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 user 2005-03-16 at 9:39:00 am Views: 226
  • #10884

    Power Amplifier Drives Multiple Inkjet

    march, 2005

    The desktop inkjet Printer is an engineering marvel. Able to place Millions
    of tiny ink Droplets precisely on a page, it creates a page of crisp type and
    vivid images rivaling silver-halide photography, which has been solidly
    entrenched for more than a century. Not so well known is the role inkjet
    printing is beginning to play in other applications — food, beverage and medical
    packaging as well as large format applications such as billboards and banners.
    It is here that power operational amplifiers, such as the Apex Microtechnology
    MP111FD, are fulfilling a crucial role in the design of inkjet printers


    The piezoelectric technique of inkjet printing employs a crystal that flexes
    whenever a voltage pulse is applied to the piezo transducer, thereby forcing a
    droplet of ink out of the nozzle.illustrates this sequence.

    Each time a voltage pulse is applied to the piezoelectric material, it
    deforms, forcing a tiny droplet 50 to 60 microns in diameter onto the surface to
    be printed. When the voltage returns to zero, the material is restored to its
    original shape, drawing ink into the reservoir and thus preparing it for the
    application of the next drop. This cycle repeats many times per second, each
    time the print head makes a pass across the page.

    Multiplexing the Nozzles

    A representative printing head configuration might employ a single power
    amplifier to drive 128 nozzles. A variation of time-division multiplexing is
    employed. We say variation because it departs somewhat from the conventional
    system of time-division multiplexing, which normally connects to just one node
    at a time. In this case, the power amplifier may be connected to any number of
    ports at any one instant — from 0 to 128.

    Depicted  are 128 MOSFET switches that connect the
    piezoelectric jet nozzles in the printhead to the power amplifier. Consequently,
    at any instant, the voltage on each piezo driver is either 0 V or 48 V,
    depending on whether that nozzle is on or off. As shown in the figure, each
    nozzle is turned on by grounding the piezoelectric transducer via the MOSFET
    switch element corresponding to the nozzle it drives. However, the high-voltage
    piezo driver remains connected at all times to all of the high sides of the 128
    nozzles via a bus.

    The MOSFET switches control the entire ensemble digitally. The switches allow
    the negative return of each piezo transducer to either float — in which case the
    companion nozzle does not dispense a droplet of ink — or to be grounded, so as
    to dispense a droplet of ink. At any instant, the printhead carrying all 128
    nozzles is emitting anywhere from 0 to 128 ink droplets as governed by the
    program instructions delivered to the bank of MOSFET switches. Perhaps numbers
    12, 84 and 128 nozzles are selected, at any instant. If so, they are all driven
    at that moment by the piezo transducer.

    Different Waveforms for Different Inks

    Various waveforms have been devised for printing various kinds of inks. These
    waveshapes are developed empirically and are then stored in a computer so that
    the optimal waveshape for each ink and its specific application can be retrieved
    at a later time.

    The simplest is a trapezoidal waveform, which has a controlled ramp on the up
    slope and a ramp with a slightly different slope on the down side. These slopes
    are well controlled but not necessarily symmetric. The rise on the up slope is
    likely to be faster, whereas on the down slope a longer fall time is necessary.
    This ensures sufficient ink will flow from the ink magazine to the nozzle
    chamber to supply ink for the next droplet to be dispensed. A representative
    waveform is shown

    Common to all designs is a waveform that must be devised that is tailored to
    the specific characteristics of the ink to be dispensed. Principal governing
    factors are the viscosity of the ink, and the shape and size of the droplet to
    be delivered to the printing surface and the mechanism of the printhead

    For some inks, it maybe necessary to double-pulse the piezo transducer to
    overcome oscillations that might hinder satisfactory ink delivery. Depending on
    the ink employed, such double-pulsing can counteract oscillations that would
    otherwise occur when the droplet leaves the jet.

    The power amplifier must be designed to deal with any arbitrary waveshape
    that may be required for a given printing solution. In other words, there is no
    single circuit that will fulfill all piezoelectric ink-driver design objectives.
    That is why an understanding of all the design issues is essential to developing
    driver circuits.

