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2016.03.25 AC/DC

Choosing an IC for Design

Design Method of PWM AC/DC Flyback Converters

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After power supply specifications have been firmed up, the second step in the design procedure involves “Selecting a power supply controller IC.”

Why do we need to select an IC? A general approach to the design of newer power supply circuit has been the use of a power supply IC (though it is not the case for power supply manufacturers). Such ICs, providing excellent control features along with various protection functions, make the design process simple and offers advantages in reducing footprint.

In this article, as a case study for the design of a PWM flyback AC/DC converter, we establish concrete power supply specifications and determine the type of power supply ICs that meet the specifications.

For power supply specifications, we set general conditions based on “a minimum set of specifications that must be established for starting the design process.” Based on those conditions, the following criteria should be adopted in selecting an IC:

Example power supply specifications Key selection features
Input voltage: 85 to 264 VAC Power supply system for wider input voltages
Output: 12 VDC±5% / 3A 36W External output power transistors
Output ripple voltage: 200mVp-p Current mode type
Voltage tolerance between primary and
 secondary isolation: 3kVAC
Optocoupler-based feedback control
Operating temperature range: 0 to 50℃ Guaranteed operating temperature range: -40 to 85℃
Minimum efficiency: 80% Switching method
Maximum input power with no load:
 0.1W
Low power type with a built-in start circuit

・Input voltage: 85 to 264 VAC
The above input specifications are based on the assumption that the device will be targeted on worldwide markets. The type of IC that we look for is one that provides a voltage tolerance capable of accommodating a wide input range while operating in a reliable manner.

・Output: 12 VDC±5% / 3A 36W
The output voltage is the 12V bus voltage, which is a standard for industrial devices. The ±5% accuracy specified is also a generally accepted parameter. The 3A output current can be accommodated. Although the use of an IC with a built-in switching transistor, for ease of understanding of the basic configuration we opt for an external power MOSFET for switching operations.

・Output ripple: 200mVp-p
This output ripple represents a standard level. We need to focus on a control IC in a current mode that can minimize the ripple.

・ Isolation voltage: 3kVAC between primary and secondary side
For feedback control for regulating the output voltage, a line is needed to return the secondary voltage (output voltage) to the primary side. In addition to the transformers, the feedback line must be isolated. A optocoupler is used to isolate the line.

・Operating temperature range: 0 to 50℃
The indicated operating temperature range reflects generally accepted specifications for various devices. To cover this range, we need to select ICs and parts that provide a wider range.

・Minimum efficiency: 80%
The indicated efficiency also represents a generally accepted value. Although for DC/DC converters, efficiency greater than 90% may be required, in the case of AC/DC converters, an 80% level, though being subject to change for improvement, is not considered very low. Delivering this level of efficiency requires switching AC/DC conversion circuits.

・ Maximum input power at no load: 0.1W
To achieve this level of power, we need to select a controller IC specifically designed for low power consumption.

So, we have to find an IC well-suited to meet these power supply specifications. To do that, we need knowledge of AC/DC conversion methods and characteristics, as well as the types and functions of controller ICs available in the market. This, however, is not as daunting a task as it may appear, provided that the specific conversion method to be adopted is fixed. Given that in this article we are concerned with a PWM flyback converter, when looking for an IC, we need to find an IC that can be used to build a flyback converter with. In the next step, we search for a PWM IC, and find an IC that meets our requirements in terms of input ranges and output specifications. Remember, websites put out by IC manufacturers support conditional searches for our use, for great convenience.

The text below describes power supply ICs capable of matching such power supply specifications.

BM1P061FJ: PWM Controller IC for AC/DC

Features

  • PWM frequency: 65kHz
  • Current mode
  • Light-load burst operation with a frequency reduction feature
  • Built-in 650V start circuit
  • VCC pin with under-voltage and over-voltage protections
  • CS pin with open circuit protection
  • CS pin with a Leading-Edge-Blanking function
  • Cycle-by-cycle overcurrent limiter
  • Overcurrent limiter with AC voltage compensation
  • Soft start function
  • Secondary overcurrent protection circuit
  • Frequency hopping function
Isolated flyback converter circuit using the BM1P061FJ

Isolated flyback converter circuit using the BM1P061FJ

The BM1P061FJ is a PWM controller IC for AC/DC converters with which isolated flyback converters can be constructed. With the IC properties as described above, the reasons for choosing the BM1P061FJ are as follows: It is a current-mode PWM-type switching current IC with a 650V-tolerant start circuit capable of accommodating DC voltages with rectified 264VAC which is compatible with the above design example; it is able to deliver both power savings and high efficiency while providing a start circuit and a switching frequency reduction function at a light load. In addition, as a member of the BM1PXXX series, it offers the flexibility to accommodate specifications changes in the midst of a design project.

Also, the figure above shows a basic circuit configuration for building a flyback converter. As may be clear from the figure, thanks to a higher degree of integration, the configuration uses an extremely limited number of external components. Further details on the IC are provided in the datasheet.

Now that the IC that can be used to implement the established power supply specifications has been fixed, let us briefly recapitulate the AC/DC conversion process and move on to the design stage.

Key Points:

・ In the case of a power supply unit, it is not uncommon that at the beginning of the design process, specifications are not completely established.

・ We need to collect as much information as possible necessary to start the design process with, and embark on the design task with adequate latitude and flexibility, on the assumption that the design could be changed along the way.

・ 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.

Design Example for PWM Flyback Converter

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