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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 on the rise. As a result, designers are increasingly focusing on converter topologies that minimize power losses. Among these, the PWM Phase Shift Control Full-Bridge Converter has gained popularity due to its ability to achieve high energy efficiency at high power levels. This paper explores the operating characteristics of MOSFET switching transistors within zero-voltage switching (ZVS) converters.
Zero-voltage switching phase-shift converters are widely used in applications such as telecom equipment power supplies, mainframes, servers, and other electronic devices that require both high power density and energy efficiency. To meet these demands, it is essential to reduce power loss and reactive power. Increasing the switching frequency is one approach, but this can lead to higher switching losses, which contradicts the goal of improving efficiency. A more effective solution is to use ZVS or zero-current switching (ZCS) converter topologies. These ensure that the switch turns on when the voltage across it is zero, thus eliminating the overlap between current and voltage waveforms that cause power loss.
ZVS offers several advantages, including linear control with constant frequency, integration of stray capacitance or resistance in the power circuit, and reduced electromagnetic interference (EMI). However, there are challenges, such as the complexity of phase shift controller design, rectifier oscillation, overshoot, and soft switching losses under light load conditions. Fortunately, the introduction of integrated controllers has simplified the design process, and the selection of dedicated switch transistors helps address issues like light load power consumption.
Certain electrical characteristics of MOSFETs make them ideal for such applications, as they help reduce the probability of failure. This article focuses on the switch sequence that is most likely to fail, providing insights into how to optimize performance and reliability.
The basic phase-shifted circuit consists of four switching transistors, with two in each bridge arm. Due to the working mode, the two bridge arms do not switch simultaneously; one always switches before the other. The first to switch is known as the "leading bridge arm," while the second is called the "lag bridge arm." For example, Q1 and Q2 represent the leading bridge arm, and Q3 and Q4 represent the lag bridge arm.
The output power is controlled by adjusting the phase shift time. When the output power is high, the phase shift time is shorter, and when it is low, the phase shift time is longer. This allows for precise control of the switching phase.
Figure 3 shows the current and voltage waveforms of a typical phase-shifted ZVS converter. Notably, the Q4 current signal consists of two parts: one flowing through the MOSFET’s channel and body diode, and the other through the internal channel. As the transformer voltage polarity changes, the current direction reverses. The leading bridge arm switch Q2 uses this sequence to transition at zero crossing, ensuring zero-voltage switching operation.
It is important to pay attention to the Q4 switch tube's current signal, especially during current reversal. Since the current has two components, the reverse recovery time (trr) of the body diode is shorter than the typical test time, making it critical to choose a MOSFET with fast reverse recovery speed.
Potential failures can occur due to the conduction of the body diode during switching. At heavy loads, the conduction time is short, but at light loads, it becomes longer. If the recovery time is insufficient, minority carriers may not fully recombine, increasing the risk of failure.
To mitigate this risk, MOSFETs with smaller trr and Qrr values should be selected. Several semiconductor technologies have been developed to address fault modes in ZVS topologies. MOSFETs with short reverse recovery times and strong dv/dt tolerance are suitable for high-frequency ZVS full-bridge applications, enhancing system reliability.
This paper discusses the operating characteristics of MOSFETs in ZVS converters, highlighting key conditions where faults may occur and identifying the most vulnerable areas within the topology. By analyzing the switching sequence, we divide the topology into two parts: the leading and lagging bridge arms. We also propose guidelines for selecting appropriate switch transistors, emphasizing the importance of trr and Qrr in the leading bridge arm. Choosing the right components improves system reliability, reduces the likelihood of failure, and supports a robust and dependable design.