Fixing device using an inverter circuit for induction heating

Description

BACKGROUND OF THE INVENTION

The present invention relates to a fixing device for a copier, printer, facsimile apparatus or similar image forming apparatus and more particularly to an induction heating type of fixing device.

An induction heating type of fixing device for use in an image forming apparatus is configured to heat the wall or core of a heat roller with Joule heat derived from induced current. Specifically, this type of fixing device includes electromagnetic induction heating means having an induction heating coil. High frequency current is fed to the induction heating coil to cause it to generate an induced flux, which in turn generates induced current (eddy current) in a conductive layer covering the heat roller. Joule heat derived from the induced current heats the surface of the heat roller to a preselected temperature. It is a common practice to produce the high frequency current by rectifying AC available with a commercial power source with a rectifying circuit and then converting it to high frequency.

A conventional inverter circuit for induction heating stabilizes the fixing temperature of the fixing device by varying frequency. A problem with this conventional scheme is that the varying frequency translates into the variation of the penetration depth of the eddy current and thereby prevents power for maintaining optimal fixing temperature from being input to the heat roller. Further, the variation of the penetration depth of the eddy current causes the heat distribution on the surface of the heat roller to vary, effecting the quality of a fixed image.

When the inverter circuit is configured for an AC 200 V application, it needs a switching device that withstands voltage two times as high as the withstanding voltage of a switching device for an AC 100 V application. A switching device for an AC 200 V application and comparable in size with a switching device for an AC 100 V application is rare or is insufficient in withstanding voltage if available. While a mold type switching device withstands high voltage, it is packaged in a size more than two times as great as the size of a 100 V switching device. This kind of switching device is not applicable to a high frequency inverter for use in a fixing device. It has therefore been difficult to realize a miniature inverter circuit adaptive to a 200 V application.

Moreover, a power control range available with the conventional inverter circuit is narrow. Therefore, when the load of the inverter circuit is light, current flowing through the induction heating coil or work coil is short and prevents current from being fully discharged from a resonance capacitor. It follows that the inverter circuit fails to perform zero voltage switching and looses its high efficiency and low noise features based on zero voltage switching.

Technologies relating to the present invention are disclosed in, e.g., Japanese Patent Laid-Open Publication No. 9-245953 and 2000-259018.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a fixing device using an inverter circuit for induction heating that achieves high efficiency and reduces the stress of a switching device as well as switching noise. In accordance with the present invention, an inverter circuit for induction heating includes a switching device that drives one end of an induction heating coil the other end of which is connected to a power source. A capacitor and a second switching device are serially connected to each other and connected to opposite ends of the induction heating coil in parallel such that one end of the capacitor is connected to the power source. A second capacitor is connected to the second switching device in parallel. The second capacitor has a capacitance of 0.1 μF to 0.4 μF. For a capacitance of 0.1 μF of the second capacitor, the induction heating coil has an inductance of 70 μH to 100 μH while the capacitor has a capacitance of 1.8 μF to 5 μF. Also, for a capacitance of 0.2 μF of the second capacitor, the induction heating coil has an inductance of 65 μH to 100 μH while the capacitor has a capacitance of 1.8 μF to 5 μF. Further, for a capacitance of 0.3 μF of the second capacitor, the induction heating coil has an inductance of 65 μH to 95 μH while the capacitor has a capacitance of 2 F to 5 F. Moreover, for a capacitance of 0.4 μF of the second capacitor, the induction heating coil has an inductance of 65 μH to 87 μH while the capacitor has a capacitance of 2.3 μF to 5 μF.

BRIEF DESCRIPTION OF THE DRAWINGS

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 circuit diagram showing a conventional inverter circuit for induction heating included in a fixing device;

FIG. 2

is a circuit diagram showing an inverter circuit for a fixing device embodying the present invention;

FIG. 3A

demonstrates mode transition unique to the illustrative embodiment;

FIG. 3B

shows waveforms associated with the mode transition of

FIG. 3A

;

FIG. 4A

shows graphs representative of an input power control characteristic particular to a prior art conventional inverter circuit;

FIG. 4B

shows graphs representative of an input power control characteristic achievable with the illustrative embodiment;

FIG. 5

is a circuit diagram showing an alternative embodiment of the present invention;

