Power supply unit and image forming apparatus

Abstract

To improve the accuracy of voltage detection without increasing power loss.SOLUTION: A power supply device comprises: a transformer T1 that has primary winding P1, secondary winding S1, and auxiliary winding P2; a main FET 1 that is connected in series with the primary winding P1; a rectifying and smoothing circuit 121 that has a diode D121 and a capacitor C121 and rectifies and smooths a voltage induced to the auxiliary winding P2; a rectifying and smoothing circuit 122 that has a diode D122 and a capacitor C122, is connected in parallel with the rectifying and smoothing circuit 121, and rectifies and smooths a voltage induced to the auxiliary winding P2; and a control unit 110 that controls the main FET 1. The control unit 110 detects the voltage induced to the auxiliary winding P2 based on an output voltage from the rectifying and smoothing circuit 121. The diode D122 has a better responsiveness than the responsiveness of the diode D121.SELECTED DRAWING: Figure 2

Description

Technical Field

[0001] The present invention relates to a power supply device and an image forming apparatus, and more particularly, to a voltage detection function of a switching power supply.

Background Art

[0002] In a switching power supply that converts an AC voltage such as a commercial AC power supply into a DC voltage, the voltage of the AC power supply (hereinafter referred to as the AC voltage) may be detected for controlling the output voltage and protecting the switching power supply itself. To detect the AC voltage, there is a method of providing an auxiliary winding in a transformer and rectifying and smoothing a voltage proportional to the AC voltage induced in the auxiliary winding for detection. However, a voltage in which a noise component generated during the switching operation of the switching element is superimposed on the voltage proportional to the AC voltage is induced in the auxiliary winding of the transformer. Therefore, there is a problem that an error occurs in the detection result of the AC voltage in the method of rectifying and smoothing the voltage of the auxiliary winding.

[0003] In order to reduce the error in the detection result of the AC voltage due to the noise component generated during the switching operation of the switching element, for example, a method as disclosed in Patent Document 1 has been proposed. That is, a method has been proposed in which the reverse recovery time of the diode used in the rectifier circuit of the auxiliary winding is defined so as not to rectify the noise component, thereby improving the detection accuracy of the AC voltage.

Prior Art Documents

Patent Documents

[0004]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0005] However, conventional methods for removing noise components require the use of diodes with significantly longer reverse recovery times than typical rectifier diodes. Therefore, conventional methods may not always be sufficiently effective depending on the voltage and frequency of the noise component. Furthermore, using diodes with long reverse recovery times increases power loss in the diode. For these reasons, there is a need to improve the accuracy of voltage detection without increasing power loss.

[0006] This invention was made under such circumstances and aims to improve the accuracy of voltage detection without increasing power loss. [Means for solving the problem]

[0007] To solve the above-mentioned problems, the present invention has the following configuration. (1) A transformer having a primary winding, a secondary winding and an auxiliary winding; a switching element connected in series with the primary winding; a first rectifier and smoothing circuit having a first diode and a first capacitor for rectifying and smoothing the voltage induced in the auxiliary winding; a second rectifier and smoothing circuit having a second diode and a second capacitor, connected in parallel with the first rectifier and smoothing circuit for rectifying and smoothing the voltage induced in the auxiliary winding; and a control unit for controlling the switching element, wherein the control unit detects the voltage induced in the auxiliary winding based on the output voltage of the first rectifier and smoothing circuit. The reverse recovery time of the first diode is longer than the reverse recovery time of the second diode. A power supply device characterized by the following features. (2) A power supply device comprising: a transformer having a primary winding, a secondary winding and an auxiliary winding; a switching element connected in series with the primary winding; a first rectifier and smoothing circuit having a first diode and a first capacitor for rectifying and smoothing the voltage induced in the auxiliary winding; a second rectifier and smoothing circuit having a second diode and a second capacitor, connected in parallel with the first rectifier and smoothing circuit for rectifying and smoothing the voltage induced in the auxiliary winding; and a control unit for controlling the switching element, wherein the control unit detects the voltage induced in the auxiliary winding based on the output voltage of the first rectifier and smoothing circuit, and the turn-on time of the first diode is longer than the turn-on time of the second diode. ( 3 ) an image forming means for forming an image on a recording material, and the (1) a means for supplying power to the image forming means or (2) above An image forming apparatus characterized by comprising the power supply device described above. [Effects of the Invention]

