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Discussion Groups | Piclist | Re: MOSFET - PIC Microcontroller

A discussion group for the PICMicro microcontroller. Also called the Microchip PIC, this list is dedicated to the use and abuse of this fine, simple, microcontroller. Close to topic posts are welcome, ie. general electronics.

MOSFET - PIC Microcontroller - devonsc - Dec 1 15:32:00 2004



Hi there,

Hope you guys don't mind me asking something simple. Just some
questions regarding reading the datasheet of a MOSFET. Need help
here. I'm searching for a MOSFET for my DC/DC Converter and I have
problem understanding the datasheet.

If it is not too much trouble, can anyone briefly explain the
following?

a.) Drain-Source Voltage
b.) Drain-Gate Voltage
c.) Gate-Source Voltage
d.) Gate-Source Threshold Voltage
d.) Drain Current

How do we refer to the minimum current required to turn on the
MOSFET? By the way, I will be feeding the MOSFET
from a PWM module in a PIC microcontroller.

Thanks in advance...Real sorry to ask about this. Please don't get
mad, guys.

Apart from that, is the voltage output from the PIC's pin
approximately 5V? How about the curent output from the PIC's pin?

Thanks in advance...






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Re: MOSFET - PIC Microcontroller - Leon Heller - Dec 4 14:08:00 2004

----- Original Message -----
From: "devonsc" <>
To: <>
Sent: Wednesday, December 01, 2004 8:32 PM
Subject: [piclist] MOSFET - PIC Microcontroller > Hi there,
>
> Hope you guys don't mind me asking something simple. Just some
> questions regarding reading the datasheet of a MOSFET. Need help
> here. I'm searching for a MOSFET for my DC/DC Converter and I have
> problem understanding the datasheet.
>
> If it is not too much trouble, can anyone briefly explain the
> following?
>
> a.) Drain-Source Voltage
> b.) Drain-Gate Voltage
> c.) Gate-Source Voltage
> d.) Gate-Source Threshold Voltage
> d.) Drain Current
>
> How do we refer to the minimum current required to turn on the
> MOSFET? By the way, I will be feeding the MOSFET
> from a PWM module in a PIC microcontroller.
>
> Thanks in advance...Real sorry to ask about this. Please don't get
> mad, guys.
>
> Apart from that, is the voltage output from the PIC's pin
> approximately 5V? How about the curent output from the PIC's pin?

FWIW, I'm using a Zetex ZVN4210A MOSFET as a driver, using a buck converter
arrangement, to put 250 mA through two strings of six LEDs. It's a
high-visibility LED beacon application. I'm turning the MOSFET on and off
with a 5V signal from a Tiny11 AVR, but a PIC would have worked just as
well. I'm not using full PWM, just different pulse widths at 38 kHz to cope
with different supply voltages and to give different brightness levels.

MOSFETs are voltage-driven devices, and take very little gate current.
Depending on the MOSFET characteristics, you might be able to drive it
directly as I am doing. The PIC output is CMOS and swings between 0 and Vcc.

Leon



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Re: MOSFET - PIC Microcontroller - Chad Russel - Dec 4 14:34:00 2004

--- devonsc <> wrote: >
> Hi there,
>
> Hope you guys don't mind me asking something simple. Just some
> questions regarding reading the datasheet of a MOSFET. Need help
> here. I'm searching for a MOSFET for my DC/DC Converter and I have
> problem understanding the datasheet.
>
> If it is not too much trouble, can anyone briefly explain the
> following?
>
MOSFET is a majority carrier, voltage controlled device, hence voltages
are more important than currents. Most common is enhancement mode
N-channel, which means than a more positive voltage on the gate
relative to the source will increase current flow. The Gate and Drain
of an N-channel should be positive relative to the source, negative on
a P-channnel

> a.) Drain-Source Voltage
voltage between the Drain and Source, your load will be on one of these
2 pins.

> b.) Drain-Gate Voltage
this is not relavent in most situations.

> c.) Gate-Source Voltage
the voltage of the gate relative to the source, this is usally strictly
limited in a MOSFET and can be + and -

> d.) Gate-Source Threshold Voltage
The voltage that enhancement begins, i.e., the MOSFET start to turn on.
This is usually speced at a value, depending on the MOSFET, often 1
ma. at a reasonable drain-source voltage.

