A
[email protected]
- Jan 1, 1970
- 0
I am using a P-channel mosfet to act as a voltage controlled resistor
to soft-start a small motor, which draws a couple amps while running
with load. My circuit quickly (<1ms) gets the mosfet gate voltage to
the point where the mosfet is passing a milliamp or so, and then it
very slowly increases Vgs to turn the mosfet on over the next 500ms.
The circuit works okay, but it expects a certain relationship of a
given change in voltage resulting in a given change in current through
the mosfet, once the mosfet is "on" (1ma or so). It doesnt really
matter if the "beginning to turn on" voltage of the mosfet changes
(i.e. via temperature or mfg batch), my circuit handles that, but it
does matter if when the mosfet is on, (a few mA passing through it),
that its "voltage to current gain" changes. For instance, if it
tripled, then my soft start would effectively ramp up three times
faster, which could cause problems, and if it went down by a factor of
three, then my soft start would take three times longer, which is also
undesirable.
So my question is, how predictable is the change in voltage to change
in current relationship for a given p channel mosfet? Temperature and
manufacturing spread are the biggest concerns here I think, if at all
(hopefully not!).
The mosfet in question is the SUB65P04 (datasheet on mouser)
to soft-start a small motor, which draws a couple amps while running
with load. My circuit quickly (<1ms) gets the mosfet gate voltage to
the point where the mosfet is passing a milliamp or so, and then it
very slowly increases Vgs to turn the mosfet on over the next 500ms.
The circuit works okay, but it expects a certain relationship of a
given change in voltage resulting in a given change in current through
the mosfet, once the mosfet is "on" (1ma or so). It doesnt really
matter if the "beginning to turn on" voltage of the mosfet changes
(i.e. via temperature or mfg batch), my circuit handles that, but it
does matter if when the mosfet is on, (a few mA passing through it),
that its "voltage to current gain" changes. For instance, if it
tripled, then my soft start would effectively ramp up three times
faster, which could cause problems, and if it went down by a factor of
three, then my soft start would take three times longer, which is also
undesirable.
So my question is, how predictable is the change in voltage to change
in current relationship for a given p channel mosfet? Temperature and
manufacturing spread are the biggest concerns here I think, if at all
(hopefully not!).
The mosfet in question is the SUB65P04 (datasheet on mouser)