We have seen how the three fundamental schemes buck, boost, and inverter are built and perform, in the previous blog post. Each of these energy sources has inherent limitations that make them less suitable for applications powered by batteries or harvested energy sources.
Figure 1: Fundamental SMPS schemes characteristics
Before diving into the SEPIC family, let’s analyze how combining the fundamental circuit schemes can create a circuit that can step down and step up voltage while preserving the polarity between the input voltage (Vin) and output voltage (Vout).
Alternatives to SEPIC
The requirement for a converter that can provide a steady Vout, while Vin can be above or below it, can be theoretically addressed in a couple of ways, as seen in Figure 2.
Figure 2: SEPIC alternatives, mixing fundamental SMPS schemes
Various approaches can be considered:
- A buck converter followed by a boost converter, or vice versa. Then, adjust the duty cycles of the two converters in series.
- A second inverter follows one unique inverter.
- A boost followed by an LDO.
Inverter as Starting Point to SEPIC
Figure 3: Fundamental inverter
To refresh on inverter topology, it's easy to identify it with the positions of the three fundamental components: Switch S, Inductor L, and Diode D.
To recognize the inverter, see the vertical element, which is an inductor; the letter I on the inductor and inverter names.
To significantly improve power loss, the standard diode D can be replaced by another switch, S' (not shown here), which operates in the opposite phase of the first switch—also known as synchronous switching. This slightly complicates the control circuit: the two switches need to be well synchronized.
We know Vout = -Vin*(1-D)/D, which allows us to either boost Vin (when D is less than 0.5) or reduce Vin (when D is greater than 0.5). However, Vout has opposite polarity compared to Vin.
From Fundamental Inverter to Inverting Flyback
We create an equivalent circuit by duplicating the inductor L into two identical coils, L1 and L2, and placing them on the same core. The response of Vout remains identical to the original buck-boost configuration.
This forms a transformer with a 1:1 ratio between the primary and secondary sides, which is essentially an inverting flyback converter--exhibiting the same behaviors as a buck-boost converter.
Later, the two coils can be dissociated from the common magnetic core, separated, and independently placed in other positions in the circuits.
With the transformer incorporated into the design, we can step up or down by adjusting the number of turns on the primary and secondary sides of the transformer. This complements the influence of the duty cycle of switch S.
Figure 4: From fundamental inverter to inverting flyback
From Inverting Flyback to Non-Inverting Flyback
With the transformer, it's simple to change the direction of the winding, particularly on the secondary side. By also reversing the diode D, we keep the same operation, but now the output has the same polarity as the input.
At this point, we have a configuration that can both step up and step down Vin with the same polarity. So, do we still need a SEPIC?
Yes, and, in fact, there are many implementations using flyback structures.
Figure 5: From inverting flyback to non-inverting flyback
Low-Side Switch Usage in Flybacks
There are now two flyback structures: one providing a negative Vout and the other yielding a positive Vout. Because the switches are connected in series on the primary side, we can position them at the bottom, making it easier and more cost-effective. Remember N-type devices are smaller than P-type.
The diode D is also in series with the secondary winding and can be moved to the ground side, which is done for the inverting version.
Can we stop here? The flyback can step down or step up and produce both positive and negative Vout. Why go any further?
The issue lies in the fact that coupled inductors, including transformers, are never ideal and always have some leakage. This leakage is prone to ringing and EMI and can cause significant ringing on the switches.
Figure 6: Use of low-side switch in flybacks
Conclusion
While basic inverters and flybacks offer various voltage conversion capabilities, they may not be suitable for clean, cheap, low-power, battery-operated applications due to various limitations. Further exploration of alternative topologies and technologies is necessary to achieve better performance in such applications. Stay tuned for the final part of the series with a destination from flyback to SEPIC.
Click here to see the previous post.
See all the blogs in the Demystifying SEPIC Converters series.
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in reply to DennisFeucht