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11-Chang_Junction_Temp_Char_LED

Junction Temperature Characterization of High Power Light Emitting Diodes

Moon-Hwan Chang, Diganta Das, and Michael Pecht*

Center for Advanced Life Cycle Engineering

Department of Mechanical Engineering

University of Maryland

College Park, MD 20742

* Corresponding Author

Abstract

High junction temperature affects the optical performance and reliability of high power LEDs. This paper presents a study of the results of junction temperature measurement tests under various drive currents and ambient temperatures carried out on 3W white InGaN high power LEDs. The goal was to improve the accuracy of junction temperature characterization of the LEDs under test. Test results showed that 200 steps of pulse with 0.061% duty cycle can be used practically to construct reference plot of junction temperature (T j) vs. forward voltage (V f) at specific current levels for 3W high power LEDs so that Joule heating of LEDs can be ignored. High power LEDs mounted on an aluminum metal printed circuit board were tested inside a temperature chamber operated in temperature steps to reach a thermal equilibrium condition between the chamber and the LEDs. The LEDs were powered by 910μs pulsed currents with two different duty ratios (0.091% and 0.061%) between 0mA and 700mA. The diode forward voltages corresponding to the short pulsed currents were monitored to correlate junction temperatures with the forward voltage responses for calibration measurement. In junction temperature measurement, the junction temperatures at different constant currents and chamber temperature steps were estimated by in-situ monitoring of forward voltage responses. As a result, we could make realistic estimates of junction temperature to effectively control conditions for long-term aging tests of high power LEDs.

Index Terms— Light emitting diode, Reliability, Forward voltage, and Junction temperature

1Introduction

High power LEDs are used for a wide variety of applications, such as display backlighting, communications, medical services, signage, and general illumination [1][2][3][4][5]. LEDs offer design flexibility; easy adjustment of line-scale, area-scale, and color dimming; high energy efficiency resulting in low power consumption; possible long lifetimes up to 50,000 hrs; micro-second-level on-off switching enabling ultrahigh speed response time; wide range of color temperature; and no low-temperature startup problems.

Despite exciting innovations driven by technological advances and ecological/energy-saving concerns, the LED industry still faces a challenge in attracting widespread adoption. The concerning issue is the lack of information and confidence regarding reliability. Data sheets of manufacturers provide user information on optical and electrical characteristics of high power LEDs at room temperature (or at 25?C junction temperature). In some cases, thermal resistance of LED assembly on a printed circuit board is also provided. Absolute maximum rating of junction temperature on data sheets is typically in the 125?C to 135?C range. Users are required to design LED based lighting systems to maintain the LEDs within this junction temperature guideline.

Use of LEDs at high junction temperature affects internal efficiency, maximum light output, reliability, peak wavelength and spectral wavelength [6]. For example, a temperature increase is related to forward voltage decrease in the I-V curve due to a decrease in the bandgap energy of the active region of LEDs as well as a decrease in series resistance occurring at high temperatures. The resistance decrease is due to higher acceptor activation occurring at elevated temperatures as well as the resulting higher conductivity of the p-type layer and active layers. When a drive current is applied to an LED, the LED junction produces heat that needs to be dissipated from the junction to keep the temperature within limit. A reliable method of estimating junction temperature is necessary to meet the absolute maximum junction temperature ratings.

Various indirect techniques and models have been attempted to estimate the junction temperature of laser diodes, including micro Raman spectroscopy, threshold voltage, thermal resistance, photothermal reflectance microscopy,

electroluminescence, and photoluminescence [6]. In earlier research conducted, the diode forward voltage has been used to assess the junction temperature of the pn-junction diode of LEDs [7][8][9]. The forward-voltage method has two steps, calibration measurement, followed by actual junction temperature measurement. Calibration measurement by using a pulse current serves as a reference for junction temperature equal to ambient (chamber) temperature and establishes the relation between the forward voltage and the junction temperature. Actual junction temperature measurement uses constant current, adding the Joule heating effect to LEDs and derives the relation between the forward voltage and the junction temperature. In the calibration measurement, a pulsed forward current drives LEDs [6][7][8] to ensure that the junction temperature is equal to the ambient temperature.

In this paper, we will show how the measurement of junction temperature by forward voltage method can be performed and improved using tests and measurement instruments, and how this information can then be used to monitor the operating conditions of high power LEDs. This work will also show that 200 steps of pulse with 0.061% duty ratio can be used to estimate junction temperature for high power LEDs.

