The image forming apparatus, such as a copier or a printer, forms an image on a recording medium, such as a plain recording sheet or an OHP (overhead projector) film. In view of speedy image formation, image quality, cost, etc., the electrophotographic method is adopted for the image forming apparatus. In the electrophotographic method, a toner image is formed on a photoconductor and the toner image is fixed to a recording medium by applying heat and pressure. The currently adopted fixing method is mostly the heating roller method in view of safety. In the heating roller method, a heating roller which is heated by a halogen heater or the like, and a pressure roller which is disposed in opposite arrangement to the heating roller are connected together under pressure to form a nip portion, and the recording medium to which the toner image is transferred is passed through the nip portion so that the toner image is fixed to the recording medium by heat and pressure.
In recent years, the environmental problem becomes important and, also in the image forming apparatus, such as a copier and a printer, the energy saving becomes important. What cannot be disregarded in considering the energy saving of the image forming apparatus is power saving of a fixing device which fixes the toner to the recording medium.
As a reduction of the power dissipation of the fixing device in a standby state of the image forming apparatus, the heating roller is maintained at a temperature that is a little lower than a fixing temperature in the standby state, and when the image forming apparatus is in use the temperature of the heating roller is immediately raised to a usable temperature for the time of use, so as to avoid waiting of the user for the temperature rise of the fixing roller.
In this case, when the fixing device is not used, a certain amount of power is supplied and excessive energy is consumed. It is said that the consumption energy at the time of the standby state is equivalent to about 70 or 80 percent of the entire consumption energy of the image forming apparatus.
It is desired to reduce the consumption energy at the time of the standby state so that power saving is attained as much as possible. The Law concerning rational use of energy is revised and strengthened in Japan, and energy-saving programs, such as the energy star and the ZESM (Zero Energy Star Mode), are enacted also in the U.S. Making the electric power supply into zero when the device is not used has been demanded.
However, the heating roller mainly uses metal rollers made of iron or aluminum, and its heat capacity is large. For this reason, if the energy consumption is made into zero at the time of standby state, a long heating time, for example, in a range from several minutes to ten and several minutes, is required to heat the heating roller to the usable temperature around about 180 degrees C., and the convenience to the user will deteriorate.
For this reason, the composition which raises the heating roller temperature promptly is needed when realizing a copier of energy saving type. For example, in the ZESM, it is required that the time for re-warming up of the heating roller is 10 seconds or less.
In order to shorten the temperature-rise time of the heating roller, it is appropriate to enlarge the rated power of the heating roller, i.e., the supply energy per unit time. Actually, many of the current high-speed image forming apparatuses with high print speed set the supply voltage to 200V.
However, the source power supply of the general-purpose outlet of the offices in Japan is 100V 15 A, and it is necessary to carry out a construction special to the power supply relation of the installation for making it correspond to 200V. Hence, setting the supply voltage to 200V is not a general solution.
Moreover, the product which raises the total supply power using two power lines of 100V 15 A is also put in practical use, but it can be installed only at the location near the place where the two plug sockets are available. For this reason, even if it is intended to heat the heating roller for a short time, the actual conditions are difficult to raise the maximum of supply energy.
Moreover, as a fixing device which realizes the short-time temperature rise, the heating roller is constituted with a plate-like ceramic heater having a small heat capacity and a film made of a heat-resistant resin wound around the circumference of the ceramic heater. Such fixing device is put in practical use for a low-speed image forming apparatus with the print speed of 30 sheets/minute or less.
However, in order to take measures to a higher speed machine, it is necessary to make the film of the heat-resistant resin thicker in order to prevent the breakage. When the film of the heat-resistant resin is thickened, the thermal conductivity of resin is worse than that of metal, and it is necessary to heat the film of the heat-resistant resin with the ceramic heater well before the recording medium enters into the nip portion between the heating roller and the pressure roller. For this reason, it is required to enlarge the area of the plate-like portion of the ceramic heater and use the power supply generating a higher power, and the actual conditions are that taking measures to the high-speed image forming apparatus is unrealizable.
