JP2012209055A - Induction heating device, fixing device and image forming device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To reduce the risk of degradation of a switching element in an induction heating device which uses a current resonance circuit.SOLUTION: The induction heating device includes: a power source part E1 which applies a power source voltage Vin between both ends of a series circuit of switching elements Q1, Q2; a second series circuit which is a series circuit of an exciting coil 531 and a capacitor C1 and is parallel-connected to the switching element Q1; diodes D1, D2 which are parallel-connected to the switching elements Q1, Q2; a control part 41 which generates current resonance by alternately performing first processing to turn on the switching element Q2 during an on-time period shorter than half of a resonance cycle and second processing to turn on the switching element Q1 during the on-time period for every switching cycle as time obtained by adding the on-time period and dead time; and a forced-off part 42 which turns off the switching element Q1 forcedly when a voltage Vs between both ends of the switching element Q1 exceeds a set voltage V1.

Description

  The present invention relates to an induction heating device, a fixing device using the induction heating device, and an image forming apparatus using the fixing device.

  Conventionally, in an induction heating apparatus using a voltage resonance circuit, a flyback voltage generated in a resonance circuit including an excitation coil is detected, and the high-temperature state of the object to be heated is determined based on the detected voltage, and it is determined as abnormal heating. In such a case, one that stops switching of the resonance circuit is known (see, for example, Patent Document 1).

  Further, in the induction heating device using the voltage resonance circuit, it is detected that the voltage applied to the switching element connected in series with the excitation coil exceeds the safe operating voltage range, and the excitation coil is supplied to the excitation coil according to the detection signal. A device that controls power supply is also known (see, for example, Patent Document 2).

  Also, in an induction heating device using a voltage resonance circuit, the actual inductance value of the exciting coil is calculated from the detection result of the high-frequency current flowing in the exciting coil, and the switching frequency is calculated from this inductance value and the input voltage of the commercial power supply. Further, it is known that the switching element is operated in a safe operating region by correcting the switching duty (for example, see Patent Document 3).

Japanese Patent Laying-Open No. 2005-244383 JP 2004-266880 A JP 2004-21174 A

  By the way, in the case of a voltage resonance circuit, since the flyback voltage generated in the exciting coil fluctuates with resonance, the occurrence of abnormality is detected by detecting this flyback voltage as described in Patent Document 1. It is possible to detect.

  In the case of the voltage resonance circuit, since the exciting coil and the switching element are connected in series, the flyback voltage generated by the exciting coil is applied to the switching element. Therefore, as described in Patent Document 2, it is possible to detect an abnormality in the flyback voltage based on the voltage applied to the switching element, and to detect the occurrence of the abnormality from the detection result.

  However, in the case of a current resonance circuit, the current value fluctuates due to resonance in a state where a constant power supply voltage is applied to the resonance circuit. Therefore, the current resonance circuit has a disadvantage that it is difficult to detect an abnormality from the flyback voltage as in the techniques described in Patent Documents 1 and 2.

  Further, the technique described in Patent Document 3 requires processing for calculating the actual inductance value of the exciting coil from the detection result of the high-frequency current flowing in the exciting coil, so that the switching element is operated in a safe operating region. The inconvenience is that the protection circuit becomes complicated.

  An object of the present invention is to use an induction heating device that can reduce the risk of deterioration or breakage of a switching element used for current resonance with a simple configuration in an induction heating device using a current resonance circuit. A fixing device and an image forming apparatus using the fixing device.

  The induction heating apparatus according to the present invention applies a preset power supply voltage between a first series circuit in which a first switching element and a second switching element are connected in series, and both ends of the first series circuit. A second circuit connected in parallel with the first switching element, the first switching element being a series circuit of a power supply unit, an exciting coil for heating a heated object by electromagnetic induction, and a first capacitor; A first diode connected in parallel with the power supply voltage in a direction opposite to the power supply voltage, and a second diode connected in parallel with the second switching element in a direction opposite to the power supply voltage. The ON time shorter than 1/2 of the resonance period of the resonance generated by the diode and the second series circuit and the time when both the first and second switching elements are to be turned off are set in advance. A first process for turning off the first switching element during the on-time and turning on the second switching element for each switching period, which is a time including a dead time, and the first switching element during the on-time. By alternately executing the second process of turning on and turning off the second switching element, the voltage between both ends of the control unit that causes current resonance and the first switching element is set to a preset setting voltage. And a forced-off unit that forcibly turns off the first switching element regardless of the processing of the control unit.

  According to this configuration, the power supply unit applies the power supply voltage between both ends of the first series circuit in which the first switching element and the second switching element are connected in series. A series circuit of an exciting coil and a first capacitor is connected in parallel with the first switching element. Then, the control unit turns off the first switching element for the on time and turns on the second switching element for each switching period, and turns on the first switching element for the on time and turns on the second switching element. By alternately executing the second process for turning off, current resonance is caused. That is, this induction heating circuit operates as a current resonance circuit. Since the forced-off unit forcibly turns off the first switching element regardless of the processing of the control unit when the voltage across the first switching element exceeds the set voltage, both ends of the first switching element When the voltage between them is a high voltage exceeding the set voltage, the first switching element is not turned on, and thus the risk of hard switching is reduced. As a result, it is possible to reduce the risk of deterioration of the switching element used for current resonance with a simple configuration.

