Design of LED junction temperature and light decay monitoring system based on LabVIEW

In this paper, the virtual instrument is applied to the measurement of LED junction temperature and light decay. The LED junction temperature and light decay monitoring system developed by LabVIEW is based on computer, coupled with sensors, data acquisition card and corresponding software. The voltage, operating current, and illuminance changes of the LEDs are monitored to automatically, continuously, and online measure and record data, and the results are displayed graphically and numerically. The system is simple, practical, high-precision, high-sensitivity, safe and reliable, easy to maintain, has good scalability and application prospects, and can be applied to many environments in the laboratory and outdoors.

1 Introduction

White light LED solid light source has the incomparable advantages of traditional light source: energy saving, environmental protection, long life, safety and reliability, etc., becoming the trend of future lighting. However, the PN junction temperature of the LED has an important influence on the performance of the LED, which causes color temperature change, wavelength red shift, forward voltage drop, etc., and affects the non-radiative recombination of electrons and holes, resulting in a decrease in optical radiation power, thus becoming The key to affecting the entry of LEDs into the general lighting field is an urgent problem to be solved. How to measure the junction temperature of LEDs, there have been many reports, such as forward voltage method, pin method, blue and white ratio method, etc. [2]. Currently used is forward voltage method, which is using LED electric transport. The temperature effect determines the junction temperature by measuring the voltage at a constant operating current. The smaller the forward voltage, the higher the junction temperature and the substantially linear relationship. The system is based on the LabVIEW graphical software platform and is developed as a virtual instrument. The purpose is to realize the indirect measurement of the junction temperature by monitoring the forward working voltage in real time during the constant current operation of the LED, and to monitor the change of the LED luminous intensity. The light decay of the LED illuminator is monitored by changes in illuminance.

2, the overall design of the system

2.1 hardware design

The LED junction temperature and light decay monitoring system converts the non-voltage signal into a voltage signal, which is read, processed, displayed, and stored by the computer after being collected by the acquisition card. The hardware design part consists of a drive circuit, an illuminometer probe, a data acquisition card, and a computer. The drive circuit is not within the system design range and is the object to be measured. The schematic diagram of system signal acquisition is shown in Figure 1.

Figure 1, system signal acquisition schematic

The circuit in this schematic diagram uses a capacitor buck LED driver circuit with good constant current. The load is the same batch of 18 low-power LEDs with the same physical parameters. All signals are collected by the acquisition card, connected to the computer through the USB interface, and then the application program completes the control and data recording and analysis of the acquisition card. By using the computer to combine the system software and the acquisition hardware, real-time data acquisition, recording, waveform display, simultaneous acquisition of multiple signals and generation of corresponding data tables can be realized.

2.2 data acquisition card

This system uses the USB-6009 multi-function data acquisition card from National Instruments of the United States, with a maximum acquisition rate of 48kS/s, 8 analog input channels, 14-bit resolution, 12 digital I/O lines, 2 Analog output, and 1 counter. In order to obtain more accurate data acquisition and avoid external interference, the system uses differential method for voltage acquisition. Three acquisition channels are formed by six input interfaces, and three voltage signals are simultaneously acquired. The corresponding channel can be configured by software, such as sampling frequency and voltage acquisition range.

2.3 Working current collection

Since the signal input of the acquisition card is a voltage signal, a resistor is connected in series in the LED circuit, and the current is converted by collecting the voltage drop across the resistor. The 0.1Ω small resistor is used here to ignore the impact on the system. The purpose of monitoring the operating current is to directly observe that the LED lamp is operating in a constant current state, and the change in junction voltage depends only on the change in junction temperature.

2.4LED working voltage acquisition and junction temperature conversion

According to the principle of forward voltage method, the relationship between junction temperature Tj and voltage Vj of LED in this experiment is obtained by other experiments:

Where Vj is the LED junction voltage (mv); Tj is the junction temperature (°C). Select 5 LEDs from any of the 18 LEDs, and measure the voltage across the two ends to obtain the operating voltage of a single LED. The scale conversion of the collected voltage signal to the junction temperature value is realized by the LabVIEW program, and the scale conversion formula is (1).

