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From: Daku on 24 Jun 2010 13:00
Not being a power electronics person, could some electronics guru
please explain the term "high side" MOSFET driver. Thanks in advance.
From: George Jefferson on 24 Jun 2010 13:39


"Daku" <dakupoto(a)gmail.com> wrote in message
news:d8dddaa9-81df-44de-8a7e-75386753ed2a(a)5g2000yqz.googlegroups.com...
> Not being a power electronics person, could some electronics guru
> please explain the term "high side" MOSFET driver. Thanks in advance.

When you drive a mosfet's gate it's gate voltage must have a special
relationship to it's source voltage. The gate voltage is referenced to it's
source voltage. If you use a n-ch mosfet in "high side", which means it's
source it no low but "high" or above ground(because it has some load tied to
source) then you have to make sure the gate voltage is relative to the
source.

For example, suppose you have a resistor attached to the source. If current
flows through the mosfet then the source voltage will increase(assuming
positive flow of course). This means that the gate voltage will also need to
increase to keep the same relative voltage.

Hence you need some driver that can look at the source voltage and create a
gate voltage relative to that.

Say the mosfet gate turn on voltage(V_gth) is 5 voltage. This is with
respect to the source. But if the source voltage is 20V then the voltage you
actually have to supply to the gate is 25 voltage. If you apply 5V(w.r.t to
ground of course) then your actually supply -15V w.r.t to the source of the
mosfet.

If the source is at 100V and you do the same then you'll have a gate voltage
of -95V whih will ruin most mosfets.

THE GATE VOLTAGE OF ANY MOSFET IS WITH RESPECT TO THE SOURCE!!!! A driver
translates ground referenced voltages to the mosfet source referened
voltages for us.

With a low side mosfet the source is grounded and hence there is no
translation/level shifting/tracking needed.

A Mosfet driver simply lets us enter a ground referenced voltage(or possible
some other reference) and it does all the work to make sure it becomes is
source referenced so that we don't have some type of "voltage mismatch"
between what we are giving the mosfet gate and what it really wants.

Note: If your highside mosfet's source voltage never goes above or below a
few voltage(Depending on it's maximum reverse voltages) then you don't
necessarily need a mosfet driver if you are simply using it as a switch.
This is because even though your gate voltage is referenced from ground and
the mosfet references it from the source, the differences are not enough to
ruin the mosfet.

e.g., suppose your source is at 5V and your gate at 10V(w.r.t. ground). The
gate is really only seeing 5V. If you put a gate voltage of 0V then it
is -5V. These are well within the ranges of most mosfets. If the source
voltage never goes above 5V or below 0V then the maximum gate-source voltage
on the mosfet will never be out of -5V to 5V and mosfets can easily handle
this.

Now change the source at 5V to the source at 50V and you'll have a dead
mosfet. Check out some mosfet datasheet and you'll see the maximum rating
and think about it for a bit and you'll see what I'm talking about. (BTW,
there are maximum and minimum voltage rating from gate to source, gate to
drain, and source to drain. which all must be respected if you want a living
mosfet)




From: John Larkin on 24 Jun 2010 17:02
On Thu, 24 Jun 2010 10:00:41 -0700 (PDT), Daku <dakupoto(a)gmail.com>
wrote:

>Not being a power electronics person, could some electronics guru
>please explain the term "high side" MOSFET driver. Thanks in advance.



+24V--------------+
|
highside
fet
switch
|
|
|
+---------- load
|
|
lowside
fet
switch
|
|
|
gnd


Like most such terms, it's a little ambiguous. Some people would stop
here, and some might say that if the upper fet were an n-channel, it
would need a more-than-24 volt "high side" gate driver.

Most "high side fet gate drivers" are intended to do just that, apply
more than 24 volts to the gate of an n-channel fet to turn it on hard.

John

From: Jamie on 24 Jun 2010 18:46
Daku wrote:

> Not being a power electronics person, could some electronics guru
> please explain the term "high side" MOSFET driver. Thanks in advance.
Assuming you are talking about using a N channel MOSFET on the
(+) rail "High side"? The GATE voltage must be above the SOURCE voltage
, greater than the specified threshold margin to fully turn on..

If you are using the source of the MOSFET as an output to some
device? This sure does present a problem of generating a signal at
the gate higher than what is going to be at the SOURCE..

High side drivers come in a few varieties. The most common I can
think of at the moment are those with isolated outputs that can couple
to the SOURCE and GATE of the Mosfet and generate their own isolated
voltage.. Because the output is isolated, the driver only needs what is
required by the MOSFET to turn on. Call it the pinch off or Threshold
voltage.. How ever, you should be driving it above that level, unless
of course, you have some special analog project in mind?



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