JP2020190622A - Image forming apparatus, and heater switching method - Google Patents

Abstract

To provide an image forming apparatus and a heater switching method that do not reduce productivity in print processing when switching the state of power supply to a plurality of heaters with an inexpensive circuit configuration having only one rectifier.SOLUTION: An image forming apparatus comprises: a plurality of heaters; a rectifier circuit that rectifies AC power to output DC power; a control unit that controls a power supplied to the heaters by PWM control; and a triac 56 that switches between supplying power to a center heater 52 and supplying power to the center heater 52 and an end heater 55. When switching from a first state where the power is supplied to the center heater 52 and the end heater 55 to a second state where the power is supplied to the center heater 52, the control unit controls time to switch to the second state to fall within a predetermined period from when the control unit stops the power supply to the plurality of heaters by the PWM control.SELECTED DRAWING: Figure 4

Description

The present disclosure relates to an image forming apparatus including a plurality of heaters for fixing an image on a sheet, and a method of switching the heaters.

In the electrophotographic image forming apparatus, the paper is heated by a heating roller in order to fix the transferred toner image on the paper (sheet). However, there are various sizes of paper processed by the image forming apparatus, and it is necessary to heat the entire heating roller when fixing a large size paper, but when fixing a small size paper, heating is required. It is sufficient to heat the center of the roller.

Therefore, Patent Document 1 discloses an image forming apparatus in which a central heater and an end heater are provided on the heating roller, and the two heaters are switched according to the size of the paper. The image forming apparatus described in Patent Document 1 employs an AC phase control method in which AC power of a commercial power source is supplied to a heater provided on a heating roller, and the temperature of the heater is adjusted by controlling the AC power. .. Therefore, in the circuit configuration of Patent Document 1, two triacs are provided as switches for switching between the two heaters provided on the heating roller.

Japanese Unexamined Patent Publication No. 2014-232247

In the image forming apparatus, in addition to adjusting the temperature of the heater by the AC phase control method, a PWM control method is known in which the temperature of the heater is adjusted by converting AC power into DC power and performing PWM control by a switching element. In the image forming apparatus adopting the PWM control method, the power factor is improved as compared with the case where the AC phase control method is adopted, the power control can be performed accurately, and the generation of noise can be suppressed.

However, in the case of an image forming device that adopts a PWM control method and switches between two heaters provided on the heating roller with only one thyrack (rectifying element), even when only one heater is to be switched, due to the nature of the thyristor. , It is necessary to temporarily stop the power supply to all heaters once.

At this time, by stopping the heater that did not need to stop the power supply, the temperature of the heater drops and the resistance becomes low, so that an inrush current is likely to occur. Therefore, when the power supply is restarted, it is necessary to gradually increase the power supply amount in order to prevent the generation of the inrush current, and as a result, there is a problem that the productivity of the printing process of the image forming apparatus is lowered. there were.

The present disclosure has been devised in view of such circumstances, and an object thereof is to reduce the productivity of printing processing when switching the power supply state to a plurality of heaters with an inexpensive circuit configuration of only one rectifying element. It is an object of the present invention to provide a method of switching an image forming apparatus and a heater which are not allowed to occur.

An image forming apparatus according to a certain aspect of the present disclosure includes a plurality of heaters for fixing an image on a sheet, a rectifying circuit that rectifies AC power from an AC power source, and outputs DC power to the plurality of heaters. The DC power output from is supplied to a plurality of heaters by PWM control, and the power is supplied to the first heater among the plurality of heaters, or the power is supplied to the first heater and the second heater. The control unit is provided with a switching unit that switches between switching by a rectifying element, and the control unit is a switching unit that supplies power to the first heater and a second state from the first state in which power is supplied to the first heater. When switching to, the switching unit is controlled so as to switch to the second state within a predetermined period after stopping the power supply to the plurality of heaters by PWM control.

A method according to a certain aspect of the present disclosure is a plurality of heaters for fixing an image on a sheet, a rectifier circuit that rectifies AC power from an AC power source and outputs DC power to a plurality of heaters, and a rectifier circuit that outputs DC power. A control unit that controls the power supplied to a plurality of heaters by PWM control, and a first state in which power is supplied to the first heater among the plurality of heaters, or power is supplied to the first heater and the second heater. It is a method of switching heaters in an image forming apparatus including a switching unit that switches between the second state of supply by a rectifying element, in which a step of stopping power supply to a plurality of heaters by PWM control and a step of stopping power supply are stopped. Then, the step of switching the rectifying element to the second state within a predetermined period is included.

In the method of switching the image forming apparatus and the heater of the present disclosure, by switching to the second state within a predetermined period after stopping the power supply to the plurality of heaters by PWM control, the productivity of the printing process is not lowered. It is possible to switch the power supply state to a plurality of heaters with an inexpensive circuit configuration of only one rectifying element.

