EcoFit Lighting Thermal Test Report

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Damian Lynch
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Warren Advanced Manufacturing Institute Kansas State University

1 EcoFit Lighting Thermal Test Report July 2009 Prepared by J. Bloomfield and K. Warren Advanced Manufacturing Institute Kansas State University KTEC Center of Excellence 510 McCall Road Manhattan, KS T F

2 Contents Introduction...3 Testing Equipment...3 Test Procedure...3 Thermocouple Placement...3 Testing Conditions...9 Measured Data...11 Conclusion

3 Introduction The Advanced Manufacturing Institute is a part of Kansas State University College of Engineering and a Kansas Technology Enterprise Corporation Center of Excellence. AMI develops products and processes, analyzes product performance, improves manufacturing efficiency, optimizes equipment design and discovers new technologies for a wide range of industries. EcoFit Lighting approached AMI to develop test methods and procedures to independently test and establish the steady state operating temperatures of various points on an LED streetlight module. The tests were performed in a thermal chamber at Kansas State University, with the streetlight module operating in a controlled temperature environment ranging from 0 C to 35 C in 5 C increments. Testing Equipment The following equipment was used to perform the test as well as process the data collected: a) Thermal Chamber at Kansas State University Institute for Environmental Research b) Omega TXDIN70 Signal Conditioners c) 30 AWG type T Thermocouple Wire d) NI LabVIEW application for Data Acquisition and processing e) NI USB 6251 Multifunction Data Acquisition device f) Microsoft Excel for post processing Test Procedure The following section illustrates how the thermocouples were mounted to the light fixture as well as the conditions of the tests that were performed. Thermocouple Placement The data acquisition device allowed for the use of sixteen analog channels. Sixteen thermocouples were placed on the streetlight module assembly in the following locations: 1) External Locations a) Nine on the LED circuit board (see Figure 1, #1 9); these thermocouples were soldered directly to the printed circuit board (PCB) core immediately adjacent to the mounting points of the LEDs; the resulting data were used to calculate the temperature of the PCB/LED solder point (T sp ) 3

4 b) One inside the acrylic cover immediately above the LEDs (see Figure 1, #10); this thermocouple was used to directly measure the ambient temperature in which the LEDs operate (T a ) c) Two on the external aluminum heat sinks directly exposed to the external environment i) One on the tip of the external fins on the power supply side (see Figure 2, #11) ii) One in the valley of two adjacent external fins on the opposite side. (see Figure 3, #12) 2) Internal Locations a) One on the internal aluminum heat sink (tip of fin) (see Figure 4, #13) b) One on the power supply heat sink directly above the power supply s heat dissipation FETs (see Figure 5, #14) c) One to measure air temperature inside light fixture housing (see Figure 6, #15) 3) One measuring ambient temperature within the thermal chamber; this thermocouple would record the simulated outdoor temperature as the thermal chamber was varied between 0 C and 35 C. To measure LED temperatures the thermocouples mounted to the circuit board were soldered directly to the board using standard 60/40 rosin core solder. The thermocouples mounted to the heat sinks were attached using a fast drying epoxy (JB Weld). The thermocouples measuring ambient conditions within both the lens and fixture housing were attached using masking tape and electrical tape. These thermocouples were taped about an inch from the end of the wire so that they could be bent into the air space. Channel Name Reference Number LED 1 1 LED 2 2 LED 3 3 LED 4 4 LED 5 5 LED 6 6 LED 7 7 LED 8 8 LED 9 9 Internal Lens Air 10 External Heat Sink Tip 11 External Heat Sink Valley 12 Internal Heat Sink 13 Power Supply 14 Internal Housing Air 15 Ambient Temp 16 Table 1 Thermocouple channels 4

5 Figure 1 Location of thermocouples on Circuit Board 5

6 11 Figure 2 Location of Heat Sink thermocouple 12 Figure 3 Location of Heat Sink thermocouple 6

7 13 Figure 4 Location of Internal Heat Sink thermocouple 14 Figure 5 Location of Power Supply thermocouple 7

8 15 Figure 6 Location of Internal Housing Air thermocouple 8

Testing Conditions Figure 7 Test Setup Performing the temperature measurement required placing the EcoFit light fixture within a controlled environment.

