JPS61239962A - Thermal head driving circuit - Google Patents

Abstract

PURPOSE:To suppress voltage drop and to enhance response by lowering internal resistance, by providing a switching circuit having two or more of electric field effect transistors connected to each other in parallel to turn a printing pulse current On and OFF and a pair of transistors subjected to totem pole connection for turning the switching circuit ON and OFF by supplying the gate charging currents of the aforementioned electric field effect transistors and discharging gate discharge currents. CONSTITUTION:In a switching circuit 21, voltage equal to or more than threshold voltage is applied to the gate of an electric field effect transistor 44 when no printing is performed and a control signal making said voltage equal to or less than said threshold voltage is applied when printing is performed. When the transistor 44 is replaced with a switch 50 and this switch 50 is turned ON, a part of the charge accumulated in the gates of electric field effect transistors 31-33 is discharged as shown by an arrow 51 to allow a transistor 46 to turn ON and the current accumulated in the gate of the transistor 31 is discharged and extinguished at a stroke as shown by an arrow 52 and the transistors 31-33 are brought to an OFF-state.

Description

DETAILED DESCRIPTION OF THE INVENTION "Field of Industrial Application" The present invention relates to a thermal head drive circuit used in a device that performs recording or display using a thermal head.

"Prior Art" Many printers, facsimile machines, and some copying machines that use image sensors record or copy using a thermal transfer recording method or a thermosensitive color recording method. A thermal head is usually used as the print head. Further, even in a display device that performs display using a magnetized latent image, a thermal head may be used as a means for applying a thermal pulse.

FIG. 7 shows a thermal head drive circuit 12 for supplying printing pulses to the thermal head 11, and a power supply circuit 13 for driving the thermal head. In the thermal heater 11, heating elements are arranged linearly in the longitudinal direction. This heating element is divided into a large number of sections in the longitudinal direction, and lead wires for applying printing pulses are connected to both ends of each section. This - category heating element will be referred to as a unit heating element. Print pulses are selectively supplied to these unit heating elements. The amount of current consumed varies greatly depending on the number of unit heating elements (heating elements) to which printing pulses are supplied at one time, that is, the ratio of energized unit heating elements to all unit heating elements (hereinafter referred to as printing ratio). Line 1 connecting power supply circuit 13 and thermal head drive circuit 12
4, thermal head drive circuit 12 and thermal head 11
When the current flowing through the line 15 connecting the lines 14 and 15 changes greatly depending on the printing ratio, the voltage applied to the thermal head 11 fluctuates due to a voltage drop due to the electrical resistance of the lines 14 and 15.

Also, inside the thermal head drive circuit 12,
A switching element that controls the on/off of current may generate voltage fluctuations of several volts as the printing ratio changes. For example, if a Darlington-connected NPN bipolar transistor is used for switching in the thermal head drive circuit 11, this element has a high collector-emitter saturation voltage, so when a current of, for example, 45 amperes flows through the thermal head 11, This results in a drive loss of approximately 2 volts.

Such voltage fluctuations depending on the printing ratio naturally make the heat generation of the thermal radar 11 unstable during operation, and affect the recording density of the recording paper. Particularly in printers or digital copying machines that perform high-speed recording or halftone reproduction, fluctuations in recording density make it difficult to maintain image quality.

In order to prevent such deterioration in printing quality due to voltage fluctuations depending on the printing ratio, the following improvements have been mainly made.

One of them is to reduce the internal resistance of the thermal head drive circuit. - An example is to use thick wires and keep their lengths as short as possible in order to reduce the voltage drop across lines 14.15 in FIG. Regarding the thermal head drive circuit 12, it has been proposed to connect bipolar transistors in parallel to lower the internal resistance of the switching circuit.

Another method is to increase or decrease the width of the print pulse to cope with voltage fluctuations caused by voltage drops in the thermal head drive circuit.

``Problems to be solved by the invention'' However, in either case, there are considerable problems in implementation. Regarding improvements to switching circuits, switching operations using bipolar transistors usually use a current drive method, and if all transistors connected in parallel do not operate uniformly at the same time, the load may concentrate on some transistors. In such a case, there is a risk that an overcurrent will flow to the transistor that operates first and damage it.