2.5 illuminance collection

Due to the photometric relationship between illuminance and luminescence intensity [3] E=I/R2, the change of LED illuminance is monitored in the axial distance, that is, the LED intensity change is monitored. The voltage signal of the illuminometer probe is collected by the acquisition card, and the illuminance is obtained by LabVIEW conversion through the relationship between the illuminance and the voltage. Through experiments and curve fitting, the relationship between the illuminance E(lx) and the illuminometer probe output voltage V(mv) is as follows:

3, system software design

This system uses LabVIEW program as the control software. The LabVIEW program is currently widely used in the field of instrument control, automated testing, data analysis and processing. It is a development software based on the graphical programming language G. When programming in this language, basically no program code is written, and instead a flowchart is used instead. The LabVIEW program combines the computer with the capture card, controls the acquisition card to collect voltage analog signals, analyzes and processes the signals, converts the signals into corresponding data, stores them in text in the computer and displays them on the front panel in various forms. .

3.1 block diagram

The LabVIEW program is programmed with a tiled sequential structure and is divided into two frames. The first frame realizes data acquisition, recording and real-time display, as shown in Fig. 2. The second frame realizes illuminance waveform playback and maximum and minimum calculation of each data, as shown in Fig. 3.

Figure 2. Block diagram of the first frame


Figure 3, the second frame of the block diagram

The first frame is programmed by timed loop and conditional structure, and is mainly composed of four parts: data acquisition module, data conversion module, real-time display module and data storage module.

(1) Data collection is completed by the sub-VI collection assistant, and the acquisition card is controlled to provide a data source for the entire program. mainly include:

Initialize the data acquisition card, start data acquisition, pause acquisition, stop acquisition, etc., which initializes the acquisition card to complete the parameter setting.

(2) The data conversion module subVI mainly realizes the conversion of the collected voltage analog signals to the junction temperature and illuminance signals respectively.

(3) Real-time display module The sub-VI is mainly composed of three waveform charts of current, junction temperature and illuminance. The display mode has graphic display and numerical display.

(4) The data storage module subVI creates a data record file in the specified directory, writes the processed data to the file and saves it, and the write mode is set to: the current collected data is automatically written after the last data, and Do not cover. The data is saved as a string and a time-ordered spreadsheet file is generated for subsequent data and process descriptions.

The second frame is mainly the data playback call module subVI, which selects the playback of the illuminance change curve and realizes the calculation of the maximum and minimum values ​​of each collected data. The direct source of playback data is to read the file saved by the previous frame program. When the user needs to analyze and process the data, the data can be recalled offline.

3.2 virtual instrument front panel

As shown in FIG. 4, the front panel mainly consists of three display windows of current, junction temperature and illumination, a data list window, an illumination change playback window and a control bar. During the acquisition process, the real-time monitoring signal can be seen in the display window; after the end of the acquisition, the data list corresponding to the time can be obtained, and the illuminance change curve is displayed in the illuminance change playback window; in the control column, the main is the collection Start and end control, and the sampling period and sampling resistance can be set as needed. During the acquisition process, the acquisition can be paused, and the number of single acquisitions and the total number of acquisitions can be displayed at the same time. When the acquisition is finished, the acquisition is determined first, and then the system is exited. At this time, the illuminance change waveform can be seen.

Figure 4, virtual instrument front panel

4, summary

Based on the LED junction temperature and light decay monitoring system developed by LabVIEW, the measured results of the LED voltage and current are consistent with the multimeter measurement results, and the illuminance system test results are also consistent with the illuminometer measurement results. Compared with other measurement methods, the system has many advantages such as real-time recording, real-time display, real-time observation and so on.

The measured results show that the whole system is simple and practical, high precision, high sensitivity, strong versatility, friendly interface, convenient data storage, stable and reliable performance, and low cost. In the LED junction temperature and illuminance, ie light attenuation monitoring, the system can be operated for a long time, long period, and provides discontinuous measurements. The junction temperature and illuminance data are displayed in real time, and the graph display wave opening can visually see the trend of change. The combination of software and computer meets the needs of data collection within a specific application. The system also has a strong expansion, which can increase other technical indicators for monitoring LED lights, such as optical parameters such as LED power and electrical parameters. Combined with the notebook computer, it is also suitable for outdoor needs in different occasions, and has a good application prospect.