[0008] According to the present invention, the accuracy of voltage detection can be improved without increasing power loss. [Brief explanation of the drawing]

[0009] [Figure 1] Figures showing the image forming apparatus for Examples 1 and 2. [Figure 2] Schematic diagram of the switching power supply in Example 1, and a graph showing the relationship between the diode response and the output voltage of the rectifier and smoothing circuit. [Figure 3] Schematic diagram of the switching power supply in Example 2, and a graph showing the relationship between the diode response and the output voltage of the rectifier and smoothing circuit. [Modes for carrying out the invention]

[0010] Hereinafter, embodiments for carrying out the present invention will be described in detail with reference to the drawings using examples. [Examples]

[0011] [Image forming apparatus] Figure 1 shows an example of the schematic configuration of an image forming apparatus. The laser beam printer 1100 (hereinafter referred to as printer 1100) comprises a photosensitive drum 1101, a charging unit 1102, and a developing unit 1103. The photosensitive drum 1101 is an image carrier on which an electrostatic latent image is formed. The charging unit 1102 uniformly charges the photosensitive drum 1101. The developing unit 1103 forms a toner image by developing the electrostatic latent image formed on the photosensitive drum 1101 with toner. The toner image formed on the photosensitive drum 1101 (on the image carrier) is transferred to a sheet P, which is a recording material supplied from a cassette 1104, by a transfer unit 1105, and the unfixed toner image transferred to the sheet P is fixed by a fuser 1106. The photosensitive drum 1101, charging unit 1102, developing unit 1103, and transfer unit 1105 constitute the image forming unit (image forming means). The fixed sheet P is discharged into tray 1107. The printer 1100 is also equipped with a power supply unit 1108, which supplies power to the drive unit such as the motor and the control unit 1109. The control unit 1109 has a CPU (not shown) and controls the image forming operation by the image forming unit and the transport operation of the sheet P. Note that the image forming apparatus to which the power supply unit of the present invention can be applied is not limited to the configuration illustrated in Figure 1.

[0012] [Power supply] The switching power supply 100 of Example 1 is characterized by having two rectifier and smoothing circuits for rectifying and smoothing the voltage VFWD0 induced in the auxiliary winding P2, and by using diodes with different response times in each rectifier and smoothing circuit. The switching power supply 100 is included in the power supply unit 1108 of the printer 1100. The circuit configuration of the switching power supply 100 will be described below, followed by the voltage VFWD0 induced in the auxiliary winding P2 and the output voltages VFWD1 and VFWD2 of the two rectifier and smoothing circuits.

[0013] First, the circuit configuration of the switching power supply 100 will be explained using Figure 2(a). The switching power supply 100 has an input smoothing capacitor Cin, an isolated transformer T1, a switching element field-effect transistor 1 (hereinafter referred to as main FET1), and a control unit 110 on its primary side. The switching power supply 100 also has a first rectifier-smoothing circuit 121 (hereinafter simply referred to as rectifier-smoothing circuit 121) and a second rectifier-smoothing circuit 122 (hereinafter simply referred to as rectifier-smoothing circuit 122). The secondary side has a diode Dout and a capacitor Cout, and has an FB circuit 130 as a feedback means for feeding back the voltage value of the output voltage Vout, which is the voltage on the secondary side, to the control unit 110 on the primary side. The switching power supply 100 outputs the output voltage Vout to the secondary side which is isolated from the AC power supply 10, and controls the main FET1 so that the output voltage Vout becomes a constant voltage.

[0014] The AC voltage from the AC power supply 10 is rectified by the diode bridge BD1 and charges the input smoothing capacitor Cin. The lower potential of the input smoothing capacitor Cin is denoted as DCL, and the higher potential as DCH. The input voltage Vin of the switching power supply 100 is the difference between DCH and DCL.

