# MOSFET GATE DRIVE?

J

#### Jim Drew

Jan 1, 1970
0
I have a question about driving the gate on standard MOSFETS. I am very
familiar with building h-bridge controllers using the typical 4 MOSFET
h-bridge configuration, but my source voltage has always been a maximum
of 12 volts. So, tieing the gate through a resistor to the positive
rail for the P-channel fets (and driving them low with another
transistor to enable the them) has always worked just fine. Now, I have
an application where the voltage is an unknown between 7.4v and 65.0v.
I know that the VGS max on most devices is +/-20v, so anything higher
than this is going to kill the MOSFET. Does anyone have any idea on
what should be used to drive the gate that will work within that voltage
range? I am not aware of any gate drivers that will work that high. I
was considering something as simple as zener diode/resistor voltage
regulation to get the voltage to a usable level. For that matter, does
the gate voltage really need to be any higher than about 10v?

Thanks for any insight!

T

#### Tim Williams

Jan 1, 1970
0
8V-ish zener from gate to source. Best results would be with a turn-off
resistor parallelling the zener, and a constant current drive (open
collector with emitter degeneration or switched current mirror). Resistor
depends on how fast you want it and how much current you can spare. You'll
need some resistance no matter what, since the gate capacitance isn't going
to fall too quickly through a zener in the "off" region, especially with
leakage current from the driver keeping it on.

Tim

F

#### Fred Bartoli

Jan 1, 1970
0
Jim Drew said:
I have a question about driving the gate on standard MOSFETS. I am very
familiar with building h-bridge controllers using the typical 4 MOSFET
h-bridge configuration, but my source voltage has always been a maximum
of 12 volts. So, tieing the gate through a resistor to the positive
rail for the P-channel fets (and driving them low with another
transistor to enable the them) has always worked just fine. Now, I have
an application where the voltage is an unknown between 7.4v and 65.0v.
I know that the VGS max on most devices is +/-20v, so anything higher
than this is going to kill the MOSFET. Does anyone have any idea on
what should be used to drive the gate that will work within that voltage
range? I am not aware of any gate drivers that will work that high. I
was considering something as simple as zener diode/resistor voltage
regulation to get the voltage to a usable level. For that matter, does
the gate voltage really need to be any higher than about 10v?

Thanks for any insight!

How about HIP4080/4081? Plus you'll only need N mosfets.

J

#### Jim Drew

Jan 1, 1970
0
Here is another delima....

I also have to be able to reduce the input voltage to a level that is
acceptable for a small 5v regulator (max input voltage for it is 20v).
A PIC micro is used to control the high voltage motor via some sort of
drive circuitry. Like I said before, I have perfectly working setup
that works with a 12-14 volt input source. The catch now is being able
to allow up to 65 volts. This also needs to be all SMT and as small as
possible.

I think what I need really is a way to reduce the incoming voltage to be
less than 20 volts, and I need probably no more than 100ma of current
for everything.

I am still looking at the idea of a zener diode/resistor voltage
regulator. I believe I saw something with a transistor added somehow as
well. Any thoughts?

J

#### Joerg

Jan 1, 1970
0
Hello Jim,

One simple method is driving them via little toroid transformers. You
just have to make sure that they cover the frequency range you are
dealing with.

Regards, Joerg

J

#### Jim Thompson

Jan 1, 1970
0
Hello Jim,

One simple method is driving them via little toroid transformers. You
just have to make sure that they cover the frequency range you are
dealing with.

Regards, Joerg

I'll have to dig thru the 20 year old paper designs I did in that
area...

Transformers driven PWM, with DC restorers on the gate side.

...Jim Thompson

K

#### Ken Smith

Jan 1, 1970
0
Jim Drew said:
I think what I need really is a way to reduce the incoming voltage to be
less than 20 volts, and I need probably no more than 100ma of current
for everything.

Consider this:

V++
!
\
/ V++
\ !
! !!-- NMOS
+----+------!!
20V /-/ ! ! !
^ ! B !
! --------C E--------
GND NPN

You may be able to do without the current limiting circuit. You can make
it slow start by putting a capacitor across the zener.

Beware that if you do both, you need a small resistor in series with the
gate of the MOSFET. With no resistors near it, the MOSFET will look for a
way to oscillate and fairly often find one.

K

#### Ken Smith

Jan 1, 1970
0
Hello Jim,

One simple method is driving them via little toroid transformers. You
just have to make sure that they cover the frequency range you are
dealing with.

