Analysis and implementation of energy-saving technology for LED display screens

The energy-saving concept of LED display screens has quietly emerged, becoming the highlight that attracts consumers' attention and the reason for its explosive growth in recent years. Of course, after the crazy growth in recent years, the industry is now in a deadlock and in a dilemma of reshuffle. In this dilemma, many companies will inevitably be seriously injured and even go bankrupt. Of course, many companies will also come out of the haze and achieve more long-term development. This is the law of development of the industry. No company can escape this disaster. How to be reborn in the disaster is a problem that every company is currently concerned about.

In this era of high calls for energy conservation, further energy conservation of LED displays has become the support point pursued by the industry. Many companies have made various improvements in this regard, and have achieved energy conservation to a certain extent. However, there is still a long way to go to achieve greater energy conservation, which requires the joint efforts of the entire industry.

Recently, a few energy-saving LED displays have appeared on the market. By improving the power supply, the energy-saving effect of LED displays has been greatly improved, which has attracted the attention of many consumers and given them high expectations. Many LED display manufacturers are eager to try and are ready to introduce this technology first to gain development opportunities. Based on the current technology, how is the energy-saving effect of energy-saving LED displays achieved?

We use a small LED module to analyze its power consumption! As shown in Figure 2, it is a small LED module with CYT62726 driver chip launched by Changyuntong Optoelectronics . Its power supply voltage is 5V. The power consumption of peripheral devices is not calculated first, because they account for a very small proportion of the entire screen. The power consumed by the entire screen is all on the light. First calculate the light power as Pled =n*U vf *I led (n is the number of channels, Uvf is the voltage drop of the LED light , and Iled is the set current value). The pin voltage drop of CYT62726 driver IC is generally about 0.6V, and the voltage drops of red, green and blue light points are 1.8V, 3.0V, and 3.0V respectively. So each channel only needs 4V (3.0+0.6V) to work normally. It can be conservatively set to 2.8V for the red channel and 3.8V for the blue and green channels. In fact, our power supply voltage is 5V, which is equivalent to adding 1V*Iled power consumption inside the IC . So as above, it can be imagined that as long as the power supply is reduced to 2.8V for the red channel, the blue and green channels can be set to 3.8V. , green 3.8V, blue 3.8V, we can save the 1V*Iled power consumption added to the IC channel, and the LED display can save energy by at least 15% when other components remain unchanged. In addition, the reduction in the heat dissipation requirements of the LED screen can also achieve a certain degree of energy saving. This is already a considerable number for a large screen, and I believe customers will be happy to accept it!

We can further analyze its energy-saving principle!

First, from the perspective of power supply, as shown in Figure 3, it is a traditional switching power supply schematic. If 5V is to be reduced to 4V, the proportion of the output voltage occupied by the rectifier Schottky forward voltage drop will inevitably increase. The lower the output voltage of the switching power supply, the lower the power supply output efficiency will be, because the proportion of the rectifier Schottky forward voltage will be higher (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%), which is not obvious for the overall energy saving effect of the LED screen. Therefore, it is obvious that the use of this power supply design principle cannot achieve the improvement of the power supply working efficiency. At the same time, 5V is the nominal value voltage, which is quite mature in market application. Enabling a new switching power supply voltage will only increase costs while reducing efficiency, and the quality is difficult to guarantee, which is difficult to achieve.

The design of power supply is a relatively mature field. Another design idea can be used to realize the power supply of display screen, such as synchronous rectification technology. The basic principle is shown in Figure 4. 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 driving a high-power synchronous rectifier, when the gate peak drive current IG (PK) is required to be ≥ 1A, a CMOS high-speed power MOSFET driver can also be used. After synchronous rectification replaces Schottky rectification, the proportion of consumption in the output power can be effectively reduced. It is necessary to use synchronous rectification technology.

When choosing an AC/DC switching power supply, you can choose a new half-bridge or full-bridge technology, which can increase the efficiency of the switching power supply to more than 90%. Of course, these technical applications can reduce the voltage to the optimal state when powering the LED display, and the efficiency of the power supply can also reach a high efficiency level. Therefore, using new power supply technology to power the LED display can achieve significant energy saving. The power cost will definitely increase. Secondly, we can carefully study the LED screen driver IC. As shown in Figure 5, the output end is a MOS switch tube (as shown in Figure 6), which controls the output port to be closed or opened. The output port voltage drop is about VDS=0.65V, which is determined by the process and materials. To reduce VDS to 0.2V or even 0.1V, the area required will inevitably increase.

In the structure of MOS tube, we can see that there is parasitic capacitance between GS and GD, and the driving of MOS tube is actually the charging and discharging of capacitance. This charging and discharging process takes some time. If the area increases, the parasitic capacitance on the MOS tube will also increase. As a result, 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 of the channel port must exist, so even if other design principles are used, it is difficult to achieve the purpose of voltage reduction.

In summary, the realization of LED energy saving display screen mainly starts from the power supply. On the existing LED display screen, half-bridge or full-bridge high-efficiency switching power supply is directly used, and the energy saving effect is remarkable with synchronous rectification. When the driver IC is in constant current state, the power supply voltage is reduced as much as possible, and the red, green and blue tubes are powered separately to achieve better energy saving effect. Of course, the application cost of this non-standard voltage power supply and new technology will inevitably increase. From the perspective of the screen driver IC, the energy saving is not obvious, and reducing the driving constant current voltage difference will also bring new problems including cost. Some IC companies promote the driver energy-saving design, which is nothing more than a sales strategy.