FIG. 6

is a circuit diagram showing another alternative embodiment of the present invention;

FIG. 7

is a circuit diagram showing a further alternative embodiment of the present invention;

FIG. 8

is a graph demonstrating the operation of the present invention;

FIG. 9

is a graph demonstrating the operation of the present invention derived from alternative device factors;

FIG. 10

is a graph demonstrating the operation of the present invention derived from other device factors; and

FIG. 11

is a graph demonstrating the operation of the illustrative embodiment derived other device factors.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

To better understand the present invention, brief reference will be made to a conventional inverter circuit for induction heating included in a fixing device and configured for a 100 V application. As shown in

FIG. 1

, the inverter circuit includes a work coil or induction heating coil L

1

, a switching device Q

1

, and a capacitor Cr. A power source E is representative of a DC power source produced by rectifying a commercial power source. A coil L

2

and a resistor R

2

, which are surrounded by a dashed line, are representative of a circuit electrically equivalent to a heat roller

1

. The switching device Q

1

is usually implemented by an IGBT (Insulated Gate Bipolar model Transistor) from the withstanding voltage and current capacity standpoint. Labeled D

1

is a parasitic diode particular to the IGBT.

In operation, the switching device Q

1

is driven by a high frequency in order to cause a high frequency current to flow through the work coil L

1

. As a result, an eddy current flows through the heat roller

1

, i.e., the coil L

2

and resistor R

2

, heating the heat roller

1

. The width of a pulse that turns on the switching device Q

1

is variable, so that necessary power can be fed. On the other hand, when the switching device Q

1

is turned off, a flyback voltage appears on the collector of the switching device Q

1

. The flyback voltage is the resonance voltage of the work coil L

1

and capacitor Cr. Therefore, although zero voltage switching is achievable, the duration of turn-off of the switching device Q

1

is determined by the time constant of the work coil L

1

and capacitor Cr and is not variable. Consequently, the heat roller

1

cannot be controlled to optimal temperature for fixation unless the frequency of the switching device Q

1

is varied. This brings about the problems discussed earlier.

Referring to

FIG. 2

, a fixing device with an inverter circuit for induction heating embodying the present invention is shown. In

FIG. 2

, symbols identical with the symbols of

FIG. 1

designate identical structural elements. As shown, the inverter circuit includes an additional capacitor Cs and a second switching device Q

2

(IGBT) connected to a work coil L

1

in parallel. A second capacitor C

1

is connected to the switching device Q

2

in parallel from the junction of the serial connection of the capacitor Cs and switching device Q

2

. Labeled Ds is a parasitic diode particular to the switching device Q

2

.

In the illustrative embodiment, the switching device Q

1

plays the role of a main switch. The capacitors C

1

and Cs are a first and a second resonance capacitor, respectively. The switching device Q

2

serves as a subswitch while the diode Ds is a reverse conducting diode associated with the subswitch Q

2

.

The principle of operation of the illustrative embodiment will be described hereinafter with reference to

FIGS. 3A and 3B

.

FIGS. 3A

shows the transition of consecutive modes

1

through

5

that the illustrative embodiment repeats at a preselected period.

FIG. 3B

shows waveforms respectively representative of a voltage between the collector and the emitter of the main switch Q

1

, a current flowing through the main switch Q

1

, a voltage between the collector and the emitter of the subswitch Q

2

(Qs), a current flowing through the subswitch Q

2

, a voltage stored in the second resonance capacitor Cs, and a current flowing through the work coil L

1

, as named from the top to the bottom.

In the mode

1

, which is a power consumption and non-resonance mode, the main switch Q

1

turns on at a time t

0

to store energy in the work coil L while feeding power to the load that generates heat, i.e., the work coil L

1

, coil L

2

, and resistor R

2

.

In the mode

2

, which is a power consumption and partial resonance mode, the main switch Q

1

turns off at a time t

1

. As a result, a closed loop including the load made up of the work coil L

1

, coil L

2

and resistor R

2

, first resonance capacitor C

1

and second resonance capacitor Cs is activated to set up a partial resonance mode. During this period of time, the capacitors C

1

and Cs are charged and discharged so as to reduce the value dv/dt of the main switch Q

1

. The main switch Q

1

can therefore turn off by ZVS (Zero Voltage Switching).

