Induction heating device with a switching power source and image processing apparatus using the same

#The present invention relates to an induction heating device of the type including a switching power source and an image processing device using the same.

#An induction heating device of the type described is applicable not only to various furnaces including a metal melting furnace, a plate heating furnace and a hardening furnace, but also to a fixing unit that fixes a toner image on a recording medium in an electrophotographic process. An image processing apparatus may be typified by a copier, a facsimile apparatus and a combination thereof. In a copier, for example, a switching power source often includes a plurality of different lines each including a converter or an inverter. The prerequisite with this kind of switching power source is that sound ascribable to noise interference be obviated. For this purpose, a particular frequency is assigned to each line while a difference in switching frequency between the lines is selected to be higher than an audible range. In practice, however, a low switching frequency must sometimes be used. A transformer included in a line whose switching frequency is low has its iron loss or hysteresis loss aggravated, resulting in a bulky, expensive configuration. Consequently, the switching power source with such a transformer makes the entire device bulky and expensive.

#The induction heating device includes an induction coil adjoining a magnetic heating member. A high-frequency current is fed to the induction coil in order to generate a magnetic flux in the heating member. The magnetic flux generates an induced current in a conductive layer formed on the heating member. The resulting Joule heat heats the surface of the heating member to a preselected temperature. To miniaturize the induction heating device and to render the amount of heat adjustable, it is necessary to use a plurality of induction coils or split induction coils and to control each induction coil independently of the others. For this purpose, it is a common practice to use a switching power source for driving the individual induction coil. The switching power source includes a plurality of inverters, or high-frequency power sources, each for controlling a particular induction coil. This, however, brings about a problem that a magnetic flux generated by any one of the induction coils effects the other induction coils. As a result, the inverters interfere with each other and fail to operate.

#The above and other objects, features and advantages of the present invention will become more apparent from the following detailed description taken with the accompanying drawings in which:

#FIG. 1 is a block diagram schematically showing a conventional switching power source including converter sections arranged on two lines;

#FIG. 2 is a schematic block diagram showing a first embodiment of the switching power source in accordance with the present invention including converter sections arranged on two lines;

#To better understand the present invention, brief reference will be made to a conventional switching power source applicable to a copier or similar image processing apparatus and including a plurality of converter lines, shown in FIG. 1. As shown, the switching power source includes two identical lines or circuitry operable independently of each other. Specifically, a fist and a second converter section 31 and 36 include switching devices Q1 and Q1, respectively. A first and a second driver 35 and 40 apply pulses, the ON width or the OFF width of which is variable, to the switching devices Q1 and Q2, respectively. In response, the switching devices Q1 and Q2 each switch, i.e., turn on or turn off an input voltage Vin. The input voltages output from the switching devices Q1 and Q2 are respectively converted to output voltages Vout1 and Vout2 via a first and a second rectifier 32 and 37. A first and a second error amplifier (EA1 and EA2) 33 and 38 respectively produce differences between the output voltages Vout1 and Vout2 and reference voltages Vz1 and Vz2 and amplify them. The differences, or errors, output from the error amplifiers 33 and 38 are respectively fed back to the drivers 35 and 40 via a first and a second controller 34 and 41 so as to stabilize the voltages Vout1 and Vout2.

#The prerequisite with a switching power source device including a plurality of converter or inverter lines, as stated above, is that sound ascribable to noise interference between the independent lines be obviated. For this purpose, it has been customary to set up a difference in switching frequency above the audible frequency range between the lines, e.g., to assign switching frequencies of 80 kHz, 110 kHz and 140 kHz to a first, a second and a third line (converter). This, however, cannot be done without using even low frequencies, as stated earlier. As a result, a transformer included in a line, to which a low switching frequency is assigned, has its iron loss or hysteresis loss aggravated and must therefore be increased in size, resulting in an increase in cost. Moreover, the entire switching power source becomes bulky and expensive.

#Referring to FIG. 2, a first embodiment of the switching power source in accordance with the present invention is shown. As shown, a first converter section 31 includes a first switching device Q1 and a first rectifier 32. A first driver 35 applies pulses, the ON width or the OFF width of which is variable, to the switching device Q1. In response, the switching device Q1 switches, i.e., turns on or turns off an input voltage Vi. The voltage Vi output from the switching device Q1 is converted to an output voltage Vout1 via the rectifier 32. A first error amplifier (EA1) 33 produces a difference between the output voltage Vout1 and a reference voltage Vz1 assigned thereto and amplifies it. The difference, or error, output from the error amplifier 33 is fed back to the driver 35 via a controller 34 so as to stabilize the voltage Vout1 at the reference voltage.