It is a figure which shows the whole structure of the image forming apparatus 1. It is a figure which shows the circuit structure of the image forming apparatus 1. It is a figure which shows the relationship between the off time of a heater and the element breakage limit time T2. It is a timing diagram of the power control to supply to each heater when the duty ratio is increased at once. It is a timing diagram of the power control to supply to each heater when the duty ratio is controlled to increase stepwise. It is a flowchart which shows the switching control procedure from a 1st state to a 2nd state. It is a flowchart which shows an example for determining the element breakage limit time T2.

Hereinafter, embodiments of the technical concept according to the present disclosure will be described with reference to the drawings. In the following description, the same parts are designated by the same reference numerals. Their names and functions are the same. Therefore, the detailed description of them will not be repeated.
[Embodiment 1]
<Overall configuration of image forming device>
FIG. 1 is a diagram showing an overall configuration of the image forming apparatus 1. In FIG. 1, the image forming apparatus 1 is, for example, a copying machine, a printer or a facsimile, or a multifunction device having these functions, and is a printing medium M (for example, paper, film, sticker, etc.) in which an image is a sheet. Print on.

The image forming apparatus 1 is roughly composed of a paper feeding unit 2, a resist roller pair 3, an image forming unit 4, a fixing unit 5, an operation / input unit 6, a control unit 7, a power supply unit 8, and the like. It is provided with an ambient temperature detection unit 9. Hereinafter, the operation of each configuration during the printing operation of the image forming apparatus 1 will be described.

An unused print medium M is loaded on the paper feed unit 2. The paper feed unit 2 feeds the print media M one by one to the transport path FP shown by the broken line in FIG. The resist roller pair 3 is provided on the transport path FP and on the downstream side of the paper feed unit 2. The resist roller pair 3 temporarily stops the print medium M sent out from the paper feeding unit 2, and then sends it out to the secondary transfer region at a predetermined timing.

The image forming unit 4 generates a toner image on the intermediate transfer belt by, for example, a well-known electrophotographic method and a tandem method. The toner image is supported by an intermediate transfer belt and conveyed toward the secondary transfer region.

The print medium M is sent from the resist roller pair 3 to the secondary transfer region, and the toner image is conveyed from the image forming unit 4. In the secondary transfer region, the toner image is transferred from the intermediate transfer belt to the print medium M.

In the fixing portion 5, the heating roller 51 and the pressure roller 53 come into contact with each other to form a nip. Further, the heating roller 51 has a plurality of heaters built in the tubular core metal. Each heater is, for example, a halogen heater, and is lit by a current supplied from the power supply unit 8.

The pressurizing roller 53 rotates under the control of the control unit 7. The heating roller 51 rotates in accordance with the rotation of the pressure roller 53. When the print medium M is fed into the nip, the print medium M is pressurized by the heating roller 51 and the pressure roller 53, and is also heated by the heating roller 51. As a result, the toner is fixed on the print medium M. After that, the print medium M is sent out toward the output tray.

The fixing unit 5 further includes, for example, a heater temperature detecting unit 54, which is a thermistor. The heater temperature detection unit 54 detects the temperature of the heater and outputs the detection result to the control unit 7. The heater temperature detection unit 54 may detect only the temperature of either the central heater 52 or the end heater 55, which will be described later, or may detect the temperatures of both heaters.

The operation / input unit 6 includes a numeric keypad, a touch panel, and the like. The user operates the operation / input unit 6 to input various information.

In the control unit 7, the CPU executes the program stored in the ROM while using the RAM as a work area. The control unit 7 performs various controls, but what is important in this embodiment is the energization control of the heater. Specifically, the control unit 7 PWM-controls the duty ratio of the switching element 831 described later so that the detection result of the heater temperature detection unit 54 becomes the target temperature. In the present embodiment, the control unit 7 is composed of a microcomputer.
<Circuit configuration>
FIG. 2 is a diagram showing a circuit configuration of the image forming apparatus 1. FIG. 2 mainly illustrates the circuit configurations of the power supply unit 8 and the fixing unit 5. The power supply unit 8 includes a rectifier circuit 81, a noise filter 82, and a chopper circuit 83. The rectifier circuit 81 is connected to a commercial power source and converts AC power into DC power.

The fixing portion 5 includes a central heater 52 and an end heater 55 in the heating roller 51. In FIG. 2, the central heater 52 and the end heater 55 are separated from each other in terms of representation in the figure. That is, in the present embodiment, the heating roller provided with both heaters is the same heating roller 51. However, the end heater 55 may be provided on the pressurizing roller 53 separately from the central heater 52.

The noise filter 82 is, for example, a π-type filter, and is longitudinally connected to the output side of the rectifier circuit 81. Specifically, the noise filter 82 includes a coil L1, a capacitor C1 and a capacitor C2. The coil L1 is connected in series with the central heater 52 and the end heater 55, and the condenser C1 and the condenser C2 are connected in parallel with the central heater 52 and the end heater 55.

The chopper circuit 83 is, for example, a step-down chopper circuit, which is connected longitudinally to the output side of the noise filter 82. In this case, the chopper circuit 83 includes a coil (reactor) L2, a reflux element D1, a switching element 831, and a drive circuit 832.

The coil L2 is connected in series between the coil L1 and the central heater 52 and the end heater 55.

