This study investigates the parameter design methodology for a flying capacitor boost (FCB) circuit in 1,500 V solar inverters. The FCB topology offers reduced voltage stress on power devices and improved efficiency compared to traditional two-level boost converters, making it ideal for high-voltage photovoltaic (PV) applications. The design focuses on optimizing critical components such as input filters, flying capacitors, and power semiconductor modules while ensuring compatibility with mainstream PV panel specifications.
Circuit Topology and Operating Modes
The FCB circuit comprises input capacitors, inductors, switching devices (T1, T2), diodes (D1, D2), and a flying capacitor (Cf). The operating modes are determined by switching states (x1, x2) as follows:
- Mode 1 (0,1): T1 OFF, T2 ON – Energy stored in L and Cf
- Mode 2 (1,0): T1 ON, T2 OFF – Energy released from L and Cf
- Mode 3 (0,0): Both switches OFF – Inductor current freewheels
- Mode 4 (1,1): Both switches ON – Inductor charges rapidly
Key Parameter Design for Solar Inverters
1. Power Semiconductor Selection
For a 1,500 V solar inverter with 60 A maximum input current, the device voltage rating is calculated as:
$$V_{ce} = k_u \cdot \frac{V_{out}}{2} + \Delta V_{ce,max}$$
Where \(k_u\) = 1.15 (safety factor), \(V_{out}\) = 1,300 V, and \(\Delta V_{ce,max}\) = 200 V. The current rating must satisfy:
$$I_{ce} = k_i \cdot I_{in,max}$$
where \(k_i\) = 1.1 and \(I_{in,max}\) = 60 A. Selected IGBT parameters are summarized in Table 1.
| Parameter | Value |
|---|---|
| IGBT Voltage Rating | 950 V |
| IGBT Current Rating | 145 A |
| Diode Voltage Rating | 1,200 V |
| Maximum Junction Temperature | 150°C |
2. Input Filter Design
The input inductance is designed to limit current ripple (\(\Delta I_L\)) to 60% of rated current:
$$L \geq \frac{V_{out}}{16f \cdot 0.6I_L}$$
With \(f\) = 16 kHz and \(I_L\) = 60 A, \(L\) ≥ 141 μH. A 150 μH inductor produces maximum ripple:
$$\Delta I_{L,max} = \frac{V_{out}}{16Lf} = 33.86\ \text{A}$$
3. Flying Capacitor Optimization
The flying capacitor voltage ripple must stay below 10% of \(V_{out}/2\):
$$C_f \geq \frac{2I_{L,max}}{0.1V_{out}f}(0.5 – |0.5 – D|)$$
At D = 0.5, this yields \(C_f\) ≥ 28.85 μF. A 42 μF capacitor ensures stable operation across all duty cycles.
Dynamic Control Strategy
The control system maintains both inductor current regulation and flying capacitor voltage balance. The governing equations are:
$$L\frac{di_L}{dt} = v_{in} + v_c(x_1 – x_2) – v_{out}(1 – x_2)$$
$$C_f\frac{dv_c}{dt} = -i_L(x_1 – x_2)$$
where \(x_1\) and \(x_2\) represent switching states. A dual-loop control architecture (Fig. 1) achieves:
- Precise current tracking through T2 duty cycle adjustment
- Capacitor voltage balance via duty cycle difference (\(\delta\)) compensation
Simulation Results
Table 2 compares design specifications with simulation results at 1,080 V input and 1,300 V output, validating the parameter selection methodology.
| Parameter | Design Value | Simulation Result |
|---|---|---|
| Inductor Ripple Current | 36 A | 31.40 A |
| Input Capacitor Ripple | 20 V | 11.17 V |
| Flying Capacitor Ripple | 65 V | 16.14 V |
Waveform analysis demonstrates stable operation across varying input voltages (800-1,300 V), with all parameters remaining within safe limits. The solar inverter maintains 98.2% efficiency at full load, significantly outperforming conventional two-level converters.
Conclusion
This work presents a systematic approach to designing flying capacitor boost circuits for 1,500 V solar inverters. By optimizing component parameters and implementing robust control strategies, the proposed solution achieves reduced voltage stress, improved efficiency, and reliable operation under varying PV input conditions. The methodology provides valuable guidance for developing high-performance solar energy conversion systems.