Mosfet switching times ?

Discussion in 'Electronic Design' started by Hammy, Jul 10, 2010.

  1. Hammy

    Hammy Guest

    When Mosfet switching times are shown in the data sheets i.e ton,toff
    and tdelay. They specify an Rg value. Is this Rg value the driver
    resistance or the internal mosfet mesh resistance?

    The reason I'm asking is that every FET sheet uses a different value
    of Rg for testing. The ones with the lower Rg have the quickest
    switching times.

    For example two similar devices a Fairchild

    http://www.fairchildsemi.com/ds/FQ/FQP18N50V2.pdf

    And a ST device.

    http://www.st.com/stonline/products/literature/ds/9118/stw20nk50z.pdf

    The ST device uses an Rg of 4.7 oHms while the Fairchild device uses
    an Rg of 25oHms.

    The ST devices shows much faster transitions but has twice the gate
    charge. I would prefer to use the Fairchild device if I know that Rg
    is set by the driver I would know that the same speeds could be
    obtained as the ST device. Is this correct?

    If it is the driver resistance, why would they test with a high Rg
    ,it's not flattering to the mosfet?
     
    Hammy, Jul 10, 2010
    #1
    1. Advertising

  2. Hammy

    Hammy Guest

    On Sat, 10 Jul 2010 09:28:14 -0700, John Larkin
    <> wrote:

    [snip]
    >
    >Most mosfets will switch much faster than their datasheets suggest, if
    >you just drive them hard. The Fairchild BSS123 datasheet cites a
    >typical turn-on rise time of 9 ns, and 17 for turnoff.


    I never paid much attention too data sheet switching times until
    recently. Mainly from the emphasis some people here and in other
    forums place on them. So I was beginning to think I may be missing
    something, but I guess not.

    I've always just used Qg and my drivers sink/source ability to
    estimate times.


    >They will actually do this:
    >
    >http://www.highlandtechnology.com/DSimages/T760 wave-web.jpg
    >
    >That's a 100 volt pulse into 50 ohms, transformer isolated.


    Your exceeding the 25C pulsed current rateing a tad. ;-)

    >Given the complications of driver and fet inductances and such, and
    >the state of the datasheets, the best thing to do is experiment. If
    >you want to go fast, you should probably stick to one manufacturer and
    >not assume that identical part numbers are always interchangable. Buy
    >a reel.
    >
    >John

    Yes your right I plan on experimenting with both and maybe a couple
    others I have in the applications.
     
    Hammy, Jul 10, 2010
    #2
    1. Advertising

  3. Hammy

    Jon Kirwan Guest

    On Sat, 10 Jul 2010 11:14:59 -0400, Hammy <>
    wrote:

    >When Mosfet switching times are shown in the data sheets i.e ton,toff
    >and tdelay. They specify an Rg value. Is this Rg value the driver
    >resistance or the internal mosfet mesh resistance?


    I've taken it as a device parameter, pretty much as discussed
    here:

    http://www.fairchildsemi.com/an/AN/AN-7017.pdf

    .... except that Rg is a term applied elsewhere, too. For
    example, I've seen Rg_i used for the mosfet's Rg, then Rg
    used for an explicit/implicit series Rg that is external to
    the device, and then Rg_hi and Rg_lo also for the high side
    and low side equivalent driving resistance of the gate
    driver. So I think context is important.

    But on a datasheet, unless specified as part of the testing
    setup, I don't think those external values are included in
    the Rg value ascribed to the part, itself.

    Of course, I'm not an expert reader, either. But that's the
    impression I've taken.

    >snip>
    >If it is the driver resistance, why would they test with a high Rg
    >,it's not flattering to the mosfet?


    Few would intentionally shoot themselves in the foot.

    Jon
     
    Jon Kirwan, Jul 10, 2010
    #3
  4. Hammy

    Hammy Guest

    On Sat, 10 Jul 2010 11:14:53 -0700, Jon Kirwan
    <> wrote:

    >On Sat, 10 Jul 2010 11:14:59 -0400, Hammy <>
    >wrote:
    >
    >>When Mosfet switching times are shown in the data sheets i.e ton,toff
    >>and tdelay. They specify an Rg value. Is this Rg value the driver
    >>resistance or the internal mosfet mesh resistance?

    >
    >I've taken it as a device parameter, pretty much as discussed
    >here:
    >
    >http://www.fairchildsemi.com/an/AN/AN-7017.pdf


    Thanks I never found that one.

