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Re: Parasitics?

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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.

-- 

Rick C

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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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