> Would someone please tell me if it is generally OK to substitute a logic
level FET
> with similar characteristics as the original FET? (ie. IRLZ44E vs.
IRFZ44E).
>
> I am thinking of using IRL1404 instead of IRFZ44E in an h-bridge for 24V
motors.
> I am attracted to the .004 ohm RDSon vs .023 ohm RDSon.  The IRL1404 has
> a VDS of only 40V vs the 60V of the IRFZ44E. Any opinions ?


Probably fairly safe.

A few points to watch whose importance will depend on the application and
how close it is "to the edge" of the device's capabilities.

- The logic level FET not only has a lower operating gate voltage
requirement but also a lower absolute permissible gate voltage. This may be
as low as 10 volts for some devices. Check the spec sheet of the new device.
If the gate drive voltage is too high you will need to reduce it in some
way - possibly a zener clamp at the gate or some attention to the drive
circuit itself.

- Depending on the frequency of operation, gate drive could be an issue.
This would only be true in very extreme environments. Is this switching at
5, 50 or 500 KHz ? :-). At extreme switching rates check the gate
capacitance in each case - if similar (factor of 2 OK) then no problems. If
the new device has a MUCH higher gate capacitance then you may wish to check
gate drive circuitry. I met a replacement FET recently that had about 10
times as much gate capacitance. In unusual cases where the gate drive
circuitry has been designed to only provide enough average current for the
intended FET then a FET which needs rather higher drive requirements MAY
load the supply excessively (which may be fine for your logic FET). This is
unlikely to happen in most real cases.

- Check other parameters such as switching times. You can get yourself into
trouble with parts which work "too well" as well as "too slowly" if snubbers
have been designed as a close fit to the circuit.


- The Vds of 40 volts is doable but you really want to watch your switching
transients. motor noise etc. 16 volts headroom is not a lot..

- You don't say how much power you are handling. The factor of 5+ decrease
in Rdson is nice but its importance depends on the I^2 you have.
If you are in the under 10 amp range then you arguably only need a modest
heatsink with the original FETs. Much above this and the lower Rdson starts
to get useful. I see the IRFZ is a 50A device so if you are anywhere near
that then the lower resistance is useful. At 30A that's  about 20 watts
versus 4 watts FET dissipation.



regards



            Russell McMahon

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