JP2012203044A - Fixing device and image forming apparatus - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a fixing device and an image forming apparatus which are reliably able to prevent breakage of switching elements resulting from low temperature environment or the like and able to be used stably and speedily even in low temperature environment.SOLUTION: Each of the switching elements Q1 and Q2 converts DC power to high frequency power and supplies this to an electromagnetic induction coil 231. The electromagnetic induction coil 231 generates induction current in a conductive layer 213 of a fixing roller 210, thereby causing a temperature increase. A switching drive part 421 switches between the respective ON states and OFF states of the switching elements Q1 and Q2 at predetermined drive frequencies belonging to the respective drive frequency ranges set in advance. An abnormality detection part 422 detects the inconsistency between the operations of the switching elements Q1 and Q2 assumed by the drive of the switching drive part 421 and the actual operations of the switching elements Q1 and Q2. If the abnormality detection part 422 detects any inconsistency, a drive frequency range changing part 423 raises the lower limit value of the drive frequency range.

Description

  The present invention relates to an induction heating type fixing device and an image forming apparatus.

  In recent years, induction heating type fixing devices have been adopted as fixing devices used in image forming apparatuses that print image data on paper by electrophotography, such as printers, copiers, and multi-function peripherals (MFPs). Has been. The fixing device heats and melts the toner image transferred onto the paper and presses the toner image onto the paper.

  The induction heating type fixing device includes a resonance circuit having an electromagnetic induction coil and a resonance capacitor, and a switching element connected to the resonance circuit. The switching element repeats a switching operation for switching between an on state and an off state to generate a pseudo high frequency from the DC power, and supplies the pseudo high frequency to the resonance circuit. The electromagnetic induction coil generates an alternating magnetic field in accordance with the pseudo high frequency, and an eddy current flows through the conductive heat generating member by the alternating magnetic field. The heat generating member is heated by Joule heat generated by the eddy current. The DC power supplied to the resonance circuit and the switching element is generated by rectifying a commercial AC power supply.

  Such an induction heating type fixing device can maintain the temperature of the fixing roller that applies heat to the sheet within an appropriate range by switching between power supply to the electromagnetic induction coil and power supply stoppage. In this type of fixing device, various techniques for realizing stable temperature control have been proposed.

  For example, Patent Document 1 listed below discloses an induction heating fixing device including a control unit that controls a high-frequency current supplied to an electromagnetic induction coil based on a temperature detected by a temperature detection unit that detects a surface temperature of a heat generating member. ing. According to this configuration, even if the resistance of the heat generating member increases as the surface temperature of the heat generating member increases, the frequency of the high frequency current and the input voltage to the inverter circuit are determined based on the surface temperature of the heat generating member. By controlling, the power supplied to the electromagnetic induction coil can be kept constant.

  Further, Patent Document 2 described later discloses a configuration including a resonance circuit in which a resonance capacitor is connected in parallel to a coil configured by a Japanese-style connection of an electromagnetic induction coil and a resonance coil. With this configuration, the Q value (Quality factor) of the resonance circuit is increased to reduce the switching loss in the switching element, and the heating time of the heat generating member can be shortened.

  Further, Patent Document 3 described later detects the voltage fluctuation of the DC voltage smoothed by the both-wave rectification bridge, and when the voltage fluctuation exceeding the fluctuation amount specified in advance is detected, the switching operation of the switching element is performed. A technique is disclosed in which the current is stopped and the energization voltage of the switching element is gradually increased to recover after the stop. With this configuration, it is possible to avoid the destruction of the switching element even if a voltage fluctuation due to lightning surge superposition or instantaneous power failure occurs in the commercial AC power supply.

JP 2004-061650 A JP 2008-103307 A JP 2004-266890 A

  In a fixing device including a resonance circuit having an electromagnetic induction coil and a resonance capacitor and a switching element connected to the resonance circuit, a frequency (hereinafter referred to as a drive frequency) for switching the switching element between an on state and an off state. When the vicinity of the resonance frequency of the resonance circuit is set, power can be efficiently supplied to the electromagnetic induction coil.

The amount of heat generated by the heat generating member is proportional to the square of the eddy current, and the eddy current is proportional to the high-frequency current flowing through the electromagnetic induction coil. Therefore, when a large amount of high-frequency current is passed through the electromagnetic induction coil, the heat generation amount of the heat generating member increases. The high-frequency current I (frequency f) flowing through the electromagnetic induction coil having the inductance value L is inversely proportional to the inductive reactance X _L (= 2πfL). Therefore, it is possible to flow a lot of high-frequency current I through the electromagnetic induction coil by reducing the inductive reactance X _L by lowering the driving frequency. That is, the lower the drive frequency, the greater the power used to generate heat from the heat generating member.

  From the above, in the induction heating type fixing device, the resonance frequency of the resonance circuit is set to the low frequency side of the variable range of the drive frequency, and when the heat generation amount of the heating member is reduced, the resonance frequency of the resonance circuit Control for driving the switching element is performed at a drive frequency that is distant from the high frequency side. Further, when increasing the amount of heat generated by the heat generating member, control is performed to drive the switching element at a drive frequency near the resonance frequency of the resonance circuit by lowering the drive frequency.

  On the other hand, the image forming apparatus is required to operate even in a low temperature environment such as 0 degrees Celsius. In order to make the image forming apparatus usable in a short time under such a low temperature environment, it is necessary to quickly raise the temperature of the heat generating member of the fixing device to a predetermined temperature. In this case, control is performed in which the switching element is driven at a low driving frequency and a large amount of high-frequency current is passed through the electromagnetic induction coil.

