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  • Author Author: rscasny
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What Is So Special About a Flyback Transformer?

rscasny

What Is so special about a flyback transformer?

The simple answer is how energy is stored in the gap of its core. Basically, it is a coupled inductor with a gapped core. During every cycle, the input voltage is applied to its primary winding. Energy is stored in the core's gap. It is then released to the secondary, which powers to the load.

imageHow Does A Flyback Transformer Work in a Circuit?

As previously stated, flyback transformers have a gapped core, which stores energy without saturating the core. This energy storage capability is its differentiating factor. It is used in a flyback controller.

Using a MOSFET as a switch, the flyback controller will open and close a MOSFET switch with a duty cycle needed to attain the target output voltage. Refer to the figure on the right. Wihen the switch is closed, current flows through the transformer's primary winding, building up a magnetic field to store energy. The winding's polarity causes the diode connected to the secondary winding (output) to be reverse biased. In this state, energy is not released to the secondary winding or the load. When the MOSFET switch is opened, the magnetic field built up around the primary collapses, thereby transferring the stored energy to the secondary winding, the diode conducts, and the energy flows to the load.

What Kind of Devices use a Flyback Transformer?

Flyback transformers are commonly used in flyback converters for voltage transformation (often, step up) and circuit isolation. But they are used in other devices such as LED lighting, battery chargers, and more.

Would you like to learn more about flyback transformers? 

element14 is enrolling in a program where you would get a free kit of flyback transformers to experiment with. We are giving away some fantastic prizes for the best blogs that describe your experiment and what you learned. Go here to learn more and enroll.

  • Back to the tech, slightly related but only partially. Circuits related to magnets, induction and related circuits make fun things for kids to do.

    This is a kids experiment (I'm still working on it). There's about 50 turns of wire on a bobbin, and kids can see an LED light up when the magnet is moved towards or away, but not when it's stationary. Similarly if a power supply is connected to the other winding on the bobbin, then the LED only lights up when power is switched on or switched off.

    Happy to write this up sometime if there's interest, but I'm sure there will be a lot more interesting stuff from the experimenting with Flyback transformers challenge.

    This is a crude circuit, simply to cut costs (like down to $1-$2 per board).

    image

  • I've seen the comment that

    "Energy is stored in the core's gap. It is then released to the secondary, which powers to the load."

    But I don't think it really explains why transformers for flyback converters usually have gaps.

    https://en.wikipedia.org/wiki/Flyback_transformer

    Links are broken in this Wiki but you can find the key one here:

    https://www.electronicdesign.com/content/article/21186397/why-have-an-air-gap

    Discussions about where the energy is actually stored get too esoteric for me:

    https://physics.stackexchange.com/questions/171955/how-does-an-inductor-store-energy

    But I'm pretty sure it isn't actually stored in the gap else you wouldn't need to use bigger cores for more power !

    MK

  • There is one guy guy on the youtube who provided analysis (review) of tons of USB phone chargers, so they may be interesting for challengers. Many his videos contains description of various flyback topologies as well as measurements on these circuits. Here is some example how flyback power supply should NOT look like