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Sci.Electronics.Basics -> MOSFETs and PWM
There are 42 messages in this thread.
You are currently looking at messages 1 to 20.
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Author: Randy DayDate: 13:05 24-02-07
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I've built some TTL motor/relay controllers
out of NPN transistors in a Darlington
configuration, and I'm considering using
mosfets instead.
+V
|_
- )|
^ )| relay
|_)|
|
_R_ |/
TTL in -|___|--|
|>
|
GND
(created by AACircuit v1.28 beta 10/06/04 www.tech-chat.de)
My experience with mosfets is zero. Is an
n-channel mosfet pretty much a drop-in
replacement for an NPN device, aside from
gate/base resistor values? If so, how do I
calculate an appropriate value for R on a
mosfet? I'm looking for full-off/full-on
switching, not voltage regulation.
The Mouser website lists the FDN5630 and
FDC655BN, which seem to have the current
capacity (and low price) I want. Are there
any other things I need to consider when
designing for mosfets, aside from ESD?
TIA
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Author: John LarkinDate: 13:49 24-02-07
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On Sat, 24 Feb 2007 18:05:50 GMT, Randy Day <randy.day@shaw.cax>
wrote:
>I've built some TTL motor/relay controllers
>out of NPN transistors in a Darlington
>configuration, and I'm considering using
>mosfets instead.
> +V
> |_
> - )|
> ^ )| relay
> |_)|
> |
> _R_ |/
> TTL in -|___|--|
> |>
> |
> GND
>(created by AACircuit v1.28 beta 10/06/04 www.tech-chat.de)
>
>My experience with mosfets is zero. Is an
>n-channel mosfet pretty much a drop-in
>replacement for an NPN device, aside from
>gate/base resistor values? If so, how do I
>calculate an appropriate value for R on a
>mosfet? I'm looking for full-off/full-on
>switching, not voltage regulation.
You don't need a gate resistor at all. Just run your logic level to
the fet's gate. There are some second-order reasons to use a gate
resistor (kill rf oscillations during the slew, slow down the edges,
protect the driver if the fet dies) so 1K, close to the fet, wouldn't
hurt.
>
>The Mouser website lists the FDN5630 and
>FDC655BN, which seem to have the current
>capacity (and low price) I want. Are there
>any other things I need to consider when
>designing for mosfets, aside from ESD?
Make sure your logic high is enough to turn the fet on hard, not
merely "gate threshold" voltage; that's on the datasheets.
Be aware that an open-gate fet could be on or off, so make sure your
driver circuit keeps the gate low when it should. Disconnected cables
can get interesting.
Most fets like this are fairly esd hard, so don't go overboard on
handling. I take no esd precautions when working in my lab, and have
never blown a mosfet. My manufacturing people do all the strap/mat
stuff, and they don't blow fets either!
John
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Author: Anthony FremontDate: 14:03 24-02-07
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Randy Day wrote:
> I've built some TTL motor/relay controllers
> out of NPN transistors in a Darlington
> configuration, and I'm considering using
> mosfets instead.
> +V
> |_
> - )|
> ^ )| relay
> |_)|
> |
> _R_ |/
> TTL in -|___|--|
> |>
> |
> GND
> (created by AACircuit v1.28 beta 10/06/04 www.tech-chat.de)
>
> My experience with mosfets is zero. Is an
> n-channel mosfet pretty much a drop-in
> replacement for an NPN device, aside from
> gate/base resistor values? If so, how do I
In this application, I'd say yes.
> calculate an appropriate value for R on a
> mosfet? I'm looking for full-off/full-on
> switching, not voltage regulation.
You will likely not need R, there is no current flowing. What you will need
is active drive, high and low. FETs are voltage controlled, the only
current to flow into the gate (base) when you turn it on is what is required
to charge up the tiny gate capacitor, after that the current flow ends. To
turn them off, the gate capacitance has to be discharged, just letting it
float won't turn it off.
> The Mouser website lists the FDN5630 and
> FDC655BN, which seem to have the current
> capacity (and low price) I want. Are there
> any other things I need to consider when
> designing for mosfets, aside from ESD?
Allot of MOSFETs require high voltages to turn them on (usually you have to
pull the gate higher than the highest voltage being switched. For example,
if you are using 12V to power the relay, you would have to pull the gate
higer than 12V. This can really suck when you want to use a microcontroller
to do the switching. Enter the "logic level" FET. They only need 4 or 5
volts to turn them on hard, even when switching higher voltages. Much more
convenient.
