacitor 207 does not need to always be higher than the output voltage Vin of the diode bridge 203. When the input voltage is the second voltage, the switching power supply 200 may be configured to have periods when the voltage of the electrolytic capacitor 207 is higher and periods when it is lower than the output voltage Vin of the diode bridge 203, depending on the external load connected to the switching power supply 200.

[0037] As described above, according to this embodiment, it is possible to improve the power factor of a switching power supply in which the primary side and secondary side are insulated. [Example]

[0038] In the first embodiment, a switching power supply was described that has a circuit configuration capable of outputting a low voltage with a high power factor and that suppresses the peak value of a sinusoidal input current waveform by setting the charging voltage of the electrolytic capacitor to always be higher than the output voltage of the diode bridge. Because the switching power supply of the first embodiment is composed only of a basic circuit, there is a concern that, depending on the power supplied to the load, a surge voltage generated when the switching element is switched may become large, resulting in a noisy switching power supply. In the second embodiment, a switching power supply with an additional noise suppression circuit will be described. The image forming apparatus in which the switching power supply of this embodiment is installed is similar to the printer 100 of the first embodiment, and the same devices and components will be described using the same reference numerals, and their description will be omitted here.

[0039] [Configuration of switching power supply] FIG. 7 is a circuit diagram showing the circuit configuration of a switching power supply 200 of this embodiment. The circuit diagram shown in FIG. 7 differs from the circuit diagram shown in FIG. 2 of the first embodiment in that diodes 701 and 704, a capacitor 702, and an auxiliary winding 703 are added to the transformer 206. Note that in the switching power supply 200 of this embodiment, components having the same configuration as those in the switching power supply 200 of the first embodiment shown in FIG. 2 are described using the same reference numerals, and descriptions thereof will be omitted here. Furthermore, descriptions of circuit operations in the switching power supply 200 of this embodiment that are the same as those in the switching power supply 200 of the first embodiment will be omitted.

[0040] 7, one end of a capacitor 702 (second capacitor) is connected to the other end of the primary winding 206b of the transformer 206 and the drain terminal of the FET 208, and the other end of the capacitor 702 is connected to the anode terminal of a diode 701. In addition, the cathode terminal of the diode 701 (second rectifying element) is connected to the positive side of the electrolytic capacitor 207 and the other end of the primary winding 206a of the transformer 206. The diode 701 and capacitor 702 are connected in series, and are connected in parallel to the primary windings 206a and 206b of the transformer 206, which are connected in series.

[0041] The anode terminal of diode 704 (third rectifying element) is connected to the source terminal of FET 208, the negative side of electrolytic capacitor 207, and output terminal 203d of diode bridge 203. The cathode terminal of diode 704 is connected to one end of auxiliary winding 703 of transformer 206. The other end of auxiliary winding 703 of transformer 206 is connected to a connection point (second connection point) where the other end of capacitor 702 and the anode terminal of diode 701 are connected. Note that diode 704 is provided to prevent backflow, so as to prevent the charge of capacitor 702 from discharging through auxiliary winding 703.

[0042] 7, when FET 208 is turned off, the current flowing between the drain and source terminals of FET 208 via primary winding 206b of transformer 206 is switched to a charging current that charges the capacitance between the drain and source terminals. As a result, the drain-source voltage of FET 208 gradually increases due to the charging current. Then, when the drain-source voltage of FET 208 exceeds the charging voltage of electrolytic capacitor 207, the current from primary winding 206b flows to electrolytic capacitor 207 via capacitor 702 and diode 701. At this time, the drain-source voltage of FET 208 is suppressed to a predetermined voltage value.

[0043] On the other hand, when FET 208 is turned on, the discharge current flowing from capacitor 702 is divided into a current flowing through FET 208 and a current flowing backward through primary windings 206a and 206b. The current flowing backward through primary windings 206a and 206b becomes a regenerative current to electrolytic capacitor 207, and a portion of the energy (charge voltage of capacitor 702) generated by the surge voltage when FET 208 is turned off is regenerated and reused in electrolytic capacitor 207. Then, as the discharge current flows, capacitor 702, from which the voltage charged by the surge voltage has been discharged, returns to the state it was in just before the charging current flowed when FET 208 was turned off, and is reset to a state where it can again store the surge voltage (surge energy).