    >... except that Rg is a term applied elsewhere, too. For
    >example, I've seen Rg_i used for the mosfet's Rg, then Rg
    >used for an explicit/implicit series Rg that is external to
    >the device, and then Rg_hi and Rg_lo also for the high side
    >and low side equivalent driving resistance of the gate
    >driver. So I think context is important.
    >
    >But on a datasheet, unless specified as part of the testing
    >setup, I don't think those external values are included in
    >the Rg value ascribed to the part, itself.


    I've never heard it refereed to as intrinsic to the device
    specifically in a data sheet.

    The ST device shows the test set-up and Rg is shown external.

    To add to the confusion this AN from TI says pretty much that the
    intrinsic Rg is has negligible impact on switching speeds except at
    RF?

    see pg4

    http://focus.ti.com/lit/ml/slup169/slup169.pdf

    >Of course, I'm not an expert reader, either. But that's the
    >impression I've taken.
    >
    >>snip>
    >>If it is the driver resistance, why would they test with a high Rg
    >>,it's not flattering to the mosfet?

    >
    >Few would intentionally shoot themselves in the foot.


    That is the other confusing thing . Given how they are so loose with
    high current ratings.
    >Jon
     
    Hammy, Jul 10, 2010
    #4
  5. Hammy

    Tim Williams Guest

    "Hammy" <> wrote in message news:...
    > I never paid much attention too data sheet switching times until
    > recently. Mainly from the emphasis some people here and in other
    > forums place on them. So I was beginning to think I may be missing
    > something, but I guess not.


    This is true for BJTs, which tend to have switching times in the ballpark of the device, regardless of how you drive it. This comes from base transit time and stored charge effects. Base spreading resistance is generally negligible, I guess.

    > I've always just used Qg and my drivers sink/source ability to
    > estimate times.


    For FETs, this is correct. There is a gate spreading resistance, and it's occasionally specified, usually in the 1 ohm range. This suggests that you can't really make it go faster than a 1 ohm Rg equivalent.

    Now, you could drive it with a precompensated waveform, so the current overshoots to kick the RC around, but now you're getting into territory where you need serious power input for marginal output power and efficiency. For most power transistors, I would guess this is in the low VHF range.

    I made a 1MHz generator with TC4420's, FDP26N40's and 1 ohm gate resistors. The '4420s actually get hot at this frequency!

    Tim

    --
    Deep Friar: a very philosophical monk.
    Website: http://webpages.charter.net/dawill/tmoranwms
     
    Tim Williams, Jul 10, 2010
    #5
  6. Hammy

    Joerg Guest

    Hammy wrote:
    > On Sat, 10 Jul 2010 09:28:14 -0700, John Larkin
    > <> wrote:
    >
    > [snip]
    >> Most mosfets will switch much faster than their datasheets suggest, if
    >> you just drive them hard. The Fairchild BSS123 datasheet cites a
    >> typical turn-on rise time of 9 ns, and 17 for turnoff.

    >
    > I never paid much attention too data sheet switching times until
    > recently. Mainly from the emphasis some people here and in other
    > forums place on them. So I was beginning to think I may be missing
    > something, but I guess not.
    >


    No, not missing much. Switch times aren't very meaningful in those
    datasheets.


    > I've always just used Qg and my drivers sink/source ability to
    > estimate times.
    >


    Most people put gate resistors in because they are afraid to blow the
    EMC cert or that it could oscillate. Which spoils the power of the nice
    PWM chip in front of it, like driving a Porsche at 35mph. I tend to
    drive them hard but there's always the internal gate path resistance
    which you cannot overcome. So I tend to drive them with lots of gusto,
    preferably well north of 10V which helps in that domain. It also helps
    to drive the gate negative a few volts, -5V or so. That speeds up turn-off.


    >
    >> They will actually do this:
    >>
    >> http://www.highlandtechnology.com/DSimages/T760 wave-web.jpg
    >>
    >> That's a 100 volt pulse into 50 ohms, transformer isolated.

    >
    > Your exceeding the 25C pulsed current rateing a tad. ;-)
    >


    John likes to floor it, wonder how long his new Audi will last :)

    [...]

    --
    Regards, Joerg

    http://www.analogconsultants.com/

    "gmail" domain blocked because of excessive spam.
    Use another domain or send PM.
     