[0015] Transformer T1 is an isolated transformer equipped with a primary winding P1 and an auxiliary winding P2 on the primary side and a secondary winding S1 on the secondary side. Energy is supplied to the secondary winding S1 from the primary winding P1 of transformer T1 by the switching operation of the main FET1. The forward voltage VFWD0 output from the auxiliary winding P2, which is a winding of the same polarity as the primary winding P1, is rectified and smoothed by rectifier and smoothing circuits 121 and 122. The number of turns of the primary winding P1 is TNP1, the number of turns of the secondary winding S1 is TNS1, and the number of turns of the auxiliary winding P2 is TNP2. In the switching power supply 100 of Embodiment 1, the polarity of the secondary winding S1 is opposite to that of the primary winding P1 because it is a flyback type, but the polarity of the secondary winding S1 may be the same as that of the primary winding P1 if the switching power supply is a forward type.

[0016] The control unit 110 is a circuit for driving the main FET 1, and is composed of a CPU 111 and a driving unit 112. The CPU 111 is an integrated general-purpose microcomputer equipped with an arithmetic unit that operates with, for example, a clock. The CPU 111 controls the setting values (for example, control start timing, period, on-duty ratio) of the control signal S10, which is a PWM signal, based on the output voltage VFWD1 output from the rectifying and smoothing circuit 121 and the output voltage VFB of the FB circuit 130. The control signal S10 is input to the driving unit 112. In the switching power supply 100 of the first embodiment, the main FET 1 is controlled by the CPU 111 and the driving unit 112, but an analog control IC or the like may be used instead of the CPU 111. The driving unit 112 is a circuit for driving the main FET 1. The driving unit 112 outputs a driving signal S20 to the gate terminal of the main FET 1 in accordance with the input control signal S10.

[0017] (Rectifying and smoothing circuit 121: for input voltage detection) The rectifying and smoothing circuit 121 is a circuit for outputting the output voltage VFWD1 for input voltage detection. The rectifying and smoothing circuit 121 is composed of a first diode D121 (hereinafter simply referred to as diode D121) and a first capacitor C121 (hereinafter referred to as capacitor C121) for rectifying and smoothing the voltage VFWD0 induced in the auxiliary winding P2. The rectifying and smoothing circuit 121 outputs an output voltage VFWD1 obtained by rectifying and smoothing the forward voltage VFWD0 induced in the auxiliary winding P2 when the main FET 1 performs a switching operation. The relationship between the voltage VFWD0 induced in the auxiliary winding P2 and the input voltage Vin has the relationship of Equation (1) using the number of turns TNP1 of the primary winding P1 and the number of turns TNP2 of the auxiliary winding P2. Therefore, the control unit 110 can detect the voltage value of the input voltage Vin by detecting the output voltage VFWD1, which is the voltage obtained by rectifying and smoothing the voltage VFWD0. VFWD0 = (TNP2 / TNP1) × Vin ··· (1)

[0018] (Rectifying and smoothing circuit 122: for power supply voltage) The rectifying and smoothing circuit 122 is a circuit for rectifying and smoothing the voltage VFWD0, just like the rectifying and smoothing circuit 121, and outputs the output voltage VFWD2 which is the power supply voltage of the driving unit 112. The rectifying and smoothing circuit 122 is composed of a second diode D122 (hereinafter simply referred to as diode D122) and a second capacitor C122 (hereinafter simply referred to as capacitor C122), just like the rectifying and smoothing circuit 121. However, it is different from the rectifying and smoothing circuit 121 in that the responsiveness of the diode D122 is better than that of the diode D121. The relationship between the responsiveness of the diode and the output voltage of the rectifying and smoothing circuit will be described using FIG. 2(b). In the switching power supply 100 of the first embodiment, the output voltage VFWD2 of the rectifying and smoothing circuit 122 is directly supplied to the driving unit 112, but a regulator circuit or the like for voltage adjustment may be connected between the rectifying and smoothing circuit 122 and the driving unit 112.

[0019] Note that in the switching power supply 100 of the first embodiment, the rectifying and smoothing circuit 121 and the rectifying and smoothing circuit 122 have the same circuit configuration. However, in order to ensure that the output voltage VFWD1 of the rectifying and smoothing circuit 121 does not exceed the rating of the input voltage of the CPU 111, a resistor for dividing the output voltage VFWD1 may be provided only in the rectifying and smoothing circuit 121. Also, in the rectifying and smoothing circuit 121, in order to increase the time constant for charging the capacitor C121 and make it easier to remove noise components, a resistor may be provided in series with the diode D121 between the diode D121 and the capacitor C121. Furthermore, by making the capacitance of the capacitor C122 larger than the capacitance of the capacitor C121, it may be easier to absorb noise components in the rectifying and smoothing circuit 122. The noise components will be described using FIG. 2(b). The FB circuit 130 is a circuit for feeding back the output voltage Vout of the switching power supply 100, which is the secondary-side voltage, to the primary-side control unit 110, and is used to keep the output voltage Vout constant. In FIG. 2(a), the ground is represented as GND.

