Patent Publication Number: US-7712318-B2

Title: Cooling apparatus, cooling method, program, computer readable information recording medium and electronic apparatus

Description:
BACKGROUND 
   1. Technical Field 
   This disclosure relates to a cooling apparatus such as a Peltier apparatus applying a cooling element such as a Peltier device, a cooling method, a program, a computer readable information recording medium such as a ROM, and an electronic apparatus such as an image forming apparatus in which the cooling method is applied. 
   2. Description of the Related Art 
   Conventionally, a cooling apparatus applying a Peltier device, having a reduced size and including no mechanical moving parts, is therefore employed for avoiding temperature rise otherwise occurring due to heat generated by circuit devices and properly controlling a temperature environment, within an apparatus in which IC devices, electronic devices, electrical components or such are assembled within a narrow space inside of a housing (such an apparatus being referred to as an ‘electronic apparatus’ hereinafter). For example, Japanese Laid-open Utility-Model Application No. 5-28657 discloses an example in which such a cooling apparatus is applied for cooling a recording sheet in a printer, a facsimile machine or such. 
   The Peltier device is a device which has a configuration in which PN thermoelectric semiconductors are bounded by cupper electrodes and in which, when a direct-current voltage is applied, heat movement occurs via the bonded surfaces, and thus, a cooling part occurs on one side and a heat generating part occurs on the other side. A common way of driving the Peltier device in the relate art is such that power supply is made by a constant voltage or in a PWM manner. For example, when the Peltier apparatus (device) is applied in an image foaming apparatus such as a copier, a printer or such, a constant DC voltage is supplied from a power supply unit as it is, or, power supply is made in a PWM manner with the use of an FET switching device or such. In a case where a temperature of a control target is measured by a sensor, the power supply is turned on or turned off in such a manner that the detected temperature is controlled for a target value. Specifically, in the PWM operation, setting of a duty is changed so that a cooling performance is changed accordingly. 
   However, as well known in the art, in a case where the Peltier device is driven in the manner that the power supply is turned on or turned off as mentioned above, degradation of the Peltier device may be accelerated when a temperature difference between a heat generating surface and a heat sink surface of the Peltier device occurs sharply as a result of a large change in the driving voltage being applied. As a result, the life of the Peltier device may be shortened. 
   Japanese Laid-open Patent Application No. 11-289111 discloses a method for solving this problem of degradation of the Peltier device. In this method, an analog feedback control circuit is provided by which a detected temperature obtained from a sensor which detects a temperature of a cooling target is fed back in such a manner that a power supply voltage to the Peltier device may change in proportion to the detected temperature. Further, in this method, a delay circuit is provided such as to provide a delay when the detected temperature is fed back. Thanks to operation of the control circuit having such a configuration, the driving voltage applied to the Peltier device is prevented from being rapidly changed, the input voltage is prevented from being changed in a step manner also in the PWM operation, and thus, a temperature change on the cooling surface generated in the Peltier device occurring in response to a temperature change in the cooling target is controlled to occur gently. Thus, the cooling operation is carried out in such a manner that rapid heat shrinkage in the Peltier device can be avoided, and thus, performance degradation can be reduced. 
   However, in the above-mentioned prior art method, the analog feed back control circuit delays the detected temperature of the cooling target as mentioned above. Accordingly, a relatively long time may be required (that is, a follow-up speed is slow) until the applying driving voltage is stabilized into a proper value upon turning-on of the Peltier device (at a time of rising up) or turning-off of the Peltier device (at a time of decaying down), or in response to a sharp temperature change occurring in the cooling target. Therefore, for a case where a subsequent step is executed after this stabilization, processing delay may occur. Further, since this method employs the analog circuit, it may be difficult to carry out adjustment of the performance of the control circuit including adjustment of the sensor for detecting the temperature. the delay circuit, and so forth. As a result, it may be difficult to optimize the circuit performance for each particular apparatus. 
   Further, in the above-mentioned prior art method, the configuration is provided such that, as mentioned above, the temperature change caused on the cooling surface of the Peltier device is controlled to occur gently. However, the essential factor causing the relevant performance degradation is a sharp occurrence of temperature difference between the heat generating surface and the heat sink surface of the Peltier device. The control in the above-mentioned prior art method may not be directly directed to avoiding this sharp occurrence of temperature difference between the heat generating surface and the heat sink surface of the Peltier device. Accordingly, this method may not be sufficiently effective to solve the relevant problem. 
   SUMMARY 
   In an aspect of this disclosure, an apparatus is provided to reduce a degradation otherwise occurring in the Peltier device, and to achieve easier setting of the applying power supply voltage, by which requirements for a relevant apparatus including a requirement for the follow-up speed can be met, for a case where the applying power supply voltage is changed when the driving power for the Pettier device is turned on or turned off, in a Peltier apparatus (cooling apparatus) in which the driving power is supplied to the Peltier device (cooling element) so that the Peltier device is driven, and thus, the control operation can be optimized for each particular apparatus. Further, a cooling method, a program, a computer readable information recording medium, and an electronic apparatus can be provided concerning the above-mentioned cooling apparatus. 
   In another aspect of this disclosure, there is provided a cooling apparatus of supplying driving power to a cooling element from a power supply unit, to drive it, that includes a driving power changing part changing a power value of the driving power supplied to the cooling element from the power supply unit, by a predetermined power value at each predetermined Lime interval. Further, an electronic apparatus employing the above-mentioned cooling apparatus for keeping a predetermined operation environment includes a sensor detecting the operation environment; and a control part controlling a driving manner of the cooling apparatus. 
   In another aspect of this disclosure, a cooling method executed by a cooling apparatus which supplies driving power to a cooling element from a power supply unit to drive it, includes a driving power changing step of changing a power value of the driving power supplied to the cooling element from the power supply unit, by a predetermined power value at each predetermined time interval. Further, a program, such as embodied in a computer readable information recording medium, includes instructions for causing a computer to execute respective steps of the above-mentioned cooling method. 
   In the Peltier apparatus (cooling apparatus) according to an exemplary embodiment of this disclosure, the driving power changing part can change the power value of the driving power to the cooling element at each predetermined time interval by the predetermined power value, and thus control the power value of the driving power for driving the Peltier device (cooling element). Accordingly, it is possible to reduce a change amount in the driving voltage when driving the Peltier device, and thus, to prevent temperature difference between a heat generating surface and a heat sink surface of the Peltier device from occurring sharply. Thereby, it is possible to avoid a degradation otherwise occurring in the Peltier device. Further, by providing a timer to change a setting of the above-mentioned driving control, it is possible to easily optimize the control operation for each particular apparatus (each particular device/element). Further, by applying the Pettier apparatus (cooling apparatus) of this disclosure for keeping an operation environment of an electronic apparatus for predetermined requirements, it is possible to improve the overall performance of the electronic apparatus. 