    Determining Design Parameters

    Because piezo elements are almost purely capacitive and therefore dissipate
    virtually no power, disposing of the heat is of paramount importance. Almost all
    the buildup of power is dissipated in the power amplifier module. This must be
    safely transferred as heat from the amplifier module, via thermally efficient
    heatsinking, in such a way that the operating temperatures within the power
    amplifier remain safely below its rated safe operating temperature.

    The Apex MP111FD power operational amplifier is particularly suited for
    driving piezoelectric transducer arrays. It is a 100-V device with a 500-kHz
    power bandwidth and a 50-A pulse capability. shows a simplified
    drawing of the MP111FD power operational amplifier in an inkjet transducer
    application with the companion passive components identified.

    Next, we examine the essential steps in designing the piezo transducer
    circuit. The first step is choosing power supply voltages. An unbalanced source
    voltage would be best. Assuming a 50-V pulse is to be delivered by the power
    amplifier, then a +62-V and a -12-V source would be appropriate. This will
    assist the charge-discharge cycle because the capacitive load presented by the
    array of tranducers must be driven back to 0 V during each pulse cycle.

    As the capacitor approaches 0 V, the -12-V potential will ensure that the
    output transistor of the power amplifier will still have sufficient potential to
    drive the capacitor back to 0 V. This minimizes the power dissipation in the
    amplifier and also contributes to signal fidelity.

    The next step is selecting passive component values. Assuming a gain of 10,
    the value of RI would be 100 Ω, RF
    is 1 kΩ and capacitor CC is 33 pF to 47 pF.

    Once those values are selected, the designer must address stability issues.
    Because the load is highly capacitive, a resistance RS may
    need to be placed in series with C1 to improve the stability of the amplifier
    circuit. You will have to experiment, but typical values for
    RS will range from 0.1 Ω to 0.5 Ω, depending on the
    capacitance of the printhead. Without RS, there may be
    considerable overshoot in the output waveform, which could affect the droplet
    shape and the overall performance of the system.

    The next task is bypassing the power supply terminals. Because the slew rate
    of the power amplifier is quite high, there is a tendency for the power supply
    to droop if no precautions are taken. It is recommended that two 1-µF ceramic
    capacitors of the leadless surface-mount type, CBP, be
    connected directly to the +VS and
    pins of the power amplifier.

    After bypass caps are added, it is necessary to enhance dynamic response. To
    do so, connect a series network comprising a 10-pFSH and a 1-kΩ resistor
    RSH in shunt with resistor RF, as
    shown in This will improve the dynamic response of the power
    amplifier. Adjust these values slightly to optimize the response.

    capacitor C

    Another step is minimizing distortion. As the amplifier slews, it forces
    energy back into the signal source. To minimize distortion that might otherwise
    occur, it is essential the signal source exhibit a low dynamic impedance of 1 W
    or less.

    Selecting a Heatsink

    It is essential to hold the junction temperatures within the power amplifier
    module below 175°C. Determining the proper heatsink is a three-step procedure.
    First, the average power consumption is determined. Then, the thermal
    resistivity in °C per watt of the heatsink is calculated. Finally, the thermal
    resistivity of the heatsink selected is checked to ensure it provides sufficient
    margin. That is, it will hold the junction temperatures to a value well below

    • Determining power dissipation

      Once the waveform for a particular ink is determined, a simplified circuit
      can be devised to perform a SPICE simulation and thereby determine the power
      that will be dissipated within the amplifier. shows a simplified
      SPICE circuit that represents the MP111FD power operational amplifier circuit.
      In this example, the amplifier is assumed to be driving four inkjet printheads

      Each printhead will be assumed to have 256 nozzles for a total of 1024
      nozzles. The total capacitance of the four heads is 1 µF. However, only every
      third nozzle is driven at any instant. Therefore, the maximum capacitance in
      this analysis is reduced to 0.33 µF. This is the load capacitance identified as

      A piece-wise linear waveform is developed by V8 that duplicates the selected
      waveform, as illustrated in  The results of the SPICE simulation

      are depicted by the three graphs in.

