In recent years, the development of power electronics has been significantly driven by advancements in wide-bandgap semiconductors, such as gallium nitride (GaN) high-electron-mobility transistors (HEMTs). These devices offer superior characteristics, including fast switching speeds, low on-resistance, and reduced parasitic capacitances, making them ideal for high-frequency applications. The single phase inverter is a critical component in various systems, including renewable energy integration, uninterruptible power supplies, and motor drives. However, traditional silicon-based inverters face limitations in efficiency and power density due to the inherent constraints of silicon devices. To address these challenges, this research explores the implementation of triangular current mode (TCM) modulation in a single phase inverter utilizing GaN HEMTs. The primary objective is to achieve zero-voltage switching (ZVS) across the entire operating range, thereby minimizing switching losses and enabling higher switching frequencies. This approach enhances both the efficiency and power density of the single phase inverter, which is essential for modern compact and high-performance applications.
The single phase inverter topology considered in this study consists of a full-bridge configuration with GaN HEMTs, as illustrated in the following diagram. This structure includes a DC input voltage source, supporting capacitors, four power switches, output filter inductors, a filter capacitor, and a load. The use of GaN devices allows for operation at elevated frequencies, which reduces the size of passive components and improves power density. However, increasing the switching frequency typically leads to higher switching losses, which can offset the benefits. Therefore, soft-switching techniques like TCM are employed to mitigate these losses. TCM modulation controls the inductor current to create resonant transitions, enabling ZVS without additional auxiliary circuits. This method simplifies the hardware design and control implementation while maximizing performance.
To understand the operational principles of TCM modulation in a single phase inverter, it is essential to analyze the half-bridge circuit, which forms the fundamental building block. The half-bridge consists of two GaN HEMTs (VT1 and VT2), an output filter inductor (L), and the load. The TCM modulation involves six distinct operational modes, each characterized by specific current and voltage behaviors. The inductor current is controlled to follow a triangular waveform, ensuring that it reaches a negative value before switching, which facilitates ZVS. The key equations governing these modes are derived to provide insights into the design and control parameters.
In Mode 1, during the interval [t0, t1], VT1 is turned on, and VT2 is off. The voltage across the inductor is given by the difference between the DC input voltage and the output voltage, leading to a linear increase in inductor current. The rate of change of current is expressed as:
$$ i_L(t) = \frac{U_{dc} – U_o(t)}{L} (t – t_0) + i_L(t_0) $$
This phase continues until the current reaches a peak value, at which point VT1 is turned off. In Mode 2, [t1, t2], both switches are off, and the parasitic capacitances of the switches (Coss1 and Coss2) resonate with the inductor. This resonance charges Coss1 and discharges Coss2 until the voltage across Coss1 reaches Udc and Coss2 reaches zero, allowing the body diode of VT2 to conduct. The resonant behavior is critical for achieving ZVS, as it ensures that the switch turns on under zero-voltage conditions.
Mode 3, [t2, t3], begins when VT2 is turned on, and the inductor current decreases linearly due to the negative output voltage applied across it. The current dynamics are described by:
$$ i_L(t) = \frac{-U_o(t)}{L} (t – t_2) + i_L(t_2) $$
As the current continues to decrease, it crosses zero and becomes negative in Mode 4, [t3, t4]. This negative current is essential for ZVS, as it provides the initial condition for the subsequent resonant transition. In Mode 5, [t4, t5], VT2 is turned off when the negative current reaches a predefined value, io. The resonant process recommences, with Coss1 discharging and Coss2 charging. The system equations during this phase are derived from Kirchhoff’s laws:
$$ L \frac{di_L(t)}{dt} + u_{s1}(t) = U_{dc} – U_o(t_4) $$
$$ C_{oss} \frac{d[u_{s1}(t) – U_{dc}]}{dt} = i_L(t) $$
Solving these equations yields the current and voltage expressions:
$$ i_L(t) = \frac{U_{dc} – U_o(t_4) – u_{s1}(t_4)}{Z_n} \sin(\omega_o t) + i_L(t_0) \cos(\omega_o t) $$
$$ u_{s1}(t) = -[U_{dc} – U_o(t_4) – u_{s1}(t_4)] \cos(\omega_o t) + i_L(t_0) Z_n \sin(\omega_o t) + U_{dc} – U_o(t_4) $$
where the characteristic impedance and angular frequency are defined as \( Z_n = \sqrt{\frac{L}{2C_{oss}}} \) and \( \omega_o = \frac{1}{\sqrt{2LC_{oss}}} \), respectively. The initial conditions at t4 are u_s1(t4) = Udc and i_L(t4) = i_o. By setting u_s1(t)_min = 0, the minimum initial current required for ZVS is derived as:
$$ i_{of} = \frac{\sqrt{U_{dc}^2 – 2U_o(t_4) U_{dc}}}{Z_n} $$
The resonant time duration, tC, is calculated as:
$$ t_C = \frac{\pi – \arcsin\left(-\frac{U_o(t_4) – U_{dc}}{\sqrt{U_o(t_4)^2 + \left(\frac{2C_{oss}\omega_n i_o}{2}\right)^2}}\right) – \arctan\left(\frac{i_o}{2C_{oss}\omega_n U_o(t_4)}\right)}{\omega_n} $$
Finally, in Mode 6, [t5, t6], the inductor current commutates through the body diode of VT1, and the current continues to decrease until the next switching cycle begins. The waveform of the inductor current in TCM modulation exhibits a triangular shape with resonant transitions, ensuring ZVS across all operating conditions. This modulation strategy allows the single phase inverter to operate at high frequencies with minimal losses, making it suitable for applications demanding high power density and efficiency.
