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[DPRG] MOSFET gate charge

Subject: [DPRG] MOSFET gate charge
From: Rick Bickle rbickle at intconsys.com
Date: Mon Jul 13 12:15:03 CDT 2009

Ray,

 

The gate of a MOSFET is basically a capacitor with respect to the
source. Thus you get current flow during a "transient" (read as: change
in voltage) across the gate. It will only draw current when being
switched on or off. (This is mainly why high speed processors draw so
much current.) For steady state applications, MOSFETS are great as they
draw virtually no current at the gate as compared with a BJT. At higher
switching speeds however, you need to supply the gate more and more
current to achieve fast switching. Check the specifications on the part
for the gate capacitance, and the Gate voltage threshold Vgs. 

 

Remember that the terminology on MOSFETS is a little different from
BJT's. Source = Emitter, Drain = Collector, and Gate = Base. MOSFETS
refer to electron flow (negative) while BJT's refer to hole flow
(positive).  The source of electrons is equivalent to the emitter of
holes.

 

Also remember that the Vgs (Gate voltage with respect to the source)
will change if you use a N-Channel MOSFET to source power to a load.
(Drain connected to VCC) This means that your MOSFET gate will have to
be at least Vgs above your load voltage. This is why N-Channel MOSFETS
require high side drivers when used In the top half of an H-Bridge motor
driver. If possible in your circuit, use the N-Channel MOSFET to sink
the load (Source connected to GND) so that your Vgs is referenced to
ground.

 

I'd love to see this laser rangefinder when it's working...

Good Luck,

 

Rick

 

 

From: dprglist-bounces at dprg.org [mailto:dprglist-bounces at dprg.org] On
Behalf Of Ray Xu
Sent: Sunday, July 12, 2009 11:58 PM
To: dprglist at dprg.org
Subject: [DPRG] MOSFET gate charge

 

Hi group,

I have one quick question that I can barely find any info on on the web:
What exactly is the MOSFET Gate charge?

My situation here is that I'm building a op amp current source circuit
for a laser diode controller, with the MOSFET "driving" the laser diode;
sourcing current into the diode.  However, when simulating my current
source circuit, I find that N-Type enhancement MOSFETs with different on
resistances (self explanatory) and different Gate Charge values will
either output the correct current, or just output some other current
that is far off than the calculated current supposed to be outputted.
I'm using MOSFETs since they're more precise than BJTs (less leakage
current, etc).

>From what I read so far, the Gate Charge is the required amount of
charge when a transient is present (?? Doesn't make any sense to me).  

Any enlightenment would be appreciated.

Thanks
Ray Xu
rayxu123 at gmail.com      

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