2Experiment: Devices and Process

For the test using the forward voltage method, 3W high power InGaN LEDs with maximum absolute junction temperature ratings of 135?C were mounted on an aluminum metal core printed circuit board (MCPCB). An MCPCB (LED board) consists of a base layer (aluminum), a dielectric layer (FR-4 layer), and a circuit layer (Cu trace layer) for higher heat dissipation than the FR-4 board. In each calibration measurement, two LEDs were lighted by a sourcemeter. Thermocouple wires were attached to the anode side of the lead for the tested LEDs. An overview of the test setup is shown in Figure 1. In the tests, the LED board was placed in a temperature chamber (Russells chamber - RB-16-3-3-LN2) and connected to a sourcemeter (Keithley 2400) generating a short pulse (910μs), a DAQ board (NI USB 9215) collecting forward voltage responses, and a data logger (Agilent 34970A) monitoring the lead temperatures. A Labview-controlled computer set was used to program a 910μs short pulse and monitor LED responses from a Keithley 2400 sourcemeter, NI USB 9215 DAQ board, and Agilent 34970A data logger. Four LEDs (LED1, LED7, LED9, and LED15) were selected and tested to reflect the location difference in the MCPCB.

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Figure 1. Schematic of the Experimental Setup

In the calibration measurement, the LED board was placed into the temperature chamber. The chamber was set at different temperature steps (20?C, 35?C, 50?C, 65?C, 80?C, 95?C, 110?C and 125?C). LED lead temperatures and chamber temperatures were monitored and recorded every second. 910μs pulses were applied on the LEDs twice in 3 seconds by using a Labview-controlled Keithley 2400 sourcemeter after confirming that the lead temperature of the LEDs was the same as the chamber temperature (i.e., thermal equilibrium condition). The duty cycle was 0.061% (for 200 peak current levels from 0mA to 700mA) or 0.091% (for a single current level). Voltage responses were stored via a NI 9215 DAQ board with 300k data in 3 seconds. Chamber temperature was then increased to the next step. The steps were repeated until the chamber temperature reached 125?C. In the calibration measurement, a V f(forward voltage) vs. T a(ambient temperature) plot was obtained at the thermal equilibrium condition between the LED junction and the ambient temperature while avoiding the impact of Joule heating into LEDs using 910μs current pulse generation.

In the junction temperature measurement, junction temperatures at each constant current and chamber temperature were estimated by the in-situ monitoring of forward voltage responses and lead temperatures of LEDs every second with a constant test current of 300mA. Chamber temperatures were selected at 20?C, 35?C, 50?C, and 65?C for 300mA. At each stage of the chamber temperature condition, 30 minutes were needed for lighting the LEDs to achieve a stable forward voltage. The junction temperature was then estimated by using the previously obtained ambient temperature (T a ) versus forward voltage (V f ) plot.

3 Experiment Results and Discussion

To reach thermal equilibrium condition between LED junction temperature and ambient temperature, LEDs need to be placed in a controlled temperature environment into temperature for some moments (i.e., dwell time). The dwell time of each temperature is around 20 minutes. The dwell time was determined experimentally, and it turned out that 20 minutes was sufficient to reach the thermal equilibrium between the LED package and ambient temperature. Pulse current was applied when the lead temperature of the LED was equal to the chamber temperature (at thermal equilibrium) conditions. After the junction has come to thermal equilibrium, a pulsed current of 910μs duration with a duty ratio 0.091% was sourced into LEDs and the voltage response was measured, as shown in Figure 2. Forward voltage response from 910μs pulse duration. Three hundred thousand data points were collected in the 3 seconds needed to detect the output signal and average the voltage responses during the pulse duration.

11-Chang_Junction_Temp_Char_LED

Figure 2. Forward voltage response from 910μs pulse duration

To verify pulse duration, a constant current of 700mA (absolute maximum rating from the manufacturer’s data sheet) was applied until it reached the maximum temperature of lead at an ambient temperature of 21.5?C. The ambient temperature was chosen because the change of the lead temperature at 21.5?C can be shown more clearly than changes at higher ambient temperatures. The temperature increased linearly until the time reached 0.28 minutes, as shown in Figure 3. During the initial 0.28 minutes, the lead temperature increased up to 33.8?C. Therefore, temperature elevation during 0.28 minutes was calculated as 11.5?C. The temperature change of the lead at 910μs was evaluated as 0.00062?C. If we assume that the junction temperature increases corresponding to the increase of the lead temperature, a 910μs pulse duration was fairly short to avoid Joule heating from the p-n junction diode of the 3W high power LEDs.

Further experiments were conducted using this pulse rate of 910μs. Two hundred pulses were applied at each chamber temperature with a 0.061% duty cycle by using a single pulse in each step, as shown in Figure 4. I-V curves obtained from 200 steps of single pulse generated by 910us pulse and 0.067% duty cycle. Without storing the Joule heating effect of LEDs, the forward voltages at different ambient temperatures as reference values can be used for estimating the junction temperature of LEDs at any specific current among 200 different current levels from 0mA to 700mA. Figure 5 shows V f (forward voltage) vs. T a (ambient temperature) plots obtained linearly from these multi-steps of 910μs pulse and chamber temperature (20?C, 35?C, 50?C, 65?C, 80?C, 95?C, 110?C, and 125?C). Two hundred steps of the I-V pulse curve also showed that when the ambient temperature (the same as the junction temperature) was higher, the forward voltage response went lower and the slope in I-V curve became steeper.

