>From: Richard Prosser <EraseMERichard.Prosserspam_OUTTakeThisOuTENERGY.INVENSYS.COM>
>Reply-To: pic microcontroller discussion list <PICLISTspam_OUTMITVMA.MIT.EDU>
>To: @spam@PICLISTKILLspamMITVMA.MIT.EDU
>Subject: Re: [EE]: Switching Higher Voltage Sources with Power MOSFETS
>Date: Mon, 9 Sep 2002 14:01:45 +1200
>
>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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>From: Richard Prosser <TakeThisOuTRichard.ProsserEraseMEspam_OUTENERGY.INVENSYS.COM>

>
>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
>

>From: Richard Prosser <Richard.ProsserEraseME.....ENERGY.INVENSYS.COM>
>Reply-To: pic microcontroller discussion list <EraseMEPICLISTMITVMA.MIT.EDU>
>To: RemoveMEPICLISTEraseMEEraseMEMITVMA.MIT.EDU
>Subject: Re: [EE]: Switching Higher Voltage Sources with Power MOSFETS
>Date: Mon, 9 Sep 2002 15:46:11 +1200
>
>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
>
>
> >From: Richard Prosser <RemoveMERichard.Prosserspam_OUTKILLspamENERGY.INVENSYS.COM>
>
> >
> >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
> >
>snip.....
>
>--
>http://www.piclist.com hint: To leave the PICList
>RemoveMEpiclist-unsubscribe-requestTakeThisOuTspammitvma.mit.edu

>From: Richard Prosser <Richard.ProsserSTOPspamspam_OUTENERGY.INVENSYS.COM>

>
>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
>

>From: fred jones <KILLspamboattowspamBeGoneHOTMAIL.COM>
>Reply-To: pic microcontroller discussion list <EraseMEPICLISTEraseMEMITVMA.MIT.EDU>
>To: @spam@PICLIST@spam@spam_OUTMITVMA.MIT.EDU
>Subject: Re: [EE]: Switching Higher Voltage Sources with Power MOSFETS
>Date: Mon, 9 Sep 2002 08:06:00 -0500
>
>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
>
>
>>From: Richard Prosser <spamBeGoneRichard.ProsserKILLspamENERGY.INVENSYS.COM>
>>Reply-To: pic microcontroller discussion list <.....PICLISTspam_OUTMITVMA.MIT.EDU>
>>To: TakeThisOuTPICLIST.....TakeThisOuTMITVMA.MIT.EDU
>>Subject: Re: [EE]: Switching Higher Voltage Sources with Power MOSFETS
>>Date: Mon, 9 Sep 2002 15:46:11 +1200
>>
>>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
>>
>>
>> >From: Richard Prosser <TakeThisOuTRichard.ProsserKILLspamspamENERGY.INVENSYS.COM>
>>
>> >
>> >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
>> >
>>snip.....
>>
>>--
>>http://www.piclist.com hint: To leave the PICList
>>.....piclist-unsubscribe-requestRemoveMEmitvma.mit.edu
>
>
>
>
>_________________________________________________________________
>Send and receive Hotmail on your mobile device: http://mobile.msn.com
>
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>[PIC]:,[SX]:,[AVR]: ->uP ONLY! [EE]:,[OT]: ->Other [BUY]:,[AD]: ->Ads

>From: Richard Prosser <RemoveMERichard.ProsserspamBeGoneENERGY.INVENSYS.COM>
>Reply-To: pic microcontroller discussion list <spamBeGonePICLIST@spam@spam_OUTMITVMA.MIT.EDU>
>To: TakeThisOuTPICLISTspamMITVMA.MIT.EDU
>Subject: Re: [EE]: Switching Higher Voltage Sources with Power MOSFETS
>Date: Mon, 9 Sep 2002 15:46:11 +1200
>
>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

>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
>
>
> >From: Richard Prosser <Richard.ProsserEraseMEENERGY.INVENSYS.COM>
>
> >
> >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.

> >current, your switching rate and what are you planning to drive it from.
>-
> >and what environment is it operating in?
> >
> >Richard P
> >
>snip.....
>
>--
>http://www.piclist.com hint: To leave the PICList
>RemoveMEpiclist-unsubscribe-requestEraseMEspam_OUTmitvma.mit.edu

>From: Richard Prosser <@spam@Richard.ProsserRemoveMEEraseMEENERGY.INVENSYS.COM>
>Reply-To: pic microcontroller discussion list <EraseMEPICLIST@spam@MITVMA.MIT.EDU>
>To: @spam@PICLISTspam_OUT.....MITVMA.MIT.EDU
>Subject: Re: [EE]: Switching Higher Voltage Sources with Power MOSFETS
>Date: Tue, 10 Sep 2002 08:44:54 +1200
>
>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
>
>
> >From: Richard Prosser <spamBeGoneRichard.ProsserEraseMEENERGY.INVENSYS.COM>
> >Reply-To: pic microcontroller discussion list <PICLISTspamBeGoneMITVMA.MIT.EDU>
> >To: RemoveMEPICLIST@spam@spamBeGoneMITVMA.MIT.EDU
> >Subject: Re: [EE]: Switching Higher Voltage Sources with Power MOSFETS
> >Date: Mon, 9 Sep 2002 15:46:11 +1200
> >
> >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
> >
> >
> > >From: Richard Prosser <.....Richard.Prosser@spam@EraseMEENERGY.INVENSYS.COM>
> >
> > >
> > >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
> > >
> >snip.....
> >
> >--
> >http://www.piclist.com hint: To leave the PICList
> >.....piclist-unsubscribe-requestRemoveMEmitvma.mit.edu
>
>
>
>
>_________________________________________________________________
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>
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>From: Richard Prosser <.....Richard.ProsserSTOPspam@spam@ENERGY.INVENSYS.COM>
>Reply-To: pic microcontroller discussion list <PICLISTEraseME@spam@MITVMA.MIT.EDU>
>To: RemoveMEPICLISTspamBeGoneMITVMA.MIT.EDU
>Subject: Re: [EE]: Switching Higher Voltage Sources with Power MOSFETS
>Date: Tue, 10 Sep 2002 08:44:54 +1200
>
>OK, let's go back to the beginning.


> >snip.....

>> Richard Prosser wrote:
.
.
>> 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 ...
.
.

> 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.

> >  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
>
>

>> 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.