To resolve the problem, Japanese Laid-Open Patent Application No. 5-232839 discloses a method in which a heating element is provided in the heating roller and an additional heating element of another system is provided in the lower portion of the pressure roller. In this method, power is supplied to the additional heating element of another system at the time of standby state.
Moreover, Japanese Laid-Open Patent Application No. 10-10913 discloses a method in which a voltage with a fixed low level is supplied to the heating roller when the fixing device changes into the standby state, in order to delay the falling of the temperature of the fixing device. Japanese Laid-Open Patent Application No. 10-282821 discloses a method in which the secondary battery which is the auxiliary power is charged at the time of standby state of the fixing device, and upon starting of the fixing device, power is supplied from the main power supply, the secondary battery and the primary battery to the fixing device, in order to shorten the temperature-rise time.
Furthermore, Japanese Laid-Open Patent Application No. 2000-315567 discloses a method in which the auxiliary power supply using a capacitor having a large capacity is used in addition to the main power supply. In this method, at the time of standby state, connection of the main power supply and the heater is cut off and connection of the main power supply and the auxiliary power supply is established to charge the auxiliary power supply. Upon starting of the heater from the standby state, power is supplied to the heater from both the main power supply and the auxiliary power supply, so that raising the temperature of the heater to a predetermined temperature is attained for a short time.
However, in the method of Japanese Laid-Open Patent Application No. 5-232839, power is supplied to the additional heating element of another system at the time of standby state. Also, in the method of Japanese laid-Open patent Application No. 10-10913, the voltage with a fixed low level is supplied to the heating roller when the fixing device changes into the standby state. It cannot be said that these methods provide sufficient power saving. Moreover, the restriction of the maximum supply voltage of the source power supply cannot be solved, and the temperature-rise time cannot be shortened.
Moreover, in the fixing device of Japanese Laid-Open Patent Application No. 10-282821, power from the main power supply, the secondary battery and the primary battery is supplied at the time of starting.
Generally, the secondary battery employs a nickel-cadmium battery or a lead battery, and such secondary battery has the property in which the operational life becomes short with large current. The capacity of such secondary battery will deteriorate if the charging and discharging is performed repeatedly. Even in a case of the nickel-cadmium battery which has, generally speaking, a long operational life, the permissible number of times of the charging and discharging repetition is about 500-1000 times. The nickel-cadmium battery must be changed by a new one within one month if the charging and discharging is repeated 20 times a day. The nickel-cadmium battery has the demerits in which the use over an extended period of time is difficult, and the time and effort of exchange must be taken and the running cost will become high. Moreover, the charging time also requires several hours for charging the large capacity at the full, and it is not suitable for the use in which the charging and discharging is performed repeatedly each day. Accordingly realization is difficult practically. Furthermore, the lead battery which uses sulfuric acid is not desirable in the image forming apparatus for office uses.
The halogen heater is usually used for heating of the fixing roller but its operational life will become short if a large amount of current is applied to the halogen heater. The permissible maximum current of the halogen heater is about 10-12 A, and it is difficult to enlarge the maximum current.
In order to obtain large power with the heating device using the halogen heater as a heating element, it is necessary to use a power supply device with a high voltage as the source power source of the heating device.
In the method of Japanese Laid-Open Patent Application No. 2000-315567, the auxiliary power supply having a large-capacitance capacitor is used in addition to the main power supply, the auxiliary power supply is charged at the time of standby state, and when starting the heater from the standby state, power is supplied to the heater from the main power supply and the auxiliary power supply. In order to prevent the electrolyzing of the solution inside the cells, the large-capacitance capacitor used in the auxiliary power supply has the characteristics that the voltage per cell is relatively low (about several volts) The voltage per cell of the aqueous solution type is as much as 1 volt, and the voltage per cell of the organic solution type is also about several volts.
For this reason, in order to heat the halogen heater as a heating element, it is necessary to use a power supply unit in which a large number of cells are connected in series to obtain a sufficiently high voltage.
With the composition in which the cells are connected in series to obtain a high voltage and a large power, even if it has energy sufficient for some cells to raise the temperature of the heating element, in order to raise voltage further, it is necessary to increase the number of the cells. There is the problem in that the cost and size of the heating device will be increased because the cells are currently expensive and the use of the large number of such cells raises the cost and size of the power supply.