A HIGH-EFFICIENCY DRIVER CIRCUIT FOR CAPACITIVE LOADS

Abstract: The invention relates to a high-efficiency driver circuit for capacitive loads, which circuit comprises a drive portion (12) connected to at least one end (X) of a capacitive electric load (11) being applied a voltage waveform. Advantageously, this circuit further comprises a switching circuit portion (13) having its output connected to the above one end (X) of the capacitive load (11) in order to supply a fraction of the overall current demanded by the load (11). Briefly, the invention is a circuit arrangement aimed at providing a highly efficient drive for the capacitive load, using a combined linear/switching setup and without debasing the quality of the waveform generated across the capacitive load.

Description: A HIGH-EFFICIENCY DRIVER CIRCUIT FOR CAPACITIVE LOADS DESCRIPTION Technical Field The present invention relates to a high-efficiency driver circuit for capacitive loads, which circuit comprises a drive portion connected to at least one end of a capacitive electric load being applied a voltage waveform. The invention relates, particularly but not exclusively, to a driver circuit for a piezoelectric type of a printer head, this description making reference to this field of application for convenience of illustration only. Back round Art As is well known in this technical field, the electric characteristic of a comprehensive class of transducers, such as those described in US Patent No. 5,895,998, can be likened to a capacitive load. • The piezoelectric heads of printers may also be viewed as, capacitive loads. ■ These printer heads are driven by applying a voltage waveform across their terminals. The waveform typically consists of a succession of linear edges. An example of this is to be found in US Patent No. 4,767,959 granted to Nippondenso Co.. The high degree of accuracy that is called for in applying said voltage waveform to the capacitive load, along with the high frequency of the drive signal, has led to the use of linear-mode driver circuits, involving high power dissipation even though the load itself may be a non- dissipative one. Representative of the structures employed for applying a voltage waveform to a capacitive element is the linear drive shown by way of example in Figure 1. In practice, the driver circuit of Figure 1 comprises an operational amplifier 2 connected to a B- or AB-class output stage 3. An AB- and B-class output stage 3 driving a capacitive load produces power dissipation according to the following relation, as first approximation : PDISS =CLOAD * VA IM * ΔV * f where, CLOAD is the value of the capacitive load, VALIM is the voltage supply to the output stage 3, ΔV is the voltage variation across the load, and f is the rate of reiteration of the waveform. It should be emphasized that the capacitive load is a non- dissipative type, and that all the electric power is dissipated through the transistors that comprise the linear drive stage. The efficiency of this conventional approach is made low, therefore, by the large amount of power that it dissipates. The technical problem that underlies this invention is to provide a driver circuit, particularly for capacitive loads, with appropriate structural and functional features for the load to be driven with high efficiency and without debasing the quality of the waveform generated across the capacitive load. Disclosure of Invention An embodiment of the present invention relates to a high- efficiency driver circuit for capacitive loads, whereby a voltage waveform formed by a succession of linear edges can be applied to a capacitive load using a combined switching and linear drive technique. In other words, the circuit of this invention incorporates a switching portion and a linear portion, these being discrete circuit portions arranged to act in parallel. Accordingly, the technical problem is solved by a driver circuit, as previously indicated, and defined in the enclosed claim 1. The features and advantages of a circuit according to the invention are made apparent by the following description of an embodiment thereof, given by way of non-limitative example with reference to the accompanying drawings. In the drawings: Brief Description of Drawings Figure 1 is a schematic diagram of a capacitive load-driving circuit according to the prior art. Figure 2 is a schematic diagram of a capacitive load-driving circuit according to the invention. Figure 3 is a comparative graph of the waveform of current draw by the load and voltage to the load. Figure 4 shows schematically one embodiment of a part of the Figure 2 circuit. Figure 5 is a flow chart illustrating sequentially one mode of operating a part of the inventive circuit. Figure 6 is another flow chart illustrating sequentially another mode of operating a part of the inventive circuit. Figures 7, 8 and 9 are respective diagrams of the circuit shown in Figure 3 at three different stages of its operation. ' Figure 10 is a comparative plot of voltage and current signals appearing in the circuit of this invention. Figure 11 is a plot of current vs. time relating to time periods when the circuit of this invention is operated in some special styles. Figure 12 shows schematically a modification of the circuit in Figure 4. Figure 13 is a plot of current vs. time for signals appearing in the circuit of Figure 12. Figures 14 and 15 are respective flow charts illustrating schematically a hysteresis type of control for the circuit of this invention. Figure 16 shows schematically an- exemplary application of the circuit of this invention. Modes for Carrying Out the Invention With reference to the drawings, in particular to the example of Figure 2, a driver circuit according to this invention for driving a capacitive electric load 