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Old 01-20-2011, 02:07 PM   #4250 (permalink)
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Originally Posted by MPaulHolmes View Post
Technically I'm over the limit of the total capacitance rating for the mic4451 driver with 10 of the irfp4668s, but it works with 22 Ohm gate resistors, so I was just trying to get something that works. According to the datasheet, I should only drive about 2.7 of those gigamos mosfets with a single mic4451, so I wasn't sure if they would be so forgiving like the mic4451 with the irfp4668. The mosfets are from different companies, and it's hard to know the way they both rate their parts.
My reading of the MIC4451/TC4451 datasheet doesn't show that as being over the limit. The limit is not the capacitance driven. It is really the instantaneous and average power dissipation in the device. Attaching a big capacitor without a resistor means all of the power is burnt inside the gate driver package. It doesn't matter how good the gate driver is -- all of the heat stays there. Using external resistors quickly shifts most of the heat there, and using a lower resistance gate driver starts paying off.

We bought a 4451 in the TO-220-5 package thinking that we could switch more/better/faster without being concerned about heat. And have it conveniently mounted on the same heatsink as the power devices. But at 20KHz with 10 ohm gate resistors we shouldn't be at the limits of the PDIP package. (OK, it was really that having Vs on the tab was more of a hassle than we wanted to deal with, and there wasn't enough motivation to use the IXYS gate driver that has a different pinout.)
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