Image forming apparatus equipped with a switching power supply

Abstract

To prevent a switching control section from being adversely affected even when a lightning surge is input to a low-voltage rated line. [Solution] The image forming device (1) includes a rectifying and smoothing circuit (112) that rectifies an AC voltage from which noise has been removed by a noise filter (Lc) and outputs the rectified voltage to a primary winding (L1) of a transformer (111); a switching control unit (121) that performs switching control; a power supply line (201) that is connected between the rectifying and smoothing circuit (112) and the primary side of the transformer (111) and inputs a high-voltage power supply voltage to a fourth terminal (VIN) of the switching control unit (121); a detection line (300) that is connected between the noise filter (Lc) and the rectifying and smoothing circuit (112), intersects with the power supply line (201), and inputs a low-voltage voltage to a fifth terminal (LS) of the switching control unit (121); and a first resistor (R10) that is connected between an intersection (500) of the detection line (300) with the power supply line (201) and a fifth terminal (LS) of the switching control unit (121) and drops the voltage input to the fifth terminal (LS).

Description

[Technical Field]

[0001] The present invention relates to an image forming apparatus that performs switching control on a switching element connected to a primary coil of a transformer. [Background technology]

[0002] Conventional switching power supplies rectify and smooth AC voltage supplied from a commercial AC power supply using a rectifying and smoothing circuit to convert it into DC voltage, which is then supplied to the primary coil of a transformer. Conventional switching power supplies have a switching control section, and are configured to perform switching control using the duty ratio of a pulse input to a switching element connected to the primary coil, so that the output voltage on the secondary side of the transformer becomes a target voltage.

[0003] Patent Document 1 discloses a technology in which an AC voltage supplied from a commercial AC power supply is full-wave rectified by a diode bridge and smoothed by a smoothing capacitor to generate a DC voltage, which is supplied to a primary coil of a transformer, and a switching control unit controls the output voltage on the secondary side of the transformer using the duty ratio of a pulse input to an NMOS. In a switching power supply, a noise filter is provided to remove noise from the AC voltage from the commercial AC power supply, and a predetermined high-voltage rated voltage and low-voltage rated voltage are input to the switching control unit via a high-voltage rated line connected between the noise filter and the primary side of the transformer, and a low-voltage rated line connected between the noise filter and the diode bridge. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Publication No. 2019-176612 Summary of the Invention [Problem to be solved by the invention]

[0005] In response to recent demands for miniaturization of switching power supplies, high-voltage rated lines are sometimes arranged to intersect with low-voltage rated lines midway.

[0006] However, when a high-voltage rated line and a low-voltage rated line are crossed, there is a problem in that a lightning surge may transfer from the high-voltage rated line to the low-voltage rated line, which has a lower rated voltage, at the crossing point, and the lightning surge input to the low-voltage rated line may have a negative effect on the switching control unit.

[0007] SUMMARY OF THE INVENTION An object of the present invention is to provide an image forming apparatus that can prevent a switching control unit from being adversely affected even when a lightning surge is input to a low-voltage rated line. [Means for solving the problem]

[0008] In order to achieve the above object, the present invention provides an image forming unit, a transformer having a primary coil and a secondary coil, the transformer outputting an output voltage of the secondary coil to the image forming unit, a noise filter removing noise from an AC voltage supplied from a commercial AC power source, a rectifying and smoothing circuit that rectifies and smoothes the AC voltage from which noise has been removed by the noise filter and outputs the resulting voltage to the primary coil of the transformer, a switching element connected to the primary coil of the transformer, a switching control unit that performs switching control on the switching element, a high-voltage rated line connected between the rectifying and smoothing circuit and the primary side of the transformer and inputting a high-voltage power supply voltage to a high-voltage input terminal of the switching control unit, a low-voltage rated line connected between the noise filter and the rectifying and smoothing circuit and intersecting the high-voltage rated line and inputting a low-voltage voltage to a low-voltage input terminal of the switching control unit, and a voltage drop component connected between an intersection of the low-voltage rated line with the high-voltage rated line and the low-voltage input terminal of the switching control unit and dropping the voltage input to the low-voltage input terminal.

