Handling Those LCDs

By William D. Kimmel, P.E.

and Daryl D. Gerke, P.E.

Kimmel Gerke Associates, Ltd.

We find LCDs showing up in all sorts of applications, in addition to the ubiquitous laptop computers and now the desktop monitors and TVs. These LCDs, and smaller versions, have become the de facto human interface for electronics, often replacing indicator lamps and (with the inclusion of touch panels) switches. The LCDs are seeing application in a range of industrial controls, medical monitors and other instrumentation.

The big problem with LCDs is that controlling EMI is difficult – effective shielding tends to block the light as well. So your choice is to pick shielding that passes some light or try to contain EMI in spite of the big hole in the shield. But any kind of opening makes for an antenna and also provides a good discharge point for ESD.

There are no real good answers for this kind of problem, but there are things that can be done. Let’s take a look at the situation and the possible alternatives.

The Problem

Figure 1 illustrates the problem. The LCD panel is driven by common mode currents on the display cable, by field coupling internal to the enclosure, or by bit switching activity within the display itself. This situation would be easily handled by a shield, except that the options for shielding are not very palatable, so we need to look for alternatives. After all, the laptops don’t use shields. How do they get by?

The answer is that the shield goes behind the display, not in front. Then the emissions can only come up the cable or originate in the LCD itself. If the common mode current on the attached cable can be effectively blocked, either by cable ferrites or effective circuit board grounding, the remaining emissions must come from the LCD itself. This is then a matter of minimizing loop areas by mounting the LCD as close to the enclosure shield as possible. This assumes that the LCD comes complete with a full metal back plate (most LCDs do), otherwise you will need to provide a supplemental plate to close the gaps.

In practice, the easiest way to produce the described shield is to ground the display frame to the enclosure around the entire perimeter. Figure 2 shows this being done with an EMI gasket, which is the most effective approach. If you are trying to get by without gaskets, you might try grounding at various points around the perimeter – the better the ground continuity, the better the performance. But you need a short fat ground in each corner as a minimum, even with small displays. Larger displays will need more contacts. Fortunately, most LCDs now come with a full metal black frame which facilitates grounding.

The bad news is that the LCD may have the metal backplate grounded to circuit ground. This works well for minimizing common mode currents generated within the display, but can make overall grounding difficult.

If you are dealing with a laptop or portable device, it may be perfectly acceptable or even desirable to have circuit board ground common with the enclosure. If you are dealing with industrial controls, this may not be palatable. As a minimum you will have to work to avoid ground loops. Grounding the backplate through capacitors at intervals is a possibility but is cumbersome to effect.

Shielding

The above approach is usually enough to reduce the emissions to an acceptable level for commercial and residential emission requirements. More stringent requirements, such as military or avionics applications, may well require a shield over the LCD. This is usually accomplished with the addition of an ITO (Indium-Tin-Oxide) screen bonded to the enclosure all around the perimeter, as shown in Figure 3. ITO thin enough to see through is not a particularly good conductor (about 50 ohms/square). We advise retaining the shield in back, blocking everything but the emissions directly from the LCD itself.

If you need still more shielding, you will need to go to a better screen. Silver coating is significantly better than ITO, and much more expensive. The last resort is a conductive screen – it makes a good shield, but does degrade the visual image, especially noticeable for high resolution images, such as would be found in a computer screen.

Whichever shield you use, you need to make sure the screen shield is grounded to the enclosure around the entire perimeter. Anything less is almost sure to fail.

ESD

Unfortunately, the LCD is quite exposed to electrostatic discharge, especially if it is equipped with a touch screen. The discharge goes not to the screen but typically to the enclosure at the edge of the screen, finding a small nick to arc to. In most cases the effect is not destructive – the discharge usually results in nonsense symbols on the screen, which remain there until refreshed by a data update.

If you are figuring on using an ITO shield or similar, grounded as described above, you should be in good shape for ESD as well. But if you don’t want to shield, you must not compromise on grounding the LCD backplate. This solution is far from being bulletproof, however.

Of course, if your data is continually being refreshed, such as with a TV or computer screen, there is no problem with nonsense characters, as they will be there no longer than one refresh cycle.

You can try to solve the problem in software. The idea is to refresh the screen on a periodic basis – once a second is adequate. If the person discharges to the screen and the contents is scrambled for a second, it is not a problem. LCDs typically require a significant initialization upload and, unfortunately, this will likely be needed each time you refresh the screen. It is not a big issue – you have already written the initialization routine. You just need to have a routine to call on it periodically to reset the display, thus keeping the screen contents the same.

Summary

If you are working with an LCD, you might as well save yourself some grief and figure out how you are going to handle the big leak. It will be there, make no mistake. You can follow some basic rules and minimize the risk.

Regardless of the extent of the emissions problem, start by grounding the LCD to the enclosure around the entire perimeter, making sure that the LCD has a metal backplate. Gasketing is preferred, but multiple ground points may be adequate. Suppressing common mode currents from the cable is important. If your measures are inadequate, you may have to go to an ITO window, or even a screen.

ESD problems may respond well to software suppression techniques.