Selection of Power Supply ICs and Design Examples

We have completed a review of basic operation relating to the design of non-isolated AC-DC converters, and so from this point we address the actual design. We begin by selecting the power supply IC that is to be used in design. As has been explained a number of times in other chapters, setting aside the power supply manufacturer, a power supply IC is used in design of the power supply circuit. Hence in order to satisfy the specifications required of the power supply, it is essential to understand what kind of power supply IC is to be used.

Power Supply ICs Used in Design

A power supply IC is selected on the assumption that the power supply input/output voltages, the load current, and other parameters are already determined. The approach is the same whether designing the non-isolated devices we are considering here, or the isolated flyback converters initially considered in the AC-DC Design Edition. The goal of this chapter is to achieve an understanding of differences in the design of non-isolated buck converters relative to the previously discussed design of isolated flyback converters. Hence for details of the previous discussion, please refer to ”Design Procedure”, ”Determining Power Supply Specifications”, and “Choosing an IC for Design” in “AC-DC Design Edition: Design Method of PWM AC-DC Flyback Converters”.

Here we shall briefly describe only the input conditions and output conditions pertaining to the design of this section.

・Input voltage: 90 VAC to 264 VAC
・Output: 20 V/0.2 A (4 W)

Based on these values, and assuming that the efficiency required, various related functions, protection functions and the like are to be considered, we shall use the following power supply IC to design the buck converter.

The following are important issues to be considered when selecting this IC. Put another way, you could say that these are critical points when selecting an IC.

• 1. Support for non-isolated circuits
• 2. Switching MOSFET incorporation, so that the need to select a MOSFET is eliminated, and the number of components is also reduced
• 3. Input/output specifications that satisfy the specifications required of the power supply
• 4. Current-mode type devices for easy stabilization of control
• 5. Internal startup circuit, low power consumption
• 6. Provision of functions to maintain high efficiency
     even under light loading
• 7. Replete protection functions
• 8. Compact package, capable of 5 W operation (specs require 4 W)

For reference, a block diagram of the IC internal circuitry is shown. However, the external circuit example is an isolated-type flyback converter. Try comparing it with the non-isolated buck converter of this section, illustrated later.

Example of Design of a Non-Isolated Buck Converter (Discontinuous Mode)

Next, the circuit diagram below is the example of a non-isolated buck converter circuit design that will be considered here.

As stated above, the input voltage ranges from 90 VAC to 264 VAC, and the output is 20 V/0.2 A (4 W). The AC input is rectified by a diode bridge, and is switched by a MOSFET incorporated directly into the IC. Rectification is again performed by D4, and the result is smoothed by L1 and C5 to obtain a DC output. Feedback of the output voltage to ensure stability is via an optocoupler, but as is clear from the diagram, the input and output are not isolated.

Operation is in the discontinuous mode. As explained in the preceding section on “Basic Operation of Buck Converters and Discontinuous Mode vs. Continuous Mode”, AC-DC buck converters generally have comparatively small output power.

From the next time, we will address selection of major components constituting the circuit and calculation of circuit constants.