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'[EE]: Driving a BUZ10 Mosfet'
2001\06\27@011102 by jeethur

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Hi,

I've been using Bipolar Transistors in a UPS application.
But now I need to use Power MOSFETs because of Its low on state resistance.
The problem is I don't know how to drive em.
Do I need to use some Driver ICs ?
I'm using a 4093 Quad NAND gate IC with a PIC.

Can I directly drive the MOSFET from the NAND Gate?

Please suggest


Jeethu Rao

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2001\06\27@042843 by Jinx

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Hi Jeethu, this link posted the other day

http://www.web-ee.com/primers/primers_frm.htm

has a MOSFET section, including "Interfacing power MOSFETs
to logic devices", which is actually a pdf of Motorola AN1102. This
deals extensively with just your question. You don't give any specs
for your application, but I'm sure you'll find what you're looking for
in that pdf, good luck

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2001\06\27@064356 by Sergi Sanchez

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> Can I directly drive the MOSFET from the NAND Gate?
>
> Please suggest
In general.. you must drive the gate arround 10~12V above source level in
order to switch on the mosfet. If you are novice with mosfets, you may try
an OMNIFET (from sgs-thomson) that works at TTL levels, and is fully
protected (short circuit, thermal, clamp voltages... etc.) please see
http://eu.st.com/stonline/products/selector/314.htm

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2001\06\27@065628 by Alan B. Pearce

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>In general.. you must drive the gate arround 10~12V above source level in
>order to switch on the mosfet. If you are novice with mosfets, you may try
>an OMNIFET (from sgs-thomson) that works at TTL levels, and is fully

Other FET that work with logic levels are available from zetec
http://www.zetec.com

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2001\06\27@090719 by Olin Lathrop

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> Can I directly drive the MOSFET from the NAND Gate?

That depends on what the gate voltage specs of the FET are.  Some FETs are
specifically called "logic level" FETs which work with 0-5V gate drive.
However, these tend to cost more and have a slightly higher on resistance
than their "normal" counterparts.  Typical "normal" FETs require around 10V
gate drive to fully turn on.  Check the data sheet.


********************************************************************
Olin Lathrop, embedded systems consultant in Littleton Massachusetts
(978) 742-9014, spam_OUTolinTakeThisOuTspamembedinc.com, http://www.embedinc.com

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2001\06\27@132935 by rottosen

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Olin Lathrop wrote:
>
> > Can I directly drive the MOSFET from the NAND Gate?
>
> That depends on what the gate voltage specs of the FET are.  Some FETs are
> specifically called "logic level" FETs which work with 0-5V gate drive.
> However, these tend to cost more and have a slightly higher on resistance
> than their "normal" counterparts.  Typical "normal" FETs require around 10V
> gate drive to fully turn on.  Check the data sheet.


Beware, it is not uncommon for a "logic" FET to be spec'ed for on
resistance with the standard 10 volt drive!

A CMOS gate does not have enough drive for the FET capacitance if you
want fast turn on and turn off times. Remember that the driver sees the
gate to source capacitance (typically several hundred picofarads) in
parallel with the Miller capacitance. The Miller capacitance can be in
the thousands of picofarads when switching high voltages quickly.


-- Rich


>
> ********************************************************************
> Olin Lathrop, embedded systems consultant in Littleton Massachusetts
> (978) 742-9014, .....olinKILLspamspam@spam@embedinc.com, http://www.embedinc.com
>
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2001\06\28@150544 by Peter L. Peres

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You can drive the BUZ directly with CD4093. Use a 10 ohm resistor in the
gate wire close to the BUZ and make sure supply is 9 <= Vcc <= 12V.

Peter

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2001\06\29@061011 by jeethur

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part 1 1408 bytes content-type:text/plain; (decoded 7bit)

Thanks Peter,

But due to design constrains, i'll have to Power the CD4093 by 5 Volts.
I've included a small piece of the schematic. So, the work around I've
devised is to
do it was to use a BC 547 NPN Transistor to drive the Mosfets.The Output
from one of the
gates of CD4093 will drive the base of the BC547 and the 547 will drive the
gate of the
MosFet. I've also incorporated the 10 Ohms Resistors you had told about. I'm
using two
Mosfets in parallel because the load is around 35 Amps and BUZ10s can only
drive around
23 amps each. Actually, its used to Drive a  Push-Pull Step Up Transformer.
And I did'nt
connect freewheeling diodes in Anti-Parallel to the Mosfets because BUZ10s
are supposed
have an internal Diode.


Have I made any mistakes ?(I make em so often!)

Please see the included schematic (MOS.gif) and suggest.

Thnx,

Jeethu Rao

{Original Message removed}
part 2 12240 bytes content-type:image/gif; (decode)


part 3 136 bytes
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2001\06\29@121538 by rottosen

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See my thoughts below.

-- Rich

Jeethu Rao wrote:
{Quote hidden}

It looks like that you are using the BC 547 as an emitter follower. When
you do this you lose about 0.7 volts of the drive needed to turn on the
FET. Even a logic FET needs as much gate-source voltage as you can give
it (up to 12 volts) to get the on resistance low.


How fast are you pulsing the FET? The 10K emitter resistor will be very
slow discharging the capacitances of the FET. I would guess that it
could take as much as 100us to turn the FET off. I estimated this by
using the time constant of 5000pf (for the FET) and the 10K resistor.


> I've also incorporated the 10 Ohms Resistors you had told about. I'm
> using two
> Mosfets in parallel because the load is around 35 Amps and BUZ10s can only
> drive around
> 23 amps each.


You must always be careful when paralleling devices like this because
they do not share the currents equally. FET's are better than bipolar
transistors doing this. Adding some resistors from the source of each
FET to ground will help the sharing. I would guess that about 0.1 ohms
would work at your 17 amps per FET.


> Actually, its used to Drive a  Push-Pull Step Up Transformer.


Are you putting the transformer winding in the source of the FET? This
is not good at high currents because the FET will have the entire gate
to source across it causing very high heat dissipation -- about 100
watts per FET(!). In fact, unless you are using a negative power supply
for the transformer there will be almost no drive to the coil at all
because of the gate to source threshold voltage.


> And I did'nt
> connect freewheeling diodes in Anti-Parallel to the Mosfets because BUZ10s
> are supposed
> have an internal Diode.


The transformer winding should be in the drain circuit. Unfortunately,
the internal diodes are then in the wrong place in the circuit to quench
the back EMF of the coil. The back EMF goes above the power supply
voltage, not below ground. You do need to add the freewheeling diodes
from the FET drain to the power supply.


>
> Have I made any mistakes ?(I make em so often!)
>
> Please see the included schematic (MOS.gif) and suggest.
>
> Thnx,
>
> Jeethu Rao
>
> {Original Message removed}

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