   Other aspects and further features of the present invention will become more apparent from the following detailed description when read in conjunction with the accompanying drawings: 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  illustrates a Peltier device driving method (upon power turning on) in an applying voltage changing control manner; 
       FIG. 2  illustrates a Peltier device driving method (upon power turning off) in the applying voltage changing control manner; 
       FIG. 3  shows an embodiment of a driving part of a Peltier apparatus in the applying voltage changing control manner; 
       FIG. 4  shows a Peltier device driving control operation flow chart in the applying voltage changing control manner; 
       FIG. 5  illustrates a Peltier device driving method (upon power turning on) in a PWM control manner; 
       FIG. 6  illustrates a Peltier device driving method (upon power turning off) in a PWM control manner; 
       FIG. 7  shows an embodiment of a driving part of a Peltier apparatus in the PWM control manner; 
       FIG. 8  shows a Peltier device driving control operation flow chart in the PWM control manner; 
       FIG. 9  shows a block diagram of a humidity/temperature-control system employing the Peltier apparatus; 
       FIG. 10  shows a block diagram of another example of a humidity/temperature control system employing the Peltier apparatus; 
       FIG. 11  shows a general configuration of a temperature control system applied for a writing unit of an image forming apparatus; 
       FIG. 12  shows a general configuration of a humidity control system applied for a photosensitive body unit of an image forming apparatus; 
       FIG. 13  shows a general configuration of a humidity control system applied for a transfer paper tray of an image forming apparatus; 
       FIG. 14  shows a general configuration of a temperature control system applied for a fixing unit of an image forming apparatus; 
       FIG. 15  shows a copier as a specific example of the image forming apparatus to which the present invention may be applied; and 
       FIG. 16  shows a hysteresis curve applicable in an on-off control of a Peltier apparatus according to the present invention. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   Embodiments of a Peltier apparatus according to the present invention are described with reference to figures. 
   As mentioned above, an object of the present invention is to solve the above-mentioned problem, i.e., to avoid a degradation of a Peltier device, to achieve easier setting of an applying supply power voltage which setting is such as to enable fulfillment of requirements of the apparatus, i.e., the Peltier apparatus, including the follow-up speed requirement, and to optimize control operation for each particular device or for each particular apparatus. The Peltier apparatus is an apparatus employing the Peltier device as an element, and is configured to utilize a cooling function of the Peltier device. In order to achieve the object of the present invention, a driving DC voltage (power supply voltage) applied to the Peltier device is controlled according to predetermined control requirements upon turning on or turning off of the power supply to an electronic apparatus. In this control scheme, basically, two control manners, i.e., an applying voltage changing control manner and a PWM control manner are proposed, in a first embodiment and a second embodiment, respectively, as will be described below. Further, third through eighth embodiments are those concerning configurations in which the Peltier apparatus according to the present invention is applied to an electronic apparatus such as an image forming apparatus. 
   The first embodiment of the present invention is described. 
   In this embodiment, as mentioned above, the applying voltage changing control manner is applied, in which the applying voltage for driving the Peltier device is changed. 
   In this manner, upon turning on or turning off of power supply to an electronic apparatus, the applying voltage to the Peltier device is increased or decreased stepwise. In this control, a time interval for which each level of the applying voltage is output can be set arbitrarily as a control requirement, for the purpose of achieving the above-mentioned object of the present invention to optimize the operation control for each particular apparatus. 
     FIG. 1  illustrates operation according to the applying voltage changing control manner, specifically showing a time chart of a change in the applying voltage upon power supply tuning on, and a change of a temperature occurring from the Peltier device in response thereto. As shown in  FIG. 1 , (A), which shows a driving trigger signal for the Peltier device, a rising up (switching) of the driving trigger signal is applied as a trigger.  FIG. 1 , (B) shows the applying voltage (driving power) for driving the Peltier device. In this example, the voltage value is increased stepwise by 3 V every step for a final target value (12 V). Thereby, the power value of the driving power increases stepwise, and thus, a temperature difference between a heat generating surface and a heat sink surface of the Peltier device increases accordingly.  FIG. 1 , (C) shows the temperature difference ΔT between the heat generating surface and the heat sink surface of the Peltier device. 
   By preventing this temperature difference ΔT from increasing sharply, it is possible to reduce a degradation occurring in the Peltier device, which is the object of the present invention as mentioned above. For this purpose, the output time interval for keeping each output level of the applying voltage shown in FIG.  1 , (B) every step is set appropriately in such a manner that the temperature change ΔT may occur gently. Then, operation according to the thus-obtained setting is carried out. Specifically, first, it is assumed that the above-mentioned trigger (rising up of the driving trigger signal shown in  FIG. 1 , (A)) occurs at a time t 0 , and respective times at which the applying voltage is switched to the subsequent one for increasing the applying voltage stepwise are t 1 , t 2  and t 3 , as shown in  FIG. 1 , (B). For example, setting is made as follows: t 1 −t 0 =20 [secs]; t 2 −t 1 =15 [secs]; and t 3 −t 2 =10 [secs]. Thereby, it is possible to obtain a gentle increasing slope of the temperature difference ΔT as shown in  FIG. 1 , (C) in which the temperature difference is prevented from increasing sharply. By thus increasing (changing) the power value of the driving power by the predetermined power value at each predetermined time interval, it is possible to obtain the desired gentle manner of change in the temperature difference ΔT. 
     FIG. 2  illustrates operation according to the applying voltage changing control manner, specifically showing a time chart of a change in the applying voltage upon power supply tuning off, and a change of a temperature occurring from the Peltier device in response thereto. As shown in  FIG. 2 , (A), which shows a driving trigger signal for the Peltier device. A decaying down (switching) of the driving trigger signal is applied as a trigger.  FIG. 2 , (B) shows the applying voltage (driving power) for driving the Peltier device. In this example, the voltage value is decreased stepwise by 3 V every step for a final target value (0 V). Thereby, the power value of the driving power decreases stepwise, and thus, a temperature difference between the heat generating surface and the heat sink surface of the Peltier device decreases accordingly.  FIG. 2 , (C) shows the temperature difference ΔT between the heat generating surface and the heat sink surface of the Peltier device. 
