JP2015097446A - Power-supply device and image formation device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To prevent blocking of voltage supply due to an inrush current to a smoothing capacitor while preventing operation of the safety vent of the smoothing capacitor.SOLUTION: A power-supply device includes: a detection section 131 detecting a ripple voltage between terminals of a smoothing capacitor 108; an overvoltage protection circuit 130 maintaining an FET 110 to an on-state when a value according to the ripple voltage detected by the detection section 131 is larger than or equal to a first predetermined value; a fuse 102 blocking an AC voltage inputted to a rectifier element 107 according to the maintenance of the on-state of the FET 110 by the overvoltage protection circuit 130; and an inrush prevention circuit 164 operating or stopping the overvoltage protection circuit 130 according to a voltage supplied from an auxiliary coil 11b.

Description

  The present invention relates to a power supply device and an image forming apparatus, and more particularly to a switching power supply and an image forming apparatus using the switching power supply.

  2. Description of the Related Art Conventionally, in a power supply device that obtains a stabilized DC voltage using a commercial AC voltage as an input, there is an overvoltage protection circuit for protecting the circuit when the voltage of the commercial AC power supply is higher than a predetermined voltage. . In addition, the overvoltage protection circuit protects the circuit from overvoltage even when a high voltage is supplied by mistake in a country or region where 100V and 200V outlets are shared. As an overvoltage protection circuit, for example, a circuit as in Patent Document 1 has been proposed.

Japanese Patent No. 4612855

  When a voltage sufficiently exceeding the rated voltage is applied to the aluminum electrolytic capacitor, which is a smoothing capacitor, the leakage current increases rapidly, and the pressure valve opens due to heat generation and an increase in internal pressure due to gas generation. In addition, the time from voltage application to valve opening operation due to increase in internal pressure varies depending on the voltage value to be applied and the time to apply. For example, when the applied voltage value slightly exceeds the rating and when it greatly exceeds, the time until the valve opening operation due to the increase in internal pressure becomes longer.

  The overvoltage protection means that detects an AC input voltage or a voltage corresponding to the AC input voltage and cuts off the voltage supply to the apparatus is a system that detects the AC input voltage regardless of the state of charge of the smoothing capacitor. In such a system, when an AC input voltage that is higher than the rated voltage or a voltage that is equal to or higher than the AC input voltage is detected for a short time, the voltage supply to the device is cut off and the operation of the device is stopped. Was. Also, when a voltage is supplied with the charge charged to the smoothing capacitor being zero, such as when the power is turned on, if an inrush current flows through the smoothing capacitor, the overvoltage protection means operates and the voltage supply is In some cases, it was blocked.

  The present invention has been made under such circumstances, and an object thereof is to prevent voltage supply from being interrupted by an inrush current to the smoothing capacitor while suppressing valve opening of the smoothing capacitor.

  In order to solve the above-described problems, the present invention has the following configuration.

  (1) Rectifying means for rectifying the input AC voltage, a smoothing capacitor for smoothing the voltage rectified by the rectifying means, a primary winding, a secondary winding and an auxiliary winding, and a primary side and a secondary winding A power supply apparatus comprising: a transformer that insulates a secondary side; a switching element that performs a switching operation for turning on or off a current flowing through the primary winding; and a control unit that controls the switching operation of the switching element, Detection means for detecting a ripple voltage between the terminals of the smoothing capacitor, and protection for maintaining the switching element in an ON state when a value corresponding to the ripple voltage detected by the detection means is equal to or greater than a first predetermined value. And a blocking means for cutting off the AC voltage input to the rectifying means in response to the switching element being maintained in the ON state by the protection means. When the power supply apparatus characterized by comprising a switching means for operating or stopping the protective means according to a voltage supplied from the auxiliary winding.

  (2) Rectifying means for rectifying the input AC voltage, a smoothing capacitor for smoothing the voltage rectified by the rectifying means, a primary winding, and a secondary winding, and a primary side and a secondary side A power supply device comprising: a transformer that insulates; a switching element that performs a switching operation for turning on or off a current flowing through the primary winding; and a control unit that controls the switching operation of the switching element. Detecting means for detecting a ripple voltage between terminals of the capacitor; and protecting means for maintaining the switching element in an ON state when a value corresponding to the ripple voltage detected by the detecting means is equal to or greater than a first predetermined value; A blocking means for cutting off the AC voltage input to the rectifying means in response to the switching element being maintained in the ON state by the protection means; Power supply for a voltage detector for detecting a voltage applied to the smoothing capacitor, and a switching means for operating or stopping the protection means in accordance with the detected voltage by said voltage detection means, comprising: a.

  (3) An image forming apparatus comprising: an image forming unit that performs image formation; and the power supply device according to (1) or (2).

  According to the present invention, it is possible to prevent the voltage supply from being interrupted by the inrush current to the smoothing capacitor while suppressing the valve opening of the smoothing capacitor.

1 is a schematic diagram of an overall configuration of an image forming apparatus according to a first embodiment. The figure which shows the power supply device of Example 1. The figure which shows the circuit operation | movement of Example 1. The figure which shows the power supply device of Example 2, The figure which shows an example of the characteristic of an input voltage and a ripple voltage The figure which shows the circuit operation of Example 2.

  Embodiments of the present invention will be described below with reference to the drawings.

  The first embodiment has a circuit configuration in which a ripple voltage generated at both ends of a smoothing capacitor is detected by a leakage current flowing before the pressure valve of the smoothing capacitor is opened, and voltage supply to the smoothing capacitor is cut off using a blocking unit. Thereby, in a present Example, the valve opening of the pressure valve of a smoothing capacitor is suppressed. Further, in this embodiment, the smoothing capacitor is changed by a voltage change caused by an inrush current that occurs when a voltage is input with the electric charge charged to the smoothing capacitor being zero, such as when the power is turned on, and a voltage exceeding the rated voltage is applied. And the ripple voltage generated at both ends of the. And it is the structure which suppresses that a voltage supply is interrupted | blocked by the rush current to the smoothing capacitor at the time of power-on.