  The second switching element is disposed on the higher potential side than the first switching element in the first series circuit, and includes a control terminal for controlling on / off of the second switching element. A switching element that is turned on when a voltage equal to or higher than a predetermined control voltage is applied between a connection point between the first switching element and the second switching element and the control terminal, and the control unit includes: A bootstrap circuit in which a starting capacitor, a third diode, and a resistor are connected in series; and a control power supply that outputs an output voltage equal to or higher than the control voltage from an output terminal, wherein one end of the starting capacitor is the first switching And the other end of the starting capacitor is connected to the cathode of the third diode. The anode of the third diode is connected to the output terminal via the resistor, and the control unit applies the charging voltage charged in the starting capacitor to the control terminal, thereby It is preferable to turn on the switching element.

  According to this configuration, when the first switching element is turned on, the starting capacitor is charged with the output voltage of the control power supply. And a control part turns ON a 2nd switching element by applying the charging voltage charged by the capacitor for starting to a control terminal. On the other hand, when the voltage across the first switching element exceeds the set voltage and the forced-off unit turns off the first switching element, the starting capacitor is not charged. Then, the control unit cannot turn on the second switching element because the control terminal cannot apply a voltage equal to or higher than the control voltage even if the charging voltage charged in the starting capacitor is applied to the control terminal. Therefore, the second switching element is turned off. As a result, the forced-off unit can turn off the second switching element only by turning off the first switching element, so that it is not necessary to separately provide a circuit for turning off the second switching element, and the circuit is simplified. Easy to do.

  In addition, it is preferable to further include a second capacitor connected in parallel with a series circuit of the second switching element and the exciting coil.

  According to this configuration, the power supply voltage is divided by the first capacitor and the second capacitor. And the resonance operation | movement of an induction heating apparatus is stabilized because the divided voltage is applied to an exciting coil.

  In addition, it is preferable to further include a third capacitor connected in parallel with the first switching element and a fourth capacitor connected in parallel with the second switching element.

  According to this configuration, the resonance operation of the induction heating device is stabilized by the third and fourth capacitors.

  The fixing device according to the present invention includes the above-described induction heating device and a heating member as the heated body that fixes the toner image to the paper by heating the paper on which the toner image is formed. .

  According to this configuration, the above-described induction heating device can be used as a fixing device.

  An image forming apparatus according to the present invention includes the above-described fixing device and an image forming unit that forms a toner image on the paper, and the heating member is a paper on which the toner image is formed by the image forming unit. Is heated to fix the toner image on the paper.

  According to this configuration, the above-described induction heating device can be used as a fixing device that fixes a toner image on a sheet in the image forming apparatus.

  An induction heating device having such a configuration, a fixing device using the induction heating device, and an image forming apparatus using the fixing device are used for current resonance with a simple configuration in an induction heating device using a current resonance circuit. The possibility that the switching element to be deteriorated or damaged can be reduced.

1 is a side view schematically showing an internal configuration of an image forming apparatus according to an embodiment of the present invention. FIG. 2 is a schematic cross-sectional view of the fixing device shown in FIG. 1. It is a circuit diagram which shows an example of a structure of the induction heating apparatus which concerns on one Embodiment of this invention. It is a signal waveform diagram for demonstrating the operation | movement at the time of normal operation | movement of the induction heating apparatus shown in FIG. It is explanatory drawing for demonstrating the resonance operation | movement of the induction heating apparatus shown in FIG. It is explanatory drawing for demonstrating the resonance operation | movement of the induction heating apparatus shown in FIG. It is explanatory drawing for demonstrating the resonance operation | movement of the induction heating apparatus shown in FIG. It is explanatory drawing for demonstrating the resonance operation | movement of the induction heating apparatus shown in FIG. It is explanatory drawing for demonstrating the subject when on-time Ton becomes longer than 1/2 of the resonance period Tcyc. It is a graph which shows an example of the relationship between a switching frequency and the electric power supplied to an exciting coil. It is a signal waveform diagram for demonstrating operation | movement of the induction heating apparatus shown in FIG. It is a circuit diagram which shows the modification of the induction heating apparatus shown in FIG.

  Embodiments according to the present invention will be described below with reference to the drawings. In addition, the structure which attached | subjected the same code | symbol in each figure shows that it is the same structure, The description is abbreviate | omitted. FIG. 1 is a side view schematically showing an internal configuration of an image forming apparatus 1 according to an embodiment of the present disclosure. The image forming apparatus 1 is a copying machine, a printer, a facsimile machine, or the like, and may be an image forming apparatus having a process for fixing a toner image placed on a sheet by pressurization and heating. The image forming apparatus 1 includes a main body unit 2, a stack tray 3 disposed on the left side of the main body unit 2, a document reading unit 4 disposed on the top of the main body unit 2, and a document reading unit 4. A document feeder 5 is provided.

  An input operation unit 6 is provided on the front part of the image forming apparatus 1. The input operation unit 6 displays a power key, a start key for a user to input a print execution instruction, a numeric keypad for inputting the number of copies to be printed, operation guide information for various copying operations, and the like. For example, the display unit 9 includes a liquid crystal display having a touch panel function.