The reflux element D1 is, for example, a diode, and is connected to the noise filter 82 side of the coil L2 in parallel with the central heater 52 and the end heater 55. More specifically, the cathode of the reflux element D1 is electrically connected between the coils L1 and L2, and its anode is electrically connected between the central heater 52 and the end heater 55 and the collector of the switching element 831. The reflux element D1 is arranged.

Further, the switching element 831 is, for example, an IGBT (Insulated Gate Bipolar Transistor) or a MOS-FET (Metal-Oxide-Semiconductor Factory), which is closer to the noise filter 82 than the recirculation element D1 and has a central heater 52 and an end. It is connected in series with the part heater 55.

More specifically, the switching element 831 is arranged so that the collector of the switching element 831 is electrically connected to the central heater 52 and the end heater 55, and its emitter is electrically connected to the output side of the rectifier circuit 81. The drive circuit 832 is connected to the gate of the switching element 831 and sets the duty ratio and the drive frequency of the switching element 831 under the control of the control unit 7. The central heater 52 and the end heater 55 are connected between the output terminals of the chopper circuit 83 as described above.

The triac 56 is a rectifying element that functions as a switch for switching the power supply state to the end heater 55. The triac 56 is turned on by applying a current to the gate. Specifically, the triac 56 is, for example, a photo triac coupler, and switches between an on state and an off state by a signal from the control unit 7.

However, the gate trigger current, which is the required gate current, is set for each triac in order to turn it on. Further, when switching from the on state to the off state, the current flowing through the triac 56 must be equal to or less than the holding current.

A thyristor having the same characteristics as the above-mentioned triac characteristics may be used as a switch. Although it is possible to configure this circuit by using parts such as relays and FETs, the relay has a large number of opening and closing times, the contact life is short, and the inrush current during opening and closing is large, so the circuit reliability. It is disadvantageous to. In addition, FET is disadvantageous in that it is expensive.

The rectifier circuit 81 full-wave rectifies the AC power supplied from the commercial power source to generate DC power. The noise filter 82 removes noise from the output current of the rectifier circuit 81. Here, the capacitors C1 and C2 of the noise filter 82 prevent the high-frequency component of the pulsed current flowing through the switching element 831 from leaking to the commercial power supply side.

When supplying power to the central heater 52 and the end heater 55, the control unit 7 inputs a control signal to the drive circuit 832. The control signal generates at least a time interval (that is, a duty ratio) for turning on the central heater 52 and the end heater 55. The drive circuit 832 generates a drive signal for turning on / off the switching element 831 at the duty ratio indicated by the control signal, and supplies the drive signal to the gate of the switching element 831.

The switching element 831 is slowed up at a frequency much higher than the frequency of the commercial power supply (eg, 20 kHz). When the switching element 831 is turned on, the DC power generated by the rectifier circuit 81 flows through the coil L2, the central heater 52, and the end heater 55 via the switching element 831. During this time, the coil L2, which is a reactance element, stores a part of the electric power flowing through itself as magnetic energy.

On the other hand, when the switching element 831 is turned off, the magnetic energy stored in the coil L2 while the switching element 831 is on is released as residual power and supplied to the central heater 52 and the end heater 55. This current returns to the coil L2 via the freewheeling element D1 as a regenerative diode.

By the operation of the power supply unit 8 as described above, the waveform of the input current to the central heater 52 and the end heater 55 becomes close to a sine wave. As a result, the power factor of the power supply unit 8 is improved, and the harmonic current is reduced from the input current.

In the circuit configuration, the power supply to the end heater 55 is switched between an on state and an off state according to the size of the seat. For example, when fixing a large-sized sheet (A3 paper or the like), it is necessary to raise the temperature of the end heater 55, and power is supplied to both the central heater 52 and the end heater 55. The state in which power is supplied to both heaters is defined as the first state.

When fixing a small-sized sheet (A4 paper in the vertical direction, etc.) from the first state, it is necessary to shift to a state in which the power supply to the end heater 55 is stopped in order to prevent excessive temperature rise. The second state is a state in which power is supplied only to the central heater 52 without supplying power to the end heater 55. In order to shift to the second state, it is necessary to switch the triac 56 from the on state to the off state.

However, due to the nature of the triac, in order to switch to the off state, it is necessary to keep the current flowing through the triac 56 below a certain holding current (in the present embodiment, the current is 0 amperes). Since the image forming apparatus 1 of the present embodiment includes the capacitor C2 (see FIG. 2), even if it is simply turned off, the electric charge is emitted from the capacitor C2 and does not fall below the holding current, and the end heater 55 Power supply to is not stopped. Therefore, the control unit 7 controls the duty ratio of the switching element 831 to 0% in order to switch the triac 56 to the off state. As a result, the DC power generated by the rectifier circuit 81 does not flow through the switching element 831.

Even after the duty ratio of the switching element 831 is controlled to 0%, the residual power stored in the coil L2, which is a reactance element, continues to flow to the triac 56. When the residual power decreases with the passage of time and the current flowing through the triac 56 becomes equal to or less than the holding current, the triac 56 can be switched to the off state.