   By preventing this temperature difference ΔT from decreasing sharply, it is possible to reduce a degradation occurring in the Peltier device, which is the object of the present invention. For this purpose, an output time interval for keeping each output level of the applying voltage shown in  FIG. 2 , (B) in each step is set appropriately in such a manner that the temperature changes ΔT may occur gently. Then, operation according to the thus-obtained setting is carried out. Specifically, it is assumed that the above-mentioned trigger (decaying down of the driving trigger signal shown in  FIG. 2 , (A)) occurs at a time t 0 , and respective times at which the applying voltage is switched to the subsequent one for decreasing the applying voltage stepwise are t 1 , t 2  and t 3 , as shown in  FIG. 2 , (B). For example, setting is made as follows: t 1 −t 0 =10 [secs]; t 2 −t 1 =15 [secs]; and t 3 −t 2 =20 [secs]. Thereby, it is possible to obtain a gentle decreasing slope of the temperature difference ΔT as shown in  FIG. 1 , (C) in which the temperature difference is prevented from decreasing sharply. By thus decreasing (changing) the power value of the driving power by the predetermined power value at each predetermined time interval, it is possible to obtain the desired gentle manner of change in the temperature difference ΔT. 
     FIG. 3  shows an embodiment of a Peltier device driving part in a Peltier apparatus for carrying out the above-described operation (see  FIGS. 1 and 2 ) according to the applying voltage changing control manner. 
   As shown in  FIG. 3 , the driving part of the Peltier apparatus includes a power supply unit  101  and a Peltier control part  201 . The Peltier control part  201  controls a driving DC voltage supplied by the power supply unit  101  for the purpose of providing a variable voltage for driving the Peltier device  301 . That is, the driving power supplied by the power supply unit  101  to the Peltier device  101  is appropriately controlled. The Peltier control part  201  includes a control part including a CPU  211  and timer counters  212 - 1  through  212 -n corresponding to respective ones of the variable voltage; and a power supply voltage decaying circuit including an FET  213  and a coil  214  by which the output voltage to the Peltier device  301  is controlled according to a control signal from the CPU  211 . 
   The CPU  211  includes, as well-known in the art for achieving a control function in various types of electronic apparatuses, a ROM or a RAM (not shown) under the control of the CPU, and operate according to instructions written in a predetermined program for controlling driving of the Peltier device  301 . When power supply is turned on and the electronic apparatus is started up, or the electronic apparatus is stopped by turning off of the power supply thereto, the Peltier apparatus provided in the electronic apparatus is started up or stopped accordingly. For this purpose, an instruction is provided to the Peltier apparatus. In response thereto, the CPU  211  starts up the program and operates according to the instructions written in the control program stored in the ROM, for example, a program for carrying out a control flow (see  FIG. 4 ) for applying the variable voltage for driving the Peltier device  301 , as described later. Then, the CPU  211  controls driving of the Peltier device  301  according to the program. 
   Operation of controlling the applying voltage for driving the Peltier device  301  in the Peltier control part  201  is carried out as follows: That is, according to the control program, the CPU  211  outputs a predetermined PWM signal to the FET  213 , and thereby, output of the voltage decaying circuit including the FET  213  and the coil  214  is controlled in such a manner that the output voltage is obtained in proportion to a duty of the PWM signal. The duty of the PWM signal means a pulse modulation factor, that is, actually, a ratio of a turn-on duration to a modulation period in the PWM signal. 
   At this time, according to the embodiment of the present invention, each output time interval of the PWM signal corresponding to the variable voltage applied to the Peltier device  301  is controlled. For this purpose, the timer counters  212 - 1  through  212 -n are provided for counting or measuring the respective ones of the output time intervals corresponding to the number of the respective PWM signals corresponding to the output steps (n times). A time interval setting can be made for each timer counter thereof to an arbitrary value. The CPU  211  manages the output time for each level of the above-mentioned stepwise control of the variable voltage (see  FIG. 1  or  2 ) by means of the group of the timer counters  212 - 1  through  212 -n, and thus, the CPU  211  can control a time interval for applying each step of the variable voltage. 
   This group of the timer counters  212 - 1  through  212 -n are applied for the starting up (turning on) process (see  FIG. 1 ) and for the stopping (turning off) process ( FIG. 2 ). However, since these processes should not occur simultaneously, it is possible to apply the timer counter group in common for each of both processes. For example, as in the example of  FIGS. 1 , (B) and  2 , (B), in which the respective time intervals are common for both processes, the timer counters can be applied without changing the time interval settings thereof. Thereby, it is possible to simplify the system. 
   A control flow of driving of the Peltier device  301  carried out by the CPU  211  of the above-described control part  50  shown in  FIG. 3  according to the control program is described now. 
     FIG. 4  shows the control flow of driving of the Peltier device  301 .  FIG. 4 , (A) shows the control flow for the power supply turning on process while  FIG. 4 , (B) shows the control flow for the power supply turning off process. 
   It is noted that, the power supply turning on process is carried out when the electronic apparatus such as an image forming apparatus is turned on. Upon turning on of the electronic apparatus, generation of a heat from various parts as described later is started. The Peltier apparatus according to the embodiment of the present invention is provided to control the thus generated heat or humid air so as to keep the temperature or humidity environment in the electronic apparatus at a predetermined level. In the same line, the power supply turning off process is carried out when the electronic apparatus is turned off. Upon turning off of the electronic apparatus, the heat or humid air is no more generated, and thus, the cooling function of the Peltier device  310  is no more required. 
   As described above, in the Peltier device driving method in the present embodiment, the applying voltage is controlled by the control of the duty of the PWM signal. This value of the duty of the PWM signal is referred to as the ‘PWM duty’, hereinafter. Specifically, the PWM duty is changed 10% every step, the applying voltage is changed by 2.4 volts every step accordingly, and the PWM duty of 50% is set for a completely started up state while the PWM duty of 0% is set for a completely stopped down state. Further, the respective times at which the PWM duty is switched (t 1 , t 2  and t 3  in the example of  FIG. 1 , (B) or  FIG. 2 , (B)) are set in such a manner that each switching is carried out at a time interval of 15 seconds every step. Accordingly, the completely starting up is achieved for a total 60 seconds from the turning on trigger or the completely stopping down is achieved for a total 60 seconds from the turning off trigger. In the present embodiment, the respective times at which the duty of the PWM signal, i.e., t 1 , t 2 , t 3 , . . . tn are obtained from the respective settings of the timer counters  212 - 1  through  212 -n, provided for the number of the output steps. However, for a case where the time interval between the applying voltage switching operations is fixed as 15 seconds every step as mentioned above, it is possible to apply a common timer counter therefor. However, in such a case, another timer counter is required for setting the completion timing of the starting up process or for the stopping down process. That is, in the above-mentioned example, the timer counter measuring 60 seconds is required other than that measuring 15 seconds. 
