The Short-Circuit Reality of Non-Isolated Stacking
Can non-isolated buck converters be connected in series outputs? The direct answer is no, you cannot stack the outputs of standard non-isolated buck converters to add their voltages if they share a common input source. If you attempt to stack a 12V and a 5V non-isolated buck converter (both fed from a shared 24V DC supply) in series, the output will not be 17V. Instead, you will create a dead short across the second converter because their ground (GND) pins are internally tied to the shared input ground.
The theoretical stacked voltage formula is straightforward: V_total = V_out1 + V_out2. For our 12V and 5V modules, V_total = 12V + 5V = 17V. However, because the non-isolated ground return path forces V_out2_GND = V_in_GND, the physical wiring required to stack them connects the positive output of Converter A directly to the common ground rail through Converter B's internal low-side MOSFET or catch diode. The actual measured voltage across the stacked load becomes a short circuit, inevitably destroying the switching components. I have personally seen the catch diode on a cheap LM2596 module literally pop off the PCB when a beginner tried this exact 17V stack from a shared 24V laptop brick.
Before we look at how to fix this, here is how the basic duty cycle math of a single non-isolated buck converter shifts when the input voltage varies by a ±20% range (common in automotive or unregulated solar arrays targeting a 12V output from a 24V nominal source):
| Input Voltage (V_in) | Variance from Nominal | Target Output (V_out) | Required Duty Cycle (D) |
|---|---|---|---|
| 19.2V | -20% | 12.0V | 62.5% |
| 21.6V | -10% | 12.0V | 55.5% |
| 24.0V | 0% (Nominal) | 12.0V | 50.0% |
| 26.4V | +10% | 12.0V | 45.4% |
| 28.8V | +20% | 12.0V | 41.6% |
Topology Comparison: Why Isolation Dictates the Math
The core reason series stacking fails on a shared bus is the lack of a transformer to break the galvanic connection. When designing multi-rail power supplies, you must choose the correct topology. Below is a data-dense comparison of common DC-DC topologies and their viability for series output stacking.
| Topology | Galvanic Isolation? | Series Stacking Viable? | Common-Ground Short Risk | Typical Controller IC Example |
|---|---|---|---|---|
| Buck (Step-Down) | No | No (on shared bus) | High (Shorts low-side FET/Diode) | LM2596, LM5160 |
| Boost (Step-Up) | No | No (on shared bus) | High (Direct DC path through inductor) | MT3608, TPS61230 |
| Flyback (Isolated) | Yes | Yes | None (Transformer breaks ground) | UCC28740, LT3748 |
| Forward (Isolated) | Yes | Yes | None | UCC2897A, LM5025 |
| SEPIC (Non-Isolated) | No | No (on shared bus) | High (Shared ground reference) | LM3478, TPS63020 |
As noted in this Analog Devices technical article on isolated vs non-isolated converters, the physical separation of the primary and secondary windings in flyback and forward topologies is what allows their outputs to be floated and stacked safely. Without that magnetic isolation, the electrical nodes remain rigidly locked to the input ground plane.
High-Voltage Shifts: 120V DC, 230V DC, and 3-Phase Equivalents
In AC power math, assumptions like power factor (PF) and phase angle fix your true power calculations. In DC-DC conversion, PF and phase are entirely meaningless. The assumption that fixes our answer here is strictly galvanic isolation and the common-ground reference. However, the consequences of ignoring this shift dramatically as you scale up to mains-equivalent DC buses.
- 120V DC Bus Equivalent: If you attempt to stack non-isolated modules to reach a 120V DC bus (often used in telecom or solar battery banks), the common-ground short still occurs. However, the resulting arc flash, inductive kickback, and MOSFET vaporization are catastrophic. The energy let-through current at 120V DC will easily weld PCB traces and destroy the input power supply.
- 230V DC Bus Equivalent: At 230V DC, non-isolated stacking is practically suicidal. Beyond the short-circuit failure, you violate basic PCB clearance and creepage distances. The common-mode voltage stress on the non-isolated controller ICs will cause internal gate-drive breakdown long before the output short even triggers.
- 3-Phase Rectified DC (approx. 540V DC): When dealing with a 3-phase AC rectified DC bus, non-isolated stacking is physically impossible. You must shift to isolated topologies (like LLC resonant or phase-shifted full-bridge) or use a multi-winding transformer. As detailed in Texas Instruments' isolated DC-DC topology overview, high-voltage bus conversion requires strict galvanic separation to manage common-mode noise and ensure safety earth grounding.
When the Series Conversion is Meaningless (and How to Fix It)
The mathematical conversion of V_1 + V_2 is entirely meaningless when the input sources share a common ground and the converters lack a transformer. You cannot mathematically add the outputs if the physical ground node is already shared; the circuit simply resolves to the highest potential node shorting through the lowest impedance path.
If your project absolutely requires stacked DC voltages from a single source, you have three viable workarounds:
- Use Isolated Converters: Swap the non-isolated buck modules for isolated flyback or push-pull modules. Because the secondary side has no electrical connection to the primary ground, you can safely wire the positive of Converter A to the negative of Converter B.
- Use a Multi-Output Topology: Instead of stacking separate boards, use a single controller with a coupled inductor or a multi-winding transformer (like a SEPIC with multiple secondary windings) to generate the required voltage rails simultaneously.
- Galvanically Isolate the Inputs: If you are stuck with non-isolated buck modules, you can stack their outputs only if their inputs are also completely isolated from one another. For example, powering Converter A from Battery 1 and Converter B from Battery 2 (with no shared ground between the batteries) allows the outputs to be stacked safely.
Always verify your ground topology with a multimeter in continuity mode before applying power. If you measure less than 1 ohm between the input GND and output GND pins of your module, it is non-isolated, and series stacking on a shared bus will result in immediate component failure.
