Flyback Converter from ROHM Simplifies Isolation Applications

February 03, 2020 by Robert Keim

The BD7J200 has a wide input voltage range and does not require an external MOSFET.

In this article, we'll use ROHM's new flyback converter to elucidate our discussion on switcher terminology and flyback conversion. 


DC/DC Converter Review

Switching regulators can be fairly complicated strictly from the perspective of circuit design and performance analysis. But to make matters worse, the terminology that engineers use to identify different types of regulators seems almost intentionally cryptic. Let’s briefly review important switcher categories:

  • Boost converter: This is my favorite category because the name is actually intuitive. Boost converters generate an output voltage that is higher than the input voltage.
  • Buck converter: The word “buck” makes me think first of money and second of male deer. As it turns out, buck also means “step-down”: a buck converter generates an output voltage that is lower than the input voltage.
  • Buck-boost: If you know what “buck” and “boost” mean, this term is fairly intuitive: a buck-boost converter can generate an output voltage that is higher or lower than the input voltage. However, “buck-boost” can also refer to a topology that is capable of generating an output voltage that is opposite in polarity to the input voltage.
  • Flyback: A flyback converter is similar to a buck-boost converter, but the output voltage is delivered to the load through a transformer.
  • SEPIC: This term comes from a single-ended primary-inductor converter. It’s a buck-boost converter whose special topology offers some advantageous features.


What Is a Flyback Converter?

The following diagram conveys the basic structure and operation of a flyback converter:


Diagram of a flyback converter using one MOSFET switch and a flyback transformer

Diagram of a flyback converter using one MOSFET switch and a flyback transformer. Image used courtesy of Digi-Key

As you can see, the switching action occurs in series with one of the coupled inductors, and this results in an alternating current that can be transferred to the other coupled inductor through changes in magnetic flux. The alternating current is then converted into a stable output voltage by a diode and a capacitor.

Note that flyback transformers are designed for phase reversal, as indicated by the polarity dots in the transformer symbol.

The flyback converter is conceptually simple and offers a low component count.

The use of a transformer as a means of coupling input energy to the load circuit provides galvanic isolation, and it also gives the designer additional flexibility, since the output voltage generated by a given flyback converter depends not only on the input voltage and the switch’s duty cycle but also on the turns ratio of the flyback transformer.


Flyback Conversion without an Optocoupler

The BD7J200 offers low-complexity, moderate-efficiency flyback conversion and is intended for industrial applications. This is the typical application circuit:


Typical application circuit of BD7J200

Typical application circuit of BD7J200. Image used courtesy of ROHM


When operating in normal mode, this PMIC uses a fixed switching frequency of 400 kHz, though keep in mind that this frequency will vary somewhat in response to temperature variations:


Switching frequency varies in response to temperature variations

Switching frequency varies in response to temperature variations. Image used courtesy of ROHM


The features list mentions a high-efficiency “light load mode,” but I’m inclined to describe the BD7J200 as a moderate-efficiency part because the only efficiency plot that appears in the datasheet indicates performance that doesn’t seem particularly impressive to me, especially for small load currents.


The efficiency of BD7J200 for various load currents

The efficiency of BD7J200 for various load currents. Image used courtesy of ROHM


The Need for Feedback

If you look again at the basic flyback topology shown above, you’ll notice that this is an open-loop configuration. The circuit generates an output voltage determined by duty cycle and turns ratio; no adjustments are made based on information that is fed back from the load circuit.

This works, but what can we do if we want improved regulation performance?

The usual solution in such situations is to incorporate feedback, such that the converter’s control circuitry can make adjustments based on measurements of the actual output voltage. With flyback converters, though, providing feedback is problematic, because feeding back information by means of normal voltage signals would compromise the galvanic isolation provided by the transformer.

One approach to flyback-converter feedback is to use an optocoupler. This maintains isolation, but it also introduces the various disadvantages associated with optocoupler technology.

The BD7J200 eliminates this problem by incorporating specialized circuitry that allows it to derive feedback information from the primary side of the transformer. I’d like to understand this feature a little bit better, but I was having difficulty extracting meaningful information from the descriptions in the datasheet. This feedback connection is implemented via the FB pin shown in the block diagram below.


Feedback connection is implemented using the highlighted FB pin

Feedback connection is implemented using the highlighted FB pin. Image used courtesy of ROHM


Do you have any experience with flyback-converter feedback strategies? Let us know in the comments section below.