Image forming apparatus having a control circuit that selectively controls power to be supplied to a plurality of heat generating blocks of a heater

An image forming apparatus includes a control circuit having a plurality of semiconductor elements configured to perform switching between ON and OFF of a plurality of heat generating blocks, and a power interrupting unit configured to be activated so as to interrupt power being supplied to the plurality of semiconductor elements when a heater overheats. Of the plurality of semiconductor elements, a first semiconductor element to supply power to a first heat generating block, is connected, in series, to a second semiconductor element to supply power to a second heat generating block. In addition, when the power interrupting unit is not activated, the second heat generating block is controlled by controlling only the second semiconductor element, and, when the power interrupting unit is not activated, the first heat generating block is controlled by controlling the first semiconductor element and the second semiconductor element.

This application claims the benefit of Japanese Patent Application No. 2017-098248, filed on May 17, 2017, and Japanese Patent Application No. 2017-223013, filed on Nov. 20, 2017, both of which are incorporated by reference herein in their entireties.

BACKGROUND OF THE INVENTION

Field of the Invention

The present invention relates to an image heating apparatus, such as a copier that uses an electrophotographic system or an electrostatic recording system, a fixing unit that is installed in such an image forming apparatus as a printer, or a gloss applying apparatus that improves a gloss level of a toner image by reheating a toner image already fixed onto a recording material. The present invention also relates to an image forming apparatus that includes this image heating apparatus.

Description of the Related Art

A conventional fixing apparatus that is included in an image forming apparatus is an apparatus having an endless belt (also called “endless film”), a flat heater that contacts an inner surface of the endless belt, and a roller that constitutes a nip portion with the heater via the endless belt. If a small sized paper is continuously printed by an image forming apparatus including this fixing apparatus, the temperature in a region of the nip portion in which paper does not pass in the longitudinal direction may gradually increase (temperature rise in non-paper passing portion). If the temperature in the non-paper passing portion increases too much, parts in the apparatus may be damaged. A method of suppressing the temperature rise in the non-paper passing portion that is proposed is a heater in which a heat generating element is disposed between two conductors arranged along the longitudinal direction, and at least one of the conductors is divided by a width corresponding to the paper size, so that heat generating is controlled for each heat generating block (Japanese Patent Application Publication No. 2017-54071).

If a plurality of thermistors (temperature detecting elements) are disposed in each of the divided heat generating blocks, however, as in Japanese Patent Application Publication No. 2017-54071, the number of wires connected with the thermistors increases as the heat generating regions increase, which may interfere with the downsizing of the apparatus.

It is an object of the present invention to provide a technique that enables downsizing of the apparatus by decreasing the number of temperature detecting elements.

SUMMARY OF THE INVENTION

In one aspect, the present invention provides an image forming apparatus including a fixing portion configured to fix an image, formed on a recording material, onto the recording material, the fixing portion including a heater that includes a substrate, a plurality of heat generating blocks arranged on the substrate in a longitudinal direction of the substrate, and a plurality of temperature detecting elements disposed on the substrate, and a control circuit configured to control power to be supplied to the plurality of heat generating blocks, the control circuit including a plurality of semiconductor elements configured to perform switching between ON and OFF of the plurality of heat generating blocks, and selectively controls the power to be supplied to the plurality of heat generating blocks by selectively controlling the plurality of semiconductor elements, wherein, out of the plurality of heat generating blocks, a first semiconductor element to supply power to a first heat generating block is connected, in series, to a second semiconductor element to supply power to a second heat generating block out of the plurality of heat generating blocks, the second heat generating block is controlled by controlling the second semiconductor element, and the first heat generating block is controlled by controlling the first semiconductor element and the second semiconductor element.

DESCRIPTION OF THE EMBODIMENTS

Hereafter, a description will be given, with reference to the drawings, of embodiments (examples) of the present invention. The sizes, materials, shapes, their relative arrangements, or the like, of constituents described in the embodiments may, however, be appropriately changed according to the configurations, various conditions, or the like, of apparatuses to which the invention is applied. Therefore, the sizes, materials, shapes, their relative arrangements, or the like, of the constituents described in the embodiments do not intend to limit the scope of the invention to the following embodiments.

FIG. 1is a schematic cross-sectional view depicting an image forming apparatus according to an example of the present invention. An image forming apparatus100of Example 1 is a laser printer that forms an image on a recording material using an electrophotographic system.

When a print signal is generated, a scanner unit21emits a laser light modulated in accordance with the image information, and scans the surface of a photosensitive drum (electrophotographic photosensitive member)19, which is charged to a predetermined polarity by a charging roller16. Thereby, an electrostatic latent image is formed on the photosensitive drum19, which is an image bearing member. When toner, which is charged to a predetermined polarity, is supplied from a developing roller17to this electrostatic latent image, the electrostatic latent image on the photosensitive drum19is developed as a toner image (developer image). On the other hand, a recording material (recording paper) P, stacked in a paper feeding cassette11, is fed one sheet at a time by a pick up roller12, and is conveyed to a resist roller pair14by a conveying roller pair13. Further, to match a timing when the toner image on the photosensitive drum19reaches a transfer position, which is determined by the photosensitive drum19and a transfer roller20(transfer member), the recording material P is conveyed from the resist roller pair14to this transfer position. While the recording material P passes through the transfer position, the toner image on the photosensitive drum19is transferred to the recording material P. Then the recording material P is heated by a fixing apparatus (image heating apparatus)200, which is a fixing portion (image heating portion), whereby the toner image is heated and fixed to the recording material P. The recording material P, which bears the fixed toner image, is discharged to a paper delivery tray31located in the upper part of the image forming apparatus100via the conveying roller pairs26and27.