> d.) Drain Current
the current flowing into the drain, and on a MOSFET also the souce
current. > How do we refer to the minimum current required to turn on the
> MOSFET?
Gate threshold voltage. No current flows in the gate, except charge
and discharge current, because the gate looks like a capacitor to the
driving circuit.

By the way, I will be feeding the MOSFET
> from a PWM module in a PIC microcontroller.

Therefore you will want a logic level MOSFET, which is one that is
fully enhanced at 5 vdc gate to source voltage. Standard MOSFET are
around 10 volts gate to source.
>
> Thanks in advance...Real sorry to ask about this. Please don't get
> mad, guys.
>
> Apart from that, is the voltage output from the PIC's pin
> approximately 5V?
Yes

> How about the curent output from the PIC's pin?
Normally limited to about 20 ma source and sink. Driving a MOSFET at
DC the current requirement is zero. When swithing it is proportional
to the switching speed and the gate capacitance of the driven MOSFET. > Thanks in advance... =====
My software has no bugs, only undocumented features.
__________________________________






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Re: MOSFET - PIC Microcontroller - sirtiffguy - Dec 4 17:48:00 2004


Only one thing to add to Chad's comments. The higher you have your
gate to source voltage (turn on voltage) the less your Rds
(Equivalent drain to source resistance) which for the DC/DC
application, you want. I would recommend going 75-85% of the max
voltage allowed (for the irf730 i think we use 15V).

If your gate voltage is not high enough, you are going to leave the
saturation region, and burn out the mosfet (add a tonne of heat).

--- In , Chad Russel <chadrussel@y...> wrote:
> --- devonsc <devonsc@y...> wrote:
>
> >
> >
> > Hi there,
> >
> > Hope you guys don't mind me asking something simple. Just some
> > questions regarding reading the datasheet of a MOSFET. Need help
> > here. I'm searching for a MOSFET for my DC/DC Converter and I
have
> > problem understanding the datasheet.
> >
> > If it is not too much trouble, can anyone briefly explain the
> > following?
> >
> MOSFET is a majority carrier, voltage controlled device, hence
voltages
> are more important than currents. Most common is enhancement mode
> N-channel, which means than a more positive voltage on the gate
> relative to the source will increase current flow. The Gate and
Drain
> of an N-channel should be positive relative to the source,
negative on
> a P-channnel
>
> > a.) Drain-Source Voltage
> voltage between the Drain and Source, your load will be on one of
these
> 2 pins.
>
> > b.) Drain-Gate Voltage
> this is not relavent in most situations.
>
> > c.) Gate-Source Voltage
> the voltage of the gate relative to the source, this is usally
strictly
> limited in a MOSFET and can be + and -
>
> > d.) Gate-Source Threshold Voltage
> The voltage that enhancement begins, i.e., the MOSFET start to
turn on.
> This is usually speced at a value, depending on the MOSFET, often
1
> ma. at a reasonable drain-source voltage.
>
> > d.) Drain Current
> the current flowing into the drain, and on a MOSFET also the souce
> current.
>
> >
> > How do we refer to the minimum current required to turn on the
> > MOSFET?
> Gate threshold voltage. No current flows in the gate, except
charge
> and discharge current, because the gate looks like a capacitor to
the
> driving circuit.
>
> By the way, I will be feeding the MOSFET
> > from a PWM module in a PIC microcontroller.
>
> Therefore you will want a logic level MOSFET, which is one that is
> fully enhanced at 5 vdc gate to source voltage. Standard MOSFET
are
> around 10 volts gate to source.
> >
> > Thanks in advance...Real sorry to ask about this. Please don't
get
> > mad, guys.
> >
> > Apart from that, is the voltage output from the PIC's pin
> > approximately 5V?
> Yes
>
> > How about the curent output from the PIC's pin?
> Normally limited to about 20 ma source and sink. Driving a MOSFET
at
> DC the current requirement is zero. When swithing it is
proportional
> to the switching speed and the gate capacitance of the driven
MOSFET.
>
> >
> > Thanks in advance... > =====
> My software has no bugs, only undocumented features. >
> __________________________________






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MOSFET - PIC Microcontroller - Devon Lee - Dec 5 15:08:00 2004

Hi there,

Quote: [I would recommend going 75-85% of the max
voltage allowed….]