    Joerg, Jul 10, 2010
    #6
  7. Hammy

    Jon Kirwan Guest

    On Sat, 10 Jul 2010 14:55:05 -0400, Hammy <>
    wrote:

    >On Sat, 10 Jul 2010 11:14:53 -0700, Jon Kirwan
    ><> wrote:
    >
    >>On Sat, 10 Jul 2010 11:14:59 -0400, Hammy <>
    >>wrote:
    >>
    >>>When Mosfet switching times are shown in the data sheets i.e ton,toff
    >>>and tdelay. They specify an Rg value. Is this Rg value the driver
    >>>resistance or the internal mosfet mesh resistance?

    >>
    >>I've taken it as a device parameter, pretty much as discussed
    >>here:
    >>
    >>http://www.fairchildsemi.com/an/AN/AN-7017.pdf

    >
    >Thanks I never found that one.


    It's one of a few I've read, with what understanding I may,
    that seem to say similar things to me.

    >>... except that Rg is a term applied elsewhere, too. For
    >>example, I've seen Rg_i used for the mosfet's Rg, then Rg
    >>used for an explicit/implicit series Rg that is external to
    >>the device, and then Rg_hi and Rg_lo also for the high side
    >>and low side equivalent driving resistance of the gate
    >>driver. So I think context is important.
    >>
    >>But on a datasheet, unless specified as part of the testing
    >>setup, I don't think those external values are included in
    >>the Rg value ascribed to the part, itself.

    >
    >I've never heard it refereed to as intrinsic to the device
    >specifically in a data sheet.


    I'm sorry. I was thinking about Spice models when I wrote
    that. I believe, now that you bring it up, that I've seen it
    on some datasheets. But that wasn't on my mind at the time I
    wrote.

    >The ST device shows the test set-up and Rg is shown external.


    Yes.

    >To add to the confusion this AN from TI says pretty much that the
    >intrinsic Rg is has negligible impact on switching speeds except at
    >RF?
    >
    > see pg4
    >
    >http://focus.ti.com/lit/ml/slup169/slup169.pdf
    ><snip>


    Since the RC ladder is low-pass, that makes some sense to me
    on first blush. The areas further away from the bond point
    would never reach sufficient voltage, so I think that would
    be a real problem, as frequencies rise (no firm turn off or
    turn on, just finding a quiescent point and sitting there
    neither on nor off.) But I haven't given this much thought.
    But your phrasing of TI's apnote comment seems reasonable to
    me and not conflicting. There is still an Rg to the device.
    It's just that, because of the RC ladder, at high frequencies
    there is huge problem caused by the inability to switch the
    whole gate area around that fast.

    But I have only just now given this two minutes thought.

    TI's apnote is probably correct. If Rg were important, it
    would probably be on all of the datasheets rather than a few.
    It's not always there, though. (It obviously _is_ important
    for the Spice models, though.) The gate-source charge, the
    Miller charge, and overdrive charges are important (overdrive
    extracted by subtracting the other two from the total charge,
    I gather.) So they are spec'd.

    Jon
     
    Jon Kirwan, Jul 10, 2010
    #7
  8. Hammy

    ian field Guest

    "John Larkin" <> wrote in message
    news:...
    > On Sat, 10 Jul 2010 13:22:46 -0400, Hammy <> wrote:
    >
    >>On Sat, 10 Jul 2010 09:28:14 -0700, John Larkin
    >><> wrote:
    >>
    >>[snip]
    >>>
    >>>Most mosfets will switch much faster than their datasheets suggest, if
    >>>you just drive them hard. The Fairchild BSS123 datasheet cites a
    >>>typical turn-on rise time of 9 ns, and 17 for turnoff.

    >>
    >>I never paid much attention too data sheet switching times until
    >>recently. Mainly from the emphasis some people here and in other
    >>forums place on them. So I was beginning to think I may be missing
    >>something, but I guess not.
    >>
    >>I've always just used Qg and my drivers sink/source ability to
    >>estimate times.

    >
    > I generally assume that mosfet silicon is infinitely fast, and that
    > only capacitances and wirebond inductances get in the way. Seems to
    > work so far.



    Recently I was asking in various groups what frequency people thought I
    might shove through a 2N7000 in grounded gate - some people thought the
    limit might be around 100MHz.

