The next step in the derivation of the switching power supply topology is to introduce a transformer in the Buck and non-isolated Flyback configurations. It is worth noting that there is no need to study the transformer-coupled Boost circuit, because once the transformer is used, the Boost and Flyback topologies become exactly the same. However, the Buck and Flyback topologies achieve very different performances with the addition of a transformer. In fact, we will change the name of the buck circuit topology with transformer to "forward" converter topology, and keep the name flyback converter. It can be seen that these names can describe the basic characteristics of the circuit more intuitively. In the forward converter, the energy pulse generated in each cycle is transmitted to the output immediately, while in the flyback converter, the energy of each pulse is temporarily stored in the magnetic core and then released to the output. The process is somewhat similar to compressing a spring and then returning it to its original position.
Both topologies have many advantages. First, the added transformer allows for safe isolation because the transformer does not pass DC current, and the output polarity can be adjusted, allowing for more flexibility in winding grounding. In addition, the turns ratio between primary and secondary windings (N=N2/N1) allows for easy voltage regulation, so now power supply designers are not limited by input and output operating ranges. A final advantage is that multiple secondary windings can be coupled into the transformer to achieve multiple voltage outputs. Please note that the gain ratio (voltage ratio) formulas for forward and flyback remain the same as the above non-isolated structure, But a turn ratio factor is added (generally, it is constant after the transformer design is completed).
One of the limitations of the single switch (single switch) topology mentioned above is that the switch drives the transformer in one direction, so the transformer needs a reset mechanism and the magnetic flux needs to return to the starting point. These reset techniques are discussed in detail later, the most common method is to use a volt-second reset equal to the energy pulse, but doing so limits the maximum duty cycle.
The difference between the two topologies is the transformer, and that difference is obvious. As mentioned before, the forward transformer is very close to the ideal transformer, so only very little energy is stored (large magnetizing inductance), so the current on the primary side is directly coupled through the transformer winding and flows to the output filter stage. In contrast, the flyback transformer has a much smaller magnetizing inductance, which needs to store all the energy required by the output load, while the secondary side current is intercepted by the output rectifier diode. When the main switch is turned off, the polarity of the winding is reversed and the stored energy flows through the output rectifier in the form of current. These differences can be clearly seen from the waveforms shown in the figure below, both circuits are operating at 50% duty cycle (note that due to the higher voltage of the flyback circuit, the number of secondary turns needs to be halved to maintain the same output voltage).