  The document reading unit 4 includes an image sensor such as a CCD (Charge Coupled Device) or a CMOS (Complementary Metal Oxide Semiconductor) and a scanner unit 13 including an exposure lamp, a document table 14 formed of a transparent member such as glass, and a document reading slit. 15. The scanner unit 13 is configured to be movable by a drive unit (not shown). When reading a document placed on the document table 14, the scanner unit 13 is moved along the document surface at a position facing the document table 14, and the document image is displayed. Image data acquired while scanning is output to an image memory (not shown). When reading the document fed by the document feeding unit 5, the document is moved to a position facing the document reading slit 15 and synchronized with the document feeding operation by the document feeding unit 5 via the document reading slit 15. The image of the original is acquired and the image data is output to the image memory.

  The document feeder 5 includes a document placement unit 16 for placing a document, a document discharge unit 17 for discharging a document whose image has been read, and one document placed on the document placement unit 16. A document transport mechanism 18 including a paper feed roller, a transport roller (not shown), and the like for feeding the paper one by one to a position facing the document reading slit 15 and discharging it to the document discharge section 17 is provided.

  Further, the document feeding unit 5 is provided so as to be rotatable with respect to the main body unit 2 so that the front side thereof can move upward. By moving the front side of the document feeder 5 upward to open the upper surface of the document table 14, the user can place a read document, for example, a book in a spread state, on the upper surface of the document table 14. ing.

  The main body 2 includes a plurality of paper feed cassettes 19, a paper feed roller 20 that feeds the paper from the paper feed cassette 19 one by one and transports it to the image forming unit 21, and an image on the paper transported from the paper feed cassette 19. And an image forming unit 21 for forming the image.

  The image forming unit 21 outputs a laser beam or the like based on the image data acquired by the scanner unit 13 to expose the photosensitive drum 22, and forms an electrostatic latent image on the surface of the photosensitive drum 22. A developing unit 24 that forms a toner image by attaching toner to the surface of the photosensitive drum 22 on which the electrostatic latent image is formed; and a transfer unit 25 that transfers the toner image on the photosensitive drum 22 to a sheet. Prepare.

  The fixing device 28 includes a belt unit 26 and a pressure roller 27 that heats the sheet on which the toner image is transferred to melt and fix the toner image on the sheet.

  In the paper conveyance path in the image forming unit 21, conveyance roller pairs 30 and 31 that convey the paper to the stack tray 3 or the discharge tray 29 are provided.

  When images are formed on both sides of a sheet, after the image is formed on one side of the sheet by the image forming unit 21, the sheet is nipped between the pair of transport rollers 30 and 31 on the discharge tray 29 side. To do. In this state, the conveyance roller pairs 30 and 31 are reversed to switch back the paper, and the conveyance roller pairs 38 and 39 send the paper to the paper conveyance path 32 and convey it again to the upstream area of the image forming unit 21. After the image is formed on the other side by 21, the paper is discharged to the stack tray 3 or the discharge tray 29.

  FIG. 2 is a schematic cross-sectional view of the fixing device 28 shown in FIG. The fixing device 28 includes an IH coil unit 530 that heats the belt 263 and the second roller 262 in addition to the belt unit 26 and the pressure roller 27 described above. The IH coil unit 530 is an example of an induction heating device.

  The belt unit 26 includes a first roller 261 disposed opposite to the pressure roller 27, a second roller 262, and a belt 263 that is wound around the first roller 261 and the second roller 262. Prepare. The pressure roller 27 sandwiches the belt 263 in cooperation with the first roller 261. A flat nip is formed between the pressure roller 27 and the belt 263.

  The belt 263 includes, for example, a nickel electroformed base material having a thickness dimension of about 30 μm or more and about 50 μm or less, a silicon rubber layer laminated on the nickel electroformed base material, and a release layer formed on the silicon rubber layer ( For example, a PFA layer). The belt 263 corresponds to an example of a heating member.

  For example, the second roller 262 is formed in a cylindrical shape having an outer diameter of 30 mm. The second roller 262 includes a cylindrical iron base and a release layer (for example, a PFA layer) having a wall thickness of 0.2 mm to 1.0 mm formed on the outer peripheral surface of the iron base.

  The first roller 261 is formed in a columnar shape, for example. The first roller 261 includes a core metal roller having an outer diameter of 45 mm made of stainless steel and a sponge layer made of silicon rubber having a thickness of 5 mm to 10 mm covering the outer peripheral surface of the core metal roller.

  The pressure roller 27 is formed in a cylindrical shape having an outer diameter of 50 mm, for example. The pressure roller 27 includes a core metal roller made of stainless steel, a sponge layer made of silicon rubber having a thickness of 2 mm to 5 mm and covering a peripheral surface of the core metal roller, and a release layer (for example, a PFA layer). The metal core of the pressure roller 27 may be formed using, for example, Fe or Al. A Si rubber layer may be formed on the core material. Further, a fluororesin layer may be formed on the surface layer of the Si rubber layer.

  The IH coil unit 530 includes an excitation coil 531 that induction heats the belt 263 and the second roller 262, and a platform 200 that supports the excitation coil 531. The IH coil unit 530 includes a pair of side cores 533, a pair of arch cores 534, and a center core 535 that define the path of magnetic lines of force in the magnetic field generated by the excitation coil 531.

  The pair of arch cores 534 and the center core 535 are arranged symmetrically with respect to a straight line L1 connecting the rotation center axis C1 of the first roller 261, the rotation center axis C2 of the second roller 262, and the rotation center axis C3 of the pressure roller 27, respectively. It is installed. A center core 535 is disposed between the pair of arch cores 534. The pair of side cores 533, the pair of arch cores 534, and the center core 535 are supported by the platform 200.