Here, the power supply to the central heater 52, which does not need to stop the power supply, is also stopped. When the power supply to the central heater 52 is stopped, the temperature of the central heater 52 is lowered, and the resistance value is lowered as the temperature is lowered. If the duty ratio of the switching element is increased from 0% at a time while the resistance value is lowered, an inrush current is generated and the switching element 831 may be damaged. In order to prevent this, a method of increasing the duty ratio of the switching element 831 stepwise can be considered, but the productivity of the image forming apparatus 1 is lowered because it takes time to wait until the temperature of the central heater 52 rises.
<Time until the switching element is damaged>
When starting power supply to the heater, in order to prevent damage to the switching element 831, generally, slow-up control is performed in which the duty ratio of the switching element 831 is gradually increased. However, when power is started to be supplied to the heater by slow-up control, it takes time to raise the temperature of the heater to a predetermined temperature, and there is a risk that the productivity of the image forming apparatus 1 may decrease.

Therefore, in the present embodiment, paying attention to the relationship between the heater off time and the generation of the inrush current, the control for starting the power supply to the heater without performing the slow-up control is adopted.

That is, the central heater 52 whose power supply is stopped keeps the temperature held before the power supply is stopped for a while. As the power supply is stopped, the air around the central heater 52 begins to take heat from the central heater 52. As a result, the temperature of the central heater 52 decreases with the passage of time. When the temperature of the central heater 52 decreases with the passage of time and reaches a resistance value at which an inrush current is generated, it becomes necessary to gradually increase the duty ratio of the switching element 831 for the first time. In other words, it is not necessary to gradually increase the duty ratio for the central heater 52 in a state where a sufficient resistance value is maintained, and the switching element 831 is not damaged even if a large amount of current is supplied at one time. ..

Therefore, it is not necessary to stepwise increase the duty ratio of the switching element 831 while the central heater 52 maintains a constant resistance value. The control unit 7 can increase the duty ratio of the switching element 831 from 0% at a time to eliminate unnecessary waiting time, and switch from the first state to the second state without reducing the productivity of the image forming apparatus 1. Be done. In the present disclosure, focusing on the resistance value and temperature of the heater and the off time of the heater, it is determined whether or not it is necessary to gradually increase the duty ratio of the switching element 831 at the start of power supply.

FIG. 3 is a diagram showing an example of the relationship between the heater off time and the element breakage limit time T2. The element breakage limit time T2 is a time during which an inrush current is generated after the power supply to the central heater 52 is stopped and the temperature drops to a temperature at which the switching element 831 is damaged.

With reference to FIG. 3, the horizontal axis represents the heater off time and the vertical axis represents the amount of inrush current supplied to the heater. The inrush current amount is an unintended surplus power supply that occurs when a constant power supply is started. The time T0 in FIG. 3 indicates the time when the power supply of the central heater 52 is stopped.

It can be seen from FIG. 3 that the amount of inrush current increases as time elapses from time T0. At time T0, the central heater 52 maintains the temperature before the power supply is stopped, but when the power supply is stopped, there is no energy source for maintaining heat, and the temperature in the surrounding air or the like is reached. Is robbed. As the temperature of the central heater 52 decreases, the resistance value of the central heater 52 also decreases. That is, the resistance value of the central heater 52 decreases with the passage of time, and the amount of inrush current increases accordingly.

With reference to FIG. 3, when an inrush current exceeding the element breakage limit current indicating the element breakage limit is generated, the switching element 831 is damaged. The time to reach the breaking temperature is the element breaking limit time T2.

When the power supply to the end heater 55 is stopped in order to shift from the first state to the second state, the power supply to the central heater 52 must be stopped due to the nature of the triac 56, and the inrush current must be stopped. There is a possibility of shifting to the state where. However, since the switching element 831 is not damaged up to the element damage limit time T2, the control unit 7 can increase the duty ratio of the switching element 831 from 0% at a time.
<Triac switching time>
If the duty ratio of the switching element 831 is increased from 0% at once by the element breakage limit time T2, the transition from the first state to the second state can be performed without lowering the productivity of the image forming apparatus 1. However, in order to increase the duty ratio of the switching element 831, the current flowing through the triac 56 needs to be equal to or less than the holding current. If the duty ratio of the switching element 831 is increased even though the current flowing through the triac 56 is not equal to or less than the holding current, the triac 56 cannot be switched to the off state and remains in the first state.

As shown in FIG. 2, in the circuit configuration, the coil L2, which is a reactance element, stores a part of the electric power flowing through itself as magnetic energy. That is, the time when the control unit 7 receives the command to switch from the first state to the second state and controls the duty ratio of the switching element 831 to 0% in order to turn off the triac 56 (time T0 in FIG. 3). Even after that, the energy stored in the coil L2, which is a reactance element, continues to be supplied to the triac 56 as electric power for a certain period of time.

Therefore, the triac 56 cannot be switched to the off state until after the residual power stored in the coil L2 is sufficiently reduced to be equal to or less than the holding power of the triac 56. The time during which the residual power is reduced to be equal to or less than the holding power of the triac 56 is defined as the triac switching time T1.

