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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