Residual toner, and the like, on the surface of the photosensitive member19are removed and cleaned by a cleaner18. A feeding tray (manual feed tray)28has a pair of recording paper control plates of which a width can be adjusted in accordance with the size of the recording paper P, so that recording paper P, other than a standard size, can be handled. A pick up roller29is a roller to feed the recording paper P from the feeding tray28. A motor30drives a roller, and the like, in the fixing apparatus200.

The above mentioned photosensitive drum19, charging roller16, scanner unit21, developing roller17, and transfer roller20constitute an image forming portion that forms a unfixed image on the recording material P. In Example 1, a developing unit that includes the photosensitive drum19, the charging roller16, and the developing roller17, and a cleaning unit that includes the cleaner18, are detachably attached to the main body of the image forming apparatus100as process cartridges15.

FIG. 2is a cross-sectional view of the fixing apparatus200in Example 1. The fixing apparatus200includes a fixing film (hereafter called “film”)202, a heater300that contacts the inner surface of the film202, a pressure roller208that constitutes a fixing nip portion N with the heater300via the film202, and a metal stay204.

The film202is a heat resistant film, referred to as an endless belt or an endless film, that is formed in a cylindrical or tubular shape, and the material of the base layer of the film is a heat resistant resin (e.g., polyimide) or a metal (e.g., stainless). An elastic layer, such as a heat resistant rubber, may be formed on the surface of the film202. The pressure roller208has a metal core209(e.g., iron, aluminum) and an elastic layer210(e.g., silicon robber). The heater300is held by a holding member201made of a heat resistant resin. The holding member201also has a guide function that guides the rotation of the film202. The metal stay204is for applying pressure of a spring (not illustrated) to the holding member201. The pressure roller208rotates in the arrow direction by being powered by the motor30. The film202is rotated by the rotation of the pressure roller208. The recording paper P, bearing the unfixed toner image, is heated while being held and conveyed by the fixing nip portion N, whereby fixing processing is performed.

The heater300includes heat generating elements (heat generating resistors)302aand302bdisposed on a later mentioned ceramic substrate305. A protecting element212(FIG. 4) is contacted to the heater300. The protecting element212is, for example, a thermoswitch or a temperature fuse, and is activated when the heater300is abnormally heated so as to interrupt the power supplied to the heater300. On a sliding surface side of the heater300facing the film202, thermistors T1(T1-1to T1-7, seeFIG. 3B) and thermistors T2(T2-2to T2-6, seeFIG. 3B) are disposed.

The configuration of the heater300according to Example 1 will be described with reference toFIGS. 3A and 3B.FIG. 3Ais a cross-sectional view of the heater300, andFIG. 3Bis a plan view of each layer of the heater300. InFIG. 3B, a conveyance reference position X0of the recording material P, in the image forming apparatus100of Example 1, is indicated. In Example 1, the conveyance reference position X0is the center of the heater300, and the recording material P is conveyed such that the center line of the recording material, in the direction perpendicular to the conveying direction, is always on the conveyance reference position X0.FIG. 3Ais a cross-sectional view of the heater300at the conveyance reference position X0.

As illustrated inFIG. 3A, the heater300has a conductor301and a conductor303, which are disposed on a substrate305. The conductor301is divided into a conductor301a, which is disposed on the upstream side of the conveying direction of the recording material P, and a conductor301b, which is disposed on the downstream side thereof. Further, in the heater300, a heat generating element302, which is heated by the power supplied via the conductor301and the conductor303, is disposed on the substrate305between the conductor301and the conductor303. This heat generating element302is divided into a heat generating element302a, which is disposed on the upstream side of the conveying direction of the recording material P, and a heat generating element302b, which is disposed on the downstream side thereof. Further, an electrode E3is disposed to supply power. Furthermore, an insulating protective glass308is disposed on the back surface layer2, and the protective glass308covers the heater300excluding the electrode E3. The heater300(substrate305) is disposed such that the longitudinal direction of the heater300is perpendicular to the conveying direction of the recording material P.

As illustrated inFIG. 3B, on the back surface layer1of the heater300, seven heat generating blocks (heating regions), each of which is constituted by a group having the conductor301, the conductor303, the heat generating element302, and the electrode E3, are disposed in the longitudinal direction of the heater300(HB1to HB7). To indicate the correspondence of these seven heat generating blocks HB1to HB7, a composing element constituting each heat generating block is denoted with a reference sign, in which a number of the corresponding heat generating block is attached at the end, such as heat generating elements302a-1to302a-7. This is the same for the heat generating element302b, the conductors301aand301b, the conductor303, and the electrode E3.

The surface protective layer308on the back surface layer2of the heater300is formed such that the electrodes E3-1to E3-7, E4and E5are exposed. To each electrode, an electric contact (not illustrated) can be connected from the back surface side of the heater300. Thereby, power can be supplied to each heat generating block independently. By dividing the heat generating block into the seven heat generating blocks like this, four heat generating regions AREA1to AREA4can be created. In Example 1, AREA1is for A5 sized paper, AREA2is for B5 sized paper, AREA3is for A4 sized paper, and AREA4is for Letter sized paper. Since the seven heat generating blocks can be controlled independently, a heat generating block, to which power is supplied, can be selected in accordance with the size of the recording paper P. The number of the heat generating regions and the number of the heat generating blocks are not limited to the numbers specified in Example 1. Further, the heat generating elements302a-1to302a-7and302b-1to302b-7in each heat generating block are not limited to a continuous pattern described in Example 1, but may be rectangular patterns with intervals.