The tests required the temperature be varied from 0 C to 35 C in 5 increments. A stand was fabricated at AMI to hold the light fixture in approximately the same orientation as in normal operation.

9 Testing Conditions Figure 7 Test Setup Performing the temperature measurement required placing the EcoFit light fixture within a controlled environment. A thermal chamber at KSU s Institute for Environmental Research (IER) allowed the light fixture to operate in such an environment. The tests required the temperature be varied from 0 C to 35 C in 5 increments. A stand was fabricated at AMI to hold the light fixture in approximately the same orientation as in normal operation. The test setup is pictured in Figure 7. The power cord and internal thermocouples entered through the mounting tube in the back of the fixture, which was then plugged to prevent airflow. To complete the tests, the thermal chamber was initially set to 10 C. The light fixture was placed inside the chamber and allowed to cool to ambient temperature overnight. The following day the light fixture was turned on at the low power setting (350 ma per LED) and allowed sufficient time to stabilize before the power setting was changed to high (525 ma per LED). Using the gathered data from the test, a mathematical equation was determined to give the transient response of the system. Equation 1 below is the general transient temperature response. After performing the first set of tests, the time constant (τ) was determined using Microsoft Excel s built in solver function. Initially it was unknown exactly how long the test should run for but typically, a good 9

10 rule of thumb is for at least five time constants. This is an exponential function approaching an asymptote, T f. This achieves 99.3% of the temperature difference or, (1 e 5 ). Equation 1 T = T ( T T ) e f f i t τ T f = Final Temperature T i = Initial Temperature t = Time increment τ = Time constant To obtain the calculated time constants shown in Table 4 the final temperature and the starting temperature were both required. The Excel Solver was used to minimize the deviation of the calculated curves from the gathered data by changing the previously mentioned parameters. As a first pass, only the time constant was allowed to be varied, giving reasonably good results. However, on the second pass the initial temperature was also allowed to be changed, which gave much better results. The calculated initial temperature was within 0.5 C of the starting temperature of the test. This can be justified because once the light fixture was turned on, the temperature rises very suddenly causing a discontinuity in an otherwise continuous curve. With a time constant derived for each channel we can ensure that all tests run long enough to capture as much data as needed to show the response of the system. With the first temperature setting completed we left the light fixture running overnight and changed the temperature of the chamber to 15 C. The remaining tests were performed in a similar fashion, letting the light fixture and data acquisition system run overnight as the ambient temperature of the chamber was changed to the next 5 increment. 10

11 Measured Data The graph below (Figure 8) shows a sample of the data that was collected. The graph shows data collected for the 15 C test. Some interesting things to note about this graph are as follows: 1. The environmental chamber temperature changed from 10 C to 15 C around 4.75 hr. 2. The large 5 jump, at 14.2 hr, when the light fixture power setting is changed from low to high power. EcoFit Lighting Temperature Testing (15 C) 60 Temperature ( C) LED 1--Avg LED 2--Avg LED 3--Avg LED 4--Avg LED 5--Avg LED 6--Avg LED 7--Avg LED 8--Avg LED 9--Avg Int Lens Air--Avg Ext QSink Tip--Avg Ext QSink Valley--Avg Int Heat Sink--Avg Power Supply--Avg Int Housing Air--Avg Ambient--Avg Time (hr) Figure 8 Showing ambient change from 10 to 15 C and transient temperatures recorded for both low and high power settings. All of the test data has been compiled into Table 2. The table shows the steady state temperatures of each channel for each power setting at each ambient temperature. 11

