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PWM DC Mosfet Selection/Circuit...

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adamey

Member
I already have a 5V logic PWM signal available to control a DC motor. What I need is the basic circuit (MOSFET and related parts) to operate a 12V DC motor that will draw a maximum of 20A.

The motor doesn't need to reverse, and all the motor controllers I seem to find online are bridges with forward/reverse functions.
 

dougy83

Well-Known Member
You can use a 5V logic MOSFET driven from the PWM signal to drive the motor. Also, a (fast) free-wheeling diode should be connected across the motor.

Depending on the PWM source characteristics (frequency, source impedance), you may need a current buffer to allow the MOSFET to switch more rapidly. There are many options, but you could use a CMOS 555 to provide the buffering (pin 8 to 5V, pin 2/6/1 to gnd, pin 4 to PWM, pin 3 to MOSFET).
 

adamey

Member
Do logic switched MOSFETs require buffers? I thought the point of a logic MOSFET is not needing high drive requirements.

Also I did a search and some have said a logic MOSFET might not have the same low turn-on resistance as a regular MOSFET? Is this true?
 

Sceadwian

Banned
Switching application often require complex drivers because the gate capacitance starts becoming a very much important part of the circuit at hi PWM frequencies. In general you're better off with a good dedicated IC FET driver to switch a FET rather than a logic level fet by itself. Logic level fets are more oriented for triggering relays or slower speed loads.

I don't know the details of the construction of a logic level FET, but to get that low turn on voltage they sacrfice something.
 

smanches

New Member
Logic level FETS are mostly for smaller loads. It's almost impossible to find them for large loads due to the charge needed to turn them on. They just aren't made for loads above about 30-50A.

They do have a slightly higher Rds_on, but it's really not that much different from a fully driven mosfet.

You might be able to find a logic level one that can handle 40A, but there aren't going to be many.

FYI, they sacrifice some gate charge and some Rds_on, but the benefit of a 5V gate is so handy.
 
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Sceadwian

Banned
If what you want is efficiency though, just use a mosfet driver, there are many fully integrated mosfet drivers out there that can be driven from lower voltage I/O lines, it takes care of the drive requirements for you.
 
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