Does it mean that operating the MOSFET at the Gate
Threshold Voltage (Vgs(threshold)) level will not
provide a good efficiency in my boost DC/DC Converter
and should apply 15V? As an example, IRF540N:

Vgs = +/- 20V
Vgs (threshold) = 2 to 4V

I’ve tried to search for lower Vgs MOSFET, it seems
that the lowest that I could find is +/-10V. And since
I’m operating the MOSFET through a PWM signal from a
PIC (output voltage of 5V), meaning I shall search for
a MOSFET with the Vgs of approximately 6V, is this
right?

The thought of switching to use a logic MOSFET:
I’ve tried searching logic MOSFETs, but by looking at
the datasheet, how do we differentiate between a
MOSFET and a logic MOSFET? For instance, FDP6030BL. I
couldn’t read the difference of a MOSFET and a logic
MOSFET from a datasheet. Any advice or guidance?
Please?

Thanks in advance…

*Gate to Source Voltage (Vgs)
*Gate Threshold Voltage (Vgs (threshold))
--- sirtiffguy <> wrote: > Only one thing to add to Chad's comments. The
> higher you have your
> gate to source voltage (turn on voltage) the less
> your Rds
> (Equivalent drain to source resistance) which for
> the DC/DC
> application, you want. I would recommend going
> 75-85% of the max
> voltage allowed (for the irf730 i think we use 15V).
>
> If your gate voltage is not high enough, you are
> going to leave the
> saturation region, and burn out the mosfet (add a
> tonne of heat).
>
> --- In , Chad Russel
> <chadrussel@y...> wrote:
> > --- devonsc <devonsc@y...> wrote:
> >
> > >
> > >
> > > Hi there,
> > >
> > > Hope you guys don't mind me asking something
> simple. Just some
> > > questions regarding reading the datasheet of a
> MOSFET. Need help
> > > here. I'm searching for a MOSFET for my DC/DC
> Converter and I
> have
> > > problem understanding the datasheet.
> > >
> > > If it is not too much trouble, can anyone
> briefly explain the
> > > following?
> > >
> > MOSFET is a majority carrier, voltage controlled
> device, hence
> voltages
> > are more important than currents. Most common is
> enhancement mode
> > N-channel, which means than a more positive
> voltage on the gate
> > relative to the source will increase current flow.
> The Gate and
> Drain
> > of an N-channel should be positive relative to the
> source,
> negative on
> > a P-channnel
> >
> > > a.) Drain-Source Voltage
> > voltage between the Drain and Source, your load
> will be on one of
> these
> > 2 pins.
> >
> > > b.) Drain-Gate Voltage
> > this is not relavent in most situations.
> >
> > > c.) Gate-Source Voltage
> > the voltage of the gate relative to the source,
> this is usally
> strictly
> > limited in a MOSFET and can be + and -
> >
> > > d.) Gate-Source Threshold Voltage
> > The voltage that enhancement begins, i.e., the
> MOSFET start to
> turn on.
> > This is usually speced at a value, depending on
> the MOSFET, often
> 1
> > ma. at a reasonable drain-source voltage.
> >
> > > d.) Drain Current
> > the current flowing into the drain, and on a
> MOSFET also the souce
> > current.
> >
> > >
> > > How do we refer to the minimum current required
> to turn on the
> > > MOSFET?
> > Gate threshold voltage. No current flows in the
> gate, except
> charge
> > and discharge current, because the gate looks like
> a capacitor to
> the
> > driving circuit.
> >
> > By the way, I will be feeding the MOSFET
> > > from a PWM module in a PIC microcontroller.
> >
> > Therefore you will want a logic level MOSFET,
> which is one that is
> > fully enhanced at 5 vdc gate to source voltage.
> Standard MOSFET
> are
> > around 10 volts gate to source.
> > >
> > > Thanks in advance...Real sorry to ask about
> this. Please don't
> get
> > > mad, guys.
> > >
> > > Apart from that, is the voltage output from the
> PIC's pin
> > > approximately 5V?
> > Yes
> >
> > > How about the curent output from the PIC's pin?
> > Normally limited to about 20 ma source and sink.
> Driving a MOSFET
> at
> > DC the current requirement is zero. When swithing
> it is
> proportional
> > to the switching speed and the gate capacitance of
> the driven
> MOSFET.
> >
> > >
> > > Thanks in advance...
> >
> >
> > =====
> > My software has no bugs, only undocumented
> features.
> >
> >
> >
> > __________________________________
> __________________________________






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RE: MOSFET - PIC Microcontroller - AHardinger - Dec 5 15:37:00 2004

Hello,

The main difference with a logic MOSFET is that it's fully turned on with a
gate voltage of 5V supplied to it. Basically, standard 5V logic circuits &
uM will be able to turn it on all the way and avoid dropping a lot of power
in the MOSFET.

