A brief analysis of the techniques to make IGBT faster

　　IGBT has large turn-off loss; tailing is a serious obstacle to high-frequency application. This problem consists of two aspects:

　　1) The problem of charge storage in the base region of GTR, the dominant device of IGBT.

　　2) Gate parasitic resistance and gate drive charge; they form an RC delay network, causing the IGBT to delay turning on and off.

　　Here, we first discuss the solution to reason 1. See Figure (1) for the solution circuit.

Figure (1)

　　The GTR of IGBT uses the base N-type semiconductor. When turned on, it is converted to P-type by applying base current, and the original PNP-type barrier area is changed to PPP path. To ensure reliable conduction, GTR is in full saturation mode of over-opening.

　　The so-called tailing caused by the base storage effect is due to the over-saturation of GTR, and the excessive conversion of the base N to P type. When turned off, since the P-type semiconductor needs to recombine the cost characteristics and even the N-type, this process causes the tailing of the device.

Figure (2)

　　This circuit uses a quasi-saturation drive mode to make the IGBT work in a quasi-saturation mode. When the IGBT enters saturation beforehand, the driving voltage will be pulled down by DC to make it exit the saturation state. On the contrary, the IGBT driving voltage rises, and VCE drops, approaching saturation. For standard IGBTs, this circuit can ensure that the on-state voltage drop of the IGBT is basically maintained at the 3.5V level, that is, the IGBT works in the quasi-linear region. In this way, the base of the GTR of the IGBT will not be overdriven, and there is almost no recombination process when it is turned off. In this way, the tailing problem of the device is almost solved! Now, the only problem is that the on-state voltage drop of the IGBT is slightly higher.

　　This method has become popular in logic ICs. All current ultra-high-speed logic circuits have this structure, including the CPU in your computer! Haha! You have already enjoyed this principle, but in fact, you don’t know it!