I am looking for an inexpensive P-Channel enhancement node SOT-23 case MOSFET that will suit my design. See http://siliconventures.net/images/P%20Channel%20MOSFET.PNG The M9 transistor will have a source voltage of between 18 and 28 volts maximum. Getting a MOSFET whose Source - Drain voltage (when off, in this case) is greater than 28 volts is no issue. I can easily get them for under $0.10. Getting one capable of handling a measly 1/2 milliamp or so is also no big deal. The problem I am having is finding one rated for more than 20 volts Soource - Gate voltage, which will be the case when the gate of M7 is high, turning the circuit on. At this point the source voltage of M9 may be as high as 28 volts, and the gate voltage will be near zero. I am doing it this way, because the source voltage of M9 is the power input voltage of the circuit, and I need the drain voltage of M9 to be as close to the power input as possible, so I can accurately measure it at the junction of R6 and R11 using an ADC.
MOSFET Needed
Started by ●March 8, 2023
Reply by ●March 8, 20232023-03-08
On a sunny day (Wed, 8 Mar 2023 01:41:21 -0800 (PST)) it happened "rhor...@gmail.com" <rhorerles@gmail.com> wrote in <b0fa0548-1a56-4a90-ac31-93138ec6ca69n@googlegroups.com>:>I am looking for an inexpensive P-Channel enhancement node SOT-23 case MOSFET >that will suit my design. See http://siliconventures.net/images/P%20Channel%20MOSFET.PNG > >The >M9 transistor will have a source voltage of between 18 and 28 volts maximum. > Getting a MOSFET whose Source - Drain voltage (when off, in this case) >is greater than 28 volts is no issue. I can easily get them for under $0.10. > Getting one capable of handling a measly 1/2 milliamp or so is also no >big deal. The problem I am having is finding one rated for more than 20 >volts Soource - Gate voltage, which will be the case when the gate of M7 is >high, turning the circuit on. At this point the source voltage of M9 may >be as high as 28 volts, and the gate voltage will be near zero. I am doing >it this way, because the source voltage of M9 is the power input voltage of >the circuit, and I need the drain voltage of M9 to be as close to the power >input as possible, so I can accurately measure it at the junction of R6 >and R11 using an ADC.Add a resistor in the drain of M7 so it forms a voltage divider with R17. That will limit the source-gate voltage of M9. I do not like this cicruit, resistor values are too high and high values make external sensitivity and any leakage a problem. Moisture. Some decoupling caps may be a good idea too.
Reply by ●March 8, 20232023-03-08
On 2023-03-08 04:41, rhor...@gmail.com wrote:> I am looking for an inexpensive P-Channel enhancement node SOT-23 case MOSFET that will suit my design. See http://siliconventures.net/images/P%20Channel%20MOSFET.PNG > > The M9 transistor will have a source voltage of between 18 and 28 volts maximum. Getting a MOSFET whose Source - Drain voltage (when off, in this case) is greater than 28 volts is no issue. I can easily get them for under $0.10. Getting one capable of handling a measly 1/2 milliamp or so is also no big deal. The problem I am having is finding one rated for more than 20 volts Soource - Gate voltage, which will be the case when the gate of M7 is high, turning the circuit on. At this point the source voltage of M9 may be as high as 28 volts, and the gate voltage will be near zero. I am doing it this way, because the source voltage of M9 is the power input voltage of the circuit, and I need the drain voltage of M9 to be as close to the power input as possible, so I can accurately measure it at the junction of R6 and R11 using an ADC. >There are a few, e.g. DMP3056L. As Jan said, an extra resistor would help, and your impedance levels are on the high side, if you want the FET to turn off rapidly. Cheers Phil Hobbs -- Dr Philip C D Hobbs Principal Consultant ElectroOptical Innovations LLC / Hobbs ElectroOptics Optics, Electro-optics, Photonics, Analog Electronics Briarcliff Manor NY 10510 http://electrooptical.net http://hobbs-eo.com
Reply by ●March 8, 20232023-03-08
On Wed, 8 Mar 2023 01:41:21 -0800 (PST), "rhor...@gmail.com" <rhorerles@gmail.com> wrote:>I am looking for an inexpensive P-Channel enhancement node SOT-23 case MOSFET that will suit my design. See http://siliconventures.net/images/P%20Channel%20MOSFET.PNG > >The M9 transistor will have a source voltage of between 18 and 28 volts maximum. Getting a MOSFET whose Source - Drain voltage (when off, in this case) is greater than 28 volts is no issue. I can easily get them for under $0.10. Getting one capable of handling a measly 1/2 milliamp or so is also no big deal. The problem I am having is finding one rated for more than 20 volts Soource - Gate voltage, which will be the case when the gate of M7 is high, turning the circuit on. At this point the source voltage of M9 may be as high as 28 volts, and the gate voltage will be near zero. I am doing it this way, because the source voltage of M9 is the power input voltage of the circuit, and I need the drain voltage of M9 to be as close to the power input as possible, so I can accurately measure it at the junction of R6 and R11 using an ADC.Add a resistor, as Jan says, if you want to stay within the abs-max of the pfet. In real life, most pfets would survive 28 g-s; they usually die around 60 or so. You can also buy protected-gate mosfets, which actually have a s-g zener, usually around 40 volts, but spec survival when the gate is over-driven.
