1. Introduction
The development of energy storage inverter has gained significant attention due to their ability to replace traditional diesel generators, which suffer from high noise, environmental pollution, and large physical footprint. Energy storage single-phase inverters, characterized by low noise, zero emissions, and high efficiency, offer a sustainable solution for portable power supply in remote areas. This paper focuses on a two-stage energy storage inverter architecture, comprising a front-end push-pull DC-DC converter and a rear-end full-bridge inverter.
2. Front-End Push-Pull DC-DC Converter
2.1 Topology and Operating Principles
The push-pull converter utilizes a center-tapped transformer to achieve voltage boosting and galvanic isolation. By leveraging the transformer’s leakage inductance ((L_s)) and resonant capacitance ((C_r)), zero-voltage switching (ZVS) and zero-current switching (ZCS) are realized, minimizing switching losses. The circuit operates in two symmetrical half-cycles:
- Phase 1 (t₀–t₁): Switch (T_1) conducts, transferring energy to the secondary side.
- Phase 2 (t₁–t₂): Both switches are off, and parasitic capacitors discharge to enable ZVS for (T_2).
The resonant frequency (f_r) is designed to exceed the switching frequency (f_s): [ f_r = \frac{1}{2\pi \sqrt{L_s C_r}} > f_s ] This ensures soft switching across varying load conditions.
2.2 Control Strategy
A voltage-closed-loop control using the SG3525 PWM controller stabilizes the DC bus voltage ((U_{dc})). The PI controller adjusts the duty cycle dynamically: [ G_T(s) = K_p \left(1 + \frac{1}{T_i s}\right) ] Key parameters of the push-pull converter are summarized in Table 1.
Table 1: Push-Pull Converter Parameters
| Parameter | Value |
|---|---|
| Input Voltage ((U_{in})) | 48 V |
| Output Voltage ((U_{dc})) | 350 V |
| Switching Frequency | 40 kHz |
| Transformer Turns Ratio | 1:9 |
| Resonant Capacitance | 280 nF |
2.3 Loss Analysis
Losses in the push-pull converter arise from MOSFET conduction, transformer core, and winding resistances. The total loss (P{\text{total}}) is: [ P{\text{total}} = P{\text{con}} + P{\text{core}} + P{\text{cu}} ] where (P{\text{con}}) is MOSFET conduction loss, (P{\text{core}}) is core loss, and (P{\text{cu}}) is copper loss.
3. Rear-End Full-Bridge Inverter
3.1 Topology and Modulation
The full-bridge inverter converts DC to AC using four IGBTs. Unipolar sinusoidal pulse-width modulation (SPWM) is employed to reduce harmonic distortion. The output voltage (u{\text{out}}) is: [ u{\text{out}} = S \cdot U_{dc} ] where (S) is the switching function.
Table 2: Full-Bridge Inverter States
| State | Switches (S1, S2, S3, S4) | Output Voltage |
|---|---|---|
| P | (1, 0, 0, 1) | (+U_{dc}) |
| N | (0, 1, 1, 0) | (-U_{dc}) |
| O | (0, 1, 0, 1) or (1, 0, 1, 0) | 0 V |
3.2 Dual-Loop Control Strategy
A voltage-current dual-loop control ensures stability and fast dynamic response:
- Voltage Outer Loop: Tracks the reference voltage using a PI controller.
- Current Inner Loop: Enhances system damping and compensates for load disturbances.
The transfer functions for the loops are: [ G_i(s) = \frac{K_p}{1 + T_i s}, \quad G_v(s) = \frac{K_v}{1 + T_v s} ] Bode plots (Figure 1) validate the controller’s phase margin and bandwidth.
4. Loss and Thermal Analysis
4.1 IGBT Loss Model
IGBT losses comprise conduction ((P{\text{con}}})) and switching ((P{\text{sw}}})) losses: [ P{\text{total}} = v{ce} \cdot i_c + \frac{1}{2} f{\text{sw}} \cdot (E{\text{on}} + E{\text{off}}}) ] where (v{ce}) is the collector-emitter voltage, (i_c) is the collector current, and (E_{\text{on/off}}}) are switching energies.
4.2 Diode Loss Model
Diode losses include forward conduction ((P{\text{con, D}}})) and reverse recovery ((P{\text{rec}}})): [ P{\text{total, D}} = v_f \cdot i_f + f{\text{sw}} \cdot E_{\text{rec}} ]
Table 3: Loss Distribution in Full-Bridge Inverter
| Component | Conduction Loss (W) | Switching Loss (W) |
|---|---|---|
| IGBT (S1) | 16.2 | 7.05 |
| Diode (D1) | 7.05 | 17.2 |
4.3 Thermal Management
A fourth-order RC thermal model predicts junction temperatures. The steady-state temperature rise (\Delta T) is: [ \Delta T = P{\text{total}} \cdot R{\text{th}} ] where (R_{\text{th}}) is the thermal resistance. Forced air cooling maintains temperatures below 90°C (Figure 2).
5. Experimental Validation
A 2.5 kW prototype was tested under resistive and inductive loads. Key results include:
- Output Voltage THD: <2% at full load.
- Efficiency: 90% at rated power.
- Thermal Performance: Maximum IGBT temperature of 57°C with active cooling.
6. Conclusion
The proposed energy storage inverter demonstrates high efficiency, low harmonic distortion, and robust thermal performance. Key innovations include:
- Soft-switching push-pull converter for reduced losses.
- Dual-loop control for precise voltage regulation.
- Optimized thermal design using forced air cooling.
Future work will explore bidirectional power flow and digital control implementation to enhance flexibility.
This study underscores the potential of energy storage inverter in enabling clean, portable power solutions, aligning with global decarbonization goals.