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'[EE]: Switching Higher Voltage Sources with Power '
2002\09\08@212403 by Michael A. Powers

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Hello,

I have a problem in which I need to switch a 24V source from zero to maximum power using MOSFETS.  The problem is, I am having trouble finding a n-channel MOSFET that can handle 24Vgs.  Many that I have found have Vgs(max) at +- 20V.  Correct me if I am wrong, but to get maximum power from the device I can't have it saturate before Vds = 24V.   Does anyone have any advice on this?

Thanks.

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2002\09\08@214545 by A.J. Tufgar

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Michael,
Use a hexfet from irf.com, there's a huge selection to choose from.
You should choose a logic or 3V gate drive.

http://ec.irf.com/ec/adirect/ir?cmd=compare&category=55&categoryName=HEXFET+Power+MOSFETs%2F%2FDiscrete+HEXFET+Power+MOSFET%2F%2FN-Channel%2F25V+-+30V

Aaron

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2002\09\08@220245 by hard Prosser

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You do not need a Vgs of this order.
The Gate voltage is what turns the FET on and normally 12V is more than
enough to do the job.
The Vds rating needs to be greater than the voltage you are switching -
possibly much greater depending on the nature of the load (e.g. inductive
loads will generate voltage spike 20 times the supply voltage without too
much difficulty.)

The device also needs to be able to handle the peak and average load
current being switched and must have sufficient heatsinking and power
capability to ensure that the die temperaure does not exceed its maximum
rating. (150 - 175C)

I can't be more specific without more info - what is your load, your load
current, your switching rate and what are you planning to drive it from. -
and what environment is it operating in?

Richard P



Hello,

I have a problem in which I need to switch a 24V source from zero to
maximum power using MOSFETS.  The problem is, I am having trouble finding a
n-channel MOSFET that can handle 24Vgs.  Many that I have found have
Vgs(max) at +- 20V.  Correct me if I am wrong, but to get maximum power
from the device I can't have it saturate before Vds = 24V.   Does anyone
have any advice on this?

Thanks.

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2002\09\08@224111 by fred jones

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I am switching an inductive load (coil on a metal detector) with 24 volts
switching through the coil and an inductive spike that reaches avalanche at
over 400V.  I'm using an IRF740.  I put an IRF840LC low capacitance version
in my circuit and inductive kickback was at 650V but didn't reach avalanche.
 I had to put a hefty heatsink on it because it gets so hot.  I'm pulsing
it with 200us on time but am going to increase it so I'm moving to a heavier
FET in a TO-247 case.  All these FETs are rated for 10V gate voltage.
Good luck


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2002\09\08@234746 by hard Prosser

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Sounds more like you need the good luck!

Have you looked at IGBT's. You should be able to get one to switch OK in
your timeframe but they are somewhat better at handling higher voltages.
There's an IR aplication note about using IGBTs for ignition systems that
may be applicable.

One possibility is that your FET is getting hot because of ringing or even
oscillation - a well designed snubber will do wonders here without killing
your risetime. Putting a 12V or so zener on the gate of the FET may assist
in protecting it.


Richard P




I am switching an inductive load (coil on a metal detector) with 24 volts
switching through the coil and an inductive spike that reaches avalanche at
over 400V.  I'm using an IRF740.  I put an IRF840LC low capacitance version
in my circuit and inductive kickback was at 650V but didn't reach
avalanche.
 I had to put a hefty heatsink on it because it gets so hot.  I'm pulsing
it with 200us on time but am going to increase it so I'm moving to a
heavier
FET in a TO-247 case.  All these FETs are rated for 10V gate voltage.
Good luck


{Quote hidden}

snip.....

