[TriEmbed] Power FETs Vol 25, Issue 27

[Shane Trent]

Glen,

The FET driver on my PCB can actually be used for logic levels down to 2v.
The 5V limit refers to the load voltage being switched. I have used it with
both a 5V Arudino and the Raspbery Pi's 3.3v outputs. If you replace the
IRF2804 with a logic FET you can use a load voltage as low as 2.75v.

I do not have much experience with logic level FETs. At low power levels
you may be better off with a 2N3904 NPN BJT since they start turning on at
0.65V or so and are rated for 200mA.

Finding a good 3.3v part is a challenge because most "Logic-Level" FETs are
based on 5V logic. The datasheets specify a gate threshold voltage but that
is just when the chip starts to conduct a small current. You really have to
study the current vs voltages family of curves for various Vgs voltages.
Basically you are going to have to put in some work to find a FET that can
carry several amps and give you a low Rds at 3.3V. If you are going to be
doing heavy lifting, more than 5A or so, it might be worth adding a gate
driver.

Shane





On Thu, Jun 18, 2015 at 9:38 AM, Glen Smith <mrglenasmith at gmail.com> wrote:

> 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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>


-- 
A blog about some of my projects.  http://fettricks.blogspot.com/
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