Stepping voltages down in circuits is incredibly important as many power supplies and microcontrollers work on different voltages. While some microcontrollers can handle a wide range of voltages, they are few and far between: most only operate on voltages between 3V – 5V. In this How-To, we will look at some different power reduction techniques and how they are used together to create efficient, effective power management circuits.
The best way to look at linear vs. switch mode regulators is to understand that they are opposites of each other. Linear regulators are great at providing low-noise supplies, whereas SMPS are incredibly noisy (as they switch at high frequencies). Linear regulators can react quickly to sudden changes on their output, whereas SMPS cannot. However, SMPS are incredibly efficient (as much as 95%): their active components that conduct power are never in their linear region (meaning there is minimal voltage drop across them and, therefore, minimal energy loss). Linear regulators, however, are incredibly inefficient when dropping large voltages (for example, 9V to 5V).
Many situations require that a designer pick one solution or the other and accept/mitigate the consequences of that choice. However, power reduction is rather forgiving and allows a designer to employ both at the same time to produce power regulation that is low-noise, efficient, and quick to react to changes.
A typical power management circuit in a commercial design will use both an SMPS and a linear regulator. The SMPS initially reduces a large input voltage (such as 15V) to a much lower one that a linear regulator can handle (such as 6V). From there, a linear regulator can then regulate this voltage down further to the final desired voltage used by microcontrollers, sensors, and other circuitry. The use of the SMPS to reduce the voltage dramatically improves the efficiency of the system by reducing the voltage enough so that when the output of the SMPS is fed into the linear regulator, there is a minimal voltage drop across the linear regulator. The linear regulator is capable of filtering out a large portion of the noise injected into the supply by the SMPS as well as providing a supply that can react to sudden changes.
We have seen how SMPS and linear regulators can be used to create power management circuitry, but how would a user construct one? One of the easiest methods is to use premade modules that include all the parts and allow for custom circuitry. However, SMPS circuitry is not easy to build, since these parts are often found in small surface mount packages that require PCBs.
The DFR0379 is an example of a premade module that can be used in projects where a DC-DC step-down converter is required to convert voltages as high as 40V into voltages between 1.25V to 37V. Based on the LM2596, the module includes a display and other supporting hardware to reduce DC voltages efficiently. However, the output of the DFR0379 is variable, which means there is no linear regulator on the output. If an efficient, low noise 5V voltage is required, then consider configuring the DC-DC module to output 7V and then connect a 7805 to the output of the module.
Linear and switched power reduction circuits each have their advantages and disadvantages, but they can be combined to help complement each other. When building linear regulator circuits, it is always best to build these from scratch. Consider purchasing DC-DC modules as opposed to designing them, however -- they are complex and often more expensive to build.