Unfortunately at 3.3V, good N-channel mosfets are hard to find. =20 Although I might try a 2n7000 in there with no gate resistor to see what=20 the output looks like. And yes, the problem is that Qinv just isn't turning off fast enough. I=20 played with base resistor values but didn't see a lot of difference... On 7/27/2011 11:18 PM, M. Adam Davis wrote: > Have you considered using an n channel mosfet for Qinv? > > Are you sure you're driving Qinv hard enough to get it to switch at > the speed you need? Hook a scope up to the input and output of Qinv > and verify that is the problem. > > -Adam > > On Wed, Jul 27, 2011 at 11:25 PM, Forrest Christian = wrote: >> I'm basically using a circuit based on the schematic from Figure 20 on >> page 20 of http://focus.ti.com/lit/ml/slup169/slup169.pdf to drive a >> high-side mosfet. Vin is around 30V. >> >> My application requires a fast (~1uS) turnoff of the FET. However, I'm >> seeing actual turnoff times much longer than that. The faster time seems >> definitely doable with reduction of Rgate to an appropriate level and a >> quick shift of the drive level into the totem pole drivers. With that in >> mind, I seem to be having problems where the level shifting transistor >> (Qinv) isn't changing state very fast. I suspect it's related to the >> issues described in the text - and specifically the saturation of the >> level shifting transistor. The text recommends moving a resistor around >> - but as the purpose of Qinv is to level shift the level from the 3.3V >> logic I'm working with up a level between 30V and 18V depending on which >> state it's in - putting half of the voltage divider below the transistor >> prevents the 3.3V from turning the transistor on. >> >> I'd like to find a circuit which will work for this which is fairly >> non-complicated. I'm not completely stuck on the P-channel device, but >> driving a N channel device seems even worse (even with a driver intended >> for this). Of course, there is every possibility I just haven't found >> the right driver yet. >> >> Ideas? Hints? >> >> -forrest >> -- >> http://www.piclist.com PIC/SX FAQ& list archive >> View/change your membership options at >> http://mailman.mit.edu/mailman/listinfo/piclist >> --=20 http://www.piclist.com PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist .