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2017.10.12 AC/DC

Selection of Power Supply ICs and Design Examples

Design Examples of Non-isolated Buck Converters

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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.

<BM2P094F: AC/DC Converter IC Included 650 V MOSFET>

■ Features

  • ・Built-in 650 V switching MOSFET
  • ・Built-in 650 V start circuit
  • ・PWM frequency: 65 kHz
  • ・Frequency hopping function
  • ・Current mode method
  • ・Burst operation when load is light /
        Frequency reduction function
  • ・VCC pin under voltage protection /
        overvoltage protection
  • ・SOURCE pin Open protection / Short protection
  • ・SOURCE pin Leading-Edge-Blanking function
  • ・Over current limiter function per cycle
  • ・Over current limiter and AC
        voltage compensation function
  • ・Soft-start function
  • ・Both isolated and non-isolated devices
        supported
  • ・Secondary over current protection
        (in the isolated structure)
  • ・Operating power
        supply voltage
        range
    VCC: 8.9 V to 26.0 V
    DRAIN: 650 V (Max.)

  • ・Operating  current
    Normal mode: 0.50 mA (Typ.)
    Burst mode: 0.40 mA (Typ.)

  • ・Operating
        temperature
    -40°C to +105°C

  • ・Package: SOP8 4.90 mm × 3.90 mm
  • ・MOSFET ON-resistance: 8.5 Ω (Typ.)
  • ・Maximum output power: 5 W

■ Applications
Wall adapters, televisions, and household appliances (vacuum cleaners, humidifiers, air cleaners, air conditioners, refrigerators, IH cooking heaters, rice cookers, etc.)

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.

A4_3_block
Internal Circuit Block Diagram of BM2P094F
(click to enlarge)

  • 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.

Key Points:

・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.

Power Supply Design Technical Materials Free Download

Power Supply Design Technical Materials Free Download

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