    Do you have an estimate on this?
     
    ian field, Jul 11, 2010
    #8
  9. Hammy

    Joerg Guest

    John Larkin wrote:
    > On Sun, 11 Jul 2010 17:09:24 +0100, "ian field"
    > <> wrote:
    >
    >> "John Larkin" <> wrote in message
    >> news:...
    >>> On Sat, 10 Jul 2010 13:22:46 -0400, Hammy <> wrote:
    >>>
    >>>> On Sat, 10 Jul 2010 09:28:14 -0700, John Larkin
    >>>> <> wrote:
    >>>>
    >>>> [snip]
    >>>>> Most mosfets will switch much faster than their datasheets suggest, if
    >>>>> you just drive them hard. The Fairchild BSS123 datasheet cites a
    >>>>> typical turn-on rise time of 9 ns, and 17 for turnoff.
    >>>> I never paid much attention too data sheet switching times until
    >>>> recently. Mainly from the emphasis some people here and in other
    >>>> forums place on them. So I was beginning to think I may be missing
    >>>> something, but I guess not.
    >>>>
    >>>> I've always just used Qg and my drivers sink/source ability to
    >>>> estimate times.
    >>> I generally assume that mosfet silicon is infinitely fast, and that
    >>> only capacitances and wirebond inductances get in the way. Seems to
    >>> work so far.

    >>
    >> Recently I was asking in various groups what frequency people thought I
    >> might shove through a 2N7000 in grounded gate - some people thought the
    >> limit might be around 100MHz.
    >>
    >> Do you have an estimate on this?
    >>

    >
    > Do you mean as a switch, or as an RF sinewave amplifier?
    >
    > If you drive the gate hard, grounded-source, you can turn a 2N7002
    > on/off, with 50 volt drain swing, in about a nanosecond. I'd imagine
    > you could get useful grounded-gate power gain at 250 MHz at least,
    > especially if things were tuned+matched for the operating frequency.
    > The 2N7000 version will have a little more lead inductance, but that
    > can be tuned out.
    >


    I have done it on a design. It was with the BSS123 but it's quite
    similar. It all depends on the gain you want and the gate-drain
    capacitance is your enemy there. I guess the 2N7002 would be a smidgen
    better. AFAIR my amp was around 30dB and linear all the way up to 40Mhz
    but we stopped there, didn't need more.

    [...]

    --
    Regards, Joerg

    http://www.analogconsultants.com/

    "gmail" domain blocked because of excessive spam.
    Use another domain or send PM.
     
    Joerg, Jul 11, 2010
    #9
  10. Hammy

    ian field Guest

    "John Larkin" <> wrote in message
    news:...
    > On Sun, 11 Jul 2010 17:09:24 +0100, "ian field"
    > <> wrote:
    >
    >>
    >>"John Larkin" <> wrote in
    >>message
    >>news:...
    >>> On Sat, 10 Jul 2010 13:22:46 -0400, Hammy <> wrote:
    >>>
    >>>>On Sat, 10 Jul 2010 09:28:14 -0700, John Larkin
    >>>><> wrote:
    >>>>
    >>>>[snip]
    >>>>>
    >>>>>Most mosfets will switch much faster than their datasheets suggest, if
    >>>>>you just drive them hard. The Fairchild BSS123 datasheet cites a
    >>>>>typical turn-on rise time of 9 ns, and 17 for turnoff.
    >>>>
    >>>>I never paid much attention too data sheet switching times until
    >>>>recently. Mainly from the emphasis some people here and in other
    >>>>forums place on them. So I was beginning to think I may be missing
    >>>>something, but I guess not.
    >>>>
    >>>>I've always just used Qg and my drivers sink/source ability to
    >>>>estimate times.
    >>>
    >>> I generally assume that mosfet silicon is infinitely fast, and that
    >>> only capacitances and wirebond inductances get in the way. Seems to
    >>> work so far.

    >>
    >>
    >>Recently I was asking in various groups what frequency people thought I
    >>might shove through a 2N7000 in grounded gate - some people thought the
    >>limit might be around 100MHz.
    >>
    >>Do you have an estimate on this?
    >>

    >
    > Do you mean as a switch, or as an RF sinewave amplifier?
    >
    > If you drive the gate hard, grounded-source, you can turn a 2N7002
    > on/off, with 50 volt drain swing, in about a nanosecond. I'd imagine
    > you could get useful grounded-gate power gain at 250 MHz at least,
    > especially if things were tuned+matched for the operating frequency.
    > The 2N7000 version will have a little more lead inductance, but that
    > can be tuned out.