[Configuration of Image Forming Apparatus]
FIG. 1 is a schematic diagram illustrating an example of an electrophotographic laser beam printer (hereinafter simply referred to as a printer) as an image forming apparatus. Here, an image forming operation in the printer 10 will be described. The sheets P are fed one by one from the sheet feed tray 11 by the sheet feed roller 12 and are transported to the process cartridge 14 by the transport roller 13 at a predetermined timing. Further, the process cartridge 14 has a photosensitive drum 15, a charging roller 16, and a developing roller 17. A charging roller 16 is disposed in pressure contact with the photosensitive drum 15, and the surface of the photosensitive drum 15 is uniformly charged by the charging roller 16. Thereafter, exposure based on image information is performed by the scanner unit 19 serving as an exposure unit, and an electrostatic latent image is formed on the surface of the photosensitive drum 15. A developing roller 17 is provided downstream of the exposure position by the scanner unit 19 in the rotation direction of the photosensitive drum 15 (the arrow direction (clockwise direction in the figure)). When the electrostatic latent image formed on the photosensitive drum 15 comes to a position facing the developing roller 17, toner is supplied to the electrostatic latent image from the developing roller 17, and a toner image (visible image) is formed on the photosensitive drum 15. Is formed.

  On the other hand, the paper P is conveyed at a timing synchronized with the moving speed of the toner image formed on the photosensitive drum 15. Then, the paper P that has reached the photosensitive drum 15 is transferred with the toner image on the photosensitive drum 15 at a transfer nip portion formed by a pressure contact portion between the photosensitive drum 15 and the transfer roller 20. The paper P on which the toner image is transferred is conveyed to the fixing device 21. The fixing device 21 includes two rollers that are pressed against each other, and fixes the toner image on the paper P by heat and pressure. Thereafter, the paper P is discharged out of the printer 10 by the paper discharge rollers 22 and 23, and a series of printing operations is completed. The cleaning device 18 is a means for cleaning the photosensitive drum 15. In the series of printing operations described above, it is necessary to drive the above-described devices such as the scanner unit 19 and the printer 10 requires a power supply device. The image forming apparatus on which the power supply device is mounted is not limited to the image forming apparatus shown in FIG. 1, and may be a color image forming apparatus such as a tandem method or a rotary method.

[Configuration of power supply unit]
FIG. 2 is a circuit configuration diagram for explaining the power supply device of this embodiment. The power supply apparatus has an overvoltage protection circuit 130 that is a protection means. In FIG. 2, the overvoltage protection circuit 130 indicated by the alternate long and short dash line includes a detection unit 131, a filter unit 132, a smoothing unit 133, and a voltage adjustment unit 134, all of which are indicated by broken lines. The detection unit 131 includes a capacitor and a resistor, and detects a ripple voltage. The filter unit 132 includes a resistor, a capacitor, and a diode. The smoothing unit 133 has a resistor and a capacitor. The voltage adjustment unit 134 has a resistance. The overvoltage protection circuit 130 further includes an N-channel field effect transistor (hereinafter simply referred to as FET) 135, a P-channel FET 138, resistors 136 and 137, and a diode 139. Further, DCH indicates a positive pole node of the smoothing capacitor 108, and DCL indicates a negative pole node of the smoothing capacitor 108, which are connected to the overvoltage protection circuit 130.

  Further, the overvoltage protection circuit 130 has an inrush suppression circuit 164 that is a switching means. As will be described later, the inrush suppression circuit 164 has a function of operating or stopping the overvoltage protection circuit 130 according to the voltage SubPower supplied from the auxiliary winding 111c of the transformer 111. The inrush suppression circuit 164 includes an N-channel FET 158, a P-channel FET 159, and resistors 160, 161, 162, and 163. The inrush suppression circuit 164 suppresses the operation of the FET 135, which is the first FET, by the inrush current that flows when a voltage is supplied from the AC inlet 101 while the electric charge charged in the smoothing capacitor 108 is zero. It is provided for.

  The current due to the AC voltage input from the AC inlet 101 is full-wave rectified by the rectifying element 107 as rectifying means, and charged to the smoothing capacitor 108. Further, the power supply IC 120 as the control means is activated by the current supplied from the activation resistor 109, and the N-channel FET 110 as the switching element is turned on or off (hereinafter referred to as switching operation). The power supply device starts operation by causing a switching current to flow through the transformer 111 by the switching operation of the FET 110. Here, the transformer 111 insulates the primary side from the secondary side, and includes a primary winding 111a, a secondary winding 111b, and an auxiliary winding 111c. When the transformer 111 is operated, the DC voltage SubPower generated by the auxiliary winding 111c of the transformer 111, the diode 118, and the electrolytic capacitor 119 is supplied as the power supply voltage of the power supply IC 120. As a result, the power supply IC 120 can continue stable operation. Note that the DC voltage SubPower is applied to the source terminal of the FET 138 of the overvoltage protection circuit 130 and one end of the resistor 161 of the inrush suppression circuit 164.

  Further, the secondary voltage transformed by the transformer 111 is smoothed by the secondary rectifier diode 122 and the electrolytic capacitor 123, and a stable DC voltage is output. The voltage control of the DC voltage output from the secondary side is performed as follows. The DC voltage on the secondary side is divided by resistors 126 and 127 and input to shunt regulator 129. Then, a feedback signal is generated by the shunt regulator 129 and the photocoupler 121, and feedback control is performed by inputting the feedback signal to the power supply IC 120. The power supply IC 120 changes the duty and frequency of the switching operation of the FET 110, so that the power supply device outputs a stable DC voltage as Vout. The power supply device includes a fuse 102, Y capacitors (line bypass capacitors) 103 and 104, and an X capacitor (across-the-line capacitor) 105, which are interruption means. The power supply device also includes a common mode coil 106, a resonance capacitor 113, a capacitor 114, resistors 115, 116, 117, resistors 124, 125, and a capacitor 128. Note that symbols A to F in FIG. 2 will be described later.

[Operation of overvoltage protection circuit]
FIG. 3A shows an example of the operation of the overvoltage protection circuit 130 that detects a change in current due to the ripple voltage between the terminals (both ends) of the smoothing capacitor 108. 3A and 3A are currents flowing downstream of the smoothing capacitor 108, FIGS. 3A and 3B are voltages across the resistance of the detection unit 131, and FIGS. Indicates the voltage across the diode. 3A and 3D show the voltage between the gate and source of the FET 135, FIGS. 3A and 3E show the voltage between the drain and source of the FET 135, and FIGS. 3A and 3F show the voltage of the diode 139. Indicates the potential of the anode. The horizontal axis is time. FIG. 3A shows a period during which a normal voltage is applied to the smoothing capacitor 108 (described as normal voltage application) and a period during which an overvoltage is applied (described as overvoltage application). Note that the symbols A to F shown in the overvoltage protection circuit 130 in FIG. 2 correspond to FIGS. 3A, 3A, 3A, and 3F.

