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Various types of digital devices can be connected, such as: Sensors, limit switches, buttons, valves, relays, light curtains and emergency stop switches, etc., and send signals to higher-level controls such as PC, PLC, CNC or DCS system. In general, AS-I does not require shielded cables and terminal resistors.
The unshielded two-wire cable used carries both the signal and the power. Generally speaking, the AS-I bus system is simple and reliable, easy to install and fast to configure, and is widely used in industrial automation fields such as logistics, automobile production, petrochemical industry, and elevator control.
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Discuss the operating characteristics of MOSFET switching transistors in zero-voltage switching (ZVS) converters
In recent years, the demand for energy-efficient and high-power systems in the switching power supply market has been steadily rising. As a result, designers are increasingly focusing on converter topologies that minimize power losses. One such popular topology is the PWM Phase Shift Control Full-Bridge Converter, which enables high energy efficiency at high power levels through soft-switching techniques. This paper aims to explore the operational characteristics of MOSFET switching transistors in zero-voltage switching (ZVS) converters, with an emphasis on their behavior under various load conditions.
Market applications for zero-voltage switching phase-shift converters include telecom equipment power supplies, mainframes, servers, and other electronic devices that require both high power density and energy efficiency. To meet these requirements, it is essential to reduce power loss and reactive power. Increasing the switching frequency can be an effective strategy, but it also leads to higher switching losses, which conflicts with the goal of improving efficiency. A more viable solution is to implement ZVS or zero-current switching (ZCS) converter topologies. These methods ensure that switches turn on under zero voltage or current conditions, thereby minimizing switching losses. Among these, ZVS is particularly advantageous as it allows the switch to conduct when the voltage across it is zero, eliminating the overlap between current and voltage waveforms that cause power loss.
ZVS offers several benefits, including linear control, constant frequency operation, integration of stray capacitance or resistance, and reduced electromagnetic interference (EMI). However, it also presents challenges, such as complex phase shift controller designs, rectifier oscillation, frequency overshoot, and increased switching losses under light loads. Recent advancements in integrated controllers have simplified the design of phase shift controllers, while the selection of specialized MOSFETs can help mitigate issues like light-load switch losses. Certain electrical properties of MOSFETs contribute to system reliability, and this article highlights the most likely failure scenarios based on switching sequences.
The basic phase-shifted circuit consists of four switching transistors arranged in two bridge arms. The leading bridge arm switches first, followed by the lagging bridge arm, ensuring that one set of switches turns off before the other turns on. This sequence is critical for achieving ZVS. As shown in Figure 1, Q1 and Q2 represent the leading bridge arm, while Q3 and Q4 form the lagging bridge arm. The output power is controlled by adjusting the phase shift time—longer shifts for lower power and shorter shifts for higher power. This method ensures precise control over the switching process.
Figure 2 illustrates the commutation sequence, where the lagging bridge arm switches only after the leading bridge arm has fully turned off. This results in a freewheeling period for the leading switches, while the lagging switches do not experience this phase. The switch sequence table (Table 1) further clarifies the timing of each transistor’s state change.
Figure 3 shows the typical voltage and current waveforms of a phase-shifted ZVS full-bridge DC-DC converter. Notably, the Q4 current signal comprises two components: one flowing through the body diode and the other through the MOSFET channel. As the transformer voltage polarity changes, the current direction reverses, and the switch transitions at zero voltage. This process reduces switching losses significantly. However, the reverse recovery time of the body diode plays a crucial role in determining the performance of the MOSFET. If the recovery time is too long, it may lead to excessive stress and potential failure.
As discussed, during the ZVS transition, the internal body diode of the MOSFET Q4 participates in the switching process, and its conduction time varies depending on the load. At heavy loads, the conduction time is short, while at light loads, it increases. This variation can affect the reliability of the switch, especially under low-load conditions. Figure 4 and Figure 5 illustrate the typical waveforms at heavy and light loads, respectively. Under light load conditions, the minority carrier recombination time may be insufficient, increasing the risk of failure.
Figure 6 demonstrates the impact of different reverse recovery times on the switching performance. Simulations show that if the recovery time is inadequate, the MOSFET may fail due to excessive stress. Therefore, selecting MOSFETs with fast reverse recovery times (trr) and low charge (Qrr) is essential for reliable operation. Several semiconductor technologies have been developed to address these issues, offering improved performance for high-frequency ZVS full-bridge applications.
Finally, this paper concludes that understanding the switching behavior of MOSFETs in ZVS converters is vital for designing robust and efficient power systems. By analyzing the switching sequence and identifying the most vulnerable points in the circuit, designers can make informed decisions about component selection. Choosing the right MOSFETs, particularly for the leading bridge arm, can significantly enhance system reliability and reduce the likelihood of failure. In summary, careful attention to the operating characteristics of MOSFETs in ZVS converters is essential for achieving a stable and efficient power supply design.