The main aspect of the design revolves around the power supply. An ideal power supply can supply a perfectly stable voltage under any output load condition and any input power with 100% efficiency. However, supplies that get close to this ideology are out of the budget of most hobbyist engineers. The assumption here is most engineers have at least one old laptop / device power supply lying around, or at least spare USB port, to provide the input to the device. By removing the mains voltages from the design, the construction can be simplified and no worry of safety is involved, as well as reducing the cost.
Every circuit has at least one power supply on the board. These are usually a Linear Regulator or a Buck (Step Down) Switched Mode Power Supply (SMPS).
A Linear Regulator is the most simple power supply. By dropping a voltage over a MOSFET, the output voltage can be regulated and maintained with very little fluctuation in output voltage (ripple). This simplicity is at a cost of power dissipation. The efficiency of the linear regulator is a function of the voltage drop and current draw through the device, and this limit is ingrained in the design of the device. The larger of a power dissipation needed, the larger the device, and requirement to extract heat from the system, and also less efficiency.
The SMPS comes in as a higher efficiency device (reaching >90% efficiency in some cases). This results in less power dissipation of the device, so less heat. However, complexity arises in the design and implementation, requiring a controller, MOSFETs and an Inductor. SMPS outputs have ripple voltages and higher switching frequency components. A well designed SMPS can have very little (<10mV) ripple, but is still unsuitable for sensitive circuitry, such as RF.
In the OSPS design, a Linear Regulator will be unsuitable on it's own due to potential power dissipation in worst case (2A @ 15V drop = 30W of heat to dissipate, for example) and would limit the operation of the device. A SMPS can be made variable, but at the expense of ripple voltage and response (assuming stability is gained at the expense of response), and would also be unsuitable on its own as a generic power supply for these reasons.
The OSPS will use both an SMPS and a Linear Regulator. By combining the two types, the power dissipation in the linear regulator can be reduced by dropping the majority of the voltage by using the SMPS. The output voltage will then gain the advantages of the Linear Regulator without complex heat dissipation.
The added bonus of this design is a current monitor can be placed between the SMPS and Linear Regulator, meaning the losses seen over the current measuring aspect can be hidden by the Linear Regulator.
The final aspect of the power supply design would see the inclusion of a Boost SMPS. This can be utilised in the case where a larger output voltage than the input is required. The compromise made with a Boost supply is in the noise, efficiency and stability of the circuit. A combined Buck/Boost supply will add the functionality with minimal extra components.
The controller of the design will be able to control the output voltage in both the Linear Regulator and SMPS. By knowing the input supply, output supply, characteristics of the internal SMPS and Linear Regulator, the controller will be able to calculate the internal voltages needed to provide the required output, and also give the limitations of the device if too much current is attempted to be drawn.