  When an overvoltage is applied to the smoothing capacitor 108, as shown in FIGS. 3A and 3A, the current flowing downstream of the smoothing capacitor 108 increases with time. The overvoltage protection circuit 130 detects a change in current due to the ripple voltage across the smoothing capacitor 108 by the detection unit 131. Since the detection unit 131 converts the current flowing through the capacitor into a voltage with a resistor, as shown in FIGS. 3A and 3B, a waveform in which the average value of the voltage is zero is detected. The signal detected by the detection unit 131 is detected by the filter unit 132 as a ripple voltage having a commercial power source frequency (hereinafter referred to as a commercial frequency) as shown in FIGS.

  Further, as described above, the DC voltage SubPower is connected to one end of the resistor 161 of the inrush suppression circuit 164 of the overvoltage protection circuit 130. One end of the resistor 160 is connected to the other end of the resistor 161, and the connection point between the resistor 161 and the resistor 160 is connected to the gate terminal of the FET 158. For this reason, when the DC voltage SubPower is generated, a value obtained by dividing the DC voltage SubPower by the resistor 161 and the resistor 160 is applied between the gate and the source of the FET 158. When the voltage obtained by dividing the DC voltage SubPower with the resistors 160 and 161 becomes equal to or higher than the voltage (hereinafter also referred to as an operation threshold) of the FET 158 that is the second FET (hereinafter also referred to as an operation threshold), The FET 158 is turned on. Further, the FET 158 is not turned on when the DC voltage SubPower is less than the operation threshold (less than the second predetermined value). A time constant is provided by the electrolytic capacitor 119 until the DC voltage SubPower is generated. That is, when the electrolytic capacitor 119 is charged, the DC voltage SubPower becomes a constant voltage value. In the state shown in FIG. 3A, it is assumed that a predetermined time has elapsed since the power supply device was turned on, the DC voltage SubPower has a constant voltage value, and the FET 158 is turned on in the inrush suppression circuit 164. It shall be.

  When the FET 158 is turned on, a current flows through the FET 158 via the resistors 162 and 163, and a potential difference is generated between the gate and source of the FET 159. When the gate-source voltage of the FET 159 exceeds the operation threshold value of the FET 159, the FET 159 is turned on, the voltage is supplied to the smoothing unit 133, and the smoothing unit 133 smoothes the DC voltage. The DC voltage smoothed by the smoothing unit 133 is input to the voltage adjusting unit 134 and divided by two resistors included in the voltage adjusting unit 134. Here, since the voltage divided by the voltage adjusting unit 134 is also a detection result of the ripple voltage of the smoothing capacitor 108, it is also referred to as a ripple voltage detection result hereinafter. The ripple voltage detection result output by the voltage adjustment unit 134 is applied to the gate terminal of the FET 135. When the ripple voltage detection result (solid line in FIGS. 3A and 3D) is equal to or higher than the operation threshold value of the FET 135 indicated by the broken line in FIGS. 3A and 3D, the FET 135 is turned on. It becomes a state. The FET 135 is not turned on when the ripple voltage detection result is less than the operation threshold (less than the first predetermined value).

(When normal voltage is applied)
With a normal input voltage, an excessive load current does not flow on the secondary side of the transformer 111, so that the ripple voltage across the smoothing capacitor 108 does not increase abnormally (FIGS. 3A and 3A). . Therefore, the detection unit 131 (FIGS. 3A and 3B), the filter unit 132 (FIGS. 3A and 3C), the smoothing unit 133, and the voltage adjustment unit 134 are detected ripple voltage detection results. (A) The voltage shown in (D) does not reach the operating threshold value of the FET 135. For this reason, the FET 135 is turned off (FIGS. 3A and 3E). When the FET 135 is turned off, the gate-source voltage of the FET 138 becomes the same potential, and the FET 138 is turned off. When the FET 138 is turned off, the DC voltage SubPower is not applied to the FET 110 via the FET 138 and the diode 139 (FIGS. 3A and 3F). Therefore, the FET 110 is controlled by the power supply IC 120 and performs a stable switching operation.

(When overvoltage is applied)
Next, a case where a voltage sufficiently exceeding the rated voltage of the smoothing capacitor 108 is applied will be described. When a voltage sufficiently exceeding the rated voltage is applied in the forward direction of the smoothing capacitor 108, the leakage current rapidly increases in the smoothing capacitor 108, and the internal pressure rises due to heat generation and gas generation. At this time, as shown in FIGS. 3A and 3A, the leakage current suddenly flows, and the internal pressure rises in a short time. The current change (FIGS. 3A and 3A) caused by the ripple voltage generated at both ends of the smoothing capacitor 108 due to the sudden leakage current is detected by the detection unit of the overvoltage protection circuit 130 as shown in FIGS. It is converted into a voltage by 131 and detected. The signal detected by the detector 131 is detected as a commercial frequency ripple voltage by the filter unit 132 (FIGS. 3A and 3C), and is smoothed to a DC voltage by the smoothing unit 133. The ripple voltage generated by the large leakage current is large, and the ripple voltage detection result generated in the voltage adjustment unit 134 gradually increases as shown in FIGS. When the ripple voltage detection result is equal to or higher than the operation threshold value of the FET 135, the FET 135 is turned on (FIGS. 3A and 3E). As a result, as shown in FIGS. 3A and 3E, the drain-source voltage of the FET 135 is saturated, and current flows from the DC voltage SubPower to the resistors 136 and 137.

  When a current flows from the DC voltage SubPower to the resistor 137, a potential difference is generated in the resistor 137. When the gate-source voltage of the FET 138 exceeds the operation threshold, the FET 138 is turned on. When the FET 138 is turned on, the DC voltage SubPower is applied to the gate terminal, which is the control terminal of the FET 110, via the FET 138 and the diode 139, as shown in FIGS. As a result, the FET 110 is kept on regardless of the control of the switching operation from the power supply IC 120. Thus, in this embodiment, even if an overvoltage exceeding the rated voltage of the smoothing capacitor 108 is input, the switching operation of the FET 110 by the power supply IC 120 continues until the gate-source voltage of the FET 135 exceeds the operation threshold. Is done.

