In recent years, the development of traditional silicon-based power devices has approached its theoretical limits, with diminishing returns on research investments aimed at improving performance. This has prompted a shift towards novel semiconductor materials, such as gallium nitride (GaN) and silicon carbide (SiC), which offer superior properties for high-frequency and high-efficiency applications. GaN high-electron-mobility transistors (HEMTs) exhibit exceptional characteristics, including low on-resistance, high electron mobility, and reduced parasitic capacitances, making them ideal for power electronics systems requiring high power density and efficiency. Among various applications, the single-phase inverter is a critical component in renewable energy systems, uninterruptible power supplies, and motor drives. However, conventional inverters face challenges in achieving high switching frequencies without compromising efficiency due to increased switching losses. To address this, triangular current mode (TCM) modulation has emerged as a promising technique to enable zero-voltage switching (ZVS) across a wide operating range, thereby reducing losses and allowing for higher switching frequencies. This article explores the implementation of TCM control in a single-phase inverter utilizing GaN devices, providing a detailed analysis of operational modes, control strategies, and experimental validation. The goal is to demonstrate how TCM modulation can enhance the performance of single-phase inverters, leading to efficiencies exceeding 98% and switching frequencies up to 300 kHz, while maintaining compact form factors.
The topology of a single-phase inverter typically consists of a full-bridge configuration, as illustrated in the following diagram. This structure includes four switching devices, output filter inductors, and a capacitor to smooth the output waveform. In this study, GaN HEMTs are employed as the switching devices due to their fast switching capabilities and low conduction losses. The TCM modulation strategy focuses on controlling the inductor current to create resonant transitions that facilitate ZVS, eliminating the need for additional snubber circuits or complex control schemes. By operating in TCM, the single-phase inverter can achieve soft switching across all load conditions, which is essential for minimizing electromagnetic interference (EMI) and improving reliability. The following sections delve into the operational principles, mathematical modeling, and experimental results of this approach, highlighting its advantages over conventional hard-switched inverters.
The single-phase inverter topology under consideration is based on a standard H-bridge design, where the DC input voltage \( U_{dc} \) is converted to an AC output voltage \( U_o \) through controlled switching of the power devices. The output filter, comprising inductors \( L_1 \) and \( L_2 \) and capacitor \( C \), ensures a sinusoidal output current with low total harmonic distortion (THD). In TCM operation, the inductor current is shaped to follow a triangular waveform, which allows the inherent capacitances of the GaN devices to resonate with the filter inductances during switching transitions. This resonant behavior enables ZVS by ensuring that the voltage across a switching device reaches zero before it is turned on. The key to implementing TCM in a single-phase inverter lies in precisely controlling the negative current during the dead time periods, which varies with load conditions and input voltage. Mathematical analysis of the switching modes reveals the conditions required for ZVS, including the minimum negative current needed to discharge the output capacitances of the devices. For instance, the resonant frequency \( \omega_o \) and characteristic impedance \( Z_n \) are derived from the inductor and capacitor values, as shown in the equations below.
To understand the TCM modulation process, consider the half-bridge segment of the single-phase inverter, which consists of two switches (e.g., VT1 and VT2) and an output inductor \( L \). The operational modes can be divided into six distinct intervals within one switching cycle, as described below:
- Interval 1 [t0, t1]: Switch VT1 is turned on, and VT2 is off. The inductor current \( i_L(t) \) increases linearly with a slope determined by the difference between the input voltage and the output voltage. The current expression is given by:
$$ i_L(t) = \frac{U_{dc} – U_o(t)}{L} (t – t_0) + i_L(t_0) $$
This mode continues until the current reaches a peak value, at which point VT1 is turned off. - Interval 2 [t1, t2]: Both switches are off, and the resonant transition begins. The output capacitances \( C_{oss1} \) and \( C_{oss2} \) of the GaN devices form a resonant circuit with the inductor \( L \). During this period, \( C_{oss1} \) is charged to \( U_{dc} \), while \( C_{oss2} \) is discharged to zero. The resonant behavior allows the voltage across VT2 to fall to zero, enabling ZVS when it is turned on later. The state equations for this interval are:
$$ L \frac{di_L(t)}{dt} + u_{s1}(t) = U_{dc} – U_o(t_1) $$
$$ C_{oss} \frac{d[u_{s1}(t) – U_{dc}]}{dt} = i_L(t) $$
Solving these equations yields the current and voltage dynamics, which are critical for determining the ZVS conditions. - Interval 3 [t2, t3]: After the resonant transition, VT2 is turned on with ZVS. The inductor current decreases linearly due to the negative voltage applied across the inductor:
$$ i_L(t) = \frac{-U_o(t)}{L} (t – t_2) + i_L(t_2) $$
This mode persists until the current crosses zero and becomes negative. - Interval 4 [t3, t4]: The inductor current continues to decrease, becoming negative. This negative current is essential for achieving ZVS in the subsequent switching cycle. The magnitude of this current is controlled to ensure that it is sufficient to discharge the device capacitances during the next resonant transition.
