In the world of switching DC/DC converters the Zeta topology is a lesser known relative of the SEPIC topology. Both converters provide a positive output voltage that can be greater than, equal to or less than V_IN while avoiding the complexity and cost of a buck-boost converter. The Zeta converter however, has the advantage of significantly reduced output ripple voltage.

Zeta and SEPIC Similarities

To compare topologies, we use the LT8471 Dual Multitopology DC/DC Converter which features two 2A/50V power switches and antiphase switching for reduced input and output ripple. The SEPIC topology is popular because it is easily implemented with many boost converter ICs (including the LT8471) and it has continuous input current, which is often desired in battery-powered systems to minimize large pulse current drain from the battery. The Zeta, with its reduced V_OUT ripple, is not compatible with most switching regulator ICs due to the negative voltages required on the V_X node (Figure 1).

Figures 1a and 1b are simplified schematics of the Zeta and SEPIC topologies using the LT8471. Each topology requires two inductors (L_1A & L_1B), two switches (Q₁ & D₁), and a flying capacitor (C_F). The Zeta rearranges the SEPIC's components such that an inductor is in series with V_OUT (which we will see significantly reducing V_OUT ripple). Note that L_1A & L_1B can be, and often are, 1:1 coupled inductors in a single package to minimize board space.

 

The Zeta and SEPIC converters use the same equation for calculating the switch duty cycle in continuous conduction mode, and given that they have the same switch current limit and combined inductor current ripple (for the two inductors), they both deliver the same maximum current to the load, which is:

 

where I_LIM is the Q1 switch current limit,  DC is the duty cycle, I_{L1_RIP} is the combined inductor current ripple and η is the conversion efficiency (typically about 80% for the Zeta and SEPIC). Additionally, the diode and power switch requirements, V_REVERSE and V_CE respectively (see Table 1), are the same for the Zeta and the SEPIC converter.

Zeta and SEPIC Differences

With a SEPIC converter (Figure 1b), the lowside Q₁' switch periodically pulls V_X' to ground, ramping up the inductor current. Q₁' then turns off, after which V_X' flies up to V_OUT+V_IN, delivering current through D₁' to the output. With the Zeta converter (Figure 1a), the Q₁ switch first pulls V_X up to V_IN then turns off, after which V_X flies down to -V_OUT. The LT8471 has a unique and flexible NPN power switch design capable of handling ‑60V on the emitter, allowing it to be used with the Zeta along with other, more familiar (Boost, Buck, SEPIC, etc.), topologies.In Figures 1a and 1b, both topologies use the same number of passive components. Note however, that the SEPIC uses a lowside power switch Q₁' similar to that used in a boost converter, while the Zeta requires a highside switch Q₁. Figure 2 shows the Zeta converter current flow and switching waveforms. The Q₁ emitter voltage toggles between V_IN and -V_OUT. Therefore, the E₁ pin must be rated to swing to -V_OUT.

 

Output Voltage Ripple

Similar to the buck and Ćuk converters, the Zeta has very low output voltage ripple. Each of these converters has an inductor in series with V_OUT preventing rapid changes in output current that would otherwise increase ripple voltage. Neglecting the ESR of C_OUT, V_OUT ripple of the Zeta operating in CCM is given by:

 

where I_{L1B_RIP} is the peak-to-peak ripple of the L_1B inductor current and F_SW is the switching frequency of the converter.

With the SEPIC, current out of the device only flows through D₁' when Q₁' is off. This makes the output current discontinuous and increases voltage ripple. Neglecting the ESR of C_OUT, V_OUT ripple of the SEPIC operating in CCM is given by:

 

where I_LOAD is the output load current and DC is the switching duty cycle.

From the equations above, and considering that I_{L1B_RIP} is typically a fraction of I_LOAD, it can be seen that the Zeta converter will have much lower V_OUT ripple than the SEPIC converter.

Input Voltage Ripple

Neglecting the ESR of C_IN, V_IN ripple of the Zeta operating in CCM is given by:

 

the SEPIC converter input voltage ripple in CCM is given by:

 

where C_IN is the input capacitor, I_{L1A'_RIP} is the input inductor's current ripple.

Therefore, in contrast to the output voltage ripple, the Zeta converter will often have higher input voltage ripple than the SEPIC converter.

Component Voltage Ratings

Table 1 compares the Zeta and SEPIC converters. It lists the voltage ratings required for the power switch (Q₁), power diode (D₁) and the flying capacitor (C_F). Note that the flying capacitor and power switch terminal voltages (Emitter and Collector) differ depending on the topology.

 

Low V_OUT Ripple Application

Figure 3 shows an example application using the LT8471 Dual Multitopology DC/DC Converter with 2A switches. This application creates +/- 5V output rails, both using low ripple topologies. Channel 1 creates the -5V output using the low output ripple ĆUK topology. Channel 2 creates the +5V output using the low output ripple Zeta topology. The input voltage can vary from 4V to 25V.

 

Using this configuration, both outputs have very low voltage ripple, which is less than 10mV, as shown in Figure 4.

 

Conclusion

When a low ripple, positive output voltage is required, consider choosing the Zeta topology. Similar to the SEPIC, it generates a regulated output voltage from an input voltage that can be greater than, equal to or less than the output voltage. Due to its unique NPN power switch design, the LT8471 DC/DC converter is nicely suited for creating Zeta DC/DC converters.