Electronics > Projects, Designs, and Technical Stuff
Driving P-Channel Mosfet
girishv:
--- Quote from: T3sl4co1l on June 26, 2020, 06:36:04 pm ---- What is the power source? If this is automotive power, mind that the supply can be quite messy. I wouldn't recommend taking gate voltage directly from it. This can be fixed easily by putting a zener diode from gate to source, say 1N5245 or equivalent. If the supply voltage jumps up suddenly, the diode clamps Vgs, and the remainder is dropped across the BJT -- which since it's limited to 40mA or so, is fine, it won't explode (at least, not instantly).
- Also if Vds(max) is pretty low, that's a concern. I wouldn't recommend less than 30V rating for automotive; >= 60V is industry standard.
Tim
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Protection: I will use a diode as suggested. I will also ensure diode, bypass capacitor and transistor are positioned close in layout.
Motor: The motor is brushed DC motor and consumes around 850mA. The +12V supply will come from a AC-DC regulator. The logic level is 3.3V. I am going to use a MOSFET with continuous drain current rating of 7A or more.
The VDS of transistor is 30V.
I want to keep the drive circuit as simple as possible. But, I am ready to change if need.
girishv:
--- Quote from: T3sl4co1l on June 26, 2020, 06:36:04 pm ---Turn-off is the bigger concern. Here you have no current from the transistor, only the resistor discharging it. We draw the equivalent RC circuit, 550pF in parallel with 10k, initially charged to ~10V. This has an RC time constant of 5.5us, which is quite long, quite a bit longer than the turn-on case. A smaller resistance may be desirable (say 2.2k or 1k). A better drive circuit isn't really necessary, but is an option.
5kHz is 200us per cycle, so you should want switching times much less than that, say, below 2us.
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I will replace the 10k resistor with 1k as suggested
girishv:
--- Quote from: langwadt on June 26, 2020, 06:39:44 pm ---why high-side drive?
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I used an existing example circuit driving a P-Channel MOSFET.
langwadt:
--- Quote from: girishv on June 27, 2020, 01:09:30 am ---
--- Quote from: langwadt on June 26, 2020, 06:39:44 pm ---why high-side drive?
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I used an existing example circuit driving a P-Channel MOSFET.
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much simpler to use an N-channel on the lowside
T3sl4co1l:
If the load does not need to be common-ground, yeah, a logic-level FET can do that without the resistor(s) and BJT.
--- Quote from: aheid on June 26, 2020, 10:43:26 pm ---For the sake of curiosity, was it something like this you meant? Ignore the load specs, I don't know motors.
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Yes, that's a good improvement, and you can save even more on the pullup resistor, for example even a 100k gives about the same results as a 1k without the follower, but at a tiny fraction of the on-state quiescent current. (Not that that's a big concern with an 850mA load already running. Pick your battles, and all.)
One downside: the gate doesn't pull up to +V, but to about 0.6V less; this makes logic-level FETs iffy (which can have Vgs(th) of (-)0.8V or smaller). It doesn't sound like the present case uses one so that's fine, but it's something to watch out for otherwise.
Tim
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