  The DC voltage SubPower is a voltage that can sufficiently turn on the FET 110. When the DC voltage SubPower is applied to the FET 110, the gate-source voltage of the FET 110 rises and the FET 110 maintains the on state. As a result, as shown in FIGS. 3A and 3F, when the fuse 102 reaches the blow time of the fuse while the diode 139 is conducting, the fuse is blown, and the fuse 102 is completely opened. The opening of the pressure valve of the smoothing capacitor 108 can be suppressed. In this embodiment, the timing at which the FET 135 operates is determined so that the fusing time of the fuse 102 is shorter than the valve opening time of the smoothing capacitor 108 (fusing time <valve opening time). To do.

(When the charge on the smoothing capacitor is zero)
On the other hand, FIG. 3B shows a transient state when a voltage is supplied from the AC inlet 101 in a state where the electric charge charged in the smoothing capacitor 108 is zero as when the power is turned off. The operation of each part of the state is shown. 3B and 3A are shown in FIGS. 3A and 3A, FIGS. 3B and 3D are shown in FIGS. 3A and 3D, and FIGS. 3B and 3F are shown in FIGS. This corresponds to FIGS. 3A and 3F. FIG. 3B (SubPower) is a diagram showing a voltage waveform of the DC voltage SubPower. As shown in FIGS. 3B and 3A, a voltage is supplied from the AC inlet 101 when the electric charge charged in the smoothing capacitor 108 is zero (the AC input voltage is changed from the OFF state to the ON state). When the transition occurs, an inrush current flows through the smoothing capacitor 108. Here, when the overvoltage protection circuit 130 does not have the inrush suppression circuit 164, the voltage between the gate and the source of the FET 135 indicated by a two-dot chain line in FIGS. 3B and 3D is the inrush current flowing in the smoothing capacitor 108. As a result, the operation threshold of the FET 135 indicated by the broken line is exceeded. Then, the overvoltage protection circuit 130 is activated and the fuse 102 is blown. Here, in FIGS. 3B and 3F, the waveform of the DC voltage SubPower when there is no inrush suppression circuit 164 is indicated by a two-dot chain line. The black circles in FIGS. 3B and 3F indicate the operating voltage of the FET 110, and the portion drawn by the alternate long and short dash line is the time until the fuse 102 is blown when the inrush suppression circuit 164 is not present (melting time). ).

  It takes time to charge the electrolytic capacitor 119 connected to the auxiliary winding 111c after the voltage is supplied from the AC inlet 101 until the DC voltage SubPower becomes a predetermined voltage. Therefore, when there is the inrush suppression circuit 164 as shown in the present embodiment, as shown by a solid line in FIG. 3B (SubPower), the gate of FET 158 is turned on until the DC voltage SubPower becomes a predetermined voltage. The source-to-source voltage does not exceed the operation threshold of the FET 158. In other words, when the DC voltage SubPower becomes a predetermined voltage, the voltage divided by the resistors 160 and 161 is configured to be equal to or higher than the operating voltage of the FET 158. Therefore, the FET 158 is not turned on until the DC voltage SubPower becomes a predetermined voltage, and the FET 159 is not turned on. Therefore, as indicated by solid lines in FIGS. 3B and 3D, the voltage of the ripple voltage detection result at point D in FIG. 2 does not exceed the operation threshold value of the FET 135, and the overvoltage protection circuit 130 does not operate. For this reason, as shown in FIGS. 3B and 3F, the DC voltage SubPower is not applied to the FET 110, and the fusing of the fuse 102 due to the inrush current to the smoothing capacitor 108 can be suppressed. Note that a period during which the overvoltage protection circuit 130 is not operated by the inrush suppression circuit 164 (a period until the DC voltage SubPower becomes a predetermined voltage) is defined as an inrush suppression period.

  As described above, the power supply apparatus according to the present embodiment has a configuration in which the overvoltage protection circuit 130 detects the ripple voltage generated at both ends of the smoothing capacitor 108 due to the leakage current that flows immediately before the pressure valve of the smoothing capacitor 108 is opened. When an overvoltage is detected by the overvoltage protection circuit 130, the FET 110 is maintained in an on state, and the fuse 102 is blown to an open state, whereby the opening of the pressure valve of the smoothing capacitor 108 can be suppressed. In this embodiment, when the electric charge charged in the smoothing capacitor 108 is zero, such as when the power is turned on, the inrush current is detected and the overvoltage protection circuit 130 operates, and the fuse 102 is blown. It is the structure which prevents that. That is, a voltage change due to an inrush current to the smoothing capacitor 108 when a voltage is supplied from the AC inlet 101, and a ripple voltage generated at both ends of the smoothing capacitor 108 immediately before the smoothing capacitor 108 is opened when a voltage exceeding the rated voltage is applied. Are distinguished. As a result, in this embodiment, it is possible to suppress interruption of voltage supply due to inrush current to the smoothing capacitor 108 when the power is turned on.

  As described above, according to this embodiment, it is possible to prevent the voltage supply from being interrupted by the inrush current to the smoothing capacitor while suppressing the valve opening of the smoothing capacitor.

  In Example 2, the ripple voltage at both ends of the smoothing capacitor and the voltage input to both ends of the smoothing capacitor are detected, and when both voltages are equal to or higher than the operating threshold, the voltage to the smoothing capacitor is used by using a cutoff means. A configuration for cutting off the supply will be described. The present embodiment suppresses the opening of the pressure valve to the smoothing capacitor by such a configuration. With this configuration, in addition to the effects of the first embodiment, the ripple voltage due to the leakage current flowing before the smoothing capacitor is opened and the ripple voltage generated when a large load current flows within the normal operation input voltage range are discriminated. be able to. Furthermore, the present embodiment is configured to include a circuit that detects a voltage corresponding to both ends of the smoothing capacitor with an input voltage detection circuit and generates a predetermined DC voltage only when the input voltage detection circuit is operating. And by using the generated DC voltage as the power supply for the cutoff means, the ripple voltage immediately before the smoothing capacitor is opened can be detected even when the control IC is powered from a separate power supply instead of from the auxiliary winding. It is the structure to do.