I've had good luck with NTE-2985 parts. 5V of drive and the on resistance
is .05 ohms max. 60V breakdown voltage and 30A max continuous current. I'm
sure there are many many others out there, but I accidentally happened upon
these whilst looking for something else.
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Author: John LarkinDate: 14:24 24-02-07
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On Sat, 24 Feb 2007 13:03:23 -0600, "Anthony Fremont"
<spam-not@nowhere.com> wrote:
>Allot of MOSFETs require high voltages to turn them on (usually you have to
>pull the gate higher than the highest voltage being switched. For example,
>if you are using 12V to power the relay, you would have to pull the gate
>higer than 12V.
Not so. The curves of drain current versus drain voltage, at fixed
gate voltage, are essentially flat. What is true is that fets with
higher max drain breakdown specs do *tend* to need more gate drive
voltage, because the oxides are thicker. I've never seen a fet spec
that went above +10 gate voltage, even to switch 600 volts, and there
are lots of mosfets around that will switch 50 volts with 3.3 volt
gate drive.
John
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Author: Anthony FremontDate: 14:38 24-02-07
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John Larkin wrote:
> On Sat, 24 Feb 2007 13:03:23 -0600, "Anthony Fremont"
> <spam-not@nowhere.com> wrote:
>
>
>
>> Allot of MOSFETs require high voltages to turn them on (usually you
>> have to pull the gate higher than the highest voltage being
>> switched. For example, if you are using 12V to power the relay, you
>> would have to pull the gate higer than 12V.
>
> Not so. The curves of drain current versus drain voltage, at fixed
> gate voltage, are essentially flat. What is true is that fets with
> higher max drain breakdown specs do *tend* to need more gate drive
> voltage, because the oxides are thicker. I've never seen a fet spec
> that went above +10 gate voltage, even to switch 600 volts, and there
> are lots of mosfets around that will switch 50 volts with 3.3 volt
> gate drive.
Oops, I stand corrected. :-) After you make the point about 600V, it does
seem ridiculous to expect that much gate voltage to turn one on. As for the
3.3V that switch 50V, how long have those been around, or are they just more
of the "logic-level" collection? While you're here, is this all that
"enhancement mode" means; logic level/low gate drive?
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On Feb 24, 2:03 pm, "Anthony Fremont" <spam-...@nowhere.com> wrote:
> Randy Day wrote:
> > I've built some TTL motor/relay controllers
> > out of NPN transistors in a Darlington
> > configuration, and I'm considering using
> > mosfets instead.
> > +V
> > |_
> > - )|
> > ^ )| relay
> > |_)|
> > |
> > _R_ |/
> > TTL in -|___|--|
> > |>
> > |
> > GND
> > (created by AACircuit v1.28 beta 10/06/04www.tech-chat.de)
>
> > My experience with mosfets is zero. Is an
> > n-channel mosfet pretty much a drop-in
> > replacement for an NPN device, aside from
> > gate/base resistor values? If so, how do I
>
> In this application, I'd say yes.
>
> > calculate an appropriate value for R on a
> > mosfet? I'm looking for full-off/full-on
> > switching, not voltage regulation.
>
> You will likely not need R, there is no current flowing. What you will need
> is active drive, high and low. FETs are voltage controlled, the only
> current to flow into the gate (base) when you turn it on is what is required
> to charge up the tiny gate capacitor, after that the current flow ends. To
> turn them off, the gate capacitance has to be discharged, just letting it
> float won't turn it off.
>
> > The Mouser website lists the FDN5630 and
> > FDC655BN, which seem to have the current
> > capacity (and low price) I want. Are there
> > any other things I need to consider when
> > designing for mosfets, aside from ESD?
>
> Allot of MOSFETs require high voltages to turn them on (usually you have to
> pull the gate higher than the highest voltage being switched. For example,
> if you are using 12V to power the relay, you would have to pull the gate
> higer than 12V. This can really suck when you want to use a microcontroller
> to do the switching. Enter the "logic level" FET. They only need 4 or 5
> volts to turn them on hard, even when switching higher voltages. Much more
> convenient.
>
> I've had good luck with NTE-2985 parts. 5V of drive and the on resistance
> is .05 ohms max. 60V breakdown voltage and 30A max continuous current. I'm
> sure there are many many others out there, but I accidentally happened upon
> these whilst looking for something else
Don't think that's correct Anthony.