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2002\09\09@090623 by fred jones

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There are literally thousands and thousands of this circuit out there
running for years without any issues.  I'm looking to improve my product,
hence moving my on time from 200us to 1ms.  I understand that a heavier
switch of some sort is needed for this change.  The IRF740 is the standard
in this product as all of my competition is using it or some are even using
an IRF 540!  Why do you think I need luck.  Do you think that it shouldn't
be getting hot?  Do you think it can't handle this short pulse kickback?
Here is the FET I am considering for replacement when I increase the pulse
width:
www.irf.com/product-info/datasheets/data/irfp32n50k.pdf
Thanks,
Fred


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2002\09\09@092043 by Justin Grimm

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I once used a irf540 (a dozen in parallel) to switch 100 amps into a
transformer with no probs

-----Original Message-----
From: Richard Prosser [EraseMERichard.ProsserspamspamspamBeGoneENERGY.INVENSYS.COM]
Sent: Monday, 9 September 2002 13:46
To: RemoveMEPICLISTKILLspamspamMITVMA.MIT.EDU
Subject: Re: [EE]: Switching Higher Voltage Sources with Power MOSFETS


Sounds more like you need the good luck!

Have you looked at IGBT's. You should be able to get one to switch OK in
your timeframe but they are somewhat better at handling higher voltages.
There's an IR aplication note about using IGBTs for ignition systems that
may be applicable.

One possibility is that your FET is getting hot because of ringing or even
oscillation - a well designed snubber will do wonders here without killing
your risetime. Putting a 12V or so zener on the gate of the FET may assist
in protecting it.


Richard P




I am switching an inductive load (coil on a metal detector) with 24 volts
switching through the coil and an inductive spike that reaches avalanche at
over 400V.  I'm using an IRF740.  I put an IRF840LC low capacitance version
in my circuit and inductive kickback was at 650V but didn't reach
avalanche.
 I had to put a hefty heatsink on it because it gets so hot.  I'm pulsing
it with 200us on time but am going to increase it so I'm moving to a
heavier
FET in a TO-247 case.  All these FETs are rated for 10V gate voltage.
Good luck


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snip.....

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2002\09\09@095927 by fred jones

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PS - I have a 2W damping resistor across the coil.


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2002\09\09@164856 by hard Prosser

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OK, let's go back to the beginning.
You were origonally asking for a FET with a Vgs(max) greater than 24V.

What you need is a FET with a Vds(max) greater than the stress you are
going to apply to it - or at least capable of handing the  energy released
under avalanche breakdown,

As I understand it you have a search coil which you connect to a 24V source
for 200uS and then turn off. The resulting pulse is detected somehow & its
shape or size depends on what material is near the coil. At present, the
FET gets warm, so, with a longer on time you felel that you will need a
larger FET.

IIRC you stated that there was also a 22ohm resistor in parallel with the
coil - sorry - I lost that part.

Now, if the coil has an inductance of about 100uH (guessed), for a 24V
supply, the initial current di/dt will be about 24/100*10^-6 = 0 .24 A/uS
(from V=Ldi/dt). and the energy stored about 16mJ (from E=1/2 L *I^2)

The final current will be limited by the FET Rds(on), the coil resistance
and the 22 ohm resistance (If in series) - say about  1.3ohms (neglecting
the resistor as I'm not sure about that).
This results in a maximum current of about 18Amps. The current rise is
exponential but this will be reached somewhat before the 200uS has elapsed
so increasing the pulse width to 1mS will have little effect unless the
resistances are significantly reduced - and the coil (ac) resistance is
probably dominant here).

You mentioned being able to generate 650V turn-off spike  which wopuld
indicate a possible turn-off time of  about 3uS (again from V=Ldi/dt).
Without knowing any details about your detector I don't know if the
turn-off time is critical to the application, but if you increase the coil
current and keep the turn off time the same, then the output voltage will
increase proportionally.


Fill in your own figures in the above but I would suggest that to increase
the coil current you will need to either decrease the coil resistance or
increase the supply voltage. Either way, you may need to increase the
turn-off time to decrease the voltage spike or add other protection to the
FET.

The FET you suggest looks like it should be quite suitable 500V / 135mohms.
&  it has an avalanche rating of 46mJ so this should be OK also - although
with higher currents you may exceed this value - (hence the warning above).

The IRF540 used in other designs would need to be turned off a bit more
carefully as IIRC the Vds(max) is only about 100V. (Although the Rds(on) is
~70 mOhm. which would assist the max current) Possible these designs us a
different detector technique?