11 is shown generally at 10 in schematic form. The circuit 10 of this invention comprises an operational amplifier 12 having its output fed back to its inverting input and further connected to one end of the load 11 (node X) to deliver a current IAMP. The non-inverting input of the amplifier 12 receives a control signal from an input terminal IN of the circuit 10. This amplifier 12 may be regarded as the essential component of the linear portion in the circuit 10. Advantageously, the circuit 10 of this invention further comprises a switching circuit portion 13 having its output connected to the capacitive load 11 to deliver a current Is - A control block 14 is provided to drive the switching circuit portion 13. Briefly, the invention is a circuit arrangement aimed to provide for the capacitive load to be drive . with high efficiency by a combine linear/ switching setup, at no loss of quality of the waveform generated across the capacitive load 11. Advantageously, the, switching circuit portion 13 functions to. supply, with high efficiency, a major fraction of the overall current draw by the load 11, while the linear circuit portion 12 assures accuracy for the voltage waveform and supplies only the difference in current, thus saving much power. The linear circuit portion 12 is to meet band and slew- rate requirements of the signal to be reproduced on the load. Figure 3 shows, by way of example, the waveform of the current draw by the capacitive load (curve 6) , as against a voltage waveform applied to the load (curve 5) . To fill the demand for current by the load, the switching portion of the circuit 10 is provided with a bridge circuit 15 of transistors Ml, M2, M3 and M4, wherein the transistors are interconnected into a highly efficient full-bridge arrangement. The bridge 15 is powered between a first or supply voltage reference VALIM and a second or ground reference GND. An inductor 16 is connected between two opposed nodes of the bridge 15. Although in the preferred embodiment of the invention being described herein by way of non- limitative example, the bridge 15 comprises MOS power transistors, a bipolar transistor bridge could be used instead. The bridge circuit 15 is coupled to the load 11 through a two- way switch M5. The control terminals of the transistors Ml, M2, M3 and M4 in the bridge circuit 15 are all connected to the control block 14, also having an output connected to the switch M5. The control block 14 acts on the transistors of the bridge 15 to produce a current profile as close as possible to the sought profile. For this reason, the control block 14 needs information about the duration, slew rate of the ramp, and value of a load CLOAD. Shown schematically in the flow chart of Figure 5 are a succession of steps illustrating the operation of the control block 14 during an upward ramp of the voltage VLOAD. The flow chart of Figure 6 likewise shows in schematic form a succession of steps illustrating the operation of the control block 14 during a downward ramp of the voltage VLOAD. The control block 14 comprises basically a logic interface coupled to a current sensor 17, and a digital technology logic network having analog output stages connected to the control terminals of the bridge circuit transistors and the switch M5. Let us see now, in greater detail, the operation of the circuit 10 according to the invention, with reference to the diagrams of Figures 7, 8, 9 and in a situation of an upward ramp at the load 11. The process steps may be divided into three discrete phases, as specified here below. 1. Pre-charging Phase TI. Since the current value required at the start of the ramp is other than zero, the inductor 16 must be pre- charged to the required value a time period TI ahead of the ramp start, and by closing two transistors, Ml and M2, of the bridge 15. During this phase, the other transistors M3, M4 and M5 are open. The current flow is represented by the dot-line curve 18. 2. Ramp Phase T2. Transistors Ml and M2 are opened and switch M5 is closed at the foot of the ramp. Thus, the current is released to the load through an intrinsic diode D4, shown in Figure 7, which is associated with transistor M4, as well as through the switch M5. During this phase, the inductor 16 is partly, or perhaps fully depleted of its initial current, and the amplifier 12 acts to provide the output voltage waveform by means of a current IAM P being equal to the difference between the current ILOAD demanded by the load and that Isw supplied from the bridge 15. The current flow is represented by the dot- line. curve 19. 3. Loop-back Phase T3. Switch M5 is opened at the ramp end, and any current left in the inductor 16 is allowed to loop back through respective intrinsic diodes D3, D4 associated with the transistors M3 and M4, thus returning a major fraction of the residual energy stored in the inductor 16 to the power supply. The current flow is represented by the dot-line curve 20. Figure 10 shows comparative plots of the current L in the inductor 16, the output current Isw from the witching circuit portion 13, and the output current IA M P from the amplifier, in relation to the three phases TI, T2 and T3 just described. As can be evinced from these plots, only a fraction of the load current comes from the linear portion 12, the rest of the current being released from the switching portion 13 in a theoretically non-dissipative manner. It should be noted that, by suitably dimensioning the transistors of the switching circuit portion 13, their dissipation can be , made trivial during the conduction phase. To enhance the efficiency of the system, thereby improving the accuracy of the switching circuit portion 13 in approaching the current profile demanded by the load, during the phase T2 above, a corrective action