Mini-LED power supply and backlight control system design

0 Introduction

In recent years, the Mini-LED field has attracted much attention from the industry. Mini-LED backlight display technology has industrial advantages that other technologies cannot match. The future development direction of Mini-LED covers large, medium and small-sized LCD display backlights and LED display screens. From the perspective of market applications, Mini-LED applications mainly have two major directions: one is the LED display screen market; the other is the backlight application market. Among them, LED display screens and large-size TVs will become the mainstream products of Mini-LED applications in the future.

Mini-LED backlit TV has become one of the key research directions in the TV industry due to its multiple partitions, excellent picture quality and thin shape. When the same lamp beads can meet the brightness of the whole machine, the dynamic scanning method is used to control the LED backlight source, and the constant current driver IC performs matrix control on the LED partitions, which can greatly reduce the number of backlight constant current driver ICs used, reduce system costs, and greatly improve the competitiveness of the product. At present, with the reduction of Mini-LED lamp bead costs and the improvement of production processes, the mass production of Mini-LED as a TV backlight source has also been greatly improved.

1 Mini-LED system design principles

With the continuous development of display technology, the requirements of LED TV for picture quality are increasing day by day. Mini-LED TV backlight technology has become a hot technology recently, and manufacturers in the industry have invested a lot of resources in research and development. In order to develop a new category of Mini-LED backlight TV products with high cost performance and excellent system, a design scheme of Mini-LED backlight TV backlight drive and power supply system with better system is designed, as shown in Figure 1.

Figure 1 Mini-LED system design schematic

The hardware platform of the Mini-LED display system is mainly composed of the mainboard, power board, MCU control board, T-CON board and light board. Among them, the mains power (usually AC 220 V) supplies power to the power board through the AC socket. After the power board works, it supplies power to the mainboard, T-CON board, MCU control board and light board. It is the power supply device of the Mini-LED system.

The main board provides control signals (SPI communication) and power standby signals for the MCU control board. One part of the MCU control board is the scanning control switch circuit, which is used to realize the scanning control of the light board. After adopting the scanning control method, the number of constant current IC controls can be greatly reduced; the other part is the light board SPI communication, which realizes the regional dimming function of the light board.

2 Key technologies of Mini-LED power supply and backlight control

2.1 Discrete dual power supply system design

At present, the power supply part of the high-voltage power supply system in the industry is easy to design. The system has many lamps. In order to optimize the system, the power supply design adopts a discrete low-voltage and high-current dual-power supply system architecture to reduce the number of lamps. At the same time, the power supply adjusts the output voltage in real time through the dual-path voltage feedback of the module light bar, so that the constant current chip is turned on with low loss, effectively reducing the heat loss of the constant current IC. The power system block diagram is shown in Figure 2.

Figure 2 Block diagram of a high-efficiency discrete dual-power supply system

① PFC circuit

The interleaved PFC scheme is adopted to reduce the electrical stress and heat loss of PFC power devices.

② LED light power supply

Adopt discrete dual power supply half-bridge resonant soft switching technology (LLC), as shown in Figure 2, LLC circuit 1 and LLC circuit 2, combined with secondary high current synchronous rectification circuit 1 and synchronous rectification circuit 1, output Vout1 and Vout2 to power the LED light board. Adopting discrete dual power supply system can improve the efficiency of main power supply and reduce heat loss caused by low voltage and high current. As shown in the circuit in the red box in Figure 2.

③ Mainboard power supply

It uses half-bridge resonant soft switching technology (LLC) to output +12 V to power the mainboard, and at the same time, through the T-CON switching circuit, it powers the screen glass T-CON board; it outputs +20 V to power the mainboard amplifier.

④Standby power supply

The burst mode solution is used to improve light load and standby efficiency. On the one hand, +5 VSB is output to the mainboard as standby power to achieve low standby power consumption, and on the other hand, it supplies power to the constant current control board.

2.2 Dynamic scanning technology of backlight source

1) Principle of dynamic scanning technology

Mini-LED backlit TV has become one of the key research directions of the TV industry due to its multiple partitions, excellent picture quality and thin shape. At present, TVs with local dimming technology generally use static control, and the constant current driver IC performs point-to-point control on the LED partitions. When the number of partitions is large, the static control method uses more backlight constant current ICs, the circuit cost is high, there are many connecting wires and the process is more complicated. When the same lamp beads can meet the brightness of the whole machine, the dynamic scanning method is used to control the LED backlight source, and the constant current driver IC performs matrix control on the LED partitions, which can greatly reduce the number of backlight constant current driver ICs used, reduce system costs, and greatly improve the competitiveness of the product.

Due to the large number of partitions, the traditional static control method is used, the number of constant current control ICs is large, the heat loss is large, and the cost is high. Therefore, the scanning method is used in the TV industry to control the light board. This solution adopts the sweep frequency LED common anode design, and the scanning MOS tube controls the opening and closing of the LED+ in time-sharing, and the MCU control board performs matrix control on the LED partitions. Here, taking the constant current driver IC of Polymer 48CH and the N scanning method design as an example, the principle block diagram of the scanning method is shown in Figure 3.

Figure 3 Mini-LED scanning principle diagram

The system is designed with N-line dynamic scanning mode. The principle block diagram of the scanning mode is shown in Figure 3. The anode power supply VLED of the LED light bar is controlled by N MOS tubes, and the drive of the MOS tube is provided by the MCU controller module port. When the power supply VCC of the MCU controller module and the constant current driver board is stable, the MCU control module initializes the register of the constant current driver board. After the initialization is completed, the MCU control module detects whether there is SPI data input from the mainboard. If so, the data information is distributed to the corresponding constant current driver board. If there is no data input, the MCU control module remains in a waiting state. The data information includes the brightness and dimming information of the LED light bar. The MCU control module detects the GCLK signal as a counting signal to control the opening and closing of the row scan MOS tube in sequence. The opening sequence of the row scan is row scan 1 to row scan N, which is opened and closed continuously. The constant current drive accurately controls each LED light partition by opening and closing the LED-port. The switching of the brightness information of each frame is realized by the Vsync signal. When the Vsync signal is detected to be at a high level, the constant current drive board refreshes the screen brightness information once. The above can realize dynamic scanning control of the LED backlight source, thereby reducing the number of constant current drive boards and reducing system costs.

This solution supports 4-wire SPI communication. The communication starts from the high-to-low transition of CS_N. The data on the serial data input (SDI) is latched on the rising edge of the serial clock (SCLK). The protocol consists of a 4-bit read/write command, a 12-bit memory address (Address), and 16-bit data (Data). The address and data are sent from the highest bit (MSB) first. The timing is shown in Figure 4.

Figure 4 Mini-LED SPI timing diagram

2) MCU control and scan switching technology

This solution uses a low-cost MCU control method to replace the FPGA control solution commonly used in the industry. The SPI frequency can support up to 100 MHz, which meets the system data transmission requirements and reserves a certain amount of expansion space for the future. At the same time, the design and development of the scanning low-latency switching scanning circuit meets the system constant current control scanning switching requirements. The MCU control and scanning switching technology system is shown in Figure 5.

Figure 5 MCU control and scan switching technology system block diagram

3 Conclusion

This article introduces in detail the power supply and backlight control system of the new display technology Mini-LED backlight when applied on TV, and gives the key technical principles, namely scanning control backlight area dimming technology, high-efficiency discrete dual power supply system design and MCU control and switching technology, which meets the system design requirements and is a design scheme for the Mini-LED backlight TV backlight drive and power supply system with a better system. It has important reference and reference significance for the development and promotion of Mini-LED TVs.