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Re: Parasitics?

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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.

Sylvia.

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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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