Regards, Joerg

Or, you can make it a bit more complex by adding a small power MOSFET like
this:

SMALL GUY
A S D !!------------
--------- -----+------!!- BIG FET
( ! ! ! ! !!-
( ------- \ !
( ! / !
( ! \ Large !
( ! ! !
-----------+-------+---------+-----------

When the A point pulses high, the body diode in the small MOSFET conducts
and charges up the gate of the big guy. If the transformer voltage then
droops, the capacitance of the FET will hold the gate voltage for quite a
while.

When the A point pulses low, the small FET turns on and the GATE of the
big MOSFET gets discharged. If the transformer then droops, the gate
voltage on the MOSFET settles back towards zero.

The resistor is only needed if you disable the drive completely. Leakages
could cause the big MOSFETs gate to drift up.

C

#### Clark

Jan 1, 1970
0
Under your search engine, look up Half bridge drivers, some will work pass
100 vdc on the hi side, and two of them will form a full bridge driver. Some
have a single input and some others have a hi and a low side drive input.
They even have protection from turning a gate on if the other Fets gate is
in the on position. Several kinds out there.

J

#### Joerg

Jan 1, 1970
0
Hello Jim,
I'll have to dig thru the 20 year old paper designs I did in that
area...

Such archaeological excavation can take a while. Had to do it a few
months ago to find a really old filter design disk. It turned out to be
easy, it was a thin 5 1/4 floppy tucked into the back of my CAD binder.
Duh! But that didn't occur to me until after several hours of digging. I
had all but forgotten that some floppy disks were indeed 'floppy'.

Transformers driven PWM, with DC restorers on the gate side.

On many H-bridges that commutate regularly you can get away without DC
restore. Sometimes I was even able to do it sans transformer, with just
a coupling capacitor.

How's the spider bit coming? Did it heal a little?

Regards, Joerg

J

#### Jim Thompson

Jan 1, 1970
0
Hello Jim, [snip]

How's the spider bit coming? Did it heal a little?

Regards, Joerg

Turned out to be a cyst that burst.

We're just home from the surgi-center, lighter by ~$3K :-( The joys of self-employment and$5K deductible ;-)

But I'm still ahead by ~$25K since I made that decision to risk a high deductible. ...Jim Thompson J #### Joerg Jan 1, 1970 0 Hello Jim, We're just home from the surgi-center, lighter by ~$3K :-(

And they probably slapped you with full fare, like $40 per bandage where the HMO pays only their negotiated rate of$7 or so. But you belong to
the part of society that plans for a rainy day and doesn't blow it all
on fancy cars.

Well, your wife can now say that you spent $3k on her around Christmas time ;-) The joys of self-employment and$5K deductible ;-)

At least we can now deduct the premiums. What's still not fair is that
we must pay the 15.3% SE tax before deducting and employees don't. IOW
we pay 15.3% more than they.

But I'm still ahead by ~$25K since I made that decision to risk a high deductible. I am thinking about that as well. Right now we pay a little over$500/mo
for two at Kaiser. Deductibles kick in mostly for hospital stays,
$200/day or so but we are never sick. When I checked high-deductible plans they often aren't that much cheaper. Regards, Joerg J #### Jim Thompson Jan 1, 1970 0 Hello Jim, And they probably slapped you with full fare, like$40 per bandage where
the HMO pays only their negotiated rate of $7 or so. But you belong to the part of society that plans for a rainy day and doesn't blow it all on fancy cars. Well, your wife can now say that you spent$3k on her around Christmas
time ;-)

At least we can now deduct the premiums. What's still not fair is that
we must pay the 15.3% SE tax before deducting and employees don't. IOW
we pay 15.3% more than they.

You're NOT incorporated ??? Look up "Key Employees" ;-)
I am thinking about that as well. Right now we pay a little over $500/mo for two at Kaiser. Deductibles kick in mostly for hospital stays,$200/day or so but we are never sick. When I checked high-deductible
plans they often aren't that much cheaper.

Regards, Joerg

I was paying $900/month for$500 deductible.

Went to $500/month for$5K deductible... for the both of us.

