Power synchronous rectification technology LED display is the key to energy saving and money saving

The energy-saving concept of led display has quietly risen, and it has become the most eye-catching highlight of consumers, and it is also the reason for the rapid growth in recent years. In this era of high energy-saving calls, the further energy-saving of LED display screens has undoubtedly become the support point for this industry to pursue.

Recently, there have been a few energy-saving LED displays on the market. Through the improvement of the power supply, the energy-saving effect of the LED display has been greatly improved, which has attracted the attention of many consumers and given them a high reputation. Looking forward to it, but whether it really saves energy needs to be studied, especially for outdoor LED displays, which generally have a relatively large area and relatively large overall power consumption. It is particularly important to reduce the power used by the display.

The picture shows the working principle of led display

1. On the basis of the current technology, how is the energy-saving effect of the energy-saving LED display realized?

Let’s take a small LED module with CYT62726 as the driver chip as an example to analyze its power consumption. Its power supply voltage is 5V, and the power consumption of peripheral devices is not calculated first, because they occupy a very small proportion in the entire screen, and the power consumed by the entire screen is on the lamp.

First calculate the power of the light point as PLED=n*Uvf*ILED (n is the number of channels, Uvf is the voltage drop of the LED light point, and ILED is the set current value), the pin voltage drop of the CYT62726 driver IC is generally about 0.6V , the voltage drops of the red, green and blue light points are 1.8V, 3.0V, and 3.0V respectively, so that each channel only needs 4V (3.0+0.6V) to work normally. If you are conservative, you can set it to 2.8V for the red light channel. The blue and green channels are 3.8V, but in fact our power supply voltage is 5V, which is equivalent to increasing the power consumption of 1V*ILED inside the IC.

So as above, it can be imagined that as long as the power supply is reduced to red 2.8V, green 3.8V, and blue 3.8V, we can save the 1V*ILED power consumption added to the IC channel. Achieving energy saving of at least 15% for the LED display, coupled with the reduction of the heat dissipation requirements of the LED screen itself, can also achieve a certain degree of energy saving, which is already a considerable number for a large screen.

2. Three problems that need to be solved for energy-saving LED display

1. The working voltage of the LED lamp is very low, usually DC 2-3.4V, then it is necessary to use a switching power supply to convert the high-voltage AC power of the mains, and how to achieve the highest conversion efficiency during the conversion process;

2. The working voltage of ordinary led display modules is usually DC 5V, while the working voltage of LED is 2-3.4V. How to effectively improve the circuit and make the voltage of the driving circuit the lowest, so as to reduce the loss of voltage on the circuit, thereby Reduce unnecessary heat loss to reduce power loss;

3. How to improve the photoelectric conversion efficiency of the LED, and reduce the driving current while ensuring the brightness of the display screen, and finally achieve the purpose of saving power; how to correctly use the luminous brightness of the display screen, so that the luminous brightness can change with the environment to save electricity.

3. Analysis of energy-saving principle of energy-saving LED display

In response to the above problems, we can see that the key to energy-saving LED display energy saving is the design of the power supply.

1. The traditional switching power supply cannot meet the needs of the LED display

Figure 3 is a schematic diagram of a traditional switching power supply. If the 5V is to be reduced to 4V, the proportion of the rectified Schottky forward voltage drop to the output voltage will inevitably increase. The lower the output voltage of the switching power supply, the rectified Schottky forward The higher the voltage proportion (the proportion X=V voltage drop/V output, the output is reduced from 5V to 4V, and the voltage drop is 0.5V, the proportion will increase from 0.1 to 0.125, an increase of 25%), and the output efficiency of the power supply will be higher. The lower it is, the overall energy-saving effect of the LED screen is not obvious.

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 a nominal value voltage, which is quite mature in the market. Using a new switching power supply voltage will only increase costs while reducing efficiency, and it is difficult to guarantee quality and implementation.

2. Power supply synchronous rectification technology can meet the power supply of led display

The design of the power supply is a relatively mature field, and another design idea can be used to realize the power supply of the display screen, such as synchronous rectification technology.

The basic principle of synchronous rectification technology is shown in Figure 4. Q10 is the power MOSFET. In the positive half cycle of the secondary voltage, Q10 is turned on, and Q10 acts as a rectifier; 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 and gate drive loss of Q10. When the switching frequency is lower than 60KHz, the conduction loss dominates; when the switching frequency is higher than 60KHz, the gate drive loss dominates. When driving a relatively high-power synchronous rectifier, a CMOS high-speed power MOSFET driver can also be used when the gate peak driving current IG(PK) is required to be ≥ 1A

After synchronous rectification replaces Schottky rectification, it can effectively reduce the proportion of output power consumption. It is necessary to adopt synchronous rectification technology

3. Half-bridge or full-bridge new technology can improve switching power supply efficienc

When choosing an AC/DC switching power supply, you can choose the new half-bridge or full-bridge technology, which can increase the efficiency of the switching power supply to more than 90%

Of course, the application of these technologies can reduce the voltage to the best state when supplying power to the LED display, and at the same time, the efficiency of the power supply can also reach a high efficiency level. Therefore, using new power supply technology to supply power to the LED display can achieve significant energy-saving effects. There will certainly be some increase in power supply costs as well

Secondly, we can carefully study the led projector screen screen driver IC. As shown in Figure 5, the output port is a MOS switch tube (as shown in Figure 6), which controls the closing or opening of the output port, and the voltage drop of the output port is about VDS=0.65V. This is determined by the process and materials. To reduce VDS to 0.2V or even 0.1V, the required area will inevitably increase

It can be seen in the structure of the MOS tube that there is a parasitic capacitance between GS and GD, and the driving of the MOS tube is actually charging and discharging the capacitor. This charging and discharging process takes a while. If the area increases, the parasitic capacitance on the MOS tube will also increase accordingly. In this way, the result is that the port response speed of the entire IC decreases. This is a problem for an LED screen driver IC. It will be the fatal weakness. Therefore, if you want to start from the IC, reduce the corner voltage, and at the same time make the driver IC have sufficient 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 still necessary to achieve the purpose of voltage drop. unattainable

4. Conclusio

To sum up, the realization of LED energy-saving display mainly starts from the power supply. The existing LED display directly adopts half-bridge or full-bridge high-efficiency switching power supply, coupled with synchronous rectification, the energy-saving effect is remarkable. In the state of constant current for the driver IC, the power supply voltage is reduced as much as possible, and the red, green and blue dies are powered separately to achieve better energy-saving effects. Of course, the application cost of this non-standard voltage power supply and new technology will inevitably rise. From the perspective of the screen driver IC, energy saving is not obvious, and reducing the driving constant current pressure difference will also bring new problems including cost.