[0020] [Relationship between Responsiveness of Diode and Output Voltage of Rectifying and Smoothing Circuit] Next, the relationship between the diode's response and the output voltage of the rectifier-smoothing circuit will be explained using Figure 2(b). Figure 2(b) shows the control signal S10, the voltage VFWD0 induced in the auxiliary winding P2, the output voltage VFWD1 of the rectifier-smoothing circuit 121, and the output voltage VFWD2 of the rectifier-smoothing circuit 122. In the graph of Figure 2(b)(i), time is shown on the horizontal axis and voltage on the vertical axis, with voltage VFWD0, output voltage VFWD1, and output voltage VFWD2 shown as solid lines. In addition, in the graph of Figure 2(b)(ii), time is shown on the horizontal axis and the signal level (high level (H), low level (L)) of the control signal S10 is shown on the vertical axis.

[0021] The on / off state of the main FET1 is controlled by a control signal S10 output from the CPU 111 of the control unit 110. The main FET1 is in the off state when the control signal S10 is low level, and in the on state when it is high level. Since the auxiliary winding P2 has the same polarity as the primary winding P1, the voltage VFWD0 induced in the auxiliary winding P2 is the value shown in equation (1) when the main FET1 is in the on state (when the control signal S10 is high level). On the other hand, the voltage VFWD0 is 0V when the main FET1 is in the off state (when the control signal S10 is low level).

[0022] In Figure 2(b)(i), as shown in the dashed lines α and β, there are periods when the voltage VFWD0 is not the value represented by equation (1) or 0V. This is due to noise generated by the switching operation of the main FET 1. In general, in switching power supplies, when the main FET switches, noise such as surge voltage is generated by parasitic components of the board patterns and elements. The noise generated by the switching operation of the main FET propagates over a wide area of ​​the board due to the common impedance of the patterns and electromagnetic field coupling between patterns and components. In the switching power supply 100, when the main FET, main FET 1, switches from the ON state to the OFF state and when it switches from the OFF state to the ON state, a surge voltage is generated, and noise components are superimposed on the voltage VFWD0 induced in the auxiliary winding P2. Therefore, there are periods when the voltage VFWD0 is neither the value represented by equation (1) nor 0V.

[0023] Incidentally, in the switching power supply 100, the reason why the output voltage differs between the rectifier-smoothing circuit 121 and the rectifier-smoothing circuit 122, despite having the same circuit configuration, is because diodes with different responsiveness are used. Generally, in switching power supplies, good responsiveness is desirable for diodes in order to achieve low power consumption. This is because diodes with good responsiveness have excellent reverse recovery characteristics, resulting in small losses due to reverse recovery current when the diode switches from a conductive state to a non-conductive state, and thus low power consumption in the diode. On the other hand, diodes with good responsiveness also have excellent turn-on characteristics, which presents the challenge of being able to rectify even very high-frequency noise components such as surge voltages.

[0024] In a rectifier circuit connected to a voltage source superimposed with noise components, using a highly responsive diode to achieve low power consumption will also rectify the noise components, thus increasing the output voltage of the rectifier-smoothing circuit by the amount of noise. Since this increase in output voltage due to noise components can lead to errors in voltage detection, it is desirable that noise components not be rectified in a rectifier-smoothing circuit that uses the output voltage for voltage detection. Therefore, in the switching power supply 100, by changing the responsiveness of the diodes used in the rectifier-smoothing circuit 121 and the rectifier-smoothing circuit 122, the rectifier-smoothing circuit 121 avoids rectifying the noise components, thereby preventing false voltage detection in the control unit 110. Specifically, the reverse recovery time of diode D121 is longer than the reverse recovery time of diode D122, and the turn-on time of diode D121 is longer than the turn-on time of diode D122.