The triac 56 cannot be switched to the off state until the triac switching time T1 is reached. That is, since the triac switching time T1 has been reached, even if the switching element 831 is raised all at once, if the element damage limit time T2 has already passed, an inrush current is generated and the switching element 831 is damaged. .. Therefore, in the present disclosure, it is determined whether to increase the duty ratio of the switching element 831 stepwise or at once from the relationship between the triac switching time T1 and the element breakage limit time T2.

The triac switching time T1 depends on the residual power stored in the coil L2, and the element breakage limit time T2 is the temperature of the central heater 52 or the surrounding temperature at the time when the duty ratio of the switching element 831 is controlled to 0%. It depends on the temperature of the air. Therefore, the triac switching time T1 may be earlier than the element damage limit time T2 depending on the surrounding environment of the image forming apparatus 1 and conditions such as a job, and the control unit 7 sets the duty ratio of the switching element 831 from 0% at a time. It can be raised. On the other hand, the triac switching time T1 may be slower than the element breakage limit time T2, and the inrush current amount shown in FIG. 3 exceeds the element breakage limit. Therefore, the control unit 7 sets the duty ratio of the switching element 831. It is necessary to control the height in stages. Hereinafter, both cases will be described in detail with reference to the figures showing the respective cases.
<When T1 is faster than T2>
FIG. 4 is a timing diagram of power control supplied to each heater when controlling to increase the duty ratio at one time. In FIG. 4, it is assumed that the triac switching time T1 is earlier than the element breakage limit time T2.

From the top, the time change of the power supplied to the end heater 55, the time change of the power supplied to the central heater 52, the time change of the detail ratio of the PWM control in the switching element 831, and the time change of the switch state of the triac 56. It is shown in the same time series.

In the period A, the image forming apparatus 1 performs the fixing process of a large-sized sheet such as A3. Therefore, since it is necessary to apply heat to fix the entire large sheet, electric power having a duty ratio of 60% is supplied to both the end heater 55 and the central heater 52. The power supply state is the first state.

When fixing to a sheet having a small size such as A4 from the first state, it is necessary to stop the power supply to the end heater 55 and shift to the second state in order to prevent overheating. In order to switch the triac 56 to the off state, it is necessary to make the current flowing through the triac 56 equal to or less than the holding current. The control unit 7 reduces the duty ratio of the switching element 831 to 0% (time T0). The off of the triac 56 in the period B means that the control unit 7 has sent a control signal for switching to the off to the triac 56, and the triac 56 itself has not been able to switch to the off state.

This is because the residual power stored in the coil L2 flows to the triac 56 during the period B, so that the triac 56 cannot be switched to the off state. When the residual power decreases and the period B ends, the triac 56 itself switches to the off state. That is, the triac switching time T1 is reached.

In FIG. 4, when the triac switching time T1 is reached, the element breakage limit time T2 has not been reached. That is, even if the duty ratio of the switching element 831 in FIG. 3 is controlled to be increased at one time, the amount of inrush current is generated within a range that does not exceed the damage limit of the element. Therefore, the control unit 7 controls the duty ratio of the switching element 831 to 60% at a time while reaching the triac switching time T1.

As a result, the power supply state is switched from the first state to the second state. Since the duty ratio of the switching element 831 is not increased stepwise, the idle time is not reduced and the productivity is not reduced.
<When T1 is slower than T2>
FIG. 5 is a timing diagram of power control supplied to each heater when the duty ratio is gradually increased. In FIG. 5, it is assumed that the triac switching time T1 is later than the element breakage limit time T2. Also in FIG. 5, the time change of the electric power and the like of each heater similar to that in FIG. 4 is shown in the same time series.

In the period A, the power supply state is the first state because the large-sized sheet is fixed. When fixing a sheet having a small size from the first state, it is necessary to shift to the second state. The control unit 7 reduces the duty ratio of the switching element 831 to 0% (time T0) in order to keep the current flowing through the triac 56 below the holding current. Since the control unit 7 controls the triac 56 to be off at the time T0, it is displayed in FIG. 4 that the triac 56 is off in the period B. However, even if the triac 56 is controlled to be off as described above, the triac 56 cannot be turned off until the triac switching time T1.

That is, in the period B, the residual power stored in the coil L2 exists and continues to flow to the triac 56, and when the residual power decreases and the period B ends, the triac 56 is switched to the off state.

However, in FIG. 5, the element breakage limit time T2 has elapsed before the triac switching time T1 is reached. That is, when the duty ratio of the switching element 831 in FIG. 3 is controlled to be increased at one time, the amount of inrush current exceeds the damage limit of the element. That is, the switching element 831 is damaged. Therefore, the control unit 7 controls so as to reach the triac switching time T1 and gradually increase the duty ratio of the switching element 831.