I'm now just shy of 66, so I'm on Medicare... $261/month premium right now for the wife only. Next August the wife will be 65 and high cost will be over ;-) "we are never sick" either... first claim in 6 years. ...Jim Thompson J #### Joerg Jan 1, 1970 0 Hello Jim, You're NOT incorporated ??? Look up "Key Employees" ;-) We live in a blue state. What can I say? Like to keep it simple, no more bureaucratic layers. I was paying$900/month for $500 deductible. Went to$500/month for $5K deductible... for the both of us. Wow. I never saw a plan for us that would drop the premium by 40%. At least not for health care. I'm now just shy of 66, so I'm on Medicare...$261/month premium right
now for the wife only. Next August the wife will be 65 and high cost
will be over ;-)

Then comes that drug plan with the mandatory donut hole. We do a lot of
ministry and visitation in nursing homes. It's sad, many of those people
had plenty of savings because of hard work and a frugal or at least
prudent lifestyle. All it takes is a serious fall, stroke, whatever and
you'll lose all that in a jiffy. $5k a month is nothing there and you still get jam packed into a small room with three others. "we are never sick" either... first claim in 6 years. Same here. Except for rather stupid reasons like poison oak exposure. When we moved here I did not know what that was. So ....$45 of copay
later I knew that I should have gotten out of those clothes right away
and not finish up the brush clearing.

Regards, Joerg

J

#### Jim Thompson

Jan 1, 1970
0
Hello Jim,

We live in a blue state. What can I say? Like to keep it simple, no more
bureaucratic layers.

[I was going to say something, but I promised :-]
Wow. I never saw a plan for us that would drop the premium by 40%. At
least not for health care.

Hospital and surgical only, no doctor visits.
Then comes that drug plan with the mandatory donut hole.

Between the two of us we only have ~$200/month, so I think Part D is likely to be useless. We do a lot of ministry and visitation in nursing homes. It's sad, many of those people had plenty of savings because of hard work and a frugal or at least prudent lifestyle. All it takes is a serious fall, stroke, whatever and you'll lose all that in a jiffy.$5k a month is nothing there and you
still get jam packed into a small room with three others.

Same here. Except for rather stupid reasons like poison oak exposure.
When we moved here I did not know what that was. So .... $45 of copay later I knew that I should have gotten out of those clothes right away and not finish up the brush clearing. Regards, Joerg I did that in poison ivy ONCE... blisters everywhere, including everywhere I scratched :-( Potassium Permanganate gauze soaks for a week. ...Jim Thompson J #### Joerg Jan 1, 1970 0 Hello Jim, We live in a blue state. What can I say? Like to keep it simple, no more bureaucratic layers. [I was going to say something, but I promised :-] Yes, I read something to that effect. That you and Win wouldn't throw snowballs at each other anymore. Or at least not with rocks in them. But seriously, it's getting nasty here at times. I mean, when the owner of a perfectly healthy surfboard manufacturer throws in the towel that should say something to the local politicians. Unfortunately it didn't. Hospital and surgical only, no doctor visits. I'd be fine with that but the premiums are still so high. Probably because it's Kahlifohniah. Between the two of us we only have ~$200/month, so I think Part D is
likely to be useless.

Don't know, depends on which meds you need. I have seen some really
cheap plans. Even Walmart out here offered one.

I did that in poison ivy ONCE... blisters everywhere, including
everywhere I scratched :-( Potassium Permanganate gauze soaks for a
week.

Well, I waited so long that the doc said the rash migrated to the inside
of my skin. Anyway, for the next season of poison oak removal he
recommended "Tecnu soap" to wash down with right after the work and that
stuff really does the trick for us.

Regards, Joerg

J

#### Jim Thompson

Jan 1, 1970
0
Hello Jim,

We live in a blue state. What can I say? Like to keep it simple, no more
bureaucratic layers.

[I was going to say something, but I promised :-]

Yes, I read something to that effect. That you and Win wouldn't throw
snowballs at each other anymore. Or at least not with rocks in them.

Rocks? Rocks? When I was a kid we used coal ;-)

(Tallmansville, WV, is just south of Philippi, WV, where my
grandparents (on my mother's side) had a HUGE farm with their own seam
of coal.)
But seriously, it's getting nasty here at times. I mean, when the owner
of a perfectly healthy surfboard manufacturer throws in the towel that
should say something to the local politicians. Unfortunately it didn't.

I'd be fine with that but the premiums are still so high. Probably
because it's Kahlifohniah.

Don't know, depends on which meds you need. I have seen some really
cheap plans. Even Walmart out here offered one.

Good point... I ought to see if my business membership at Sam's Club
has any "benefits" ;-)
Well, I waited so long that the doc said the rash migrated to the inside
of my skin. Anyway, for the next season of poison oak removal he
recommended "Tecnu soap" to wash down with right after the work and that
stuff really does the trick for us.

Regards, Joerg

...Jim Thompson

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