Annirak Wrote:
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>
> MOSFETs, even "logic level" ones, are spec'd at
> 10V Vgs. They (almost) all accept gate drives up
> to 20V. The IRLB8743 is pretty good as a logic
> level FET, but you're still not getting great
> performance out of it. It dumps 3W as heat if you
> drive it at 30A, Vgs = 4.5. If you were to drive
> that same FET at 12V, you could probably cut that
> down to 2W or even 1.5W; I'd be more specific, but
> IRF's curves are cut off below 0.1V. If you were
> going to do that, you could probably use a
> lower-cost FET.
>
> So what does a level translator actually cost,
> both in complexity and in price? PWM on an
> extruder or a heated bed doesn't need to be run
> faster than 1kHz and, at that frequency, a level
> shifter is a $0.10 mosfet and a $0.01 resistor.
> And you could easily make up the cost difference
> on a lower cost MOSFET.
>
> Are we quibling over $0.11? Or is two discretes
> too complex?
First off, that's 2 discretes per output, which if you have a board with 2 fans and 2 extruders and 1 HB out means 10 discretes.
The other thing is this expands the board size.
Both of these are not insurmountable, and they're not an issue IMO, but they do need to be accounted for.
FWIW: One of the issues with using 12V to drive the gate on a FET is that you then assume that we're using 12V to drive other parts of the printer, which for some of us is definitely not the case.
My printer runs entirely off 24V, and I know a lot of people who use 15V to get their 12V heated beds to heat properly.
On something RAMPS-like you could simply use a 9V regulator to get an intermediate voltage to feed the FET drivers and the voltage input for Arduino board (eg: a Due).
On other designs it means you either have to:
- Limit it to 12V only (annoying)
- Provide an intermediate voltage that otherwise would not be necessary (bumps up the price if populated)
- Use components that have a greater voltage range (which may bump up their price)
As it is, I see lots of RAMPS boards with 16V capacitors on them (instead of 35V or 50V caps) because they're cheaper. I also see lots of cheap FET's that make it onto RAMPS boards because once again they're significantly cheaper than either the original or the preferred alternatives. This leads to all sorts of annoying issues when people do things that these people don't account for, or just leads to all sorts of random failures.
Anyway, as I said, I'm definitely NOT against the idea of using level shifters to drive the FET's, we just need to get it done right first time to avoid issues down the track.
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>
>Quote
The voltage required to switch the MOSFET.
> That's individual and can be extracted from the
> data sheet. There are only few which do at 3.3V
> and even fewer of them can handle 50A and up,
> which is required to use it for switching the 15A
> of a heated bed without additional
> cooling.
> MOSFETs, even "logic level" ones, are spec'd at
> 10V Vgs. They (almost) all accept gate drives up
> to 20V. The IRLB8743 is pretty good as a logic
> level FET, but you're still not getting great
> performance out of it. It dumps 3W as heat if you
> drive it at 30A, Vgs = 4.5. If you were to drive
> that same FET at 12V, you could probably cut that
> down to 2W or even 1.5W; I'd be more specific, but
> IRF's curves are cut off below 0.1V. If you were
> going to do that, you could probably use a
> lower-cost FET.
>
> So what does a level translator actually cost,
> both in complexity and in price? PWM on an
> extruder or a heated bed doesn't need to be run
> faster than 1kHz and, at that frequency, a level
> shifter is a $0.10 mosfet and a $0.01 resistor.
> And you could easily make up the cost difference
> on a lower cost MOSFET.
>
> Are we quibling over $0.11? Or is two discretes
> too complex?
First off, that's 2 discretes per output, which if you have a board with 2 fans and 2 extruders and 1 HB out means 10 discretes.
The other thing is this expands the board size.
Both of these are not insurmountable, and they're not an issue IMO, but they do need to be accounted for.
FWIW: One of the issues with using 12V to drive the gate on a FET is that you then assume that we're using 12V to drive other parts of the printer, which for some of us is definitely not the case.
My printer runs entirely off 24V, and I know a lot of people who use 15V to get their 12V heated beds to heat properly.
On something RAMPS-like you could simply use a 9V regulator to get an intermediate voltage to feed the FET drivers and the voltage input for Arduino board (eg: a Due).
On other designs it means you either have to:
- Limit it to 12V only (annoying)
- Provide an intermediate voltage that otherwise would not be necessary (bumps up the price if populated)
- Use components that have a greater voltage range (which may bump up their price)
As it is, I see lots of RAMPS boards with 16V capacitors on them (instead of 35V or 50V caps) because they're cheaper. I also see lots of cheap FET's that make it onto RAMPS boards because once again they're significantly cheaper than either the original or the preferred alternatives. This leads to all sorts of annoying issues when people do things that these people don't account for, or just leads to all sorts of random failures.
Anyway, as I said, I'm definitely NOT against the idea of using level shifters to drive the FET's, we just need to get it done right first time to avoid issues down the track.