SEPICs

Overview

SEPIC (single-ended primary inductance converter) is a topology of switch-mode power supply (SMPS) which can both up and down-convert, similar to a buck/boost. It can be viewed as a boost converter followed by a buck-boost converter.

The basic schematic of a SEPIC (single-ended primary inductance converter). Capacitor C1 is shown as polarized, but in some cases may be unpolarized to prevent problems if the voltage changes polarity (which can occur if the switch has a high duty cycle).

Like a Ćuk converter, a SEPIC has one switch and two inductors. Its advantages over a buck-boost alone is that it has a non-inverted output voltage, DC decoupling from input to output (through a series power-transferring capacitor), which makes it easier to handle things such as short circuits on the output, and true turnoff of the output (when the switch is off, the output truly goes to 0V).

Like other SMPS, the SEPIC converter uses a switching element to control the output. The power transferring capacitor between input and output is sometimes called the AC capacitor.

Output Voltage

In continuous-conduction mode (CCM), the equation linking the input voltage $V_{IN}$, output voltage $V_{OUT}$ and duty cycle $D$ of a SEPIC is:

$$\begin{align*} V_{OUT} = \frac{D}{1-D} V_{IN} \end{align*}$$

Like before, this equation assumes all components are ideal. This equation is identical to the one for an inverting buck-boost except for the negative sign (a buck-boost inverts the output, while a SEPIC does not).

Inductor(s)

The SEPIC has two inductors, just like the Ćuk converter. They can either be wound on separate cores and not share any magnetic field (uncoupled inductors), or be wound on the same core and share a magnetic field (a coupled dual-winding inductor). Using a coupled dual-winding inductor has the advantages of reducing the component count, and lowering the total inductance requirements, but can be hard to find for high-power requirements. Coupled inductors used in a SEPIC also benefit from some leakage inductance, which reduces the AC losses.

The equations are different for coupled and un-coupled inductor designs. For a coupled inductor, the equation to calculate the inductance $L$ is:

$$\begin{align*} L = \frac{V_{IN(max)}^2 d_{min}^2}{2f_s P_{OUT(min)}(1 + d_{min}\frac{1 - n}{n})} \end{align*}$$

And for two uncoupled inductors:

$$\begin{align*} L_1 = \frac{d_{min} V_{IN(max)}^2 n}{2f_s P_{OUT(min)}} \end{align*}$$ $$\begin{align*} L_2 = \frac{(1 - d_{min}) V_{OUT}^2}{2f_s P_{OUT(min)}} \end{align*}$$

The above equations determine the minimum inductance required for CCM operation at maximum input voltage and minimum load (the worst-case scenario for a SEPIC).

Note

Even though the equations above show this, it is still worth pointing out that in a decoupled design, the inductances do not have to be the same value. This is a common misconception; this rule only applies to the coupled SEPIC design.

Capacitor

Sometimes the AC capacitor needs a series RC snubber circuit to make the SEPIC stable. A low resistance between 1-10R and a large capacitance between 50-1000uF can sometimes fix this.