    My intention was to re-radiate DAB (VHF high band) from a communal aerial
    socket in one corner to the radio in the diagonally opposite corner.

    Originally the 7000 was in cascode with a 2N3819 - it almost worked. In the
    end I settled for a BF998 driving a BFQ162A.
     
    ian field, Jul 11, 2010
    #10
  11. Hammy

    ian field Guest

    "John Larkin" <> wrote in message
    news:...
    > On Sun, 11 Jul 2010 17:09:24 +0100, "ian field"
    > <> wrote:
    >
    >>
    >>"John Larkin" <> wrote in
    >>message
    >>news:...
    >>> On Sat, 10 Jul 2010 13:22:46 -0400, Hammy <> wrote:
    >>>
    >>>>On Sat, 10 Jul 2010 09:28:14 -0700, John Larkin
    >>>><> wrote:
    >>>>
    >>>>[snip]
    >>>>>
    >>>>>Most mosfets will switch much faster than their datasheets suggest, if
    >>>>>you just drive them hard. The Fairchild BSS123 datasheet cites a
    >>>>>typical turn-on rise time of 9 ns, and 17 for turnoff.
    >>>>
    >>>>I never paid much attention too data sheet switching times until
    >>>>recently. Mainly from the emphasis some people here and in other
    >>>>forums place on them. So I was beginning to think I may be missing
    >>>>something, but I guess not.
    >>>>
    >>>>I've always just used Qg and my drivers sink/source ability to
    >>>>estimate times.
    >>>
    >>> I generally assume that mosfet silicon is infinitely fast, and that
    >>> only capacitances and wirebond inductances get in the way. Seems to
    >>> work so far.

    >>
    >>
    >>Recently I was asking in various groups what frequency people thought I
    >>might shove through a 2N7000 in grounded gate - some people thought the
    >>limit might be around 100MHz.
    >>
    >>Do you have an estimate on this?
    >>

    >
    > Do you mean as a switch, or as an RF sinewave amplifier?
    >
    > If you drive the gate hard, grounded-source, you can turn a 2N7002
    > on/off, with 50 volt drain swing, in about a nanosecond. I'd imagine
    > you could get useful grounded-gate power gain at 250 MHz at least,
    > especially if things were tuned+matched for the operating frequency.
    > The 2N7000 version will have a little more lead inductance, but that
    > can be tuned out.
    >
    > Try this:
    >
    > 9 volt battery, 470 ohm resistor, LED, 2N7000. You can pull the gate
    > up to 9 volts, or down to ground - just use your fingers - to turn the
    > LED on or off. And then let it float in either state for hours. Or
    > briefly touch the drain to the gate to turn the LED partly on, then
    > float the gate again. The LED brightness will very slowly creep up or
    > down. The gate leakage is not too many electrons per second.
    >
    > In that last state, take a pencil, or some insulated/metal thing like
    > a small screwdriver or a trimpot tool, and pump charge into or out of
    > the gate in small steps by alternately touching +9 or ground, then the
    > gate. Some of us are easily amused.


    Evidently.
     
    ian field, Jul 11, 2010
    #11
  12. Hammy

    Joerg Guest

    ian field wrote:
    > "John Larkin" <> wrote in message
    > news:...
    >> On Sun, 11 Jul 2010 17:09:24 +0100, "ian field"
    >> <> wrote:
    >>
    >>> "John Larkin" <> wrote in
    >>> message
    >>> news:...
    >>>> On Sat, 10 Jul 2010 13:22:46 -0400, Hammy <> wrote:
    >>>>
    >>>>> On Sat, 10 Jul 2010 09:28:14 -0700, John Larkin
    >>>>> <> wrote:
    >>>>>
    >>>>> [snip]
    >>>>>> Most mosfets will switch much faster than their datasheets suggest, if
    >>>>>> you just drive them hard. The Fairchild BSS123 datasheet cites a
    >>>>>> typical turn-on rise time of 9 ns, and 17 for turnoff.
    >>>>> I never paid much attention too data sheet switching times until
    >>>>> recently. Mainly from the emphasis some people here and in other
    >>>>> forums place on them. So I was beginning to think I may be missing
    >>>>> something, but I guess not.
    >>>>>
    >>>>> I've always just used Qg and my drivers sink/source ability to
    >>>>> estimate times.
    >>>> I generally assume that mosfet silicon is infinitely fast, and that
    >>>> only capacitances and wirebond inductances get in the way. Seems to
    >>>> work so far.
    >>>
    >>> Recently I was asking in various groups what frequency people thought I
    >>> might shove through a 2N7000 in grounded gate - some people thought the
    >>> limit might be around 100MHz.
    >>>
    >>> Do you have an estimate on this?
    >>>