[0009] In this configuration, a voltage drop component is provided between the intersection of the low-voltage rated line and the high-voltage rated line and the low-voltage input terminal of the switching control unit, and the voltage drop component is capable of dropping the voltage input to the low-voltage input terminal. Unlike conventional switching power supplies, this configuration allows the high voltage of a lightning surge to be reduced to a low voltage before being input to the switching control unit, even if the lightning surge transitions from the high-voltage rated line to the low-voltage rated line at the intersection. Therefore, the switching power supply of this configuration can prevent a lightning surge from adversely affecting the switching control unit even if it is input to the low-voltage rated line. [Effects of the Invention]

[0010] According to the present invention, even if a lightning surge is input to a low-voltage rated line, it is possible to prevent the switching control unit from being adversely affected. [Brief explanation of the drawings]

[0011] [Figure 1] 1 is a cross-sectional view showing a schematic configuration of an image forming apparatus according to an embodiment of the present invention. [Figure 2] FIG. 2 is a block diagram showing the electrical configuration of the image forming apparatus. [Figure 3] 1 is a circuit diagram of an AC / DC conversion circuit that is a switching power supply. [Figure 4] FIG. 10 is a circuit diagram of an AC / DC conversion circuit according to a modified example in which a capacitor is provided as a voltage drop component. [Figure 5] FIG. 10 is a circuit diagram of an AC / DC conversion circuit according to a modified example in which a Zener diode is provided as a voltage drop component. DETAILED DESCRIPTION OF THE INVENTION

[0012] Hereinafter, an embodiment of the present invention will be described with reference to the drawings.

[0013] <General configuration of image forming apparatus> FIG. 1 is a cross-sectional view showing a schematic configuration of an image forming apparatus 1. As shown in FIG. The image forming apparatus 1 is, for example, a laser printer that uses toner to form an image on a sheet S. In this example, the image forming apparatus 1 is a monochrome printer that performs image formation processing for a monochrome image. The image forming apparatus 1 may also be, for example, a color printer that performs image formation processing for a full-color image.

[0014] As shown in FIG. 1, the image forming apparatus 1 includes a housing 2, a paper feed unit 20, an image forming unit 4, discharge rollers 5, and a discharge tray 7. The paper feed unit 20 supplies a sheet S toward registration rollers 21. The sheet S is paper on which an image formation process is performed. The registration rollers 21 align the leading edge of the sheet S supplied from the paper feed unit 20 and transport the sheet S toward the image forming unit 4.

[0015] The image forming unit 4 forms an image, in this example, a developer image, on the sheet S fed by the paper feed unit 20. The image forming unit 4 includes an exposure unit 10, a transfer unit 19, a charging unit 18, a developing unit 13, a fixing unit 6, a photosensitive drum 17, and a high-voltage power supply board 8 (see FIG. 2).

[0016] The exposure unit 10 includes a laser light source (not shown), a polygon mirror 11, and a reflecting mirror 12. The polygon mirror 11 is a rotating polygonal mirror with six reflecting surfaces on the side surfaces of a regular hexagonal prism. The polygon mirror 11 deflects the light beam LB emitted from the laser light source in a direction toward the photosensitive drum 17.

[0017] The exposure unit 10 deflects the light beam LB using a polygon mirror 11. The exposure unit 10 emits the light beam LB from the polygon mirror 11 to the surface of the photosensitive drum 17 via a reflecting mirror 12. The exposure unit 10 scans the surface of the photosensitive drum 17 with the light beam LB to expose the photosensitive drum 17. An electrostatic latent image that constitutes a toner image is formed on the photosensitive drum 17.

[0018] The transfer unit 19 includes a transfer roller that sandwiches the sheet S between itself and the photosensitive drum 17. The transfer unit 19 transfers the toner image from the photosensitive drum 17 to the sheet S. The charging unit 18 includes, for example, a charging wire and a grid unit (not shown). In the charging unit 18, a charging voltage is applied to the charging wire by the high-voltage power supply board 8. In the charging unit 18, a corona discharge is generated by applying a grid voltage to the grid unit. This causes the surface of the photosensitive drum 17 to be uniformly charged. The developing unit 13 includes a developing roller 14 and a toner cartridge 15 that contains toner.

[0019] <Image formation operation> In the image forming apparatus 1 configured as described above, an electrostatic latent image based on print data is formed on the surface of the photosensitive drum 17 by the light beam LB from the exposure unit 10. The development roller 14 supplies toner from inside the toner cartridge 15 to the surface of the photosensitive drum 17. This makes the electrostatic latent image visible, and a toner image is formed on the surface of the photosensitive drum 17. A sheet S fed from the paper feed unit 20 is transported to a transfer position between the photosensitive drum 17 and the transfer unit 19. This causes the toner image formed on the surface of the photosensitive drum 17 to be transferred onto the sheet S.

