52 An IGBT combines the strengths of a BJT and a MOSFET into a single device. The input is essentially a voltage-controlled MOSFET gate with high input impedance. The output stage of the BJT portion of the design offers very high-power gain and output current flow. The most common IGBT styles are the original punch-through (PT) topology, with the non-punch-through (NPT) version more suitable for hard switching applications. More recently, developers have introduced technology enhancements such as field-stop (FS), trench gates and integrated freewheeling diodes (FRD), built into the structure to provide forward and reverse bias operation in the same package. The IGBT operates in a similar way to the MOSFET, where applying the ‘on’ signal to the gate of the n-channel MOSFET introduces a conduction state. Current then flows from the emitter to the base of the PNP transistor at the second stage of the IGBT. The base current reduces the ‘on’ resistance of the MOSFET. Applying a positive voltage from the emitter to the gate terminal causes electrons to flow towards the gate terminal. Once the voltage reaches or exceeds the threshold voltage, electrons will flow towards the gate to form a conductive channel that allows current to flow from the collector to the emitter. As electrons flow from the emitter to the collector, positive ions from the substrate are attracted to the drift region towards the emitter. The IGBT is frequently used in inverters with a voltage of 600 V and is often a better choice in high-frequency applications. MOSFET switching frequency is limited by the travel of electrons across the drift region, and the time required to charge the input gate and Miller capacitances. The IGBT has other advantages over the power MOSFET and BJT. It has a very low on-state voltage drop and better current density in the ‘on’ state. This allows for a smaller die size, with the possibility of more economical manufacturing costs. Driving IGBTs is simple and requires low power. It is easier to control the IGBT voltage-driven input in high-voltage and high-current applications compared with the current-controlled BJT. The conduction, forward-blocking and reverse-blocking capabilities of the IGBT are superior to the BJT. The MOSFET’s small voltage drop has an advantage in low-current applications, whereas the IGBT is better than the MOSFET in high-current applications. Low-voltage MOSFETs have a much lower ‘on’ resistance than IGBTs. These factors make MOSFETs ideal for switching power supplies and other applications that operate at about 100 kHz and at a low current density. Conversely, IGBTs are superior solutions in inverters that operate under 20 kHz with a high current density as it is easier to drive the IGBT at lower operating frequencies since the input capacitance is approximately 10% of that of a MOSFET with similar ratings. Along with these operational differences is the element of efficiency. However, the forward characteristic of the MOSFET is strongly dependent on temperature. Power MOSFETs have junction temperature limitations, and close attention should be paid to the maximum junction temperature specified in the data sheet. MOSFETs generally require a heat sink nearby to dissipate heat. This introduces an added expense, and requires extra space and additional design components. IGBTs are much more efficient thermally and do not require heat sinks. However, IGBTs can be damaged by extended power pulses and heattransfer conditions. If a hotspot is formed by excessive current concentration in the gate area, the cells within and surrounding the hotspot Deep insight | Power semiconductors March/April 2024 | E-Mobility Engineering Gate drivers are key for power semiconductors (Image courtesy of Semikron) Leadless packages boost thermal performance (Image courtesy of Infineon Technologies)
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