[TriEmbed] Power FETs Vol 25, Issue 27

[Shane Trent]

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
>
>
> _______________________________________________
> Triangle, NC Embedded Computing mailing list
> TriEmbed at triembed.org
> http://mail.triembed.org/mailman/listinfo/triembed_triembed.org
> TriEmbed web site: http://TriEmbed.org
>
>


-- 
A blog about some of my projects.  http://fettricks.blogspot.com/
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://mail.triembed.org/pipermail/triembed_triembed.org/attachments/20150618/cba8a4e6/attachment.htm>