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Topic: SMD MOSFET to power down my SMD H-Bridge (Read 5 times) previous topic - next topic

SouthernAtHeart

Thanks dc42,
I really would like to understand this stuff better. So my scenario is #3, since my motor voltage is 6 volts and my Arduino voltage is 4.5 volts.
This photo is what I've found from another thread. Is this correct?  I've read that the pull up resistor on the MOSFET's gate is needed to turn it off. But the continuous current drain across that 10K resistor will defeat the purpose of attaining ultra low quiescent current.  My motor voltage is 6 volts, so across 10K, that'd be a 600 uA, no?

dc42

That circuit is OK, although personally I would reduce the value of the pullup resistor to +6V to 2k2 or 4K7, at the expense of slightly increased current consumption when the H-bridge is powered up. This resistor will only draw current when the transistor is turned on and the H-bridge is powered up, so it doesn't defeat the purpose of switching the H-bridge supply.

As you have a little under 6V gate drive available, you will need to use a logic level P-channel mosfet.
Formal verification of safety-critical software, software development, and electronic design and prototyping. See http://www.eschertech.com. Please do not ask for unpaid help via PM, use the forum.

SouthernAtHeart

Ok, I think I got it!  Here's a photo of what I've got, and the components.  Oh, I don't have it...  I just discovered the 2N3904  transistor doesnt come in SMD at Digikey. What would a good NPN transistor be to turn on/off my MOSFET?  I have some BSS138W-7-F in stock. Would they be a good choice?  If not ill be ordering from Digkey anyway.
and does the P channel mosfet look appropriate?

BSS1338 transistor: (I guess it's not a transistor, but an NPN MOSFET)
http://www.diodes.com/datasheets/ds30206.pdf


The P channel MOSFET (DMP2305U)
http://www.diodes.com/datasheets/ds31737.pdf

H-Bridge
http://www.vishay.com/docs/70007/si9986.pdf

dc42

Looks OK to me. The Vgs rating of that P-channel mosfet is only 8V, uncomfortablly close to the 6V drive it is getting, so it is probably better to spit the 2K2 resistor into 2 x 1K and drive the gate from the junction of the two. Alternatively, pick a mosfet with a higher Vgs rating but designed for 5V gate drive, such as TSM2313CX.

For the transistor, use either a bipolar NPN transistor such as BC817 and omit the 10K resistor to ground, or a small signal mosfet such as your BSS138W and omit the 10K series resistor..
Formal verification of safety-critical software, software development, and electronic design and prototyping. See http://www.eschertech.com. Please do not ask for unpaid help via PM, use the forum.

dc42

PS - don't forget to add some good decoupling capacitors between the source terminal of the P-channel mosfet and the ground connection of the H-bridge.
Formal verification of safety-critical software, software development, and electronic design and prototyping. See http://www.eschertech.com. Please do not ask for unpaid help via PM, use the forum.

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