The gate of a MOSFET is much like a capacitor. When you switch the device
on the capacitor is going to suck up current and the actual gate voltage
will rise slowly, in terms of nano seconds. The current requirements from
your drive circuit depends of how fast you need to turn the MOSFET on or
off. The more current you have available, the faster you can pump up the
gate capacitor. Likewise, when you want to turn off the MOSFET you need to
suck the charge out of the gate capacitor. Note, that if you are turning
the MOSFET on and off, the MOSFET actually consumes no drive current at all.
It gives back what you put into it, technically speaking. In reality that
current is converted to heat in resistance residing in the gate drive
circuits and/or gate resistors. You should have a resistor in series with
the gate to prevent oscillations from occurring and the limit the peak
current pulled from the gate drive circuit. When you turn on the MOSFET the
capacitor will be empty and will look like a short circuit for a very brief
time. A resistor will limit this peak current to something safe for a PIC.

A lot of times designers will only drive the gate up to 70% of the rated
gate voltage because MOSFETS are usually turned on as much as they possible
can be below their rated gate voltages. Driving the voltage up higher than
that %70-%80 just means you have to pump more current into the gate
capacitor and it takes longer to turn them on and off. I can't see a way
for you to control the gate voltage applied to the MOSFET when you drive it
from a PIC, and unless you are switching at a very high frequency with a
large MOSFET, you shouldn't worry about hitting just %70-%80.

I have a formula for figuring out how much drive current is needed to switch
a MOSFET given the frequency you want to operate it at and the gate
capacitance. However, I don't have access to my books at the moment, so if
you want it please let me know and I will dig it out for you.

Aaron

-----Original Message-----
From: Devon Lee [mailto:]
Sent: Sunday, December 05, 2004 12:09 PM
To:
Subject: [piclist] MOSFET - PIC Microcontroller
Hi there,

Quote: [I would recommend going 75-85% of the max
voltage allowed....]

Does it mean that operating the MOSFET at the Gate
Threshold Voltage (Vgs(threshold)) level will not
provide a good efficiency in my boost DC/DC Converter
and should apply 15V? As an example, IRF540N:

Vgs = +/- 20V
Vgs (threshold) = 2 to 4V

I've tried to search for lower Vgs MOSFET, it seems
that the lowest that I could find is +/-10V. And since
I'm operating the MOSFET through a PWM signal from a
PIC (output voltage of 5V), meaning I shall search for
a MOSFET with the Vgs of approximately 6V, is this
right?

The thought of switching to use a logic MOSFET:
I've tried searching logic MOSFETs, but by looking at
the datasheet, how do we differentiate between a
MOSFET and a logic MOSFET? For instance, FDP6030BL. I
couldn't read the difference of a MOSFET and a logic
MOSFET from a datasheet. Any advice or guidance?
Please?

Thanks in advance...