[0020] The sheet S onto which the toner image has been transferred is transported to the fixing unit 6 by the photosensitive drum 17 and the transfer unit 19. The fixing unit 6 includes a heating roller 22 having a heater and a pressure roller 23. The fixing unit 6 thermally fixes the toner image on the sheet S transported from the photosensitive drum 17 and the transfer unit 19. The sheet S onto which the toner image has been thermally fixed is discharged onto the discharge tray 7 by discharge rollers 5.

[0021] <Electrical configuration of image forming apparatus> Fig. 2 is a block diagram showing the electrical configuration of the image forming apparatus 1. As shown in Fig. 2, the image forming apparatus 1 includes a main circuit board MK, an image forming unit 4, and a power supply board 3 that supplies a DC voltage to the image forming unit 4.

[0022] The main board MK includes an ASIC (application specific integrated circuit) with a CPU, a ROM, a RAM, and an application circuit MK1.

[0023] The CPU of the ASIC controls the image forming unit 4 and other components in accordance with the control program read from the ROM. The ROM stores the programs executed by the CPU of the ASIC. The RAM serves as a working area from which the various control programs are read, or temporarily stores data.

[0024] One terminal of the commercial AC power supply P is connected to the first input terminal T1. The other terminal of the commercial AC power supply P is connected to the second input terminal T2. The AC / DC conversion circuit 31 on the power supply board 3 is connected to the first input terminal T1 and the second input terminal T2, respectively, and receives AC voltage from the commercial AC power supply P. The AC / DC conversion circuit 31 converts the AC voltage into a DC voltage of 24 V. The DC voltage is supplied to the image forming unit 4 via the application circuit MK1 on the main board MK.

[0025] The application circuit MK1 supplies a 24V DC voltage supplied from the power supply board 3 to the high-voltage power supply board 8 of the image forming unit 4, the photosensitive drum 17, and the exposure unit 10. The DC voltages supplied to the photosensitive drum 17 and the exposure unit 10 are used as voltages to drive them. The high-voltage power supply board 8 boosts the 24V DC voltage supplied from the application circuit MK1 to a high voltage such as 8 KV. The boosted voltage is supplied to the charging unit 18, the developing unit 13, and the transfer unit 19.

[0026] The fixing unit 6 of the image forming unit 4 is supplied with an AC voltage from a commercial AC power supply P.

[0027] <Circuit overview of switching power supply> The AC / DC conversion circuit 31 constitutes a so-called switching power supply. Hereinafter, it will be simply referred to as the "switching power supply 31" where appropriate. The circuit configuration of the switching power supply 31 is shown in FIG. 1. In FIG. 1, the switching power supply 31 converts AC voltage supplied from a commercial AC power supply P via the first input terminal T1, the second input terminal T2, the noise filter LC, and the power supply lines PS1 and PS2 into DC voltage. The switching power supply 31 includes a transformer 111, and is divided by the transformer 111 into a primary side on the left side of the figure where a primary winding L1 is provided and a secondary side on the right side of the figure where a secondary winding L2 is provided. The transformer 111 also has an auxiliary winding L3 provided on its primary side. The primary winding L1 is an example of a primary coil, and the secondary winding L2 is an example of a secondary coil.

[0028] <Primary side configuration 1> The switching power supply 31 includes a primary side rectifying and smoothing circuit 112 and a switching element 113 on the primary side of a transformer 111. The primary side rectifying and smoothing circuit 112 is an example of a rectifying and smoothing circuit.

[0029] <Primary rectifier smoothing circuit> The primary side rectifying and smoothing circuit 112 includes a diode bridge 114 and a smoothing capacitor C1.

[0030] The diode bridge 114 is a circuit configured by connecting four diodes D1, D2, D3, and D4 in a bridge configuration. Specifically, in the diode bridge 114, a series circuit of two diodes D1 and D2 is connected in parallel with a series circuit of the remaining two diodes D3 and D4. Power supply lines PS1 and PS2 are connected to the connection point of the diodes D1 and D2 in one series circuit and the connection point of the diodes D3 and D4 in the other series circuit, respectively. One connection point between the series circuit of the diodes D1 and D2 and the series circuit of the diodes D3 and D4 is connected to one end, which is the upper end in the figure, of the primary winding L1 of the transformer 111 via a wiring W1, and the other connection point is connected to the primary-side ground line G3.