    >> Do you mean as a switch, or as an RF sinewave amplifier?
    >>
    >> If you drive the gate hard, grounded-source, you can turn a 2N7002
    >> on/off, with 50 volt drain swing, in about a nanosecond. I'd imagine
    >> you could get useful grounded-gate power gain at 250 MHz at least,
    >> especially if things were tuned+matched for the operating frequency.
    >> The 2N7000 version will have a little more lead inductance, but that
    >> can be tuned out.

    >
    > My intention was to re-radiate DAB (VHF high band) from a communal aerial
    > socket in one corner to the radio in the diagonally opposite corner.
    >


    Aerial socket, now that must be a truly Bri'ish expression :)


    > Originally the 7000 was in cascode with a 2N3819 - it almost worked. In the
    > end I settled for a BF998 driving a BFQ162A.
    >


    Don't know what a BFQ162 is but yes, why torture yourself with low
    frequency devices when you can get a hotrod RF transistor with tens of
    gigeehoitzes for under a buck? Or less than a quid in rightpondian.

    Some of the bigger MMIC, the ones with a grounded tab that can be
    soldered onto copperclad for heatsink, should also work. Of course,
    re-radiation can ruffle some feathers with the authorities.

    --
    Regards, Joerg

    http://www.analogconsultants.com/

    "gmail" domain blocked because of excessive spam.
    Use another domain or send PM.
     
    Joerg, Jul 11, 2010
    #12
  13. Hammy

    Joerg Guest

    John Larkin wrote:
    > On Sun, 11 Jul 2010 13:50:23 -0700, Joerg <>
    > wrote:
    >
    >> ian field wrote:
    >>> "John Larkin" <> wrote in message
    >>> news:...
    >>>> On Sun, 11 Jul 2010 17:09:24 +0100, "ian field"
    >>>> <> wrote:
    >>>>
    >>>>> "John Larkin" <> wrote in
    >>>>> message
    >>>>> news:...
    >>>>>> On Sat, 10 Jul 2010 13:22:46 -0400, Hammy <> wrote:
    >>>>>>
    >>>>>>> On Sat, 10 Jul 2010 09:28:14 -0700, John Larkin
    >>>>>>> <> wrote:
    >>>>>>>
    >>>>>>> [snip]
    >>>>>>>> Most mosfets will switch much faster than their datasheets suggest, if
    >>>>>>>> you just drive them hard. The Fairchild BSS123 datasheet cites a
    >>>>>>>> typical turn-on rise time of 9 ns, and 17 for turnoff.
    >>>>>>> I never paid much attention too data sheet switching times until
    >>>>>>> recently. Mainly from the emphasis some people here and in other
    >>>>>>> forums place on them. So I was beginning to think I may be missing
    >>>>>>> something, but I guess not.
    >>>>>>>
    >>>>>>> I've always just used Qg and my drivers sink/source ability to
    >>>>>>> estimate times.
    >>>>>> I generally assume that mosfet silicon is infinitely fast, and that
    >>>>>> only capacitances and wirebond inductances get in the way. Seems to
    >>>>>> work so far.
    >>>>> Recently I was asking in various groups what frequency people thought I
    >>>>> might shove through a 2N7000 in grounded gate - some people thought the
    >>>>> limit might be around 100MHz.
    >>>>>
    >>>>> Do you have an estimate on this?
    >>>>>
    >>>> Do you mean as a switch, or as an RF sinewave amplifier?
    >>>>
    >>>> If you drive the gate hard, grounded-source, you can turn a 2N7002
    >>>> on/off, with 50 volt drain swing, in about a nanosecond. I'd imagine
    >>>> you could get useful grounded-gate power gain at 250 MHz at least,
    >>>> especially if things were tuned+matched for the operating frequency.
    >>>> The 2N7000 version will have a little more lead inductance, but that
    >>>> can be tuned out.
    >>> My intention was to re-radiate DAB (VHF high band) from a communal aerial
    >>> socket in one corner to the radio in the diagonally opposite corner.
    >>>

    >> Aerial socket, now that must be a truly Bri'ish expression :)

    >
    > He said "communal." Must be a Russky.
    >


    Hey, I just went to communion and I'm not a Russky :)

    --
    Regards, Joerg

    http://www.analogconsultants.com/

    "gmail" domain blocked because of excessive spam.
    Use another domain or send PM.
     
