Firing circuit for thermal inkjet-printing nozzle

[0001] Thermal inkjet printing devices, such as inkjet printers, operate by appropriately firing ink from an inkjet printing nozzle to form an image on a medium such as paper. Ink is fired from a given inkjet printing nozzle by means of an initiating circuit for the inkjet printing nozzle. The initiating circuit includes a heating resistor and a switch. When the switch is closed, current flows through the heating resistor, which heats the ink and causes it to fire from the correct nozzle. Current initiating circuit solutions are known as the "low voltage side switch" of the initiating circuit in which the switch side is always connected to ground and the heating resistor side is always connected to a voltage source. However, such solutions can be problematic. If, for example, the heating resistor of a given nozzle fails, the resulting voltage shortage can damage other initiating circuits. EP-A-1 142 715 discloses a high voltage side switch circuit.

BRIEF DESCRIPTION OF THE DRAWINGS [0002] The drawings referred to herein form part of the description of the invention. The features shown in the drawing are intended to illustrate only some and not all embodiments of the invention.

FIG. 1 is a schematic of a DC mode initiating circuit for an inkjet printing nozzle, according to an embodiment of the invention.

FIG. 2 is a diagram illustrating parasitic resistance that is the result of the operation of a plurality of initiation circuits simultaneously activated, according to an embodiment of the invention.

FIG. 3 is a graph showing the DC characteristics of a DC mode high-voltage side switch, according to an embodiment of the invention.

FIG. 4 is a graph showing the characteristics of alternating current (AC) in constant current mode of the high voltage side switch, according to an embodiment of the invention.

FIG. 5 is a block diagram of representative inkjet printing devices according to an embodiment of the invention.

FIG. 6 is a flowchart of how to use a high voltage side switch, DC mode initiating circuit for a thermal inkjet printing nozzle, according to an embodiment of the invention.

FIG. 7 is a diagram of a basic method for making inkjet printing devices for carrying out the invention. DETAILED DESCRIPTION OF THE DRAWINGS [0003] In the following, by way of example, detailed examples of the invention, reference is made to the accompanying drawings, which form part of the present description of the invention, and in which specific examples of the application of the invention are illustrated by way of illustration. These embodiments are described in sufficient detail to enable those skilled in the art to practice the invention. Other embodiments may be used, and logical, mechanical and other changes may be made without departing from the scope of the present invention. Therefore, the following detailed description should not be construed as limiting, and the scope of the present invention is defined only by the appended claims. [0004] FIG. 1 shows an initiating circuit 100 for a thermal inkjet nozzle, according to an embodiment of the invention. The initiating circuit 100 includes a switch 102 and a heating resistor 104. However, the dashed line defining the initiating circuit 100 in FIG. 1 includes a movable plate 108 that separates the heating resistor 104 from the ink 114, the initiation circuit 100 in one embodiment of the invention does not include a movable plate 108 and / or ink 114. In addition, although the dashed line delimiting the initiation circuit 100 in FIG. 1 does not include a voltage enabling circuit 116 that converts the initial logic signal at 120 to a higher voltage, the initiating circuit 100 of the invention includes the voltage enabling circuit 116.

[0005] Switch 102 in one embodiment of the invention is a metal-oxide semiconductor (MOS) transistor, such as a transverse MOS (LDMOS) transistor. The switch 102 has a first terminal 122 connected to a voltage source 106, and a second terminal 124 connected to a heating resistor 104. The switch 102 is hereinafter referred to as the high voltage side switch because it is connected to voltage source 106, as opposed to, for example, the heating resistor 104, and the initiating circuit 100 is hereinafter referred to as the high voltage side initiating circuit.

[0006] The switch 102 comprises a transistor, such as, for example, an MOS and / or LDMOS transistor, having a drain D at terminal 122, source S at terminal 124 of switch 102, gate - G also designated gate 128, and preferably substrate B also designated substrate 126 in FIG. 1. The drain is therefore connected to voltage source 106 and the source to heating resistor 104. Substrate 126 is preferably connected to the source. According to the invention, the transistor operates in DC mode as will be described. The threshold voltage is determined between the gate and the transistor source.