[Configuration of power supply unit]
FIG. 4A is a circuit configuration diagram for explaining the power supply device of this embodiment. In addition, the same code | symbol is attached | subjected to the same structure as the structure demonstrated in Example 1, and description is abbreviate | omitted. The transformer 111 of this embodiment has a primary winding 111a and a secondary winding 111b, and does not have an auxiliary winding. The input voltage detection circuit 140 as voltage detection means indicated by a broken line portion in FIG. 4A includes a time constant filter, an N-channel FET 143 connected to the time constant filter, and resistors 148 and 149. Further, the input voltage detection circuit 140 includes an FET 144, a PNP bipolar transistor (hereinafter simply referred to as a transistor) 145, and resistors 150 and 151. Here, the time constant filter includes a Zener diode 141 connected to DCH, a resistor 146 connected to the Zener diode 141, and a capacitor 142. The resistors 148 and 149 detect the voltage across the smoothing capacitor 108 when the FET 143 is turned on. The FET 144 performs an on / off operation according to a voltage detection result generated at both ends of the resistor 149. The operation of the transistor 145 is switched according to the state of the FET 144. The power supply IC 120 shown in FIG. 4A is stably operated by the voltage supply from the separate power supply 152 after being activated by the activation resistor 109. The input voltage detection circuit 140 of this embodiment includes an NPN bipolar transistor (hereinafter referred to as a transistor) 153, resistors 154, 155, and 156, and a Zener diode 157.

  Here, FIG. 4B shows the relationship between the voltage applied to both ends of the smoothing capacitor 108 and the ripple voltage. 4B, the vertical axis represents the ripple voltage [V], and the horizontal axis represents the voltage [V] applied across the smoothing capacitor 108. In general, when the output voltage on the secondary side of the transformer 111 is constant, if the voltage applied to both ends of the smoothing capacitor 108 is increased, the switching current flowing on the primary side of the transformer 111 is reduced. For this reason, the ripple voltage generated at both ends of the smoothing capacitor 108 becomes small. Therefore, as shown in FIG. 4B, the voltage applied to both ends of the smoothing capacitor 108 and the ripple voltage are represented by a right-down characteristic.

  Further, when the output voltage consumed by Vout on the secondary side of the transformer 111 is increased, a large switching current flows also on the primary side of the transformer 111. For this reason, depending on the power supply device, the ripple voltage within the normal operating voltage range may increase. As an example, FIG. 4B shows a case where the load current consumed by Vout on the secondary side of the transformer 111 is large. In FIG. 4B, Vrip is a ripple voltage due to a leakage current flowing before the smoothing capacitor 108 is opened, and Vmax is a maximum ripple voltage generated when a large load current flows within the normal operating voltage range. . The overvoltage protection circuit 130 according to this embodiment includes a detection unit 131, a filter unit 132, a smoothing unit 133, a voltage adjustment unit 134, and an inrush suppression circuit 164. When the maximum ripple voltage Vmax is larger than the ripple voltage Vrip, the overvoltage protection circuit 130 may operate by detecting the maximum ripple voltage Vmax. For this reason, in this embodiment, the overvoltage protection circuit 130 and the input voltage detection circuit 140 are provided as means for distinguishing between the ripple voltage due to the leakage current and the ripple voltage within the normal operating voltage range.

[Power supply operation]
The operation of the power supply device of this embodiment will be described. The circuit conditions are as follows. The withstand voltage of the smoothing capacitor 108 is a DC voltage V2 [V], and a leakage current starts to flow when V3 [V] or higher. The Zener voltage of the Zener diode 141 is Vz1 [V], the Zener voltage of the Zener diode 157 is Vz2 [V], and the sum of Vz1 and Vz2 is configured to be a voltage V2 (= Vz1 + Vz2). The Zener voltage Vz2 is a voltage that can sufficiently turn on the FET 143. The FET 144 is designed by resistors 148 and 149 so as to operate when a voltage of V2 [V] or more (third predetermined value or more) is applied across the smoothing capacitor 108. The FET 135 is designed to operate when the ripple voltage across the smoothing capacitor 108 is equal to or higher than Vo [V] (first predetermined value) that is the operation threshold of the FET 135. That is, the overvoltage protection circuit 130 operates when the ripple voltage across the smoothing capacitor 108 becomes equal to or higher than Vo [V].

(1) When a voltage of V1 or more and less than V2 is applied (V1 ≦ V <V2)
Since the normal operation is the same as that of the first embodiment, it is omitted. FIG. 5A shows an example of operations of the overvoltage protection circuit 130 and the input voltage detection circuit 140 when a voltage of V1 or more and less than V2 is applied to the smoothing capacitor 108. FIGS. 5A, 5A, 5D and 5A are respectively shown in FIGS. 3A, 3A, 3D, 3D. a) It is the same as (F) and will not be described. FIG. 5A (Vth) is a waveform of the detection voltage Vth output from the input voltage detection circuit 140 to the overvoltage protection circuit 130. The same applies to FIGS. 5B to 5D. FIG. 5A shows respective waveforms when the load current consumed by Vout on the secondary side of the transformer 111 is small and large when a normal voltage is applied to the smoothing capacitor 108.

  The voltage applied across the smoothing capacitor 108 is less than V2, and does not exceed V2 (= Vz1 + Vz2) which is the sum of the Zener voltages of the Zener diode 141 and the Zener diode 157. For this reason, since no current flows through the resistor 146 via the Zener diode 141, the gate-source voltage of the FET 143 is lower than the operating threshold value of the FET 143. Accordingly, the FET 143 is turned off, and the gate-source voltage of the FET 144 is lower than the operating voltage of the FET 144, so that the FET 144 is also turned off. Since the base-emitter voltage of the transistor 145 does not exceed the operation threshold value of the transistor 145, the transistor 145 is also turned off. Therefore, as shown in FIG. 5A (Vth), the voltage Vth is not supplied to the overvoltage protection circuit 130 via the transistor 145.

  Further, since the voltage Vth is not supplied to the overvoltage protection circuit 130, the gate-source voltage of the FET 158 is lower than the operation threshold value of the FET 158, and the FET 158 is turned off. Therefore, no current flows between the drain and source of the FET 158 via the resistors 162 and 163, and the gate-source voltage of the FET 159 does not exceed the operating threshold value of the FET 159. For this reason, the FET 159 is also turned off. Therefore, as shown in FIGS. 5A and 5D, the gate-source voltage of the FET 135 does not exceed the operation threshold value of the FET 135, and the FET 135 is turned off. As a result, the FET 138 is turned off, and the voltage Vth is not supplied to the FET 110 via the FET 138 as shown in FIGS. 5A and 5F, and the FET 110 performs a stable switching operation by the control of the power supply IC 120. Do. Thus, when the voltage applied across the smoothing capacitor 108 is not less than V1 and less than V2, the input voltage detection circuit 140 does not output the detection voltage Vth, and the overvoltage protection circuit 130 does not operate.

