Which topology

Part 12 · Voltage Regulation

Key point: the job picks the cell. Down, up, either side, a negative rail, or isolation. Continuous means an inductor in that cable. Rectified means the current is a pulse.

Green fits. Amber can do the job and is the heavier cell. Rose cannot. Each later page is that cell’s switch node and currents.

1
Buck fits. Its output is continuous.
Preset 12 V → 5 V, isolation off. Fits: Buck. Overkill: SEPIC, Zeta, Flyback, Forward. No: Boost, Inverting buck-boost, Ćuk. Buck’s input is rectified and its output is continuous. SEPIC and Zeta can step down. They are amber: a second inductor a pure step-down does not need.
2
Boost fits. Zeta quiets the load.
Preset 12 V → 24 V, isolation off. Fits: Boost, SEPIC, Zeta. Overkill: Flyback, Forward. No: Buck, Inverting buck-boost, Ćuk. Boost and SEPIC keep a continuous input and a rectified output. Zeta swaps the ports: rectified input, continuous output.
3
Buck and boost are both in No.
Preset 12 V → 12 V, isolation off. Fits: SEPIC, Zeta. Overkill: Flyback, Forward. No: Buck, Boost, Inverting buck-boost, Ćuk. A rail that can sit on either side of the other leaves buck and boost in No. SEPIC’s input is continuous. Zeta’s output is continuous.
4
Ćuk keeps both cables continuous.
Preset 12 V → −12 V, isolation off. Fits: Inverting buck-boost, Ćuk. Overkill: none. No: Buck, Boost, SEPIC, Zeta, Flyback, Forward. The inverting buck-boost rectifies both ports. Ćuk’s input and output are both continuous.
5
Forward’s output is continuous.
Preset 48 V → 5 V, isolation on. Fits: Flyback, Forward. Overkill: none. No: Buck, Boost, Inverting buck-boost, SEPIC, Ćuk, Zeta. Both ports of a flyback are rectified. Forward’s input is rectified and its output is continuous.

A four-switch buck-boost is the IC people buy when VIN crosses VOUT. It is not a row here. A flipped flyback secondary can make a negative rail; this table keeps flyback same-sign. The next pages are one cell at a time: the animation, the switch node, and the currents.

Eight cells, same drawings as their lessons

Green fits. Amber can do it and is the heavier cell. Dim cannot. Continuous is a triangle. Rectified is a pulse.

\(V_{\mathrm{IN}}\) (V) \(V_{\mathrm{OUT}}\) (V) Isolation