   For the power supply turning on process, as shown in  FIG. 4 , (A), the CPU  211  responds to the Peltier device driving turning on trigger (see  FIG. 1 , (A)), and then transmits the PWM signal of duty 10% to the FET  213 , which controls the power supply from the power supply unit  101  in the PWM duty of 10%, and thus applies the driving voltage of 2.4 V to the Peltier device  301  via the coil  214  (Step S 101 ). 
   At the same time as that of the output of the PWM duty 10%, the CPU  211  starts up the first-stage timer counter  212 - 1  (with the time interval setting of 15 seconds) which counts up by 1 second each (Step S 102 ), and determines whether or not 15 seconds have elapsed from the starting up (whether or not the count value of the timer counter has reached 16 seconds) each time of the 1 second counting up operation (Step S 103 ). 
   When the first-stage timer counter  212 - 1  has counted 15 seconds, the CPU  211  switches the driving voltage applied to the Peltier device  301  (Step S 104 ). Specifically, the CPU  211  first disables the output of the PWM signal, increases the duty of the PWM signal further by 10% for further increasing the driving voltage by 2.4 V as a second voltage of 4.8 V, and then, enables the thus-updated PWM signal. Further, after the second-stage time counter  212 - 2  is started up (Step S 102 ) and this timer counter has counted 15 seconds (Yes in Step S 103 ), the setting of the PWM signal is again updated so that the duty is further increased by 10%, and thus, the applying voltage to the Peltier device  103  is increased further by 2.4 V so that 7.2 V is applied. 
   Thus, the duty of the PWM signal is increased by 10% at each elapse of 15 seconds. Then, after 60 seconds have elapsed from the turning on trigger (Yes in Step S 105 ), that is, after the loop of Steps S 102  through S 104  is repeated three times, and the fourth-stage timer counter  212 - 4  reaches a time-up state, the setting of the duty of the PWM signal is increased to 50%, and the applying voltage to the Peltier device  301  reaches 12 V (Step S 106 ). Then, the control flow for the turning on process is finished. 
   For the power supply turning off process, as shown in  FIG. 4 , (B), the CPU  211  responds to the Peltier device driving turning off trigger (see  FIG. 2 , (A)), and then transmits the PWM signal of the duty 40% to the FET  213 , which controls the power supply from the power supply unit  101  in the PWM duty of 40%, and applies the driving voltage of 9.6 V after decaying down from 12 V by 2.4 V, to the Peltier device  301  via the coil  214  (Step S 201 ). 
   At the same time as that of the output of the PWM duty 40%, the CPU  211  starts up the first-stage timer counter  212 - 1  (with the time interval setting of 15 seconds) which counts up by 1 second each (Step S 202 ), and determines whether or not 15 seconds have elapsed from the starting up (whether or not the count value of the timer counter has reached 16 seconds) each time of the 1 second counting up operation (Step S 203 ). 
   When the first-stage timer counter  212 - 1  has counted 15 seconds, the CPU  211  switches the driving voltage applied to the Peltier device  301  (Step S 204 ). Specifically, the CPU  211  first disables the output of the PWM signal, decreases the duty of the PWM signal further by 10% for further decreasing the driving voltage by 2.4 V for a second voltage of 7.2 V, and then, enables the thus-updated PWM signal. Further, after the second-stage time counter  212 - 2  is started up (Step S 202 ) and this timer counter has counted 15 seconds (Yes in Step S 203 ), the setting of the PWM signal is again updated so that the duty is further decreased by 10%, and thus, the applying voltage to the Peltier device  103  is decreased further by 2.4 V so that 4.8 V is applied. 
   Thus, the duty of the PWM signal is decreased by 10% at each elapse of 15 seconds. Then, after 60 seconds have elapsed from the turning off trigger (Yes in Step S 205 ), that is, after the loop of Steps S 202  through S 204  is repeated three times, and the fourth-stage timer counter  212 - 4  reaches a time-up state, the setting of the duty of the PWM signal is decreased to 0%, and the applying voltage to the Peltier device  301  finally reaches 0 V (Step S 206 ). Then, the control flow for the turning off process is finished. 
   The above-mentioned second embodiment of the present invention is described now. 
   In this embodiment, as mentioned above, the PWM control manner is applied, in which the applying voltage for driving the Peltier device is changed. 
   In this manner, upon turning on or turning off of the power supply, the PWM duty is increased or decreased stepwise for a target value. In this control, a time interval for which each level of the applying voltage is output can be set arbitrarily as a control requirement, for the purpose of achieving the above-mentioned object of the present invention. 
     FIG. 5  illustrates operation according to the PWM control manner, specifically showing a time chart of a change in the applying voltage upon power supply tuning on, and a change of a temperature occurring in the Peltier device in response thereto. As shown in  FIG. 5 , (A), which shows a driving trigger signal for the Peltier device, a rising up (switching) of the driving trigger signal is applied as a trigger.  FIG. 5 , (B) shows the applying voltage (driving power) for driving the Peltier device. In this example, the PWM duty is increased stepwise by 25% each step for a final target value (100%). Thereby, the power value of the driving power increases stepwise, and thus, a temperature difference between the heat generating surface and the heat sink surface of the Peltier device increases accordingly.  FIG. 5 , (C) shows the temperature difference ΔT between the heat generating surface and the heat sink surface of the Peltier device. 
   By preventing this temperature difference ΔT from increasing sharply, it is possible to reduce a degradation occurring in the Peltier device, which is the object of the present invention. For this purpose, the output time interval for keeping each PWM duty of the applying voltage shown in  FIG. 5 , (B) every step is set appropriately in such a manner that the temperature changes ΔT may occur gently as shown in  FIG. 5 , (C). Then, operation according to the thus-obtained setting is carried out. Specifically, it is assumed that the above-mentioned trigger (rising up of the driving trigger signal shown in  FIG. 5 , (A)) occurs at a time t 0 , and respective times at which the PWM duty is switched to the subsequent one for increasing the PWM duty stepwise are t 1 , t 2  and t 3 , as shown in  FIG. 5 , (B). For example, setting is made as follows: t 1 −t 0 =20 [secs]; t 2 −t 1 =15 [secs]; and t 3 −t 2 =10 [secs]. Thereby, it is possible to obtain a gentle increasing slope of the temperature difference ΔT as shown in  FIG. 5 , (C) in which the temperature difference is prevented from increased sharply. Thus, by increasing (changing) the power value of the driving power by the predetermined power value (by changing the PWM duty of the driving power) at each predetermined time interval, it is possible to obtain the desired gentle change of the temperature difference ΔT. 