Typically the gate threshold for a 30A MOSFET is 2-4 volts. For
example take a look at a BUZ11
http://www.fairchildsemi.com/ds/BU/BUZ11.pdf, maybe you're defining
this as a logic level fet too. ;)
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Author: Anthony FremontDate: 15:43 24-02-07
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maxfoo@punkass.com wrote:
> Don't think that's correct Anthony.
> Typically the gate threshold for a 30A MOSFET is 2-4 volts. For
> example take a look at a BUZ11
> http://www.fairchildsemi.com/ds/BU/BUZ11.pdf, maybe you're defining
> this as a logic level fet too. ;)
Vgs(th) for the NTE part is 2V max. .05 Ohms Rds(on) is at 5V gate drive.
At 4V drive, the on resistance is .07 Ohms max. Sounds like logic-level to
me. ;-)
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Author: John PopelishDate: 16:02 24-02-07
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Randy Day wrote:
> I've built some TTL motor/relay controllers
> out of NPN transistors in a Darlington
> configuration, and I'm considering using
> mosfets instead.
> +V
> |_
> - )|
> ^ )| relay
> |_)|
> |
> _R_ |/
> TTL in -|___|--|
> |>
> |
> GND
> (created by AACircuit v1.28 beta 10/06/04 www.tech-chat.de)
>
> My experience with mosfets is zero. Is an
> n-channel mosfet pretty much a drop-in
> replacement for an NPN device, aside from
> gate/base resistor values? If so, how do I
> calculate an appropriate value for R on a
> mosfet? I'm looking for full-off/full-on
> switching, not voltage regulation.
>
> The Mouser website lists the FDN5630 and
> FDC655BN, which seem to have the current
> capacity (and low price) I want. Are there
> any other things I need to consider when
> designing for mosfets, aside from ESD?
It takes more voltage to turn a mosfet on than it takes to
turn on a darlington, but the gate looks like a small
capacitor, rather than two diodes in series to ground, so
instead of putting a current limiting resistor between the
logic output and the gate, put it between the +5 supply and
the logic output, so that the logic high voltage will be the
full logic supply. 2.2k to 10k should help get the fet
turned on more than just the TTL output would do.
And you will have to find a mosfet that is called a logic
level type. This means that its on state resistance is
specified with a logic level gate drove. Most other mosfets
have the on resistance specified with 10 volts of gate drive.
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Author: John FieldsDate: 16:26 24-02-07
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On Sat, 24 Feb 2007 14:43:42 -0600, "Anthony Fremont"
<spam-not@nowhere.com> wrote:
>maxfoo@punkass.com wrote:
>
>> Don't think that's correct Anthony.
>> Typically the gate threshold for a 30A MOSFET is 2-4 volts. For
>> example take a look at a BUZ11
>> http://www.fairchildsemi.com/ds/BU/BUZ11.pdf, maybe you're defining
>> this as a logic level fet too. ;)
>
>Vgs(th) for the NTE part is 2V max. .05 Ohms Rds(on) is at 5V gate drive.
>At 4V drive, the on resistance is .07 Ohms max. Sounds like logic-level to
>me. ;-)
---
Regardless, you'll need to know how long it takes to charge up the
gate capacitance between when the switch is "ON" and how long it'll
take to discharge it when it's "OFF", since during that time the FET
will be required to dissipate extraordinary power.
--
JF
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Author: Anthony FremontDate: 17:10 24-02-07
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John Fields wrote:
> On Sat, 24 Feb 2007 14:43:42 -0600, "Anthony Fremont"
> <spam-not@nowhere.com> wrote:
>
>> maxfoo@punkass.com wrote:
>>
>>> Don't think that's correct Anthony.
>>> Typically the gate threshold for a 30A MOSFET is 2-4 volts. For
>>> example take a look at a BUZ11
>>> http://www.fairchildsemi.com/ds/BU/BUZ11.pdf, maybe you're defining
>>> this as a logic level fet too. ;)
>>
>> Vgs(th) for the NTE part is 2V max. .05 Ohms Rds(on) is at 5V gate
>> drive. At 4V drive, the on resistance is .07 Ohms max. Sounds like
>> logic-level to me. ;-)
>
> ---
> Regardless, you'll need to know how long it takes to charge up the
> gate capacitance between when the switch is "ON" and how long it'll
> take to discharge it when it's "OFF", since during that time the FET
> will be required to dissipate extraordinary power.