However, the inductance L of the electromagnetic induction coil has temperature dependence, and becomes a low inductance under a low temperature environment. Therefore, the smaller the inductive reactance X _L of the electromagnetic induction coils in comparison with normal temperature environment in a low temperature environment, while become more high-frequency current I flows that, the resonance frequency f ₀ of the resonant circuit due to a decrease in the inductance L = 1 / 2π (LC) ^1/2 is increased. That is, when control is performed to lower the drive frequency of the switching element for the purpose of increasing the amount of heat generated by the heat generating member in a low temperature environment, a situation occurs in which the drive frequency is lower than the resonance frequency f _{0 of the} resonance circuit. obtain. In this case, the control for lowering the driving frequency of the switching element is control for driving the switching element at a driving frequency away from the resonance frequency f ₀ . In this state, there are often cases where it is inevitable that the switching loss of the voltage and current of the switching element becomes large (so-called hard switching operation state). When such a switching loss occurs in a state where a driving frequency is low and a large high-frequency current flows through the resonance circuit, the switching element is destroyed due to Joule heat generated due to the switching loss.

  The same problem also occurs when the DC voltage used when the switching element generates the pseudo high frequency drops. FIG. 8 is a diagram illustrating an example of the drive frequency dependence of the high-frequency power used for heat generation (generation of surreal heat) in the heat generating member. In FIG. 8, the DC voltage applied to the switching element when the switching element generates the pseudo high frequency is changed as a parameter. In FIG. 8, the horizontal axis corresponds to the drive frequency, and the vertical axis corresponds to the high frequency power.

As can be understood from FIG. 8, when the DC voltage decreases at the same drive frequency, the power supplied to the heat generating member is significantly reduced. At this time, in order to maintain the power supplied to the heat generating member, it is necessary to control the drive frequency to be smaller. For example, when the DC voltage is 100 V, it is assumed that the output power P ₀ is obtained at the drive frequency f ₂ . In this case, when the DC voltage is reduced to 95 V, the same output power P ₀ cannot be obtained unless the drive frequency is set to f ₁ (f ₁ <f ₂ ). When such control occurs in a low temperature environment, for example, the switching element is more easily broken.

  Such destruction of the switching element may be prevented, for example, by setting a lower limit value in the variable range of the drive frequency so that the drive frequency is not set to a frequency lower than the resonance frequency. However, such a lower limit value needs to be set with a margin in consideration of the temperature dependence of the resonance frequency and the characteristic variation of each device. That is, a frequency that is considerably higher than the resonance frequency in a low-temperature environment must be set as the lower limit value. In such a setting, for example, in a situation where the electromagnetic induction coil has a high inductance and the resonance frequency is lowered in a high temperature environment, the drive frequency is set far away from the resonance frequency. In other words, in spite of the resonance system having the ability to input a large amount of power to the heat generating member, it becomes an inefficient state in which only a small amount of power is input, and the ability to output excessive power that is not used. There is a problem in that the cost is high because the resonance system must be provided.

  In addition, the techniques disclosed in the above-mentioned Patent Documents 1 and 2 may enable stable power supply to the electromagnetic induction coil and may shorten the heating time. When the phenomenon occurs, it is impossible to avoid the destruction of the switching element when the above phenomenon occurs. Further, in the technique disclosed in Patent Document 3, it is possible to prevent the switching element from being destroyed when the voltage is lowered, but the above phenomenon occurs when the power supply voltage is stable in a low temperature environment. In this case, the destruction of the switching element cannot be avoided. In addition, in an environment where noise frequently superimposes on the power supply, the switching operation frequently stops, so that there is a problem that a stable operation cannot be realized.

  The present invention has been made in view of the problems of the prior art as described above, and can surely prevent the switching element from being destroyed under a low temperature environment or the like, and can be stably and quickly used under a low temperature environment. It is an object of the present invention to provide a fixing device and an image forming apparatus that can be configured.

  In order to achieve the above object, the fixing device according to the present invention employs the following technical means. The present invention is an induction heating type fixing device, and includes a heating member, a resonance circuit, a switching element, a switching drive unit, an abnormality detection unit, and a drive frequency range changing unit. The resonant circuit has an electromagnetic induction coil and a resonant capacitor. The electromagnetic induction coil is disposed close to the heat generating member and generates an induced current in the heat generating member. The switching element converts DC power into high-frequency power and supplies it to the electromagnetic induction coil. A switching drive part switches the ON state (conduction state) and OFF state (cut-off state) of a switching element with the predetermined drive frequency which belongs in the drive frequency range set beforehand. The abnormality detection unit detects a mismatch between the operation of the switching element assumed by driving by the switching drive unit and the actual operation of the switching element. The drive frequency range changing unit increases the lower limit value of the drive frequency range set in the switching drive unit when the abnormality detection unit detects the mismatch.

  According to this fixing device, when a mismatch between the operation of the switching element assumed by driving by the switching drive unit and the actual operation of the switching element is detected in a low temperature environment or the like, the setting is set in the switching drive unit. The lower limit value of the driving frequency range is automatically increased. Thereby, destruction of a switching element can be prevented reliably. In this configuration, only when there is a possibility that the switching element may be destroyed, the lower limit value of the drive frequency range is increased to suppress the power input capability. Electric power can be supplied to the heat generating member without suppressing the charging capacity. As a result, it can be stably operated regardless of the ambient temperature environment.

  In the fixing device, for example, the abnormality detection unit may employ a configuration that detects the mismatch based on the potential of the output terminal of the switching element whose potential varies between an on state and an off state. Further, the drive frequency range changing unit may be configured to increase the lower limit value of the drive frequency range in a stepwise manner.

