Technology is taught by everyone: Research on the oscillation problem of LED display_led display manufacturer|Huaze Optoelectronics

by:XY Screens     2021-09-07
The LED display effect and life span largely depend on the performance of the driver chip. When LED display manufacturers are importing LED display driver chips, they pay more attention to the display effect, such as whether the low gray display is consistent, whether the gradation transition is uniform, whether there are dark bright spots in the diagonal scan, open and short circuits, etc., these tests The item will be presented to the end customer.   The test of the display effect is the most important step, but it is also the first step.  A deeper understanding of the driver chip can enable us to have a clearer assessment of the performance and life of our LED display. Now, more and more screen factory engineers are beginning to pay attention to the working environment and working methods of the chip itself when importing driver chips. For example, the consistency of the drive current, the consistency of the REXT voltage, the chip VDD spike, the waveform of the drive port, and so on.    The OUT drive port of the drive chip is directly connected to the LED light and is the core part of the drive chip. In view of the many hazards caused by oscillation to the LED display, this article will briefly introduce the port oscillation problem of some driver chips, and analyze its causes and improvement methods.  Test method  The power supply is a 5V40A power supply with a single module. When the module lights up normally, use an oscilloscope probe to connect the ground terminal to the ground wire inlet of the module, and the probe touches the drive port of the drive chip.  Test result  A mainstream driver chip in the market has a port oscillation phenomenon on a module with greater interference (such as P10 static, etc.). 0. SM16016/SM16026 did not oscillate on a board with a large interference. 0. A mainstream IC had an oscillation phenomenon on a board with a large interference. Oscillation harmed the port’s oscillation and brought many harms:    chip with poor display effect The internal constant current feedback loop can achieve the ideal constant current effect under stable working conditions. The port oscillates, and the chip cannot reach a constant current. What needs to be explained is that such a chip that is easy to oscillate will still oscillate when it is low dust, which will seriously affect the low dust effect. For full-color screens with high consistency requirements, the screen factory spent money on constant-current chips, which is even worse than using constant-voltage chips.  The service life of the LED lamp is greatly reduced.  It can be seen from the oscillation waveform that the negative electrode of the LED lamp has been working under a sine wave with a voltage difference of 2V and a frequency of about 20MHz. Large power spikes, large transient currents, and continuous extremely high frequency switching will reduce the life of the LED lamp.  The power supply fluctuates greatly, reducing the power supply life.  If the 5V40A power supply works at full load, that is, 40A current load, then for the power supply, the current change will be the ampere-level current that continues to oscillate around 20MHz. The capacitance at the output end of the DC power supply, under the high-frequency large current fluctuations, the equivalent internal resistance heats up, which reduces the lifespan, which in turn causes a larger module spike. It is not only the power supply that is damaged, but all the components and drive chips on the module.  EMI exceeds the standard    a sine-like wave with a continuous frequency of 20MHz, the current of each drive port varies from 0mA to 40mA, and the power supply is ampere-level current that continues to oscillate around 20MHz. This will result in the superposition of the dBuV value of the entire cabinet at the frequency of about 20MHz, which will eventually cause the EMI to easily exceed the standard at this frequency.  Abnormal noise of the module capacitor  Capacitor noise is often caused by the heating and damage of the capacitor. The same applies to power capacitors, port oscillations will more likely cause capacitor damage and abnormal noise.  Oscillation reasons and preventive measures   Oscillation reasons  Take the constant current source driver chip application program as an example to explain the OUT port oscillation phenomenon.  0. System application diagram (including parasitic parameters)   Inductance L0, L1...LN is the parasitic inductance on the system power trace, and the capacitor C0 is the trace parasitic capacitance.   When the system power supply VIN is powered on, after the OUT port voltage drops more than the LED lamp voltage drop, the OUT port starts to output current, and the current on the parasitic inductance L0~LN changes, so the voltage on the system power trace appears ripples.   In the above figure, the output current of the OUT port is turned on, and a voltage surge is generated on the parasitic inductance L of the port. At the same time, there is a voltage ripple in the power supply VIN, so a relatively large voltage ripple appears at the OUT port of the chip. All the LED lights on the LED display are lit at the same time, that is, all the drive ports on the screen turn on the output current. At this moment, the voltage ripple source of the OUT port: 1) The parasitic inductance of the chip OUT port generates the voltage ripple of the OUT port; 2) The parasitic inductance on each power supply or ground wire causes a voltage ripple on the system power supply VIN, which is transmitted in series to the chip OUT port.   In addition, the practice of increasing the response time of the drive port will cause the problem of poor low-gray display.  Precautions  From the above analysis, it can be seen that the measures to avoid port oscillation are:   1. Stability of the driver chip: Replace the driver chip with better stability to avoid oscillation from the source.  2, module wiring optimization, reduce parasitic inductance: try to use the ground wire, the power wire copper method, but limited by the board space limitation, there is little room for improvement.  3, the module capacitance increases: it will increase the cost.  Conclusion  Because of the poor internal stability of some LED display driver chips on the market, under the influence of the parasitic inductance on the board, the problem of driving port oscillation occurs. This will cause problems such as poor display effect, shortened LED lamp and power supply life, higher EMI, and abnormal noise of capacitors.  Using a high-stability driver chip can avoid port oscillation problems.
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