If I was actually going to try and switch 30A with it, I guess I'd use a
gate-driver of some type. I have some in my shack that are made by
Microchip (IIRC) that I used for tinkering with some IGBTs. They have very
fast switching times and will supply much larger currents (>3A) during the
transition. The NTE transistors that I mentioned seem to work fine being
driven by a PIC at 5V. I was only switching a couple of amps though without
a heatsink.
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Author: Randy DayDate: 17:28 24-02-07
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John Larkin wrote:
[snip]
Wow! Lots of really excellent advice from everyone!
> You don't need a gate resistor at all. Just run your logic level to
> the fet's gate. There are some second-order reasons to use a gate
> resistor (kill rf oscillations during the slew, slow down the edges,
> protect the driver if the fet dies) so 1K, close to the fet, wouldn't
> hurt.
I think I'll put the 1k in for driver protection.
For peace of mind, if nothing else. ;)
> Make sure your logic high is enough to turn the fet on hard, not
> merely "gate threshold" voltage; that's on the datasheets.
The FDC655 lists 33 mOhms Rds(on) at 4.5v, which
seems more than adequate. I think I'll go with it.
>
> Be aware that an open-gate fet could be on or off, so make sure your
> driver circuit keeps the gate low when it should. Disconnected cables
> can get interesting.
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Author: John LarkinDate: 19:27 24-02-07
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On Sat, 24 Feb 2007 13:38:48 -0600, "Anthony Fremont"
<spam-not@nowhere.com> wrote:
>John Larkin wrote:
>> On Sat, 24 Feb 2007 13:03:23 -0600, "Anthony Fremont"
>> <spam-not@nowhere.com> wrote:
>>
>>
>>
>>> Allot of MOSFETs require high voltages to turn them on (usually you
>>> have to pull the gate higher than the highest voltage being
>>> switched. For example, if you are using 12V to power the relay, you
>>> would have to pull the gate higer than 12V.
>>
>> Not so. The curves of drain current versus drain voltage, at fixed
>> gate voltage, are essentially flat. What is true is that fets with
>> higher max drain breakdown specs do *tend* to need more gate drive
>> voltage, because the oxides are thicker. I've never seen a fet spec
>> that went above +10 gate voltage, even to switch 600 volts, and there
>> are lots of mosfets around that will switch 50 volts with 3.3 volt
>> gate drive.
>
>Oops, I stand corrected. :-) After you make the point about 600V, it does
>seem ridiculous to expect that much gate voltage to turn one on. As for the
>3.3V that switch 50V, how long have those been around, or are they just more
>of the "logic-level" collection? While you're here, is this all that
>"enhancement mode" means; logic level/low gate drive?
>
If you draw the "saturation" curve of a mosfet, Id versus Vd, there's
some drain voltage where it stops being resistive and goes into
constant-current mode. That voltage tends to be about the same as the
applied gate voltage. To get it to saturate below X volts, you need at
least X volts of gate drive. You can see that effect in the curves on
the datasheets. Maybe that's what you were referring to. But you can
switch a bazillion volts of load with just a 5 or 10 volt gate swing,
if the final drain current isn't too high.
Logic-level fets are fairly new, last 10 years maybe. Classic
n-channel mosfets fets were barely turned on at 3-4 volts, and not
fully enhanced til 6-10 volts; p-channels were worse. The new parts
use IC technology (cellular layout, polysilicon gates, thin oxide) to
get low thresholds, at the expense of breakdown voltage.
Enhancement mode means the drain-source channel is inherently
nonconducting, "normally off", and applying a gate voltage makes it
conduct. Depletion mode devices are already conducting, "normally on"
and require a negative voltage to turn them off. Most depletion
devices are jfets or gaasfets, although a few people (like Supertex)
make depletion mosfets, which can be very handy in certain apps.
John
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Author: John LarkinDate: 19:32 24-02-07
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On 24 Feb 2007 11:42:23 -0800, maxfoo@punkass.com wrote:
>On Feb 24, 2:03 pm, "Anthony Fremont" <spam-...@nowhere.com> wrote:
>> Randy Day wrote:
>> > I've built some TTL motor/relay controllers
>> > out of NPN transistors in a Darlington
>> > configuration, and I'm considering using
>> > mosfets instead.
>> > +V
>> > |_
>> > - )|
>> > ^ )| relay
>> > |_)|
>> > |
>> > _R_ |/
>> > TTL in -|___|--|
>> > |>
>> > |
>> > GND
>> > (created by AACircuit v1.28 beta 10/06/04www.tech-chat.de)
>>
>> > My experience with mosfets is zero. Is an
>> > n-channel mosfet pretty much a drop-in
>> > replacement for an NPN device, aside from
>> > gate/base resistor values? If so, how do I
>>
>> In this application, I'd say yes.
