JPH06230702A - Heater driving device - Google Patents

Abstract

PURPOSE:To suppress the rush current of a heater and to prevent the deterioration and malfunction of parts by controlling switching time in such a manner that heater current is made small right after heater on and thereafter, the heater current is made to increase. CONSTITUTION:This heater driving device has an AC power source input terminal 11, a noise filter 12, a bridge type rectifier 13, a condenser 14 and a coil 15. A noise preventive circuit is formed, out of the condenser 14 and the coil 15. The heater driving device has a resistor 18 for voltage drop, a smoothing condenser 19 and a photocoupler 20 and has further an electric, heating element layer 4b of a fixing device heater 4, a thermistor 4d as the temp. detecting element for the fixing device heater 4, a one-chip microprocessor 26, a power source limiting resistor 27of a photocoupler 20, etc. The switching time by a switching transistor 16 for switching the electric power subjected to current rectification by the bridge type rectifier 13 is controlled by a control circuit 17 in such a manner that the heater current is made small right after heater on and, thereafter, the heater current is made to increase.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a heater driving device for driving a fixing heater used in a fixing device of an image forming apparatus such as an electrophotographic device and an electrostatic recording device.

[0002]

2. Description of the Related Art Conventionally, for supplying electric power to a heater of an image heat fixing device, a large amount of electric power is supplied to the heater, so that the heater is directly supplied from a commercial AC power source via a triac.

[0003]

However, when an AC voltage is directly applied to the heater, a large rush current flows at the moment the heater is turned on.

This rush current is particularly large when a halogen lamp using a tungsten filament is used as a heater. If such a rush current flows, it has an adverse effect of shortening the life of the power switch and the heater drive circuit in the apparatus.

Further, since the voltage of the AC power supply line to which the device is connected is lowered, there is an adverse effect that a malfunction occurs in another device connected to the same AC power supply line.

[0006]

SUMMARY OF THE INVENTION The present invention for solving the above-mentioned problems is a rectifying means for rectifying AC power in a heater driving means for driving a heater for heating and fixing an image on a recording material, and the rectifying means. And a control means for controlling the switching time by the switching means such that the heater current is reduced immediately after the heater is turned on and then the heater current is increased immediately after the heater is turned on. .

[0007]

Embodiments of the present invention will be described below with reference to the drawings.

FIG. 4 is a sectional view of a heat roller fixing device using the heater driving device according to the embodiment of the present invention.

Reference numeral 4a is a heat roller made of an aluminum cylinder, 4b is a heater, 4c is a sponge pressure roller, and 4d is a thermistor for detecting the surface temperature of the heat roller 4a. Heater 4b
Uses a halogen lamp to heat the heat roller 4a from the inside. In the electrophotographic image forming apparatus, unfixed paper is fixed by passing between the heat roller 4a and the pressure roller 4c.

1 and 2 are circuit diagrams of a heater driving device of this embodiment.

In the circuit of FIG. 1, 11 is an AC power supply input terminal, 12 is a noise filter, 13 is a bridge rectifier, 14 is a capacitor, and 15 is a coil. The capacitor 14 and the coil 15 form a noise prevention circuit. .

Reference numeral 16 is a switching transistor, and 17 is a control circuit, which compares the detection voltage input from the V _I terminal with a built-in voltage standard, and PWM-modulates the output of the built-in oscillator according to the error voltage. Output from.

Reference numeral 18 is a voltage drop resistor, and 19 is a smoothing capacitor, which supplies power to the V terminal of the circuit 17.

Reference numeral 20 is a photocoupler, the output of which is connected to the V _C terminal of the circuit 17. V _C terminal is circuit 1
It has a function to control the PWM output of 7 on / off.

Reference numeral 4b is an electric heating layer of the fixing device heater 4 described above. The output PWM voltage of the transistor 16 is divided and smoothed by the resistors 22 and 23 and the capacitor 24, and the average value of the output PWM voltage is extracted.

Reference numeral 4d is a thermistor as a temperature detecting element of the above-mentioned fixing device heater 4, which detects the heater temperature.

Reference numeral 26 is a one-chip microprocessor I
/ O port, ROM, RAM, built-in A / D converter. For example, μPD7811 from NEC Corporation corresponds to this. Reference numeral 27 is a power supply limiting resistor of the photocoupler.