[0044] Meanwhile, the current flowing from capacitor 702 to FET 208 flows via diode 704 to auxiliary winding 703, and the current energy of the discharge current is stored in auxiliary winding 703. The energy stored in auxiliary winding 703 is then converted into a secondary current the next time FET 208 is turned off, and the energy stored by the current flowing through primary windings 206a and 206b is added to the converted secondary current. As described above, switching power supply 200 shown in FIG. 7 is configured to regenerate energy in electrolytic capacitor 207 when FET 208 is turned on and off. In this way, by adding diodes 701 and 704, capacitor 702, and auxiliary winding 703, switching power supply 200 is able to suppress the surge voltage of FET 208 while regenerating the energy of the surge voltage, thereby becoming a highly efficient switching power supply.

[0045] 7, the input current waveform can be made to be the current waveform shown in Fig. 3[A](a) by always setting the voltage of electrolytic capacitor 207 higher than the output voltage of diode bridge 203. When a product equipped with switching power supply 200 of this embodiment and an electronic device equipped with a capacitor-input type switching power supply are connected to a common outlet 601 (Fig. 6), the combined current waveform of both devices can be made closer to a sine waveform, and the power factor can be increased.

[0046] As described above, according to this embodiment, it is possible to improve the power factor of a switching power supply in which the primary side and secondary side are insulated. [Example]

[0047] In the first and second embodiments, a switching power supply was described that has a circuit configuration capable of outputting a low voltage with a high power factor, and that suppresses the peak value of the sine waveform of the input current by setting the voltage of the electrolytic capacitor to be always higher than the output voltage of the diode bridge. By suppressing the peak value of the sine waveform of the input current, it is possible to improve the power factor of the combined waveform of the input current with another electronic device 600 that is equipped with a capacitor-input type switching power supply. In the third embodiment, an image forming apparatus is described that is equipped with a capacitor-input type switching power supply as a main power supply and the high-power-factor switching power supply described in the first and second embodiments as a sub-power supply.

[0048] [Configuration of image forming device] Fig. 8 is a diagram showing the configuration of an image forming apparatus with all optional devices installed. In Fig. 8, image forming apparatus 800 shows the minimum configuration for performing image forming operations, and image forming apparatus 800 can print on a sheet P based on received image data in response to a print request sent from a computer (not shown) or the like. Note that image forming apparatus 800 is similar to printer 100 shown in Fig. 1 of the first embodiment, and therefore a description of the image forming operation will be omitted.

[0049] An image forming apparatus 800 shown in FIG. 8 is equipped with an image scanner 802, an input option device 803, and an output option device 804. The image scanner 802 is an image reading device that reads an original placed on a glass table and realizes a copy function by printing the read image on a sheet P. The input option device 803 can store a large number of sheets P, including sheets P of various sizes, so that the sheet P can be specified when printing. The output option device 804 is an optional device that has multiple sorter bins 804a, 804b, and 804c and has a sorting function that sorts and outputs printed sheets P. Note that the output option device is not limited to one with a sorting function, and may be an optional processing device that processes sheets P, such as an output option device with a function to staple multiple sheets P on which images have been formed.

[0050] 8, image forming apparatus 800 is equipped with switching power supply 801 (corresponding to low-voltage power supply 108 in FIG. 1), which is a low-voltage power supply device, and output option device 804 is equipped with switching power supply 805, which is also a low-voltage power supply device. In the image forming apparatus of FIG. 8, switching power supply 801 (first power supply device) is configured as a main power supply, and switching power supply 805 (second power supply device) is configured as a sub-power supply. For example, if a single switching power supply 801 is configured to supply power to all option devices, the power that can be supplied from switching power supply 801 will be excessive in the case of image forming apparatus 800 alone, with no option devices installed. As a result, the cost of switching power supply 801 will not be optimal. Therefore, when multiple option devices are installed as shown in FIG. 8, a configuration using two switching power supplies 801 and 805 is likely to reduce overall costs. In this embodiment, switching power supply 801 is a capacitor input type switching power supply, and switching power supply 805 is a high power factor type switching power supply described in embodiments 1 and 2, and they are configured to receive AC voltage from a common AC power supply. As a result, the current waveform obtained by combining the current waveforms of the input currents of switching power supplies 801 and 805 has an approximately sinusoidal wave shape as shown in Fig. 3[A] described in embodiment 1, making it possible to achieve a high power factor.