(2) When a voltage of V2 or more and less than V3 is applied (V2 ≦ V <V3)
Next, the case where the voltage generated at both ends of the smoothing capacitor 108 is V2 or more and less than V3 will be described. FIG. 5B shows a state in which a normal voltage (V1 ≦ V <V2) is applied to the smoothing capacitor 108, an overvoltage (V2 ≦ V <V3) is applied, and the normal voltage is applied again. ing. In this case, a voltage slightly exceeding the withstand voltage V2 of the smoothing capacitor 108 is applied to both ends of the smoothing capacitor 108. When a voltage of V2 or more and less than V3 is applied to both ends of the smoothing capacitor 108 for a short time, a current flows through the Zener diode 141, and the voltage across the Zener diode 141 is set to a constant value equal to the Zener voltage Vz1. The current flowing at this time flows to the Zener diode 157 via the resistor 146, and tries to make the voltage across the Zener diode 157 a constant value equal to the Zener voltage Vz2. Accordingly, the voltage across the resistor 156 and the capacitor 142 becomes the Zener voltage Vz2 of the Zener diode 157. Since a voltage higher than the operation threshold of the FET 143 is applied between the gate and the source of the FET 143, the FET 143 is turned on, and the gate-source voltage of the FET 144 is determined by the divided voltage of the resistor 148 and the resistor 149. In addition, in order for the voltage across the resistor 156 to be Vz2, it takes time to fully charge the capacitor 142. Therefore, the voltage across the resistor 156 becomes Vz2 after a predetermined time has elapsed since the voltage was applied to the DCH, and the FET 143 is turned on.

  Further, since the voltage generated by the voltage division of the resistors 148 and 149 exceeds the operation threshold value of the FET 144, the FET 144 is turned on. Further, the base terminal of the transistor 153 becomes the Zener voltage Vz2 of the Zener diode 157. Therefore, the transistor 153 is turned on, and a voltage Vdc obtained by subtracting the base-emitter saturation voltage of the transistor 153 from the Zener voltage Vz2 is applied to both ends of the resistor 155. Therefore, current flows from the transistor 153 to the resistors 150 and 151, and a potential difference is generated between both ends of the resistor 150. The base-emitter voltage of the transistor 145 exceeds the operation threshold due to the potential difference generated across the resistor 150. Therefore, the transistor 145 is turned on, and the voltage Vdc (predetermined voltage) is set to the voltage Vth via the transistor 145 after a predetermined time as shown in FIG. 5B (Vth). To be supplied.

  When the voltage Vth is supplied to the overvoltage protection circuit 130, a value obtained by dividing the voltage Vth by the resistor 161 and the resistor 160 is applied between the gate and source of the FET 158. When the voltage applied between the gate and source of the FET 158 exceeds the operation threshold value of the FET 158, the FET 158 is turned on. When the FET 158 is turned on, a current flows through the FET 158 via the resistor 162 and the resistor 163, and a potential difference is generated between the gate and source of the FET 159. When the gate-source voltage of the FET 159 exceeds the operation threshold value of the FET 159, the FET 159 is turned on, the voltage is supplied to the smoothing unit 133, and is smoothed to a DC voltage.

  On the other hand, as shown in FIG. 4B, the ripple voltage generated when a voltage not lower than V2 and lower than V3 is applied across the smoothing capacitor 108 is smaller than the minimum ripple voltage Vmin. That is, it is smaller than the ripple voltage Vo at which the FET 135 operates. For this reason, the ripple voltage detection result detected by the detection unit 131, the filter unit 132, the smoothing unit 133, and the voltage adjustment unit 134 is smaller than the operation threshold value of the FET 135 as shown in FIGS. For this reason, the FET 135 remains off. Therefore, no current flows through the resistors 136 and 137, the gate-source voltage of the FET 138 becomes the same potential, and the FET 138 remains off. As a result, the voltage Vth is not applied to the FET 110 via the FET 138 as shown in FIGS. 5B and 5F, and the FET 110 performs a stable switching operation under the control of the power supply IC 120. As described above, when the voltage applied across the smoothing capacitor 108 is V2 or more and less than V3, the input voltage detection circuit 140 outputs the detection voltage Vth, but the overvoltage protection circuit 130 does not operate.

(3) When a voltage of V3 or higher is applied (V3 ≦ V)
Next, a case where the voltage generated across the smoothing capacitor 108 is V3 or higher will be described. FIG. 5C shows an example of operations of the overvoltage protection circuit 130 and the input voltage detection circuit 140 when a voltage of V3 or higher is applied. When a voltage V3 sufficiently exceeding the withstand voltage V2 of the smoothing capacitor 108 is applied across the smoothing capacitor 108, a current flows through the Zener diode 141, and the voltage across the Zener diode 141 is set to a constant value equal to the Zener voltage Vz1. To do. The current flowing at this time flows to the Zener diode 157 via the resistor 146, and tries to make the voltage across the Zener diode 157 a constant value equal to the Zener voltage Vz2. Accordingly, the voltage across the resistor 156 and the capacitor 142 becomes the Zener voltage Vz2 of the Zener diode 157. Since a voltage higher than the operation threshold of the FET 143 is applied between the gate and the source of the FET 143, the FET 143 is turned on, and the gate-source voltage of the FET 144 is determined by the divided voltage of the resistors 148 and 149.

  Further, since the voltage generated by the divided voltage of the resistors 148 and 149 exceeds the operation threshold value of the FET 144, the FET 144 is turned on. Further, the base terminal of the transistor 153 becomes the Zener voltage Vz2 of the Zener diode 157. Therefore, the transistor 153 is turned on, and a voltage Vdc obtained by subtracting the base-emitter saturation voltage of the transistor 153 from the Zener voltage Vz2 is applied to both ends of the resistor 155. Therefore, current flows from the transistor 153 to the resistors 150 and 151, and a potential difference is generated between both ends of the resistor 150. The voltage difference generated between both ends of the resistor 150 causes the base-emitter voltage of the transistor 145 to exceed the operation threshold, so that the transistor 145 is turned on. Then, as shown in FIG. 5C (Vth), after a predetermined time has elapsed, the voltage Vdc is supplied to the overvoltage protection circuit 130 as the voltage Vth through the transistor 145.

