Groups > sci.electronics.design > #452229

Re: Parasitics?

From	Sylvia Else <sylvia@not.at.this.address>
Newsgroups	sci.electronics.design
Subject	Re: Parasitics?
Date	2017-03-17 14:51 +1100
Message-ID	<ej14q3Fov9nU1@mid.individual.net> (permalink)
References	<oaf7ro$efu$1@dont-email.me> <ej0s1jFngurU1@mid.individual.net> <oafka4$5on$1@dont-email.me>

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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.

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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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