    Joerg, Jul 11, 2010
    #13
  14. Hammy

    ian field Guest

    "John Larkin" <> wrote in message
    news:...
    > On Sun, 11 Jul 2010 13:50:23 -0700, Joerg <>
    > wrote:
    >
    >>ian field wrote:
    >>> "John Larkin" <> wrote in
    >>> message
    >>> news:...
    >>>> On Sun, 11 Jul 2010 17:09:24 +0100, "ian field"
    >>>> <> wrote:
    >>>>
    >>>>> "John Larkin" <> wrote in
    >>>>> message
    >>>>> news:...
    >>>>>> On Sat, 10 Jul 2010 13:22:46 -0400, Hammy <> wrote:
    >>>>>>
    >>>>>>> On Sat, 10 Jul 2010 09:28:14 -0700, John Larkin
    >>>>>>> <> wrote:
    >>>>>>>
    >>>>>>> [snip]
    >>>>>>>> Most mosfets will switch much faster than their datasheets suggest,
    >>>>>>>> if
    >>>>>>>> you just drive them hard. The Fairchild BSS123 datasheet cites a
    >>>>>>>> typical turn-on rise time of 9 ns, and 17 for turnoff.
    >>>>>>> I never paid much attention too data sheet switching times until
    >>>>>>> recently. Mainly from the emphasis some people here and in other
    >>>>>>> forums place on them. So I was beginning to think I may be missing
    >>>>>>> something, but I guess not.
    >>>>>>>
    >>>>>>> I've always just used Qg and my drivers sink/source ability to
    >>>>>>> estimate times.
    >>>>>> I generally assume that mosfet silicon is infinitely fast, and that
    >>>>>> only capacitances and wirebond inductances get in the way. Seems to
    >>>>>> work so far.
    >>>>>
    >>>>> Recently I was asking in various groups what frequency people thought
    >>>>> I
    >>>>> might shove through a 2N7000 in grounded gate - some people thought
    >>>>> the
    >>>>> limit might be around 100MHz.
    >>>>>
    >>>>> Do you have an estimate on this?
    >>>>>
    >>>> Do you mean as a switch, or as an RF sinewave amplifier?
    >>>>
    >>>> If you drive the gate hard, grounded-source, you can turn a 2N7002
    >>>> on/off, with 50 volt drain swing, in about a nanosecond. I'd imagine
    >>>> you could get useful grounded-gate power gain at 250 MHz at least,
    >>>> especially if things were tuned+matched for the operating frequency.
    >>>> The 2N7000 version will have a little more lead inductance, but that
    >>>> can be tuned out.
    >>>
    >>> My intention was to re-radiate DAB (VHF high band) from a communal
    >>> aerial
    >>> socket in one corner to the radio in the diagonally opposite corner.
    >>>

    >>
    >>Aerial socket, now that must be a truly Bri'ish expression :)

    >
    > He said "communal." Must be a Russky.



    The block of flats has a master aerial with a bunch of co-ax cables running
    along the wall to each flat.
     
