In recent years, the LED display industry has been struggling with each other in terms of price. In order to save costs, manufacturers have simplified their production processes and cut corners, leading to a big discount in the effect and life of LED displays.

When the output channel is closed, the parasitic capacitance at the channel will be continuously charged due to the continuous current effect of the parasitic inductance, resulting in high voltage burrs. At this time, a reverse voltage loaded on the lamp bead is formed with the output of the tube, so the blanking voltage of the tube affects the reverse voltage of the lamp bead at the same time. When the voltage at the constant current output channel is fixed, the higher the blanking voltage of the tube, the lower the reverse voltage of the lamp bead. Normally, the nominal reverse voltage of the lamp bead is 5V. In fact, the manufacturer's test shows that the back voltage below 1.4V can greatly reduce the bad points caused by the back voltage. Therefore, the blanking voltage for the back voltage problem of the lamp bead cannot be too low, generally not lower than VCC-2V.  

1. Short circuit caterpillar

When the LED display screen is short circuited, a column of long lights will appear, which is generally called short circuited caterpillar. When the middle LED display lamp beads are short circuited, the LED lamp beads in the same column will form a path as shown in the figure below when scanning to the line. If the pressure difference between VLED and point A is greater than the LED display lamp bead lighting value, a column of constantly bright caterpillars will be formed.

The biggest difference between the short circuit caterpillar and the open circuit caterpillar is that as long as the screen is in the scanning state, the short circuit caterpillar will appear no matter whether the LED lamp beads display images, while the open circuit caterpillar will appear only when the open circuit lamp beads are lit. Generally, by increasing the tube blanking voltage, the differential pressure is less than the forward voltage VF of the LED display, that is, VLED-VH<VF. Generally, the forward voltage of red LED beads is 1.6~2.4V, and that of green and blue LED beads is 2.4~3.4V. It is tested that the red lamp bead can be turned on at 1.4V, so taking the red lamp bead as an example, when VH>VCC － 1.4V, the problem of short circuit caterpillar can be completely solved. When VCC-2V<VH<VCC-1.4V, only one red LED below the short circuit point is weakly lit on the LED electronic display screen.  

2. Open Cross

When the LED electronic display screen has an open circuit in the scanning screen and this point is lit, the channel OUT1 voltage is pulled down to below 0.5V. If the scanning line potential blanking voltage VH is 3.5V, a conductive path will be formed for this column of lamp beads to form an open circuit caterpillar.

When the lamp bead is open circuited, the channel OUT1 voltage is pulled down to below 0.5V or even 0V, and affects the column parasitic capacitance Cr through the parasitic capacitors C1 and C2. When the Cr potential is pulled down, the LED in the same row as the open circuit lamp bead appears to be dimmed.  

Turning down the blanking voltage of the blanking tube can effectively solve the problem of open circuit cross, that is, the blanking voltage VH<1.4V. Some pipes in the industry also use the method of adjustable blanking voltage to reduce the blanking voltage to 1.4V or below to solve the open circuit cross, but this will lead to increased LED back voltage and accelerate the damage of LED lamp beads and short circuit caterpillars.  

3. The lamp bead VF value is too high

The constant lighting of the column caused by the large VF value of the lamp bead is also a problem that puzzles users. Normally, the nominal forward voltage VF of the green light is 2.4~3.4V. Normally, the differential pressure between the anode and cathode of the green light is 1.8V, which can make it light up. However, the high blanking voltage VH of the tube will cause the column to light up constantly.

With the lamp bead forward voltage VF1=3.4V as the column, when scanning to the secondary lamp bead, VOUT and VLED1 are turned on simultaneously,

Channel terminal voltage: VOUT=VLED1 － VF1

Voltage at both ends of other row lamp beads in this column: V △=VH － VOUT=VH － VLED1 ＋ VF1

If V △>1.8V, the column may be always on, that is, VH － VLED1 ＋ VF1>1.8V, where VLED=VCC (the voltage drop of the tube is ignored). Therefore, VH>VCC － 1.6V is not conducive to solving the problem of column being always on due to the large VF value of the lamp bead.  

4. High contrast coupling

In the high contrast coupling of LED electronic display screen, the highlight screen is superimposed on the low light background. The area where the low light screen and the highlight screen go together appears to be color biased and dark. As shown in Figure 9, the dotted line is the superimposed highlight screen. The high contrast coupling phenomenon is caused by the mutual interference between the column channels through the tubes. The high contrast coupling can be improved to a certain extent by designing the clamping voltage, that is, keeping it at a certain level after the discharge, so as to lower the tube blanking voltage. However, this design method will bring problems such as dark short circuit column, low gray and red, and large VF value of lamp beads. To improve the high contrast coupling from the perspective of row driving, the blanking voltage can be lowered, but it will lead to the problem of large lamp bead back voltage and short circuit caterpillar.

5. Tube blanking voltage selection

Therefore, to comprehensively deal with various application problems, LED electronic display screen is a more reasonable choice when the blanking voltage is 3V~3.4V (VCC=5V). It can meet the design requirements of various scanning modules of users, so as to reasonably solve a variety of application problems.

To sum up, the selection of the blanking voltage of LED electronic display screen in the tube faces the challenges of the above six problems, and there are certain challenges respectively. The blanking voltage cannot be too high or too low. Generally, the open circuit cross is detected and eliminated by the constant current drive, because the low blanking voltage will reduce the reliability of the lamp bead in long-term use and the life of the LED display. The following table summarizes the appropriate range of blanking voltage under various conditions.

Author: kewei

This article from:http://www.ledtransparency.com/news.html