The mode

3

a

is a power consumption and diode Ds conduction, resonance mode. In this mode, when the voltage of the first resonance capacitor C

1

becomes zero, the reverse conducting diode Ds of the subswitch Q

2

(Qs) turns on. As a result, a closed loop including the load made up of the work coil L

1

, coil L

2

and resistor R

2

, second resonance capacitor Cs and diode Ds is activated.

The mode

3

b

following the mode

3

a

is a power consumption and subswitch Q

2

conduction, resonance mode. In this mode, The current flowing through the subswitch Q

2

becomes zero at a time t

3

. The subswitch Q

2

therefore successfully turns on by ZVS and ZCS (Zero Current Switching). By maintaining the subswitch Q

2

turned on during one period of the inverter, it is possible to allow the main switch Q

1

to operate with a constant frequency even if the duration of conduction of the main switch Q

1

is made variable.

In the mode

4

, which is a power consumption and partial resonance mode, the subswitch Q

2

turns off at a time t

4

. At this time, a closed loop including the load, i.e., the work coil L

1

, coil L

2

and resistor R

2

, first resonance capacitor C

1

and second resonance capacitor Cs is activated to set up a partial resonance mode. By charging and discharging the capacitor C

1

and Cs during this period of time, it is possible to reduce the value dv/dt of the subswitch Q

2

and therefore to implement turn-off by ZVS.

In the mode

5

, which is a power regeneration and non-resonance mode, the sum of the voltage of the first resonance capacitor C

1

and that of the second resonance capacitor Cs tends to increase above the power source voltage Ed at a time t

5

. At this instant, the reverse conducting diode D

1

is biased forward and sets up the mode

5

. The current flowing through the main switch Q

1

becomes zero at the time t

0

and again sets up the mode

1

. At this time, the main switch Q

1

turns on by ZVS and ZCS.

The modes

1

through

5

are repeated at a preselected period, as stated above. The additional switching device Q

2

and capacitors Cs and C

1

allow the duration of turn-off to be variable and therefore realizes power control based on PWM (Pulse Width Modulation), which uses fixed frequency. It is therefore possible to maintain the penetration depth of eddy current in the heat roller constant. This insures stable fixation than enhances image quality.

One of major advantages achievable with the illustrative embodiment will be described hereinafter. The subswitch Q

2

and second resonance capacitor Cs lower voltage at the time of turn-off and therefore lower voltage to act on the main switch Q

1

and subswitch Q

2

. It follows that the illustrative embodiment is practicable with devices for 100 V applications and therefore realizes a miniature inverter circuit. This implements a miniature fixing device adaptive to an AC 200 V power source system.

Japanese Patent Laid-Open Publication No. 9-245953 mentioned earlier teaches a circuit similar to the circuit of FIG.

2

and in which the capacitor C

1

and work coil L

1

of the illustrative embodiment are connected in parallel.

FIGS. 4A and 4B

compare the prior art circuit of the above document and the illustrative embodiment as to the voltage to act on the subswitch Q

2

determined by simulation. For the simulation, an input voltage was assumed to be 280 V. Specifically,

FIGS. 4A and 4B

pertain to the prior art circuit and illustrative embodiment, respectively.

FIGS. 4A and 4B

each sow the peak VceQs of the voltage acting on the subswitch Q

2

in accordance with a pulse width (Duty Factor) that varies in accordance with the input voltage Pin.

As shown in

FIG. 4A

, for input power Pin of 3 kW, the duty of the prior art circuit is 0.48 while a peak voltage VceQs corresponding to such a duty is about 660 V. By contrast, as shown in

FIG. 4B

, the duty of the illustrative embodiment is 0.375 for the input power Pin of 3 kW; a peak voltage corresponding to the duty of 0.375 is as low as 490 V. The peak voltage of 490 V is lower than the peak voltage of 660 V by 170 V. The illustrative embodiment is therefore operable with an input voltage and a voltage range impractical with the prior art circuit. This is because the maximum withstanding voltage of switching devices is generally 900 V or so.