12 0 C 5 C 10 C 15 C 20 C 25 C 30 C 35 C Ambient Temperature P ower Setting LED 1 L ED 2 LED 3 LED 4 LED 5 LED 6 LED 7 LED 8 LED 9 Int L ens Air E xt Qsink Tip E xt Qsink Valley Int Heat Sink Power Supply Int Housing Air Low 30.3 C 28.7 C 28.6 C 28.5 C 29.8 C 30.7 C 27.9 C 30.6 C 29.2 C 15.5 C 23.3 C 20.6 C 25.1 C 31.2 C 3.8 C 0.4 C High 42.1 C 40.1 C 40.3 C 39.8 C 42.1 C 42.7 C 39.4 C 42.6 C 41.2 C 21.4 C 32.0 C 28.1 C 34.9 C 41.4 C 5.6 C 0.0 C Low 34.3 C 32.7 C 32.7 C 32.5 C 33.8 C 34.7 C 31.9 C 34.6 C 33.2 C 19.6 C 27.4 C 24.7 C 29.2 C 35.3 C 7.8 C 3.5 C High 46.6 C 44.6 C 44.9 C 44.3 C 46.5 C 47.1 C 43.8 C 47.0 C 45.6 C 26.0 C 37.1 C 33.3 C 39.4 C 45.7 C 10.3 C 4.2 C Low 40.5 C 38.8 C 38.7 C 38.4 C 39.7 C 40.6 C 37.9 C 40.5 C 39.2 C 25.4 C 33.8 C 31.1 C 35.4 C 41.0 C 13.9 C 10.4 C High 52.5 C 49.9 C 50.7 C 49.7 C 52.4 C 52.5 C 49.8 C 52.4 C 51.3 C 31.4 C 43.3 C 39.0 C 45.2 C 50.5 C 16.0 C 9.5 C Low 44.9 C 43.1 C 43.1 C 42.8 C 44.0 C 44.9 C 42.1 C 44.8 C 43.4 C 29.8 C 38.1 C 35.5 C 39.7 C 45.2 C 18.5 C 15.0 C High 56.8 C 54.6 C 54.7 C 54.0 C 56.2 C 56.7 C 53.5 C 56.6 C 55.1 C 35.6 C 47.2 C 43.4 C 49.5 C 55.1 C 20.3 C 14.7 C Low 49.3 C 47.5 C 47.3 C 46.9 C 48.2 C 49.0 C 46.3 C 48.9 C 47.5 C 34.1 C 42.4 C 39.7 C 44.0 C 49.3 C 23.3 C 19.3 C High 60.8 C 58.6 C 58.8 C 58.1 C 60.1 C 60.5 C 57.4 C 60.4 C 59.1 C 39.9 C 51.3 C 47.6 C 53.5 C 59.0 C 24.8 C 19.2 C Low 53.5 C 51.5 C 51.5 C 51.0 C 52.3 C 52.9 C 50.3 C 52.8 C 51.5 C 38.4 C 46.6 C 44.0 C 48.2 C 53.2 C 27.8 C 23.9 C High 65.0 C 62.7 C 63.0 C 62.3 C 64.2 C 64.6 C 61.6 C 64.5 C 63.2 C 44.5 C 55.6 C 51.8 C 57.8 C 63.1 C 29.5 C 24.1 C Low 57.6 C 55.6 C 55.6 C 55.2 C 56.2 C 56.9 C 54.4 C 56.9 C 55.6 C 42.7 C 50.8 C 48.2 C 52.3 C 57.2 C 32.1 C 28.5 C High 69.0 C 66.6 C 66.8 C 66.0 C 68.0 C 68.4 C 65.3 C 68.3 C 67.0 C 48.6 C 59.4 C 55.6 C 61.6 C 66.7 C 33.7 C 28.4 C Low 63.0 C 60.9 C 60.7 C 60.3 C 61.5 C 62.1 C 59.6 C 62.0 C 60.8 C 48.1 C 56.0 C 53.5 C 57.5 C 62.2 C 37.9 C 34.4 C High 74.4 C 71.9 C 72.1 C 71.4 C 73.3 C 73.6 C 70.6 C 73.5 C 72.2 C 54.1 C 64.9 C 61.3 C 67.1 C 71.9 C 39.9 C 34.7 C Ambient Table 2 Steady State Temperatures The following table illustrates the temperature differential of each channel with respect to the environmental chamber temperature setting, which represents the outdoor temperature in field operation. On average, the LEDs are approximately 27 above ambient for the low power setting and around 40 above ambient on the high power setting. This table also verifies that the temperature of the light fixture changes according to the change in the environmental chamber temperature as expected. 