The AC power source input from the terminal 11 is rectified by the rectifier 13 and has the waveform shown in FIG. When PWM modulation is applied to this pulsating wave by the transistor 16, the waveform shown in FIG. 3B is obtained. The waveform of (b) of FIG.
When the voltage is divided by 23 W and smoothed by the capacitor 24, an average voltage corresponding to the input AC voltage is obtained as shown in FIG.

[0019] The voltage compared to standard voltage in the circuit 17, the lower the voltage V _I widen the PWM modulation, a voltage of V _I is to narrow the width of the PWM modulation is higher. As a result, the voltage applied to the heater 4b becomes constant regardless of the fluctuation of the AC voltage applied to the terminal 11.

Next, the circuit 17 will be described with reference to FIG.

30 is a sawtooth wave generator, 31 and 32 are error amplifiers, 33 and 34 are comparators, 35 is an AND circuit, 36 is a photocoupler, 37 is a three-terminal regulator, 38 is a transistor, 39 is a capacitor, and 40-5.
0 is a resistor.

First, the input from the V _I terminal will be described.

The voltage applied to the V _I terminal is the error amplifier 3
The voltage becomes a voltage that determines the slice level at which the comparator 33 slices the sawtooth wave amplified by 1 and output from the sawtooth wave generator 30. A PWM-modulated pulse output is output to the output of the comparator 33. The PWM pulse output passes from the AND gate 35 through the photo coupler 36 to V _D.
It is output from the terminal.

The voltage dividing resistors 45 and 46 form the reference voltage of the error amplifier 31.

Next, the operation when the terminal V _C becomes high level, that is, when the heater 4b is turned on will be described.

Until V _C becomes high level, the AND gate 35 is closed and no output is made from the terminal V ₀ . That is, the input voltage at the terminal V ₁ is also 0V.

When V _C becomes high level, _first , the input voltage of the terminal V ₁ is still 0 V, so the output of the comparator 33 remains at the high level.

Further, the transistor 38 is turned on, and the charging current starts flowing through the capacitor 39. Then the error amplifier 3
+ Input of 2 is pulled up to high level and error amplifier 3
The output of 2 also becomes high level, and the output of the comparator 34 becomes low level. Then, as the capacitor 39 is charged, the + input voltage of the error amplifier 32 gradually decreases, and along with that, the pulse width of the output of the comparator 34 increases.

Since two of the three inputs of the AND gate 35 are at high level, the output pulse of the comparator 34 passes through the gate 35, passes through the photocoupler 36, and is output to the V ₀ terminal. When the capacitor 39 is completely charged, the + input voltage of the error amplifier 32 becomes 0V, and the output of the comparator 34 remains at the high level.

In this state, a voltage has already been applied to the V _I terminal, and the voltage determined by the voltage dividing resistors 45 and 46 and V
The PWM pulse width of the comparator 33 is controlled so that the _I inputs become equal. In this way, when the heater is applied, the average value of the voltage gradually increases from zero at first and becomes a constant voltage.

If the time from reaching zero to a constant value is set longer than the time during which the rush current of the heater flows, the heater is gradually heated and the internal resistance increases, so that a large rush current flows. Does not flow.

FIG. 5 shows a waveform when the heater is ON.

FIG. 5A shows the voltage at the terminal V _C (heater on), FIG. 5B shows the + input voltage of the error amplifier 32, and FIG.
(C) shows the envelope of the current flowing through the heater 4b.

FIG. 6 is a circuit diagram of the second embodiment.

In this embodiment, the rectified output of the rectifier 13 as the rectifying means of the heater driving circuit shown in FIG. 1 is branched so as to be shared with another power source in the printing apparatus (for example, the power source for the CPU 26). . Further, by setting the input voltage detection terminal V _I of the control circuit 17 to the low level, the PWM operation is performed only when the heater is rushed, and thereafter the transistor 16 is kept completely turned on.

This is because the switching loss of the transistor 16 increases and the temperature rises by itself when the PWM operation is always performed.
Prevents the generation of electrical noise that may affect other devices.

The fixing heater has a very low impedance as a rectifier load, whereas the low-voltage power source consumes little power and has a high impedance as a rectifier load. DC / DC as low-voltage power supply
A converter is required, but its input voltage waveform must be fairly DC.

