Because the end-use environment often is not well-known during the IC evaluation stage, and the environment can vary greatly from application to application, thermal derating graphs are included in data sheets for pulse-wide-modulated (PWM) controller ICs with internal MOSFETs. The derating curves show how much power can be drawn from the chip under given airflow and ambient temperature. To ensure the application does not overload the PWM controller, a thermal derating box can be created on an evaluation board to do thermal derating. This physical box with a fan inside allows uniform airflow inside the box as well as calibration of the airflow. This, in turn, provides a practical way of evaluating the thermal derating for the PWM chip on an evaluation board. Test results will show a close match to actual operating conditions.
Without some type of standardized control of the environment, test results from different vendors and different packages will be inconsistent. One approach would be to actually measure thermal performance of the module under different ambient temperature and airflow conditions and publish the results so that designers can choose correct thermal derating based on their applications. Thermal performance can be estimated for a given size of module, including heatsink, and for a given airflow. The associated physics principles limit the maximum dissipated power. This can be used as a basis for choosing the appropriate IC for the end application.
To perform the actual testing, create a standard thermal derating box (Fig. 1). Adjust the box dimensions so that the largest evaluation board can fit inside the box with sufficient clearance around the box. The linear dimensions of the box should be at least 1 ft × 1 ft. Set the height of the box at 3 in. to accommodate the fan inside the box. Two or more fans can be used to provide uniform airflow inside the box. The material used for the box should have low thermal conductivity, such as polycarbonate, polypropylene or glass epoxy sheet. To ensure the box is rigid, its thickness should be at least 3.2 mm.
Place the box horizontally inside a thermal chamber. The box can be calibrated by measuring airflow at the front of the box using an average airflow meter (Fig. 2). Airflow measurement is done at the left, center and right side to ensure airflow is uniform inside the box. When the module is placed inside the box, make sure that fan is at least 2 in. away from it. If necessary, adjust airflow by using a voltage-controlled variable speed fan.
Thermal performance of the PWM ICs starts with using a standard evaluation board. This board can be mounted on a bigger board so that it can be placed conveniently inside the box. The bigger board should be elevated at least 1 in. from the bottom of the box to ensure airflow is uniform under the board.
Once the box is inside the oven, determine if oven airflow will affect the airflow inside the derating box. If it will, then place a larger box over the test enclosure to maintain uniform airflow inside the box. One approach is to mount the derating box so that the box's fan airflow is at a right angle to oven airflow. This will keep interference to a minimum.
To assemble the box, use common fasteners and adhesives — just be sure they can withstand higher temperatures in the oven. Place a thermocouple at the geometric center of the IC. Note that the attachment of the thermocouple to the IC is critical, because it is imperative the thermocouple does not act as a heatsink. The best approach is to use a minimal amount of thermal epoxy to attach the thermocouple wire to the IC.
The meter measuring the thermocouple output should float electrically so that temperature readings are unaffected by any voltage applied to the IC. Thermocouple wire size can be AWG 30 or smaller, and the wires should be routed in such a way that they interfere minimally with the airflow. The same holds true for the power and any other wires coming from the board under test.
The box can be calibrated with an airflow meter with the evaluation board mounted inside the thermal derating box. Tables 1 and 2 show the calibration data. Figs. 3a and 3b show plots of the data, respectively. Almost uniform airflow is measured inside the box. Once the box is ready, mount the evaluation board on the test board and put everything inside the thermal chamber. Run the chamber for some time so that it reaches thermal equilibrium. Next, load and run the evaluation board for some time to stabilize temperature readings. If two readings taken 5 min. apart do not change more than 0.2°C, then assume thermal equilibrium is reached. Thermal derating with no airflow is harder to determine, since natural convection is unstable — especially at higher IC temperatures and higher power levels.
To verify performance of the test box, experiments were performed to measure the thermal performance of two Maxim PWM ICs, the MAX15035 and MAX8686. Test results are shown in Figs. 4a and 4b, respectively. The MAX15035 is 15-A step-down regulator with internal switches, and the MAX8686 is a single/multiphase, step-down, dc-dc converter that can deliver up to 25 A per phase. The MAX15035 evaluation board is a 4-layer board measuring 2.4 in. × 2.4 in. with 2-oz copper, while the MAX8686 board contains six layers that measures 3.5 in. × 3.0 in. with 2-oz copper.
By capturing this real-world data, designers can more accurately determine the necessary thermal derating for their application. This, in turn, makes it easier to choose the right part based on their operating environmental conditions. Fig 5 shows thermal derating of MAX8686 with 3.3-V output. The same set up is used to measure thermal performance at higher output voltage. It is interesting to note that you can draw higher output power with higher output voltage as efficiency is improved for higher output voltages. The MAX8686 data sheet can be checked to see efficiency differences at 1.2-V and 3.3-V outputs.
|Fan supply (V)||LFM - Left||LFM - Center||LFM - Right|
|Airflow measurement with box only||3.3||102||98||91|
|Fan supply (V)||LFM - Left||LFM - Center||LFM - Right|
|Airflow measurement with evaluation board inside box||3.3||91||90||89|