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AC/DC Design Method of PWM AC/DC Flyback Converters

Number of visits: Date:2017-02-08 18:39:09
In this series, with the title [Design Method of PWM AC/DC Flyback Converters], we have reviewed the basics of isolated flyback converters, and have explained design procedures, specification finalization, selection of power supply ICs, and actual circuit design and board layout. This final article ends with [Summary]

A switching power supply circuit is in essence an analog circuit, and design principles are the same as for analog circuits. However, because such circuits include switching of large currents, it is important that the design take into account harmonics, EMI and other issues.

As an AC/DC converter, high voltages of up to hundreds of volts are handled, and so in contrast with sensor circuits and the like, components with high withstand voltages must be used. Hence knowledge of isolation and high-voltage components is mandatory.

From the standpoint of an isolated power supply, transformer design is necessary. Although transformers are simple components, they tend to be avoided by those without experience in handling them. However, they are fundamental and extremely important components of power supply circuits.

In determining component values, there were values that could be found using formulas, and values for which general values were used in light of the general circuit conditions, based on experience. Unfortunately there is no easy way to decide on component values determined from empirical rules, including methods for determining margins. One must repeat the cycle of design, prototyping and evaluation to gain the experience required.

In any case, because the circuits are analog circuits, there are numerous methods for determining component values in “a general roundabout way”, and there may have been some methods that did not seem persuasive, but on the other hand there are no values for which a specific exact value has to be chosen to ensure circuit operation. In combinations of components where there is a degree of leeway, the best way to think of design is as tuning the circuit so as to approach an optimum. In other words, if three designers undertake design of a power supply with the same specifications, a basic circuit configuration such as for example a flyback converter might be the same, but the details will be different for each of the three circuits. This may be regarded as another interesting aspect of the design of analog circuits and power supply circuits.

Below, topics that have been explained in this series and in particular points described beginning from [2. Design Procedure] are summarized. This is information that should be comprehended in the respective processes, and so taking a few moments to review these items is recommended.

<Design Method of AC/DC PWM Flyback Converters>

  1. Isolated Flyback Converter Basics
  2. Design Procedure

    Key points:

    ・In the case of power supplies, often the specifications are not completely determined at the time design is begun, but to the extent possible, design should be started after finalizing specifications.

    ・In actual power supply design, the power supply IC plays a major role, and the circuit and components used are largely determined by the IC used.

    ・In judgments leading up to mass production, along with decisions about what is or is not compatible with the requested specifications, adjustments involving trade-offs are also necessary.

  3. Determining Power Supply Specifications

    Key points:

    ・Information necessary to begin design is collected to the extent possible, and with the premise that there will be changes, design is begun given appropriate margins and flexibility.

  4. Choosing an IC for Design

    Key points

    ・In recent power supply design, the circuit and components are largely determined by the power supply IC used, and so circuit examples and other data can be referenced to obtain an overall idea of the circuit design.

  5. Designing Isolated Flyback Converter Circuits:
    • Transformer Design (Calculating numerical values)

      Key points:

      ・Basically, the design of a transformer that conforms to the circuit being designed will be necessary.

      ・Although some engineers may shy away from designing a transformer due to the tediousness of the task, support available from IC and transformer manufacturers can be tapped into.

    • Transformer Design (Structural Design) – 1
    • Transformer Design (Structural Design) – 2

      Key points:

      ・After calculating numerical values, we move on to the design of a specific transformer structure.

      ・Once a rough structural design is completed in addition to the calculations of numerical values, the process of finalization can be speeded up with the help available from the manufacturers of transformers.

    • Selecting Critical Components – MOSFET related – 1

      Key points:

      ・Select a switching transistor (MOSFET) principally based on the drain-source voltage, the peak current, loss due to on-resistance, and allowable power dissipation for the package.

      ・Because it is difficult to make a rigorous selection based exclusively on paper and pencil calculations, you need empirical formulas and measurements on actual devices.

    • Selecting Critical Components – MOSFET related – 2

      Key points:

      ・The circuits for the control of switching transistors (MOSFET) operation are based on the specifications of the power supply IC.

      ・Power supply IC datasheets provide circuits and component values determination methods; such information should be consulted in designing a circuit.

    • Selecting Critical Components – CIN and Snubber

      Key points:

      ・The input capacitor CIN is important to compensate for input currents that are drawn in due to momentary input power shutoffs and switching.

      ・A snubber circuit is fundamentally essential to protect switching transistors from surges that occur at the input.

    • Selecting Critical Components – Output Rectifier and Cout

      Key points:

      ・The basic circuit operation is the same for a diode-rectifying DC/DC converter.

      ・The output rectifying diode should be a Schottky diode or fast-recovery diode, for which losses are small.

      ・An electrolytic capacitor used as an output capacitor should be adequately examined with respect to the effect of ripple currents on service lifetime.

    • Selecting Critical Components – VCC of IC

      Key points:

      ・A power supply IC requires a low-voltage DC power supply (referred to as VCC etc.) for the control circuit within the power supply IC; in general, auxiliary windings of a transformer (also called the VCC windings, third windings, etc.) are used.

      ・Often, simple diode rectification is used to generate the power supply.

      ・In conforming to the IC specifications, the voltage must be stepped down from a higher voltage particularly when converting from AC to DC, and care must be taken not to exceed the rated voltage.

    • Selecting Critical Components – IC Settings Etc.

      Key points:

      ・In a design using a power supply IC, in addition to basic components (diode bridges, transformer and the like), components are also necessary to set the functions of the power supply IC and the like.

      ・The values of the components used with the power supply IC should be determined according to the data sheet, application notes, design manuals, and the like.

      ・These are often used to set threshold levels of protection circuits, limiting values and the like; components are primarily resistors and capacitors.

    • Addressing EMI and Output Noise

      Key points:

      ・Switching power supplies are potential EMI sources, and measures must be taken with respect to both conductive noise and radiated noise.

      ・From the standpoint of EMC, measures principally address emission (noise radiation).

      ・Countermeasures begin with installation of noise filters, but the measures taken will be related to the board layout and basic components.

  6. Example Board Layout

    Key points:

    ・In designing a switching power supply, the board layout greatly affects the power supply performance and EMC.

    ・In essence, lines through which large currents flow are made wide and short, with small loops.

    ・Control signal lines are placed separately from lines with large amounts of noise, and are not positioned directly below transformers


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