MOSFET Usage
in [Design]
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From: ian field on 26 May 2010 10:38 "Ron M." <strmbrgr2(a)hotmail.com> wrote in message news:57432e16-bcdf-4a2f-b78a-7b526bfa7784(a)c7g2000vbc.googlegroups.com... On May 25, 5:25 pm, "ian field" <gangprobing.al...(a)ntlworld.com> wrote: > "Ron M." <strmbr...(a)hotmail.com> wrote in message > > news:f45e170c-7062-4a9b-ab08-d38ac1b3b997(a)y12g2000vbg.googlegroups.com... > On May 25, 7:56 am, neddie <seegoo...(a)yahoo.com> wrote: > > > > > > > On May 25, 1:32 am, "Ron M." <strmbr...(a)hotmail.com> wrote: > > > > On May 22, 10:42 am, "Joe G \(Home\)" <jo...(a)optusnet.com.au> wrote: > > > > > "Ron M." <strmbr...(a)hotmail.com> wrote in message > > > > >news:a67274a4-1270-45cf-8c90-4f139be8ad3c(a)y12g2000vbr.googlegroups.com... > > > > > >I am in the process of designing a device that uses 30A MOSFET for > > > > >the > > > > > output. I need it to apply a 12 Vdc source to a switchable load on > > > > > the > > > > > ground side. Are there any major issues using a MOSFET in this > > > > > way? > > > > > What is the load type, inductive (motor etc) capacitive, resistive? > > > > > What is the peak current on switch on? > > > > > Have you done thermal calcuations of the MOSFET and heat sink? > > > > > What is the operating temprature range? Do the thermal calcualtions > > > > meet > > > > the MOSFET thermal requirements? > > > > > What is the operating voltage? Is the max_Vds of the MOSEFT much > > > > greater > > > > than the operating voltage. > > > > > What is the Vgs_turn_voltage? Is it much greater than the MOSFET > > > > Vgs_turn_on_threshold? > > > > > If you answer these questions then you are well on your way to > > > > operating the > > > > MOSFET correctly. > > > > > PS... good clues on MOSFET operation may be obtained from here. > > > > >http://www.btipnow.com/events/training.php > > > > > Joe > > > > I knew this would go crazy if didn't spell things out. OK here we go. > > > The MOSFET will be switching a 2 ohm (resistive) load for approx 100 - > > > 200 ms. It will be controlled via 555 output. The MOSFET needs to have > > > plus attached all the time and the load will be between the MOSFET and > > > ground and will be several loads switched in one at a time. Basically > > > this is the output for a sequencer that will be used to provide power > > > to several different strings of devices (9 outputs in sequence by 20 > > > sequences). The different sequences will be switched in manually using > > > diode matrixing and siwtched grounds (relay). I know the MOSFET can > > > handle the amount of current and power factors being used. Just wasn't > > > sure if the plus could be connected to the MOSFET all the time and the > > > load between the MOSFET and ground. Haven't used them much and wanted > > > to save myself some grief during the design stages. They aren't that > > > expensive $1.80 each but why blow em up if you don't gotta. THANKS for > > > all the responses. Hopefully this will be enough for an informed > > > opinion. THANKS folks. > > > Do you want to use N-channel or P-channel mosfets. Using the latter > > would be easier > > for switching grounded loads.- Hide quoted text - > > > - Show quoted text - > > The actual device is an NTE2376 30A MOSFET crossed over to an > IRFP250NPbf. The device is an N-channel Enhancement type. Just want to > know if I can place the load on the Source side instead of the drain > side without undue stress on the component. It will be pulsed for > 100-200 ms into a 2 ohm resistive load @ 12 vdc. provided by battery. > This is a portable application for firing e-matches in fireworks in a > sequential manner. > > ************* > > Shouldn't be a problem, but if you want to ground one side of the load and > still use an N-channel MOSFET, you have to devise an extra supply rail > above > Vdd. > > Look at the data sheet - there is a minimum Vgs to achieve the headline > RDSon.- Hide quoted text - > > - Show quoted text - I checked the data sheet and am having difficulty decyphering what you mean. The Vgs is +/- 20 volts MAX but the Gate Threshold Voltage is between 2 and 4 volts. Like I said I'm kind of new to designing with MOSFET's. I'm more of a bi-polar kind of tech. Boy!! That didn't sound right but I guess you get the gist of what I mean. Right - before you do anything else, protect the gate by connecting a 20V zener from gate to source. If you're using logic level MOSFETs, Vgs of about 8V should absolutely guarantee the headline RDSon, so lets assume you're using a 12V lead acid battery to fire the igniters, you could either build a blocking oscillator voltage booster to get the higher voltage you need for the gate pull-up supply - or you could simply use a 9V alkaline battery which will last almost its shelf life because of the very low currents involved. Once you have your 9V battery on top of your 12V rail (21V gate pull-up supply) you connect the rail via a pull-up resistor, 100k would do, now all you need is a switching transistor sitting on the ground rail to switch the MOSFET's gate - I would suggest including a small "gate stopper" resistor in series with the switching transistor's collector.