[0031] One electrode of the smoothing capacitor C1 is connected to the wiring W1, and the other electrode is connected to the ground line G3.

[0032] The AC voltage supplied from the commercial AC power supply P has noise removed by a noise filter LC, and then is full-wave rectified by a diode bridge 114. The full-wave rectified voltage is smoothed by a smoothing capacitor C1, generating a DC voltage between the wiring W1 and the ground line G3.

[0033] <Switching element> The switching element 113 is an N-channel MOSFET (NMOS), and its drain terminal is connected to the other end of the primary winding L1 (the lower end in the figure) via a wiring W2. The source terminal of the switching element 113 is connected to a ground line G3. The switching element 113 performs switching operation (on / off) according to the voltage input to the gate terminal.

[0034] When the switching element 113 is turned on, a current flows through the primary winding L1 of the transformer 111, and energy is accumulated in the primary winding L1. When the switching element 113 is then turned off, the energy accumulated in the primary winding L1 is released, generating an electromotive force in the primary winding L1, and a secondary voltage according to the turns ratio between the primary winding L1 and the secondary winding L2 is generated in the secondary winding L2 of the transformer 111. The secondary voltage is generated in pulses as the switching element 113 is repeatedly turned on and off.

[0035] <Secondary side configuration> The switching power supply 31 includes a secondary-side rectifying and smoothing circuit 115 and a constant voltage circuit 116 on the secondary side of the transformer 111 .

[0036] <Secondary side rectification smoothing circuit> The secondary-side rectifying and smoothing circuit 115 includes a rectifying diode D5 and a smoothing capacitor C2. One end of the secondary winding L2 of the transformer 111, which is the upper end in the figure, is connected to the image forming unit 4 via a wiring W3, and the other end is connected to a secondary-side ground line G2, indicated by "0 V" in the figure. The rectifying diode D5 is interposed in the middle of the wiring W3 with its anode facing the secondary winding L2. One electrode of the smoothing capacitor C2 is connected to the wiring W3 on the cathode side of the rectifying diode D5, and the other electrode is connected to the ground line G2. The secondary voltage generated in the secondary winding L2 in a pulsed manner is rectified and smoothed by the secondary-side rectifying and smoothing circuit 115 and converted into a DC voltage.

[0037] <Constant voltage circuit> The constant voltage circuit 116 includes a shunt regulator IC (Integrated Circuit) 17, resistors R2, R3, and R4, and a photocoupler PC. The resistors R2 and R3 are connected in series. One end of the series circuit of the resistors R2 and R3 is connected to a wiring W3, and the other end is connected to a ground line G2. A reference voltage is preset in the shunt regulator IC17, and a voltage obtained by dividing the voltage output from the output terminal and outputting the voltage at the junction of the resistors R2 and R3 is input as a feedback voltage. The anode of the light-emitting diode PD of the photocoupler PC is connected to the wiring W3 via the resistor R4, and the cathode is connected to the shunt regulator IC17. With this configuration, a constant voltage (Output) is output from the output terminal and output to the image forming unit 4 connected to the output terminal. When the feedback voltage exceeds the reference voltage, the light-emitting diode PD of the photocoupler PC emits light.

[0038] <Primary side configuration 2> The switching power supply 31 includes a switching control unit 121 on the primary side of the transformer 111. The switching control unit 121 is made up of an IC incorporating a CPU and the like, and has a first terminal FB, a second terminal OUT, a third terminal VCC, a fourth terminal VIN, and a fifth terminal LS.

[0039] The phototransistor PT on the light-receiving side of the photocoupler PC is connected to the first terminal FB. When the feedback voltage on the secondary side exceeds the reference voltage and the light-emitting diode PD of the photocoupler PC emits light, the phototransistor PT becomes conductive and a voltage is input to the first terminal FB. The current flowing through the light-emitting diode PD increases or decreases depending on the potential difference between the feedback voltage on the secondary side and the reference voltage, so the amount of light emitted by the light-emitting diode PD increases or decreases. As the amount of light emitted increases or decreases, the current flowing through the phototransistor PT changes, and so the voltage input to the first terminal FB changes.

