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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