In addition, a separate drive circuit is provided to drive such a switching circuit, but conventional drive circuits may be configured to consume a certain amount of power even when the switching circuit is in the off state. There was a drawback in that it placed an unnecessary burden on the power supply. Furthermore, if the drive circuit or switching circuit breaks down for some reason, current continues to flow through the thermal head, potentially burning out the heating element, so appropriate protective measures have been required.

Furthermore, regarding the correction of the print pulse width, there are the following problems. In a recording device or a display device using a thermal head, in order to improve printing quality, the width of the printing pulse is corrected by taking into account various external conditions such as the amount of heat stored in a unit heating element and the temperature of the thermal head substrate. These various print pulse correction data are input in parallel to a print pulse generation circuit that determines the width of the print pulse. Therefore,
Some bits of a predetermined number of parallel correction data allocated for controlling the print pulse generation circuit are used for correction of power supply fluctuations. However, it is difficult to unnecessarily increase the number of bits of correction data; the more external conditions are used for correction, the more the number of bits allocated to each correction decreases, making it difficult to set a sufficiently wide dynamic range of correction. There was also the problem of not being able to do so.

The present invention has been made in view of the above points, and an object of the present invention is to provide a thermal head drive circuit having a switching circuit with low internal resistance, suppressing voltage drop, and improving responsiveness. .

A further object of the present invention is to provide a thermal head drive circuit that suppresses the power consumed by the drive circuit when the switching circuit is switched off, has a circuit protection function, and has improved reliability. .

"Means for Solving the Problems" The thermal head drive circuit of the present invention includes a switching circuit that switches a printing pulse current to be supplied to the thermal head, a drive circuit that drives this switching circuit, and a power supply for this drive circuit. It consists of a drive power switch circuit provided at the input section. The switching circuit has two or more field effect transistors connected in parallel to each other to turn on and off the printing pulse current, and the drive circuit supplies the gate charging current of the field effect transistor and discharges the gate discharge current to perform switching. It has a pair of transistors connected in a totem pole to turn on and off the circuit. The drive power switch circuit is capable of cutting off the power to the drive circuit when the drive circuit is not operating; for example, it has a switch that is turned on so that the drive power is connected to the drive circuit only when the switching circuit is on. are doing.

In this way, when the drive power switch circuit connects the power to the drive circuit only when the switching circuit is on, the power consumption of the drive circuit when the switching circuit is off can be suppressed. Further, since this drive power switch circuit shuts off the power supply as necessary, abnormal operation of the drive circuit is suppressed, so that the reliability of the circuit can be improved.

Furthermore, since field effect transistors are used in the switching circuit, it is possible to sufficiently reduce the internal resistance of the switching circuit. Furthermore, since the impedance of the drive circuit is low, the switching response of the field effect transistor is fast and high-speed switching is possible.

"Embodiment" (Circuit Configuration) FIG. 1 is a wiring diagram showing an embodiment of a thermal head drive circuit and its peripheral circuits of the present invention. This circuit consists of two drive circuits 18.12 and two switch circuits 2. ．．
2. and two drive power switch circuits 31.3
□ and the thermal head 11 and print data generation section 1
It consists of 6.

The thermal head 11 used in this embodiment is a so-called thick film alternating lead wire type recording head. On the substrate of this thermal head 11, as shown in FIG. 2, one elongated heating resistor 21 is formed. Two types of electrodes 22 and 23 are attached alternately to the heating resistor 21 at predetermined intervals. One of these electrodes 22 is connected to the first and second common electrodes Cl through diodes 24, respectively.
They are alternately connected to C2. The other electrode 23 is connected to a shift register mounted on the substrate of the thermal head.
It is connected to each driver (switching element) of the driver 25.