  Further, when the heating member reaches a predesignated temperature, or when the switching element is continuously driven by the switching drive unit in a state in which the above mismatch is not detected by the abnormality detection unit, the drive frequency range is reached. The changing unit may adopt a configuration in which the lower limit value of the changed drive frequency range is changed to a value before the change. As a result, when the ambient temperature rises due to heat generated by the heat generating member and the resonance frequency of the resonance circuit decreases, it is possible to automatically cancel the suppression of the power input capability.

  The fixing device further includes a voltage detection unit that detects a power supply voltage, and when the voltage detection unit detects a decrease in the power supply voltage, the drive frequency range changing unit is configured to change the drive frequency range set in the switching drive unit. A configuration in which the lower limit value is increased can also be employed. In this configuration, when it is predicted that the operation of the switching element assumed by the driving by the switching drive unit and the actual operation of the switching element will be inconsistent due to a decrease in the power supply voltage, the occurrence is prevented. be able to.

  In another aspect, the present invention can also provide an image forming apparatus including the above-described fixing device.

  According to the present invention, it is possible to reliably prevent a switching element from being destroyed in a low temperature environment or the like, and to realize a fixing device and an image forming apparatus that can be stably and promptly usable even in a low temperature environment. Can do.

1 is a schematic configuration diagram showing the overall configuration of a multifunction machine according to an embodiment of the present invention. 1 is a schematic configuration diagram illustrating a fixing device according to an embodiment of the present invention. The figure which shows the hardware constitutions of the multifunctional device in one Embodiment of this invention. 1 is a schematic diagram illustrating a fixing driving unit of a fixing device according to an embodiment of the present invention. The figure explaining operation | movement of the abnormality detection part with which the fixing device in one Embodiment of this invention is provided. The figure explaining operation | movement of the abnormality detection part with which the fixing device in one Embodiment of this invention is provided. The figure which shows the temperature rising process of the fixing device in one Embodiment of this invention. The figure which shows the relationship between the emitted-heat amount of a heat generating member, and the drive frequency of a switching element

  Hereinafter, an embodiment of the present invention will be described in more detail with reference to the drawings. Hereinafter, the present invention is embodied as a digital multifunction peripheral and a fixing device included in the digital multifunction peripheral.

  FIG. 1 is a schematic configuration diagram illustrating an example of the overall configuration of a digital multifunction peripheral according to the present embodiment. As shown in FIG. 1, the multifunction peripheral 100 includes a main body 101 including an image reading unit 120 and an image forming unit 140, and a platen cover 102 attached above the main body 101. A document table 103 is provided on the upper surface of the main body 101, and the document table 103 is opened and closed by a platen cover 102. Further, the platen cover 102 includes a document conveying device 110.

  An image reading unit 120 is provided below the document table 103. The image reading unit 120 reads an image of a document with the scanning optical system 121 and generates digital data (image data) of the image. The document can be placed on the document table 103 or the document transport device 110. The scanning optical system 121 includes a first carriage 122, a second carriage 123, and a condenser lens 124. The first carriage 122 is provided with a linear light source 131 and a mirror 132, and the second carriage 123 is provided with mirrors 133 and 134. The light source 131 illuminates the document. The mirrors 132, 133, and 134 guide reflected light from the document to the condenser lens 124, and the condenser lens 124 forms an optical image on the light receiving surface of the line image sensor 125. In the scanning optical system 121, the first carriage 122 and the second carriage 123 are provided so as to be able to reciprocate in the sub-scanning direction 135. By moving the first carriage 122 and the second carriage 123 in the sub-scanning direction 135, the image of the document placed on the document table 103 can be read by the image sensor 125. When reading an image of a document set on the document conveying device 110, the image reading unit 120 temporarily fixes the first carriage 122 and the second carriage 123 according to the image reading position, and passes the image reading position. Are read by the image sensor 125. The image sensor 125 generates image data of a document corresponding to, for example, each color of R (red), G (green), and B (blue) from the light image incident on the light receiving surface.

  The image forming unit 140 prints image data obtained by the image reading unit 120 or image data received from another device (not shown) connected to the network 162 via the network adapter 161 on a sheet.

  The image forming unit 140 includes a photosensitive drum 141. The photosensitive drum 141 rotates in one direction at a constant speed. Around the photosensitive drum 141, a charger 142, an exposure unit 143, a developing unit 144, and an intermediate transfer belt 145 are arranged in this order from the upstream side in the rotation direction. The charger 142 uniformly charges the surface of the photosensitive drum 141. The exposure device 143 irradiates the surface of the uniformly charged photoconductor drum 141 with light according to the image data, and forms an electrostatic latent image on the photoconductor drum 141. The developing device 144 attaches toner to the electrostatic latent image formed on the photosensitive drum 141 to form a toner image on the photosensitive drum 141. The intermediate transfer belt 145 transfers the toner image on the photosensitive drum 141 onto a sheet. When the image data is a color image, the intermediate transfer belt 145 transfers each color toner image onto the same sheet. The RGB color image is converted into C (cyan), M (magenta), Y (yellow), and K (black) format image data, and the image data of each color is input to the exposure unit 143.

  The image forming unit 140 feeds paper from the manual feed tray 151, the paper feed cassettes 152, 153, and 154 to the transfer unit between the intermediate transfer belt 145 and the transfer roller 146. Various sizes of paper can be placed or stored in the manual feed tray 151 and the paper feed cassettes 152, 153, and 154. The image forming unit 140 selects a sheet specified by the user or a sheet corresponding to the automatically detected document size, and pulls out the selected sheet from the manual feed tray 151 and the cassettes 152, 153, and 154 by the feeding roller 155. The pulled-out paper is sent to the transfer unit by the conveyance roller 156 and the registration roller 157. The sheet on which the toner image is transferred is conveyed to the fixing device 200 by the conveyance belt 147. The fixing device 200 has a fixing roller 210 and a pressure roller 220 with a built-in heater, and fixes a toner image on a sheet by heat and pressing force. The image forming unit 140 discharges the paper that has passed through the fixing device 200 to the paper discharge tray 148.