can be applied to prevent decay of the current released through the inductor 16. This is achieved by keeping the transistor Ml closed for suitably selected time periods, shown at T* in Figure 11. In this way, the value of the current released to the load through the inductor 16 can be sustained, thereby to reduce the amount of current to be sustained by the linear portion 12, more dissipative. The time periods T* can be controlled by either calculating the evolvement of the current in the inductor 16 with time, or more simply by utilizing a feedback variable. As a schematic, example, the diagram of Figure 12 represents ' a modification of the circuit 10 where the output current IAMP from the linear portion 12 is used as the feedback variable. A sensor 17 is provided downstream of the linear portion 12 and upstream of the node X, and is connected to the control block 14 to have the value of the current IAMP assessed. Based on the ' value of the current IAMP, the bridge 15, specifically transistor Ml during the voltage upward ramp or transistor M4 during the downward voltage ramp, is switched over. The feedback variable may be controlled in any of several ways. For example, transistor Ml may be closed at predetermined times, and opened again upon the current IAMP crossing a predetermined threshold. A different form of control provides for the use of hysteresis. For example, as the output current from the linear portion 12 exceeds a suitable threshold I HI G H , transistor Ml (or alternatively, transistor M4 during the downward ramp) is kept closed until said current reaches a lower threshold ILOW at which the transistor Ml (or M4) is opened. In this way, the hysteresis, type of control will be: IHYST = IHIGH - ILOW. The type of control and the corresponding waveforms are shown in Figure 13 for a predetermined time period denoted by T2. The value of the current IHYST will be a compromise choice, a small value involving fast switching of the half-bridge circuit legs, whereas a large value may lead to increased dissipation by the linear portion 12. Shown in Figures 14 and 15 are respective flow charts illustrating in schematic form the hysteresis type of control, as relating to an upward ramp and a downward ramp, respectively. It should be noted that using a single driver circuit 10, as described hereinabove and shown in the drawings, will be adequate only when the time interval between any two adjacent ramps can produce a current loop-back through the diodes at the end of the first ramp and pre- charge the inductor 16 to a required current value before the next ramp begins. If the above condition fails to occur, the switching circuit portion 13 must be duplicated to provide a bridge circuit Bl effective to inject a current during the upward ramps, as shown schematically in Figure 16. Also, represented in Figure 16 is a modified embodiment where diodes Dl l, D12 in bridge Bl and D13, D14 in bridge B2 are substituted for two transistors in each of the bridge circuits Bl and B2. The last-mentioned embodiment can make for much easier control of the current loop-back phase. Thus, the circuit of this invention does solve the technical problem in either of its embodiments and offers a number of advantages, foremost among which is the fact that the capacitive load 11 can be driven with high efficiency without debasing the quality of the waveform generated across the load.

Claims: CLAIMS 1. A high-efficiency driver circuit for capacitive loads, which circuit comprises a drive portion (12) connected to at least one end (X) of a capacitive electric load (11) being applied a voltage waveform, characterized in that it further comprises a switching circuit portion (13) having its output connected to said one end (X) of the capacitive load (11) to supply a fraction of the overall current demanded by the load (11). 2. A circuit according to Claim 1, characterized in that it comprises a control block (14) for driving the switching circuit portion (13). 3. A circuit according to Claim 1, characterized in that said switching circuit portion (13) comprises a transistor bridge (15) being powered between a first or supply voltage reference and a second or ground reference. 4. A circuit according to Claim 1, characterized in that an inductor (16) is connected between two opposed nodes of said bridge circuit (15). 5. A circuit according to Claim 3, characterized in that said bridge circuit (15) is coupled to the load (11) through a two-way switch (M5). 6. A circuit according to Claim 5, characterized in that the control terminals of the transistors in the bridge circuit (15) are all connected to outputs of the control block (14), said control block also having an output connected to the switch (M5). 7. A circuit according to Claim 2, characterized in that it comprises a current sensor (17) connected in downstream of said linear portion (12) and connected to said control block (14). 8. A circuit according to Claim 3, characterized in that said bridge circuit (15) comprises diodes and transistors. 9. A circuit according to Claim 3, characterized in that it comprises an additional bridge circuit (B2) effective to draw current from the load (11) during the downward ramps of said waveform.

Inventors: BATTAGLIN LUCA, DIAZZI CLAUDIO, GALLINA PIETRO, PEDUTO VITTORIO, SABA GIANCARLO, ZINCO GIANCARLO, BATTAGLIN, LUCA, DIAZZI, CLAUDIO, GALLINA, PIETRO, PEDUTO, VITTORIO, SABA, GIANCARLO, ZINCO, GIANCARLO
Applicants: BATTAGLIN LUCA, DIAZZI CLAUDIO, GALLINA PIETRO, PEDUTO VITTORIO, SABA GIANCARLO, ST MICROELECTRONICS SRL, ZINCO GIANCARLO, BATTAGLIN, LUCA, DIAZZI, CLAUDIO, GALLINA, PIETRO, PEDUTO, VITTORIO, SABA, GIANCARLO, STMICROELECTRONICS S.R.L., ZINCO, GIANCARLO
Application year: 2001
Main classification: H03K4
IPC codes: H01L41, H03K4, H03K17