In FIG. 2, when this thermal head receives print data equivalent to half of one line of print data 27 from the print data generation section 16, it sets it in a shift register in a shift register driver 25. , and turns on/off the dry noise corresponding to the bits according to the converted parallel data. At this time, a printing pulse is supplied to the common electrode C1, and the corresponding portion of the heating resistor 21 is energized to perform printing. Thereafter, the contents of the shift register driver 25 are changed by the remaining half of the print data 27 for the same line, and a print pulse is supplied to the common electrode C2. The remaining printing for one line will now be done. - Now, returning to FIG. 1, the first common electrode CI is commonly connected to the drains of three field effect transistors (n-channel enhancement type MOSFETs) 31 to 33 in the switching circuit 21. These field effect transistors 31
The source of ~33 is the output voltage v of a power supply circuit (not shown).
0 is applied. Resistors 34 to 36 are connected to the gates of the field effect transistors 31 to 33, respectively, and the other ends of these are connected to the output terminal of the drive circuit 11. Further, a Zener diode 37 for overvoltage protection is connected in parallel with the field effect transistors 31 to 33. Furthermore, the gate resistor 34~
The other end of the transistor 36 is grounded via a silicon diode 41 for suppressing the spike voltage that occurs when the field effect transistors 31 to 33 are turned off. Further, the sources of the field effect transistors 31 to 33 are grounded via a smoothing capacitor 42.

On the other hand, the drive circuit 1□ includes a field effect transistor 44 that receives and receives the control signal 43, and a field effect transistor 44 that is provided at the subsequent stage.
A pair of so-called totem-pole connected conjunctive transistors 45 often used in output circuits such as TTL (transistor-transistor-logic).
．． It has 46. This field effect transistor 44 also uses an n-channel enhancement type MO3FET, and its drain is grounded. A pull-up resistor 48 connected to the source side of this field effect transistor 44
This is provided to limit the switching current. In addition, the power supply side of the pull-up resistor 48 is connected to a smoothing capacitor 47.
is grounded through.

This drive circuit 1. In this case, the power supply voltage ■1 is the drive power switch circuit 3. It is configured to be applied via. This drive power switch circuit 3I consists of a pair of so-called Darlington-connected PNP) transistors 1.62 and two resistors 63.6 connected between the base and emitter of each of these transistors 6162.
It is equipped with 4. a power supply with a transistor 61 connected to its emitter and a drive circuit 11 connected to its collector;
The transistor 620 has a resistor 65 that adjusts its base current and an NPN (NPN) that turns the base current on and off.
Ranjistaro 6 is connected. This transistor 6
Reference numeral 6 is provided to drive Darlington transistors 61 and 62 with a small current of TTL level.

A TTL level drive signal 701 is applied to the base of this transistor 660 to buffer driver buffer 1. This control signal 701 is connected to be applied via the
is the inverter buffer dry buffer 2. It is connected to the drive circuit 11 via a manual connection. That is, the control signal 701 is transmitted to the drive power switch circuit 3. while inputting it to the drive circuit 11 by reversing its polarity.
It is configured to be manually operated.

The drive circuit 12 connected to the second common electrode C2, the switch circuit 2□, and the drive power switch circuit 32
The circuit configuration is exactly the same as the circuit configuration described for the first common electrode C1, and the explanation thereof will be omitted.

(Circuit Operation) The thermal head drive circuit having the above configuration operates as follows. A control signal 70 for the first common electrode CI that is manually supplied to the drive circuit 11 and the drive power switch circuit 31. and the control signal 702 for the second common electrode C2 which is manually input to the drive circuit 12 and the drive power switch circuit 3, are synchronized with the recording timing and are synchronized with each other, as explained earlier using FIG. It is designed to occur exclusively in

Switching circuit 1 and switching circuit 2 are not printed. (when turning off) A voltage higher than the threshold voltage is applied to the gate of the field effect transistor 44, and a so-called negative control signal 43 which becomes lower than the threshold voltage when printing is applied. Therefore, the gate of the field effect transistor 44 normally exceeds the threshold voltage and is in the switched-on state. At this time, the circuit including the two subsequent transistors 45 and 46 operates as shown in FIG. In this circuit, the field effect transistor 44 is replaced with a switch 50, and since the transistor 45 does not operate, it is shown by a broken line. Further, only a part of the switching circuit 2° is shown.

When the switch 50 is on, the switching circuit 21
A part of the charge accumulated in the gates of the field effect transistors 31 to 33 becomes the base current of the transistor 46 through the gate resistors 34 to 36 and is discharged as shown by an arrow 51. This discharge current turns on transistor 46. Then, as shown by arrow 52, the current accumulated in the gate of field effect transistor 31 is discharged and disappears all at once, and as a result, field effect transistors 31 to 33 are turned off.