On a sliding surface layer1of the heater300(on the surface of the substrate305at the opposite side to the surface on which the heat generating elements are disposed), thermistors T1-1to T1-7and thermistors T2-2to T2-6are disposed as temperature detecting elements to detect the temperature of each heat generating block of the heater300. Each of the thermistors T1-1to T1-7, which are mainly used for controlling the temperature of each heat generating block, is disposed at the center of each heat generating block (center of the substrate in the longitudinal direction). The thermistors T2-2to T2-6are edge thermistors for detecting the temperature of a non-paper passing region (edges) when recording paper, which is narrower than the heat generating region, is fed. Therefore, each of the thermistors T2-2to T2-6is disposed in a position closer to the outer side of each heat generating block with respect to the conveyance reference position X0, excluding the heat generating blocks on both ends in which the heat generating region is narrow. One end of each of the thermistors T1-1to T1-7is connected to the respective conductor ET1-1to ET1-7for detecting the resistance value of the thermistor, and the other end thereof is commonly connected to the conductor EG9. One end of each of the thermistors T2-2to T2-6is connected to the respective conductor ET2-2to ET2-6, and the other end thereof is commonly connected to the conductor EG10. In this way, the width L of the heater300tends to increase as the number of thermistors and number of conductors increase.

On the sliding surface layer2of the heater300, a surface protective layer309, coated by glass having slidability, is disposed. The surface protective layer309is disposed, excluding both end portions of the heater300, so as to create electrical contact in each conductor of the sliding surface layer1.

FIG. 4is a circuit diagram depicting a control circuit400of the heater300of Example 1. A commercial alternating current (AC) power supply401is connected to the image forming apparatus100. The power supply voltages Vcc1and Vcc2are direct current (DC) power supplies generated by an AC/DC convertor (not illustrated), which is connected to the AC power supply401. The AC power supply401is connected to the heater300via relays430and440and triacs (semiconductor elements)441to447. The triacs441to447are turned ON/OFF by control signals FUSER1to FUSER7from a central processing unit (CPU)420. The drive circuits of the triacs441to447are not illustrated. Power supply to the plurality of heat generating elements can be selectively controlled by selectively controlling the triacs441to447, which are a plurality of semiconductor elements, whereby a plurality of heat generating blocks, which are divided in the longitudinal direction, can be selectively heated independently.

The temperature detecting circuit of the thermistors will be described. The conductors EG9and EG10are connected to the ground potential. The voltages for the thermistors T1-1to T1-7and T2-2to T2-6shown inFIG. 3are divided into voltages of Th1-1to Th1-7and Th2-2to Th2-6and voltages for the resistors451to457and462to466, which are pulled up to Vcc1respectively. The divided voltages are detected by the CPU420as Th1-1to Th1-7signals and Th2-2to Th2-6signals. Then the voltages are converted into temperature information by the information that is set in an internal memory of the CPU420in advance, whereby the temperature is detected.

In the internal processing, the CPU420calculates power to be supplied using proportion integral (PI) control, for example, based on the set temperature and the detected temperatures by the thermistors T1-1to T1-7. The ON timings of the FUSER1to7signals are generated by the CPU420, based on the timing signal ZEROX synchronizing with the zero potential of the AC power supply401generated by a zero cross detecting unit421. Based on the zero cross timing of the AC power supply401, the detected temperatures are converted into the phase angle (phase control) and wave number (wave number control) corresponding to the power to be supplied, and the triacs441to447are controlled based on the control conditions.

Relays430and440and the protecting circuit will be described. The relays430and440are power interrupting units that are activated when the heater300overheats due to a failure, or the like.

An operation of the relay430will be described. When the CPU420sets an RLON signal to High, a transistor433turns ON, the current is supplied from the power supply Vcc2to the secondary side coil of the relay430, and the primary side contact of the relay430turns ON. When the CPU420sets the RLON signal to Low, the transistor433turns OFF, and current that flows from the power supply voltage Vcc2to the secondary side coil of the relay430is interrupted, and the primary side contact of the relay430turns OFF. The resistor434is a resistor to limit the base current of the transistor433. This operation is also the same for the relay440and the transistor435.

The operation of a safety circuit using the relay430and the relay440will be described. When the detected temperature by any one of the thermistors T1-1to T1-7exceeds a predetermined value that is set, a comparison unit431activates a latch unit432, and the latch unit432sets the RLOFF1signal to Low, and latches the RLOFF1signal. When the RLOFF1signal becomes Low state, the transistor433maintains the OFF state even if the CPU420sets the RLON signal to High, and, therefore, the relay430can maintain the OFF state (safe state). In the same manner, when the detected temperature by any one of the thermistors T2-2to T2-6exceeds a predetermined value that is set, a comparison unit437activates a latch unit436, and the latch unit436sets the RLOFF2signal to Low, and latches the RLOFF2signal.

A relationship between a configuration of the heater drive circuit using the triacs441to447and the number of thermistors will be described here. The triac441that drives the heat generating block HB1is connected in series with the triac442that drives the adjacent heat generating block HB2. If only the triac442is driven, only the heat generating block HB2is heated. If both of the triacs441and442are driven, the heat generating blocks HB1and HB2are heated. In this configuration, it is unlikely that only the heat generating block HB1is heated. Since the triacs441and442are connected in series, in order to drive the heat generating block HB1, which is disposed on the outer side of the heat generating block HB2in the longitudinal direction of the heater300, the heat generating region can be selected depending on the paper size.