- Interval 5 [t4, t5]: When the negative current reaches a predefined value \( i_o \), VT2 is turned off. Another resonant transition occurs, where \( C_{oss1} \) is discharged and \( C_{oss2} \) is charged. The equations governing this interval are:
$$ i_L(t) = \frac{-U_o(t_4)}{Z_n} \sin(\omega_o t) + i_o \cos(\omega_o t) $$
$$ u_{s1}(t) = U_o(t_4) \cos(\omega_o t) + i_o Z_n \sin(\omega_o t) + U_{dc} – U_o(t_4) $$
Here, \( Z_n = \sqrt{\frac{L}{2C_{oss}}} \) and \( \omega_o = \frac{1}{\sqrt{2LC_{oss}}} \). The minimum initial current \( i_{of} \) required for ZVS is derived as:
$$ i_{of} = \frac{\sqrt{U_{dc}^2 – 2U_o(t_4) U_{dc}}}{Z_n} $$
Additionally, the resonant time \( t_C \) is calculated to ensure proper timing for the switching commands. - Interval 6 [t5, t6]: The inductor current flows through the body diode of VT1, and the current magnitude decreases until the next cycle begins. This interval completes the switching cycle, and the process repeats for subsequent cycles.
The waveform of the inductor current in TCM modulation resembles a triangle, with positive and negative peaks that facilitate soft switching. The switching frequency varies with the load and input voltage to maintain ZVS, which is a key advantage of TCM over fixed-frequency methods. For the single-phase inverter, this variable frequency operation requires careful control to avoid excessive frequency variations that could impact the output power quality. The control strategy involves sensing the output current and adjusting the switching instants based on the calculated negative current thresholds. This ensures that the single-phase inverter operates efficiently across its entire load range.
To validate the theoretical analysis, an experimental prototype of the single-phase inverter was developed using GaN HEMTs. The key parameters of the prototype are summarized in Table 1. The design focuses on achieving high power density and efficiency, with a maximum switching frequency of 500 kHz and a target output of 220 V AC at 50 Hz. The filter components were selected to minimize losses and provide adequate harmonic attenuation. The control algorithm was implemented using a digital signal processor (DSP) to realize the TCM modulation in real-time.
| Parameter | Value |
|---|---|
| DC Input Voltage \( U_{dc} \) | 360 V |
| AC Output Voltage \( U_g \) | 220 V (50 Hz) |
| Maximum Switching Frequency \( f_s \) | 500 kHz |
| Filter Inductors \( L_1 \), \( L_2 \) | 40 μH |
| Parasitic Resistance of Inductors | 1 mΩ |
| Filter Capacitor \( C \) | 5 μF |
| Load Resistance | 165 Ω |
| Rated Power | 300 W |
The experimental results demonstrate the effectiveness of TCM modulation in achieving ZVS for the single-phase inverter. The switching frequency varied between 50 kHz and 300 kHz depending on the load conditions, with the highest frequency observed at light loads. This variability is characteristic of TCM and contributes to the reduction of switching losses. The output voltage and current waveforms were measured to be sinusoidal with low distortion, as shown in the captured data. The efficiency of the single-phase inverter was evaluated across different load levels and compared with a conventional hard-switched inverter. The results, summarized in Table 2, indicate a peak efficiency of 98.5% at full load, which is significantly higher than the 96% efficiency achieved with hard switching. This improvement is attributed to the elimination of switching losses through ZVS.