[0051] As described above, the image forming apparatus of this embodiment is equipped with all optional devices and is configured with two switching power supplies, a main power supply and a sub-power supply. One of the two switching power supplies is a conventional capacitor-input type switching power supply, and the other is a high-power-factor type switching power supply described in Examples 1 and 2. This power supply configuration allows the composite waveform of the input currents of the switching power supplies to approach a sine wave, thereby improving the power factor. In this embodiment, the switching power supply 805 installed in the output option device 804 is a high-power-factor type switching power supply, and supplies power to the output option device 804. Meanwhile, power is supplied to the image forming apparatus 800, including the image scanner 802 and the input option device 803, by the capacitor-input type switching power supply 801 installed in the image forming apparatus 800. Note that, for example, the same effect can be achieved by installing a high-power-factor type switching power supply in the input option device 803 and supplying power using the switching power supply 801 and the two high-power-factor type switching power supplies.

[0052] As described above, according to this embodiment, it is possible to improve the power factor of a switching power supply in which the primary side and secondary side are insulated. [Explanation of symbols]

[0053] 203 Diode Bridge 204 Inductor 205 Diode 206 Trans 207 Electrolytic Capacitor 208 Field Effect Transistor

Claims

1. A power supply device that, when connected to a first commercial AC power source, generates a supply voltage to be supplied to a predetermined load based on a first AC voltage input from the first commercial AC power source, a transformer having a first primary winding, a second primary winding, and a secondary winding, the primary side and the secondary side being insulated from each other; a rectifier circuit having a first output terminal and a second output terminal, and full-wave rectifying the first AC voltage; a first series circuit in which an inductor having a predetermined inductance value and a first rectifying element are connected in series, the first series circuit being connected between the first output terminal and a first connection point to which one end of the first primary winding and one end of the second primary winding are connected; a switching element having one end connected to the other end of the second primary winding and the other end connected to the second output terminal, the switching element being switched between an on state and an off state; a first capacitor having one end connected to the other end of the first primary winding and the other end connected to the second output terminal; Equipped with a power supply that is connectable to a second commercial AC power supply that outputs a second AC voltage that is smaller than the first AC voltage, and that generates the supply voltage to be supplied to the predetermined load based on the second AC voltage; a power supply device connected to the first commercial AC power supply and having the first AC voltage input to the rectifier circuit, wherein, at a point in time when the rectifier circuit outputs an output voltage that is a peak value of an output waveform, the voltage value of the first capacitor is higher than the peak value.

2. 2. The power supply device according to claim 1, wherein the number of turns of said first primary winding is greater than the number of turns of said second primary winding.

3. the first rectifying element is a diode, the inductor has one end connected to the first output terminal and the other end connected to the anode terminal of the diode; 2. The power supply device according to claim 1, wherein the cathode terminal of the diode is connected to the first connection point.

4. the transformer has an auxiliary winding; a second series circuit in which a second capacitor and a second rectifying element are connected in series, the second series circuit being connected between the other end of the first primary winding and the other end of the second primary winding; a third series circuit in which a third rectifying element and the auxiliary winding are connected in series, the third series circuit being connected between the other end of the switching element and a second connection point at which the second capacitor and the second rectifying element are connected; 4. The power supply device according to claim 3, further comprising:

5. the second rectifying element is a diode, the second capacitor has one end connected to the other end of the second primary winding and one end of the switching element, and the other end connected to the anode terminal of the diode; 5. The power supply device according to claim 4, wherein the cathode terminal of the diode is connected to the other end of the first primary winding and one end of the first capacitor.