However, since the impedance of the fixing heater is low, a very large capacitor is required to properly smooth the rectifier output, which is not realistic.

Therefore, in the circuit of this example, the DC / DC converter 51 is connected via the diode 50. In this way, by inserting the diode 50, the impedance seen from the load side rises and the smoothing effect of the capacitor 52 can be fully utilized.

In this way, two loads having very different load impedances can be connected in parallel to the output of the rectifier 13, which is effective for cost reduction.

FIG. 7 is a circuit diagram of the third embodiment.

The same parts as those in FIG. 1 are designated by the same reference numerals and the description thereof will be omitted. 53 is a photo interrupter, 54 is a choke coil, 55 is a fast recovery diode, 56 is a noise prevention capacitor, and 57 and 58 are resistors.

In this embodiment, the inrush power supply flowing to the heater 4b is prevented and the current flowing in the steady state is made constant. When a current flows through the heater 4b, a potential difference is generated across the resistor 57. A part of the current flowing through the resistor 57 is divided into the LED portion of the photocoupler 53 and the resistor 58. This photocoupler 53 is provided on the power input side of the choke coil 15 because the switching waveform is smoothed by the filter at this location, and it has a pulsating flow of the frequency of the original AC power supply and is affected by switching noise. Because there is no.

The pulsating current waveform optically transmitted from the LED of the photocoupler 53 to the phototransistor is smoothed by the resistor 59 and the capacitor 24, and AC is applied to the V _I terminal of the control circuit 17.
The voltage corresponding to the average value of the current is input.

The internal operation of the control circuit 17 is the same as that described in the first embodiment with reference to FIG.

In this embodiment, the switching transistor 16 is placed on the GND side of the load (heater 4b).
Therefore, since the potential difference between the base of the transistor 16 and the V ₀ terminal of the circuit 17 is small, it is not necessary to insulate with the photo coupler 36 in FIG. 2, and there is an advantage that the photo coupler 36 can be omitted.

The coil 54 smoothes the rising and falling waveforms of the switching current flowing through the heater 4b, which helps prevent noise. The capacitor 56 is also for noise prevention. The diode 55 is for absorbing the reverse excitation voltage generated in the coil 54 when the transistor 16 is turned off.

In this embodiment, the effect of preventing the inrush current when the heater is turned on is the same as that in the first embodiment. In the first embodiment, the heater voltage is constant, but the present embodiment is different in that the constant current is constant.

In both cases, if the resistance of the heater is constant, the power consumption of the heater can be made constant.

In this embodiment, the switching transistor 16
Is at the GND potential, which facilitates connection with the control circuit 17.

[0051]

As described above, according to the present invention, the inrush current of the heater is suppressed, deterioration of parts such as switches in the device through which the inrush current flows is prevented, and the voltage fluctuation of other devices connected to the same AC line is prevented. It is possible to prevent the malfunction.

[Brief description of drawings]

FIG. 1 is a circuit diagram of a heater driving device according to an embodiment of the present invention.

FIG. 2 is a detailed diagram of a control circuit 17 in FIG.

3A is a waveform after rectification of an input AC voltage, FIG. 3B is its PWM waveform, and FIG. 3C is its smoothed waveform.

FIG. 4 is a sectional view of a fixing device to which an embodiment of the present invention is applied.

5A is a heater-on signal, and FIG. 5B is a control circuit 1.
7 shows the + input waveform of the error amplifier 32, and (c) shows the envelope of the heater current waveform.

FIG. 6 is a circuit diagram of another embodiment of the present invention.

FIG. 7 is a circuit diagram of another embodiment of the present invention.

[Explanation of symbols]

 4b Halogen heater 4d Thermistor 13 Rectifier 16 Switching transistor 17 Control circuit equipped with PWM modulator 20,36 Photocoupler 26 Microprocessor 31,32 Error amplifier 33,34 Comparator 35 AND gate

Claims (2)

[Claims] 1. A heater driving means for driving a heater for heating and fixing an image on a recording material, a rectifying means for rectifying AC power, a switching means for switching the power rectified by the rectifying means, and a heater ON. Immediately after that, the heater drive device is provided with a control means for controlling the switching time by the switching means so that the heater current is reduced and then the heater current is increased. 2. The heater driving device according to claim 1, wherein the heater is a halogen heater having a tungsten filament.