  When the voltage Vth is supplied to the overvoltage protection circuit 130, a value obtained by dividing the voltage Vth by the resistor 161 and the resistor 160 is applied between the gate and source of the FET 158. When the voltage applied between the gate and source of the FET 158 exceeds the operation threshold value of the FET 158, the FET 158 is turned on. When the FET 158 is turned on, a current flows to the FET 158 via the resistors 162 and 163, and a potential difference is generated between the gate and the source of the FET 159. When the gate-source voltage of the FET 159 exceeds the operation threshold value of the FET 159, the FET 159 is turned on, the voltage is supplied to the smoothing unit 133, and is smoothed to a DC voltage.

  Further, since a voltage exceeding the withstand voltage V2 of the smoothing capacitor 108 (voltage of V3 or more) is applied to both ends of the smoothing capacitor 108, the leakage current increases rapidly as shown in FIG. The internal pressure increases due to heat generation and gas generation. The detection unit 131 detects a change in current due to the ripple voltage (for example, the ripple voltage Vrip in FIG. 4B) at both ends of the smoothing capacitor 108 due to the sudden leakage current. The signal detected by the detection unit 131 is detected as a commercial frequency ripple voltage by the filter unit 132 and smoothed to a DC voltage by the smoothing unit 133. As shown in FIGS. 5C and 5D, since the ripple voltage generated by the leakage current is equal to or higher than Vo (see FIG. 4B), the ripple voltage detection result generated in the voltage adjustment unit 134 also increases, and the FET 135 The FET 135 is turned on when the operation threshold is exceeded. As a result, the gate-source voltage of the FET 138 exceeds the operating threshold value, and the FET 138 is turned on. Accordingly, as shown in FIGS. 5C and 5F, the voltage Vth is applied to the FET 110 via the FET 138 and the diode 139.

  The voltage Vth is a voltage that can sufficiently turn on the FET 110. When the voltage Vth is applied to the FET 110, the gate-source voltage of the FET 110 increases. As a result, when the fuse blowing time is reached, the fuse 102 is blown, the fuse 102 is completely opened, and the opening of the pressure valve of the smoothing capacitor 108 can be suppressed. Thus, when the voltage applied across the smoothing capacitor 108 is V3 or higher, the input voltage detection circuit 140 outputs the detection voltage Vth, and the overvoltage protection circuit 130 also operates.

(When the charge on the smoothing capacitor is zero)
On the other hand, FIG. 5D shows that the voltage charged to the smoothing capacitor 108 is zero as in the case of power-on, and a voltage of V2 or more (rated voltage or more) is supplied to the smoothing capacitor 108 from the AC inlet 101. It shows the operation in a transitional state when it is done. As shown in FIGS. 5D and 5A, when a voltage is supplied from the AC inlet 101 while the electric charge charged in the smoothing capacitor 108 is zero, an inrush current flows through the smoothing capacitor 108. Here, when the overvoltage protection circuit 130 does not have the inrush suppression circuit 164, the ripple voltage detection result is equal to or higher than the operation threshold value of the FET 135 indicated by the broken line, as indicated by a two-dot chain line in FIGS. Become. Then, the overvoltage protection circuit 130 operates and the fuse 102 is blown. Here, in FIGS. 5D and 5F, the waveform of the voltage Vth when the inrush suppression circuit 164 is not provided is indicated by a two-dot chain line. 5D and 5F, the black circles indicate the operating voltage of the FET 110, and the portion drawn with a one-dot chain line indicates the fusing time of the fuse 102.

  It takes time to charge the capacitor 142 until the voltage Vdc reaches a predetermined voltage after the voltage is supplied from the AC inlet 101. That is, as indicated by a solid line in FIG. 5D (Vth), a predetermined time (referred to as an inrush suppression period) is required until the voltage Vth reaches the predetermined voltage (Vdc). In the inrush suppression period, the gate-source voltage of the FET 158 does not exceed the operation threshold value of the FET 158, so that the FET 159 is not turned on. Therefore, the potential indicated by the solid line in FIGS. 5D and 5D does not exceed the operation threshold of the FET 135, and the overvoltage protection circuit 130 does not operate. For this reason, as indicated by solid lines in FIGS. 5D and 5F, the voltage Vth is not applied to the FET 110, and the fusing of the fuse 102 due to the inrush current can be suppressed. Thus, when an inrush current flows through the smoothing capacitor 108, the input voltage detection circuit 140 outputs the detection voltage Vth, but the overvoltage protection circuit 130 does not operate.

  As described above, the power supply device of this embodiment includes the overvoltage protection circuit 130 and the input voltage detection circuit 140. The overvoltage protection circuit 130 of this embodiment detects a ripple voltage generated at both ends of the smoothing capacitor 108 due to a leakage current that flows immediately before the pressure valve of the smoothing capacitor 108 is opened. In addition, the input voltage detection circuit 140 of this embodiment detects a voltage corresponding to both ends of the smoothing capacitor 108. When both the overvoltage protection circuit 130 and the input voltage detection circuit 140 operate (FIG. 5C), the FET 110 is kept on and the fuse 102 is opened. Specifically, when the FET 135 of the overvoltage protection circuit 130 is equal to or higher than the operation threshold and the input voltage detection circuit 140 outputs the voltage Vth, the FET 110 is maintained in the on state.

  In the first embodiment, the overvoltage protection circuit 130 uses the DC voltage SubPower generated by the auxiliary winding 111c of the transformer 111 as the power supply when the power supply IC 120 controls the switching operation of the FET 110. On the other hand, in this embodiment, the transformer 111 does not have an auxiliary winding, and the power supply IC 120 is supplied with a voltage from another power supply 152. The overvoltage protection circuit 130 of this embodiment is supplied with the voltage Vth generated by the input voltage detection circuit 140 using the input voltage detection circuit 140 as a power supply, not the separate power supply 152. Therefore, in the configuration of this embodiment, even if the transformer 111 does not have an auxiliary winding, the smoothing capacitor is used by the overvoltage protection circuit 130 and the input voltage detection circuit 140 regardless of the state of the separate power supply 152. The opening of the pressure valve 108 can be suppressed.

