In recent years, the depletion of fossil fuels and growing environmental concerns have accelerated the development of renewable energy sources. Solar power, in particular, has gained significant attention due to its safety, abundance, and low technical barriers. Photovoltaic (PV) systems require efficient conversion of DC power to AC power, making the performance of solar inverters critical. Among various topologies, non-isolated inverters are popular for their simplicity and cost-effectiveness, but they often suffer from leakage currents that increase system losses and pose safety risks. Existing solutions, such as H-bridge and HERIC topologies, partially mitigate leakage currents but fail to eliminate them entirely. To address this, we propose a novel single-phase solar inverter incorporating a flying capacitor, which ensures common grounding between the AC and DC sides, thereby completely eliminating leakage currents. This paper details the topology, operational principles, control strategy, and validation through simulations and experiments, highlighting the advantages of our design in terms of efficiency, cost, and performance.
The proposed solar inverter topology, as illustrated in the figure below, consists of five switching devices (S1 to S5), a DC-side stabilizing capacitor (Cin), a flying capacitor (CFC), a filter inductor (L1), a filter capacitor (C), and a load resistor (RL). This structure is simpler than conventional diode-clamped or H-bridge solar inverters, reducing component count and cost. The key innovation lies in the common-ground connection between the AC output and DC input, which shorts the parasitic capacitance of the PV panel to ground, effectively nullifying leakage currents. The flying capacitor enables voltage balancing and facilitates energy transfer during negative half-cycles, enhancing overall efficiency. We analyze this solar inverter in depth, focusing on its operational modes, component selection, and control techniques to achieve high-quality AC output.
To understand the operation of this solar inverter, we divide its working modes into four intervals based on the direction of the inductor current (iL1) and output voltage (uo). Each interval involves distinct switching states for energy transfer and freewheeling, optimized to minimize switching losses. All switches are modulated at high frequency only during half-cycles, reducing overall power dissipation. The switching sequences are controlled using an improved unipolar sinusoidal pulse width modulation (SPWM) technique, which simplifies the control logic and ensures smooth transitions between modes. Below, we describe each operational interval in detail, supported by analytical expressions and a summary table.
In Interval I, where iL1 > 0 and uo > 0, the PV panel supplies energy to the load and charges the flying capacitor. During the energy transfer phase, switches S1 and S3 are turned on, allowing current to flow through PV-S1-S3-L1-RL, charging the inductor. In the freewheeling phase, S5 is activated, enabling the inductor to discharge via S5. Similarly, in Interval II (iL1 < 0, uo > 0), the flying capacitor is charged through PV-S1-CFC-S4, while energy is transferred from the filter capacitor to the load during the active phase, and freewheeling occurs through S4 and S5. For Interval III (iL1 < 0, uo < 0), the flying capacitor discharges to power the load via CFC-S2-RL-L1-S5 in the energy transfer phase, and freewheeling involves S1, S4, and S5. In Interval IV (iL1 > 0, uo < 0), the filter capacitor charges the flying capacitor and inductor during energy transfer, and freewheeling is handled by S4. The switching states for all intervals are summarized in Table 1, where “1” denotes on-state and “0” off-state.