Hope this helps -

Richard P





There are literally thousands and thousands of this circuit out there
running for years without any issues.  I'm looking to improve my product,
hence moving my on time from 200us to 1ms.  I understand that a heavier
switch of some sort is needed for this change.  The IRF740 is the standard
in this product as all of my competition is using it or some are even using
an IRF 540!  Why do you think I need luck.  Do you think that it shouldn't
be getting hot?  Do you think it can't handle this short pulse kickback?
Here is the FET I am considering for replacement when I increase the pulse
width:
www.irf.com/product-info/datasheets/data/irfp32n50k.pdf
Thanks,
Fred


{Quote hidden}

at
>over 400V.  I'm using an IRF740.  I put an IRF840LC low capacitance
version
>in my circuit and inductive kickback was at 650V but didn't reach
>avalanche.
>   I had to put a hefty heatsink on it because it gets so hot.  I'm
pulsing
{Quote hidden}

inductive
> >loads will generate voltage spike 20 times the supply voltage without
too
> >much difficulty.)
> >
> >The device also needs to be able to handle the peak and average load
> >current being switched and must have sufficient heatsinking and power
> >capability to ensure that the die temperaure does not exceed its maximum
> >rating. (150 - 175C)
> >
> >I can't be more specific without more info - what is your load, your
load
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2002\09\09@182103 by fred jones

picon face
Actually, you have me confused with the original poster, however I enjoyed
your information and appreciate you looking over my shoulder on my circuit
changes.
Thanks,
Fred


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2002\09\09@182918 by hard Prosser

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Ah-ha - it is now becoming clear!
   - Yes I was confused on that

Richard P



Actually, you have me confused with the original poster, however I enjoyed
your information and appreciate you looking over my shoulder on my circuit
changes.
Thanks,
Fred


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2002\09\10@161951 by Mike Singer

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18Amps chaotically oscillating in the large
open coil - you may use it to jam hostile
radio broadcasts in all radio-frequency regions.

Mike :-)


fred jones <spamBeGoneboattowKILLspamspam@spam@HOTMAIL.COM> wrote:
.
.
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2002\09\10@180030 by Peter L. Peres

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On Tue, 10 Sep 2002, Mike Singer wrote:

> 18Amps chaotically oscillating in the large
>open coil - you may use it to jam hostile
>radio broadcasts in all radio-frequency regions.

You are very funny sometimes. There is a self resonant frequency of the
search coil, as you know, and people who do this tune it so it is in one
of the permitted frequencies. The chaos is elsewhere.

Peter

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2002\09\11@014841 by Mike Singer

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Yes, may be you are right. The only thing I'm struggling
to understand: for what heck these RF guys use sine
RF signal and all this chebyshev filters to feed emitting
devices.
Why not just switch chaotically large open coil by FET to
work at 303.825 Mhz for example, just adjusting slightly
"self resonant  frequency of the search coil".

73 ( Best regards)

Mike.

Peter L. Peres wrote:
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2002\09\11@022714 by Jim

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>  Why not just switch chaotically large open coil by FET to
>  work at 303.825 Mhz for example, just adjusting slightly

Because a "chaotically" (nice term!) switched coil would
have a very wide signal (wide bandwidth) interferring with
other nearby signals (and servcies), and, a simple coil is
not an efficient radiating device.

A Chebyshev filter after the output amplifier (usually
termed a PA or power amplifier) knocks down harmonic
energy before reaching the antenna port.

This is so that the harmonics (X2, x3 etc frequencies) do
not bother other radio and TV services operating in other
bands. Butterworth filters could be used, but part-for-part
(generally) don't yield as much attenuation (per unit
frequency or outside the passband) as a Chebyshev filter
is capable of.

For *really* wild results and marked attenuation outside
of the passband - I like the Elliptical filters ...

RF Jim


{Original Message removed}

2002\09\11@120942 by Mike Singer

picon face
Hi Jim,

It was a joke answer to Peter L. Peres statement that
switching on/off large open coil with FET could hold it
oscillating  at "one of the permitted frequencies".