[0007] The heating resistor 104 is also hereinafter referred to as inkjet thermal resistor. The heating resistor 104 has a first terminal 130 connected to switch 102 and a second terminal 132 connected to ground 110. The board 108 may be a tantalum plate, or another type of plate. The board 108 is also connected to ground 112. The switch 102 controls the activation of the heating resistor 104. When the switch 102 is on, approximately constant current flows through the heater resistor 104, as will be described. The heating resistor 104 heats the ink 114 on the other side of the plate 108, expanding the ink 114 and eventually causing it to fire. When current flows through the heater resistor 104, it is believed that the heater resistor is activated or initialized. The switch 102, as such, controls the activation of the heating resistor 104.

[0008] The switch 102 is turned on when a voltage greater than the threshold voltage of the switch 102 is applied to the gate 128. The voltage enabling circuit 116 controls whether voltage is supplied to the gate 128. In particular, the voltage switching circuit 116 is connected between voltage source 118 providing "VppLogic" voltage and grounding 122. The initiating logic signal is provided to ground 120 when the thermal inkjet printing nozzle specific to the initiating circuit 100 is to fire the ink. The initiating logic signal has a lower voltage than the voltage desired for gate 128 of the switch 102. For example, the initiating logic signal may be five volts, while the VppLogic voltage may be 32 V. As such, the voltage enabling circuit 116 converts the lower voltage of the initial logic signal into the higher voltage of VppLogic.

[0009] Thus, when a high initiating logic signal, such as 5 V, reaches point 120, the output voltage of the voltage enabling circuit 116 is VppLogic, such as 32 V. The switch 102 is closed and causes current flows through the heating resistor 104 and ink 114 is fired. When the low initial logic signal, such as zero, reaches point 120, the output voltage of the voltage enabling circuit 116 is also zero. The switch 102 is open and no current flows through the heater resistor 104. Thus, ink 114 is not fired.

[0010] The voltage source 106 provides a Vpp voltage that is preferably equal to or greater than the VppLogic voltage, but may be, in some cases, less than the VppLogic voltage, as will be described in more detail. The switch 102 operates in DC mode, at least for the reason that, first, the Vpp voltage provided by the voltage source 106 is not less than the VppLogic voltage that is supplied to the gate 128 of the switch 102, by more than the threshold voltage of the switch 102. For example , the threshold voltage of the switch 102 may be 1.2 V. Thus, if the VppLogic voltage is 32 V, it means that the Vpp voltage is not less than 32-1.2 = 30.8 V. Thus, a Vpp voltage not less than the VppLogic voltage by more than the threshold voltage - and in some embodiments, the Vpp voltage actually equal to or greater than the VppLogic voltage ensures that the switch 102 operates in DC mode. Additionally, switch 126's substrate 126 may be connected to a source at terminal 124 of switch 102.

[0011] In the case where the switch 102 operates in constant current mode, the current flowing through the heating resistor 104 when it is activated (i.e. when it initiates), has at least approximately the same value. In other words, a switch 102 operating in constant current mode means that at least approximately constant current flows through the heating resistor 104 at the time of activation. The voltage at terminal 130 of the heating resistor 104 follows the voltage at gate 128 of the switch 102, regardless of the voltage variation Vpp on the drain of the switch 102 so that the voltage at the terminal 130 of the heating resistor 104 is equal to the voltage at gate 128 minus the threshold voltage of switch 102. Threshold voltage switch 102 is the voltage between gate 128 and the source of switch 102 when it is on.

[0012] The voltage at terminal 130 of the heating resistor 104 therefore needs adjustment, due to the switch 102 operating in constant current mode and being in the configuration of the source follower, or in the mode of the source follower, in which the voltage at the source follows the voltage at gate 128. Thus, the mode the source follower in which the switch 102 operates, provides the switch 102 with DC operation in one embodiment. In the case where the ground 110 is a local non-stabilized ground, the terminal 132 of the heater resistor 104 is also unstable. In contrast, if the 110 ground is a stabilized total ground, the terminal 132 of the heater resistor 104 is stabilized to zero. When the heating resistor 104 is not activated and does not initiate, its voltage level is at least substantially equal to the ink voltage level 114 because the board 108, and thus the ink, is connected to the local ground 112. As a result, when the heating resistor 104 malfunctions , this only affects the initiating circuit 100 and the ink jet nozzle specific to the initiating circuit 100, and does not affect any of the adjacent initiating circuits or nozzles.

