JPH0659741B2 - Thermal head drive circuit - Google Patents

Description

The present invention relates to a thermal head drive circuit used in an apparatus for recording or displaying using a thermal head.

“Prior Art” Many printers, facsimile machines, and some copying machines using image sensors perform recording or copying using a thermal transfer recording method or a thermal color recording method. Usually, a thermal head is used as a print head. Further, even in a display device that performs display by using a magnetization latent image, a thermal head may be used as a heat pulse applying unit.

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

Also, inside the thermal head drive circuit 12,
The switching element for controlling the on / off of the current may cause a voltage fluctuation of about several volts with the fluctuation of the printing ratio. For example, NPN with Darlington connection
Bipolar transistor is used for thermal head drive circuit 11
When used for switching, the element has a high collector-emitter saturation voltage, so that when a current of, for example, 45 amperes is applied to the thermal head 11, a drive loss of about 2 volts occurs.

Such voltage fluctuations depending on the printing ratio naturally make the heat generation of the thermal head 11 unstable and affect the recording density of the recording paper. Especially in a printer or a digital copying machine that performs high-speed recording or halftone reproduction, fluctuations in recording density make it difficult to maintain image quality.

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

One of them is to reduce the internal resistance of the thermal head drive circuit. One example is to use thick wires and minimize their length to reduce the voltage drop across lines 14 and 15 of FIG. Regarding the thermal head drive circuit 12, it has been proposed to connect bipolar transistors in parallel to reduce the internal resistance of the switching circuit.

The other is to deal with a voltage fluctuation caused by a voltage drop in the thermal head drive circuit by increasing or decreasing the width of the print pulse.

“Problems to be solved by the invention” However, in each case, there are considerable problems in implementation. Regarding the improvement of the switching circuit, the switching operation by the bipolar transistor normally uses the current driving method, and if all the transistors connected in parallel do not operate uniformly at the same time, the load may concentrate on some transistors. In such a case, an overcurrent may flow through the transistor that operates first and may be damaged.

Further, a separate drive circuit is provided to drive such a switching circuit, but a conventional drive circuit may be configured to consume a certain amount of power even when the switching circuit is in an off state. There was a drawback that it wasted a burden on the power supply. Further, if the drive circuit or the switching circuit fails for some reason, current may continue to flow in the thermal head, and the heating element may be burned. Therefore, appropriate protection measures have been required.

Further, 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, the width of the print pulse is corrected by taking into consideration various external conditions such as the amount of heat stored in the unit heating element and the temperature of the thermal head substrate in order to improve printing quality. Such 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 assigned for controlling the print pulse generating circuit are used for power supply fluctuation correction. However, it is difficult to unnecessarily increase the number of bits of the correction data, and as the number of external conditions used for correction increases, the number of bits assigned to each correction relatively decreases, and the correction dynamic range is set sufficiently wide. There was also the problem of not being able to do it.

The present invention has been made in view of the above points, and an object thereof is to provide a thermal head drive circuit having a switching circuit in which the internal resistance is reduced to suppress the voltage drop and the response is improved. .

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

"Means for Solving Problems" A thermal head drive circuit of the present invention is a switching circuit for switching a print pulse current to be supplied to a thermal head, a drive circuit for driving the switching circuit, and a power supply for the drive circuit. And a drive power switch circuit provided in the input section. The switching circuit has two or more field effect transistors that are connected in parallel with each other to turn on and off the print pulse current, and the drive circuit supplies the gate charging current of the field effect transistor and discharges the gate discharging current to switch the switching circuit. Turn on
It has a pair of totem pole-connected transistors that are turned off. The drive power switch circuit can shut off the power supply to the drive circuit when the drive circuit does not operate.For example, the drive power switch circuit has a switch that turns on so that the drive power supply is connected to the drive circuit only when the switching circuit is on. is doing.

In this way, when the drive power switch circuit connects the power source 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. In addition, the drive power switch circuit shuts off the power supply as necessary, so that abnormal operation of the drive circuit is suppressed, so that the reliability of the circuit can be improved.

Further, since the field effect transistor is used in the switching circuit, the internal resistance of the switching circuit can be made sufficiently low. Moreover, 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 connection diagram showing an embodiment of a thermal head drive circuit and its peripheral circuits of the present invention. This circuit comprises two drive circuits ₁ 1, _{1 2} and the two switching circuits ₂ 1, 2
₂ and two drive power switch circuits 3 ₁ , 3 ₂ ,
It is composed of a thermal head 11 and a print data generator 16.