[0025] In the switching power supply 100, the responsiveness of diode D122 in the rectifier-smoothing circuit 122 is improved compared to diode D121 in the rectifier-smoothing circuit 121, thereby absorbing the noise component of the voltage VFWD0. Because the noise component is absorbed by the rectifier-smoothing circuit 122, which is connected in parallel with the rectifier-smoothing circuit 121, the rectifier-smoothing circuit 121 does not have the noise component superimposed on it, and can rectify and output only a voltage proportional to the input voltage Vin. On the other hand, since the rectifier-smoothing circuit 122 also rectifies the noise component, the output voltage VFWD2 of the rectifier-smoothing circuit 122 becomes a higher voltage Vpeak1 than the output voltage VFWD1 of the rectifier-smoothing circuit 121 (VFWD2 = Vpeak1 > VFWD1). Unlike the output voltage VFWD1, which is used to detect the input voltage Vin, the output voltage VFWD2 is used as the power supply voltage for the drive unit 112, so as long as the output voltage VFWD2 does not exceed the voltage rating of the drive unit 112, it does not have any effect even if the output voltage VFWD2 is high. Furthermore, by using a highly responsive diode for diode D122, there is no need to use a less responsive diode for diode D121. This allows for improved voltage detection accuracy without increasing power loss in the rectifier and smoothing circuit for voltage detection. In other words, diode D121 only needs to be relatively less responsive than diode D122.

[0026] Unlike the circuit configuration of the present invention, it is also possible to omit the second rectifier-smoothing circuit in the auxiliary winding and use the voltage induced in the auxiliary winding solely for detecting the input voltage. In such a circuit configuration, using a diode with low responsiveness in the single rectifier-smoothing circuit reduces voltage detection errors due to switching noise of the main FET. However, this increases the voltage detection error compared to the configuration with two rectifier-smoothing circuits as in the present invention. This is because, by connecting rectifier-smoothing circuits 121 and 122 in parallel as in the present invention, the noise component of the voltage VFWD0 induced in the auxiliary winding P2 is absorbed by the rectifier-smoothing circuit 122, which has good diode responsiveness. Compared to the case with one rectifier-smoothing circuit with low diode responsiveness, the case with two rectifier-smoothing circuits allows the rectifier-smoothing circuit using a diode with good responsiveness to suppress the voltage spike induced in the auxiliary winding during the switching operation of the main FET. Therefore, connecting two rectifier-smoothing circuits in parallel, as in the present invention, and using a highly responsive diode in one of the circuits, improves the accuracy of voltage detection compared to a circuit configuration with only one rectifier-smoothing circuit and a less responsive diode. Furthermore, since a diode is generally considered better if it is highly responsive, there is a wider variety of diodes available with better responsiveness, while using a less responsive diode limits the types of diodes that can be used. For this reason, a configuration with two rectifier-smoothing circuits with different diode responsiveness allows for a wider variety of diodes to be used compared to a system with only one rectifier-smoothing circuit and a less responsive diode.

[0027] Therefore, according to Example 1, noise components are absorbed by the second rectifier-smoothing circuit, resulting in higher voltage detection accuracy than when there is only one rectifier-smoothing circuit. Furthermore, a switching power supply capable of high-precision voltage detection without increasing power loss can be realized. As described above, according to Example 1, the accuracy of voltage detection can be improved without increasing power loss.

[0028] In the above embodiment 1, the voltage VFWD2 output from the rectifier-smoothing circuit 122 was used as the power supply voltage for the drive unit 112, but this is not limited to this. For example, the voltage VFWD2 may be supplied directly to the CPU 111 and used as the power supply voltage for the CPU 111. Alternatively, while using the voltage VFWD2 as the power supply voltage for the drive unit 112, the voltage VFWD2 may be further reduced through a step-down circuit such as a regulator circuit and then used as the power supply voltage for the CPU 111. The uses of the voltage VFWD2 are not limited to these, and the voltage VFWD2 may be supplied to locations other than the drive unit 112 and the CPU 111. [Examples]

[0029] The switching power supply 200 of Example 2 differs from the switching power supply 100 of Example 1 in the polarity of the auxiliary winding of the transformer and the feedback means for the output voltage Vout. The switching power supply 200 is included in the power supply unit 1108 of the printer 1100. The circuit configuration of the switching power supply 200 of Example 2 will be described below, followed by a description of the relationship between the voltage VFLB0 induced in the auxiliary winding P3 and the output voltages VFLB1 and VFLB2 of the two rectifier and smoothing circuits. Circuit configurations similar to those of the switching power supply 100 will use the same reference numerals and will not be described further.