  The pair of side cores 533 and the pair of arch cores 534 are configured using, for example, ferrite. Further, the center core 535 is configured using, for example, a columnar conductive shaft 538 and a cylindrical magnetic cylinder 539 covering the conductive shaft 538.

  The platform 200 includes a coil support part 201 including a curved surface along the peripheral surface of the second roller 262. The coil support unit 201 supports the excitation coil 531 that generates a magnetic field for induction heating the belt 263 and the second roller 262.

  The exciting coil 531 forms a looped coil surface in a space surrounded by the coil support portion 201, the side core 533, and the arch core 534.

  As described above, the coil support portion 201 is formed along the arcuate outer surface of the belt 263 on the second roller 262. The exciting coil 531 is arranged so as to surround the coil support portion 201. As a result, the exciting coil 531 is continuously provided along the curved coil support part 201 and forms a loop-shaped coil surface having a substantially arc-shaped cross section. Then, substantially half of the second roller 262 is surrounded by the exciting coil 531.

  When a high frequency current flows through the exciting coil 531, the belt 263 and the second roller 262 are induction heated. Further, the drive mechanism (not shown) drives and rotates the first roller 261 in the clockwise direction in FIG. 2, whereby the second roller 262 is driven and rotated, and the heated belt 263 is replaced with the pressure roller 27 and the first roller 261. Head toward the nip. As a result, the paper P nipped by the pressure roller 27 and the belt 263 is heated, and the toner image is fixed on the paper P.

  The fixing device 28 may include a heating roller that does not use a belt instead of the belt unit 26.

  FIG. 3 is a circuit diagram showing an example of the configuration of the induction heating device 40 according to one embodiment of the present invention. 3 includes a power supply unit E1, a switching element Q1 (first switching element), a switching element Q2 (second switching element), a diode D1 (first diode), a diode D2 (second diode), An excitation coil 531, a capacitor C1 (first capacitor), a control unit 41, and a forced-off unit 42 are provided.

  The power supply unit E1 is a DC power supply circuit that generates a power supply voltage Vin of 141 V DC from a commercial AC power supply voltage, for example.

  The switching elements Q1, Q2 are, for example, IGBTs (Insulated Gate Bipolar Transistors). As the switching elements Q1 and Q2, various switching elements such as FET (Field Effect Transistor) and bipolar transistor can be used. Switching elements Q1 and Q2 are connected in series to form a first series circuit.

  The collector of the switching element Q2 is connected to the positive output terminal of the power supply unit E1, the emitter of the switching element Q2 is connected to the collector of the switching element Q1, and the emitter of the switching element Q1 is connected to the negative output terminal of the power supply unit E1. It is connected. The negative output terminal of the power supply unit E1 is a circuit ground.

  Further, the cathode of the diode D1 is connected to the collector of the switching element Q1, the anode of the diode D1 is connected to the emitter of the switching element Q1, and the switching element Q1 and the diode D1 are connected in parallel. The cathode of the diode D2 is connected to the collector of the switching element Q2, the anode of the diode D2 is connected to the emitter of the switching element Q2, and the switching element Q2 and the diode D2 are connected in parallel.

  One end of the exciting coil 531 is connected to the collector of the switching element Q1, and the other end of the exciting coil 531 is connected to the emitter of the switching element Q1 via the capacitor C1. That is, a second series circuit that is a series circuit of the exciting coil 531 and the capacitor C1 is connected in parallel with the switching element Q1.

  When the inductance of the exciting coil 531 is L and the capacitance of the capacitor C1 is C, the resonance frequency fr of the current resonance circuit configured using the exciting coil 531 and the capacitor C1 is expressed by the following equation (1). The

fr = 1 / {2 × π × (L × C) ^1/2 } (1)
Here, the inductance L of the exciting coil 531 includes the self-inductance of the exciting coil 531 itself, the second roller 262, the belt 263, the side core 533, the arch core 534, the center core 535, and the exciting coil 531 that exist around the exciting coil 531. And mutual inductance generated by magnetic coupling between the two.

  The control unit 41 includes a microcontroller 411, a gate driver 412, a bootstrap circuit 413, and a control power supply E2. The control power supply E2 is a power supply circuit that outputs a control voltage, for example, 15V, for turning on the switching elements Q1, Q2.

  The bootstrap circuit 413 includes a resistor Rb, a diode D3 (third diode), and a capacitor Cb (starting capacitor) connected in series in this order. One end of the capacitor Cb is connected to the connection point P1 of the switching elements Q1 and Q2, and the other end of the capacitor Cb is connected to the cathode of the diode D3. The anode of the diode D3 is connected to one end of the resistor Rb, and the other end of the resistor Rb is connected to the control power source E2. The connection point P2 between the capacitor Cb and the diode D3 is connected to the gate driver 412.

  The gate driver 412 is a drive circuit that turns on and off the switching elements Q1 and Q2 in accordance with a control signal from the microcontroller 411. In order to turn on the IGBTs such as the switching elements Q1 and Q2, it is necessary to apply a voltage equal to or higher than a predetermined control voltage (for example, 15 V) between the gate (control terminal) and the emitter (source in the case of MOSFET). is there.

  However, since the switching elements Q1 and Q2 are connected in series, a voltage of 15V is applied to the gate of the switching element Q2 when the switching element Q1 is on in order to turn on the switching element Q2 on the high potential side. However, when the switching element Q1 is off, it is necessary to apply a voltage obtained by adding 15 V to the voltage Vs at the connection point P1 of the switching elements Q1 and Q2 to the gate of the switching element Q2.

  Therefore, the gate driver 412 cooperates with the bootstrap circuit 413 to generate gate signals G1 and G2 for turning on and off the switching elements Q1 and Q2 in accordance with a control signal from the microcontroller 411. Output to the gates of the switching elements Q1, Q2.

  As the gate driver 412, for example, a driver element manufactured by Toshiba Corporation, M81709 can be used.

  The microcontroller 411 is a control in which, for example, a central processing unit (CPU), a read only memory (ROM), a random access memory (RAM), a timer circuit, and a pattern generator capable of generating an arbitrary signal waveform are integrated on one chip. Circuit. The microcontroller 411 outputs, for example, a control signal S1 for controlling on / off of the switching element Q1 to the gate driver 412 via the AND gate 421 by executing a control program stored in a built-in ROM in advance. A control signal S2 for controlling on / off of the element Q2 is output to the gate driver 412.

  FIG. 3 conceptually shows the internal configuration of the gate driver 412. The gate driver 412 shown in FIG. 3 includes buffers B1 and B2 and switching elements SW1 and SW2. The switching element SW1 is connected between the control power supply E2 and the gate of the switching element Q1, and the switching element SW2 is connected between the connection point P2 and the gate of the switching element Q2.

  Note that the control signal S1a output from the AND gate 421 is input to the gate driver 412, but in order to simplify the description, the control signal S1 output from the microcontroller 411 is input to the gate driver 412 as it is. It will be described as being done.

  Since the emitter of the switching element Q1 on the low potential side is connected to the ground, when the control signal S1 is at the high level, the buffer B1 turns on the switching element SW1, and when the control signal S1 is at the low level, the buffer B1 By turning off the switching element SW1, on / off of the switching element Q1 can be controlled in accordance with the control signal S1.

  On the other hand, when the switching element Q1 is turned on, a current flows from the control power supply E2 to the ground via the resistor Rb, the diode D3, the capacitor Cb, and the switching element Q1, and the capacitor Cb is charged to the control voltage 15V of the control power supply E2. Then, the voltage at the connection point P2 becomes a voltage obtained by adding the terminal voltage (charge voltage) of the capacitor Cb to the voltage at the connection point P1, that is, the emitter voltage of the switching element Q2.

  Thus, when the capacitor Cb is charged to the control voltage 15V, the buffer B2 turns on the switching element SW2 when the control signal S2 is at a high level regardless of whether the switching element Q1 is on or off. Can be applied between the emitter and base of the switching element Q2. As a result, the switching element Q2 can be turned on regardless of whether the switching element Q1 is on or off.

  The forced-off unit 42 includes an AND gate 421, a comparator 422, a reference voltage source E3, and resistors R1 and R2. The connection point P1 is connected to the ground through a series circuit of resistors R1 and R2. As a result, the voltage at the connection point P1, that is, the voltage Vs between both terminals of the switching element Q1, is divided by the resistors R1 and R2, and is level-converted into the input voltage range of the comparator 422. ) Terminal.

  The reference voltage source E3 is connected to the plus (+) terminal of the comparator 422. The reference voltage source E3 outputs, as a reference voltage, a voltage obtained by multiplying a set voltage V1 set in advance as a voltage Vs to turn off the switching element Q1 by a voltage dividing ratio of the resistors R1 and R2. Thereby, the comparator 422 compares the voltage Vs with the set voltage V1 indirectly by comparing the voltage divided by the resistors R1 and R2 with the reference voltage output from the reference voltage source E3. .

  The comparator 422 outputs a low level signal S3 to the AND gate 421 when the voltage Vs exceeds the set voltage V1, and outputs a high level signal S3 to the AND gate 421 when the voltage Vs is less than the set voltage V1. Output.

  For example, the reference voltage source E3 may generate a reference voltage by resistance-dividing the output voltage of the control power source E2, or may be a constant voltage circuit that generates a reference voltage. The set voltage V1 is set to, for example, 50V when the power supply voltage Vin is 141V.

  The AND gate 421 outputs a control signal S1a obtained by ANDing the control signal S1 and the signal S3 to the gate driver 412. Thereby, when the voltage Vs exceeds the set voltage V1, a low level signal S3 is output from the comparator 422 to the AND gate 421, and the control signal S1a is forcibly set to the low level. As a result, the switching element Q1 is forcibly turned off by the gate driver 412.

  Next, operation | movement of the induction heating apparatus 40 comprised in this way is demonstrated. FIG. 4 is a signal waveform diagram showing the gate signals G1 and G2, the voltage Vs, and the coil current IL flowing through the exciting coil 531 during normal operation of the induction heating device 40 shown in FIG. The horizontal axis shows the passage of time.

  FIG. 4 shows that the switching element Q1 is turned on when the gate signal G1 is at a high level (H), and the switching element Q1 is turned off when the gate signal G1 is at a low level (L). Further, the switching element Q2 is turned on when the gate signal G2 is at a high level (H), and the switching element Q2 is turned off when the gate signal G2 is at a low level (L).

