Flyback DC DC Converter Basics

Go to flyback dc-dc converter.
Even though some authors presents flyback converter as transformer isolated boost converter, it is more closer to reality that flyback dc-dc converter can be explained as transformer isolated buck-boost converter. In order to prove this, let’s have a look at buck-boost topology.

flyback-dc-dc-converter

Now, if we split inductor to form transformer, configuration look like this:

fly_bck_bst02

Given transformer isolated buck-boost converter can be slightly modified, so that output can be positive. In order to achieve this, we need to move main switch down, wind transformer on opposite way, and to orientate clamping diode to opposite way as well.

fly_bck_bst03

In this way, flyback dc-dc converter is obtained. When transformer ratio is one to one, transfer characteristic of flyback dc-dc converter is identical to buck-boost converter. In CCM, output is (when transformer ratio is 1:1)

fly_eq01

For different transformer ratio, generally N:1, in CCM output is

fly_eq02

When flyback transformer has purpose to transform voltage and not only to be used for galvanic isolation, flyback converter looks like in figure bellow.

flyb_dc_dc_converter

Flyback transformer is like any other high frequency transformer, but with more challenging task. It needs to store energy during main switch “ON” time,

fly_bck_bst04

and to release energy to the load that was stored in flux, during “OFF” time.

fly_bck_bst05

For that reason, an air gap is necessary in many practical situation in order to avoid transformer saturation. Generally, flyback converters are applicable in situations when overall power doesn’t exceeds 250W.

Go to flyback dc-dc converter.

External links:

Flyback Converter on eetimes
Flyback Converter on dos4ever
Flyback Converter on mouser

Posted in electronic

Leave a Reply

Your email address will not be published. Required fields are marked *

*