F o r w a r d v o l t a g e (V )

Time (s)

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Figure 3. Pulse duration verification at 25?C ambient temperature with 700mA constant current

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Figure 4. I-V curves obtained from 200 steps of single pulse generated by 910us pulse and 0.067% duty cycle

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Figure 5. Forward voltage vs. chamber temperature of LED7 and LED15

The values of slopes and intercepts in the duty ratio 0.091% were compared with those in the duty ratio of 0.061%, as shown in Table 1. It turned out that the average values and standard deviations of the slope a 2 and intercept b 2 from 300mA result obtained by 200 steps of 910μs pulse with 0.061% duty are similar to average values and standard deviations of the slope a1 and intercept b1 from a single current level (at 300mA) with 0.091% duty ratio. This result confirmed that 200 steps of pulse with a 0.061% duty ratio can be practically used to construct a reference plot of T j vs. V f at a specific current level between 0mA to 700mA. The minor difference was caused by the variation of chamber temperature (0.4?C) at each step of ambient temperature during the experiments.

T e m p e r a t u r e (°C )Time (min.)

C u r r e n t (A )Voltage (V)

F o r w a r d v o l t a g e (V )

Chamber temperature (°C)

F o r w a r d v o l t a g e (V )

Chamber temperature (°C)

Table 1. Comparison of slope and intercept at 300mA between 0.091% duty cycle test and 0.061% duty cycle test result

T j = a 1 + b 1 × V f

T j = a 2 + b 2 × V f

Duty cycle 0.091% at 300mA Duty cycle 0.061% at 300mA a 1 b 1

a 2

b 2 LED1 1655.9 -506.0 LED1 1661.8 -507.1 LED7 1881.4 -580.9 LED7 1869.0 -576.1 LED9 1535.6 -463.6 LED9 1539.3 -463.9 LED15 1509.1 -458.6 LED15 1515.1 -459.8 a 1 b 1 a

2 b 2 Average 1645.5 -502.

3 Average 1646.3 -501.7 Standard deviation 169.8

56.6 Standard deviation 161.8

54.0

In Figure 6, the voltage responses at constant current 300mA show that the Joule heating effect changes the voltage characteristics of LEDs. A constant current of 300mA is useful to combine the high ambient temperature effect and the Joule heating effect from LEDs. The test results of junction temperature measurement at a constant current of 300mA demonstrated that the junction temperature reached the maximum absolute rating of 135?C at around 65?C chamber temperature and 80?C of lead temperature, as shown in Figure 7.

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Figure 6. Forward voltage response at constant current 300mA

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Figure 7. Junction temperature measurement at constant current 300mA

F o r w a r d v o l t a g e (V )

Chamber temperature (°C)

J u n c t i o n t e m p e r a t u r e (°C )

L e a d t e m p e r a t u r e (°C )Chamber temperature (°C)

4Conclusions

In this study, junction temperatures of 3W high-power LEDs mounted on an aluminum metal core printed circuit board were estimated at various ambient temperatures and drive currents with 910μs pulsed currents and a duty ratio of 0.061%/ 0.091%. A pulse duration of 910μs was able to generate short-pulse current without generating Joule heating effects from the LED die. The test results showed that 200 steps of pulse with 0.061% duty ratio can be used practically to construct a reference plot of junction temperature (T j) vs. forward voltage (V f) at a specific current level between 0mA and 700mA for 3W high power LEDs. From the relationship between junction temperature and the forward voltage, the junction temperature of high power LEDs was directly estimated to control conditions for long-term aging tests and the reliability of high power LEDs.

This approach shows a direct way of estimating junction temperature by considering forward voltage and junction temperature. It doesn’t need to use temperature data of reference points those in the previous thermal resistance approach. Also, once results of multi-current steps for all range of operating currents are obtained, junction temperature at specific current and ambient temperature can be derived without experiments at each single current. The previous approach could not obtain junction temperatures at different current levels as it derived junction temperature at a single current. This result helps LED designers and customers to estimate junction temperature without the uncertainties that exist in current methods.

5Acknowledgments

The authors would like to thank the more than 100 companies and organizations that support research activities at the Center for Advanced Life Cycle Engineering at the University of Maryland annually, specifically the CALCE Prognostics and Health Management Group. Also, the authors would like to thank Shivangi Parikh and Mark Zimmerman for copyediting.

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