To validate the theoretical analysis, an experimental prototype of the single phase inverter was developed. The key parameters of the prototype are summarized in Table 1. The setup includes GaN HEMTs, filter components, and a digital controller to implement the TCM modulation. The DC input voltage was set to 360 V, and the output was designed for 220 V AC at grid frequency. The filter inductors and capacitor were selected to minimize losses and ensure stable operation up to 500 kHz switching frequency.
| Parameter | Value |
|---|---|
| DC Input Voltage (Udc) | 360 V |
| Output Voltage (Ug) | 220 V AC |
| Maximum Switching Frequency (fs) | 500 kHz |
| Filter Inductors (L1, L2) | 40 μH |
| Parasitic Resistance (R1, R2) | 1 mΩ |
| Filter Capacitor (C) | 5 μF |
| Load Resistance | 165 Ω |
The experimental results demonstrate the effectiveness of TCM modulation in the single phase inverter. The switching frequency varied cyclically, with a maximum value of 300 kHz, as observed from the PWM waveforms. This variable frequency operation is a hallmark of TCM, enabling soft-switching across different load conditions. The output voltage and current waveforms were sinusoidal with low distortion, confirming the inverter’s ability to deliver high-quality power. The inductor current exhibited the characteristic triangular shape with negative segments, which is crucial for achieving ZVS.
The ZVS operation was verified through voltage and current measurements across the switches. The waveforms showed that the switch voltage dropped to zero before turn-on, indicating successful soft-switching. This significantly reduces switching losses and allows for higher efficiency. The efficiency of the single phase inverter was measured under various load conditions and compared with a conventional hard-switched inverter. The results, summarized in Table 2, indicate that the TCM-based inverter achieved a peak efficiency of 98.5%, with improvements particularly notable at light loads. This highlights the advantage of ZVS in enhancing overall performance.
| Load Condition | Efficiency with TCM (%) | Efficiency without ZVS (%) |
|---|---|---|
| Light Load (20% rated) | 97.8 | 94.2 |
| Medium Load (50% rated) | 98.3 | 96.5 |
| Full Load (100% rated) | 98.5 | 97.1 |
The implementation of TCM modulation in a single phase inverter based on GaN devices presents several design considerations. The control algorithm must accurately regulate the inductor current to maintain the triangular waveform and ensure the negative current required for ZVS. This involves real-time monitoring of current and voltage signals, which can be challenging at high frequencies. Digital signal processors (DSPs) or field-programmable gate arrays (FPGAs) are typically used to execute the control laws with high precision. Additionally, the parasitic elements of the circuit, such as the inductor’s equivalent series resistance and the switch capacitances, must be carefully modeled to optimize performance.
Another aspect to consider is the impact of load variations on the single phase inverter’s operation. Under light loads, the required negative current for ZVS is smaller, but the switching frequency may increase, leading to higher core losses in the inductors. Conversely, at heavy loads, the current amplitudes are larger, which can increase conduction losses. Therefore, a balanced design that accounts for these trade-offs is essential. The use of GaN HEMTs mitigates some of these issues due to their low conduction losses and fast switching capabilities. However, the layout and thermal management must be optimized to handle the high power densities.
In comparison to other soft-switching techniques, such as phase-shifted modulation or resonant converters, TCM offers a simpler implementation without additional components. For instance, in a single phase inverter, TCM relies on the inherent resonant behavior of the circuit elements, whereas other methods may require auxiliary switches or transformers. This reduces the cost and complexity, making TCM an attractive option for commercial applications. Moreover, the variable switching frequency in TCM can be managed through advanced control strategies, such as frequency clamping or adaptive dead-time control, to avoid excessive frequency variations that might interfere with electromagnetic compatibility (EMC) standards.
Future work on single phase inverters with TCM modulation could focus on improving the control dynamics and extending the operating range. For example, integrating model predictive control (MPC) or artificial intelligence (AI) techniques could enhance the response to transient conditions, such as sudden load changes or input voltage fluctuations. Additionally, the application of TCM in multi-level inverters or three-phase systems could be explored to scale up the power rating while maintaining high efficiency. The ongoing development of GaN technology, with higher voltage ratings and improved thermal characteristics, will further push the boundaries of what is achievable with single phase inverters.
In conclusion, the research demonstrates that TCM modulation effectively enables ZVS in a single phase inverter based on GaN HEMTs, leading to significant improvements in efficiency and power density. The experimental prototype achieved a peak efficiency of 98.5% and operated at switching frequencies up to 300 kHz, validating the theoretical analysis. The single phase inverter’s performance under various load conditions confirms the robustness of the TCM approach. This work contributes to the advancement of high-frequency power conversion systems, with potential applications in renewable energy, electric vehicles, and compact power supplies. The continued optimization of control strategies and device technologies will further enhance the capabilities of single phase inverters in meeting the demands of modern power electronics.