Reply by ●March 9, 20232023-03-09
PS about that right side MOSFET with the high value series resistor in the gate: try using your smartphone next to it... It REALLY needs decoupling caps. That gate input, when not in a 100% shielded housing, will react to any RF, if the signal is big enough unexpected things may happen.
Reply by ●March 9, 20232023-03-09
On Wednesday, March 8, 2023 at 12:00:58 PM UTC-5, John Larkin wrote:> On Wed, 8 Mar 2023 01:41:21 -0800 (PST), "rhor...@gmail.com" > <rhor...@gmail.com> wrote: > > >I am looking for an inexpensive P-Channel enhancement node SOT-23 case MOSFET that will suit my design. See http://siliconventures.net/images/P%20Channel%20MOSFET.PNG > > > >The M9 transistor will have a source voltage of between 18 and 28 volts maximum. Getting a MOSFET whose Source - Drain voltage (when off, in this case) is greater than 28 volts is no issue. I can easily get them for under $0.10. Getting one capable of handling a measly 1/2 milliamp or so is also no big deal. The problem I am having is finding one rated for more than 20 volts Soource - Gate voltage, which will be the case when the gate of M7 is high, turning the circuit on. At this point the source voltage of M9 may be as high as 28 volts, and the gate voltage will be near zero. I am doing it this way, because the source voltage of M9 is the power input voltage of the circuit, and I need the drain voltage of M9 to be as close to the power input as possible, so I can accurately measure it at the junction of R6 and R11 using an ADC. > Add a resistor, as Jan says, if you want to stay within the abs-max of > the pfet. In real life, most pfets would survive 28 g-s; they usually > die around 60 or so. > > You can also buy protected-gate mosfets, which actually have a s-g > zener, usually around 40 volts, but spec survival when the gate is > over-driven.Guess it's too much to ask what the sample rate might be. It surely not a high science instrumentation project. Higher voltages that vary over such large range are likely susceptible to large coupled transients. There are plenty of ways to use a single N-FET with bootstrapped gate to circumvent the special "MOSFET needed" non-requirement. That will get around the zero quiescent and minimum operating current requirement.
Reply by ●March 9, 20232023-03-09
On Thursday, March 9, 2023 at 2:03:30 AM UTC-5, Jan Panteltje wrote:> PS > about that right side MOSFET with the high value series resistor in the gate: > try using your smartphone next to it... > It REALLY needs decoupling caps. > > That gate input, when not in a 100% shielded housing, will react to any RF, > if the signal is big enough unexpected things may happen.If he drops it in a coffee cup, unexpected things will happen too.
Reply by ●March 9, 20232023-03-09
On a sunny day (Thu, 9 Mar 2023 05:07:24 -0800 (PST)) it happened Fred Bloggs <bloggs.fredbloggs.fred@gmail.com> wrote in <5b315f99-cc6e-4c4d-a43b-7277a29c455cn@googlegroups.com>:>On Thursday, March 9, 2023 at 2:03:30=E2=80=AFAM UTC-5, Jan Panteltje wrote: >> >PS >> about that right side MOSFET with the high value series resistor in the gate: >> >try using your smartphone next to it... >> It REALLY needs decoupling caps. >> >> That gate input, when not in a 100% shielded housing, will react to any RF, >> >if the signal is big enough unexpected things may happen. > >If he drops it in a coffee cup, unexpected things will happen too.You clealy have no RF experience. So shut up.
Reply by ●March 9, 20232023-03-09
On Thursday, March 9, 2023 at 10:07:46 AM UTC-5, Jan Panteltje wrote:> On a sunny day (Thu, 9 Mar 2023 05:07:24 -0800 (PST)) it happened Fred Bloggs > <bloggs.fred...@gmail.com> wrote in > <5b315f99-cc6e-4c4d...@googlegroups.com>: > >On Thursday, March 9, 2023 at 2:03:30=E2=80=AFAM UTC-5, Jan Panteltje wrote: > >> > >PS > >> about that right side MOSFET with the high value series resistor in the gate: > >> > >try using your smartphone next to it... > >> It REALLY needs decoupling caps. > >> > >> That gate input, when not in a 100% shielded housing, will react to any RF, > >> > >if the signal is big enough unexpected things may happen. > > > >If he drops it in a coffee cup, unexpected things will happen too. > You clealy have no RF experience. > So shut up.I have way more than you're capable of even beginning to acquire, lightweight.
Reply by ●March 9, 20232023-03-09
On Thu, 09 Mar 2023 06:54:56 GMT, Jan Panteltje <alien@comet.invalid> wrote:>PS >about that right side MOSFET with the high value series resistor in the gate: >try using your smartphone next to it... >It REALLY needs decoupling caps. > >That gate input, when not in a 100% shielded housing, will react to any RF, >if the signal is big enough unexpected things may happen.I doubt it. Fet capacitances will keep gate RF levels down to millivolts.