As a result, the power supply state is switched from the first state to the second state. Since the switching element 831 is increased stepwise, no inrush current is generated, but the time for the duty ratio to reach 60% is time T3. When compared with FIG. 4, an idle time of the period C occurs in addition to the period B, and the productivity of the image forming apparatus 1 decreases.
<Switching control procedure from the first state to the second state>
Hereinafter, the switching control procedure from the first state to the second state will be described using a flowchart. FIG. 6 is a flowchart showing a switching control procedure from the first state to the second state. After starting the printing process, the control unit 7 determines whether or not the printing process of the sheet such as A3 paper has been accepted (step S1). That is, the control unit 7 determines whether or not power supply to the end heater 55 is necessary.

When the printing process of a sheet such as A3 paper is not accepted (NO in step S1), power is supplied only to the central heater 52 to perform the printing process (step 11). That is, since printing of a small sheet such as A4 is accepted, power is supplied only to the central heater 52 to perform the fixing process in order to prevent excessive temperature rise. The power supply state is the second state.

When the printing process of the sheet such as A3 paper is accepted (YES in step S1), the control unit 7 switches the triac 56 to the on state (step S2). The control unit 7 supplies electric power to the end heater 55 and the central heater 52, and performs a printing process (step S3). That is, in order to print a large-sized sheet, the fixing process is performed with the power supply state set to the first state.

The control unit 7 determines whether or not the printing process of the sheet such as A4 paper has been accepted (step S4). That is, it is determined whether or not the power supply to the end heater 55 is unnecessary. When the printing process of the sheet such as A4 paper is not accepted (NO in step S4), the control unit 7 returns the process to step S3.

When the printing process of the sheet such as A4 paper is accepted (YES in step S4), the control unit 7 reduces the detail ratio of the switching element 831 to 0% (step S5). The control unit 7 determines whether or not the triac switching time T1 is earlier than the element breakage limit time T2 (step S6).

When the triac switching time T1 is earlier than the element breakage limit time T2 (YES in step S6), the control unit 7 switches the triac 56 to the off state when the triac switching time T1 elapses (step S7). The control unit 7 increases the duty ratio of the switching element 831 from 0% to 60% at once (step S8). After switching to the second state and reaching the duty ratio of 60%, the control unit 7 supplies power only to the central heater 52 to perform printing processing (step S11).

When the triac switching time T1 is later than the element breakage limit time T2 (NO in step S6), the control unit 7 switches the triac 56 to the off state when the triac switching time T1 elapses (step S9). The control unit 7 gradually increases the duty ratio of the switching element 831 from 0% to 60% (step S10).

After switching to the second state and reaching the duty ratio of 60%, the control unit 7 supplies power only to the central heater 52 to perform printing processing (step S11).

In this way, it is determined whether the duty ratio is increased at once or stepwise in relation to the triac switching time T1 and the element breakage limit time T2. In any case, the generation of inrush current can be prevented and the switching element 831 is not damaged.
<How to determine the element damage limit time T2>
As shown in FIGS. 3 to 6, the element breakage limit time T2 has an important role in the present disclosure. This is because the relationship between the element breakage limit time T2 and the triac switching time T1 determines whether the control unit 7 can increase the duty ratio of the switching element 831 at once or increase it stepwise.

Here, a method of determining the element breakage limit time T2 will be described. As shown in FIG. 3, the element breakage limit time T2 is determined by the resistance value of the central heater 52. The resistance value of the central heater 52 is determined by the temperature of the central heater 52. Therefore, the element breakage limit time T2 is affected by the temperature of the central heater 52.

The heater temperature detection unit 54 shown in FIG. 1 can detect the temperature of the central heater 52. The heater temperature detection unit 54 may detect the temperature of the central heater 52 based on the amount of infrared rays.

The higher the temperature of the central heater 52, the higher the resistance value, and the lower the temperature, the lower the resistance value. Therefore, when the control unit 7 reduces the duty ratio of the switching element 831 to 0% (time T0 in FIGS. 3 to 5), the higher the temperature of the central heater 52, the longer the time to reach the element breakage limit time T2. Will be slow.

On the other hand, since the amount of toner used and the number of sheets to be processed are small, it is assumed that electric power is supplied to the central heater 52 with a duty ratio lower than that of the normal printing process. If the temperature of the central heater 52 is already low at the time T0, the time until the element breakage limit time T2 is reached becomes faster.

Further, when the temperature of the air around the central heater 52 is low (for example, less than 0 ° C.), the speed at which the temperature of the central heater 52 drops becomes high. On the other hand, when the temperature of the air around the central heater 52 is high (for example, 35 ° C. or higher), the rate at which the temperature of the central heater 52 drops slows down.

As described above, the temperature around the image forming apparatus 1 also affects the time until the element breakage limit time T2 is reached. The ambient temperature of the image forming apparatus 1 is detected by the ambient temperature detecting unit 9. Therefore, the element breakage limit time T2 can be determined from the temperature of the central heater 52 and the ambient temperature of the image forming apparatus 1. Hereinafter, how to determine the element breakage limit time T2 will be described using a flowchart. FIG. 7 is a flowchart showing an example for determining the element breakage limit time T2.

