Re: What Driver Voltage to Drive a Mosfet? (SMPS app)

On Tue, 31 Jul 2007 04:04:58 -0700, Wimpie <wimtel@xxxxxxxxx> wrote:

On 31 jul, 02:08, D from BC <myrealaddr...@xxxxxxxxx> wrote:
On Tue, 31 Jul 2007 00:07:53 +0100, Eeyore



<rabbitsfriendsandrelati...@xxxxxxxxxxx> wrote:

D from BC wrote:

My power mosfet has an absolute gate spec of 30VDC.
Most of the Mosfet data*** gate behavior graphs only go to 10V.

Is there a drive voltage that'll provide low switching loss?
Looks like there's a 6V to 30V range to choose from.

As others have said, the trick is to turn it on *FAST*. Age proportion of the
losses are during turn-on and turn-ff as opposed to conduction.

So... what you *should* be asking (given a sensible choice of voltage drive) is
gate CURRENT ! The more amps the better.

Graham

I'm picturing driving a mosfet like this..

+----Rsource---+
| |
| |
Vdriver Cgate
| |
| |
+--------------+

V is from the mosfet driver rail voltage.
Rsource is mostly internal mosfet driver resistance.

Here's my confusing thing...

Given that the mosfet driver can change state fast with currents up to
14A peak and the objective to fast charge/discharge the gate for low
Pds switching loss ...It kinda looks like one should make V (the
mosfet driver rail) as high as possible for the fastest charging.

D from BC

Hi "D from BC"

Basically driving the MOSFET with high positive and negative voltage
will result in faster switching times

Driving your MOSFET with high voltage (for example 20 V) will turn-on
the mosfet faster, but increases the drive losses significantly (so
your driver will consume more power).

The gate of the MOSFET has internal resistance (the gate of standard
SMPS MOSFETs is made of polysilicon instead of metal). Even when you
would use a driver with infinite current capability, the drive current
is limited by the MOSFET's internal gate resistance (and dI/dt is
limited by inductances).

You can get some idea of the internal Rgate by looking into the
simulation model of the device you are going to use.

If you want to speed up turn-off, you may use a negative gate drive.
In that case you can pull more current out of the gate (but with
higher gate drive loss).

Before designing your driver circuitry I would recommend you to first
evaluate the losses for you SMPS topology. Maybe you are spending
money (and power) in an almost perfect drive circuitry, while the
reduction in switching loss is insignificant with respect to other
losses.

Unnecessary fast switching will generate more HF noise, so you have to
spend more components to meet EMC requirements.

You mentioned 600 kHz (and higher). Are your using a (semi) resonant
or a zero-voltage-switching topology?

The driver IC has been (probably) designed for the IXYS MOSFETS. They
have several types that have very low internal gate resistance and can
be used into the HF/VHF frequency range.

Best regards,

Wim
PA3DJS
www.tetech.nl

Hey...that's different.. It's been suggested in this thread to only
charge up the gate to a voltage for needed drain current.
That way there's less to discharge.
Little charges and little discharges can be done fast.

But... Every Rseries and Lseries (driver internal,trace and internal
mosfet R&L) limits the current therefore limits the charge rate which
in turn sets the mosfet switching speed.
The slower the switching speed...the more Pds switching loss..

Someday I'll learn how to balance the mosfet driver heat, the mosfet
heat and the EMI.
I guess when I burn my finger on the IXYS (IXDD414) mosfet driver,
it's max'd out. :)
(Assuming it's worthy to get the mosfet driver that hot.)

Picking a mosfet driver supply voltage has become a 3 bears story for
me..
Too big...too small...just right..

About resonance...
I'm not using a resonant design.
The power inductors I'm using do ring a bit and I do have to try to
keep inductor parallel capacitance to a min with proper windings.

I could convert to a resonant design and switch at "dead" times but
I'd like to experiment with non-resonant first.

Ahh....you suspect IXYS drivers best with IXYS mosfets...I'll take a
peak at the IXYS mosfets sometime.


D from BC
.