The printer of Example 1 includes the safety circuit using the thermistors, so that the heater300does not heat up to an abnormal temperature even if an abnormality occurs to the control of the heater300due to a malfunction of the CPU420, or the like. In other words, the safety circuit is included so that even if one component does not function due to failure, the abnormality of the heater300is detected, and the relays430and440are turned OFF to protect the heater300. In the heat generating block HB3, for example, two thermistors T1-3and T2-3are disposed. Further, a comparison unit437and a latch unit436, to which the voltage signals Th1-3and Th2-3in accordance with the resistance values of these thermistors, are included. Because of these configurations, even if either one of the thermistors fails, the voltage signal from the other thermistor is inputted to the comparison unit437and the latch unit436. Therefore the abnormal temperature relay430or440can be activated to protect the heater300. In the heat generating block HB2,4,5and6as well, two thermistors are disposed in the same manner. In the heat generating block HB1, on the other hand, only one thermistor (T1-1) is disposed. The triacs441and442are connected in series, however, so that the heat generating block HB2is always heated whenever the heat generating block HB1is heated. Therefore, unless a disconnection occurs in the heat generating block HB1at point P indicated inFIG. 4, the heat generating block HB1alone does not abnormally heat up. In other words, while the heat generating block HB1is heated, the heat generating block HB2is always heated. If the heat generating block HB1abnormally heats up because of the failure of the thermistor T1-1, the heat generating block HB2also abnormally heats up, and hence, the abnormal heat generating can be detected by the thermistor T1-2and the thermistor T2-2disposed in the heat generating block HB2. In other words, the temperature of the heat generating block HB1can be managed using the thermistor T1-1alone. This is the same for the heat generating block HB7, and a description thereof is omitted. Further, the heat generating regions of the heat generating blocks HB1and HB7are small, and hence, one thermistor is used for both the edge thermistor to detect the temperature of the non-paper passing region (edge) and the thermistor for temperature control.

As described above, according to Example 1, the heat generating block HB1, which is driven by the semiconductor element441in a subsequent stage of the semiconductor element442to drive the heat generating block HB2, is disposed at least in one of a plurality of heat generating blocks HB1to HB7. Because of this configuration, the heater300can be protected even if the number of thermistors is decreased.

In Example 1, the triac441for driving the heat generating block HB1, which is located on the outer side (edge side) of the heat generating block HB2in the longitudinal direction, is connected in series to the triac442for driving the heat generating block HB2. The configuration to which the present invention can be applied is not limited, however, to this configuration. For example, the triac442for driving the heat generating block HB2, which is located on the outer side (edge side) of the heat generating block HB3in the longitudinal direction, may be connected in series to the triac443for driving the heat generating block HB3. By this configuration, the number of thermistors for detecting the temperature of the heat generating block HB2can be less than the number of thermistors for detecting the temperature of other heat generating blocks.

FIG. 5is a control flow chart according to Example 1. When a print request is received in step S500, the following steps start. In step S501, the RLON signal is outputted at High level to turn the relays430and440ON. In step S502, the CPU420reads the target temperature Ta stored in the internal memory of the CPU420(not shown). In step S503, a critical temperature when the temperature of the non-paper passing portion rises (risen temperature on the edge) Tmax, is read from the internal memory. In step S504, a paper size sensor (not illustrated) in the paper feeding cassette11detects the size of the recording paper P that is set in the paper feeding cassette11. In steps S505-1to S505-4, the paper size is determined, and, in steps S506-1to S506-4, a heat generating region (heating region), corresponding to each paper size, is determined, and a triac corresponding to the heat generating region is controlled. If the temperatures detected by the thermistors T2-2to T2-6(edge thermistors) exceed the critical temperature Tmax of the temperature rise in the non-paper passing portion in S507, the throughput is decreased in step S508, so as to prevent the failure of the fixing apparatus200caused by overheating. The steps from S502to S508are repeated until the print job ends in step S509, and, if the print job ends, the RLON signal is outputted at Low level in step S510, and the relays430and440are turned OFF.

As described above, the number of thermistors can be decreased in a heat generating block in which semiconductor elements to drive the heater are connected in series in two stages, and, therefore, the width L of the heater300can be decreased, and the fixing apparatus200can be downsized.

Example 2 of the present invention will be described. A control circuit700and a heater600in Example 2 are different from the control circuit400described in Example 1 in terms of the heat generating regions, which are connected in two stages in series. A composing element of Example 2 that is the same as Example 1 is denoted with a same reference symbol, and a description thereof is omitted. Matters that are not explained particularly in Example 2 are the same as those in Example 1.

The configuration of the heater600according to Example 2 will be described with reference toFIGS. 6A and 6B.FIG. 6Ais a cross-sectional view of the heater600(cross-sectional view of an area near the conveyance reference position X0inFIG. 6B), andFIG. 6Bis a plan view of each layer of the heater600. As illustrated inFIG. 6B, in Example 2, in the sliding surface layer1, the number of thermistors is one only in the heat generating block HB5, unlike Example 1. The reason for this is described with reference toFIG. 7. In Example 2, a thermistor T3-4is added to the heat generating block HB4of Example 1. This is for detecting the temperature rise in the non-paper passing portion when A5 sized paper is fed in the paper passing region AREA1in a state of being shifted to one side from the conveyance reference position X0, in the longitudinal direction of the heater600.

FIG. 7is a circuit diagram depicting a control circuit700of the heater600of Example 2. In Example 2, the triac445, for driving the heat generating block HB5, is connected in series to a subsequent stage of the triac443for driving the heat generating block HB3. The heat generating block HB3and the heat generating block HB5are symmetrical with respect to the conveyance reference position X0in the longitudinal direction of the substrate305, and hence, even when AREA2is heated, the heat generating can be controlled without being affected by this driving configuration. By connecting the triacs445and443like this, even if a disconnection occurs at point S, the thermistor T2-5can detect the abnormal heat generating of the heater600and stop the heater600, just like Example 1, and, therefore, the number of thermistors can be decreased compared with other heat generating blocks.

FIG. 8is a control flow chart according to Example 2. The steps in S500to S503are the same as Example 1. In this flow chart, a case of detecting the B5 size, which corresponds to AREA2, in the paper size detection in step S801will be described. When the triacs443to445corresponding to the B5 size are controlled, the power supply ratio between the triac443and the triac445is controlled to be 100:100 in step S802. In step S803, when the temperatures detected by the thermistors T2-3and T2-5, which are edge thermistors of the heat generating blocks HB3and HB5, are Th2-3and Th2-5, it is checked whether the difference of Th2-3and Th2-5exceeds the temperature difference TΔ, which was set in advance in step S800. If the temperature of the thermistor T2-5is high and the temperature difference exceeds TΔ, for example, it is regarded that the recording paper P was shifted toward the heat generating block HB3in step S804, and the power supply ratio of the triacs443and445is decreased to 100:50, so as to suppress the temperature rise at the non-paper passing portion. In step S805, the temperature rise at the non-paper passing portion is detected, just like Example 1, and it is checked whether the detected temperature of the thermistors T2-5and T2-3exceed the threshold Tmax. If the detected temperatures exceed the threshold Tmax, the throughput is decreased in step S508, and control is continued. The above series of controls are repeated until the print job ends.

As described above, when a pair of heat generating blocks, which are disposed symmetrically with respect to the conveyance reference X0of the recording paper, are connected in series and driven, the number of thermistors can be decreased just like Example 1, even if the heat generating blocks are not adjacent to each other.

Example 3 of the present invention will be described. Example 3 is a modification of the drive configuration of Example 2, and the semiconductor element on the second stage, out of the semiconductor elements connected in series, is shorted. In Example 3, the recording paper P is not shifted because of the conveying guide (not illustrated), and hence, the semiconductor element in the second stage may be shorted without disposing the triac445in a subsequent stage, as in Example 2. A composing element of Example 3 that is the same as Examples 1 and 2 is denoted with a same reference symbol, and a description thereof is omitted. Matters that are not explained particularly in Example 3 are the same as those in Example 1 and Example 2.

The configuration of a heater900according to Example 3 will be described with reference toFIGS. 9A and 9B.FIG. 9Ais a cross-sectional view of the heater900(cross-sectional view of an area near the conveyance reference position X0inFIG. 9B), andFIG. 9Bis a plan view of each layer of the heater900. As illustrated inFIG. 9B, the number of thermistors of the heat generating block HB3is smaller by one than that of Example 2 on the sliding surface layer1.

FIG. 10is a circuit diagram depicting a control circuit901of the heater900of Example 3. Even if a disconnection occurs at point T, the thermistor T2-5can detect the abnormal state and protect the heater900. In the same manner, even if a disconnection occurs at point U, the thermistor T1-3can protect the heater900. In other words, even if the number of thermistors is less than that of the other heat generating blocks1,2,4, and,6and7, the abnormality state of the heater900can be detected, and the heater900can be protected.

As described above, the number of thermistors can be decreased, even in the configuration in which the semiconductor element in a subsequent stage, out of the semiconductor elements connected in series, is shorted, and, therefore, the width of the heater900can be decreased, and the fixing apparatus200can be downsized.

Further, in Example 3, the supply of power to the respective heat generating elements that heat the heat generating block HB3and the heat generating block HB5, which are disposed symmetrically with respect to the conveyance reference position X0of the recording material in the longitudinal direction of the substrate, is controlled by controlling a single triac443. The configuration to which the present invention can be applied is not limited, however, to this configuration. For example, the supply of power to of the heat generating elements302a-2and302b-2for heat generating the heat generating block HB2and the supply power to the heat generating elements302a-6and302b-6for heat generating the heat generating block HB6, may be controlled by controlling a single triac442.

Example 4 of the present invention will be described. A control circuit904of a heater903of Example 4 has a configuration combining Example 1 and Example 3. A composing element of Example 4 that is the same as Examples 1 to 3 is denoted with the same reference symbol, and a description thereof is omitted. Matters that are not explained particularly in Example 4 are the same as those in Examples 1 to 3.

The configurations of the control circuit904of the heater903according to Example 4 will be described with reference toFIGS. 11A and 11B.FIG. 11Ais a cross-sectional view of the heater903(cross-sectional view of an area near the conveyance reference position X0inFIG. 9B), andFIG. 9Bis a plan view of each layer of the heater903. As illustrated inFIG. 11B, the heater903of Example 4 has less thermistors on the sliding surface layer1as compared with Example 1 and Example 3.

FIG. 12is a circuit diagram depicting a control circuit904of the heater903. The heat generating blocks HB1, HB3, HB5and HB7have one thermistor respectively based on the configuration described in Examples 1 and 3. Further, in Example 4, the triac441and the triac447are disposed in the fixing apparatus200. Thereby the number of AC lines, which connect the control circuit904and the fixing apparatus200, can be decreased, and hence, the number of pins of the connector and the number of wires can be decreased. In the same manner, the triacs442to446may also be disposed in the fixing apparatus200.

As described above, the heater904can be protected in the abnormal state using less thermistors, since a plurality of heat generating blocks connected in series are driven. Therefore, the width of the heater904can be decreased, and the fixing apparatus200can be downsized. Further, the wires can be decreased by disposing the triacs inside the fixing apparatus200, and, as a result, the image forming apparatus100can be downsized.

In Examples 1 to 4, the configuration is for protecting the heater from one failure, but the present invention is not limited to one failure, and may have a configuration that protects the heater from two or more failures. Further, the semiconductor elements that are connected in series are not limited to two stages, but may be three or more stages.