>>
>> > calculate an appropriate value for R on a
>> > mosfet? I'm looking for full-off/full-on
>> > switching, not voltage regulation.
>>
>> You will likely not need R, there is no current flowing. What you will need
>> is active drive, high and low. FETs are voltage controlled, the only
>> current to flow into the gate (base) when you turn it on is what is required
>> to charge up the tiny gate capacitor, after that the current flow ends. To
>> turn them off, the gate capacitance has to be discharged, just letting it
>> float won't turn it off.
>>
>> > The Mouser website lists the FDN5630 and
>> > FDC655BN, which seem to have the current
>> > capacity (and low price) I want. Are there
>> > any other things I need to consider when
>> > designing for mosfets, aside from ESD?
>>
>> Allot of MOSFETs require high voltages to turn them on (usually you have to
>> pull the gate higher than the highest voltage being switched. For example,
>> if you are using 12V to power the relay, you would have to pull the gate
>> higer than 12V. This can really suck when you want to use a microcontroller
>> to do the switching. Enter the "logic level" FET. They only need 4
or 5
>> volts to turn them on hard, even when switching higher voltages. Much more
>> convenient.
>>
>> I've had good luck with NTE-2985 parts. 5V of drive and the on resistance
>> is .05 ohms max. 60V breakdown voltage and 30A max continuous current. I'm
>> sure there are many many others out there, but I accidentally happened upon
>> these whilst looking for something else
>
>Don't think that's correct Anthony.
>Typically the gate threshold for a 30A MOSFET is 2-4 volts. For
>example take a look at a BUZ11
>http://www.fairchildsemi.com/ds/BU/BUZ11.pdf, maybe you're defining
>this as a logic level fet too. ;)
>
>
>
>
The drain current at "gate threshold" is only 1 mA. The Rdson at 10
amps is spec'd with +10 on the gate. So this is of course not a
logic-level fet, because it wouldn't be prudent to run a beast like
this with just 3.3 or 5 volts on the gate.
John
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Author: Anthony FremontDate: 20:27 24-02-07
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John Larkin wrote:
> On Sat, 24 Feb 2007 13:38:48 -0600, "Anthony Fremont"
> If you draw the "saturation" curve of a mosfet, Id versus Vd, there's
> some drain voltage where it stops being resistive and goes into
> constant-current mode. That voltage tends to be about the same as the
> applied gate voltage. To get it to saturate below X volts, you need at
> least X volts of gate drive. You can see that effect in the curves on
> the datasheets. Maybe that's what you were referring to. But you can
> switch a bazillion volts of load with just a 5 or 10 volt gate swing,
> if the final drain current isn't too high.
As you can see, I'm not much on analog that's for sure. :-) I was thinking
that I had read somewhere that conventional FETs worked the way I described,
and it sounds like you might be saying that they sort of do if you need them
"really turned on". ;-) And I was under the impression that
"ordinary"
P-channels would have to have the gate pulled below ground for the same
reasons.
> Logic-level fets are fairly new, last 10 years maybe. Classic
> n-channel mosfets fets were barely turned on at 3-4 volts, and not
> fully enhanced til 6-10 volts; p-channels were worse. The new parts
> use IC technology (cellular layout, polysilicon gates, thin oxide) to
> get low thresholds, at the expense of breakdown voltage.
Logic level FETs sure are convenient for guys like me that like to use PICs.
I also like the high-side drivers that are available, pretty slick features.
> Enhancement mode means the drain-source channel is inherently
> nonconducting, "normally off", and applying a gate voltage makes it
> conduct. Depletion mode devices are already conducting, "normally on"
> and require a negative voltage to turn them off. Most depletion
> devices are jfets or gaasfets, although a few people (like Supertex)
> make depletion mosfets, which can be very handy in certain apps.
Ok, so it really doesn't have anything to do with what makes the FET turn on
better at lower gate voltages?
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Author: Anthony FremontDate: 20:30 24-02-07
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John Larkin wrote:
> On 24 Feb 2007 11:42:23 -0800, maxfoo@punkass.com wrote:
>
>> On Feb 24, 2:03 pm, "Anthony Fremont" <spam-...@nowhere.com>
wrote:
>>> Randy Day wrote:
>>>> I've built some TTL motor/relay controllers
>>>> out of NPN transistors in a Darlington
>>>> configuration, and I'm considering using
>>>> mosfets instead.