*Gate to Source Voltage (Vgs)
*Gate Threshold Voltage (Vgs (threshold))
--- sirtiffguy <> wrote: > Only one thing to add to Chad's comments. The
> higher you have your
> gate to source voltage (turn on voltage) the less
> your Rds
> (Equivalent drain to source resistance) which for
> the DC/DC
> application, you want. I would recommend going
> 75-85% of the max
> voltage allowed (for the irf730 i think we use 15V).
>
> If your gate voltage is not high enough, you are
> going to leave the
> saturation region, and burn out the mosfet (add a
> tonne of heat).
>
> --- In , Chad Russel
> <chadrussel@y...> wrote:
> > --- devonsc <devonsc@y...> wrote:
> >
> > >
> > >
> > > Hi there,
> > >
> > > Hope you guys don't mind me asking something
> simple. Just some
> > > questions regarding reading the datasheet of a
> MOSFET. Need help
> > > here. I'm searching for a MOSFET for my DC/DC
> Converter and I
> have
> > > problem understanding the datasheet.
> > >
> > > If it is not too much trouble, can anyone
> briefly explain the
> > > following?
> > >
> > MOSFET is a majority carrier, voltage controlled
> device, hence
> voltages
> > are more important than currents. Most common is
> enhancement mode
> > N-channel, which means than a more positive
> voltage on the gate
> > relative to the source will increase current flow.
> The Gate and
> Drain
> > of an N-channel should be positive relative to the
> source,
> negative on
> > a P-channnel
> >
> > > a.) Drain-Source Voltage
> > voltage between the Drain and Source, your load
> will be on one of
> these
> > 2 pins.
> >
> > > b.) Drain-Gate Voltage
> > this is not relavent in most situations.
> >
> > > c.) Gate-Source Voltage
> > the voltage of the gate relative to the source,
> this is usally
> strictly
> > limited in a MOSFET and can be + and -
> >
> > > d.) Gate-Source Threshold Voltage
> > The voltage that enhancement begins, i.e., the
> MOSFET start to
> turn on.
> > This is usually speced at a value, depending on
> the MOSFET, often
> 1
> > ma. at a reasonable drain-source voltage.
> >
> > > d.) Drain Current
> > the current flowing into the drain, and on a
> MOSFET also the souce
> > current.
> >
> > >
> > > How do we refer to the minimum current required
> to turn on the
> > > MOSFET?
> > Gate threshold voltage. No current flows in the
> gate, except
> charge
> > and discharge current, because the gate looks like
> a capacitor to
> the
> > driving circuit.
> >
> > By the way, I will be feeding the MOSFET
> > > from a PWM module in a PIC microcontroller.
> >
> > Therefore you will want a logic level MOSFET,
> which is one that is
> > fully enhanced at 5 vdc gate to source voltage.
> Standard MOSFET
> are
> > around 10 volts gate to source.
> > >
> > > Thanks in advance...Real sorry to ask about
> this. Please don't
> get
> > > mad, guys.
> > >
> > > Apart from that, is the voltage output from the
> PIC's pin
> > > approximately 5V?
> > Yes
> >
> > > How about the curent output from the PIC's pin?
> > Normally limited to about 20 ma source and sink.
> Driving a MOSFET
> at
> > DC the current requirement is zero. When swithing
> it is
> proportional
> > to the switching speed and the gate capacitance of
> the driven
> MOSFET.
> >
> > >
> > > Thanks in advance...
> >
> >
> > =====
> > My software has no bugs, only undocumented
> features.
> >
> >
> >
> > __________________________________
> __________________________________
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Re: MOSFET - PIC Microcontroller - Chad Russel - Dec 5 18:43:00 2004

I really do not like to disagree with another PIClister, but the
maximum Vgs allowed it just that. You touch it for a microsecond and
goodbye MOSFET. I try whenever possible to leave a 2:1 safety margin.
If Vgs is +/-20, I will keep it to +/-10 if possible. Maximum Vgs only
tells you how much abuse the device will take.

You need a MOSFET that is saturated at 5 volts Vgs. Normally these
have an L in the part number. Threshold voltage is not truly an
indication of logic level, the threshold and tranconductance determine
if it is Logic level or not.

Usually the manufacture will show Rdson at a gate voltage of 5 for
logic level devices.

