Achieving a small power rail solution size is one of the highest priorities for embedded system engineers, especially for those designing industrial and communications equipment such as drones or routers. Compared to models released a few years ago, currently available drones are much lighter and have smaller fuselages, while routers are now more portable and compact with a built-in power adapter. As equipment size shrinks, engineers are looking for ways to shrink the power supply solution. In this technical article, I’ll provide a few tips to help you make your power rail design smaller, while demonstrating how to resolve the resulting thermal performance challenges.

**Shrinking the package**

One obvious way to reduce your solution size is to choose an integrated circuit (IC) in a smaller package. Small-outline (SO)-8 and small-outline transistor (SOT)-23-6 packages are common for 12-V voltage rail DC/DC converters. They are typically very reliable. However, if you work in an industry where every millimeter counts – such as the drone market – you may be looking for an even smaller DC/DC converter. The SOT-563 package is almost 260% smaller than the SOT-23-6, and 700% smaller than the SO-8 package. Figure 1 compares the size of the mechanical outline of all three packages.

**Figure 1: Mechanical outline sizes of three converter packages**

Apart from choosing a smaller package, another approach to reducing your solution size is to reduce the output inductor and capacitor. Equations 1 and 2 calculate the output inductance (L_{OUT}) and output capacitance (C_{OUT})

_{RIPPLE}is the maximum allowed peak-to-peak ripple voltage and f

_{sw}is the switching frequency of the converter. Because L

_{OUT}and C

_{OUT}are both inversely proportional to f

_{sw}, the larger the switching frequency, the smaller the L

_{OUT}and C

_{OUT}. Smaller inductance or capacitance means engineers can select an inductor or a capacitor of a smaller size. Converters with a higher f

_{sw}can work with these smaller inductors and capacitors.

**Addressing thermal performance**

_{DS(on)}. Equation 3 calculates the temperature rise on a DC/DC converter:

_{LOSS}is the total power loss of the converter and R

_{ΘJA}is the junction-to-ambient thermal resistance. Consider a 2-A load converter with an average R

_{DS(on)}change from 100 mΩ to 50 mΩ. The power loss of this device will result in a 200-mW decrease, which will bring a 16°C cooldown on a typical SOT-563 board with a thermal resistance of 80°C/W. Therefore converters with a lower R

_{DS(on)}offer better working conditions at a lower temperature.

**Turning theory to practice**

**Figure 2: Power-stage architecture of an embedded system**

**Figure 3: Solution sizes of converters with different packages**

_{DS(on)}of the integrated metal-oxide semiconductor field-effect transistor (MOSFET) in the TPS562231 is 95 mΩ (high side) and 55 mΩ (low side). Figure 4 is a thermal image of the full load temperature rise of a 12-V input on the TPS562231 evaluation board.

**Figure 4: Thermal image of the TPS562231 with a 12-V input voltage**

**Additional resources**

- Read the technical article, “3 tips for reducing wireless network interference in access point equipment.”
- Check out the application note, “Optimizing DC-DC Layout For SOT Package To Improve Thermal And Voltage Spike.”
- Find out more about thermals in the application note “SOT23 Package Thermal Consideration.”