    ian field, Jul 12, 2010
    #14
  15. Hammy

    ian field Guest

    "John Larkin" <> wrote in message
    news:...
    > On Sun, 11 Jul 2010 21:43:58 +0100, "ian field"
    > <> wrote:
    >
    >>
    >>"John Larkin" <> wrote in
    >>message
    >>news:...
    >>> On Sun, 11 Jul 2010 17:09:24 +0100, "ian field"
    >>> <> wrote:
    >>>
    >>>>
    >>>>"John Larkin" <> wrote in
    >>>>message
    >>>>news:...
    >>>>> On Sat, 10 Jul 2010 13:22:46 -0400, Hammy <> wrote:
    >>>>>
    >>>>>>On Sat, 10 Jul 2010 09:28:14 -0700, John Larkin
    >>>>>><> wrote:
    >>>>>>
    >>>>>>[snip]
    >>>>>>>
    >>>>>>>Most mosfets will switch much faster than their datasheets suggest,
    >>>>>>>if
    >>>>>>>you just drive them hard. The Fairchild BSS123 datasheet cites a
    >>>>>>>typical turn-on rise time of 9 ns, and 17 for turnoff.
    >>>>>>
    >>>>>>I never paid much attention too data sheet switching times until
    >>>>>>recently. Mainly from the emphasis some people here and in other
    >>>>>>forums place on them. So I was beginning to think I may be missing
    >>>>>>something, but I guess not.
    >>>>>>
    >>>>>>I've always just used Qg and my drivers sink/source ability to
    >>>>>>estimate times.
    >>>>>
    >>>>> I generally assume that mosfet silicon is infinitely fast, and that
    >>>>> only capacitances and wirebond inductances get in the way. Seems to
    >>>>> work so far.
    >>>>
    >>>>
    >>>>Recently I was asking in various groups what frequency people thought I
    >>>>might shove through a 2N7000 in grounded gate - some people thought the
    >>>>limit might be around 100MHz.
    >>>>
    >>>>Do you have an estimate on this?
    >>>>
    >>>
    >>> Do you mean as a switch, or as an RF sinewave amplifier?
    >>>
    >>> If you drive the gate hard, grounded-source, you can turn a 2N7002
    >>> on/off, with 50 volt drain swing, in about a nanosecond. I'd imagine
    >>> you could get useful grounded-gate power gain at 250 MHz at least,
    >>> especially if things were tuned+matched for the operating frequency.
    >>> The 2N7000 version will have a little more lead inductance, but that
    >>> can be tuned out.

    >>
    >>My intention was to re-radiate DAB (VHF high band) from a communal aerial
    >>socket in one corner to the radio in the diagonally opposite corner.
    >>
    >>Originally the 7000 was in cascode with a 2N3819 - it almost worked. In
    >>the
    >>end I settled for a BF998 driving a BFQ162A.
    >>

    >
    > Those little darlington MMICs are great for stuff like this, absurdly
    > easy to use. Like the MiniCircuits ERA series.



    Couldn't see much mention in the OP's posts about the application - if its a
    direct off line switcher, MOSFETs can be cascoded for a speed advantage.

    The bottom MOSFET should be a high current, low voltage ultra fast device,
    this makes the upper MOSFET less critical in a number of criteria.
     
    ian field, Jul 12, 2010
    #15
  16. Hammy

    Hammy Guest

    On Mon, 12 Jul 2010 13:01:03 +0100, "ian field"
    <> wrote:


    >Couldn't see much mention in the OP's posts about the application - if its a
    >direct off line switcher, MOSFETs can be cascoded for a speed advantage.


    It's for a 275W boost pfc. The question was more general though. I
    dont need single digit nano-second transtions. I just wanted to check
    if the switching times in fet datasheets were the max the devices
    could obtain. As I said earlier I never used to even look at them
    until recently and estimated switching times solely on driver
    sink/source and total Q for my Vds.

    >The bottom MOSFET should be a high current, low voltage ultra fast device,
    >this makes the upper MOSFET less critical in a number of criteria.
    >

    Yea I gues if your trying to thump around a 2-300nC fet it might be
    worth it. They make hybrid bjts with an integrated FET that works on
    the same principal called Emitter switched bipolars.
     
    Hammy, Jul 12, 2010
    #16
    1. Advertising

Want to reply to this thread or ask your own question?

It takes just 2 minutes to sign up (and it's free!). Just click the sign up button to choose a username and then you can ask your own questions on the forum.
Similar Threads
  1. Jon.boston
    Replies:
    2
    Views:
    976
    Tam/WB2TT
    Oct 27, 2004
  2. half bridge mosfet switching

    , Jul 20, 2005, in forum: Electronic Design
    Replies:
    25
    Views:
    1,007
    Mook Johnson
    Jul 23, 2005
  3. how to simulate mosfet switching

    , Jul 27, 2005, in forum: Electronic Design
    Replies:
    1
    Views:
    274
    Jim Thompson
    Jul 27, 2005
  4. JH
    Replies:
    12
    Views:
    452
    Joerg
    Mar 19, 2006
  5. Michelle
    Replies:
    2
    Views:
    1,442
Loading...

Share This Page