  The user can use the operation panel 170 provided on the upper front of the main body to give a copy start or other instruction to the multi-function device 100 as described above, or to check the status and settings of the multi-function device 100. .

  FIG. 2 is a schematic configuration diagram showing the fixing device in the present embodiment. As shown in FIG. 2, the fixing device 200 has a configuration in which a fixing roller 210 and a pressure roller 220 are arranged to face each other and have a columnar rotational axis parallel to each other. In addition, an induction heating unit 230 is disposed on the opposite side of the pressure roller 220 in the vicinity of the fixing roller 210.

  The rotation shaft of the fixing roller 210 is rotationally driven in one direction (the direction of the arrow shown in FIG. 2) at a constant speed by a driving unit (not shown). The rotation shaft of the pressure roller 220 is urged in the direction of the rotation shaft of the fixing roller 210 by an urging means (not shown), and the pressure roller 220 also rotates as the fixing roller 210 rotates. .

  The fixing roller 210 includes an internal roller 211 and a fixing belt 212 disposed on the outer peripheral surface of the internal roller 211. Although not particularly limited, in the present embodiment, a sponge roller in which silicone rubber is foamed is used as the inclusion roller 211. The fixing belt 212 has an elasticity for improving the releasability of the toner image and improving the fixability of the color image on the conductive layer 213 where the eddy current (induction current) is generated by the alternating magnetic field generated by the induction heating unit 230. It has a structure in which the layer 214 is disposed. Any material can be used for the conductive layer 213 functioning as a heat generating member as long as it can generate heat by eddy current. Here, nickel having a film thickness of about 40 μm is used. The material of the elastic layer 214 is not particularly limited, but here, a silicone rubber having a thickness of about 200 to 300 μm is used.

  The pressure roller 220 includes an elastic material having a small thermal conductivity on the outer peripheral surface. Although not particularly limited, in this embodiment, a roller having an outer diameter of 35 mm provided with an elastic layer 221 made of silicone rubber having a thickness of about 3.5 mm is used as the pressure roller 220. The elastic layer 221 is pressed against the fixing roller 210 by the urging force to form a nip portion 240. When the paper 241 on which the toner image 242 is formed is carried into the nip portion 240, the toner image 242 is fixed on the paper 241 by heating by the fixing roller 210 and pressure by the fixing roller 210 and the pressure roller 220. .

  The induction heating unit 230 has a structure in which the electromagnetic induction coil 231 and the ferrite core 233 are accommodated in the casing 232. The casing 232 has a semi-cylindrical shape and is disposed so as to cover the surface of the fixing roller 210. The electromagnetic induction coil 231 is disposed in a spiral shape over the entire width of the fixing roller 210 in the rotation axis direction, generates an alternating magnetic field that varies with time by driving a fixing driving unit described later, and causes the conductive layer 213 to generate heat. The heat generation amount of the conductive layer 213 is controlled by the fixing driving unit according to the surface temperature of the fixing roller 210. The surface temperature of the fixing roller 210 is detected by a temperature sensor 250 disposed on the upstream side immediately before the nip portion 240. The ferrite core 233 is arranged in a state that constitutes a magnetic path that efficiently guides the magnetic flux generated around the electromagnetic induction coil 231 toward the fixing roller 210. Although not shown, a semi-cylindrical ferrite core is disposed on the surface of the induction heating unit 230 opposite to the fixing roller 210 so as to cover the surface of the casing 232.

  FIG. 3 is a hardware configuration diagram of a control system in the multifunction machine. The multifunction peripheral 100 according to the present embodiment includes a CPU (Central Processing Unit) 301, a RAM (Random Access Memory) 302, a ROM (Read Only Memory) 303, an HDD (Hard Disk Drive) 304, an original transport device 110, and an image reading unit 120. A driver 305 corresponding to each operation driving unit in the image forming unit 140 is connected via an internal bus 306. The ROM 303, the HDD 304, and the like store a control program, and the CPU 301 controls the multifunction peripheral 100 in accordance with instructions of the control program. For example, the CPU 301 uses the RAM 302 as a work area, and controls the operation of each operation drive unit by exchanging data and commands with the driver 305. The HDD 304 is also used for storing image data obtained by the image reading unit 120 and image data received through the network adapter 161.

  An operation panel 170 and various sensors 307 are also connected to the internal bus 306. The operation panel 170 receives a user operation and supplies a signal based on the operation to the CPU 301. The sensor 307 includes various sensors such as an open / close detection sensor for the platen cover 102, a document detection sensor on the document table 103, a temperature sensor for the fixing device 200, a detection sensor for the conveyed paper or document, and a sensor for detecting a change in power supply voltage. Including sensors. For example, the CPU 301 executes a program stored in the ROM 303 to realize each function of the multifunction peripheral 100 and controls the operation of the multifunction peripheral 100 according to signals from these sensors. The CPU 301 implements each unit (functional block) described later by executing a program stored in the ROM 303, for example.

  FIG. 4 is a schematic diagram illustrating a fixing driving unit provided in the fixing device of the present embodiment. As shown in FIG. 4, the fixing driver 400 includes a two-stone inverter circuit 401 and a rectifier circuit 402. The rectifier circuit 402 removes the power noise of the AC power supplied from the commercial AC power supply 403, rectifies it, and converts it into DC power. The inverter circuit 401 converts the DC power output from the rectifier circuit 402 into high frequency power and supplies it to the electromagnetic induction coil 231.