Next, in order to turn on this switching circuit, a control signal 70. as shown in FIG. changes from low level to high level. This causes the control signal 43 to change to low level. When the control signal 43 is supplied to the drive circuit 11, the gate of the field effect transistor 44 becomes lower than the threshold voltage, the charge at the gate is discharged, and the switch 1 is turned off. At this time, the circuit including the two subsequent transistors 45 and 46 is connected to the third transistor.
It is shown in FIG. 4 in the same manner as in the figure. When the switch 50 is turned off, the voltage on the side of the pull-up resistor 48 connected to the switch 50 rises, causing a base current to flow through the transistor 45. This is connected to the arrow 53 through the gate resistors 34 to 36.
The current flows toward the gate of the field effect transistor 31 as shown in FIG. This current turns on transistor 45. Then, a gate charging current flows as indicated by an arrow 54. When charge is accumulated in the gate and the potential of the gate exceeds the threshold voltage, the field effect transistors 31 to 33 are turned on all at once. The resistors 34 to 36 are used to suppress fluctuations in the gate currents of the field effect transistors 31 to 34. For example, when the threshold voltage of the field effect transistors 31 to 34 is set to 24 volts, the power supply voltage (1) of the drive circuit is selected to be about 50 volts. The power supply voltage Vo of the switching circuit is a voltage suitable for applying to the unit heating element of the thermal head, and is usually selected to be about 20 volts.

Incidentally, the loss between the drains and sources of the field effect transistors 31 to 33 is influenced by their on-resistance. Now, assuming that the ambient temperature of the field effect transistors 31 to 33 is 25 degrees Celsius, their on-resistance is about 0.06Ω. Therefore, the output voltage■. If a maximum current of 45 amperes flows from the power supply circuit to the first common electrode C1, then 15 amperes each divided into three will flow to each field effect transistor 31 to 33, and the drive loss will be the maximum for each. It becomes 0.9 volts (0.06 x 15 volts).

In other words, in this example, the printing ratio varies from 0 to 0.
A voltage drop of up to 9 volts will occur. In conventional bipolar transistors, this drive loss is approximately 2 volts at maximum, so this means that this can be halved.

Of course, if a field effect transistor with a lower on-resistance is used, the drive loss can be reduced accordingly. Furthermore, if this switching circuit is configured with a larger number of field effect transistors, the maximum current per transistor can be reduced accordingly, and drive loss can be further reduced.

Furthermore, in the present invention, since a pair of totem-pole connected transistors is used in the drive circuit, the resistance of the circuit that flows the gate charging/discharging current for turning on/off the field effect transistor of the switching circuit is small. Therefore, charging and discharging are performed quickly, and the switching start-up speed is extremely fast.

For example, when the drive circuit is composed of only the field effect transistor 44, a rise time of 30 microseconds is required when the printing pulse current is 33 A, as shown by a curve 56 shown in FIG. This is because the circuit resistance for passing the gate charging current is high, and the mirror capacitance (capacitance between the drain and gate) of the parallel circuit of field effect transistors is large, on the order of several thousand picofarads.The temperature of the unit heating element increases rapidly. However, in the present invention, the rise time is shortened to 5 microseconds as shown by curve 57.

Now, in the above operation, when the drive circuit 11 (FIG. 1) does not turn on the switching circuit 21, that is, in the so-called standby state, the field effect transistor 44 is turned on, so the power supply voltage is (2) If , is applied, a constant current will always flow. This results in unnecessary power consumption by the pull-up resistor 48 in the standby state.

Therefore, in the present invention, a drive power switch circuit 31 is provided at the power input section of this drive circuit.

Returning to the circuit shown in Figure 1, drive power switch circuit 3
The operation of 1 will be explained.

Switching circuit 2. In order to turn off the drive circuit 1. The control signal 43 input to is at high level. Therefore, drive power switch circuit 3. A control signal 70. is low level. At this time,
Transistor 66 is off, and Darlington-connected transistors 61 and 62 are both off. Therefore, the power supply voltage ■1 is the drive circuit 1. is not applied.

On the other hand, when the switching circuit is turned on, the control signal 43 input to the drive circuit 11 becomes low level, and the control signal 701 input to the drive power switch circuit 31 becomes high level.

