MOSFETs versus IGBTs

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MOSFETs and IGBTs each have their own set of strengths and weaknesses when used in switching audio amplifiers.

MOSFETs only employ “majority carriers“.  For an N-channel MOSFET (the most commonly used due to the much higher mobility of electrons versus holes) the carriers are electrons.  This is a great advantage from a switching speed point of view, as the MOSFET may be turned off very quickly since there are no “minority carriers” to remove from the conducting channel, in order to return it to a non-conducting state.  However, the downside is that no “conductivity modulation” takes place – i.e. the presence of increasing numbers of minority carriers tends to decrease the effective resistance of the conducting channel, much as happens with diodes, or BJTs, or…

IGBTs utilize both majority and minority carriers.  From a switching speed point of view this is a disadvantage because the minority carriers must now be swept from the conducting channel before the IGBT can return to a non-conducting state.  The is the often referred to “current tail” of IGBTs.  Conversely, IGBTs do enjoy the benefits of conductivity modulation, so while MOSFETs are suffering Rds(on)·I² losses, the IGBT suffers only Vce(sat)·I losses.  Even better, the Vce(sat) of the IGBT tends to be roughly constant with temperature (sometimes even decreasing a bit), while the Rds(on) of a MOSFET can increase by up to 2.5 times with increasing temperature.

What does this mean for audio?  Given the high peak-to-average ratio of audio program material, the IGBT is a natural choice, since the losses are roughly proportional to current, instead of proportional to the square of current, as with a MOSFET.  The only problem is the higher switching frequency of switching amplifiers results in extra dissipation due to the minority charge of the IGBT.  This may not be an issue at all in the power supply portion of the switching amplifier, but in the amplifier section it is a little more tricky.  However, with a little ingenuity it is possible to combine the strengths of both devices in the amplifier section as well.  This promises to provide amazingly high efficiency in higher power switching amplifiers.

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