  The inverter circuit 401 uses the DC capacitors output from the rectifier circuit 402 to connect resonance capacitors C1 and C2 connected in parallel to a pair of switching elements Q1 and Q2 and switching elements Q1 and Q2 connected in series, respectively. Prepare. The resonant capacitors C1 and C2 are connected in series with each other. Resonance capacitors C1 and C2 have substantially the same capacitance. The electromagnetic induction coil 231 (inductor L1) is connected in parallel with the low-side switching element Q2 (resonance capacitor C2). As the switching elements Q1 and Q2, any element that switches between an on state and an off state according to an input signal can be used. In this embodiment, an IGBT (Insulated Gate Bipolar Transistor) is used as the switching element. The switching elements Q1 and Q2 are employed.

  The resonant capacitors C1 and C2 resonate with the inductance component of the electromagnetic induction coil 231 when the switching elements Q1 and Q2 are switched, and the switching is performed in a state where the potential difference applied to both ends of the switching elements Q1 and Q2 is zero. Functions as a snubber capacitor that realizes operation (soft switching operation). That is, the resonance capacitors C1 and C2 and the inductor L constitute the resonance circuit 411.

  A driving signal is input from the switching driving unit 421 to input terminals of the switching elements Q1 and Q2 (terminals to which driving signals are input: here, gate terminals). The drive signal is a signal for switching between the ON state and the OFF state of the switching element at a predetermined drive frequency, and the switching drive unit 421 inputs the drive signal to each of the switching elements Q1 and Q2 independently. The drive frequency is configured to be changeable within a drive frequency range designated in advance. The switching drive unit 421 is configured to determine a drive frequency (power to be input to the electromagnetic induction coil 231) based on the surface temperature of the fixing roller 210 acquired by the temperature sensor 250.

  The switching drive unit 421 outputs a drive signal so that the switching element Q2 is turned off when the switching element Q1 is turned on, and the switching element Q1 is turned off when the switching element Q2 is turned on. . In addition, in order to surely prevent both of the switching elements Q1 and Q2 from being turned on, a short-circuit prevention period (dead) in which both the switching elements Q1 and Q2 are turned off immediately before any of the switching elements is turned on. Time) is provided.

  Further, the fixing device 200 according to the present embodiment includes an abnormality detection unit 422. The abnormality detection unit 422 detects a mismatch between the operation of the switching element assumed by driving by the switching drive unit 421 and the actual operation of the switching element. Although not particularly limited, here, the abnormality detection unit 422 monitors the potential Vs at the connection S between the emitter terminal of the High-side switching element Q1 and the collector terminal of the Low-side switching element Q2, as will be described in detail below. Thus, the mismatch is detected. The connection portion S is an output terminal (emitter terminal) whose potential varies between the on state and the off state in the switching element Q1, and the output whose potential varies between the on state and the off state in the switching element Q2. Terminal (collector terminal).

  Further, the fixing device 200 includes a drive frequency range changing unit 423. When the abnormality detection unit 422 detects the mismatch, the drive frequency range change unit 423 increases the lower limit value of the drive frequency range set in the switching drive unit 421.

  Note that the switching drive unit 421, the abnormality detection unit 422, and the drive frequency range change unit 423 are realized, for example, when the above-described CPU 301 executes a program stored in the ROM 303 or the HDD 304.

  Subsequently, an abnormality detection operation by the abnormality detection unit 422 will be described. FIG. 5 shows a state where the operation of the switching elements Q1 and Q2 assumed by the driving of the switching drive unit 421 and the actual operation of the switching elements Q1 and Q2 match, that is, the soft switching operation is normally performed. 6 is a time chart showing a state in which the current state (hereinafter referred to as a normal state as appropriate) is present. FIG. 5 schematically shows the gate signal (gate terminal potential) of the switching element Q1, the gate signal (gate terminal potential) of the switching element Q2, the potential Vs of the connection point S, and the current flowing through the electromagnetic induction coil 231. Yes. The gate signal for turning on the switching element is “high (high potential)”, and the drive signal for turning off the switching element is “low (low potential)”.

As shown in FIG. 5, the High-side switching element Q1 and the Low-side switching element Q2, "high" signal is input alternately with dead time t _d. The reciprocal of the value obtained by doubling the sum of the time (on time) t _on when the “high” signal is input and the dead time t _d corresponds to the drive frequency by the switching drive unit 421.

  When the switching element Q1 is in the on state and the switching element Q2 is in the off state, the DC voltage output from the rectifier circuit 402 is applied to the connection point S. That is, the potential Vs at the connection point S is “high level (high potential state)”. In addition, when the switching element Q1 is in the off state and the switching element Q2 is in the on state, the connection point S is in conduction with the low potential side (here, the ground potential) of the rectifier circuit 402. That is, the potential Vs of the connection point S becomes “low level (low potential state)”.

  Further, when the switching element Q1 is switched from the on state to the off state, the potential Vs at the connection point S decreases with the discharge of the resonance capacitor C2 (resonance between the capacitor C2 and the inductor L1), and the switching element Q2 has “high”. The potential Vs of the connection point S has dropped to the “low level” until the “signal” is input. That is, when the switching element Q2 is switched from the off state to the on state, the potential difference between the emitter and the collector of the switching element Q2 is zero.