      Plot 1 depicts the amplified output voltage waveform V(1). Plot 2 is the
      current pulse train I(C1) applied to the load capacitor C1, whereas Plot 3 shows
      the average power dissipated in the amplifier versus time:

      Average Power = AVG {[(V1)-V(6)]*[I(V6)]+[(V(1)-V(4)]*[I(V7)]} (Eq. 1)

      Plot 3, as governed by this equation, is the average of the voltage across
      each output transistor multiplied by the current through each transistor at each
      instant in time. The result at the end of the period is the average power that
      the heatsink must dissipate due to the load. It is this average power, 68 W,
      which is of interest in determining the heatsink requirement.

      Because the pulse rate frequency is 30 kHz, its period is 33.33 µs. Notice
      that in Plot 2 the current pulses end after 16 µs. The remainder of the period
      is dead time. Therefore, the time interval for the average is greater than the
      time over which instantaneous energy is being delivered. Therefore, 68 W is the
      average power that must be dissipated by the amplifier over the full period.

    • Determining heatsink requirements

      By referring to the data sheet for the MP111FD, the ac thermal resistance is
      determined to be 0.65°C/W. To calculate the temperature rise of the junctions of
      the output transistors above the case temperature, multiply the thermal
      resistance of the MP111FD by the average power dissipated:

      0.65°C/W * 68 W = 44.2°C (Eq. 2)

      To determine the permissible case temperature, assume that a normal ambient
      temperature within the printer will be 30°C. The maximum operating case
      temperature of the MP111FD is 85°C. Thus, subtract the ambient from the maximum
      operating temperature to determine the permissible case temperature rise:

      85°C – 30°C = 55°C (Eq. 3)

      Therefore, the permissible case temperature rise is 55°C.

      Although the load, C1, dissipates 68 W of power in the amplifier, the
      heatsink will have to dissipate the quiescent power dissipation of the amplifier
      as well as the power delivered by the pulse train.

      The quiescent power dissipation of the MP111FD with the operating conditions
      given is approximately 11 W. This quiescent power is due to the operating power
      supply voltages and the quiescent current in the amplifier. Therefore, the
      actual amount of power that must be dissipated is the sum of the two, or 79 W.
      The thermal resistance of the heatsink required is governed by this

      (X°C/W) * (79 W) = 55°C (Eq. 4)

      Where X is the required heatsink thermal resistance in °C/W; 79 is the total
      amplifier dissipation in watts; and 55 is the permitted temperature rise of the
      amplifier in °C.

      Solving equation 4 for X yields a thermal resistance of 0.696°C/W. Thus, any
      heatsink having a thermal resistance of 0.696°C/W or less will be acceptable in
      this application.

    • Confirming the maximum junction temperature

      Although a maximum junction temperature of 175°C is allowed, for long-term
      reliability, a lower temperature would be better. Check that a heatsink with a
      thermal resistance of 0.696°C/W will hold the junction temperature of the output
      transistors below 175°C.

      As previously mentioned, the 68 W of power dissipation due to the load causes
      a temperature rise of 44.2°C in the output transistors. The total junction
      temperature with the selected heatsink is then the sum of the maximum case
      temperature and the temperature rise in the output transistors:

      85°C + 44.2°C = 129.2°C (Eq. 5)

      Where 85°C is the case temperature with the selected heat sink and 44.2°C is
      the temperature rise of the output transistors due to the load. Because a
      junction temperature of 175°C is the maximum allowed, there will be a margin of
      45.8°C — acceptable for the heatsink in this application.

      If the power module is to be located near the printhead and the ink is
      heated, the operating ambient will be well above room temperature of the
      traditional 25°C. In this case, liquid cooling or forced air may be

      Note that for illustration purposes, the quiescent current in the output
      stage and some other fine details have been neglected. However, Apex has an
      online power design spreadsheet that can easily help you with all the details of
      arriving at a heatsink thermal resistance for your particular application. Log
      on to WWW. APEXMICROTECH.COM And look for
      “Circuit Design Software” under the “Support” icon.

      As we have shown, devising a drive circuit for inkjet circuits requires
      tailoring a waveform that will optimize the delivery of the ink droplets
      delivered by a particular printhead. Then, by following the sequence of steps
      described, the designer will be able to configure a driver circuit that will
      provide the necessary current pulse train and preserve the fidelity of the
      waveform delivered to the printhead, while addressing the resulting thermal