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

In FIG. 2, when the thermal head is supplied with print data corresponding to half of the print data 27 for one line from the print data generator 16, this thermal head is set in the shift register in the shift register driver 25. , The on / off control of the drivers corresponding to these pits is performed according to the converted parallel data. At this time, the common electrode C
1 is supplied with a printing pulse, and the corresponding portion of the heating resistor 21 is energized to perform printing. After that, the content of the shift register driver 25 is changed by the remaining half of the print data 27 on the same line, and the print pulse is supplied to the common electrode C2. With this, the remaining printing for one line is performed.

Now back to FIG. 1, the first common electrode C1 is commonly connected to the three drain of the field effect transistor (n-channel enhancement-type MOSFET) 31 to 33 of the switching circuit _{2 1.} The output voltage V _{0 of a} power supply circuit (not shown) is applied to the sources of these field effect transistors 31 to 33. Is connected to resistors 34 to 36 to the gate of the field effect transistor 31 to 33, these other ends are connected to the output terminal of the drive circuit 1 _1. A Zener diode 37 for overvoltage protection is connected in parallel with the field effect transistors 31 to 33. Further, the other ends of the gate resistors 34 to 36 are connected to the field effect transistor 3
It is grounded via a silicon diode 41 for suppressing a spike voltage generated when the transistors 1 to 33 are turned off. The source sides of the field effect transistors 31 to 33 are grounded via a smoothing capacitor 42.

On the other hand, the drive circuit 1 ₁ has a field-effect transistor 44 that receives the control signal 43, provided in a subsequent stage, T
A so-called totem pole-connected complementary pair of transistors 45 often used in output circuits such as TL (transistor / transistor / logic).
Has 46. An n-channel enhancement type MOSFET is also used for the field effect transistor 44, and the drain is grounded. The pull-up resistor 48 connected to the source side is the field effect transistor 44.
Is provided to limit the switching current of the. The power source side of the pull-up resistor 48 is a smoothing capacitor 47.
Grounded through.

The drive circuit 1 _1, the power supply voltages V ₁ is configured to be applied through the drive power switching circuit 3 _1. This drive power switching circuit 3 _1, a pair of PNP transistors so-called Darlington connection 6
1 and 62, and two resistors 63 and 64 connected between the bases and emitters of these transistors 61 and 62, respectively. Is connected as the transistor 61 turns on and off the circuit between the drive circuit 1 ₁ connected to the connected power source and the collector to the emitter, the base of transistor 62, a resistor for adjusting the base current 65 and an NPN transistor 66 for turning on / off the base current are connected. This transistor 66 is TTL
Darlington transistors 61 and 62 with small level current
It is provided to drive the.

The TTL level drive signal 70 ₁ is connected to the base of the transistor 66 via the buffer driver 71 _1, and at the same time, the control signal 70 _{1 is connected.}
Is connected to the drive circuit 1 ₁ via the inverter buffer driver 72 ₁ . That is, the control signal 70 ₁ is configured to be input to the drive power supply switch circuit 3 ₁ while inverting its polarity and input to the drive circuit 1 ₁ .

Second connected with the drive circuit 1 ₂ and the switch circuit 2 ₂ and the drive power switching circuit 3 ₂ to the common electrode C2,
The circuit configuration is exactly the same as that described for the first common electrode C1, and the description thereof is omitted.

(Circuit Operation) The thermal head drive circuit having the above configuration operates as follows. Drive circuit 1 ₁ and drive power switch circuit 3
₁ and the control signal 70 ₁ for the first common electrode C1 input to
When the drive circuit _{1 2} and the drive power switching circuit _{3 2}
The second of the control signals 70 _{and second} common electrode C2, and as described with reference to FIG. 2 above, so as to generate mutually exclusively in synchronism with the recording timing are input to the preparative .

The gate of the switching when not printing the circuit 1 ₁ (when keep off the switching circuit 2 ₁₎ field effect transistor 44, joined by a threshold voltage or higher, so-called negative polarity equal to or less than the threshold voltage when the printing Control signal 43 is added. Therefore, the gate of the field effect transistor 44 always exceeds the threshold voltage and is in a switch-on state. At this time, the circuit including the two transistors 45 and 46 in the subsequent stage operates as shown in FIG. In this circuit, the field effect transistor 44 is replaced with the switch 50, and the transistor 45 does not operate. Further, the switching circuit 2 ₁ illustrated only partially.

