AN-1566| Application Note

Techniques for Thermal Analysis of Switching Power Supply Designs

Techniques for Thermal Analysis of Switching Power Supply Designs
To reduce time-to-market and component count, power management ICs with integrated power transistors such as National's new SIMPLE SWITCHER regulators (LM5576, LM25576, and others) are often preferred over controllers with external FETs. However, with the power transistor onboard, it's important to do careful thermal analysis of the power IC to make sure the silicon temperature does not exceed the maximum allowable junction temperature. Integrated circuits are rated up to a maximum `die' temperature. Operation at higher temperatures will put the IC out of specification and possibly destroy it. There are three main ways of thermally analyzing a given design. The following article explains the different approaches, and discusses the precision of each approach.

National Semiconductor Application Note 1566 Frederik Dostal Applications Engineer February 2007

The Analytical Approach
The analytical approach is a good way to get a rough estimate of the die temperature of a given switching regulator. One approach is to calculate the losses the switching regulator IC generates. For step down regulators the following formulas can be used. There are bias losses which are mainly the ground pin current times the input voltage: Pbias = Iq VIN The power conduction losses are the losses of the built in transistors while fully turned on and a rough estimation is: Pcond = duty cycle Rdson IOUT2 The switching losses are the losses that occur during the transition times of the internal transistor before and after the on time and can be estimated by: Pswitch = (IOUT VIN)/2 F (tLH + tHL) Where F is the switching frequency and tLH and tHL are the transition times from low to high or high to low. All the individual losses are sometimes difficult to calculate due to incomplete information regarding parameters such as the exact rise time, exact Rdson during the on time and other parasitics which
are not easily characterized. Sometimes it is easier to take the over all efficiency of a given power converter board and to subtract the losses of the external components such as the external schottky diode, the inductor, current flowing through the external resistive divider, and possibly the capacitors depending on the ESR. Once we know the losses of the switching regulator IC, the thermal analysis can be started. The individual datasheets give the thermal resistance from the junction of the IC to case (or PCB), which is referred to as JC. The units are degrees centigrade per Watt, and knowing the ambient temperature as well as the dissipated power on the die gives the temperature of the die. The resistance value JC has a lot to do with the package the silicon is housed in but it also includes the

size of the die, the die attach material, and bond wire type and number. This is the reason why there is not one JC per package type, and why the junction to resistance has to be thermally measured with each individual newly released IC product. The junction to ambient thermal resistance, JA, depends greatly on the design of the printed circuit board around the IC. Generally, datasheets give information about the PCB and layout situation in which the given thermal resistance is valid. The precision of the analytical approach depends greatly on the complexity of the formulas as well as on the precision of data of components available to the designer. In many cases, it is more precise to use a practical approach with measurements in the lab rather than mathematical models which lack accuracy due to many unknowns.

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FIGURE 1. Typical Efficiency at 5V VOUT vs IOUT and VIN

The Simulation Approach
To simplify thermal predictions, National's WEBENCH online simulation tool includes a module called WebTHERM which offers thermal modeling of many switching regulator ICs, including National's new LM557x and LM2557x SIMPLE SWITCHER regulators. The thermal simulation results are given in a colorful thermal graph where hotspots can easily be detected and the temperature of each point on the board can be found. Heat sinks can be added to improve thermal dissipation. Also, airflow can be adjusted using fans from different directions. Figure 2 shows a screenshot of a thermal simulation result with WebTHERM. This approach is very simple and gives a good idea of how heat dissipates across a board. It also helps to understand where hotspots exist in individual designs.

AN-1566

2007 National Semiconductor Corporation

300046

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