[0030] [Power supply] First, the circuit configuration of the switching power supply 200 will be explained using Figure 3(a). Transformer T2 is an isolated type transformer equipped with a primary winding P1 and an auxiliary winding P3 on the primary side, and a secondary winding S1 on the secondary side. Transformer T2 differs from transformer T1 in Example 1 in that the auxiliary winding P3 has opposite polarity to the primary winding P1. The voltage VFLB0 induced in the auxiliary winding P3 is rectified and smoothed by a first rectifier-smoothing circuit 221 (hereinafter simply referred to as rectifier-smoothing circuit 221) and a second rectifier-smoothing circuit 222 (hereinafter simply referred to as rectifier-smoothing circuit 222). The number of turns of the primary winding P1 is TNP1, the number of turns of the secondary winding S1 is TNS1, and the number of turns of the auxiliary winding P3 is TNP3. In the switching power supply 200 of Example 2, the secondary winding S1 has opposite polarity to the primary winding P1 and the same polarity as the auxiliary winding P3.

[0031] The control unit 210 is a circuit for driving the main FET 1 and consists of a CPU 211 and a drive unit 112. The CPU 211 is an integrated general-purpose microcomputer equipped with an arithmetic unit that operates on a clock, for example. The CPU 211 controls the set values ​​(e.g., control start timing, period, on duty cycle) of the control signal S10, which is a PWM signal, based on the voltage VFLB1 output from the rectifier and smoothing circuit 221. The control signal S10 is input to the drive unit 112. Note that, as with the CPU 111 in Embodiment 1, an analog control IC or the like may be used instead of the CPU 211.

[0032] (Rectifier and smoothing circuit 221: for output voltage detection) The rectifier-smoothing circuit 221 is a circuit for outputting a voltage VFLB1 for output voltage detection. The rectifier-smoothing circuit 221 consists of a first diode D221 (hereinafter simply referred to as diode D221) and a first capacitor C221 (hereinafter simply referred to as capacitor C221) for rectifying and smoothing the voltage VFLB0 induced in the auxiliary winding P3. The rectifier-smoothing circuit 221 outputs an output voltage VFLB1 obtained by rectifying and smoothing the flyback voltage VFLB0 induced in the auxiliary winding P3 when the main FET 1 is in switching operation. The voltage VFLB0 induced in the auxiliary winding P3 and the output voltage Vout have the relationship shown in equation (2) using the number of turns TNS1 of the secondary winding S1 and the number of turns TNP3 of the auxiliary winding P3. Therefore, the control unit 110 can detect the voltage value of the output voltage Vout by detecting the output voltage VFLB1, which is the voltage obtained by rectifying and smoothing the voltage VFLB0. VFLB0=(TNP3 / TNS1)×Vout···(2)

[0033] (Rectifier and smoothing circuit 222: for power supply voltage) The rectifier-smoothing circuit 222, like the rectifier-smoothing circuit 221, is a circuit for rectifying and smoothing the voltage VFLB0, and outputs the output voltage VFLB2, which is the power supply voltage of the drive unit 112. The rectifier-smoothing circuit 222, like the rectifier-smoothing circuit 221, is composed of a second diode D222 (hereinafter simply referred to as diode D222) and a second capacitor C222 (hereinafter simply referred to as capacitor C222). However, it differs from the rectifier-smoothing circuit 221 in that the response of diode D222 is better than that of diode D221. Details of the relationship between the response of the diode and the output voltage will be described later. In the switching power supply 200 of Embodiment 2, the output voltage VFLB2 of the rectifier-smoothing circuit 222 is supplied directly to the drive unit 112, but a regulator circuit for voltage adjustment may be connected between the rectifier-smoothing circuit 222 and the drive unit 112. Furthermore, similar to Example 1, the rectifier-smoothing circuit 221 may be provided with a voltage divider resistor for the output voltage VFLB1 and a resistor for changing the time constant. In addition, the capacitance of capacitor C222 may be made larger than the capacitance of capacitor C221.

