I apologize for asking the same question twice. I'm just not very good at this stuff. It took me awhile to grasp the concept that it would take a different voltage to turn the top on than the bottom. I want to avoid the complication of driver circuits or charge pumps (mostly for size). Does the IRL5602 look like a suitable p-channel for an upper stage? Thanks for the help! -----Original Message----- From: pic microcontroller discussion list [mailto:PICLIST@MITVMA.MIT.EDU]On Behalf Of Dan Michaels Sent: Friday, July 20, 2001 9:16 AM To: PICLIST@MITVMA.MIT.EDU Subject: Re: [EE]: IRL540 MOSFET Question Phil, as you see below, I already told him he does not have enuf voltage to turn the upper MOSFET on when stacking 2 n-channels, but ???????????? Eisermann, Phil wrote: >Remember that gate voltage of the upper FET has to be +Vt (+5) above the >source voltage. And the on-state resistance increases with decreasing gate >voltage (or decreases with increasing gate voltage). If the upper FET is >just barely on, then Rds will be relatively high, resulting in increased >power dissipation. > >I didn't follow this thread. Are you using heatsinks? PWM? Do you only have >5V available? I would probably use a bjt level shifter to drive the gate at >10V or higher (charge pump would work well enough for this), or go to >p-channel fets for the high-side switches. Alternatively, International >Rectifier and others make a variety of H-bridge driver chips that will >generate the voltage for you. > >I'd also be highly tempted to use a FET with a lower Rds rating, and drive >it with the maximum voltage possible. Look at the datasheet, and find where >it shows a graph of Ids vs. Vds for varying levels of Vgs (gate voltage). >Even though the FET will turn on with 5V and is rated for 36A, it's not >going to conduct 36V with 5V of gate drive. I'd estimate that each FET is >dropping well over 2V with your 5V gate drive. Don't use the curves for >Tj=25C, because the junction temperature will never be 25C under use, >especially if you've got 30A going through the device. Not unless you do >some heavy-duty cooling (and i don't mean just a heatsink), anyway. > >> -----Original Message----- >> From: Hartung, Greg [mailto:ghartung@ADT.COM] >> Sent: Thursday, July 19, 2001 5:13 PM >> To: PICLIST@MITVMA.MIT.EDU >> Subject: Re: [EE]: IRL540 MOSFET Question >> >> >> Well, I forgot to mention I am using the IRL540N, which is >> the logic >> level version. Anyway, I tore the circuit apart last night >> and started with >> the basics. Would 5v turn the motor on thru a single 540n? >> Yes, but only >> if the FET was sinking the motor, not sourcing it. >> Attempting to source >> current for the motor resulted in IRL meltdown. So in a >> cascaded scenario >> with the motor between source and drain, only one of the FETs >> works. Vmotor >> is a 7.2v RC battery. Motors will draw about 20-30A. >> heeeeeeeeeeeeeeeeeeeeeelp >> >> -----Original Message----- >> From: pic microcontroller discussion list >> [mailto:PICLIST@MITVMA.MIT.EDU]On Behalf Of Dan Michaels >> Sent: Wednesday, July 18, 2001 2:05 PM >> To: PICLIST@MITVMA.MIT.EDU >> Subject: Re: [EE]: IRL540 MOSFET Question >> >> >> Hartung, Greg wrote: >> > I have been trying to a get 4 of these working in an >> H-bridge also, but >> >perhaps I've been lead astray. I was lead to believe that >> the gate voltage >> >needed to be 5v+Vds. Is 5v enough to turn a 540 on completely? >> > >> >> Greg, >> >> From what I can tell from the datasheet, the answer is yes. The Vgs >> threshold is 4v max, so if you tie the 540 source to gnd, >> then 5v out of >> a PIC should do it. However, if you are also using a 540 for the upper >> transistor in the bridge, then you will not be able to turn >> it on without >> a larger voltage. >> >> Better probably to have p-chan MOSFETs in the upper level, >> with "source" >> wired to Vpower - ie, mirror image to n-chan MOSFET below - and both >> gates wired together. Then the same 5v in will turn the upper >> one off, and >> the n-chan in the lower position on - and vice versa for 0v >> in. This is >> exactly how the output stage in any CMOS chip works - here the >> PMOSFET/upper - NMOSFET/lower h-bridge is basically emulating same. >> >> Note - this will only work for Vpower = 5v. If you want to run the >> h-bridge at higher voltage, then you will need a level-shifter - >> std npn inverter with collector pullup to Vpower and collector >> also tied to the MOSFET gates should work, I think. >> >> Also, you might look into using "logic-level" MOSFETs, with >> Vgs threshold only ~2v, rather than 4v. >> >> cheers, >> - dan michaels >> www.oricomtech.com >> ========================= >> >> -- >> http://www.piclist.com hint: The PICList is archived three different >> ways. See http://www.piclist.com/#archives for details. >> >> -- >> http://www.piclist.com#nomail Going offline? 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