The configuration of each of the above examples may be combined as much as possible.

Example 5 of the present invention will be described with reference toFIGS. 13A and 13BtoFIGS. 15A and 15B. Example 5 is a configuration example in which the number of thermistors in HB1and HB7in the heater300, described in Example 1, can be further decreased than the number in Example 1. The heater of Example 5 includes a control circuit1001, in which a disconnection detecting portion1002, which can detect the disconnection at point P, and a disconnection detecting portion1003, which can detect the disconnection at point Q, are provided to the control circuit400of Example 1. A composing element of Example 5 that is the same as Examples 1 to 4 is denoted with a same reference symbol, and a description thereof is omitted. Matters that are not explained particularly in Examples 1 to 4.

FIGS. 13A and 13Bshow a cross-sectional view and a plan view of a heater1000. The number of thermistors in each of the heat generating blocks HB2to HB6is three, which is one more compared with Example 1, on the sliding surface layer1inFIG. 13B. If there are three thermistors in each heat generating block, the abnormality of the heater1000can be detected even if two components fail and cannot function. On the other hand, the number of thermistors in HB1and HB7is one, which is two less than the other heat generating blocks. The reason for this arrangement will be explained with reference toFIG. 14.

FIG. 14is a circuit diagram depicting the control circuit1001of the heater1000in Example 5. The voltages applied to all the thermistors T1-1to T1-7, T2-2to T2-6and T3-2to T3-6inFIG. 14are all divided by the resistors451to457,462to466and472to476pulled up to Vcc1respectively. The divided voltages are detected by the CPU420as Th1-1to Th1-7signals, Th2-2to Th2-6signals and Th3-2to Th3-6, and the temperature is detected. In Example 5, the disconnection detecting portion1002and the disconnection detecting portion1003are disposed so as to detect the disconnection at point P and point Q. The detected signal Di1002, Di1003, Di1004, and Di1005of each disconnection detecting portion1002and1003are connected to the latch units432and436or the CPU420. When a disconnection is detected, the disconnection detecting portion1002outputs the disconnection detecting signals Di1002and Di1004, and the disconnection detecting portion1003outputs the disconnection detecting signals Di1003and Di1005. When Di1004and Di1005are output, the latch units432and436are activated and set the RLOFF1signal and RLOFF2signal to Low, latch these signals, and turn the relays430and440OFF. When Di1002and Di1003are output, the CPU420outputs the FUSER1to FUSER7signals so that the triacs441to447are turned OFF. The internal circuits of the disconnection detecting portion1002and the disconnection detecting portion1003will be described with reference toFIGS. 15A and 15B.

Here, the relationship between the disconnection detection and the number of thermistors, which is a characteristic of Example 5, will be described. In Example 5, just like Example 1, the triacs441and447, which drive the heat generating blocks HB1and HB7, are connected to the triacs442and446in series, which drive the adjacent heat generating blocks HB2and HB6respectively. Therefore, unless one failure, in which disconnection occurs at point P and point Q, is generated, the heat generating blocks HB1and HB7alone do not abnormally heat up. Therefore, the number of thermistors in HB1and HB7can be decreased by one, compared with the other heat generating elements, just like Example 1. Further, in Example 5, the disconnection detecting portions1002and1003, for detecting whether the disconnection occurred at point P and point Q, are included. Therefore, the heat generating blocks HB1and HB7alone will never abnormally heat up, unless a first failure in which disconnection occurs at points P and Q, and a second failure in which the disconnection detecting portions fails, are generated. Hence, the number of thermistors in HB1and HB7can be decreased by two, compared with the other heat generating elements.

FIGS. 15A and 15Bshow an internal circuit of the disconnection detecting portion1002shown inFIG. 14. The internal circuit of the disconnection detecting portion1003is the same as that of the disconnection detecting portion1002, and hence, a description thereof is omitted.FIG. 15Ais a diagram depicting a circuit in which the signal Di1002, output from the disconnection detecting portion1002, is connected to the CPU420, and the signal Di1004is connected to the latch units432and436. Inside the disconnection detecting portion1002, a detection resistor1010is connected near point P, as a second current detecting portion to detect the current that flows through point P. Further, a resistor1013and an AC coupler1015, which propagates a signal detected by the detection resistor1010to the secondary side, are connected in parallel with the detection resistor1010. Furthermore, inside the disconnection detecting portion1002, a detection resistor1011is disposed as a first current detecting portion so that the current to the triac441can be detected. Also in parallel with the detection resistor1011, a resistor1014and an AC coupler1016, which propagates the signal detected by the detection resistor1011to the secondary side, are connected. The current path to supply current to the heat generating resistors302a-2and302b-2branches in the middle of the line connecting the triac442and the heat generating resistors302a-2and302b-2, and is connected to the heat generating resistors302a-1and302b-1via the triac441. In other words, the first current path, which supplies the current from the branch point to the heat generating resistors302a-1and302b-1located downstream of the branch post, and the second current path, which supplies the current from the branch point to the heat generating resistors302a-2and302b-2located downstream of the branch point, are branched from the third current path located upstream of the branch point.