[0040] A second terminal OUT of the switching control unit 121 is connected to the gate terminal of the switching element 113. The switching control unit 121 controls the switching operation of the switching element 113 based on the input voltage of the first terminal FB, which changes depending on the feedback voltage on the secondary side. This keeps the feedback voltage on the secondary side at a constant value as a target voltage.

[0041] <Primary side configuration 3> Furthermore, the switching power supply 31 includes a voltage generating circuit 122 and an overvoltage protection circuit 123 on the primary side of the transformer 111 .

[0042] <Voltage generation circuit> The voltage generating circuit 122 includes a rectifier diode and a smoothing capacitor (not shown). When a pulsed secondary voltage is generated in the secondary winding L2, a pulsed AC voltage is generated in the auxiliary winding L3. The AC voltage generated in the auxiliary winding L3 is output as either a positive or negative voltage by the rectifier diode, and this positive or negative voltage is further smoothed by the smoothing capacitor and output as a DC voltage.

[0043] <Overvoltage protection circuit> The overvoltage protection circuit 123 includes an NPN transistor and a Zener diode (not shown). When the voltage output from the output terminal of the transformer 111 is a normal voltage in a steady state, a current flows through the Zener diode due to the DC voltage, and a constant voltage that is lower than the breakdown voltage of the Zener diode by the base-emitter voltage of the NPN transistor is input to the third terminal VCC of the switching control unit 121 as a drive voltage. For example, if an abnormality such as a circuit failure occurs in the switching power supply 31, the voltage output from the output terminal begins to rise, and accordingly, the voltage induced in the auxiliary winding L3 begins to rise. When the voltage of the signal input to the third terminal VCC exceeds a predetermined voltage value identified as an overvoltage, the switching control unit 121 activates the overvoltage protection function, stops the switching operation of the switching element 113, and reduces the voltage output from the output terminal. As a result, when the voltage induced in the auxiliary winding L3 exceeds the overvoltage detection level, the switching operation of the switching element 113 is stopped, and the secondary side is protected against overvoltage.

[0044] <Primary side configuration 4> Furthermore, the switching power supply 31 includes power supply lines 201 and 2202 and a detection line 300 on the primary side of the transformer 111.

[0045] <Power line> The power supply line 201 is connected between the rectifying and smoothing circuit 112 and the primary winding L1 of the transformer 111, and inputs a predetermined high-voltage rated voltage to the fourth terminal VIN of the switching control unit 121. The fourth terminal VIN is an example of a high-voltage input terminal, and the high-voltage rated voltage is an example of a high-voltage power supply voltage. The power supply line 201 is an example of a first high-voltage rated line and also an example of a high-voltage rated line. The power supply line 202 connects between the noise filter Lc and the rectifying and smoothing circuit 112 and the first power supply line 201. The power supply line 202 is an example of a second high-voltage rated line, and is also an example of a high-voltage rated line.

[0046] A voltage for initial drive is supplied to the switching control unit 121 via power supply lines 201 and 202. After the initial drive is completed, the switching control unit 121 is driven by a DC voltage generated as a drive voltage from the voltage generation circuit 122 based on the AC voltage generated in the auxiliary winding L3, which is input to the third terminal Vcc via the overvoltage protection circuit 123.

[0047] <Detection line> The detection line 300 is connected between the noise filter Lc and the rectifying and smoothing circuit 112, and inputs a predetermined low-voltage rated voltage to a fifth terminal LS of the switching control unit 121. The switching control unit 121 detects whether or not an AC voltage is being supplied from the commercial AC power supply P based on whether or not a sine wave is contained in the AC voltage guided via the detection line 300. The fifth terminal LS is an example of a low-voltage input terminal, the low-voltage rated voltage is an example of a low-voltage voltage, and the detection line 300 is an example of a low-voltage rated line.

[0048] <Structure where power supply lines and detection lines intersect> In this embodiment, in response to recent demands for miniaturization of the switching power supply 31, the power supply line 201 and the detection line 300 are arranged to cross at the crossing portion 500. The crossing of the power supply line 201 and the detection line 300 means that wiring such as a jumper wire of the power supply line 201 and the detection line 300 is laid out so as to cross over the crossing portion 500. If such an intersection structure exists, if a lightning surge is input, it will transition from the power line 201 to the detection line 300, which has a lower rated voltage, at the intersection point 500, and the lightning surge input to the detection line 300 may have a negative effect on the switching control unit 121.

