h bridge using n and p-mosfet

hi,
m using a H-bridge for 12v,1.6A motor in which upper two mosfet r p-
type n lower r n- type. i m using pic o/p for h bridge so using 4
transisters with 4 mosfets.pnp with gate of n mosfet and npn conncted
with gate of p-mosfet.i m connecting the gates of alternate opposite
mosfets ie one p type with opposite n type mosfet together.
my problem is that te both p type mosfet are always high irrespective
of the signal on gate(either high o low) and one of n type mosfet
sourc is not high when its getting hig signal on gate.
pls advice

 

[Jamie]

sneha.kathuria@gmail.com wrote:

hi,
m using a H-bridge for 12v,1.6A motor in which upper two mosfet r p-
type n lower r n- type. i m using pic o/p for h bridge so using 4
transisters with 4 mosfets.pnp with gate of n mosfet and npn conncted
with gate of p-mosfet.i m connecting the gates of alternate opposite
mosfets ie one p type with opposite n type mosfet together.
my problem is that te both p type mosfet are always high irrespective
of the signal on gate(either high o low) and one of n type mosfet
sourc is not high when its getting hig signal on gate.
pls advice

I couldn't quite follow your explanation but i'll give you what I know

that has to be done.
First of all
They need to be all the same types
Enhanced mode. This means they are normally off..

The P-Fets Source needs to be on the + rail, you need to apply
- or common to the gate to turn them on.
The N-Fets Source needs to be on the - Rail and you need + to
turn them on.
In all cases, the Drains lead to the Motor.

make sure the full turn on voltage of the Fets are well below your
rail voltages you have.
for example, lets say your rails are 5 Volts, you should have Fets
that turn on at 2.8.. 3 volts (fully on), Logic types.
If you need to run a separate supply for the FETS due to the need of
higher voltage, the use open collector drivers from the Pic with a pull
up resistor on each gate from its level of voltage it needs, this will
protect the PIC


With the respect for the P-channel fet, as long as the + Rail is higher
than the required - voltage gate requirement, the Gate will turn on just
by pulling it to common/-rail side.

Also, with respect to PIC's, I may have my uC's mixed up how ever, I
think there is a function in there to force the pic to turn on the
internal pull up's for the outputs, otherwise, you'll only get an open
collector effect which will drive only the P-channel..

P.S.
Make sure you have some dead time between direction changes to allow
for the gates to discharge. Some will actually apply the outputs in
reverse of each FET to quickly discharge it before reversing it. Sort of
a turn off point in the bridge.
So what ever code you use to turn off the bridge, you do so for a
minimum time before switching directions. Other wise, this will cause
the fets to heat up due to a short time of shorting..
Some use inductors from the Drain of each fet to absorb this how
ever, the use of inductors can be damaging if bad connections exist.
Make sure you have protection diodes in place.




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