At this time, a base current flows through the transistor 66, and the transistor 66 is turned on. Therefore,
A base current flows through the Darlington-connected transistors 61 and 62, turning on the transistor 61. As a result, the power supply voltage ■1 is applied to the drive circuit 1°, and the drive circuit 1. works fine as described earlier.

As described above, the thermal head drive circuit of the present invention includes the switching circuit 2. When the drive circuit 1. is off, that is, when there is no printing operation, the drive circuit 1. The power supply voltage is not applied to the

Therefore, the drive circuit does not consume unnecessary power when no printing operation is performed. Moreover, even if the drive circuit 1° operates abnormally for some reason when no printing is performed, the power supply voltage is cut off by the drive power switch circuit 31, so the switching circuit 21 will not be turned on. . Therefore, it also has a function to ensure safety in case of malfunction.

Although the case where the application pulse is supplied to the first common electrode CI has been described above, the same applies to the case where the application pulse is supplied to the second common electrode C2.

Although the above embodiment describes a thermal head drive circuit used in a so-called thick film alternating lead wire type thermal head, the present invention can be applied to any thermal head drive circuit that supplies relatively large current printing pulses. Of course.

Further, the connection of the pair of transistors in the drive circuit may be appropriately changed so that they have the same function, or, for example, a diode 57 may be inserted for stabilizing the operation as shown in FIG. It goes without saying that the polarity type, method of use, etc. of the field effect transistor may be changed as appropriate.

Furthermore, the drive power switch circuit 3I has one PN
P) It may be replaced with a suitable switching element such as a transistor. And the switching control is
The control signal may be generated by a system different from the control signal manually input to the drive circuit 1□. In that case, even if there is an abnormality in the control signal input to the drive circuit, the drive circuit and power supply can be cut off. The timing of turning on this switch does not have to be only when the drive circuit is operated, but may be any suitable time that includes the operation time of the drive circuit.

[Effects of the Invention] As described above, according to the present invention, the drive loss of the switching element can be reduced, so the generation of thermal energy can be reduced accordingly, and a large-scale heat dissipation device is not required. In addition, in order to drive a large current, bipolar transistors require 1/10 to 1/20 of the collector current.
However, this is no longer necessary, and the drive circuit can be made smaller.

Furthermore, since the thermal head drive circuit of the present invention uses field effect transistors, the switching speed is reduced to 1/2 to 1/15 of that of a bipolar type circuit. Furthermore, since the field effect transistor is driven by a pair of transistors connected in a totem pole connection, the rise time of the switching current is extremely fast, enabling high-speed printing.

Furthermore, since the drive circuit and the power supply can be cut off except during the printing operation, unnecessary power is not consumed and reliability is high.

[Brief explanation of the drawing]

Fig. 1 is a wiring diagram showing an embodiment of the thermal head drive circuit of the present invention and its peripheral circuit, Fig. 2 is a wiring diagram of the thermal head used therein, and Figs. 3 and 4 are the operation of the drive circuit. An explanatory diagram, FIG. 5 is a characteristic diagram showing the switching current rise characteristic of the thermal head drive circuit of the present invention, and FIG.
The figure is a main part wiring diagram showing a modified example of the drive circuit, and FIG. 7 is a peripheral block diagram of a general thermal head drive circuit. ' II % 12... Drive circuit, 21.22... Switching circuit,
3. 3.・・・・・・Drive power switch circuit,
11... Thermal head, 31.32.33... Field effect transistor,
45.46... A pair of transistors connected to a totem pole, 52... Gate discharge current, 54... Gate charging current. Applicant Fuji Xerox Co., Ltd.

Claims (1)

[Claims] 1. A switching circuit that switches the printing pulse current to be supplied to the thermal head, a drive circuit that drives this switching circuit, and a drive circuit that is provided at the power input section of this drive circuit and is required to operate the drive circuit. a drive power switch circuit capable of cutting off the power supply when the drive A thermal head drive circuit characterized in that the circuit includes a pair of totem-pole connected transistors that supply a gate charging current to the field effect transistor and discharge a gate discharge current to turn on and off a switching circuit. 2. The thermal head drive circuit according to claim 1, wherein the drive power switch circuit is turned on to connect the drive power to the drive circuit only when the switching circuit is on.