  Further, when the switching element Q2 is switched from the on state to the off state, the potential Vs at the connection point S rises along with the discharge of the resonance capacitor C1 (resonance between the capacitor C1 and the inductor L1), and the switching element Q1 has “high”. The potential Vs of the connection point S has risen to “high level” until the “” signal is input. That is, when the switching element Q1 is switched from the off state to the on state, the potential difference between the emitter and the collector of the switching element Q1 is zero.

  By repeating the above operation, high frequency power corresponding to the drive frequency is applied to the electromagnetic induction coil 231 and a high frequency current (coil current) corresponding to the drive frequency flows.

On the other hand, FIG. 6 shows a state where the operation of the switching elements Q1 and Q2 assumed by the driving of the switching drive unit 421 does not match the actual operation of the switching elements Q1 and Q2, that is, the soft switching operation is normal. It is a time chart which shows the state (henceforth an abnormal state suitably) which is not implemented. Similarly to FIG. 5, FIG. 6 also schematically shows the gate signal of the switching element Q <b> 1, the gate signal of the switching element Q <b> 2, the potential Vs at the connection point S, and the current flowing through the electromagnetic induction coil 231. This state corresponds to a state where the drive frequency of the switching elements Q1 and Q2 is controlled to be lowered in a low temperature environment and the drive frequency is lower than the resonance frequency f _{0 of the} resonance circuit.

Similarly to FIG. 5, “high” signals are alternately input to the high-side switching element Q1 and the low-side switching element Q2 with the dead time t _d interposed therebetween. However, in FIG. 6, when the switching element Q <b> 1 switches from the on state to the off state, the potential Vs at the connection point S does not decrease to the “low level” until the “high” signal is input to the switching element Q <b> 2. Can be understood (arrow A in FIG. 6). This indicates that there is a potential difference between the emitter and the collector of the switching element Q2 when the switching element Q2 switches from the off state to the on state. Similarly, when the switching element Q2 is switched from the on state to the off state, it is understood that the potential Vs of the connection point S has not risen to the “high level” until the “high” signal is input to the switching element Q1. Yes (FIG. 6, arrow B). This indicates that there is a potential difference between the emitter and the collector of the switching element Q1 when the switching element Q1 switches from the off state to the on state.

  That is, in the arrow portions A and B, not a soft switching operation but a switching operation (hard switching operation) in a state where there is a potential difference between the emitter and the collector of the switching element occurs. When such a hard switching operation occurs under a condition where a driving frequency is low and a large high-frequency current flows, a large switching loss (current × voltage) occurs in the switching element, leading to destruction of the switching element.

  Therefore, in this embodiment, the abnormality detection unit 422 monitors the potential Vs to detect whether or not the above-described abnormal state is present.

  For example, the abnormality detection unit 422 confirms whether or not the potential Vs is “low level” at the rising edge (the arrow A portion in FIG. 6) where the switching element Q2 is switched from the off state to the on state. In this case, the abnormality detection unit 422 determines that the state is normal when the potential Vs is “low level”, and determines that the state is abnormal when the potential Vs is not “low level”. Note that whether or not the potential Vs is “low level” can be easily determined based on, for example, whether or not the potential Vs is equal to or higher than a preset threshold value.

  Further, the abnormality detection unit 422 confirms whether or not the potential Vs is “high level” at the rising edge (the arrow B portion in FIG. 6) where the switching element Q1 is switched from the off state to the on state. It can also be determined whether or not it is in a state. In this case, for example, the abnormality detection unit 422 determines that the state is normal if it exceeds a preset threshold value, and determines that the state is abnormal if it is equal to or less than the threshold value.

  Note that the abnormality detection unit 422 may determine whether or not it is an abnormal state by combining the above two determinations. Further, the abnormality detection unit 422 can also determine whether or not the switching element Q1 or Q2 is in an abnormal state based on the presence or absence of a potential difference between the emitter and the collector at the rise when the switching elements Q1 and Q2 are switched from the off state to the on state. In this case, for example, the abnormality detection unit 422 determines that the state is normal if there is no potential difference greater than or equal to a preset threshold value between the emitter and collector, and there is a potential difference greater than or equal to the preset threshold value between the emitter and collector. It is determined that the state is abnormal.

  When the abnormality detection unit 422 detects an abnormal state, the abnormality detection unit 422 notifies the drive frequency range change unit 423 to that effect. The drive frequency range changing unit 423 that has received the notification changes the lower limit value of the drive frequency range specified in advance in the switching drive unit 421 to a higher value. For example, if the lower limit value of the drive frequency range specified in advance for the switching drive unit 421 is 35 kHz, the drive frequency range changing unit 423 sets the lower limit value to a value higher by, for example, 500 Hz. Although not particularly limited, in the present embodiment, the switching drive unit 421 is configured to stop the switching operation while the drive frequency range changing unit 423 changes the drive frequency range (for example, about 500 μsec).

  At this time, when the switching drive unit 421 is driving the switching elements Q1 and Q2 at a drive frequency smaller than the lower limit after the change of the drive frequency range, the drive frequency is forcibly increased to the lower limit drive frequency after the change. To do. Thus, when the abnormality detection unit 422 detects the cancellation of the abnormal state (that is, the normal state), the switching drive unit 421 detects the switching elements Q1 and Q2 in the drive frequency range changed by the drive frequency range change unit 423. To drive.