     FIG. 6  illustrates operation according to the PWM control manner, specifically showing a time chart of a change in the applying voltage upon power supply tuning off, and a change of a temperature occurring in the Peltier device in response thereto. As shown in  FIG. 6 , (A), which shows a driving trigger signal for the Peltier device, a decaying down (switching) of the driving trigger signal is applied as a trigger.  FIG. 6 , (B) shows the applying voltage (driving power) for driving the Peltier device. In this example, the PWM duty is decreased stepwise by 25% for each step for a final target value (0%). Thereby, the power value of the driving power decreases stepwise, and thus, a temperature difference between the heat generating surface and the heat sink surface of the Peltier device decreases accordingly.  FIG. 6 , (C) shows the temperature difference ΔT between the heat generating surface and the heat sink surface of the Peltier device. 
   By preventing this temperature difference ΔT from decreasing sharply, it is possible to reduce a degradation occurring in the Peltier device, which is the object of the present invention. For this purpose, an output time interval for keeping each PWM duty of the applying voltage shown in  FIG. 6 , (B) every step is set appropriately in such a manner that the temperature changes ΔT may occur gently as shown in  FIG. 6 , (C). Then, operation according to the thus-obtained setting is carried out. Specifically, it is assumed that the above-mentioned trigger (decaying down of the driving trigger signal shown in  FIG. 6 , (A)) occurs at a time t 0 , and respective times at which the PWM duty is switched to the subsequent one for decreasing the PWM duty stepwise are t 1 , t 2  and t 3 , as shown in  FIG. 6 , (B). For example, setting is made as follows: t 1 −t 0 =10 [secs]; t 2 −t 1 =15 [secs]; and t 3 −t 2 =20 [secs]. Thereby, it is possible to obtain a gentle decreasing slope of the temperature difference ΔT as shown in  FIG. 6 , (C) in which the temperature difference is prevented from decreasing sharply. Thus, by decreasing (changing) the power value of the driving power by the predetermined power value (by controlling the PWM duty of the driving power) at each predetermined time interval, it is possible to obtain the desired gentle change of the temperature difference ΔT. 
     FIG. 7  shows an embodiment of a Peltier device driving part in a Peltier apparatus for carrying out the above-described operation (see  FIGS. 5 and 6 ) according to the above-described PWM control manner. 
   As shown in  FIG. 7 , the driving part of the Peltier apparatus includes a power supply unit  101  and a Peltier control part  201 . The Peltier control part  201  controls a driving DC voltage supplied by the power supply unit  101  for the purpose of providing the driving voltage with the variable PWM duty for driving the Peltier device  301 . That is, the driving power supplied by the power supply unit  101  to the Peltier device  101  is controlled as mentioned above. The Peltier control part  201  includes a control part including a CPU  211  and timer counters  212 - 1  through  212 -n corresponding to the respective PWM duty steps; and a switching driving device  215  controlled by a control signal from the CPU  211  to apply the driving voltage to the Peltier device  301  in the PWM manner. 
   The CPU  211  includes, as well-known in the art for achieving a control function in various types of electronic apparatuses, a ROM or a RAM (not shown) under the control of the CPU, and executes instructions written in a program provided for controlling driving of the Peltier device  301 . When power supply is turned on and the electronic apparatus is started up, or the electronic apparatus is stopped down by turning off of the power supply, the Peltier apparatus is started up or stopped down accordingly. For this purpose, an instruction is provided to the Peltier apparatus. In response thereto, the CPU  211  starts up the program for executing instructions written in the control program stored in the ROM, for example, a program for carrying out a control flow (see  FIG. 8 ) for applying the driving voltage to the Peltier device  301  in the PWM manner as described later. Then, the CPU  211  controls driving of the Peltier device  301  according to the program. 
   Operation of controlling the driving power applied to the Peltier device  301  in the Peltier control part  201  in the PWM manner is as follows: The predetermined PWM signal is output to the switching driving device  215  according to the control program, and, then, according to the duty of the PWM signal, the switching driving device  215  carries out switching operation and thus modulates the power supply provided to the Peltier device  205 . Thus, the driving power applied to the Peltier device  301  is controlled as mentioned above. 
   At this time, according to the second embodiment of the present invention, each output time interval of a respective PWM duty step for modulating the power supply applied to the Peltier device  301  is controlled. For this purpose, the timer counters  212 - 1  through  212 -n are provided for counting or measuring the respective ones of the output time intervals corresponding to the number of the respective PWM duty steps corresponding to the output steps (n times). A time interval setting can be made for each timer counter to an arbitrary value. The CPU  211  manages the output time for each PWM duty of the above-mentioned stepwise control of the PWM duty (see  FIG. 5  or  6 ) by means of the group of the timer counters  212 - 1  through  212 -n, and thus, the CPU  211  can control a time interval for which each PWM duty step. 
   This group of the timer counters  212 - 1  through  212 -n are applied for the starting up process (see  FIG. 1 ) and the down stopping process ( FIG. 2 ). However, since these processes should not occur simultaneously, it is possible to apply the timer counter group in common for these processes. For example, as in the example of  FIGS. 5 , (B) and  6 , (B), in which the respective time intervals are common for these processes, the timer counters can be applied without changing the time interval settings. Thereby, it is possible to simplify the system. 
   A control flow of driving the Peltier device  301  carried out by the CPU  211  of the above-described control part  50  shown in  FIG. 7  according to the control program is described now. 
     FIG. 8  shows the control flow of driving the Peltier device  301 .  FIG. 8 , (A) shows the control flow for the power supply turning on process while  FIG. 8 , (B) shows the control flow for the power supply turning off process. 
   As described above, in the Peltier device driving method in the present embodiment, the PWM duty is controlled so as to control the driving power applied the Peltier device  301 . Specifically, the PWM duty is changed 20% every step, and then, the duty of 100% is set for a completely started up state while the duty of 0% is set for a completely stopped down state. Further, the respective times at which the duty of the PWM signal is switched (t 1 , t 2  and t 3  in the example of  FIG. 6 , (B) or  FIG. 6 , (B)) are set in such a manner that each switching is carried out at a time interval of 15 seconds. Accordingly, the completely starting up is achieved for a total 60 seconds from the turning on trigger or the completely stopping down is achieved for a total 60 seconds from the turning off trigger. In the present embodiment, the respective times at which the duty of the PWM signal, i.e., t 1 , t 2 , t 3 , . . . tn are obtained from the respective settings of the timer counters  212 - 1  through  212 -n, provided for the number of the output steps. However, for a case where the time interval between the applying voltage switching operations is fixed as 15 seconds every step as mentioned above, it is possible to apply a common timer counter therefor. However, in such a case, another timer counter is required for setting the completion timing of the starting up process or for the stopping down process. That is, in the above-mentioned example, the timer counter measuring 60 seconds is required other than that measuring 15 seconds. 