[0034] [Relationship between diode response and output voltage of rectifier / smoothing circuit] Next, the relationship between the diode's response and the output voltage of the rectifier-smoothing circuit will be explained using Figure 3(b). Figure 3(b) shows the control signal S10, the voltage VFLB0 induced in the auxiliary winding P3, the output voltage VFLB1 of the rectifier-smoothing circuit 221, and the output voltage VFLB2 of the rectifier-smoothing circuit 222. The graph in Figure 3(b)(i) shows time on the horizontal axis and voltage on the vertical axis, showing voltage VFLB0, output voltage VFLB1, and output voltage VFLB2. The graph in Figure 3(b)(ii) shows time on the horizontal axis and signal level (high level (H), low level (L)) on the vertical axis.

[0035] The on / off state of the main FET1 is controlled by a control signal S10 output from the CPU 211 of the control unit 210. The main FET1 is in the off state when the control signal S10 is low level, and in the on state when it is high level. Since the auxiliary winding P3 has opposite polarity to the primary winding P1, the voltage VFLB0 induced in the auxiliary winding P3 is the value shown in equation (2) when the main FET1 is in the off state (when the control signal S10 is low level). On the other hand, the voltage FFLB0 is 0V when the main FET1 is in the on state (when the control signal S10 is high level). Note that in Figure 3(b), there is a period in the voltage VFLB0 where the voltage is neither 0V nor the value shown in equation (2), but this is due to switching noise of the main FET1, similar to the voltage VFWD0 explained in Figure 2(b). In Figure 3(b)(i), the noise components due to the switching noise of the main FET1 are shown by the dashed circles γ and δ.

[0036] Incidentally, in the switching power supply 200, the reason why the output voltages of the rectifier-smoothing circuit 221 and the rectifier-smoothing circuit 222 are different is because diodes with different responsiveness are used, for the same reason as in the switching power supply 100 of Example 1. That is, in the rectifier-smoothing circuit 222 as well, the diode D222 with good responsiveness also rectifies the noise component. For this reason, the output voltage VFLB2 of the rectifier-smoothing circuit 222 is a higher voltage Vpeak2 than the output voltage VFLB1 of the rectifier-smoothing circuit 221 (VFLB2 = Vpeak2 > VFLB1).

[0037] The reason for wanting to accurately detect the voltage in the rectifier-smoothing circuit 221 is to achieve miniaturization and cost reduction of the circuit board. In a switching power supply that generates a DC voltage from a commercial AC power supply, it is necessary to feed the output voltage back to the primary side in order to keep the output voltage constant, but the primary and secondary sides must be isolated from each other. For this reason, a switching power supply requires an isolation transformer or a photocoupler or similar element. On the other hand, since switching power supplies generally require miniaturization and cost reduction of the circuit board, it is desirable to avoid including unnecessary elements. In a switching power supply that generates a DC voltage from a commercial AC power supply, an isolation transformer is indispensable as a means of transmitting power from the primary side to the secondary side. On the other hand, an isolation element using light, such as a photocoupler, is merely a means of transmitting information between the primary and secondary sides, so if it can be omitted, it is advantageous in terms of miniaturization and cost reduction of the circuit board. Therefore, in order to miniaturize and reduce the cost of the circuit board, a dedicated feedback circuit using a photocoupler or the like may be omitted, and the output voltage may be fed back using the auxiliary winding of the isolation transformer that transmits power. However, even when a dedicated feedback circuit is omitted, it is often unacceptable to compromise the accuracy of the output voltage, so it is necessary to improve the accuracy of voltage detection in the auxiliary winding.

[0038] Since the switching power supply 200 of Example 2 does not have an FB circuit, it is possible to reduce the size of the board compared to the switching power supply 100 of Example 1, which has a dedicated FB circuit. Furthermore, a rectifier and smoothing circuit 222 using a diode that is cheaper, smaller, and has better response than elements such as photocouplers is connected in parallel to the rectifier and smoothing circuit 221 for detecting the output voltage Vout. As a result, the switching power supply 200 can achieve high-precision voltage detection without increasing power loss due to the rectifier and smoothing circuit. In addition, by using the output voltage VFLB2 of the rectifier and smoothing circuit 222 as the power supply voltage of the drive unit 112, the power supply voltage generation circuit of the primary side circuit can also be omitted, and further reduction in board size is expected.