[0013] FIG. 2 shows why the Vpp voltage may be less than the VppLogic voltage, according to an embodiment of the invention, so that operation in DC mode of the high-voltage side switch circuit is beneficial. In particular, FIG. 2 shows a series of initiation circuits 202A, 202B, ..., 202N, collectively referred to as initiating circuits 202. Each of the initiating circuits 202 may be an example of an initiating circuit 100 in FIG. 1. Initiation circuits 202, as such, have high-voltage side switches 204A, 204B, ..., 204N, collectively referred to as switches 204, heating resistors 206A, 206B, ..., 206N, collectively referred to as heating resistors 206. Initialization circuits 202 can be 88 or more.

[0014] The VppLogic voltage is largely constant at 32 V. However, the Vpp voltage is lower than VppLogic because of the parasitic resistance 208. The parasitic resistance 208 increases with the number of initiating circuits 202 currently activated. Thus, the parasitic resistance 208 increases with the number of switches 208 that are currently closed, so the parasitic resistance 208 increases depending on the number of heating resistors 206 that are currently activated. Therefore, the voltage Vpp provided by the voltage source 106 in FIG. 1, is lowered depending on the number of initiating circuits 202 that are currently simultaneously activated.

[0015] In this situation, having switches 204 operating in constant current mode, it is ensured that the voltage on the heating resistors 206, and thus the current flowing through the heating resistors 206, is stabilized, regardless of the voltage drop Vpp. However, it should be noted that the Vpp voltage should not drop more than the threshold voltage below the VppLogic voltage used to turn on switches 204 to ensure that switches 204 remain in constant current mode as described. Thus, the operation of the switches 204 in DC mode stabilizes the voltage on the heating resistors 206 and the current flowing through them, which is advantageous.

[0016] It should be noted in particular that having a Vpp voltage greater than the VppLogic voltage by more than the threshold voltage (as opposed to the Vpp voltage being only not less than the VppLogic voltage by more than the threshold voltage), the effect of parasitic resistance on the initiating circuits is effectively minimized 202. In addition, when designing the initiating circuits 202, the parasitic resistance may be concentrated on or to the parasitic resistance 208 shown in FIG. 2. Other parasitic resistances, such as those on or near ground 110, not shown in FIG. 2, are minimized by comparison when designing the initiating circuits 202.

[0017] FIG. 3 shows a graph 300 that shows the DC characteristic of the switch 102 of FIG. 1 when operating on the high-voltage side, in constant current mode, according to an embodiment of the invention. The y axis 302 determines the voltage at the voltage source V of the switch 102, relative to the voltage VppLogic at the gate 128 of the switch 102. Thus, the y axis 302 shows how much the voltage of the source falls below VppLogic. The x-axis 304 determines the voltage Vpp on the drain of switch 102 relative to the voltage VppLogic. Thus, the x-axis 304 shows how much Vpp voltage falls below VppLogic, simulating parasitic resistance 208 in FIG. 2 as described which increases with the number of initiating circuits 202 activated. For example, in FIG. 3 VppLogic voltage is maintained at 29 V.

[0018] Therefore, as shown at point 306 of diagram 300, the source voltage drops by only 91.2 mV, or by 0.343%, with a drop of 1.2 V by Vpp. However, if the whole decrease by

1.2 Vpp voltage would be noticeable at terminal 130 of resistor 104, then the drop would be greater, by 4.5%. Thus, the operation of the switch 102 in DC mode is advantageous because it provides stabilization for such voltage at the source of the switch 102, and therefore at the terminal 130 of the heating resistor 104.

[0019] As shown in graph 300, when the Vpp voltage drops by more than 1.2 V, the source voltage follows the Vpp voltage almost volt-by-volt. This is the range in which the VppLogic voltage exceeds the Vpp voltage by more than the threshold voltage of the switch 102. Therefore, for effective regulation of the voltage of the voltage source V, the switch 102 should operate in constant current mode so that the Vpp voltage is not less than the VppLogic voltage by more than threshold voltage of switch 102.