  On the other hand, when the abnormality detection unit 422 detects an abnormal state, if the switching drive unit 421 is driving the switching elements Q1 and Q2 at a drive frequency higher than the lower limit value after the change of the drive frequency range, the abnormal state is The abnormality detection unit 422 continues to detect the abnormal state without being resolved. In this case, the drive frequency range changing unit 423 sets the lower limit value of the drive frequency range to a value higher by a predetermined amount (for example, 500 Hz). In other words, the drive frequency range changing unit 423 increases the lower limit value of the drive frequency range of the switching drive unit 421 until the abnormality detection unit 422 detects that the abnormality is detected. Note that the amount of increase in the lower limit value may be varied according to the number of detections by the abnormality detection unit 422, or the amount of increase in one time may be set large so that the abnormal state is reliably eliminated. Although not particularly limited, in this embodiment, the drive frequency range changing unit 423 causes the abnormality detection unit 422 to detect an abnormality even though the lower limit value of the drive frequency range is increased by three levels (for example, 500 Hz three times). When the state is continuously detected, the switching drive unit 421 is instructed to stop the switching operation. In this configuration, when hard switching occurs due to other causes such as breakage of the electromagnetic induction coil 231, the operation of increasing the lower limit value of the drive frequency range is repeated indefinitely. Destruction can be prevented from occurring.

  According to the above configuration, the lower limit value of the drive frequency range is changed when an abnormal state of the switching elements Q1 and Q2 is detected. Therefore, the destruction of the switching elements Q1 and Q2 can be surely prevented. Further, since the lower limit value of the drive frequency range is changed only when an abnormal state of the switching elements Q1 and Q2 is detected, a large margin is provided in the drive frequency range so that the drive frequency is not set to a frequency lower than the resonance frequency. It is not necessary to set the lower limit value to include, and the performance of the resonance system (resonance circuit) can be maximized. Therefore, in a room temperature environment or a high temperature environment, it is possible to supply power to the heat generating member without suppressing the power input capability. As a result, it can be stably operated regardless of the ambient temperature environment.

  By the way, the above-described change of the drive frequency range may be performed when a drop in the power supply voltage (the voltage of the commercial AC power supply 403 or the output voltage of the rectifier circuit 402) occurs. As described above, when the power supply voltage is lowered, the alternating magnetic field supplied from the electromagnetic induction coil 231 to the fixing roller 210 is significantly reduced. At this time, in order to maintain the alternating magnetic field supplied to the fixing roller 210, the switching drive unit 421 reduces the drive frequency within the drive frequency range. When such control occurs, the above-described abnormal state may occur suddenly. Therefore, in the fixing device 200, when the voltage detection unit 424 that detects the power supply voltage (here, the output voltage of the rectifier circuit 402) detects a decrease in the power supply voltage, the drive frequency range change unit 423 supplies the switching drive unit 421 with the drive frequency range change unit 423. Increase the lower limit of the set drive frequency range.

  For example, it is assumed that the output voltage of the rectifier circuit 402 is 100 V when it is normal, and the lower limit value of the drive frequency range specified in advance for the switching drive unit 421 is 35 kHz. In this case, when the output voltage of the rectifier circuit 402 reaches 90 V, the drive frequency range changing unit 423 sets the lower limit value of the drive frequency range to, for example, 35.5 kHz. Further, when the output voltage of the rectifier circuit 402 becomes 85V, the drive frequency range changing unit 423 sets the lower limit value of the drive frequency range to, for example, 36 kHz. Furthermore, when the output voltage of the rectifier circuit 402 reaches 80 V, the drive frequency range changing unit 423 sets the drive frequency range lower limit value to, for example, 36.5 kHz.

  In this configuration, when the power supply voltage drop is detected, the lower limit value of the drive frequency range is changed according to the voltage drop amount. For this reason, when it is predicted that the operation of the switching elements Q1 and Q2 assumed by the driving by the switching drive unit 421 and the actual operation of the switching elements Q1 and Q2 are unexpectedly inconsistent, it reacts promptly. Can be prevented.

  In addition, although the change of the drive frequency range lower limit value by the drive frequency range change unit 423 has been described above, the output power from the inverter circuit 401 to the electromagnetic induction coil 231 is suppressed when the lower limit value is changed. It is in a state. Therefore, when it is no longer necessary to change the lower limit value, it is preferable to quickly return to the lower limit value before the change. Therefore, in the fixing device 200, the abnormality detection unit 422 detects that the fixing roller 210 has reached a predetermined temperature, or that the switching elements Q1 and Q2 have been driven in a normal state for a predetermined period. Then, the drive frequency range changing unit 423 changes the lower limit value of the changed drive frequency range to the value before the change. Thereby, for example, when the ambient temperature rises due to heat generated by the fixing roller 210 and the resonance frequency of the resonance circuit 411 decreases, and the switching elements Q1 and Q2 are not likely to be in an abnormal state (or become abnormal). When it is possible to predict that the possibility has disappeared), it is possible to automatically cancel the suppression of the power input capability. Here, when the temperature sensor 250 detects 100 ° C. or when driving of the switching elements Q1 and Q2 in a normal state continues for 10 seconds, the driving frequency range changing unit 423 is set to the switching driving unit 421. The lower limit value of the drive frequency range is restored to the initial drive frequency range lower limit value. In addition, when the drive frequency range lower limit value is changed due to the power supply voltage drop described above, when the voltage detection unit 424 detects a normal voltage, the drive frequency range change unit 423 detects the initial drive frequency. What is necessary is just to return to a range lower limit.

  FIG. 7 is a diagram showing a temperature rising process of the fixing roller 210 in the fixing device 200 described above. In FIG. 7, the power actually output to the electromagnetic induction coil 231 by driving by the switching drive unit 421 is indicated by a broken line, and the required output power is indicated by a dotted line. Further, the surface temperature of the fixing roller 210 is indicated by a solid line. In FIG. 7, the horizontal axis corresponds to time. The left vertical axis corresponds to the surface temperature, and the right vertical axis corresponds to the output power. The ambient environment is an air temperature of 10 ° C. and a humidity of 15%.