| Load Condition (% of Full Load) | TCM Efficiency (%) | Hard-Switching Efficiency (%) |
|---|---|---|
| 20% | 97.8 | 94.5 |
| 50% | 98.2 | 95.8 |
| 80% | 98.4 | 96.2 |
| 100% | 98.5 | 96.0 |
Further analysis of the ZVS achievement is provided through the switching waveforms, which show the voltage across a GaN device falling to zero before the gate signal is applied. This confirms the soft-switching operation and validates the theoretical models. The inductor current waveform exhibits the triangular shape characteristic of TCM, with the negative current portion being precisely controlled to ensure ZVS. The mathematical expressions for the resonant transitions were verified through these measurements, with the observed resonant times matching the calculated values within a 5% error margin. For instance, the resonant time \( t_C \) derived from Equation (8) was found to be approximately 150 ns in practice, which aligns with the theoretical prediction based on the parameters in Table 1.
In addition to efficiency, the power density of the single-phase inverter was assessed by comparing the volume of the prototype with that of a conventional design. The use of GaN devices and TCM modulation allowed for a 30% reduction in the size of the magnetic components, as the higher switching frequency enabled smaller inductors and capacitors. This makes the single-phase inverter suitable for space-constrained applications, such as electric vehicle onboard chargers or portable power systems. The thermal performance was also improved due to reduced losses, with the peak temperature rise measured at 15°C above ambient under full load, compared to 25°C for a hard-switched counterpart.
Despite the advantages, implementing TCM in a single-phase inverter presents challenges, such as the need for accurate current sensing and fast control loops. The variable switching frequency can also complicate EMI filter design, as the harmonic spectrum shifts with load variations. To address this, advanced control techniques, such as adaptive dead-time control and frequency stabilization algorithms, were incorporated into the DSP code. These measures ensured stable operation across the entire load range without compromising performance. The single-phase inverter prototype demonstrated robustness in various test scenarios, including step-load changes and input voltage fluctuations.
In conclusion, the application of TCM modulation to a single-phase inverter based on GaN devices offers significant benefits in terms of efficiency, power density, and reliability. By enabling ZVS across all operating conditions, the single-phase inverter achieves peak efficiencies of 98.5% and switching frequencies up to 300 kHz, which are difficult to attain with conventional methods. The experimental results validate the theoretical analysis and confirm the practicality of this approach for modern power electronics systems. Future work could focus on optimizing the control strategy for wider input voltage ranges and integrating the single-phase inverter with renewable energy sources to enhance overall system performance. The success of this research underscores the potential of GaN technology and advanced modulation techniques in advancing the state of the art in single-phase inverter design.
The mathematical foundation of TCM modulation can be extended to other converter topologies, such as three-phase inverters or bidirectional DC-DC converters, to further explore its capabilities. For example, the generalized equations for ZVS conditions in a single-phase inverter can be adapted to multi-level converters by considering the equivalent resonant networks. The key parameters, such as the characteristic impedance \( Z_n \) and resonant frequency \( \omega_o \), remain central to these analyses. Additionally, the efficiency improvements demonstrated in this study highlight the importance of soft-switching techniques in reducing energy losses and mitigating thermal management challenges. As power electronics continue to evolve, the integration of wide-bandgap devices like GaN with innovative control strategies like TCM will play a crucial role in meeting the demands for higher performance and compactness in single-phase inverter applications.
In summary, this research provides a comprehensive framework for designing and implementing TCM-controlled single-phase inverters using GaN HEMTs. The detailed modal analysis, experimental validation, and performance comparisons establish a solid foundation for future developments in high-frequency power conversion. The single-phase inverter prototype serves as a benchmark for achieving high efficiency and power density, paving the way for broader adoption in industrial and consumer electronics. The insights gained from this work can guide engineers in optimizing single-phase inverter systems for specific applications, ensuring that they meet the ever-increasing requirements for energy efficiency and miniaturization.