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
> =========================
>
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