  Hereinafter, in order to simplify the description, description of individual operations of the microcontroller 411, the gate driver 412, and the bootstrap circuit 413 will be omitted, and the control unit 41 outputs the gate signals G1 and G2, whereby the switching elements Q1, Description will be made assuming that Q2 is turned on / off.

  The control unit 41 performs switching during the on-time Ton for each switching cycle Tsw that is a time obtained by adding the on-time Ton shorter than 1/2 of the resonance cycle Tcyc obtained as the reciprocal of the resonance frequency fr and the dead time Td. A second process (timing t1 to t2) for turning on the element Q1 and turning off the switching element Q2, and a first process (timing t3 to t4) for turning off the switching element Q1 and turning on the switching element Q2 during the on time Ton; Execute alternately.

  The control unit 41 provides a dead time Td for turning off both the switching elements Q1 and Q2 between the first process and the second process, so that the switching elements Q1 and Q2 are simultaneously turned on and a short-circuit current flows. Is preventing. Further, the control unit 41 adjusts the amount of power supplied to the exciting coil 531 by increasing or decreasing the on-time Ton.

  Next, the resonance operation of the induction heating device 40 will be described. First, at timing t3, the control unit 41 sets the gate signal G1 to the low level and the gate signal G2 to the high level to turn off the switching element Q1 and turn on the switching element Q2. Then, as indicated by a current I1 in FIG. 5, a current flows from the power source E1 to the switching element Q2, the exciting coil 531, and the capacitor C1, and current resonance is started.

  Next, at timing t4 when the on-time Ton has elapsed from timing t3, the control unit 41 turns off the switching element Q2. Then, as indicated by current I2 in FIG. 6, the current I2 flows in the forward direction of the diode D1 due to the back electromotive force of the exciting coil 531. At this time, since the voltage Vs is equal to the forward voltage of the diode D1, it is about 0.7 V which is almost zero.

  Next, at the timing t5 when the dead time Td has elapsed from the timing t4, the control unit 41 turns on the switching element Q1 while keeping the switching element Q2 off with the gate signal G1 at the high level and the gate signal G2 at the low level. Let Then, as a result of the soft switching that is turned on when the voltage Vs is substantially zero, the switching element Q1 is reduced in power loss in the switching element Q1, and the possibility that the switching element Q1 is deteriorated or damaged is reduced. Is done.

  Thus, when the forward current flows through the diode D1 and the voltage Vs is almost zero, the switching cycle Tsw, the on time Ton, and the dead time Td are set in advance so that the switching element Q1 is turned on. ing. The on time Ton is shorter than 1/2 of the resonance period Tcyc.

  Further, at the timing t5, the voltage Vs does not reach the set voltage V1, so the comparator 422 outputs a high level signal S3 to the AND gate 421. As a result, since the same signal as the control signal S1 is output from the AND gate 421 to the gate driver 412 as the control signal S1a, the switching element Q1 is not forcibly turned off by the forced-off unit 42.

  Since the timing t5 and the timing t1 are the same operation, the operation after the timing t5 will be described below with reference to the signal waveforms at the timings t1 to t3. After timing t1 (t5), a current I2 due to resonance between the exciting coil 531 and the capacitor C1 flows, and when a predetermined time elapses, the direction of the current flowing due to secondary resonance is reversed. At this time, since the switching element Q1 is already on, the current I3 shown in FIG. 7 flows through the switching element Q1.

  Then, at the timing t2 when the on-time Ton has elapsed from the timing t1, the control unit 41 sets the gate signal G1 to the low level to turn off the switching element Q1. Then, the resonance current cannot flow through the switching element Q1, and the resonance current flows through the diode D2, as shown as a current I4 in FIG. At this time, since the voltage between both terminals of the switching element Q2 is equal to the forward voltage of the diode D2, it is about 0.7V which is almost zero.

  Next, at the timing t3 when the dead time Td has elapsed from the timing t2, the control unit 41 turns on the switching element Q2 while keeping the switching element Q1 turned off by setting the gate signal G2 to the high level. As a result, the switching element Q2 becomes soft switching that is turned on with the voltage between the terminals being almost zero. As a result, power loss in the switching element Q2 is reduced, and the switching element Q2 may be deteriorated or damaged. Reduced.

  As described above, the switching element Q2 is switched so that the switching element Q2 is turned on when a forward current flows through the diode D2 and the voltage between both terminals of the switching element Q2 is almost zero. A cycle Tsw, an on time Ton, and a dead time Td are set in advance.

  Thereafter, the operation after the timing t3 described above is repeated. In FIG. 4, processing at timings t3 to t4 corresponds to an example of the first processing, and processing at timings t1 to t2 corresponds to an example of the second processing.

  FIG. 9 is an explanatory diagram for explaining a problem when the on-time Ton is longer than ½ of the resonance period Tcyc due to fluctuations in the resonance frequency fr. In FIG. 9, the operation when the induction heating device 40 does not include the forced-off unit 42 is described.

  First, similarly to the timing t3 in FIG. 4, at the timing t13 in FIG. 9, the control unit 41 sets the gate signal G1 to the low level and the gate signal G2 to the high level to turn off the switching element Q1 and turn on the switching element Q2. Then, as indicated by a current I1 in FIG. 5, a current flows from the power source E1 to the switching element Q2, the exciting coil 531, and the capacitor C1, and current resonance is started.

  Here, at time t14 when 1/2 of the resonance period Tcyc has elapsed from timing t13 (timing t16) and the on-time Ton has elapsed thereafter from timing t13, the control unit 41 turns off the switching element Q2.

