AN1703 APPLICATION NOTE GUIDELINES FOR USING STS MOSFET SMD PACKAGES R.Gulino 1. ABSTRACT The trend from through-hole packages to low-cost SMD-applications is marked by the improvement of chip technologies.Silicon instead of heatsin is therefore possible in many cases. Many applications today use PCBs assembled with SMD-technologies, the emphasis being on Power ICs in SMD packages mounted on single-sided PCBs laminated on one side. The printed circuit board (PCB) itself becomes the heatsink. In early fabrications a solid heatsink was either screwed or clamped to the power package. It was easy to calculate the thermal resistance from the geometry of the heatsink. In SMD-technology, this calculation is much more difficult because the heat path must be evaluated: chip (junction) - lead frame - case or pin - footprint - PCB materials (basic material, thickness of the laminate) - PCB volume - surroundings. As the layout of the PCB is a main contributor to the result, a new technique must be applied. Surface mount board layout is a critical portion of the total design. The footprint for the semiconductor packages must be the correct size to ensure proper solder connection interface between the board and the package. The power dissipation for a SMD device is a function of the drain pad size, which can vary from the minimum pad size for soldering to a pad size given for maximum power dissipation. The measurements achieved on SMD packages for different drain pad size show that by increasing the area of the drain pad the power dissipation can be increased. Although one can achieve improvements in power dissipation with this method, the tradeoff is to use valuable board area. 2 Next we consider the common ST MOSFET SMD packages (D PAK, DPAK, SOT-223, SO-8, PowerSO- 8, PowerFLAT (5x5) and (6x5), TSSOP8 and PowerSO-10) with their recommended footprints. For each of these packages we will show the power dissipation for the minimum footprint and for a large 2 2 drain pad area (1in or 600mm ) using the max measured R . THJ-PCB We will show the maximum allowable power dissipation versus drain pad area for different T -T values J A as well. Finally we will make a thermal performance comparison for all SMD packages analysed. Information regarding the mechanical dimensions for each SMD package can be found on the related datasheets. 2. THERMAL MEASUREMENTS The most practical method of optimizing thermal performance is to characterize the MOSFET on the PCB where it will be used. The basis of this method is to dissipate a known amount of power in the MOSFET, and to measure the amount of temperature rise this causes in the junction, given the data June 2003 1/22AN1703 - APPLICATION NOTE required to calculate the junction to PCB thermal resistance (C/W). The procedure has two main steps. First is the characterization of the body diode. Second is the temperature rise measurements and calculation of the thermal resistance. As an inherent part of the MOSFET structure, the body diode makes the ideal sensor to measure the junction temperature, since the forward voltage V varies with temperature, approximately -2.2mV/C. F For this the diode s temperature coefficient is needed to get an accurate representation of the junction temperature. The forward voltage is measured with a low level current flowing through it to ensure there is no self heating, which would make the junction temperature measurement inaccurate. The MOSFET being characterized is soldered on to the thermal test PCB and has the gate shorted to the source, to insure the MOSFET cannot turn on. The copper mounting pad reaches the remote connection points through fine traces that do not contribute significant thermal dissipation but serve the purpose of electrical connections. Figure 1 illustrate the schematic of R measurements where the MOSFET s drain and source are TH connected with two power supplies. Figure 1: Schematic of temperature measurements HEHEAATTINGING S SEENSNSINGING PPmosmos D.U.TD.U.T.. HEHEATINGATING CCURRURREENNTT SeSennssee VfVf CuCurrrrenentt The first power supply forces current through the body diode to heat the junction with a fixed power level, when the switch is in theheatin position. The second power supply provides the sensing current for measuring the junction temperature, through the V measurement, when the switch is in the sensin F position. The measure of R is based on the well know relationship between the power being dissipated in THJ-PCB the devices and the relevant arising junction temperature: TT-T J C R == THJ-PCB P P D D This computation is made from pulsed operation to steady state in order to achieve the whole thermal transient response of the device under test. The applied power P is fixed by the equipment in terms of D magnitude and time duration, so it is an input data. 2/22