>>>> +V
>>>> |_
>>>> - )|
>>>> ^ )| relay
>>>> |_)|
>>>> |
>>>> _R_ |/
>>>> TTL in -|___|--|
>>>> |>
>>>> |
>>>> GND
>>>> (created by AACircuit v1.28 beta 10/06/04www.tech-chat.de)
>>>
>>>> My experience with mosfets is zero. Is an
>>>> n-channel mosfet pretty much a drop-in
>>>> replacement for an NPN device, aside from
>>>> gate/base resistor values? If so, how do I
>>>
>>> In this application, I'd say yes.
>>>
>>>> calculate an appropriate value for R on a
>>>> mosfet? I'm looking for full-off/full-on
>>>> switching, not voltage regulation.
>>>
>>> You will likely not need R, there is no current flowing. What you
>>> will need is active drive, high and low. FETs are voltage
>>> controlled, the only current to flow into the gate (base) when you
>>> turn it on is what is required to charge up the tiny gate
>>> capacitor, after that the current flow ends. To turn them off, the
>>> gate capacitance has to be discharged, just letting it float won't
>>> turn it off.
>>>
>>>> The Mouser website lists the FDN5630 and
>>>> FDC655BN, which seem to have the current
>>>> capacity (and low price) I want. Are there
>>>> any other things I need to consider when
>>>> designing for mosfets, aside from ESD?
>>>
>>> Allot of MOSFETs require high voltages to turn them on (usually you
>>> have to pull the gate higher than the highest voltage being
>>> switched. For example, if you are using 12V to power the relay,
>>> you would have to pull the gate higer than 12V. This can really
>>> suck when you want to use a microcontroller to do the switching.
>>> Enter the "logic level" FET. They only need 4 or 5 volts to turn
>>> them on hard, even when switching higher voltages. Much more
>>> convenient.
>>>
>>> I've had good luck with NTE-2985 parts. 5V of drive and the on
>>> resistance is .05 ohms max. 60V breakdown voltage and 30A max
>>> continuous current. I'm sure there are many many others out there,
>>> but I accidentally happened upon these whilst looking for something
>>> else
>>
>> Don't think that's correct Anthony.
>> Typically the gate threshold for a 30A MOSFET is 2-4 volts. For
>> example take a look at a BUZ11
>> http://www.fairchildsemi.com/ds/BU/BUZ11.pdf, maybe you're defining
>> this as a logic level fet too. ;)
>>
>>
>>
>>
>
> The drain current at "gate threshold" is only 1 mA. The Rdson at 10
> amps is spec'd with +10 on the gate. So this is of course not a
> logic-level fet, because it wouldn't be prudent to run a beast like
> this with just 3.3 or 5 volts on the gate.
You are referring to the BUZ11 right?
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Author: LionelDate: 20:52 24-02-07
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On Sat, 24 Feb 2007 13:03:23 -0600, "Anthony Fremont"
<spam-not@nowhere.com> wrote:
I'm a complete amateur WRT MOSFETs, so I'm finding this discussion
interesting. I hope you'll bear with me if this question sounds dumb:
>Allot of MOSFETs require high voltages to turn them on (usually you have to
>pull the gate higher than the highest voltage being switched. For example,
>if you are using 12V to power the relay, you would have to pull the gate
>higer than 12V. This can really suck when you want to use a microcontroller
>to do the switching.
If you're driving the gate from 5V CMOS logis (eg; a micro), can you
get around this by putting in a series diode after the logic output, &
a large (1M+) pullup resistor from the MOSFET gate to your 12V (or
whatever) relay supply?
--
W "Some people are alive only because it is illegal to kill them."
. | ,. w ,
\|/ \|/ Perna condita delenda est
---^----^---------------------------------------------------------------
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Author: John LarkinDate: 21:04 24-02-07
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On Sun, 25 Feb 2007 12:52:57 +1100, Lionel <usenet@imagenoir.com>
wrote:
>On Sat, 24 Feb 2007 13:03:23 -0600, "Anthony Fremont"
><spam-not@nowhere.com> wrote:
>
>I'm a complete amateur WRT MOSFETs, so I'm finding this discussion
>interesting. I hope you'll bear with me if this question sounds dumb:
>
>>Allot of MOSFETs require high voltages to turn them on (usually you have to
>>pull the gate higher than the highest voltage being switched. For example,
>>if you are using 12V to power the relay, you would have to pull the gate
>>higer than 12V. This can really suck when you want to use a microcontroller
>>to do the switching.