  Then, as indicated by current I2 in FIG. 6, the current I2 flows in the forward direction of the diode D1 due to the back electromotive force of the exciting coil 531, but the resonance period Tcyc has already been reached from timing t13 corresponding to the start of the resonance period. Since 1/2 has elapsed, the direction of the resonance current is reversed before the dead time Td elapses, and the current I4 shown in FIG. 8 starts to flow through the diode D2. Then, the voltage Vs rises toward the power supply voltage Vin (= 141 V) of the power supply unit E1, and becomes a high voltage.

  In this state, when the dead time Td elapses and the timing t15 is reached, the control unit 41 sets the gate signal G1 to the high level to turn on the switching element Q1. Then, the switching element Q1 is so-called hard switching which is turned on in a state where a high voltage is applied between both terminals. Therefore, the switching element Q1 may be damaged.

  Similarly, if the on-time Ton at the timings t11 to t12 is longer than ½ of the resonance period Tcyc, the switching element Q2 may be hard-switched and damaged at the timing t13.

  FIG. 10 shows an example of the relationship between the switching frequency fs (= 1 / (2 × Tsw)) that is the reciprocal of twice the switching period Tsw and the power supplied to the exciting coil 531, that is, the heat generation amount of the fixing device 28. It is a graph to show.

  The horizontal axis of FIG. 10 indicates the switching frequency fs, and the vertical axis indicates the power supplied to the exciting coil 531. The inductance of the exciting coil 531 is 30 μH, and the capacitance of the capacitor C1 is 0.68 μF. Then, the resonance frequency fr is about 35 kHz.

  As shown in FIG. 10, the relationship between the switching frequency fs and the power supplied to the exciting coil 531 varies depending on the temperature of the belt 263 as well as the power supply voltage Vin. The resonance frequency fr also changes due to the influence of the component characteristics of the exciting coil 531 and the capacitor C1, the variation of assembly, and the temperature of the belt 263.

  If the switching frequency fs is lower than the resonance frequency fr, the on-time Ton becomes longer than ½ of the resonance period Tcyc, which may cause hard switching. Therefore, it is necessary to set the switching frequency fs so that the switching frequency fs does not overlap with the variation range of the resonance frequency fr, for example, the range of 34 kHz to 36 kHz.

  In order to prevent the switching frequency fs from being below the resonance frequency fr including the characteristic variation as described above, a sufficiently large margin may be provided so that the switching frequency fs is sufficiently larger than the resonance frequency fr.

  However, as shown in FIG. 10, the power supplied to the exciting coil 531 increases when the switching frequency fs is small and the resonance frequency fr is close. Therefore, if a large margin is provided so that the switching frequency fs is larger than the resonance frequency fr, the performance of the induction heating device 40 heating the fixing device 28 is deteriorated.

  In view of this, the induction heating device 40 includes the forced-off unit 42, so that the switching frequency fs can be easily set to a very close frequency close to the resonance frequency fr. This facilitates increasing the amount of heat generated by the fixing device 28.

  FIG. 11 is a signal waveform diagram for explaining the operation of the induction heating apparatus 40 shown in FIG. The horizontal axis shows the passage of time. The operations at timings t1 to t3 are the same as the timings t1 to t3 shown in FIG.

  Then, when 1/2 of the resonance period Tcyc becomes shorter than the on-time Ton due to variations in the characteristics of parts, the influence of temperature, etc., 1/2 of the resonance period Tcyc elapses from the timing t3, and then the on-time Ton from the timing t3. At the timing t4 when elapses, the microcontroller 411 sets the control signal S2 to the low level and turns off the switching element Q2.

  Then, as indicated by current I2 in FIG. 6, the current I2 flows in the forward direction of the diode D1 due to the back electromotive force of the exciting coil 531, but the resonance period Tcyc has already been reached from the timing t3 corresponding to the start of the resonance period. Since 1/2 has elapsed, the direction of the resonance current is reversed before the dead time Td elapses, and the current I4 shown in FIG. 8 starts to flow through the diode D2. Then, the voltage Vs rises toward the power supply voltage Vin (= 141 V) of the power supply unit E1, and becomes a high voltage.

  In this state, when the dead time Td elapses and the timing t5 is reached, the microcontroller 411 attempts to turn on the switching element Q1 by setting the control signal S1 to a high level. However, at the timing t5, the voltage Vs exceeds the set voltage V1 (= 50V), so the comparator 422 outputs a low level signal S3 to the AND gate 421. As a result, the control signal S1a output from the AND gate 421 is set to the low level and output to the gate driver 412.

  As a result, regardless of the operation of the microcontroller 411, the switching element Q1 is forcibly maintained in the off state. As a result, the switching element Q1 is prevented from being hard-switched, so that the risk of damage to the switching element Q1 is reduced.

  Furthermore, as described above, when the switching element Q1 is turned on, the capacitor Cb is charged to the control voltage 15V. Then, the gate driver 412 applies the control voltage 15V between the emitter and base of the switching element Q2 by applying the voltage at the connection point P2 of the bootstrap circuit 413 to the switching element Q2, so that the switching element Q2 is turned on. It has become.