Interestingly, could your posting help Peter?

Thank you.
Mike.

Jim wrote:
{Quote hidden}

> {Original Message removed}

2002\09\11@122435 by Jim

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Ya know - they used to do it that way - before
'spark' was banned!

We have some old docs in the FCC collection of historic
regulations that address this - and truly, if anything
was 'chaotic' it was spark-excited tank (tuned LC)
circuits!

Now we are coming full-circle, there are propositions
for WB (wide-band) RF transmissions for wide-band,
real-time 2-way data transfer in the microwave/millimeter
wave spectrum.

Ever heard the saying: "What comes around - goes around."

I used to take contention with that with 'the old guys'.
Now I am an 'old guy' and see the wisdom of that ... <BIG GRIN>

RF Jim


{Original Message removed}

2002\09\11@171754 by Peter L. Peres

picon face
On Wed, 11 Sep 2002, Mike Singer wrote:

> Hi Jim,
>
> It was a joke answer to Peter L. Peres statement that
>switching on/off large open coil with FET could hold it
>oscillating  at "one of the permitted frequencies".
>
> Interestingly, could your posting help Peter?

Basic electromagnetics question: A parallel RLC resonant circuit formed by
a flat aircoil of inductance L and its parasitic capacitance C is
connected through a switch to a DC voltage source U. After the current in
the coil reaches its limiting value Imax determined by U/Rcoil, the switch
is opened within negligible time. Describe the waveform of the current in
the coil, knowing that Rcoil << Zl at the resonant frequency of the
circuit.  Analyze the current and voltage in the time and frequency domain
over time (Bode plot). Analyze the energy equation of the circuit knowing
that the wavelength of the self resonant frequency of the circuit is very
large wrt. the physical dimensions of the circuit.

Peter

PS: There is a lot of chaos in more than one place.

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2002\09\11@171802 by Peter L. Peres

picon face
On Wed, 11 Sep 2002, Jim wrote:

>Ya know - they used to do it that way - before
>'spark' was banned!
>
>We have some old docs in the FCC collection of historic
>regulations that address this - and truly, if anything
>was 'chaotic' it was spark-excited tank (tuned LC)
>circuits!
>
>Now we are coming full-circle, there are propositions
>for WB (wide-band) RF transmissions for wide-band,
>real-time 2-way data transfer in the microwave/millimeter
>wave spectrum.
>
>Ever heard the saying: "What comes around - goes around."
>
>I used to take contention with that with 'the old guys'.
>Now I am an 'old guy' and see the wisdom of that ... <BIG GRIN>

Of course you know that there are people who build and operate SG
transmitters for fun in your country ? A SG transmitter can be made to
meet FCC rules with a lot of filtering, so it can be done.

Peter

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2002\09\11@180759 by Olin Lathrop

face picon face
> Basic electromagnetics question: A parallel RLC resonant circuit formed by
> a flat aircoil of inductance L and its parasitic capacitance C is
> connected through a switch to a DC voltage source U. After the current in
> the coil reaches its limiting value Imax determined by U/Rcoil, the switch
> is opened within negligible time. Describe the waveform of the current in
> the coil, knowing that Rcoil << Zl at the resonant frequency of the
> circuit.  Analyze the current and voltage in the time and frequency domain
> over time (Bode plot). Analyze the energy equation of the circuit knowing
> that the wavelength of the self resonant frequency of the circuit is very
> large wrt. the physical dimensions of the circuit.

Vcoil ~= sin(wt) * e**(-Kt)

> PS: There is a lot of chaos in more than one place.

This system is not chaotic.  If all the parameters were well known, it would
be reasonably predictable.  The whole point of chaotic systems is that there
is more than one possible response, and the choice of response can not be
predicted in any one instance.  At best, each response can be given a
probability.


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2002\09\11@184153 by Peter L. Peres

picon face
On Wed, 11 Sep 2002, Mike Singer wrote:

> Yes, may be you are right. The only thing I'm struggling
>to understand: for what heck these RF guys use sine
>RF signal and all this chebyshev filters to feed emitting
>devices.
> Why not just switch chaotically large open coil by FET to
>work at 303.825 Mhz for example, just adjusting slightly
>"self resonant  frequency of the search coil".