Another major advantage of the illustrative embodiment is that the switching devices Q

1

and Q

2

each turn on and turn off when voltage and current both are zero, realizing ZVS and ZCS. The switching devices Q

1

and Q

2

therefore involve a minimum of switching loss, making the inverter circuit efficient and free from noticeable switching noise.

The illustrative embodiment differs from the embodiment shown in

FIG. 2

in that the positional relation between the work coil portion and the switching device Q

1

is inverted in the up-and-down direction. While the illustrative embodiment operates in the same manner as the embodiment of

FIG. 2

, it is characterized in that one end of the work coil L

1

is connected to ground.

Reference will be made to

FIG. 5

for describing an alternative embodiment of the present invention. In

FIG. 5

, symbols identical with the symbols of

FIG. 2

designate identical structural elements. As shown, this embodiment is identical with the embodiment of

FIG. 2

except that it additionally includes an inductor La and a capacitor Ca connected to the work coil L

1

in parallel. The circuit of

FIG. 5

operates in the same manner as the circuit of

FIG. 2

except that a current fed to the inductance L

1

partly flows to the inductor La. While the capacitor Ca is shown in

FIG. 4

as being serially connected to the inductor La, the capacitor Ca may be omitted if the omission does not effect the operation of the inverter circuit.

In the previous embodiment shown in

FIG. 2

, the range that implements ZVS is, in principle, dependent on whether or not the first capacitor C

1

(resonance capacitor Cr in the prior art circuit,

FIG. 1

) can be fully charged and discharged. More specifically, the above range is dependent on the value of resonance initial current that flows through, e.g., the work coil just before the partial resonance mode. In this case, the work coil is representative of the inductance of the closed loop formed in the partial resonance mode. It follows that when voltage is lowered in the circuits shown in

FIGS. 1 and 2

, the initial current value (magnetic energy) stored in the work coil L

1

becomes short and makes ZVS impracticable.

In light of the above, the illustrative embodiment causes the inductor La serially connected to the work coil L

1

to increase the resonance initial current value, thereby broadening the ZVS range.

FIG. 6

shows another alternative embodiment of the present invention. In

FIG. 6

, symbols identical with the symbols of

FIGS. 2 and 5

designate identical structural elements. As shown, in the illustrative embodiment, the inductor La and a third switching device Q

3

are serially connected to each other and connected to the work coil L

1

in parallel. As for the rest of the configuration, the illustrative embodiment is identical with the embodiment shown in FIG.

2

. The illustrative embodiment differs from the embodiment shown in

FIG. 5

in that the third switching device Q

3

is substituted for the capacitor Ca. A diode D

3

is associated with the switching device Q

3

.

The illustrative embodiment causes the third switching device Q

3

to turn on only in a light load condition or in an operating condition not lying in the ZVS range. The illustrative embodiment may also include the capacitor Ca,

FIG. 5

, and serially connect it to the third switching device Q

3

, if desired. Because the third switching device Q

3

turns on only in the above particular condition, the illustrative embodiment enhances efficiency while preserving the broader control range.

A further alternative embodiment of the present invention of the present invention will be described with reference to FIG.

7

. In

FIG. 7

, symbols identical with the symbols of

FIG. 2

designate identical structural elements. As shown, in the illustrative embodiment, one end of the work coil L

1

is connected to ground. The switching device Q

1

serially connected to the work coil L

1

is connected to the positive terminal of the power source E. The capacitor Cs and switching device Q

2

serially connected to each other are connected to the work coil L

1

in parallel. The capacitor C

1

is connected to the switching device Q

2

in parallel from the junction of the serial connection of the capacitor Cs and switching device Q

2

. The parasitic diode Ds is associated with the switching device Q

2

. Further, the heat roller

1

, which is the load of the work coil L

1

, and the work coil L

1

are spaced by a gap g of 3 mm or less.

The illustrative embodiment differs from the embodiment shown in

FIG. 2

except that the positional relation between the work coil portion and the switching device Q

1

is inverted in the up-and-down direction. While the illustrative embodiment operates in the same manner as the embodiment of

FIG. 2

, it is characterized in that one end of the work coil L

1

is connected to ground.

In the circuit shown in

FIG. 2

, not only the high-frequency voltage driven by the switching device Q

1

but also the power source voltage constantly act on the work coil L

1

, increasing the total voltage to act on the work coil L

1

. By contrast, in the illustrative embodiment, the voltage acting on the work coil L

1

is lower than the above voltage by the power source voltage.