0 C 5 C 10 C 15 C 20 C 25 C 30 C 35 C Ambient Temperature Power S etting L ED 1 L E D 2 LED 3 L ED 4 L ED 5 L ED 6 LED 7 LED 8 L ED 9 Int L ens Air Ext Qsink Tip Ext Qsink Valley Int Heat Sink Power S upply Int Housing Air Low 30.3 C 28.7 C 28.6 C 28.5 C 29.8 C 30.7 C 27.9 C 30.6 C 29.2 C 15.5 C 23.3 C 20.6 C 25.1 C 31.2 C 3.8 C 0.4 C High 42.1 C 40.1 C 40.3 C 39.8 C 42.1 C 42.7 C 39.4 C 42.6 C 41.2 C 21.4 C 32.0 C 28.1 C 34.9 C 41.4 C 5.6 C 0.0 C Low 29.3 C 27.7 C 27.7 C 27.5 C 28.8 C 29.7 C 26.9 C 29.6 C 28.2 C 14.6 C 22.4 C 19.7 C 24.2 C 30.3 C 2.8 C 1.5 C High 41.6 C 39.6 C 39.9 C 39.3 C 41.5 C 42.1 C 38.8 C 42.0 C 40.6 C 21.0 C 32.1 C 28.3 C 34.4 C 40.7 C 5.3 C 0.8 C Low 30.5 C 28.8 C 28.7 C 28.4 C 29.7 C 30.6 C 27.9 C 30.5 C 29.2 C 15.4 C 23.8 C 21.1 C 25.4 C 31.0 C 3.9 C 0.4 C High 42.5 C 39.9 C 40.7 C 39.7 C 42.4 C 42.5 C 39.8 C 42.4 C 41.3 C 21.4 C 33.3 C 29.0 C 35.2 C 40.5 C 6.0 C 0.5 C Low 29.9 C 28.1 C 28.1 C 27.8 C 29.0 C 29.9 C 27.1 C 29.8 C 28.4 C 14.8 C 23.1 C 20.5 C 24.7 C 30.2 C 3.5 C 0.0 C High 41.8 C 39.6 C 39.7 C 39.0 C 41.2 C 41.7 C 38.5 C 41.6 C 40.1 C 20.6 C 32.2 C 28.4 C 34.5 C 40.1 C 5.3 C 0.3 C Low 29.3 C 27.5 C 27.3 C 26.9 C 28.2 C 29.0 C 26.3 C 28.9 C 27.5 C 14.1 C 22.4 C 19.7 C 24.0 C 29.3 C 3.3 C 0.7 C High 40.8 C 38.6 C 38.8 C 38.1 C 40.1 C 40.5 C 37.4 C 40.4 C 39.1 C 19.9 C 31.3 C 27.6 C 33.5 C 39.0 C 4.8 C 0.8 C Low 28.5 C 26.5 C 26.5 C 26.0 C 27.3 C 27.9 C 25.3 C 27.8 C 26.5 C 13.4 C 21.6 C 19.0 C 23.2 C 28.2 C 2.8 C 1.1 C High 40.0 C 37.7 C 38.0 C 37.3 C 39.2 C 39.6 C 36.6 C 39.5 C 38.2 C 19.5 C 30.6 C 26.8 C 32.8 C 38.1 C 4.5 C 0.9 C Low 27.6 C 25.6 C 25.6 C 25.2 C 26.2 C 26.9 C 24.4 C 26.9 C 25.6 C 12.7 C 20.8 C 18.2 C 22.3 C 27.2 C 2.1 C 1.5 C High 39.0 C 36.6 C 36.8 C 36.0 C 38.0 C 38.4 C 35.3 C 38.3 C 37.0 C 18.6 C 29.4 C 25.6 C 31.6 C 36.7 C 3.7 C 1.6 C Low 28.0 C 25.9 C 25.7 C 25.3 C 26.5 C 27.1 C 24.6 C 27.0 C 25.8 C 13.1 C 21.0 C 18.5 C 22.5 C 27.2 C 2.9 C 0.6 C High 39.4 C 36.9 C 37.1 C 36.4 C 38.3 C 38.6 C 35.6 C 38.5 C 37.2 C 19.1 C 29.9 C 26.3 C 32.1 C 36.9 C 4.9 C 0.3 C Ambient Table 3 Steady State Temperature Differential Knowing the environmental chamber temperature and the operating temperature differential from Table 3 allows for predicting steady state operating temperatures. Table 4 below shows the time constants (τ, equation 1) that were calculated for each test where environmental chamber temperature was held constant. Time constants for 0 C and 35 C are not shown because chamber temperature changed while testing was in progress and equation 1 could not be fit to the transient measurements to accurately represent the time constant for the streetlamp module. With this information it is possible to approximate, using the 5τ rule for 99.3% final temperature, how long it takes to reach the operating temperature differential. 12

