Driver IC or power supply: energy-saving LED display design

    Recently, a few energy-saving LED displays have appeared on the market. Do these energy-saving LED displays really save energy? How to design energy-saving LED displays? Let's analyze how energy-saving displays are designed.

    First of all, from the perspective of power supply, if 5V is to be reduced to 4V, then the proportion of Schottky forward voltage drop in the output voltage will definitely increase, which will also reduce the voltage of the switch power supply output. Because the higher the proportion of the rectifier Schottky forward voltage (its proportion X = V voltage drop / V output, the output is reduced from 5V to 4V, and its voltage drop is 0.5V, then its proportion will increase from 0.1 to 0.125, an increase of 25%), the lower the power supply output efficiency, which is not too obvious in the overall energy saving of the LED screen. Therefore, it is obviously impossible to improve the working efficiency of the power supply by adopting this power supply design principle. At the same time, 5V is the nominal voltage, which is quite mature in market application. If the new power supply voltage is enabled, although it reduces efficiency and increases cost, it is difficult to guarantee quality, so it is difficult to achieve such practice.

    The design of power supply is a relatively mature field. Another design idea can be used to realize the power supply of the display screen, such as synchronous rectification technology. Q10 is a power MOSFET. In the positive half cycle of the secondary voltage, Q10 is turned on and Q10 plays a rectifying role; in the negative half cycle of the secondary voltage, Q10 is turned off. The power loss of the synchronous rectification circuit mainly includes the conduction loss of Q10 and the gate drive loss. When the switching frequency is lower than 60KHz, the conduction loss is dominant; when the switching frequency is higher than 60KHz, the gate drive loss is dominant. When designing low-voltage, high-current output AC/DC or DC/DC converters, the use of synchronous rectification technology can significantly improve the power supply efficiency, even up to 95%.

    Secondly, we can carefully study the LED screen driver IC, which controls the output port to be turned on or off. The output port voltage drop is VDS=0.65V, which is determined by the process and materials. To reduce VDS to 0.2V or even 0.1V, the required area must increase. In the structure of the MOS tube, we can see that there is a parasitic capacitor between GS and GD, and the driving of the MOS tube is actually the charging and discharging of the capacitor. This charging and discharging process takes a period of time. If the area increases, the parasitic capacitance on the MOS tube will also increase. In this way, the consequence is that the port response speed of the entire IC decreases, which will be a fatal weakness for an LED screen driver IC.

    Therefore, if we want to start from the IC, reduce the turning voltage, and make the driver IC have enough response speed, the decisive factor is the process, which is difficult to achieve. Some people think that other design principles can be used, but if it is a constant current IC, the internal circuit may be different, but the switch tube at the channel port must exist, so even if other design principles are used, it is difficult to achieve the purpose of voltage reduction.

    From the above description, it is not difficult to find that in fact, the energy saving of LED should mainly start from the power supply, that is, the input of power supply voltage must be reduced in the constant current state of driver IC, and the red, green and blue tubes are powered separately, which can also achieve energy saving, but the cost will increase. It is really difficult to start from IC, that is to say, the energy saving of LED has nothing to do with IC, but it is just an innovation in power supply. We might as well make such an assumption that using a power supply on ordinary driver IC can actually achieve the energy saving effect requirements.