The secondary side of the AC coupler1015is connected to the power supply Vcc1via a pull up resistor1017, and is then connected to the CPU420via a damping resistor1025. When AC current is supplied to point P, AC voltage is applied to both ends of the detection resistor1010, and the applied voltage signal is transferred to the secondary side via the AC coupler1015. Here, the AC photocoupler is used for the AC coupler1015to transfer the signal of the full wave AC current to the secondary side, but a regular photocoupler may be used if only a signal of a half wave current is transferred. The signal transferred to the secondary side becomes a pulse signal, and is outputted to the CPU420as the disconnection detecting signal Di1002. The CPU420determines that disconnection occurred if the pulsed disconnection detection signal Di1002from the disconnection detecting portion1002is not detected, even if the FUSER1signal is turned ON and the triac442is turned ON, and that disconnection did not occur if the pulsed disconnection signal Di1002is detected. When the CPU420determines that disconnection occurred, the FUSER1and FUSER2are turned OFF to interrupt power being supplied to the triacs441and442. The waveforms will be described in detail with reference toFIG. 15B. The pulse signals transferred to the secondary side by the AC coupler1015and the AC coupler1016pass through the resistors1018and1022respectively, smoothed by the capacitors1019and1023and resistors1020and1024, and connected to the comparator1025. When the current is flowing in the detection resistor1011, even if current is not flowing in the detection resistor1010, it is likely that the route passing through point P is more likely disconnected. In this case, inFIG. 15B, the voltage at the negative (−) terminal of the comparator1025exceeds the voltage at the positive (+) terminal, the output Di1004signal becomes LOW, and the latch units432and436are activated. The waveforms will be described in detail with reference toFIG. 15B.

FIG. 15Bis a waveform chart depicting the operation of the circuit inFIG. 15A. A waveform1101indicates the voltage detected by the detection resistor1010, a waveform1102indicates the voltage detected by the detection resistor1011, and a waveform1103indicates the Di1002signal output from the disconnection detecting portion1002. The solid line of waveform1104indicates the voltage that is applied to the − terminal of the comparator1025, and the dotted line thereof indicates the voltage that is applied to the + terminal of the comparator1025. When the triac442is in the OFF state and power is OFF, the voltage is not generated (0V) at the detection resistor1010, and, as a result, the transistor of the secondary side AC coupler1015is not activated. Therefore, the Di1002signal becomes as indicated by the waveform1103, and the voltage is pulled up to Vcc1. Further, when the triac442is turned ON and power is turned ON, voltage is generated at the detection resistor1010, as indicated by the waveform1101. As a result, the transistor of the secondary side AC coupler1015is activated, and makes the Di1002signal LOW, and, therefore, the output Di1002signal becomes the pulsed signal, as indicated by the waveform1103. The CPU420can determine whether the current is supplied to the detection resistor1010or not, by detecting this pulsed waveform. When the disconnection is generated at point P, voltage is not generated at the detection resistor1010, even if the triac442is turned ON, and hence, the waveform1101and the waveform1103indicate the same waveform as the waveform when power is OFF. Therefore, when the waveform1103is not the pulsed waveform, even if the triac442is turned ON and power is ON, by the CPU420, it can be determined that disconnection is generated at point P, and the power of the triac442can be turned OFF.

When the triac442is in the OFF state and power is OFF, the transistor of the secondary side AC coupler1015is not activated. Therefore, the voltage at the − terminal of the comparator1025becomes a constant voltage that is determined by the voltage division by the resistors1017,1018, and1020, as indicated by the solid line of the waveform1104. In the same manner, the voltage is not generated at the detection resistor1011, and hence, the voltage at the + terminal of the comparator1025also becomes a constant voltage that is determined by the voltage division by the resistors1021,1022, and1024, as indicated by the dotted line of the waveform1104. Here, the resistance values of the resistors1017,1018, and1020and the resistors1021,1022, and1024are set so that the voltage at the + terminal is greater than the voltage at the − terminal. Since the voltage at the + terminal is greater than the voltage at the − terminal, the output of the comparator1025becomes the open collector output, and the latch unit does not perform the latch operation. When the triac442is turned ON and power is turned ON, a voltage is generated at the detection resistor1010, as indicated by the waveform1101. As a result, the transistor of the secondary side AC coupler1015is activated, and the voltage at the − terminal of the comparator1025gradually decreases, as indicated by the solid line of the waveform1104. Further, when the triac441is turned ON and power is turned ON, voltage is generated at the detection resistor1011, as indicated by the waveforms1102. Hence, the voltage at the + terminal of the comparator1025gradually decreases, as indicated by the dotted line of the waveform1104. Here, the resistance values of the detection resistors1010and1011have been adjusted so that the voltage at the + terminal is greater than the voltage at the − terminal. Since the voltage at the + terminal is greater than the voltage at the − terminal, the output of the comparator becomes the open collector output, and the latch unit does not perform the latch operation. When the disconnection is generated at point P, the voltage is not generated at the detection resistor1010even if the triac442is turned ON, and hence, the transistor of the secondary side AC coupler1015is not activated. Therefore, the voltage at the − terminal gradually increases, as indicated by the solid line of the waveform1104. Since the triac441is continuously ON even if disconnection is generated at point P, the voltage at the + terminal remains in the power ON state, as indicated by the dotted line of the waveform1104. As a result, the voltage at the − terminal of the comparator eventually exceeds the voltage at the + terminal after the disconnection at point P, as indicated by the waveform1104. Then, the output of the comparator becomes LOW, whereby the latch units432and436are activated.

As described above, according to Example 5, in the heat generating blocks HB1and HB2, which are driven by the semiconductor elements in subsequent stages of the semiconductor elements to drive the heat generating blocks HB2and HB6, the disconnection detecting portions, to detect disconnection in HB2and HB6, are disposed. Thereby, even if the number of thermistors in the heat generating blocks HB1and HB2is less than the other heat generating blocks, the heater300can be protected even when two failures occur.

Example 6 of the present invention will be described with reference toFIGS. 16A and 16B. Example 6 is a configuration in which the disposed position of the detection resistor1012and the connection position of Di1002are different in the circuit of the disconnection detecting portion1002described inFIG. 15Aof Example 5. The other configuration is the same as Example 5. A composing element of Example 6 that is the same as Examples 1 to 5 is denoted with the same reference symbol, and a description thereof is omitted. The rest is the same as Examples 1 to 5.

