The internal structure of MOSFETs and IGBTs differs significantly, which directly influences their respective application fields. One key distinction is that MOSFETs are typically capable of handling large currents, often reaching the kiloampere range, but they generally have lower voltage blocking capabilities compared to IGBTs. On the other hand, IGBTs are designed for high-power applications where both current and voltage levels are substantial. While IGBTs can operate at frequencies up to 100 kHz, they lag behind MOSFETs in terms of switching speed, which can reach hundreds of kHz, even MHz or higher in RF applications.
In terms of application, MOSFETs are commonly used in high-frequency power supplies such as switching power supplies, ballasts, induction heating systems, and communication power supplies. IGBTs, on the other hand, are more suited for industrial applications like welding machines, inverters, and electroplating power supplies. The performance of these devices heavily depends on the choice of power semiconductor components, including switching transistors and rectifiers.
Although there is no one-size-fits-all solution when choosing between IGBTs and MOSFETs, analyzing their performance in specific SMPS applications can help identify the optimal parameters. This article explores several key factors, such as switching losses in both hard-switching and soft-switching ZVS (Zero-Voltage Switching) topologies. It also discusses the three main types of power switching losses—conduction loss, turn-on loss, and turn-off loss—and how they relate to circuit and device characteristics.
Additionally, the article explains how the recovery characteristics of diodes play a critical role in determining conduction switching losses in both MOSFETs and IGBTs. It highlights how the performance of the diode affects the efficiency of hard-switching topologies. While the conduction characteristics of IGBTs and power MOSFETs are quite similar, IGBTs tend to have longer voltage drop times due to the minority carrier effects in the PNP BJT section of their structure.
The Eon energy consumption listed in IGBT datasheets represents the integral of the product of the collector current and VCE over each switching cycle. It includes losses related to saturation and is further divided into Eon1 and Eon2. Eon1 accounts for losses not associated with diode recovery, while Eon2 includes those related to the diode’s recovery during hard switching. Testing Eon2 involves measuring the energy dissipated during diode recovery, typically using a dedicated test circuit.
In hard-switching scenarios, the gate drive voltage, impedance, and the recovery characteristics of the rectifier diode all influence Eon switching loss. For example, in a conventional CCM boost PFC circuit, the diode's reverse recovery time (Trr) and charge (QRR) are crucial in controlling Eon energy. Choosing a diode with minimal Trr and QRR, along with soft recovery characteristics, helps reduce electrical noise and voltage spikes caused by rapid current changes.
For hard-switching applications, the body diode recovery of MOSFETs can be a limiting factor, especially in high-frequency SMPS designs. In contrast, IGBT packages often include matched diodes optimized for specific applications, allowing for better performance in high-speed SMPS systems. Adjusting the gate drive impedance can also help manage Eon loss, although this must be balanced against potential increases in EMI.
Conduction losses are another important consideration. At higher junction temperatures, IGBTs generally exhibit lower conduction losses than MOSFETs of similar size. However, this comparison should be made under identical current densities and worst-case operating conditions. For example, at 125°C, an IGBT may outperform a MOSFET above a certain current threshold.
Turn-off losses are also significant, particularly in IGBTs, where the tail current caused by minority carriers leads to higher energy dissipation. In contrast, MOSFETs typically have lower turn-off losses due to their faster switching behavior. Techniques like ZVS and ZCS topologies can help reduce these losses, though their effectiveness varies depending on the device type.
Ultimately, selecting the right power switch requires careful consideration of factors such as circuit topology, operating frequency, ambient temperature, and physical constraints. There is no universal solution, and the best choice often depends on the specific requirements of the application. Both MOSFETs and IGBTs have their strengths and weaknesses, and the decision should be made based on a thorough evaluation of the system's needs.
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