Structural design of power supply control part for OLED luminescent material testing

1. Introduction

The general structure of an organic electroluminescent (EL) device, or organic light-emitting diode ( OLED ), is to sandwich a layer of organic electroluminescent medium between a metal cathode and a transparent anode. When a certain voltage is applied between the electrodes, this layer of luminescent medium will emit light. The display made by applying OLED to a flat panel display is called an organic luminescent display, also called an OLED display . Compared with LCD , OLED has a series of advantages such as active luminescence, no viewing angle problem, light weight, small thickness, high brightness , high luminous efficiency , rich luminescent materials, easy to achieve color display, fast response speed, high dynamic picture quality, wide operating temperature range, flexible display, simple process, low cost, strong earthquake resistance, etc., so it is called the ideal display of the future by experts.

Although OLED has made great progress and brought new hope to the field of flat panel display, OLED technology is still in the development stage, and organic light-emitting materials are still the most important limiting factor of OLED. Since the microscopic world of organic electroluminescence is difficult to observe directly, it can only provide a certain basis for analyzing the luminescence mechanism by measuring driving voltage, current, brightness, luminous efficiency and other parameters. The focus of this paper is to use single-chip microcomputer control and power conversion technology, and use a self-designed driving power supply with adjustable voltage, frequency and duty cycle as a test platform for analyzing organic electroluminescent materials. At the same time, different software modules such as real-time adjustment of voltage and frequency are designed to enable the power supply to work under different driving modes, realizing the performance test of cold light sheets (organic electroluminescent media) in different states.

2. Test power supply hardware structure

The test power supply used in this article is an AC pulse power supply, which is divided into two parts based on circuit function: the main circuit and the auxiliary circuit.

The main circuit includes: chopping voltage regulation and full-bridge conversion circuit, which generate AC pulse voltage with adjustable peak voltage, frequency and duty cycle.

The auxiliary circuit consists of the following parts:

(1) Control circuit, which realizes the generation of control signals for the chopper tube and the frequency modulation tube, and has the functions of over-current protection interruption, A/D sampling of the potentiometer setting value and manual reset.

(2) The driving circuit amplifies the pulse signal generated by the control circuit and provides it to each switch tube, while isolating the main circuit from the control circuit.

(3) Buffer circuit, which reduces the power consumption of each switch tube at the switching moment and improves the safety of the switch tube at the switching moment.

(4) Overcurrent protection circuit to prevent excessive transient current from damaging components when the load is short-circuited.

(5) Peak voltage sampling circuit, which provides the display circuit with peak voltage sampling values ​​in the range of 0 to 5 V.

(6) Auxiliary power supply, which provides power for the control circuit, drive circuit and display circuit. The block diagram of the power supply circuit is shown in Figure 1.

3. Hardware structure design of power supply control part of cold light film test

3.1 Introduction to control chip

The control chip of the driving power supply adopts the PIC microcontroller produced by the American Microchip Company . The hardware system design of this series of microcontrollers is simple and the instruction system design is concise. At present, many well-known semiconductor companies have developed a series of microcontrollers that are pin-compatible with the PIC series microcontrollers.

For example, the SX series of SCENIX in the United States, the EM78P series of EMC in Taiwan , and the MDT series of MDT in Taiwan.

3.2 Introduction to control circuit design

PIC819 is used as the control chip, and the crystal oscillator is 20 MHz. The execution time of one instruction is 0.25 μs. Figure 2 shows the chopper voltage regulation control circuit, and Figure 3 shows the frequency modulation control circuit.

4. Software structure design of power supply control part of cold light film test

In order to realize the brightness-voltage and brightness-frequency testing of cold light films, this paper designs different software modules such as real-time adjustment of voltage and frequency to realize the performance testing of cold light films in different states.

4.1 Real-time voltage regulation

The voltage regulation is controlled by the PWM output port of PIC819. The tasks it completes are:

(1) The PIC has its own A/D converter port, and the PWM duty cycle is set by adjusting the potentiometer.

(2) In order to prevent the voltage from suddenly increasing to the set value when the power is turned on, causing excessive instantaneous impact current to damage the cold light film, the program sets a slow start program in the initial state so that the voltage can slowly rise from 0 to the set voltage value after the power is turned on.

(3) When the output current of the main circuit is too large, the overcurrent protection circuit triggers the interrupt control terminal of the PIC, and the interrupt protection program clears the PWM port to 0, making the chopper output voltage 0.

The PWM chopping frequency is 20 kHz, and the output duty cycle is adjusted by the potentiometer. Since the 168 V AC input is stepped down by the main circuit transformer, the voltage after rectification and filtering is 200 V (under load), so the duty cycle is controlled at 0-100%, and the 0-200 V voltage can be chopped and output. That is, the PWM pulse width is from 0 to 50 microseconds. According to the calculation formula of the PIC chip PWM pulse width register, the PWM pulse width register assignment range is 00H to OFAH.

The program design process is shown in Figure 4.

The program design idea is: Initialize and assign the PWM pulse width register to 0. Through A/D conversion, the PWM setting value is collected into the single-chip microcomputer, and converted into the PWM pulse width register and the value to be set is stored in the register. Then compare the value of the register with the value of the PWM pulse width register, and gradually increase the value of the PWM pulse width register to equal the value of the register. Adjust the timing time by 20 ms every time the value of the PWM pulse width register is added by "1". In this way, the slow start program is completed. Then the A/D sampling is cyclically performed, the converted setting value is stored in the register and compared with the value of the PWM pulse width register, and the value of the PWM pulse width register is continuously adjusted to make it the same as the value of the register. In this way, the pulse width of the PWM can be adjusted in real time, that is, the voltage of the chopping output can be adjusted in real time.

4.2 Real-time frequency adjustment

The frequency adjustment is realized by another PICl6F819 chip, and the control circuit is shown in Figure 3. In order to make the waveform stable, the program adopts unstructured design.

The design idea is: combine the switch control states of the two groups of tubes into 4 cyclic states. In each state, pins RA2 and RA4 have only a unique value: 01, 00, 10, 00, and set their state time in a cycle. In each cycle, execute the A/D sampling program. Calculate each state time according to the sampling frequency setting value and the duty cycle setting value. Then insert the above work into each state time to complete. In this way, real-time adjustment and real-time sampling can be achieved, and the output pin state will not be affected. The on-time ton = cycle T × duty cycle D, the off-time toff = cycle T minus the on-time ton program flow is shown in Figure 5.

Conclusion

The unique advantages of OLED are in line with the development direction of ideal displays in the future. Based on the characteristics of OLED organic electroluminescent media, this paper uses a self-designed power supply as a test platform and realizes research-based testing and application-based measurement of organic electroluminescent media through different modes of software and hardware combination control, providing a good test platform for further research and application of inorganic electroluminescent materials.