FIG. 16Ais a diagram depicting the disconnection detecting portion1002, and a current detection resistor1010to detect current that flows through point P is connected near point P. Further, inFIG. 16Aa detection resistor1012(third current detecting portion) is disposed immediately after the triac442, that is, on the third current path before branching into the first current path and the second current path, so that whether the current is supplied from the triac442or not can be determined. In each of the detection resistors1010and1012, the AC couplers1015and1016are connected in parallel, and the detection signal transferred to the secondary side is smoothed by the capacitors1019and1023and the resistors1020and1024, and are connected to comparators1030and1031, respectively. The output of the comparator1030is connected to the + terminal of the comparator1031via a transistor1034and resistors1032and1033.

When the current is not flowing in the detection resistor1010, even when the current is flowing in the detection resistor1012, it is likely that the route passing through point P is disconnected. In this case, inFIG. 16A, the voltage at the − terminal of the comparator1031exceeds the voltage at the + terminal, the output Di1004signal becomes LOW, and the latch units432and436are activated. At this time, the output of the Di1002signal, which is connected to the CPU420, also becomes LOW. When the Di1002signal becomes LOW, even if the triac442is ON, the CPU420determines that disconnection is generated at point P, and turns FUSER1and FUSER2OFF so as to interrupt power supply to the triacs441and442. The waveforms will be described in detail with reference toFIG. 16B.

FIG. 16Bis a waveform chart depicting the operation of the circuit shown inFIG. 16A. InFIG. 16B, a waveform1105indicates the voltage detected by the detection resistor1010, and the waveform1106indicates the voltage detected by the detection resistor1012. The solid line of a waveform1107indicates the voltage that is applied to the − terminal of the comparator1030, and the dotted line thereof indicates the voltage that is applied to the + terminal of the comparator1030. The solid line of a waveform1108indicates the voltage that is applied to the − terminal of the comparator1031, and the dotted line thereof indicates the voltage that is applied to the + terminal of the comparator1031. When the triac442is in the OFF state and power is OFF, the voltage is not generated at the detection resistor1012, and, as a result, the transistor of the secondary side AC coupler1016is not activated. Therefore, the voltage at the − terminal of the comparator1030becomes a constant voltage that is determined by the voltage division by the resistors1021,1022and1024, as indicated by the solid line of the waveform1107. Here, the resistors1021,1022and1024, and the resistors1026and1027have been adjusted so that the voltage at the − terminal of the comparator1030is greater than the voltage at the + terminal of the comparator1030. Hence, the output of the comparator1030becomes LOW, and the transistor1034is activated, and the + terminal of the comparator1031becomes HIGH voltage. The voltage at the − terminal of the comparator1031becomes a constant voltage that is determined by the voltage division by the resistors1017,1018, and1020, as indicated by the solid line of the waveform1108, since voltage is not generated at the detection resistor1010. Here, the resistors1017,1018, and1020have been adjusted so that the voltage at the + terminal of the comparator1031is greater than the voltage at the − terminal of the comparator1031. Since the voltage at the + terminal is greater than the voltage at the − terminal, the output of the comparator1031becomes the open collector output, and the latch units432and436do not perform the latch operation. When the triacs442and441are turned ON and power is turned ON, voltage is generated at the detection resistor1012. As a result, voltage at the − terminal of the comparator1030gradually decreases, as indicated by the solid line of the waveform1107. In the same manner, voltage is generated at the detection resistor1010, and hence, voltage at the − terminal of the comparator1031also gradually decreases, as indicated by the solid line of the waveform1108. When the voltage at the + terminal of the comparator1030exceeds the voltage at the − terminal of the comparator1030, the output of the comparator1030becomes the open collector output. As a result, the transistor1034is turned OFF and the voltage that is applied to the + terminal of the comparator1031changes to the voltage determined by the resistors1028and1029, as indicated by the waveform1108. Here, the resistors1028and1029have been adjusted so that the voltage that is applied to the + terminal of the comparator1031is greater than the voltage that is applied to the − terminal thereof. Since the voltage at the + terminal is greater than the voltage at the − terminal, the output of the comparator1031becomes the open collector output, and the latch units432and436do not perform the latch operation. When disconnection is generated at point P, the voltage that passes through the detection resistor1012decreases, and voltage at the − terminal of the comparator1030gradually increases, as indicated by the solid line of the waveform1107. Current is flowing, however, to the triac441, and hence, the increase of voltage at the − terminal of the comparator1030remains within a certain range. Even at this time, the resistors1026and1027have been adjusted so that the voltage that is applied to the + terminal of the comparator1030is greater than the voltage that is applied to the − terminal thereof, and hence, the output of the comparator1030becomes the open collector output. The voltage at the − terminal of the comparator1031, on the other hand, voltage increases, as indicated by the solid line of the waveform1108, since disconnection is generated at point P. Since the voltage at the + terminal of the comparator1031does not change, the voltage at the − terminal of the comparator1031eventually exceeds the voltage at the + terminal after the disconnection at point P, and the output of the comparator1031becomes LOW, whereby the latch units432and436and the CPU420are activated.

As described above, according to Example 6, in the circuit of the disconnection detecting portion1002, the disconnection at point P can be detected even if the disposed position of the detection resistor1012and the connection position of Di1002are different.

Each of the above examples may be combined with each other if possible.

For example, the disconnection detecting portion in Example 5 or Example 6 may be added to the circuit configuration of Example 2 (between the triacs443and445inFIG. 7) or circuit configuration of Example 4 (between the triacs442and441inFIG. 12).