Because they are not allowed to use spark gap transmitters anymore ;-).
However metal seekers, mine seekers and geophysics equipment as well as
our friends in khaki with their radars ARE autorized to do just that when
needed. The coil is not chaotically large, It just has a size, and a
number of turns, you know ? The chaos is still elsewhere ...

Peter

>73 ( Best regards)

no need to translate that

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2002\09\11@193351 by Mike Singer

picon face
Peter L. Peres wrote:
.
> Because they are not allowed to use spark gap transmitters
> anymore ;-).
> However metal seekers, mine seekers and geophysics equipment
> as well as our friends in khaki with their radars ARE autorized to
> do just that when needed.

You forgot some List members. They use massive "spark gap"
style of postings. Rare, but massive sparks of a noise at full
bandwidth.

12.,   (Do you understand?)

Mike.

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2002\09\11@195648 by Nathan Hurst

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On Wed, 11 Sep 2002 18:07:52 -0400
"Olin Lathrop" <olin_piclistspam_OUTspam@spam@EMBEDINC.COM> wrote:

> > PS: There is a lot of chaos in more than one place.
>
> This system is not chaotic.  If all the parameters were well known, it
> would be reasonably predictable.  The whole point of chaotic systems
> is that there is more than one possible response, and the choice of
> response can not be predicted in any one instance.  At best, each
> response can be given a probability.

No, chaos is simply that the system is very sensitive to small changes
in input.  A common requirement is that two systems differing by epsilon
will tend to differ by e^kt epsilon after time t, k (Lyapunov exponent)
being positive.

Considering you have a negative power of e: Vcoil ~=
sin(wt) * e**(-Kt) in there I'd say it isn't likely to be chaotic.

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2002\09\11@204128 by Russell McMahon

face
flavicon
face
>  You forgot some List members. They use massive "spark gap"
> style of postings. Rare, but massive sparks of a noise at full
> bandwidth.

CQ CQ CQ DX DE ZLIANC CQ CQ ..............  :-)

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2002\09\12@154011 by Peter L. Peres

picon face
On Wed, 11 Sep 2002, Olin Lathrop wrote:

{Quote hidden}

Of course your response is 100% correct, but the point was that this is a
needle in respnose to needling by another list member, in this thread
(about broadcasting noise by switching a coil). He introduced the term
chaotic here, I was merely paraphrasing.

Or to put it in another way, with a cheap enough oscope any waveform looks
chaotic ;-)

Peter

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2002\09\12@181614 by Peter L. Peres

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On Wed, 11 Sep 2002, Olin Lathrop wrote:

>Vcoil ~= sin(wt) * e**(-Kt)

Yes, but more importantly, with reefrence to the 'chaos' and 'rf spectrum
pollution' another list member referred to, the Bode plot of this is a
nice bell curve (gauss I think) that corresponds to the 'modulation'
induced by the decay. The 3dB etc width of this curve is as usual given by
the Q of the RLC circuit, according to the usual formula for Bw. Thus
there is very little chaos and the spectrum pollution can be controlled
very well. Incidentally this is the same thing that makes spark gap
transmitters possible and legal (if filtered enough) at least for VLF CW.
With enough filtering ANY squarewave will become a suitably filtered
signal that will pass FCC tests for transmitters.

Peter

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2002\09\12@192010 by Peter L. Peres

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On Thu, 12 Sep 2002, Mike Singer wrote:

>Peter L. Peres wrote:
>.
>> Because they are not allowed to use spark gap transmitters
>> anymore ;-).
>> However metal seekers, mine seekers and geophysics equipment
>> as well as our friends in khaki with their radars ARE autorized to
>> do just that when needed.
>
> You forgot some List members. They use massive "spark gap"
>style of postings. Rare, but massive sparks of a noise at full
>bandwidth.
>
>12.,   (Do you understand?)

Yes, yes, please stop ;-)

Peter

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