[0049] <Voltage drop component placement> In this embodiment, a first resistor R10 that drops the voltage input to the fifth terminal LS of the switching control unit 121 is provided between an intersection 500 of the detection line 300 with the power line 201 and the fifth terminal LS of the switching control unit 121. One end of the first resistor R10 is connected to the detection line 300, and the other end is connected to ground. The first resistor R10 is an example of a voltage drop component.

[0050] <Additional resistor placement> In this embodiment, a second resistor R20 is further disposed between an intersection 500 of the detection line 300 with the power supply line 201 and a connection portion of the detection line 300 with the first resistor R10.

[0051] <Effects of the embodiment> As described above, in this embodiment, the first resistor R10 is connected between the intersection 500 of the detection line 300 with the power supply lines 201 and 202 and the fifth terminal LS of the switching control unit 121. The first resistor R10 can drop the voltage input to the fifth terminal LS. According to this embodiment, unlike conventional switching power supplies, even if a lightning surge transits from the power supply line 201 to the detection line 300, which has a lower rated voltage, at the intersection 500, the high voltage of the lightning surge can be reduced to a low voltage before being input to the switching control unit 121. Therefore, even if a lightning surge is input to the detection line 300, it is possible to prevent it from adversely affecting the switching control unit 121.

[0052] Particularly in this embodiment, the rectifying and smoothing circuit 112, the primary winding L1 of the transformer 111, and the fourth terminal VIN of the switching control unit 121 are connected by first power supply lines 201 and 202. A first resistor R10 is provided between an intersection 500 of the detection line 300 with the first power supply line 201 and the fifth terminal LS. According to this embodiment, even if a lightning surge that has passed through the rectifying and smoothing circuit 112 transitions from the first power supply line 201 to the detection line 300 at the intersection point 500, the first resistor R10 can drop the voltage to a low level before inputting it to the switching control unit 121.

[0053] In particular, in this embodiment, the second power supply line 202 connects between the noise filter Lc and the rectifying and smoothing circuit 112 and the first power supply line 201. According to this embodiment, even if a lightning surge that has passed through the noise filter Lc is introduced from the second power supply line 202 to the first power supply line 201 and then transitions from the first power supply line 201 to the detection line 300 at the intersection 500, the first resistor R10 can drop the surge to a low voltage before it is input to the switching control unit 121.

[0054] In this embodiment, a first resistor R10 is provided as a voltage drop component, and one end of the first resistor R10 is connected to the detection line 300 and the other end is connected to ground. According to this embodiment, the high voltage of the lightning surge that has transitioned to the detection line 300 at the intersection 500 can be reduced to a low voltage before being input to the switching control unit 121 by the voltage dividing effect of the first resistor R10.

[0055] In particular, in this embodiment, a second resistor R20 is provided on the detection line 300, and the second resistor R20 is disposed between an intersection 500 with the power supply line 201 and a connection portion with the first resistor R10. According to this embodiment, in addition to the voltage drop effect of the first resistor R10, the potential difference generated before and after the second resistor R20 on the detection line 300 can be used to more reliably drop the high voltage of a lightning surge to a lower voltage.

[0056] In particular, in this embodiment, the power supply lines 201 and 202 that provide the initial drive voltage for the switching control unit 121 are provided as high-voltage rated lines, and the detection line 300 that enables the switching control unit 121 to detect whether or not AC voltage is being supplied from the commercial AC power supply P is provided as a low-voltage rated line. According to this embodiment, even if a lightning surge transitions from the power supply lines 201, 202 with a high rated voltage to the detection line 300 with a low rated voltage at the intersection 500, the high voltage of the lightning surge can be reduced to a low voltage before being input to the switching control unit 121.

[0057] The present invention is not limited to the above-described embodiment, and various modifications are possible without departing from the spirit and technical concept of the present invention. Such modifications will be described below in order. The same reference numerals will be used to designate parts equivalent to those in the above-described embodiment, and descriptions will be omitted or simplified as appropriate.

[0058] (1) When using a capacitor as a voltage drop component In this modification, as shown in FIG. 4, in addition to the first resistor R10 described above, a capacitor C10 having one end connected to the detection line 300 and the other end connected to ground is provided as a voltage drop component. According to this modified example, the high voltage of the lightning surge that has transitioned to the detection line 300 at the intersection 500 can be reduced to a low voltage before being input to the switching control unit 121 by using the potential difference generated by the charge stored in the capacitor C10.