  Therefore, when the switching element Q1 is maintained in the off state by the forced-off unit 42, the capacitor Cb is discharged from, for example, the resistors R1 and R2 without being charged, so that the voltage between both terminals of the capacitor Cb is changed. The voltage is lower than that required to turn on the switching element Q2. As a result, the gate driver 412 cannot turn on the switching element Q2, and the switching element Q2 is turned off.

  As described above, the forced-off unit 42 forcibly turns off the switching element Q1, so that the switching element Q2 is also turned off. As a result, the risk of hard switching occurring in the switching elements Q1 and Q2 can be reduced.

  In addition, although the example which provides the forced OFF part 42 only with respect to the switching element Q1 of the low voltage side was shown, you may provide the forced OFF part 42 also with respect to the switching element Q2 of the high voltage side. Then, when the voltage between both terminals of the switching element Q2 exceeds the set voltage V1, the switching element Q2 may be forcibly turned off.

  In addition, you may provide the capacitor C2 in parallel with the series circuit of the switching element Q2 and the exciting coil 531 like the induction heating apparatus 40a shown in FIG. The capacitor C2 has substantially the same capacitance as the capacitor C1. Thereby, the power supply voltage Vin is divided by the capacitors C1 and C2, and the divided voltage is applied to one end of the exciting coil 531, thereby improving the stability of resonance.

  Further, a capacitor C3 connected in parallel with the switching element Q1 and a capacitor C4 connected in parallel with the switching element Q2 may be further provided. This improves the stability of resonance.

  In addition, in the induction heating apparatus 40a shown in FIG. 12, it is good also as a structure which is not provided with the capacitor C2, and is good also as a structure which is not provided with the capacitors C3 and C4.

DESCRIPTION OF SYMBOLS 1 Image forming apparatus 2 Main body part 3 Stack tray 18 Document conveying mechanism 21 Image forming part 22 Photosensitive drum 23 Optical unit 24 Developing part 25 Transfer part 26 Belt unit 27 Pressure roller 28 Fixing device 29 Discharge tray 30, 31 Conveying roller pair 32 Paper transport path 38 Transport roller pair 40, 40a Induction heating device 41 Control unit 42 Forced off unit 200 Platform 201 Coil support unit 261, 262 Roller 263 Micro-controller 412 Gate driver 413 Bootstrap circuit 421 And gate 422 Comparator 530 Coil unit 531 Excitation coil 533 Side core 534 Arch core 535 Center core 538 Conductive shaft 539 Magnetic cylinder B1, B2 Buffer C1, C2, C3, C4, Cb Capacitor D1 , D2, D3 Diode E1 Power supply E2 Control power supply E3 Reference voltage source fr Resonance frequency fs Switching frequency G1, G2 Gate signal IL Coil current P Paper Q1, Q2 Switching elements R1, R2, Rb Resistors S1, S1a, S2 Control signal S3 Signal SW1, SW2 Switching element Tcyc Resonance cycle Td Dead time Ton On time Tsw Switching cycle V1 Setting voltage Vin Power supply voltage Vs Voltage

Claims (6)

A first series circuit in which a first switching element and a second switching element are connected in series;
A power supply unit that applies a preset power supply voltage between both ends of the first series circuit;
A second circuit that is a series circuit of an exciting coil and a first capacitor that heats the object to be heated by electromagnetic induction, and is connected in parallel with the first switching element;
A first diode connected in parallel with the first switching element in a direction opposite to the power supply voltage;
A second diode connected in parallel with the second switching element in a direction opposite to the power supply voltage;
A switching cycle that is a time obtained by adding an ON time shorter than ½ of a resonance cycle of resonance generated by the second series circuit and a dead time set in advance as a time to turn off both the first and second switching elements. Every time, a first process of turning off the first switching element and turning on the second switching element during the on time, and turning on the first switching element and turning off the second switching element during the on time. A controller that causes current resonance by alternately executing the second process;
An induction including a forcible off unit that forcibly turns off the first switching element regardless of the processing of the control unit when the voltage across the first switching element exceeds a preset voltage. Heating device. The second switching element is
The first series circuit includes a control terminal that is disposed on a higher potential side than the first switching element and controls on / off of the second switching element, and includes the first switching element and the first switching element. 2 is a switching element that is turned on when a voltage equal to or higher than a predetermined control voltage is applied between the connection point of the switching element and the control terminal,
The controller is
A bootstrap circuit in which a starting capacitor, a third diode, and a resistor are connected in series;
A control power supply that outputs an output voltage equal to or higher than the control voltage from an output terminal;
One end of the starting capacitor is connected to a connection point between the first switching element and the second switching element, the other end of the starting capacitor is connected to a cathode of the third diode, and an anode of the third diode Is connected to the output terminal via the resistor,
The controller is
The induction heating device according to claim 1, wherein the second switching element is turned on by applying a charging voltage charged in the starting capacitor to the control terminal.   The induction heating device according to claim 1, further comprising a second capacitor connected in parallel with a series circuit of the second switching element and the exciting coil. A third capacitor connected in parallel with the first switching element;
The induction heating device according to any one of claims 1 to 3, further comprising a fourth capacitor connected in parallel with the second switching element. The induction heating device according to any one of claims 1 to 4,
A fixing device comprising: a heating member as the heated body that fixes the toner image on the paper by heating the paper on which the toner image is formed. A fixing device according to claim 5;
An image forming unit that forms a toner image on the paper;
The heating member is
An image forming apparatus for fixing the toner image on the paper by heating the paper on which the toner image is formed by the image forming unit.

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