Good luck,
Chad
--- Devon Lee <> wrote: > Hi there,
>
> Quote: [I would recommend going 75-85% of the max
> voltage allowed….]
>
> Does it mean that operating the MOSFET at the Gate
> Threshold Voltage (Vgs(threshold)) level will not
> provide a good efficiency in my boost DC/DC Converter
> and should apply 15V? As an example, IRF540N:
>
> Vgs = +/- 20V
> Vgs (threshold) = 2 to 4V
>
> I’ve tried to search for lower Vgs MOSFET, it seems
> that the lowest that I could find is +/-10V. And since
> I’m operating the MOSFET through a PWM signal from a
> PIC (output voltage of 5V), meaning I shall search for
> a MOSFET with the Vgs of approximately 6V, is this
> right?
>
> The thought of switching to use a logic MOSFET:
> I’ve tried searching logic MOSFETs, but by looking at
> the datasheet, how do we differentiate between a
> MOSFET and a logic MOSFET? For instance, FDP6030BL. I
> couldn’t read the difference of a MOSFET and a logic
> MOSFET from a datasheet. Any advice or guidance?
> Please?
>
> Thanks in advance…
>
> *Gate to Source Voltage (Vgs)
> *Gate Threshold Voltage (Vgs (threshold)) >
> --- sirtiffguy <> wrote:
>
> >
> > Only one thing to add to Chad's comments. The
> > higher you have your
> > gate to source voltage (turn on voltage) the less
> > your Rds
> > (Equivalent drain to source resistance) which for
> > the DC/DC
> > application, you want. I would recommend going
> > 75-85% of the max
> > voltage allowed (for the irf730 i think we use 15V).
> >
> > If your gate voltage is not high enough, you are
> > going to leave the
> > saturation region, and burn out the mosfet (add a
> > tonne of heat).
> >
> > --- In , Chad Russel
> > <chadrussel@y...> wrote:
> > > --- devonsc <devonsc@y...> wrote:
> > >
> > > >
> > > >
> > > > Hi there,
> > > >
> > > > Hope you guys don't mind me asking something
> > simple. Just some
> > > > questions regarding reading the datasheet of a
> > MOSFET. Need help
> > > > here. I'm searching for a MOSFET for my DC/DC
> > Converter and I
> > have
> > > > problem understanding the datasheet.
> > > >
> > > > If it is not too much trouble, can anyone
> > briefly explain the
> > > > following?
> > > >
> > > MOSFET is a majority carrier, voltage controlled
> > device, hence
> > voltages
> > > are more important than currents. Most common is
> > enhancement mode
> > > N-channel, which means than a more positive
> > voltage on the gate
> > > relative to the source will increase current flow.
> > The Gate and
> > Drain
> > > of an N-channel should be positive relative to the
> > source,
> > negative on
> > > a P-channnel
> > >
> > > > a.) Drain-Source Voltage
> > > voltage between the Drain and Source, your load
> > will be on one of
> > these
> > > 2 pins.
> > >
> > > > b.) Drain-Gate Voltage
> > > this is not relavent in most situations.
> > >
> > > > c.) Gate-Source Voltage
> > > the voltage of the gate relative to the source,
> > this is usally
> > strictly
> > > limited in a MOSFET and can be + and -
> > >
> > > > d.) Gate-Source Threshold Voltage
> > > The voltage that enhancement begins, i.e., the
> > MOSFET start to
> > turn on.
> > > This is usually speced at a value, depending on
> > the MOSFET, often
> > 1
> > > ma. at a reasonable drain-source voltage.
> > >
> > > > d.) Drain Current
> > > the current flowing into the drain, and on a
> > MOSFET also the souce
> > > current.
> > >
> > > >
> > > > How do we refer to the minimum current required
> > to turn on the
> > > > MOSFET?
> > > Gate threshold voltage. No current flows in the
> > gate, except
> > charge
> > > and discharge current, because the gate looks like
> > a capacitor to
> > the
> > > driving circuit.
> > >
> > > By the way, I will be feeding the MOSFET
> > > > from a PWM module in a PIC microcontroller.
> > >
> > > Therefore you will want a logic level MOSFET,
> > which is one that is
> > > fully enhanced at 5 vdc gate to source voltage.
> > Standard MOSFET
> > are
> > > around 10 volts gate to source.
> > > >
> > > > Thanks in advance...Real sorry to ask about
> > this. Please don't
> > get
> > > > mad, guys.
> > > >
> > > > Apart from that, is the voltage output from the
> > PIC's pin
> > > > approximately 5V?
> > > Yes
> > >
> > > > How about the curent output from the PIC's pin?
> > > Normally limited to about 20 ma source and sink.
> > Driving a MOSFET
> > at
> > > DC the current requirement is zero. When swithing
> > it is
> > proportional
> > > to the switching speed and the gate capacitance of
> > the driven
> > MOSFET.
> > >
> > > >
> > > > Thanks in advance...
> > >
> > >
> > > =====
> > > My software has no bugs, only undocumented
> > features.
> > >
> > >
> > >
> > > __________________________________
> > >
> >
> >
> >
> >
>
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=== message truncated === =====
My software has no bugs, only undocumented features.
__________________________________





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