6. the third rectifying element is a diode, the diode has an anode terminal connected to the other end of the switching element and a cathode terminal connected to one end of the auxiliary winding; 6. The power supply device according to claim 5, wherein the other end of the auxiliary winding is connected to the second connection point.

7. 7. The power supply device according to claim 6, wherein when the switching element is turned off, the current flowing through the second primary winding flows to the second series circuit, and the second capacitor is charged, thereby suppressing surge voltage.

8. 8. The power supply device according to claim 7, wherein when the switching element is turned on, a current from the second capacitor flows through the switching element to the auxiliary winding of the third series circuit.

9. the switching element is a field effect transistor, one end of the switching element is a drain terminal of a field effect transistor, 9. The power supply device according to claim 8, wherein the other end of the switching element is a source terminal of a field effect transistor.

10. an image forming unit for forming a toner image on a recording material; a power supply device that generates a supply voltage to be supplied to the image forming unit based on a first AC voltage input from a first commercial AC power supply when the power supply device is connected to the first commercial AC power supply; Equipped with The power supply device a transformer having a first primary winding, a second primary winding, and a secondary winding, the primary side and the secondary side being insulated from each other; a rectifier circuit having a first output terminal and a second output terminal, and full-wave rectifying the first AC voltage; a first series circuit in which an inductor having a predetermined inductance value and a first rectifying element are connected in series, the first series circuit being connected between the first output terminal and a first connection point to which one end of the first primary winding and one end of the second primary winding are connected; a switching element having one end connected to the other end of the second primary winding and the other end connected to the second output terminal, the switching element being switched between an on state and an off state; a first capacitor having one end connected to the other end of the first primary winding and the other end connected to the second output terminal; and the power supply device is connectable to a second commercial AC power supply that outputs a second AC voltage that is smaller than the first AC voltage, and generates the supply voltage to be supplied to the predetermined load based on the second AC voltage; When the image forming apparatus is connected to the first commercial AC power supply and the first AC voltage is input to the rectifier circuit, at the time when the rectifier circuit outputs an output voltage that is a peak value of an output waveform, the voltage value of the first capacitor is higher than the peak value.

11. the transformer of the power supply device has an auxiliary winding; The power supply device a second series circuit in which a second capacitor and a second rectifying element are connected in series, the second series circuit being connected between the other end of the first primary winding and the other end of the second primary winding; a third series circuit in which a third rectifying element and the auxiliary winding are connected in series, the third series circuit being connected between the other end of the switching element and a second connection point at which the second capacitor and the second rectifying element are connected; 11. The image forming apparatus according to claim 10, further comprising:

12. In an image forming apparatus to which an optional device for processing recording materials on which images have been formed is connected, an image forming unit for forming a toner image on a recording material; a first power supply device for supplying power to the image forming unit; a second power supply device that generates a supply voltage to be supplied to the optional device based on a first AC voltage input from the first commercial AC power supply when connected to the first commercial AC power supply; Equipped with The second power supply device a transformer having a first primary winding, a second primary winding, and a secondary winding, the primary side and the secondary side being insulated from each other; a rectifier circuit having a first output terminal and a second output terminal, and full-wave rectifying the first AC voltage; a first series circuit in which an inductor having a predetermined inductance value and a first rectifying element are connected in series, the first series circuit being connected between the first output terminal and a first connection point to which one end of the first primary winding and one end of the second primary winding are connected; a switching element having one end connected to the other end of the second primary winding and the other end connected to the second output terminal, the switching element being switched between an on state and an off state; a first capacitor having one end connected to the other end of the first primary winding and the other end connected to the second output terminal; and the second power supply device is connectable to a second commercial AC power supply that outputs a second AC voltage that is smaller than the first AC voltage, and generates the supply voltage to be supplied to the predetermined load based on the second AC voltage; When the image forming apparatus is connected to the first commercial AC power supply and the first AC voltage is input to the rectifier circuit, at the time when the rectifier circuit outputs an output voltage that is a peak value of an output waveform, the voltage value of the first capacitor is higher than the peak value.

13. 13. The image forming apparatus according to claim 10, wherein the number of turns of the first primary winding is greater than the number of turns of the second primary winding.

Images