>
>If you're driving the gate from 5V CMOS logis (eg; a micro), can you
>get around this by putting in a series diode after the logic output, &
>a large (1M+) pullup resistor from the MOSFET gate to your 12V (or
>whatever) relay supply?
Nope. A diode doesn't give voltage gain. You can shift the gate swing
up by one diode drop, so it swings 0.6 low to 5.6 high. but that
doesn't help a lot.
John
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Author: John LarkinDate: 21:08 24-02-07
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On Sat, 24 Feb 2007 19:30:46 -0600, "Anthony Fremont"
<spam-not@nowhere.com> wrote:
>John Larkin wrote:
>> On 24 Feb 2007 11:42:23 -0800, maxfoo@punkass.com wrote:
>>
>>> On Feb 24, 2:03 pm, "Anthony Fremont"
<spam-...@nowhere.com> wrote:
>>>> Randy Day wrote:
>>>>> I've built some TTL motor/relay controllers
>>>>> out of NPN transistors in a Darlington
>>>>> configuration, and I'm considering using
>>>>> mosfets instead.
>>>>> +V
>>>>> |_
>>>>> - )|
>>>>> ^ )| relay
>>>>> |_)|
>>>>> |
>>>>> _R_ |/
>>>>> TTL in -|___|--|
>>>>> |>
>>>>> |
>>>>> GND
>>>>> (created by AACircuit v1.28 beta 10/06/04www.tech-chat.de)
>>>>
>>>>> My experience with mosfets is zero. Is an
>>>>> n-channel mosfet pretty much a drop-in
>>>>> replacement for an NPN device, aside from
>>>>> gate/base resistor values? If so, how do I
>>>>
>>>> In this application, I'd say yes.
>>>>
>>>>> calculate an appropriate value for R on a
>>>>> mosfet? I'm looking for full-off/full-on
>>>>> switching, not voltage regulation.
>>>>
>>>> You will likely not need R, there is no current flowing. What you
>>>> will need is active drive, high and low. FETs are voltage
>>>> controlled, the only current to flow into the gate (base) when you
>>>> turn it on is what is required to charge up the tiny gate
>>>> capacitor, after that the current flow ends. To turn them off, the
>>>> gate capacitance has to be discharged, just letting it float won't
>>>> turn it off.
>>>>
>>>>> The Mouser website lists the FDN5630 and
>>>>> FDC655BN, which seem to have the current
>>>>> capacity (and low price) I want. Are there
>>>>> any other things I need to consider when
>>>>> designing for mosfets, aside from ESD?
>>>>
>>>> Allot of MOSFETs require high voltages to turn them on (usually you
>>>> have to pull the gate higher than the highest voltage being
>>>> switched. For example, if you are using 12V to power the relay,
>>>> you would have to pull the gate higer than 12V. This can really
>>>> suck when you want to use a microcontroller to do the switching.
>>>> Enter the "logic level" FET. They only need 4 or 5 volts to
turn
>>>> them on hard, even when switching higher voltages. Much more
>>>> convenient.
>>>>
>>>> I've had good luck with NTE-2985 parts. 5V of drive and the on
>>>> resistance is .05 ohms max. 60V breakdown voltage and 30A max
>>>> continuous current. I'm sure there are many many others out there,
>>>> but I accidentally happened upon these whilst looking for something
>>>> else
>>>
>>> Don't think that's correct Anthony.
>>> Typically the gate threshold for a 30A MOSFET is 2-4 volts. For
>>> example take a look at a BUZ11
>>> http://www.fairchildsemi.com/ds/BU/BUZ11.pdf, maybe you're defining
>>> this as a logic level fet too. ;)
>>>
>>>
>>>
>>>
>>
>> The drain current at "gate threshold" is only 1 mA. The Rdson at 10
>> amps is spec'd with +10 on the gate. So this is of course not a
>> logic-level fet, because it wouldn't be prudent to run a beast like
>> this with just 3.3 or 5 volts on the gate.
>
>You are referring to the BUZ11 right?
>
Yup. The typical curves show it conducting pretty hard, 8 or 9 amps,
with +5 on the gate, but that's not guaranteed. If your load is just a
couple of amps, it's probably safe to go with +5 drive. But I wouldn't
bet a production run on typical performance curves.
At 3.3, it might conduct just a few mA, if that.