When the switch 50 is turned on, part of the charge accumulated in the gate of the field effect transistor 31 to 33 switching circuits 2 _1, as indicated by the arrow 51 is the base current of the transistor 46 through the gate resistor 34 to 36 To be discharged. This discharge current turns on the transistor 46. Then, as shown by the arrow 52, the current accumulated in the gate of the field effect transistor 31 is discharged at once and disappears, and as a result, the field effect transistors 31 to 33 are turned off.

Next, in order to turn on this switching circuit, as shown in FIG. 1, the control signal 70 ₁ changes from low level to high level. As a result, the control signal 43 changes to low level. When the control signal 43 to the drive circuit 1 ₁ is supplied, the gate of the field effect transistor 44 becomes the threshold voltage or less, the charge of the gate is discharged, the state of switch-off. At this time 2 of the latter stage
A circuit including the individual transistors 45 and 46 is shown in FIG. 4 in the same manner as in FIG. When the switch 50 turns off,
The voltage on the side of the pull-up resistor 48 connected to the switch 50 rises, and a base current flows through the transistor 45. This flows toward the gate of the field effect transistor 31 as indicated by an arrow 53 through the gate resistors 34 to 36.
This current turns on the transistor 45. Then, the gate charging current flows as shown by an arrow 54. When electric 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 the field effect transistors 31 to 31.
4 is used to suppress the fluctuation of the gate current. For example, when the threshold voltage of the field effect transistors 31 to 34 is 24 volts, the power supply voltage V ₁ of the drive circuit is selected to be about 50 volts. The power supply voltage V ₀ of the switching circuit is a voltage suitable for being applied to the unit heating element of the thermal head, and is usually selected to be about 20 volts.

By the way, the loss between the drain and the source of the field effect transistors 31 to 33 depends on the ON resistance thereof. Now, assuming that the ambient temperature of the field effect transistors 31 to 33 is 25 degrees Celsius, the ON resistance of these is about 0.06Ω. Therefore, if a maximum current of 45 amps is assumed to flow from the power supply circuit of the output voltage V ₀ to the first common electrode C1, then 15 amps each obtained by dividing these into three flow into the field effect transistors 31 to 33, and the drive is performed. The maximum loss is 0.9 volt (0.06 × 15 volt). That is, in the case of this example, a voltage drop of 0 to 0.9 V occurs depending on the printing ratio. In the conventional bipolar transistor, this drive loss is about 2 volts at maximum, which means that it can be reduced to half. Of course, if a field effect transistor having a lower ON resistance is used, the drive loss can be reduced accordingly. Further, if the switching circuit is composed of a larger number of field effect transistors, the maximum current per one is reduced accordingly, and the drive loss can be further reduced.

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

For example, if the drive circuit is composed of only the field effect transistor 44, a rise time of 30 microseconds is required when the print pulse current is 33 A as shown by the curve 56 in FIG. This is because the circuit resistance for flowing the gate charging current is high, and the mirror capacitance (capacitance between drain gates) of the parallel circuit of the field effect transistors is as large as several thousand picofarads. In order to raise the temperature of the unit heating element at a high speed, it is preferable that this rise time is shorter. However, in the case of the present invention, the rise time is shortened to 5 microseconds as shown by the curve 57.

Now, in the above operation, the drive circuit 1 ₁ (first
Figure) does not turn on the switching circuit 2 _1, i.e., when a so-called standby state, the field effect transistor 44 is turned on, when the power supply voltages V ₁ through the pull-up resistor 48 is applied, always constant The current is flowing. This means that the pull-up resistor 48 consumes unnecessary power in the standby state.

Therefore in the present invention are provided drive power switching circuit 3 ₁ to the power input of the drive circuit.

Returning to the circuit of FIG. 1, the drive power switch circuit 3
The operation of _{No. 1} will be described.

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

On the other hand, when the switching circuit is turned on, the control signal 43 input to the drive circuit 1 ₁ becomes low level, and the control signal 70 ₁ input to the drive power supply switch circuit 3 ₁ becomes high level. At this time, the transistor 6
A base current flows through 6, and this transistor 66 is switched on. Therefore, a base current flows through the Darlington-connected transistors 61 and 62, and the transistor 61 is turned on. This drive circuit 1 _first supply voltage V ₁ is applied to the drive circuit 1 ₁ operates normally as previously described.