| Mode | S1 | S2 | S3 | S4 | S5 |
|---|---|---|---|---|---|
| Ia (Energy Transfer) | 1 | 0 | 1 | 1 | 0 |
| Ib (Freewheeling) | 1 | 0 | 0 | 1 | 1 |
| IIa (Energy Transfer) | 1 | 0 | 1 | 1 | 0 |
| IIb (Freewheeling) | 1 | 0 | 0 | 1 | 1 |
| IIIa (Energy Transfer) | 0 | 1 | 0 | 0 | 1 |
| IIIb (Freewheeling) | 1 | 0 | 0 | 1 | 1 |
| IVa (Energy Transfer) | 0 | 1 | 0 | 0 | 1 |
| IVb (Freewheeling) | 1 | 0 | 0 | 1 | 1 |
The selection of the flying capacitor is crucial for maintaining voltage stability and output quality in this solar inverter. The capacitor discharges during the negative half-cycle, and its energy must approximate the load demand. Let u1 be the voltage across CFC at time t0 (start of discharge), and u2 be the voltage after a duration Δt. The energy balance equation is given by:
$$ \frac{1}{2} C_{FC} (u_1^2 – u_2^2) = \int_{t_0}^{t_0 + \Delta t} u_o i_{L1} dt $$
Assuming sinusoidal output voltage and current, uo = Uo sin(ωt) and iL1 = IL1 sin(ωt), where Uo and IL1 are peak values, and output frequency f = 50 Hz with switching frequency fs = 20 kHz, Δt = 1/fs = 0.05 ms. Solving for CFC:
$$ C_{FC} = \frac{2 \int_{t_0}^{t_0 + \Delta t} U_o I_{L1} \sin^2(\omega t) dt}{u_1^2 – u_2^2} $$
For our design, Uo = 110√2 V, IL1 = 2 A, u1 = 200 V. To limit voltage drop to 1%, we set u2 = 198 V, resulting in CFC ≈ 470 μF. In practical applications, multiple solid capacitors can be series-parallel connected to achieve this value while ensuring safety and reliability. This choice balances cost and performance, minimizing output distortion in the solar inverter.
Control of the solar inverter is achieved through an improved unipolar SPWM technique combined with a voltage instantaneous value closed-loop system. As shown in the control block diagram, the reference modulation signal uoref is compared with the actual output voltage uo, and the error is processed by a PID controller to generate the modulation signal ur. This signal is then compared with unipolar triangular carriers to produce control signals uc for the switches. The SPWM logic ensures that S3 is turned on when ur exceeds the positive carrier, S5 when ur is below the positive carrier, S1 and S4 when ur exceeds the negative carrier, and S2 when ur is below the negative carrier. This modulation scheme drives the solar inverter through the four operational intervals, producing a high-quality AC output after LC filtering. The closed-loop control enhances robustness against load variations, making the solar inverter suitable for real-world PV systems.
To validate the proposed solar inverter, we developed a simulation model in MATLAB with the following parameters: input voltage 200 V, output voltage 110 V, output frequency 50 Hz, rated power 200 W, Cin = 220 μF, CFC = 470 μF, L1 = 5 mH, C = 4.7 μF, RL = 60.5 Ω, switching frequency 20 kHz, and parasitic capacitance to ground 100 nF. The simulation results, as depicted in the waveforms, confirm the effectiveness of the topology and control. The output voltage and inductor current are sinusoidal with minimal distortion, and the leakage current is negligible, approximately zero. These findings demonstrate that the solar inverter successfully converts DC to AC while eliminating leakage currents, aligning with our theoretical analysis.
Furthermore, we constructed an experimental prototype with identical parameters to verify practical performance. The control signals for the five switches, output voltage, inductor current, and leakage current were measured under rated load conditions. The results show clean switching sequences, sinusoidal output waveforms, and virtually no leakage current, corroborating the simulation outcomes. Additionally, we evaluated the efficiency of the solar inverter across output powers ranging from 50 W to 200 W. The efficiency curve indicates values above 90% throughout, peaking at 94.3% at 200 W. This high efficiency, combined with the complete elimination of leakage currents, underscores the superiority of our solar inverter design for photovoltaic applications.
In conclusion, we have presented a novel single-phase solar inverter with a flying capacitor that addresses the leakage current issue in non-isolated topologies. The common-ground connection and optimized SPWM control ensure high performance, simplicity, and cost-effectiveness. Simulation and experimental results validate the design, showing excellent output quality and negligible leakage currents. Future work could focus on extending this topology to three-phase systems or integrating advanced control strategies for grid-connected solar inverters. This research contributes to the development of efficient and safe solar energy conversion systems, promoting the adoption of renewable energy technologies.