John
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On Feb 24, 7:32 pm, John Larkin
<jjlar...@highNOTlandTHIStechnologyPART.com> wrote:
> On 24 Feb 2007 11:42:23 -0800, max...@punkass.com wrote:
>
>
>
>
>
> >On Feb 24, 2:03 pm, "Anthony Fremont" <spam-...@nowhere.com>
wrote:
> >> Randy Day wrote:
> >> > I've built some TTL motor/relay controllers
> >> > out of NPN transistors in a Darlington
> >> > configuration, and I'm considering using
> >> > mosfets instead.
> >> > +V
> >> > |_
> >> > - )|
> >> > ^ )| relay
> >> > |_)|
> >> > |
> >> > _R_ |/
> >> > TTL in -|___|--|
> >> > |>
> >> > |
> >> > GND
> >> > (created by AACircuit v1.28 beta 10/06/04www.tech-chat.de)
>
> >> > My experience with mosfets is zero. Is an
> >> > n-channel mosfet pretty much a drop-in
> >> > replacement for an NPN device, aside from
> >> > gate/base resistor values? If so, how do I
>
> >> In this application, I'd say yes.
>
> >> > calculate an appropriate value for R on a
> >> > mosfet? I'm looking for full-off/full-on
> >> > switching, not voltage regulation.
>
> >> You will likely not need R, there is no current flowing. What you will
need
> >> is active drive, high and low. FETs are voltage controlled, the only
> >> current to flow into the gate (base) when you turn it on is what is
required
> >> to charge up the tiny gate capacitor, after that the current flow ends.
To
> >> turn them off, the gate capacitance has to be discharged, just letting it
> >> float won't turn it off.
>
> >> > The Mouser website lists the FDN5630 and
> >> > FDC655BN, which seem to have the current
> >> > capacity (and low price) I want. Are there
> >> > any other things I need to consider when
> >> > designing for mosfets, aside from ESD?
>
> >> Allot of MOSFETs require high voltages to turn them on (usually you have
to
> >> pull the gate higher than the highest voltage being switched. For
example,
> >> if you are using 12V to power the relay, you would have to pull the gate
> >> higer than 12V. This can really suck when you want to use a
microcontroller
> >> to do the switching. Enter the "logic level" FET. They only
need 4 or 5
> >> volts to turn them on hard, even when switching higher voltages. Much
more
> >> convenient.
>
> >> I've had good luck with NTE-2985 parts. 5V of drive and the on resistance
> >> is .05 ohms max. 60V breakdown voltage and 30A max continuous current.
I'm
> >> sure there are many many others out there, but I accidentally happened
upon
> >> these whilst looking for something else
>
> >Don't think that's correct Anthony.
> >Typically the gate threshold for a 30A MOSFET is 2-4 volts. For
> >example take a look at a BUZ11
> >http://www.fairchildsemi.com/ds/BU/BUZ11.pdf, maybe you're defining
> >this as a logic level fet too. ;)
>
> The drain current at "gate threshold" is only 1 mA. The Rdson at 10
> amps is spec'd with +10 on the gate. So this is of course not a
> logic-level fet, because it wouldn't be prudent to run a beast like
> this with just 3.3 or 5 volts on the gate.
>
> John-
Come on, the BUZ11 isn't a beast, It's a beauty. It's quite versatile
and cheap too.(84cents at digikey). One of the least expensive N-
Mosfet's available.
Here's a PWM using a 555 and a BUZ11.
http://www.uoguelph.ca/~antoon/circ/pwm555.html
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Author: Anthony FremontDate: 21:25 24-02-07
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John Larkin wrote:
> On Sat, 24 Feb 2007 19:30:46 -0600, "Anthony Fremont"
> <spam-not@nowhere.com> wrote:
>
>> John Larkin wrote:
>>> The drain current at "gate threshold" is only 1 mA. The Rdson at
10
>>> amps is spec'd with +10 on the gate. So this is of course not a
>>> logic-level fet, because it wouldn't be prudent to run a beast like
>>> this with just 3.3 or 5 volts on the gate.
>>
>> You are referring to the BUZ11 right?
>>
>
> Yup. The typical curves show it conducting pretty hard, 8 or 9 amps,
> with +5 on the gate, but that's not guaranteed. If your load is just a
> couple of amps, it's probably safe to go with +5 drive. But I wouldn't
> bet a production run on typical performance curves.
>
> At 3.3, it might conduct just a few mA, if that.
Just wanted to be sure. The NTE part I mentioned turns on hard at 4V drive.
They aren't the cheapest things, but so far I like them ok for a jelly-bean
part. Do you have a favorite cheapie logic-level part in this current
range?
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