After starting the process of determining the element breakage limit time T2, the control unit 7 determines whether or not the temperature of the central heater 52 is equal to or higher than the predetermined temperature (step S12). The predetermined temperature is a constant temperature required for fixing a relatively small amount of toner among the temperatures of the central heater 52 for fixing the toner. It may be determined by an experiment or the like. That is, when the temperature of the central heater 52 is lower than the predetermined temperature, the element breakage limit time T2 is reached earlier than when the temperature is equal to or higher than the predetermined temperature.

When the temperature of the central heater 52 is lower than the predetermined temperature (NO in step S12), the control unit 7 determines that the element breakage limit time T2 is 50 ms (step S13). When the temperature of the central heater 52 is equal to or higher than the predetermined temperature (YES in step S12), the control unit 7 determines whether or not the ambient temperature of the image forming apparatus 1 is equal to or higher than the reference temperature (step S14). The reference temperature is a temperature for determining the rate at which the temperature of the central heater 52 drops, and may be determined by an experiment or the like. Moreover, a plurality of reference temperatures may be provided.

When the ambient temperature of the image forming apparatus 1 is lower than the reference temperature (NO in step S14), the control unit 7 determines that the element breakage limit time T2 is 100 ms (step S15). When the ambient temperature of the image forming apparatus 1 is equal to or higher than the reference temperature (YES in step S14), the control unit 7 determines that the element breakage limit time T2 is 300 ms (step S16).

The time set in step S13, step S15, and step S16 in the flowchart is only an example and may be set based on experimental data.

Further, as a method of determining the predetermined period, it may be determined based on the content of the job executed before switching from the first state to the second state. For example, power is supplied to both heaters for printing a sheet such as A3 paper, but when the amount of toner to be fixed is extremely small, it can be fixed at a heater temperature lower than that of a normal fixing process. On the contrary, when a large amount of toner must be fixed on a sheet such as A3 paper and the number of copies of the sheet such as A3 paper is large, the temperature required by the heater becomes high.

In this way, the control unit 7 may set the element breakage limit time T2 in consideration of the content of the job executed before switching from the first state to the second state.
<Summary>
As described above, the image forming apparatus 1 according to the present embodiment rectifies the AC power from the plurality of heaters for fixing the image on the sheet and the AC power source, and outputs the DC power to the plurality of heaters. The rectifier circuit 81, the control unit 7 that controls the power supplied to the plurality of heaters by PWM control of the DC power output from the rectifier circuit 81, and the central heater 52 among the plurality of heaters, or the central heater It includes a triac 56 that switches whether to supply power to the 52 and the end heater 55.

When the triac 56 switches from the first state in which power is supplied to the central heater 52 and the end heater 55 to the second state in which power is supplied to the central heater 52, the control unit 7 sets the time for switching to the second state. , The triac 56 is controlled so as to be performed within a predetermined period after the power supply to the plurality of heaters by PWM control is stopped.

According to this, the image forming apparatus 1 according to the present embodiment reduces the productivity of the printing process when the power supply state to a plurality of heaters is switched with an inexpensive circuit configuration of only one rectifying element. It is possible to switch the heater without.

Further, the predetermined period may be shorter than the time during which the inrush current to the central heater 52 and the end heater 55 is generated. According to this, the switching can be performed before the time when the inrush current is generated.

Further, a heater temperature detecting unit 54 for detecting the temperature of the central heater 52 or the end heater 55 may be further provided, and the control unit 7 may determine a predetermined period based on the detection result of the heater temperature detecting unit 54. According to this, according to this, a predetermined period can be determined based on the temperature of the central heater 52 or the end heater 55.

Further, the heater temperature detection unit 54 may detect the temperature of the central heater 52 or the end heater 55 based on the amount of infrared rays. According to this, the central heater 52 or the end heater 55 can be detected based on the amount of infrared rays.

Further, an ambient temperature detection unit 9 for detecting the ambient temperature of the image forming apparatus 1 may be further provided, and the control unit 7 may determine a predetermined period based on the detection result of the ambient temperature detection unit 9. According to this, a predetermined period can be determined based on the ambient temperature of the image forming apparatus 1.

Further, the time for switching to the second state may be the time required to turn on the triac 56 in order to switch to the second state. According to this, by switching to the second state at the time required to turn on the triac 56, it is possible to switch at the optimum timing.

Further, the triac 56 may be a thyristor or other rectifying element having similar properties. According to this, the rectifying element can be used as a switch other than the triac.

Further, the control unit 7 may be composed of a microcomputer. Further, the control unit 7 may perform control to gradually increase the duty ratio of PWM control when the triac 56 cannot switch to the second state within a predetermined period. According to this, even in a situation where an inrush current that damages the element is generated, it is possible to prevent the element from being damaged by controlling it stepwise.

Further, the control unit 7 may determine a predetermined period based on the job content executed by the image forming apparatus 1, and the job content is the type and execution of the job executed before switching from the first state to the second state. It may be frequency. According to this, a predetermined period can be set based on the job content before switching.

Further, a plurality of heaters are provided in the fixing portion 5 for fixing the image on the sheet, the central heater 52 heats the central portion of the fixing portion 5, and the end heater 55 heats the end portion of the fixing portion 5. .. According to this, a region can be divided in the fixing portion 5, and the temperature can be adjusted for each region.