  As can be understood from FIG. 7, when the fixing device 200 is activated (at “8 seconds” in FIG. 7), an output of 1000 W is requested. In contrast, the fixing device 200 increases the lower limit value of the driving frequency range to prevent the switching elements Q1 and Q2 from being in an abnormal state. As a result, the actual output power is suppressed to a value smaller than 1000W. After that, when 10 seconds elapses, the normal state continues for 10 seconds, so the drive frequency range changing unit 423 returns the lower limit value of the drive frequency range to the initial set value. As a result, in FIG. 7, it can be understood that 1000 W, which is the output power as requested, is output after the time point of “18 seconds”. As a result, the fixing device 200 can be quickly activated in a low temperature environment. In FIG. 7, the output power gradually increases between “8 seconds” and “18 seconds”. This is because the ambient temperature rises due to the heat generated by the fixing roller 210 and the resonance circuit 411 This is because the resonance frequency is gradually lowered.

Table 1 below evaluated the start-up state of the fixing device 200 when the power supply voltage was intentionally lowered at an air temperature of 10 ° C. and a humidity of 15%, with the time for the surface temperature to reach 160 ° C. as the start-up time. It is a result. For comparison, in the table, the starting time when the lower limit value of the drive frequency range is not changed is also shown as a comparative example. In the comparative example of 80V, the hard switching operation immediately after the start of the start-up is remarkable, and the switching element may be destroyed, so it is determined that the start-up is impossible.

  As shown in Table 1, in the fixing device 200 of the present embodiment, the start-up time is shorter than that of the comparative example measured using the conventional configuration at any power supply voltage. This is because, in the comparative example, since a remarkable hard switching operation did not occur, startup was possible without causing destruction of the switching element, but the difference between the drive frequency and the resonance frequency was large. It is thought to be caused. In other words, although the driving can be performed at a low driving frequency, since the switching loss is large, the state in which the power applied to the electromagnetic induction coil 231 is reduced continues for a long time, and thus the startup time is considered to be longer.

  On the other hand, in the fixing device 200 of the present embodiment, the drive frequency lower limit value is changed to a high value when the abnormality detection unit 422 detects a hard switching operation even if it is not remarkable. For this reason, although the driving is performed at a higher driving frequency than in the comparative example, since the switching loss is small, the decrease in the power applied to the electromagnetic induction coil 231 is suppressed. As a result, the rise in temperature rises faster than in the comparative example. It can be considered that the system can be started up in a short time due to a synergistic effect of the increase in heat generation efficiency caused by the decrease in the resonance frequency accompanying the temperature increase.

  As described above, according to the present invention, the fixing device and the image forming apparatus that can reliably prevent the switching element from being destroyed in a low temperature environment or the like and can be stably and promptly used even in a low temperature environment. Can be realized.

  The above-described embodiments do not limit the technical scope of the present invention, and various modifications and applications other than those already described are possible within the scope of the present invention. For example, an arbitrary circuit configuration can be adopted for the inverter circuit as long as the technical idea of the present invention is applicable.

  In the above-described embodiment, the present invention is embodied as a digital multi-function peripheral. However, the present invention is not limited to a digital multi-function peripheral, and the present invention is applied to any image processing apparatus such as a printer or a copier having the above-described fixing device. You can also

  According to the present invention, the switching element can be surely prevented from being destroyed in a low temperature environment or the like, and can be stably and promptly used in a low temperature environment, which is useful as a fixing device and an image forming apparatus.

100 MFP 200 Fixing device 210 Fixing roller 213 Conductive layer (heat generating member)
220 Pressure roller 230 Induction heating unit 231 Electromagnetic induction coil 250 Temperature sensor 400 Fixing drive unit 401 Inverter circuit 411 Resonance circuit 421 Switching drive unit 422 Abnormality detection unit 423 Frequency range change unit 424 Voltage detection unit Q1, Q2 Switching elements C1, C2 Resonant capacitor L1 Inductor (electromagnetic induction coil 231)

Claims (6)

An induction heating type fixing device,
A heating member;
An electromagnetic induction coil disposed adjacent to the heat generating member and generating an induced current in the heat generating member;
A resonant circuit having the electromagnetic induction coil and a resonant capacitor;
A switching element that converts DC power into high-frequency power and supplies the electromagnetic induction coil;
A switching drive unit that switches an on state and an off state of the switching element at a predetermined drive frequency belonging to a preset drive frequency range;
An anomaly detector that detects an inconsistency between an operation of the switching element assumed by driving by the switching driver and an actual operation of the switching element;
When the abnormality detection unit detects the mismatch, a drive frequency range change unit that increases a lower limit value of the drive frequency range set in the switching drive unit,
A fixing device.   The fixing device according to claim 1, wherein the abnormality detection unit detects the inconsistency based on a potential of an output terminal of the switching element in which a potential varies between an on state and an off state.   The fixing device according to claim 1, wherein the drive frequency range changing unit gradually increases a lower limit value of the drive frequency range.   The driving when the heating member reaches a predesignated temperature, or when the switching drive by the switching driving unit continues for a predesignated period in a state where the mismatch is not detected by the abnormality detection unit The fixing device according to claim 1, wherein the frequency range changing unit changes the lower limit value of the changed drive frequency range to a value before the change. A voltage detection unit for detecting a power supply voltage;
5. The drive frequency range change unit increases the lower limit value of the drive frequency range set in the switching drive unit when the voltage detection unit detects a decrease in power supply voltage. 6. The fixing device according to 1.   An image forming apparatus comprising the fixing device according to claim 1.

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