   For the power supply turning on process, as shown in  FIG. 8 , (A), the CPU  211  responds to the Peltier device driving turning on trigger (see  FIG. 5 , (A)), and then transmits the PWM signal of the duty 20% to the switching driving device  215 , which controls the power supply from the power supply unit  101  in the PWM duty of 20%, and applies the driving power to the Peltier device  301  (Step S 301 ). 
   At the same time as that of the output of the PWM duty 20%, the CPU  211  starts up the first-stage timer counter  212 - 1  (with the time interval setting of 15 seconds) which counts up by 1 second every step (Step S 302 ), and determines whether or not 15 seconds have elapsed from the starting up (whether or not the count value of the timer counter has reached 16 seconds) each time of the 1 second counting up operation (Step S 303 ). 
   When the first-stage timer counter  212 - 1  has counted 15 seconds, the CPU  211  switches the PWM duty of the driving power applied to the Peltier device  301  (Step S 304 ). Specifically, the CPU  211  first disables the output of the PWM signal, increases the duty of the PWM signal further by 20% for 40%, and then, enables the thus-updated PWM signal. Further, after the second-stage time counter  212 - 2  is started up (Step S 302 ) and this timer counter has counted 15 seconds (Yes in Step S 303 ), the setting of the PWM signal is again updated so that the PWM duty is further increased by 20% so that the PWM duty of 60% is applied 
   Thus, the duty of the PWM signal is increased by 20% at each elapse of 15 seconds. Then, after 60 seconds have elapsed from the turning on trigger (Yes in Step S 305 ), that is, after the loop of Steps S 302  through S 304  is repeated three times, and the fourth-stage timer counter  212 - 4  reaches a time-up state, the setting of the duty of the PWM signal is made by which the power supply to the Peltier device  301  is increased to 100% (Step S 306 ). Then, the control flow for the turning on process is finished. 
   For the power supply turning off process, as shown in  FIG. 8 , (B), the CPU  211  responds to the Peltier device driving turning off trigger (see  FIG. 6 , (A)), and then transmits the PWM signal of the duty 80% to the FET  213 , which controls the power supply from the power supply unit  101  in the PWM duty of 80%, thus decayed from 100% by 20%, to the Peltier device  301  (Step S 401 ). 
   At the same time as that of the output of the PWM duty 80%, the CPU  211  starts up the first-stage timer counter  212 - 1  (with the time interval setting of 15 seconds) which counts up by 1 second each step (Step S 402 ), and determines whether or-not 15 seconds have elapsed from the starting up (whether or not the count value of the timer counter has reached 16 seconds) each time of the 1 second counting up operation (Step S 403 ). 
   When the first-stage timer counter  212 - 1  has counted 15 seconds, the CPU  211  switches the PWM duty for controlling the driving power applied to the Peltier device  301  (Step S 404 ). Specifically, the CPU  211  first disables the output of the PWM signal, decreases the duty of the PWM signal further by 20% for 60%, and then, enables the thus-updated PWM signal. Further, after the second-stage time counter  212 - 2  is started up (Step S 302 ) and this timer counter has counted 15 seconds (Yes in Step S 403 ), the setting of the PWM signal is again updated so that the duty is further decreased by 20%, and thus, the driving power applied to the Peltier device  103  is decreased further by 20% for 40% 
   Thus, the duty of the PWM signal is decreased by 20% at each elapse of 15 seconds. Then, after 60 seconds have elapsed from the turning off trigger (Yes in Step S 405 ), that is, after the loop of Steps S 402  through S 404  is repeated three times, and the fourth-stage timer counter  212 - 4  reaches a time-up state, the setting of the duty of the PWM signal for controlling the driving power applied to the Peltier device  301  is decreased finally to 0% (Step S 406 ). Then, the control flow for the turning off process is finished. 
   A third embodiment of the present invention is described. 
   In this embodiment, the Peltier apparatus in the first or the second embodiment described above is actually applied in the electronic apparatus. 
   Since a cooing apparatus as the Peltier apparatus described above applying the Peltier device may have a effectively reduced size, and includes no mechanical moving element, such a cooling apparatus is suitable for avoiding temperature rise otherwise occurring due to a heat generated by circuit devices or such, a temperature or a humidity is thus controlled within a housing of an electronic apparatus and thus, an adequate operation environment can be kept, in the electronic apparatus in which an IC circuit, an electronic component, an electric unit or such for which a temperature or a humidity should be kept to a predetermined level is mounted in a narrow space within the housing. In the third embodiment, as an embodiment which can be preferably applied as such an electronic apparatus, a Peltier apparatus such as that according to the first or second embodiment described above is applied as a temperature control system or a humidity control system which may be mounted in the electronic apparatus. 
     FIG. 9  shows a block diagram of a humidity/temperature control system according to the third embodiment. It is noted that a temperature control system and a humidity control system have configurations basically the same as one another although a control target is different between the temperature environment and the humidity environment, and thus, which sensor, a temperature sensor or a humidity sensor, should be applied is different. Accordingly, hereinafter, a general expression ‘temperature/humidity’ is applied for the purpose of simplification. 
   In the system shown in  FIG. 9 , humidity/temperature sensors  401 ,  402  for detecting a humidity or a temperature of an operation environment to be controlled and an A/D converter  216  are added to the Peltier apparatus ( FIG. 7 ) according to the second embodiment. Detection signals of the humidity/temperature sensors  401 ,  402  are converted into digital signals by means of the A/D converter  216 , and then are input to the CPU  211 . The CPU  211  executes humidity/temperature control according to a control program, and, with the use of an additional sub-sequence of the control program, carries out control of driving the Peltier device  301 . The CPU  211  may be a common one also used to control the entirety of the electronic apparatus, or may be one specially provided for controlling the Peltier apparatus part of the electronic apparatus. 
   Operation of the humidity/temperature control system shown in  FIG. 9  is described now. The CPU  211  has the detection values of the humidity/temperature sensors  401 ,  402  fed back thereto via the A/D converter  216 , and, based on the thus-obtained humidity/temperature information (detection results), the CPU  211  obtains an error thereof from a target value (an adequate humidity/temperature range). Therewith, the CPU  211  controls driving of the Peltier apparatus by turning on or turning off of the same (on-off control). In this on-off control, the driving control for the turning on process is carried out as shown in  FIG. 4 , (A) or  FIG. 8 , (A); or, the driving control for the turning off process is carried out as shown in  FIG. 4 , (B) or  FIG. 8 , (B), according to the first or the second embodiment described above. 
   Further, a hysteresis is provided between an upper limit and a lower limit in the target value applied for the on-off control operation. In other words, a hysteresis is provided between a turning-on trigger threshold and a turning-off trigger threshold. By thus providing the hysteresis between the upper limit and the lower limit, it is possible to reduce the number of times of the on-off control operations. As a result, it is possible to elongate the operation life of the Peltier device  301 . 
