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Re: Parasitics?

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On Saturday, March 18, 2017 at 5:45:07 AM UTC-4, rickman wrote:
> On 3/18/2017 4:36 AM, bloggs.fredbloggs.fred@gmail.com wrote:
> > On Friday, March 17, 2017 at 12:57:57 AM UTC-4, rickman wrote:
> >> On 3/16/2017 11:51 PM, Sylvia Else wrote:
> >>> On 17/03/2017 2:16 PM, rickman wrote:
> >>>> On 3/16/2017 9:21 PM, Sylvia Else wrote:
> >>>>> On 17/03/2017 10:44 AM, rickman wrote:
> >>>>>> I'm running a simulation where I'm pumping charge into a capacitor to
> >>>>>> generate a voltage.  A transistor emitter/source is connected to the
> >>>>>> positive rail and the collector/drain is connected to a 1 nF cap
> >>>>>> through
> >>>>>> a 47k resistor.  When the base/gate is driven high to shut off the
> >>>>>> current I see the current go up initially then taking almost 2 us to
> >>>>>> shut off.  I've tried 2N3906 and BSS84 and a couple others selected
> >>>>>> randomly.  The switching period is around 8 uS.
> >>>>>>
> >>>>>> Is this from the parasitic capacitance in the transistor?  The BSS84
> >>>>>> only has 1 nC gate charge and still the effect is very pronounced.
> >>>>>>
> >>>>>> Is there a way to mitigate this effect other than using a larger cap
> >>>>>> and
> >>>>>> smaller resistors?
> >>>>>>
> >>>>> When you try to drive the gate high, the gate-drain capacitance has to
> >>>>> be charged, and the only way than can happen is for current to flow
> >>>>> through R3. A back of the envelope calculation suggests that will take
> >>>>> about the time you're seeing.
> >>>>>
> >>>>> To get it to switch more quickly, you need to reduce R3, or provide some
> >>>>> other path for the current.
> >>>>>
> >>>>> A diode in series with R3, and a 1K resistor from the drain to ground
> >>>>> improves things substantially.
> >>>>
> >>>> That actually makes things worse by substituting the capacitance of the
> >>>> diode through the 1k resistor for the capacitance of the transistor.
> >>>>
> >>>> I was trying to use a very low power approach by injecting charge onto a
> >>>> capacitor which would have a very low drain from it.  But if low value
> >>>> resistors have to be added, I can just use a dithered pullup/pulldown
> >>>> instead.  That may also have current injection, but it should be
> >>>> approximately equal in both directions because of the push-pull output.
> >>>>
> >>>
> >>> I had in mind this. The current through R3 reverses, then returns to
> >>> zero in 190ns.
> >>
> >> Yes, that's what I modeled.  I see large current spikes on both edges of
> >> the pulse.  Regardless, I won't be using a discrete transistor.  I'll be
> >> using the output from an FPGA.  Depending on the pulse width of the
> >> output, the current in R2 is around 1 mA which is as much as the entire
> >> rest of the project.
> >>
> >> I can use a large resistor in series with the output into the cap and
> >> drive it high and low for a range of duty cycles.  That will get me what
> >> I need.  This circuit only needs to source voltage, so I thought it
> >> would use less current to only pull high.  We'll see what the real
> >> circuit does when I use the actual FPGA output.  It doesn't have to be
> >> perfectly linear with duty cycle, but it does need to at least be
> >> monotonic.
> >
> > Uh-huh, and what is the problem you can't simply drive this flea power RC from the FPGA directly? You do know how an RC works? Right?
> 
> I'm not sure what you are talking about.  I will be driving the RC 
> directly from the FPGA output.
> 
> I don't have FPGA outputs in the simulation, so I was using a FET as a 
> model.  Since it appears there are issues with charge injection when 
> driving a high impedance, I will just have to work that out on the bench.

Huh? You think that circuit is representative of an FPGA CMOS output? Don't think so...

> 
> -- 
> 
> Rick C

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Thread

Parasitics? rickman <gnuarm@gmail.com> - 2017-03-16 19:44 -0400
  Re: Parasitics? bloggs.fredbloggs.fred@gmail.com - 2017-03-16 16:48 -0700
  Re: Parasitics? Sylvia Else <sylvia@not.at.this.address> - 2017-03-17 10:59 +1100
    Re: Parasitics? rickman <gnuarm@gmail.com> - 2017-03-16 20:19 -0400
  Re: Parasitics? Sylvia Else <sylvia@not.at.this.address> - 2017-03-17 12:21 +1100
    Re: Parasitics? rickman <gnuarm@gmail.com> - 2017-03-16 23:16 -0400
      Re: Parasitics? Sylvia Else <sylvia@not.at.this.address> - 2017-03-17 14:51 +1100
        Re: Parasitics? rickman <gnuarm@gmail.com> - 2017-03-17 00:57 -0400
          Re: Parasitics? bloggs.fredbloggs.fred@gmail.com - 2017-03-18 01:36 -0700
            Re: Parasitics? rickman <gnuarm@gmail.com> - 2017-03-18 05:45 -0400
              Re: Parasitics? bloggs.fredbloggs.fred@gmail.com - 2017-03-18 06:33 -0700
                Re: Parasitics? rickman <gnuarm@gmail.com> - 2017-03-18 12:22 -0400
                Re: Parasitics? Sylvia Else <sylvia@not.at.this.address> - 2017-03-24 13:30 +1100
                Re: Parasitics? rickman <gnuarm@gmail.com> - 2017-03-24 00:36 -0400
                Re: Parasitics? Sylvia Else <sylvia@not.at.this.address> - 2017-03-24 16:10 +1100
  Re: Parasitics? "Tim Williams" <tiwill@seventransistorlabs.com> - 2017-03-17 01:42 -0500
    Re: Parasitics? rickman <gnuarm@gmail.com> - 2017-03-17 04:11 -0400
  Re: Parasitics? Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net> - 2017-03-17 09:53 -0400
    Re: Parasitics? Cursitor Doom <curd@notformail.com> - 2017-03-17 18:52 +0000

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