13 Ambient Temperature L E D 1 LED 2 L E D 3 LED 4 LED 5 LED 6 LED 7 LED 8 LED 9 Int Lens Air Ext QS ink Tip E xt QSink Valley Int Heat Sink P ower S upply 5 C 40.2 min 39.5 min 40.7 min 41.1 min 39.0 min 40.1 min 39.2 min 39.8 min 40.1 min 46.5 min 41.3 min 37.6 min 40.1 min 41.3 min 82.6 min 10 C 37.0 min 37.4 min 37.8 min 37.1 min 37.3 min 38.1 min 36.9 min 37.1 min 38.6 min 38.5 min 39.0 min 39.1 min 38.2 min 39.6 min 44.8 min 15 C 37.6 min 34.9 min 34.7 min 34.3 min 35.4 min 36.8 min 35.6 min 36.4 min 35.7 min 34.1 min 35.9 min 36.2 min 35.8 min 39.6 min 36.6 min 20 C 39.8 min 40.2 min 40.8 min 40.5 min 39.4 min 40.2 min 39.8 min 40.3 min 40.1 min 39.5 min 41.5 min 40.1 min 40.5 min 42.1 min 54.1 min 25 C 36.8 min 36.5 min 38.2 min 37.3 min 36.9 min 38.2 min 37.0 min 37.7 min 38.2 min 41.1 min 39.5 min 37.1 min 37.1 min 42.8 min 52.3 min 30 C 38.9 min 39.8 min 38.5 min 38.5 min 38.8 min 39.1 min 38.1 min 38.8 min 39.1 min 41.9 min 39.0 min 40.3 min 38.2 min 41.1 min 44.5 min Average 38.4 min 38.0 min 38.4 min 38.1 min 37.8 min 38.8 min 37.8 min 38.3 min 38.6 min 40.3 min 39.4 min 38.4 min 38.3 min 41.1 min 52.5 min Int Housing Air Table 4 Calculated Time Constants Based on the direct thermocouple measurements from solder point thermocouples 1 9 attached directly to the PCB core immediately adjacent to the LEDs (T sp ), the LED junction temperature (T j ) can be directly calculated based on the relationship provided by the LED manufacturer (Equation 2) for the specific model of LEDs tested (Cree XP E). This relationship is defined as follows: Equation 2 T = T + R ( V I ) j sp j sp T j = LED junction temperature T sp = Solder point temperature (measured) f f R j-sp = thermal resistance between the LED and the solder point (9 C/W) V f = Forward voltage across LEDs I f = Forward current supplied to LEDs (350 ma low power; 525mA high power) Because nine LED temperatures were collected during the testing procedure, the highest measured solder point temperature (T sp ) at any of the nine LED test points was used to calculate the junction temperature (T j ). Example: At a thermal chamber temperature of 15 C with the unit operating at low power (350 ma), the junction temperature T j is calculated using Equation 2 as follows: T T T T j j j j = Tsp + R j = 55.2 C sp ( V I ) = 44.9 C + 9 C / W = 44.9 C C f f ( 3.3V 0.350A) 13

