Using flyback topology in power designs

Most of the embedded engineers are aware of the two standard power supply topology types, Linear and switched. We are aware of their application in the circuits and make a choice between them based on the circuit power requirements. While using linear regulator is a straight forward decision, there are several topology in switching mode power supply which for a beginner is little confusing to understand. The primary challenge in any power supply design is to have a small form factor circuit, low cost, low loss (high efficiency). In addition to board level specifications, system level specifications also play a major role in the design as it has to pass the specified compliance. The main topology that come under the switching mode power supplies are:

• Buck converter
• Boost converter
• Buck-Boost converter
• Fly-back topology
• Push-pull converter

Buck, Boost and Buck-boost converters are basically non-isolated type of converters. Fly-back and push-pull have a transformer which provides the galvanic isolation between the input and output and hence these are isolated type of converters.

Considering above scenarios, let us assume that we have a circuit in which 100W power need to be supplied where the power rails on the board are to be derived from a wide input range as shown in the below. 

In this scenario, it is always preferred to have use a fly-back topology. The main advantage of flyback topology is that it can generate multiple power rails from a single transformer element in the circuit. Also, the input range is so wide that a single fly-back can work for your solution to meet the operating voltage ranges. The basic topology of fly back converter is as shown below:

Fly-back is basically derived from the buck and boost converter topology. When the switch 'S' is ON, the primary side of the transformer charges and when switch is OFF, the voltage that was stored will transform to the second side. During the ON state of the switch when primary is charging, the capacitor at the output supplies to the load. This is similar to the boost topology. The transformer here works on the principle of mutual coupling. The transformer at the secondary basically steps down the voltage to the desired voltage level. Also, the required voltage levels can be derived based on the tapping of the secondary. 

The additional circuitry that is required in fly back converter is the feedback that is required to control the voltage and current in the circuit. this to maintain the isolation is opto-coupled. The complete fly-back converter with LT3573 as the sensing and control element is as follows:

Salient points in fly back converter:

• Fly back works in two modes of operation, continuous and discontinuous conduction modes.
• Fly back topology uses a low side transistor for the switching operation. It is placed basically at the lower side of the primary transformer winding.
• For a wide input range and multiple outputs and high power, fly back is one of the preferred topology on the board.
• We must be aware that the inductor is always an EMI component which requires careful PCB layout and placement to meet the compliance criteria.
• Fly back can be comfortably used for powers up to 100W. In terms of current, the components becomes bulky with current more than 10A.

The below link from Texas Instruments has been the best informative one for a long time now for the power engineers:

http://www.ti.com/lit/sg/sluw001f/sluw001f.pdf