This time, we shall present an example of board layout for this design case study, and give an overall summary to conclude the " AC/DC Design Examples of Non-isolated Buck Converters" of the AC/DC converter design edition.
Board Layout Example
It was explained in another corner that the board layout in switching regulator design is extremely important, regardless of whether the design is for AC/DC or for DC/DC conversion. Here again, it bears repeating that a switching power supply, as an analog circuit (although in recent years there are also digital power supplies), itself emits noise while also being sensitive to noise. Moreover, switching noise affects other equipment as EMI, and a layout must be used that avoids noise emission to the extent possible.
Below, the board layout for this design case study is shown. This time the circuit design is for a "non-isolated" device, but the basic concept is the same. It is necessary to carefully consider the fact that in a switching power supply circuit there are paths in which large currents are switched, and there are control signal paths that are sensitive to noise. In the board wiring layout, efforts must be made to prevent the large-current paths from emitting noise, and to keep the control signal paths from being affected by noise, insofar as possible.
Due to the importance of board layout, an example of basic board wiring layout is provided in the data sheet for the power supply IC and in design materials. In not a few cases, data that can be used immediately as-is, such as a Gerber file, is provided, and such resources should be used wherever possible. However, it must not be forgotten that, no matter what kind of provided example is used in design and manufacture, the performance of actual equipment must of course be checked.
Below are listed articles related to "basic knowledge" of board layout presented in Tech Web. Please refer to them as well.
Summary of “AC/DC Design Examples of Non-isolated Buck Converters”
Here we conclude this chapter. We end by summarizing the key points of the sections we have covered thus far.
＜AC/DC Design Examples of Non-isolated Buck Converters＞
・The design of non-isolated AC/DC converters is described.
・An example circuit known as a diode-rectified or asynchronous-rectified buck converter is considered.
・Step-down converter operation is in either a continuous mode or a discontinuous mode.
・In DC/DC conversion, the continuous mode is generally used, but in AC/DC conversion at about 60 W, the discontinuous mode is often used.
・Design begins with selection of a power supply IC that satisfies the power supply specifications.
・It is important to understand the differences with isolated-type circuits.
・The rated voltage of the input capacitor is selected considering that a voltage equal to 1.41 times the maximum input voltage is applied.
・It should be remembered that in addition to stabilizing VCC, the VCC capacitor also determines the startup time.
・The inductor should be set such that the operating mode is discontinuous mode
・The inductance is determined from the VIN minimum condition and the maximum value of ton.
・The inductor current is determined from the VIN maximum condition and the minimum on-time.
・The switching current limiting resistor R1 required in the example circuit is determined.
・In calculating R1, the numerical values used when calculating the inductance L1 are necessary.
・An output capacitor is selected based on the ripple current and capacitor impedance, so as to satisfy the output ripple voltage design target.
・Aluminum electrolytic capacitors are components with limited lifetimes, and the lifetime is shortened by large ripple currents.
・Normally, a device capable of fast switching is used as the output rectifying diode. Here, a fast recovery diode is used.
・An output rectifying diode is in essence selected based on consideration of the rated voltage and losses.
・EMI countermeasures involve adding input filters, capacitors at switches (across drain and source), and snubbers at output rectifying diodes.
・LC filters are added to the output to address output noise.
・The board layout also has a considerable impact, and should be studied as well.
・The board layout has a great influence on performance and noise in switching power supply design, regardless of whether the design is for AC/DC or for DC/DC conversion.