From: oparr on 26 May 2010 15:04 > Once you have your 9V battery on top of your 12V rail (21V gate pull-up > supply) you connect the rail via a pull-up resistor Geez! Using a P-Channel MOSFET like the IRF5305 would be so much simpler. Why has he latched onto the use of a N-Channel MOSFET? On May 26, 10:38 am, "ian field" wrote: >
From: ian field on 26 May 2010 15:11 <oparr(a)hotmail.com> wrote in message news:8885382a-f931-45f8-977b-4cbb931c0864(a)q8g2000vbm.googlegroups.com... > Once you have your 9V battery on top of your 12V rail (21V gate pull-up > supply) you connect the rail via a pull-up resistor Geez! Using a P-Channel MOSFET like the IRF5305 would be so much simpler. Why has he latched onto the use of a N-Channel MOSFET? *************** Sounds like he already has the N-channel devices, and is using a significant number, so changing route halfway and buying up a batch of P-channel devices might be too much over budget. P-channel devices are more expensive and less efficient than N-channel and the OP might not have been able to find a reasonably priced P device that he liked for the current rating he wanted.
From: oparr on 26 May 2010 15:42 > Sounds like he already has the N-channel devices OP, can you confirm? If so, then you put the cart before the horse. > P-channel devices are more expensive and less efficient than N-channel and > the OP might not have been able to find a reasonably priced P device that he > liked for the current rating he wanted. You **may** be giving the OP too much credit. On May 26, 3:11 pm, "ian field" wrote: >
From: Ron M. on 26 May 2010 16:02
On May 26, 3:11 pm, "ian field" <gangprobing.al...(a)ntlworld.com> wrote: > <op...(a)hotmail.com> wrote in message > > news:8885382a-f931-45f8-977b-4cbb931c0864(a)q8g2000vbm.googlegroups.com... > > > Once you have your 9V battery on top of your 12V rail (21V gate pull-up > > supply) you connect the rail via a pull-up resistor > > Geez! Using a P-Channel MOSFET like the IRF5305 would be so much > simpler. Why has he latched onto the use of a N-Channel MOSFET? > > *************** > > Sounds like he already has the N-channel devices, and is using a significant > number, so changing route halfway and buying up a batch of P-channel devices > might be too much over budget. > > P-channel devices are more expensive and less efficient than N-channel and > the OP might not have been able to find a reasonably priced P device that he > liked for the current rating he wanted. Ok Ian. If I only need a max of 6 amps and am driving it with 12 on gate would that be sufficient for the channel to open enough? I think the easiest way to do this is just build a test circuit and try it. All it will cost me is $1.80. It would be simpler and then I would know for sure. I'm still just having difficulty grasping the extra rail concept. I see the specs but can't figure out why I have to go so much higher than the source for it to conduct enough for my needs. I think a remedial class on MOSFET operation is definitely in order for me. |