Common Voltage Drop

 Overview
To print dots on thermal media a printhead must be supplied with an appropriate heater voltage (VH). Each driver IC switches a number of heaters to the common ground, causing current to flow through the specific heaters. The printhead has a system of common conductors to carry this current to and from the heaters, as shown in the diagram below. There is a big difference in the amount of current flow between a print line of a few dots and one with the most dots printed. There can be a significant voltage drop in the common conductors when many dots are printing. This means there is less voltage available at individual heaters, so less print energy is provided, compared to the same heater when fired on a line with fewer dots.

 Heater Current Conductors

The power connector is attached to a Flexible Printed Circuit (FPC) which has conductor paths formed from a relatively thick metal layer. The FPC carries the current to the outside edges of the ceramic wafer, where it flows through solder joints onto conductor patterns deposited on the ceramic wafer. The heater current flows through the relatively narrow pattern on either side of the heater line, then down the relatively narrow lateral common bus. Heater elements are shown below as a line of red dots, which are individually connected to the lateral common bus and to the driver IC, shown below as checkerboard rectangles. The path of the current back to ground is distributed among many ground conductors, one from each IC, across the width of the ceramic wafer, so there are no common conductors on the ground returns on the ceramic wafer. The relatively thick and wide conductors on the FPC have relatively low resistances, so there is little common drop associated with the FPC.
 

common bus structure

To minimize the common voltage drop, a thick conductor pattern, shown above in green, is screened onto the wafer. The widths of this common bus pattern vary with the amount of ceramic area available on the various printhead series. Of course the length of the lateral common bus varies with the print width of the printhead. Even with the additional conducting cross section provided by the screened layer, common voltage drop is still an issue.


 Print Image Symptoms

Common voltage drop is only a problem when a print density difference is observed. Most thermal printing is bimodal, meaning that a dot is either black or white, not gray. Bimodal media is designed to have a sharp transition from white to dark, followed by a flat region where additional print energy does not result in increased darkness. This bimodal property which smoothes out the effect of print energy variation due to dot-to-dot resistance variation also smoothes out the effect of print energy variation from common voltage drop. So 100% black printing which maximizes the amount of common drop also minimizes its visible effect. To force the symptom to be visible, use a gray pattern that fires most dots but leaves a regular pattern of white dots, so that size variations of black dots due to energy variation will be visible. Print many lines of this gray pattern, then continue printing but leaving a large white rectangle with gray on either side. If common drop is a problem, the gray adjacent to the white rectangle will be darker than the gray above it before the start of the white rectangle. The difference between the number of dots printing on a line with the white rectangle and on a line with no white rectangle causes a difference in common voltage drop. The use of a gray pattern makes the difference in common voltage drop visible.

If the lateral common bus is long, another symptom of common drop can be an overall lightness of print in the middle of the page. To test for it, simply print a gray pattern and compare the center to the edges. Dots on the edges will not be affected by common drop on the lateral common bus. Dots in the center will have the maximum effect from common drop occurring over the half-width of the lateral common bus.


 Simultaneous Dots Limitation
In the "maximum ratings" section of Kyocera thermal printhead specifications, there is an item titled "Number of printable dots simultaneously". This number is either the total number of heaters on the printhead or some smaller limit. When it is a smaller number, it indicates that common voltage drop will be a visible problem when more than that number of dots is exceeded. However, the only problem is visual. The printhead will not have a shorter life when this limit is exceeded. It is an arbitrary threshold for an effect that varies continuously with the number of dots. Grey patterns may reveal the problem with fewer dots. Bar codes or fonts with thick stems may print without difference if the limit is exceeded.

 Compensation for Common Drop
A printer controller can compensate for the visible effects of common drop by counting the number of dots on a print line and extending the pulse width as a function of this count. If the amount of compensation is not too large and the customer requests, Kyocera will change the "Number of printable dots simultaneously" to the total number of heaters on the printhead and add "With compensation" to the specification item. The limit to this approach is that it can cause heaters on the ends of the printhead to exceed their maximum operating conditions, since the end heaters will receive the full compensation, but will not experience the full common drop because their current hardly flows through the lateral common buss. With multi-pulse gray scale printing, compensation can take the form of adding pulses to the part of the image in the center of the print line.

 Eliminating Common Drop

Kyocera has several series of printheads that do not have visible common drop artifacts. The KGT series features a second FPC that reinforces the lateral common bus. This FPC is wrapped around the front edge of the heat sink, so its plane is perpendicular to the plane of the ceramic wafer. It is connected to the common bus at many points along their length and receives current from the main FPC via wide straps running on the reverse side of the heat sink. Other benefits of this design are that the platen roller can be much longer than the print line and the printhead need not be much wider than the media. The KWT series of wide format printheads uses two or three separate ceramic wafers which are carefully assembled to form a single heater line. The KWT series uses a second FPC much like the KGT series.

The KDE series real edge printheads and the KCE series corner edge printheads have the common bus on the reverse side of the ceramic wafer, where it can be as wide as needed to avoid common drop. The common bus receives current through relatively wide FPC straps that run from the FPC through channels cut into the ceramic side of the heat sink, at each end of the printhead.

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