     FIG. 16  shows one example of the hysteresis provided between the turning-on threshold T ON  and the turning-off threshold T OFF . In this scheme, the Peltier apparatus is turned on when the detected temperature increases and then reaches the turning-on threshold T ON . On the other hand, the Peltier apparatus is turned off when the detected temperature decreases and then reaches the turning-off threshold T OFF . 
   A fourth embodiment of the present invention is described now. 
   According to the fourth embodiment, the control function of the above-described third embodiment is improved. 
   According to the third embodiment described above, a humidity/temperature environment can be controlled for a fixed condition with the use of a cooling function of the Peltier device. However, by feeding air which is once cooled by the Peltier device or dried air by means of a fan so as to rapidly change the operation environment, and also, by changing air flow of the fan depending on a power supply amount to the Peltier device  301 , it is possible to further improve the efficiency. 
     FIG. 10  shows a block diagram of a humidity/temperature control system according to the fourth embodiment. In the system shown in  FIG. 10 , fans  303 ,  304  and  305 , a motor driving circuit  218  and a D/A converter  217  are added to the humidity/temperature control system shown in  FIG. 9  according to the third embodiment. As a result of controlling the fan motor control circuit  218 , air flows of the fans are controlled. At this time, the air flows are controlled depending on the power supply amount to the Peltier device  301 . Specifically, the CPU  211  outputs a control instruction for controlling revolving speeds of the fans, which is determined depending on the power supply amount to the Peltier device  305 , for example, the PWM duty value mentioned above for controlling the driving power to the Peltier device. The control instruction is converted into an analog value by the D/A converter  217 , which is then input to the motor driving circuit  218 . The CPU  211  executes humidity/temperature control according to a control program, and, with the use of an additional sub-sequence of the control program, carries out control of driving the Peltier device  301 . The CPU  211  may be a common one also used for controlling the entirety of the electronic apparatus, or may be one specially provided for controlling the Peltier apparatus part of the electronic apparatus. 
   Operation of the humidity/temperature control system shown in  FIG. 10  is described now. The CPU  211  has the detection values of the humidity/temperature sensors  401 ,  402  fed back thereto via the A/D converter  216 , and, based on the thus-obtained humidity/temperature information (detection results), the CPU  211  obtains an error thereof from a target value (an adequate humidity/temperature range). Therewith, the CPU  211  controls driving of the Peltier apparatus by turning on or turning off thereof (on-off control). In this on-off control, the driving control for the turning on process is carried out as shown in  FIG. 4 , (A) or  FIG. 8 , (A); or the driving control for the turning off process is carried out as shown in  FIG. 4 , (B) or  FIG. 8 , (B), according to the first or the second embodiment described above. 
   At the same time of carrying out the on-off control, the CPU  211  reduces the fans&#39; air flows by providing a control instruction to the fan motor control circuit  218  to lower the revolution speeds thereof, when the power supply to the Peltier device  301  is reduced (that is, when the applying voltage becomes low or the PWM duty is reduced). There, the D/A converter  217  is applied to control the motor driving circuit  218  for controlling the revolution speeds of the fans with the analog value. However, any other well-known method may be applied to control the revolution speeds of the fans. 
   A fifth embodiment of the present invention is described. 
   In the fifth embodiment, the temperature control system applying the Peltier apparatus as in the above-described fourth embodiment is applied to an image forming apparatus. 
   As mentioned above, the temperature control system according to the fourth embodiment is suitable to control temperature or such within an electronic apparatus in which IC circuits, electronic parts, electric components or such are assembled in a narrow space, and to keep proper operation environment thereof. According to the fifth embodiment, as the above-mentioned electronic apparatus to which the temperature control system according to the present invention can be suitably applied, an image forming apparatus is applied for example. The image forming apparatus may be an electrophotographic color laser printer, a copier (i.e., an apparatus in which optical writing is-carried out on a photosensitive body by means of a laser, and an electrostatic latent image thus produced is developed by means of toner) or such. In the fifth embodiment, the temperature control system according to the present invention is applied to control a temperature environment of a laser diode in a writing unit used to expose the photosensitive body. 
     FIG. 11  shows a general configuration of the temperature control system according to the fifth embodiment. A basic configuration and function of such an image forming apparatus such as a color laser printer, a copier or such itself is well-known, and thus, parts/components other than those necessary to describe the configuration of the present embodiment are omitted. 
   In  FIG. 11 , the control target is a temperature environment in a writing unit  511 , in which a laser diode  512  is loaded for which a temperature environment should be controlled for a fixed condition. 
   The temperature control system in this embodiment includes a Peltier part  501 , a control part (control substrate)  502  supplying power to the Peltier part  501 , and a temperature sensor  402  and so forth connected to the control part  502 . The control part  502  has a configuration the same as that of the fourth embodiment (see  FIG. 10 ). 
   The Peltier part  501  has a configuration in which heat radiating fins  302  and fans  303 ,  304  are provided on both sides of a Peltier device  301 . When a voltage is applied to the Peltier device  301 , heat is generated from a heat generating surface of the Peltier device  301 , which heat is discharged to the outside of the machine by the fan  303 . On the other hand, low-temperature air generated from a heat sink surface of the Peltier device  301  is injected to the writing unit  511  via a duct  306  by means of the fan  304 . Further, high-temperature air in the writing unit  511  is discharged via a duct  307  by means of a fan  305 . The temperature in the writing unit  511  is monitored by means of the temperature sensor  402 . Based on the detection result of the temperature sensor  402 , a CPU of the control part  502  controls the Peltier part  501 , so that the temperature environment of the laser diode  512  in the writing unit  511  can be controlled for a fixed condition. By thus controlling the temperature environment of the laser diode  512 , it is possible to avoid a degradation of the laser diode  512 , and also, to avoid a writing position error otherwise occurring due to a temperature change. 
   The control target is not limited to the temperature environment of the laser diode  512 . Other than it, it is possible to apply the same control method for controlling environments of various types of sensors, provided in such a type of an image forming apparatus, which sensors require well-controlled temperature environments. 
   The various types of sensors as the control targets may include the following sensors, for example: 
   A photosensor which is applied in control of conveyance of original paper or transfer paper, that is, an object detecting sensor in a non-contact type including an LED and a photodiode or a phototransistor; 
   A toner adhesion detecting sensor, i.e., a sensor detecting a toner density adhering to the photosensitive body, to a medium such as transfer paper or such. With the use of its detection result, appropriate correction is made, and thus, the image quality is improved. 
   A color drift sensor. That is, in a tandem-type color image forming apparatus or such, a color drift (positional error) among respective colors is detected. With the use of its detection result, appropriate correction is made, and thus, the image quality is improved. 