The thermal head drive circuit as in the present invention, the switching circuit 2 ₁ is off, that is, so as not to apply the supply voltage to the drive circuit 1 ₁ when not the printing operation.

Therefore, the drive circuit does not consume unnecessary power when the printing operation is not performed. Moreover, even if the drive circuit 1 ₁ is unusual behavior for some reason when no printing operation, the power supply voltage is cut off by the drive power switching circuit 3 _1, it would turn on the switching circuit 2 ₁ There is no. Therefore, it also has a function of ensuring safety in case of malfunction.

The case where the application pulse is supplied to the first common electrode C1 has been described above, but the same applies to the case where the application pulse is supplied to the second common electrode C2.

Although the thermal head drive circuit used in the so-called thick film alternating lead wire type thermal head has been described in the above embodiment, the present invention can be applied to all thermal head drive circuits that supply a print pulse of a relatively large current. Of course.

Further, the connection of the pair of transistors of the drive circuit may be appropriately changed to have the same function, or a diode 57 for stabilizing the operation may be inserted as shown in FIG. 6, for example. It goes without saying that the polarity type and usage of the field effect transistor may be changed appropriately.

Furthermore, the drive power supply switch circuit _{3 1,} one PNP
It may be replaced with an appropriate switching element such as a transistor. And the switching control may be made of a different system from the control signal input to the drive circuit 1 _1. In that case, even if there is an abnormality in the control signal input to the drive circuit, the drive circuit and the power supply can be cut off. The timing for turning on the switch is not limited to the time when the drive circuit is operated, but may be an appropriate time including the operation time of the drive circuit.

[Advantages of the Invention] As described above, according to the present invention, since the drive loss of the switching element can be reduced, the generation of heat energy can be reduced accordingly, and a large-scale heat dissipation device becomes unnecessary. In addition, since the bipolar transistor drives a large current, it is 1/10 to 1/20 of the collector current.
Although a large-capacity base current was required, this becomes unnecessary and the drive circuit can be downsized accordingly. Furthermore, since the field effect transistor is used in the thermal head drive circuit of the present invention, the switching speed is reduced to 1/2 to 1/15 as compared with the bipolar type. Further, since the field effect transistor is driven by a pair of totem pole-connected transistors, the rise time of the switching current is extremely fast, and high-speed printing is possible.

Since the drive circuit and the power supply can be shut off except during the printing operation, no wasteful power is consumed and the reliability is high.

[Brief description of drawings]

FIG. 1 is a connection diagram showing an embodiment of a thermal head drive circuit and its peripheral circuits of the present invention, FIG. 2 is a connection diagram of a thermal head used for the same, and FIGS. 3 and 4 are operations of the drive circuit. FIG. 6 is a characteristic diagram showing a switching current rising characteristic of the thermal head drive circuit of the present invention, FIG.
FIG. 7 is a wiring diagram of a main part showing a modified example of the drive circuit, and FIG. 7 is a block diagram of the periphery of a general thermal head drive circuit. 1 ₁ , 1 ₂ ...... Drive circuit, 2 ₁ , 2 ₂ ...... Switching circuit, 3 ₁ , 3 ₂ ...... Drive power supply switch circuit, 11 ...... Thermal head, 31, 32, 33 ...... Field effect transistor, 45 , 46 ... a pair of totem pole-connected transistors, 52 ... Gate discharge current, 54 ... Gate charge current.

Claims (2)

[Claims] 1. A switching circuit for switching a print pulse current to be supplied to a thermal head, a drive circuit for driving the switching circuit, and a drive circuit provided in a power source input section of the drive circuit when it is not necessary to operate the drive circuit. And a drive power supply switch circuit capable of cutting off the power supply, wherein the switching circuit has two or more field effect transistors which are connected in parallel with each other to turn on / off the print pulse current, and the drive circuit comprises: A thermal head drive circuit having a pair of totem pole-connected transistors that supply a gate charge current of a field effect transistor and discharge a gate discharge current to turn on / off a switching circuit. 2. The thermal head drive circuit according to claim 1, wherein the drive power supply switch circuit is turned on so that the drive power supply is connected to the drive circuit only when the switching circuit is on.