# Re: Formula for minimum drive current for mosfet

news:7e10d5cc-4430-45df-8654-c9ace3961ffa@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
On Apr 10, 4:44 pm, Fred Bloggs <nos...@xxxxxxxxxx> wrote:
bill.slo...@xxxxxxxx wrote:
On 11 apr, 01:33, "Jon Slaughter" <Jon_Slaugh...@xxxxxxxxxxx> wrote:

Whats the formula?

I = 1/2*F*Q*V?

Trying to figure out if I can drive

http://www.fairchildsemi.com/pf/FD/FDD8424H.html

with a uC directly? (I think it can supply up to 20mA or so)

V = 12V if I use pullup and 5V if not.

Check out the data sheet. Figure 7 shows the typical gate charge
required to get the gate voltage up to a level where the part is
turned on - something like 10nC. The worst case total gate charge
listed earlier in the data sheet is 24uC.

20mA s going to take 1.2usec to deliver that 24uC of charge - this is
slow switching by MOSFET standards, and you won't want to switch that
slowly very often, because if you do there is a real risk that the
switch will overheat.

--
Bill Sloman, Nijmegen

That thing looks like a cross conduction hazard and half at that
switching speed, both FETs come on at less than 2V from their source
rails, wonder if he's tying the gates together, definitely would want to
switch as fast as possible then...- Hide quoted text -

- Show quoted text -

Jon, the problem is the amount of time it takes the mosfet(s) to
transition from on to off and back from off to on again. During each
switching action of a mosfet it acts like a resistor for the
duration. Heat! So you want the duration of each switching event as
short as possible, regardless of whether these events occur at 100Hz
or 100kHz.

Its not so simple. I do see that now as I stated in the last post I made. I
thought when they said frequency they meant something else and not the
switchign time. (although you do make it much clearer than they did)

But I cannot switch at any frequency and have as fast a transition as I
want. I must drive the gates of the mosfet with BJT's and, of course, they
have there own limits.

But that's only part of the problem. Fred brings up a good point
Now, when your drive has the gates pulled all the way to the rail
(either one), that's no problem.
But it IS a problem during the switching transition, because both
mosfets are partly turned on providing a path -- not through the load,
but directly across the power rails. You don't want slow switching
here.

Yes, I know.. its basically just like CMOS(I guess it is CMOS) and there is
a short between tranistions.

But there are a few solutions. One is to switch the mosfets on sequentially
instead of at the same time. Al I loose is a bit of power to the motor
momentarily but inertia should smooth it out. (basically delay the PWM a bit
until the transition of the first fet is finished)

But you do make a good point. The problem is, I do have limitations. I do
see bill's point now about using a uC to drive the fets as its probably just
way to low. Ultimately I'd like a formula for drive. (is it a simple RC
circuit(Approximately) or more?)

So a weak drive is bad.
For motor drive, a kilohertz is probably way plenty, and this lower
frequency is better so that you don't put your mosfets in the hot seat
so often.

Well, I do understand that(and its more clear now that you said it). The
thing about that slow of a frequency, from what I've read, is that its
audiable(Which may or may not be an issue depending on how loud it is).

Basically I need to maximize transition time and minimize frequency given
all the contraints. (> 20khz(probably), surge current by drivers(bjt's),
least power dissipation(Although I guess as long as its below max I'm ok)).

Ok, I guess I see the confusion. The subject is minimum drive current for
mosfets. I guess this is bad. I shouldn't require a minimum but a maxium
within the mosfet drivers specs. (basically what I wanted was a formula to
see the relationship)

What I plan on doing is use bjt's to drive the gates in emitter follower...
but I need to configure them for optimal conditions. (max drive current but
within device specs)

Anyways, thanks for the post. Its more clear now but I need to let it settle
in. (I "knew" all the concepts before but there was just no glue tieing them
together ;/)

(Although I'd still like to drive them with the uC if possible because that
is the simplest method... sure it might not be most efficient but if I'm
still able to get reasonble power dissipation then its not *wrong* but just
not optimal)

Thanks,
Jon

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