   By keeping temperature environments of these sensors by means of the above-mentioned temperature control system according to the present invention, it is possible to obtain stable detection outputs, and, with the use thereof, the image forming operation can be kept in an adequate condition. 
   A sixth embodiment of the present invention is described next. 
   In the sixth embodiment, the humidity control system applying the Peltier apparatus as in the above-described fourth embodiment is applied to the image forming apparatus. 
   In the sixth embodiment, different from the above-described fifth embodiment, a humidity environment of the photosensitive body unit of the image forming apparatus such as an electrophotographic color laser printer or a copier is controlled by means of the humidity control system according to the present invention. 
     FIG. 12  shows a general configuration of the humidity control system according to the sixth embodiment. 
   In  FIG. 12 , the control target is a humidity environment in a photosensitive body unit  521 , in which the photosensitive body  522  is loaded for which a humidity environment should be controlled for a fixed condition. 
   The humidity control system in this embodiment includes a Peltier part  501 , a control part (control substrate)  502  supplying power to the Peltier part  501 , and a humidity sensor  401  and so forth connected to the control part  502 . The control part  502  has a configuration the same as that of the fourth embodiment (see  FIG. 10 ). 
   The Peltier part  501  has a configuration in which heat radiating fins  302  and fans  303 ,  304  are provided on the both sides of a Peltier device  301 . When a voltage is applied to the Peltier device  301 , heat is generated from a heat generating surface of the Peltier device  301 , which heat is discharged to the outside of the machine by the fan  303 . On the other hand, air around a heat sink surface of the Peltier device  301  is cooled and thus dried. The thus-obtained dried air is injected to the photosensitive body unit  521  via a duct  306  by means of the fan  304 . Further, humid air in the photosensitive body unit  521  is discharged via a duct  307  by means of a fan  305 . The humidity in the photosensitive body unit  521  is monitored by means of the humidity sensor  402 . Based on the detection result of the humidity sensor  402 , a CPU of the control part  502  controls the Peltier part  501 , so that the humidity environment of the photosensitive body  522  in the photosensitive body unit  521  can be controlled for a fixed condition. 
   A seventh embodiment of the present invention is described next. 
   Also in the seventh embodiment, the humidity control system applying the Peltier apparatus as in the above-described fourth embodiment is applied to the image forming apparatus. 
   In the seventh embodiment, different from the above-described fifth embodiment, a humidity environment of a transfer paper tray of the image forming apparatus such as an electrophotographic color laser printer or a copier is controlled by means of the humidity control system according to the present invention. 
     FIG. 13  shows a general configuration of the humidity control system according to the seventh embodiment. 
   In  FIG. 13 , the control target is a humidity environment in the transfer paper tray  531 , in which transfer paper  532  is loaded for which a humidity environment should be controlled for a fixed condition. 
   The humidity control system in this embodiment includes a Peltier part  501 , a control part (control substrate)  502  supplying power to the Peltier part  501 , and a humidity sensor  401  and so forth connected to the control part  502 . The control part  502  has a configuration the same as that of the fourth embodiment (see  FIG. 10 ). 
   The Peltier part  501  has a configuration in which heat radiating fins  302  and fans  303 ,  304  are provided on both sides of a Peltier device  301 . When a voltage is applied to the Peltier device  301 , heat is generated from a heat generating surface of the Peltier device  301 , which heat is discharged to the outside of the machine by the fan  303 . On the other hand, air around a heat sink surface of the Peltier device  301  is cooled and thus dried. The thus-obtained dried air is injected to the transfer paper tray  531  via a duct  306  by means of the fan  304 . Further, humid air in the transfer paper tray  531  is discharged via a duct  307  by means of a fan  305 . The humidity in the transfer paper tray  531  is monitored by means of the humidity sensor  402 . Based on the detection result of the temperature sensor  402 , a CPU of the control part  502  controls the Peltier part  501 , so that the humidity environment of the transfer paper  532  in the transfer paper tray  531  can be controlled for a fixed condition. 
   An eighth embodiment of the present invention is described next. 
   In the eighth embodiment, the temperature control system applying the Peltier apparatus as in the above-described fourth embodiment is applied to the image forming apparatus. 
   In the eighth embodiment, different from the above-described fifth embodiment, temperature environment of a fixing device of the image forming apparatus such as an electrophotographic color laser printer or a copier is controlled by means of the temperature control system according to the present invention. 
     FIG. 14  shows a general configuration of the temperature control system according to the eighth embodiment. 
   In  FIG. 14 , the control target is a temperature environment in the fixing device  541 , in which heating rollers  542  and  543  are loaded for which a temperature environment should be controlled to a fixed condition. 
   The temperature control system in this embodiment includes a Peltier part  501 , a control part (control substrate)  502  supplying power to the Peltier part  501 , and a temperature sensor  402  and so forth connected to the control part  502 . The control part  502  has a configuration the same as that of the fourth embodiment (see  FIG. 10 ). 
   The Peltier part  501  has a configuration in which heat radiating fins  302  and fans  303 ,  304  are provided on both sides of a Peltier device  301 . When a voltage is applied to the Peltier device  301 , heat is generated from a heat generating surface of the Peltier device  301 , which heat is discharged to the outside of the machine by the fan  303 . On the other hand, low-temperature air generated from a heat sink surface of the Peltier device  301  is injected to the fixing device  541  via a duct  306  by means of the fan  304 . Further, high-temperature air in the fixing device  541  is discharged via a duct  307  by means of a fan  305 . The temperature in the fixing device  541  is monitored by means of the temperature sensor  402 . Based on the detection result of the temperature sensor  402 , a CPU of the control part  502  controls the Peltier part  501 , so that the temperature environment of the heating roller  542  in the fixing device  541  can be controlled for a fixed condition. 
     FIG. 15  shows a copier  601  corresponding to a specific example of the image forming apparatus according to any one of the above-described sixth through eighth embodiments of the present invention. The copier  601  shown includes the writing unit  511  (in which the laser diode  512  is loaded) of  FIG. 11 , the photosensitive body unit  521  (in which the photosensitive body  522  is loaded) of  FIG. 12 , the transfer paper tray  531  (in which the transfer paper  532  is loaded) of  FIG. 13  and the fixing device  541  (in which the heating rollers  542  and  543  are loaded) of  FIG. 14 . 
   Further, the present invention is not limited to the above-described embodiments, and variations and modifications may be made without departing from the basic concept of the present invention claimed below. 
   The present application is based on Japanese Priority Applications Nos. 2004-125572 and 2005-110214, filed on Apr. 21, 2004 and Apr. 6, 2005, respectively, the entire contents of which are hereby incorporated herein by reference.