How to Go from Flyback to SEPIC: Part 4 of 4

We have come to the final destination in the SEPIC family. The journey started with a basic and fundamental SMPS. The beauty of the approach is the ability to see SEPIC, Cuk, and Zeta converters as part of a normal flow. 

Figure 1: Basic flow of SMPS to SEPIC 

This blog post focuses on the transformation from flybacks to Cuk, SEPIC, and Zeta converters.

Note that inverter, flyback, Zeta, Cuk, and SEPIC have a similar structure and output formulation:

Vout = +/- D/(1-D) 

From Flyback to SEPIC 

To prevent the ringing caused by less-than-ideal coupled inductors, add a large capacitor, or flying capacitor, between the primary and secondary windings of the 1:1 transformer T. 

This apparent simple modification transforms the circuit into a SEPIC structure! 

There are two versions: one is derived from the non-inverting flyback and is specifically known as a SEPIC, while the other comes from the inverting flyback and is called a Cuk. The Cuk can also be thought of as an inverting SEPIC, just as the SEPIC can be referred to as an inverting Cuk. 

One can observe a SEPIC, Cuk, or Zeta are recognized by noting there is a capacitor between the primary and the secondary coils--even with two independent inductors. 

When examining Vout versus Vin in continuous conduction mode (CCM), it's similar to the calculation for the buck-boost. For the Cuk, it's the same as the buck-boost, and for the SEPIC, it's reversed in polarity. 

Figure 2: Flyback to SEPIC and Cuk

SEPIC with Dissociated Transformer  

Figure 3: SEPIC redrawn 

A SEPIC circuit drawn with a 1:1 transformer is not the typical configuration. It’s more commonly seen with two separate coils with varying degrees of coupling. 

The circuits displayed here represent the same SEPIC converter. The approach we've adopted, incorporating a transformer derived from the basic buck-boost, automatically provides the Vout equation in relation to Vin. 

 The two inductors don't necessarily have to be coupled, but uncoupled inductors are less efficient. For equivalent performance, the size of uncoupled inductors needs to be approximately twice as large as well-coupled inductors. 

Differentiating between SEPIC and Cuk can be a challenge because they are quite similar. Both the SEPIC and the Cuk have the primary inductor connected directly to Vin, but the Cuk can be identified by its two coils: one connected to Vin for the primary coil and the other to Vout for the secondary coil. 

Cuk Converter with Dissociated Transformer  

Figure 4: Cuk redrawn 

The Cuk converter is directly derived from the inverting flyback. It's also sometimes referred to as a negative SEPIC; similarly, the Cuk can be called a negative SEPIC. 

Many papers describe the Cuk as a converter where both the input and output act like current sources. 

Because of this configuration, switching noise is somewhat concealed for both Vin and Vout, thanks to the inductors in series with them. The Cuk design provides a relatively quiet operating environment for both the input and output stages. However, its main drawback is that its Vout has the opposite polarity compared to Vin; like what the inverter is doing. 

Zeta Converter – A SEPIC Variant 

Figure 5: Zeta converter – a variant of SEPIC 

The Zeta converter is another variation of the SEPIC structure. The change is quite simple. It involves interchanging the positions of certain elements in the circuit, which doesn't alter its behavior. 

On the primary side, the switch and the inductor positions are swapped, and on the secondary side, the diode and the secondary inductor are switched. These two changes ensure that the Vout polarity remains the same as Vin, with Vout/Vin = D/(1-D), which is identical to the SEPIC equation. 

Note that the input is directly switched on and off, which can easily generate and propagate noise to the input stage. However, the output is better shielded with the secondary coil in series, making the Zeta structure superior to its cousin, the SEPIC, in this aspect. 

Conclusion 

The four blog posts in this series describe how a SEPIC, Cuk, and Zeta can simply derive from the basic fundamental inverter. We have done that in a progressive step-by-step construction, demystifying this group of converters.  

To conclude, we list and compare the main characteristics of each structure: 

Figure 6: Comparison SEPIC, Cuk, Zeta performances 

See all the blogs in the Demystifying SEPIC Converters series.