[0020] FIG. 4 shows a graph 400 that shows the alternating current (AC) characteristics of the switch 102 in FIG. 1 when operating on the high-voltage side, in a DC mode configuration, according to an embodiment of the invention. The y-axis 402 determines the percentage change in energy supplied to a single heating resistor when the resistor is on or activated for one microsecond. The x 404 axis determines the voltage drop Vpp relative to the voltage VppLogic, which arises as a result of a single heating resistor or initiating circuit activated on the left side of Figure 400, and as a result of a large number of heating resistors or initiating circuits activated on the right side of Figure 400.

[0021] The voltage drop Vpp also arises as a result of the parasitic resistance 208 as described. Thus, switch 102 operates in DC mode, and the maximum voltage drop Vpp relative to VppLogic is equal to the threshold voltage value of switch 102, or 1.2 V, in the example of FIG. 4, which occurs when a large number of heating resistors are activated. For comparison, when only one heating resistor is activated, the voltage drop Vpp relative to the voltage VppLogic is close to zero.

[0022] Line 406 on diagram 400 determines the percentage change in energy supplied to the heating resistor 104 that is activated when switch 102 is operating in constant current mode. At the point where the right side of line 406 meets the baseline determining zero percent, the increase in energy delivered to the heating resistor 104 is 8.2% when only one heating resistor is activated, compared to activating multiple heating resistors. Thus, by comparison, in a low-voltage side switch configuration, the increase in energy supplied to the heating resistor 104 is 18.8% when only one resistor is activated, compared to activating multiple heating resistors. Thus, the DC mode in the high-voltage side switch configuration of the initiating circuit 100 provides better stabilization of the energy supplied to the heating resistor 104 during activation, regardless of the number of firing circuits or heating resistors firing.

[0023] FIG. 5 shows a block diagram of a representative inkjet printing device 500 that may include a high voltage side switch of the initiating circuit in DC mode as described, according to an embodiment of the invention. The inkjet printing device 500 may be, for example, an inkjet printer. The illustrated inkjet printing device 500 includes one or more inkjet printheads 502, and one or more inkjet cartridges 508. Those skilled in the art will find that the inkjet printing device 500 can and will typically contain other components in addition to those described in FIG. 5.

[0024] Inkjet printheads 502 include one or more matrices 504, and a series of inkjet printers 506A, 506B, ..., 506N, hereinafter generally referred to as inkjet printers 506. Dies 504 are semiconductors or other types of substrate on which they produce initiating circuits 202 as described. 506 ink nozzles are suitable for 502 initiating circuits. Thus, each of the initiating circuits 502 controls the firing of ink from the respective nozzles 506. The ink is provided by the ink cartridge 508. The ink cartridge 508 in one embodiment may be integrated with the ink printhead 502 as part of an ink cartridge that is not specifically depicted on FIG. 5.

[0025] FIG. 6 shows a method 600 of using one or more side switches with high voltage initiating circuit in DC mode as described, according to an embodiment of the invention. The required switch-on voltage is supplied to the side switch with a high initiating circuit voltage for the 602 inkjet nozzle. For example, a low voltage initiating logic signal can be provided that is converted to a higher switching voltage that is provided to the high voltage side switch of the initiating circuit. In response, at least approximately constant current flows through the heating resistor of the initiation circuit such that the secondary switching ink is fired from the thermal inkjet printing nozzle specific to the initiating circuit 604.

[0026] Basic process 602 and 604 is preferably shown generally for all initiating circuits of an inkjet printhead. For example, voltage is selectively delivered to each side switch with high voltage additional initiating circuits for additional thermal inkjet print nozzles 606. As a result, for each additional initiating circuit that is activated, at least approximately constant current flows through the heating resistor of the initiating circuit causing the ink to be fired from the correct inkjet nozzle (608).

[0027] FIG. 7 illustrates the basic method of performing 700. First, an initiating circuit is made for the inkjet nozzle on the matrix (702). This includes constructing a high-voltage side switch on the matrix (704) and a low-side side heating resistor (706). The initiation circuit is thus constructed in a high-voltage mode of the side switch with a high voltage initiating circuit switch as described. Additional initiating circuits are later made on the same or different matrices (708).

[0028] Thus, printheads can be made using these matrices (710). In one embodiment, the ink cartridges may be constructed to include those ink print heads (712) that may contain ink supplies. Finally, inkjet printing devices can be constructed to include inkjet printheads and / or inkjet cartridges (714). The inkjet printing device may be an inkjet printer or other type of inkjet printing device.

18791 / EP / 09

EP 1 881 900