[0039] Therefore, according to Example 2, noise components are absorbed by the second rectifier-smoothing circuit 222, resulting in higher voltage detection accuracy than when there is only one rectifier-smoothing circuit. At the same time, a switching power supply capable of high-precision voltage detection can be realized without increasing power loss. As described above, according to Example 2, the accuracy of voltage detection can be improved without increasing power loss.

[0040] In the above embodiment 2, the voltage VFLB2 output from the rectifier-smoothing circuit 222 was used as the power supply voltage for the drive unit 112, but this is not limited to this. For example, the voltage VFLB2 may be supplied directly to the CPU 211 and used as the power supply voltage for the CPU 211. Alternatively, while using the voltage VFLB2 as the power supply voltage for the drive unit 112, the voltage VFLB2 may be further reduced through a step-down circuit such as a regulator circuit and then used as the power supply voltage for the CPU 211. The uses of the voltage VFLB2 are not limited to these, and the voltage VFLB2 may be supplied to locations other than the drive unit 112 and the CPU 211. [Explanation of symbols]

[0041] 110 Control Unit 121 Rectifier smoothing circuit 122 Rectifier smoothing circuit C121 Capacitor C122 Capacitor D121 Diode D122 Diode T1 Transformer

Claims

1. A transformer having a primary winding, a secondary winding, and an auxiliary winding, A switching element connected in series with the primary winding, A first rectifier and smoothing circuit having a first diode and a first capacitor for rectifying and smoothing the voltage induced in the auxiliary winding, A second rectifier-smoothing circuit having a second diode and a second capacitor, connected in parallel with the first rectifier-smoothing circuit, for rectifying and smoothing the voltage induced in the auxiliary winding, A control unit for controlling the switching element, Equipped with, The control unit detects the voltage induced in the auxiliary winding based on the output voltage of the first rectifier-smoothing circuit, A power supply device characterized in that the reverse recovery time of the first diode is longer than the reverse recovery time of the second diode.

2. A transformer having a primary winding, a secondary winding and an auxiliary winding, A switching element connected in series with the primary winding, A first rectifier and smoothing circuit having a first diode and a first capacitor for rectifying and smoothing the voltage induced in the auxiliary winding, A second rectifier-smoothing circuit having a second diode and a second capacitor, connected in parallel with the first rectifier-smoothing circuit, for rectifying and smoothing the voltage induced in the auxiliary winding, A control unit for controlling the switching element, Equipped with, The control unit detects the voltage induced in the auxiliary winding based on the output voltage of the first rectifier-smoothing circuit, A power supply device characterized in that the turn-on time of the first diode is longer than the turn-on time of the second diode.

3. The auxiliary winding has the same polarity as the primary winding. The power supply device according to claim 1 or 2, characterized in that the control unit detects the voltage input to the primary winding based on the voltage induced in the auxiliary winding.

4. The power supply device according to claim 3, further comprising a feedback means for feeding back the voltage output from the secondary winding to the control unit.

5. The secondary winding has the opposite polarity to the primary winding. The auxiliary winding has opposite polarity to the primary winding. The power supply device according to claim 1 or 2, characterized in that the control unit detects the voltage output from the secondary winding based on the voltage induced in the auxiliary winding.

6. The power supply device according to claim 1 or 2, characterized in that the second capacitor has a larger capacitance than the first capacitor.

7. The power supply device according to claim 1 or 2, characterized in that the first rectifier-smoothing circuit has a resistor connected in series with the first diode between the first diode and the first capacitor.

8. The system includes a drive unit for driving the switching element, The power supply device according to claim 1 or 2, characterized in that the second rectifier-smoothing circuit generates a power supply voltage to be supplied to the drive unit.

9. The power supply device according to claim 1 or 2, characterized in that the second rectifier-smoothing circuit generates a power supply voltage to be supplied to the control unit.

10. Image forming means for forming an image on a recording material, An image forming apparatus comprising a power supply device according to claim 1 or claim 2 that supplies power to the image forming means.

Images