[TriEmbed] Power FETs Vol 25, Issue 27

[Glen Smith]

Shane,

I can't speak for anyone else, but I don't think it was YOU that was
stumbling. I know I was, but I will re-read it this evening when I can
devote my full attention to it.

One question: it sounds like your little breakout board can be used with
its current FET (IRF2084) for a 5v signal and up to 30vdc load, but you
would use a different FET for a 3.3v signal. Is there a specific FET you
would recommend that has the right pinout for an R-Pi 3.3v application?

Thanks

On Thu, Jun 18, 2015, 8:55 AM Shane Trent via TriEmbed <
triembed at triembed.org> wrote:

> Brian,
>
> Yes, the MIC5014 is under the FET. I fold over the FET to make it more
> compact and expose the TO-2220 tab for convenient mounting to a heat-sink
> if needed. Generally you can get 1W - 1.5W of dissipation from a TO-220
> package without a heat-sink. Keeping the I^2 * Rds power dissipation low is
> one of the reasons I like to use over-sized MOSFET's in my projects.
>
> There are two primary differences in using a FET driver vs using an IO pin
> with a logic-level FET. First, this FET driver will fully enhance a even a
> non-logic FET as long as the chip is powered by at least 5V (my application
> uses for 12V, note you can still power the FET driver from 30V and drive
> the input with a 2V logic signal) so I can use a wider range of FETs.
> Looking at Figure 1 on the FQP30N06L datasheet you can see that this FET is
> not fully enhanced at 3.3V which is not surprising since the Rds for the
> FET is quoted at Vgs = 10V (not an problem until you reach higher currents)
>
> Second, the FET driver will drive the gate of the FET faster that you can
> typically pull it up and down using a logic output. My application involved
> applying variable power to a solenoid via pulse-width modulation (PWM) so I
> wanted to make sure I could quickly switch the FET. This differences become
> more important at higher currents because high current FETs have a larger
> input capacitance and are harder to drive quickly. For example, the IRF2084
> that I used has an on resistance of 2 milliohms vs the FQP30N06L with 35
> milliohms (at 10V gate drive!). But this comes with a larger gate
> capacitance 6450 pF vs the FQP30N06L with a typical value of 800 pF. So
> even if the Pi could drive the IRF2804 to the higher gate voltage needed,
> the circuit would be slower since the gate capacitance is 8 times higher.
>
> The PCB offers another advantage that may not be clear at first glance.
> You can mount the PCB at the load (or even on the load in my application)
> and switch it by supplying two wires (logic ground and your control
> signal). This means that the high current switching and the PWM of the gate
> drive are localized at the load and not traveling though long cables. This
> improves the performance of the switching by not having the gate drive
> loaded by long cables and not having power losses and reduced voltage to
> the load due to voltage drop in the power cables to and and from the FET if
> not mounted at the load.
>
> Surely something I said was not clear. Please feel free to ask for
> clarification if I stumbled through something.
>
> Shane
>
> On Thu, Jun 18, 2015 at 7:45 AM, Grawburg via TriEmbed <
> triembed at triembed.org> wrote:
>
>> Shane,
>>
>> So the FET driver is under the FET itself?
>> If the FQP30N06L I am using switches with the 3.3VDC from the Pi, what
>> advantage does your combo offer (disregarding the other components)?
>>
>>
>> Brian
>>
>>
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>>
>>
>
>
> --
> A blog about some of my projects.  http://fettricks.blogspot.com/
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