Generally, in an induction heating type fixing device, a hollow cylindrical heat roller concentrically surrounds a work coil or induction heating coil. The heat roller, which is the load of the work coil, is conductive and connected to ground. Therefore, when a power source voltage acts on the work coil, as in the embodiment shown in

FIG. 2

, high voltage acts on the work coil. It follows that the work coil and heat roller cannot be brought excessively close to each other from the safety or breakdown voltage standpoint. By contrast, the illustrative embodiment allows the gap between the work coil L

1

and the heat roller

1

to be reduced because of the lower voltage to act on the work coil L

1

. More specifically, in the illustrative embodiment, the gap g between the work coil L

1

and the heat roller

1

is selected to be 3 mm for realizing an efficient fixing device.

Further, because one end of the work coil L

1

is connected to ground, the circuit elements connected to the work coil L

1

are also connected to ground. The illustrative embodiment therefore reduces high frequency noise more than the embodiment shown in FIG.

1

.

In each of the embodiments shown in

FIGS. 2 and 5

through

7

, the switching device Q

1

repeats switching, as described with reference to FIG.

3

B. If the switching voltage VcdQ

1

and current i

1

exceed the withstanding current and withstanding voltage of the switching device Q

1

, then the switching device Q

1

breaks. It is therefore necessary to select the values of the first and second resonance capacitors C

1

and Cs and the value of the inductance L

1

of the work coil that obviate the above occurrence.

However, to lower the peak voltage, it is necessary to reduce the inductance L

1

, to increase the value of the second resonance capacitor Cs, and to reduce the value of the first resonance capacitor C

1

. On the other hand, to lower the peak current, it is necessary to increase L

1

, to reduce Cs, and to increase C

1

. In this manner, the conditions for lowering the peak voltage and those for lowering the peak current are contradictory to each other, as well known in the art.

Moreover, the various factors mentioned above must satisfy the previously stated ZVS. It is therefore difficult to determine optimal factors by experiments or simple arithmetic operations.

We therefore conducted simulations in a range implementing the optimal factors of the various elements under operating conditions that satisfy ZVS. Specifically, the simulations were conducted with a switching voltage of 700 V or below and a switching current of 700 A or below, which are customary with a switching device, for use in a fixing device belonging to the class concerned. Such a switching voltage and switching current are, however, only illustrative.

FIG. 8

shows the results of simulations. In

FIG. 8

, Cs and L

1

are varied with respect to C

1

of 0.1 μF. A curve with circles is representative of the ZVS condition while a curve with squares is representative of a current condition. Further, a curve with triangles is representative of a voltage condition. In a range indicated by arrows in

FIG. 8

, the factors satisfy all of the required conditions.

Specifically, as

FIG. 8

indicates, when capacitance of the first resonance capacitor C

1

is 0.1 μF, the optimal factor of the work coil L

1

is 70 μH to 100 μH while the optimal factor of the second resonance capacitor Cs is 1.8 μF to 5 μF.

Likewise,

FIG. 9

shows the result of simulation conducted by varying the factor of the second resonance capacitor Cs and that of the work coil L

1

for the capacitance of 0.2 μF of the first resonance capacitor C

1

. In a range indicated by arrows in

FIG. 9

, the factors satisfy all of the required conditions. Specifically, for the capacitance of 0.2 μF of the first resonance capacitor C

1

, the optimal factor of the work coil L

1

is between 65 μH and 100 μH while the factor of the second resonance capacitor Cs is between 1.8 μF and 5 μF.

Further,

FIG. 10

shows the result of simulation conducted by varying the factor of the second resonance capacitor Cs and that of the work coil L

1

for the capacitance of 0.3 μF of the first resonance capacitor C

1

. In a range indicated by arrows in

FIG. 9

, the factors satisfy all of the required conditions. Specifically, for the capacitance of 0.3 μF of the first resonance capacitor C

1

, the optimal factor of the work coil L

1

is between 65 μH and 95 μH while the factor of the second resonance capacitor Cs is between 2 μF and 5 μF.