14 Since the thermal resistance(r j-sp ), forward voltage(v f ), and forward current (I f ) do not change with outdoor temperature, it follows by Equation 2 that each LED junction temperature is 10.4 C and 15.6 C hotter than the measured solder point temperature at low power and high power respectively. The calculated LED junction temperatures are presented in Table 5. 0 C 5 C 10 C 15 C 20 C 25 C 30 C 35 C Ambient Temperature Power Setting LED 1 LED 2 LED 3 LED 4 Low 40.7 C 39.1 C 39.0 C 38.8 C 40.2 C 41.1 C 38.3 C 41.0 C 39.6 C High 57.7 C 55.7 C 55.9 C 55.4 C 57.7 C 58.3 C 55.0 C 58.2 C 56.8 C Low 44.7 C 43.1 C 43.1 C 42.9 C 44.2 C 45.1 C 42.3 C 45.0 C 43.6 C High 62.2 C 60.1 C 60.4 C 59.9 C 62.1 C 62.7 C 59.4 C 62.6 C 61.2 C Low 50.9 C 49.2 C 49.1 C 48.8 C 50.1 C 51.0 C 48.3 C 50.9 C 49.6 C High 68.1 C 65.5 C 66.3 C 65.3 C 68.0 C 68.1 C 65.4 C 68.0 C 66.9 C Low 55.3 C 53.5 C 53.5 C 53.2 C 54.4 C 55.3 C 52.5 C 55.2 C 53.8 C High 72.4 C 70.2 C 70.3 C 69.6 C 71.8 C 72.3 C 69.1 C 72.2 C 70.7 C Low 59.7 C 57.9 C 57.7 C 57.3 C 58.6 C 59.4 C 56.7 C 59.2 C 57.9 C High 76.4 C 74.2 C 74.4 C 73.7 C 75.7 C 76.1 C 73.0 C 76.0 C 74.7 C Low 63.9 C 61.9 C 61.8 C 61.4 C 62.7 C 63.3 C 60.7 C 63.2 C 61.9 C High 80.6 C 78.3 C 78.6 C 77.9 C 79.8 C 80.2 C 77.1 C 80.1 C 78.8 C Low 68.0 C 66.0 C 66.0 C 65.6 C 66.6 C 67.3 C 64.8 C 67.3 C 66.0 C High 84.6 C 82.2 C 82.4 C 81.6 C 83.6 C 84.0 C 80.9 C 83.9 C 82.6 C Low 73.4 C 71.3 C 71.1 C 70.7 C 71.9 C 72.5 C 70.0 C 72.4 C 71.2 C High 90.0 C 87.5 C 87.7 C 87.0 C 88.9 C 89.2 C 86.2 C 89.1 C 87.8 C Table 5 Calculated LED junction temperatures based on Equation 2 and measurements from Table 2 LED 5 LED 6 LED 7 LED 8 LED 9 14

15 Conclusion The primary purpose of the tests performed for this report was to determine the steady state operating temperatures of the EcoFit streetlight module over a range of outdoor temperature conditions. To complete the tests the light fixture was placed in a thermally controlled chamber and operated at 5 C increments from 0 C to 35 C. The data was then recorded and compiled into the tables showing steady state temperatures and steady state temperature differentials. The measured steady state temperatures were utilized to calculate LED junction temperatures at each tested temperature setting. A secondary experiment was also performed, in tandem with the primary, to determine the time constant of the system at the measured locations. Where chamber temperature was held stable throughout the test the measured data over time was used to calculate time constants for each channel. With the time constant, a reasonable transient approximation of temperature can be calculated using equation 1. A benefit of calculating the time constant is that it will now be possible to approximate how long the light fixture will take to take to come to a steady state temperature for any given outdoor temperature (assuming a steady outdoor temperature and no wind) using the operating temperature differentials observed during this testing. On average, the time required for steady state temperature to be reached from power up in a constant outdoor temperature ranges from 3 to 3 ½ hours. However, a typical scenario might include a falling outdoor temperature as the streetlamp module powers up and climbs to steady operating temperature. This meeting in the middle will shorten the time it takes the streetlamp module to reach steady state. 15

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