  Further, in the present embodiment, it is possible to determine a ripple voltage generated when a large load current flows within a normal operation input voltage range and a ripple voltage immediately before the smoothing capacitor 108 is opened. This is effective when a large load current flows within the normal operation input voltage range of the smoothing capacitor 108 and when there is no significant difference in ripple voltage immediately before the smoothing capacitor 108 is opened due to overvoltage. Here, when a large load current flows within the normal operation input voltage range of the smoothing capacitor 108, for example, the normal operation input voltage range may be wide or the secondary side load current of the transformer 111 may be large.

  In this embodiment, the input voltage detection circuit 140 is provided on the smoothing capacitor 108 side (downstream side) from the rectifying element 107, but the input voltage detection circuit 140 only needs to be able to detect a voltage corresponding to both ends of the smoothing capacitor 108. Therefore, for example, a full-wave rectifier circuit, a half-wave rectifier circuit, or the like may be provided on the AC inlet 101 side (upstream side) from the rectifier element 107 to detect a voltage corresponding to the smoothing capacitor 108.

  Further, the configuration including the input voltage detection circuit 140 according to the present embodiment may be applied to the configuration of the power supply apparatus including the transformer 111 having the auxiliary winding 111c according to the first embodiment. That is, the power supply IC 120 of this embodiment may be configured to be supplied with a voltage from the auxiliary winding 111c.

  Further, the first and second embodiments described above are configured to detect the ripple voltage generated at both ends of the smoothing capacitor 108 and determine immediately before opening the valve based on the detected ripple voltage. However, the present invention is not limited to this, and for example, it may be determined immediately before opening the valve using the integrated value of the ripple voltage generated across the smoothing capacitor 108. In other words, the threshold for determining immediately before opening the valve may be any other value as long as the value corresponds to the ripple voltage generated at both ends of the smoothing capacitor 108.

  As described above, according to this embodiment, it is possible to prevent the voltage supply from being interrupted by the inrush current to the smoothing capacitor while suppressing the valve opening of the smoothing capacitor.

102 fuse 108 smoothing capacitor 110 FET
130 Overvoltage protection circuit 164 Inrush suppression circuit

Claims (13)

Rectifying means for rectifying the input AC voltage;
A smoothing capacitor for smoothing the voltage rectified by the rectifying means;
A transformer having a primary winding, a secondary winding and an auxiliary winding, and insulating the primary side and the secondary side;
A switching element that performs a switching operation to turn on or off a current flowing through the primary winding;
Control means for controlling the switching operation of the switching element;
A power supply device comprising:
Detecting means for detecting a ripple voltage between terminals of the smoothing capacitor;
Protection means for maintaining the switching element in an ON state when a value corresponding to the ripple voltage detected by the detection means is equal to or greater than a first predetermined value;
A blocking means for cutting off the AC voltage input to the rectifying means in response to the switching element being maintained in the ON state by the protection means;
Switching means for operating or stopping the protection means according to the voltage supplied from the auxiliary winding;
A power supply apparatus comprising:   The switching means stops the protection means when the voltage supplied from the auxiliary winding is less than a second predetermined value, and the voltage supplied from the auxiliary winding becomes equal to or higher than the second predetermined value. 2. The power supply device according to claim 1, wherein the protection means is operated in the event of a failure.   The protection means outputs a voltage supplied from the auxiliary winding to the control terminal of the switching element when a value corresponding to the ripple voltage detected by the detection means is equal to or greater than the first predetermined value. The power supply apparatus according to claim 2, wherein the switching element is maintained in an on state. The protection means includes a first FET that operates at the first predetermined value or more,
4. The power supply device according to claim 3, wherein when the first FET operates, a voltage supplied from the auxiliary winding is output to a control terminal of the switching element. 5. The switching means has a second FET,
5. The power supply device according to claim 2, wherein the second predetermined value is an operation threshold value of the second FET. 6. Rectifying means for rectifying the input AC voltage;
A smoothing capacitor for smoothing the voltage rectified by the rectifying means;
A transformer having a primary winding and a secondary winding and insulating the primary side and the secondary side;
A switching element that performs a switching operation to turn on or off a current flowing through the primary winding;
Control means for controlling the switching operation of the switching element;
A power supply device comprising:
Detecting means for detecting a ripple voltage between terminals of the smoothing capacitor;
Protection means for maintaining the switching element in an ON state when a value corresponding to the ripple voltage detected by the detection means is equal to or greater than a first predetermined value;
A blocking means for cutting off the AC voltage input to the rectifying means in response to the switching element being maintained in the ON state by the protection means;
Voltage detecting means for detecting a voltage applied to the smoothing capacitor;
Switching means for operating or stopping the protection means according to the voltage detected by the voltage detection means;
A power supply apparatus comprising:   The switching unit stops the protection unit when the voltage detected by the voltage detection unit is less than a second predetermined value, and the voltage detected by the voltage detection unit becomes equal to or higher than the second predetermined value. The power supply apparatus according to claim 6, wherein the protection means is operated in the event of a failure.   The protection means has a predetermined value when a value corresponding to the ripple voltage detected by the detection means is not less than the first predetermined value and the voltage detected by the voltage detection means is not less than a third predetermined value. The power supply apparatus according to claim 7, wherein a voltage is output to a control terminal of the switching element to maintain the switching element in an on state. The protection means includes a first FET that operates at the first predetermined value or more,
The power supply device according to claim 8, wherein the predetermined voltage is output to a control terminal of the switching element by the operation of the first FET. The switching means has a second FET,
The power supply device according to any one of claims 7 to 9, wherein the second FET operates at the second predetermined value or more.   As for the time from when the voltage detected by the detection means becomes equal to or higher than the first predetermined value to when the blocking means blows, the value corresponding to the ripple voltage detected by the detection means becomes equal to or higher than the first predetermined value. The power supply device according to any one of claims 1 to 10, wherein a time period from when the smoothing capacitor is opened to when the smoothing capacitor is opened is shorter.   The power supply device according to claim 1, wherein the control unit controls the switching operation of the switching element based on a voltage output from the secondary winding. Image forming means for forming an image;
A power supply device according to any one of claims 1 to 12,
An image forming apparatus comprising:

Images