[0059] (2) When using a Zener diode as a voltage drop component In this modified example, as shown in FIG. 5, in addition to the aforementioned first resistor R10, a Zener diode Z10 is provided as a voltage drop component. The Zener diode Z10 has one end connected to the detection line 300 and the other end connected to ground, and a current flows when the voltage output by the detection line 300 exceeds a threshold. According to this modified example, the high voltage of the lightning surge that has transitioned to the detection line 300 at the intersection 500 can be reduced to a low voltage before being input to the switching control unit 121 by using the potential difference that occurs across the Zener diode Z10.

[0060] (3) Other In addition to the above, the methods according to the above embodiments and modifications may be used in appropriate combination.

[0061] Although not specifically illustrated, the present invention can be implemented with various modifications within the scope of the invention. [Explanation of symbols]

[0062] 1. Image forming device 4 Image forming unit 31 AC / DC conversion circuit (an example of a switching power supply) 111 Trans 112 Primary side rectifying and smoothing circuit (example of rectifying and smoothing circuit) 113 Switching element 201 Power line (first high voltage rated line, example of high voltage rated line) 202 Power line (second high voltage rated line, example of high voltage rated line) 211 Switching control section 300 detection line (example of low voltage rated line) 500 intersections C10 Capacitor (an example of a voltage drop component) L1 Primary winding (an example of a primary coil) L2 Secondary winding (an example of a secondary coil) Lc noise filter LS 5th terminal (an example of a low voltage input terminal) R10 First resistor (an example of a voltage drop component) R20 2nd resistor VIN 4th terminal (example of high voltage input terminal) Z10 Zener diode (an example of a voltage drop component)

Claims

1. an image forming unit; a transformer having a primary coil and a secondary coil, the transformer outputting an output voltage of the secondary coil to the image forming unit; a noise filter that removes noise from the AC voltage supplied from the commercial AC power supply; a rectifying and smoothing circuit that rectifies and smoothes the AC voltage from which noise has been removed by the noise filter and outputs the resulting voltage to a primary coil of the transformer; a switching element connected to the primary coil of the transformer; a switching control unit that performs switching control on the switching element; a high-voltage rated line connected between the rectifying and smoothing circuit and the primary side of the transformer, for inputting a high-voltage power supply voltage to a high-voltage input terminal of the switching control unit; a low-voltage rated line connected between the noise filter and the rectifying and smoothing circuit, intersecting the high-voltage rated line and inputting a low-voltage voltage to a low-voltage input terminal of the switching control unit; a voltage drop component connected between an intersection of the low-voltage rated line with the high-voltage rated line and the low-voltage input terminal of the switching control unit, and dropping a voltage input to the low-voltage input terminal; An image forming apparatus characterized by:

2. 2. The image forming apparatus according to claim 1, The high voltage rated wire is a first high-voltage rated line connecting between the rectifying and smoothing circuit and the primary side of the transformer, and between the high-voltage input terminal; The voltage drop component is connected between the low-voltage rated line and the first high-voltage rated line at an intersection thereof and the low-voltage input terminal; An image forming apparatus characterized by:

3. 3. The image forming apparatus according to claim 2, The high voltage rated wire is a second high-voltage rated line connecting between the noise filter and the rectifying and smoothing circuit and the first high-voltage rated line; An image forming apparatus characterized by:

4. 2. The image forming apparatus according to claim 1, The voltage drop component is a first resistor having one end connected to the low-voltage rated line and the other end connected to ground; An image forming apparatus characterized by:

5. 2. The image forming apparatus according to claim 1, The voltage drop component is a capacitor having one end connected to the low-voltage rated line and the other end connected to ground; An image forming apparatus characterized by:

6. 2. The image forming apparatus according to claim 1, The voltage drop component is a Zener diode having one end connected to the low-voltage rated line and the other end connected to ground, through which a current flows when the voltage output from the low-voltage rated line exceeds a threshold value; An image forming apparatus characterized by:

7. 2. The image forming apparatus according to claim 1, Furthermore, a second resistor disposed in the low-voltage rated line between an intersection with the high-voltage rated line and a connection with the voltage drop component; Equipped with An image forming apparatus characterized by:

8. 2. The image forming apparatus according to claim 1, The high voltage rated wire is a power supply line for supplying an initial drive voltage to the switching control unit; The low voltage rated line is a detection line for the switching control unit to detect whether the AC voltage is being supplied from the commercial AC power supply; An image forming apparatus characterized by:

Images