Furthermore,

FIG. 11

shows the result of simulation conducted by varying the factor of the second resonance capacitor Cs and that of the work coil L

1

for the capacitance of 0.4 μF of the first resonance capacitor C

1

. In a range indicated by arrows in

FIG. 9

, the factors satisfy all of the required conditions. Specifically, for the capacitance of 0.4 μF of the first resonance capacitor C

1

, the optimal factor of the work coil L

1

is between 65 μH and 87 μH while the factor of the second resonance capacitor Cs is between 2.3 μF and 5 μF.

The ranges of the factors are determined in the manner described in order to select optimal devices. This realizes a miniature fixing unit that allows its inverter to operate with optimal efficiency. While the capacitance of the first resonance capacitor C

1

was selected to be 0.1 μF to 0.4 μF for simulation, such a range is substantially optimal from the inverter operation standpoint.

In summary, it will be seen that the present invention provides a fixing device that allows its inverter for induction heating to operate with optimal efficiency in the event of PWM power control. Also, the fixing device allows a resonance initial current value to be increased to broaden a ZVS range. Further, the fixing device enhances efficiency while preserving a broad control range, and reduces high frequency, switching noise.

Various modifications will become possible for those skilled in the art after receiving the teachings of the present disclosure without departing from the scope thereof.

Claims

1. A fixing device comprising:an inverter circuit for induction heating, the inverter circuit comprising: a first switching device that drives one end of an induction heating coil the other end of which is connected to a power source; a first capacitor and a second switching device serially connected to each other and connected to opposite ends of the induction heating coil in parallel such that one end of said first capacitor is connected to the power source; and a second capacitor connected to said second switching device in parallel; wherein said second capacitor has a capacitance of 0.1 μF, said induction heating coil has an inductance of 70 μH to 100 μH, and said first capacitor has a capacitance of 1.8 μF to 5 μF.
2. A fixing device comprising:an inverter circuit for induction heating, the inverter circuit comprising: a first switching device that drives one end of an induction heating coil the other end of which is connected to a power source; a first capacitor and a second switching device serially connected to each other and connected to opposite ends of the induction heating coil in parallel such that one end of said first capacitor is connected to the power source; and a second capacitor connected to said second switching device in parallel; wherein said second capacitor has a capacitance of 0.2 μF, said induction heating coil has an inductance of 65 μH to 100 μH, and said first capacitor has a capacitance of 1.8 μF to 5 μF.
3. A fixing device comprising:an inverter circuit for induction heating, the inverter circuit comprising: a first switching device that drives one end of an induction heating coil the other end of which is connected to a power source; a first capacitor and a second switching device serially connected to each other and connected to opposite ends of the induction heating coil in parallel such that one end of said first capacitor is connected to the power source; and a second capacitor connected to said second switching device in parallel; wherein said second capacitor has a capacitance of 0.3 μF, said induction heating coil has an inductance of 65 μH to 95 μH, and said first capacitor has a capacitance of 2 μF to 5 μF.
4. A fixing device comprising:an inverter circuit for induction heating, the inverter circuit comprising: a first switching device that drives one end of an induction heating coil the other end of which is connected to a power source; a first capacitor and a second switching device serially connected to each other and connected to opposite ends of the induction heating coil in parallel such that one end of said first capacitor is connected to the power source; and a second capacitor connected to said second switching device in parallel; wherein said second capacitor has a capacitance of 0.4 μF, said induction heating coil has an inductance of 65 μH to 87 μH, and said first capacitor has a capacitance of 2.3 μF to 5 μF.

Priority Claims (1)

Number	Date	Country	Kind
2000-140312	May 2000	JP

US Referenced Citations (2)

Number	Name	Date	Kind
6018154	Izaki et al.	Jan 2000	A
6246843	Nanataki et al.	Jun 2001	B1

Foreign Referenced Citations (1)

Number	Date	Country
2000-259018	Sep 2000	JP

Fixing device using an inverter circuit for induction heating

Information

Patent Number

Date Filed

Date Issued

Inventors

Original Assignees

Examiners

Agents

CPC

US Classifications

Field of Search

US

International Classifications

Abstract

Description

Claims

Priority Claims (1)

US Referenced Citations (2)

Foreign Referenced Citations (1)