Further, the heater switching method according to the present embodiment includes a plurality of heaters for fixing an image on a sheet and a rectifier circuit 81 that rectifies AC power from an AC power source and outputs DC power to the plurality of heaters. A control unit 7 that controls the DC power output from the rectifier circuit 81 to a plurality of heaters by PWM control, and a first unit that supplies power and an end heater 55 to the central heater 52 among the plurality of heaters. This is a heater switching method in an image forming apparatus including a triac 56 for switching between a state and a second state for supplying electric power to the central heater 52. The heater switching method includes a step of stopping the power supply to the plurality of heaters by PWM control and a step of switching to the second state within a predetermined period after the power supply is stopped.

According to this, in the heater switching method according to the present embodiment, when switching from the first state to the second state, the triac 56 is controlled and switched within a predetermined period after the power supply is stopped. With an inexpensive circuit configuration, the heater can be switched without reducing the productivity of the printing process when switching the heater.

It should be considered that the embodiments disclosed this time are exemplary in all respects and not restrictive. The scope of the present invention is shown by the scope of claims, not the above description, and is intended to include all modifications within the meaning and scope equivalent to the scope of claims.

1 Image forming device, 2 Feeding unit, 3 Resist roller pair, 4 Image forming unit, 5 Fixing unit, 6 Operation / input unit, 7 Control unit, 8 Power supply unit, 9 Ambient temperature detection unit, 51 Heating roller, 52 Center Heater, 53 Pressurizing roller, 54 Heater temperature detector, 55 End heater, 56 Triac, 81 Rectifier circuit, 82 Noise filter, 831 Switching element, C1, C2 capacitor, D1 Recirculation element, L1, L2 coil, M Printing medium , T1 triac switching time, T2 element damage limit time.

Claims (14)

It is an image forming device
With multiple heaters to fix the image on the sheet,
A rectifier circuit that rectifies AC power from an AC power supply and outputs DC power to the plurality of heaters.
A control unit that controls the power supplied to the plurality of heaters by PWM control of the DC power output from the rectifier circuit.
A switching unit for switching between supplying power to the first heater and supplying power to the first heater and the second heater among the plurality of heaters by a rectifying element is provided.
The control unit
When switching from the first state in which power is supplied to the first heater and the second heater to the second state in which power is supplied to the first heater in the switching unit, the time for switching to the second state is set. An image forming apparatus that controls the switching unit so that it is performed within a predetermined period after stopping the power supply to the plurality of heaters by PWM control. The image forming apparatus according to claim 1, wherein the predetermined period is a time shorter than a time during which inrush currents to the plurality of heaters are generated. Further, a heater temperature detecting unit for detecting the temperature of at least one of the plurality of heaters is provided.
The control unit
The image forming apparatus according to claim 1 or 2, wherein the predetermined period is determined based on the detection result of the heater temperature detection unit. The image forming apparatus according to claim 3, wherein the heater temperature detecting unit detects the temperature of at least one of the plurality of heaters based on the amount of infrared rays. Further, an ambient temperature detection unit for detecting the ambient temperature of the image forming apparatus is provided.
The control unit
The image forming apparatus according to any one of claims 1 to 4, wherein the predetermined period is determined based on the detection result of the ambient temperature detecting unit. The image forming apparatus according to any one of claims 1 to 5, wherein the time for switching to the second state is the time required to turn on the rectifying element in order to switch to the second state. .. The image forming apparatus according to any one of claims 1 to 5, wherein the rectifying element is a triac. The image forming apparatus according to any one of claims 1 to 5, wherein the rectifying element is a thyristor. The image forming apparatus according to any one of claims 1 to 8, wherein the control unit is composed of a microcomputer. The control unit
The method according to any one of claims 1 to 9, wherein when the switching unit cannot switch to the second state within the predetermined period, the duty ratio of the PWM control is controlled to be gradually increased. Image forming device. The control unit
The image forming apparatus according to any one of claims 1 to 10, wherein the predetermined period is determined based on the content of a job executed by the image forming apparatus. The image forming apparatus according to claim 11, wherein the job content is a type and execution frequency of a job executed before switching from the first state to the second state. The plurality of heaters are provided in a fixing portion for fixing an image on a sheet.
The image forming apparatus according to any one of claims 1 to 12, wherein the first heater heats a central portion of the fixing portion, and the second heater heats an end portion of the fixing portion. .. PWM control of a plurality of heaters for fixing an image on a sheet, a rectifier circuit that rectifies AC power from an AC power source and outputs DC power to the plurality of heaters, and the DC power output from the rectifier circuit. A control unit that controls the electric power supplied to the plurality of heaters, and a first state of supplying electric power to the first heater and the second heater among the plurality of heaters, or a first state of supplying electric power to the first heater. It is a method of switching a heater in an image forming apparatus including a switching unit that switches between two states by a rectifying element.
A step of stopping the power supply to the plurality of heaters by PWM control, and
A method for switching a heater, which comprises a step of switching the rectifying element to the second state within a predetermined period after stopping the power supply.

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