Design Procedure for AC/DC Conversion Circuits (Overview)
Selecting a Power Supply Control IC
Once specifications are fixed, we move on to select a power supply IC that can meet those specifications. In this section, we assume the use of a power supply IC.
Based on a set of requirements, you select a power supply IC by determining whether the AC/DC system to be employed is a transformer or switching type, a step up or step down, or flyback or forward. In other words, selecting a power supply IC means determining the specific power supply system to be used. Because basically a power supply IC is compatible only with a specific system, you end up selecting an IC suitable for a selected system. In a design using a power supply IC, the IC represents a significant part of investment, and the circuits and parts to be used are substantially determined by the IC selected. In other words, it would not be an overstatement to say that a design is a function of the IC.
Power supply ICs are available in a wide variety of types, including the functions that they provide. An important point in selecting a power supply IC is to find an IC that supports the necessary functions for the power supply being designed. In particular, implementing protection functions using an external circuit requires a larger number of additional parts and a greater foot print than the power supply IC itself, not to mention the additional time and effort that would be needed in the design and evaluation steps. For these reasons, adding functions using discrete parts may not be a realistic option. Taking an approach involving an adroit use of the power supply IC can lead to increased design efficiency.
In selecting a power supply IC, the question often asked is: “Is it better too big than too small”? In most cases, a diverse set of requirements can be covered with high voltage tolerance and high-power capable ICs. In answer to that question, yes it may be a good strategy to adopt a large unit to take care of those requirements that can be met with smaller units. Bear in mind, however, that in terms of efficiency and optimization of external parts such an approach may not be the best.
As a final point, ICs for switching power supplies are not truly compatible between manufacturers. Although such ICs may be similar in configuration, importantly they differ in pin assignment. Unlike the 78 series of linear regulators pin compatible between manufacturers, basically these ICs are not interchangeable. If parts must be changed during the design process, especially after board layout has been completed, it amounts to redesigning the whole unit. Therefore, selection of an IC must be performed with adequate study.
・The selection of a power supply IC can have a significant impact on the design process.
・As a general rule, for greater design efficiency, protection functions incorporated into the IC should be utilized.