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HV OpAmp

Started by Jim Thompson October 11, 2015
HV OpAmp...

<http://www.analog-innovations.com/SED/HV_OpAmp_2015-10-11.png>

Can be built to arbitrary HV supplies, just change Q1 and Q2 to higher
voltage devices.
		
                                        ...Jim Thompson
-- 
| James E.Thompson                                 |    mens     |
| Analog Innovations                               |     et      |
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On Sun, 11 Oct 2015 09:35:54 -0700, Jim Thompson
<To-Email-Use-The-Envelope-Icon@On-My-Web-Site.com> wrote:

>HV OpAmp... > ><http://www.analog-innovations.com/SED/HV_OpAmp_2015-10-11.png> > >Can be built to arbitrary HV supplies, just change Q1 and Q2 to higher >voltage devices. > > ...Jim Thompson
You might need some loop compensation to drive real loads. What's the sinewave output look like with a load? HV things often have to drive capacitive loads, and arcs happen, so I generally have a cap of my own across the output. At higher voltages, R1 and R2 will fry. If you make them really big, things get weak and slow. Bipolars need base current, so R1 has a bad tradeoff. And buying HV resistors is a nuisance. Putting lots of low voltage resistors in series takes, well, lots of resistors. Up to +-200 volts, just use a MOC8204, much simpler and no crossover distortion: perfect class AB. https://dl.dropboxusercontent.com/u/53724080/Optos/HVamp.JPG It's also interesting to cascode a couple of low voltage optoisolators with a couple of depletion fets. https://dl.dropboxusercontent.com/u/53724080/Optos/HV_Dep_Cascode.JPG Even better still, do the PV+mosfet thing that I posted. 1500 volt DPAK mosfets are affordable, and can output a lot of current. https://dl.dropboxusercontent.com/u/53724080/Optos/HV_Reg_4.jpg No HV resistors or associated dissipation. Swings to both rails. Explicit current limits. Zero quiescent current. This can drive a capacitive load with microwatts of power dissipation on the high side... the main power dissipation is in the feedback resistor.
On Sun, 11 Oct 2015 11:26:34 -0700, John Larkin
<jjlarkin@highlandtechnology.com> Gave us:

>On Sun, 11 Oct 2015 09:35:54 -0700, Jim Thompson ><To-Email-Use-The-Envelope-Icon@On-My-Web-Site.com> wrote: > >>HV OpAmp... >> >><http://www.analog-innovations.com/SED/HV_OpAmp_2015-10-11.png> >> >>Can be built to arbitrary HV supplies, just change Q1 and Q2 to higher >>voltage devices. >> >> ...Jim Thompson > >You might need some loop compensation to drive real loads. What's the >sinewave output look like with a load? HV things often have to drive >capacitive loads, and arcs happen, so I generally have a cap of my own >across the output.
HV applications all differ from application to application. The proper HV supply ONLY supplies what is need to perform the required HV function. Extra storage on the output is typically a bad thing. HV Arc suppression is properly managed by a series bulk form resistive current limiting element (read carbon composition). It doesn't actually stop the arc, it merely stops the upstream circuit element damage from occurring. Your cap on the output will not stop all of your multiplier caps and diodes from fusing. Only an arc suppression current limiting series resistor of bulk form media will do that job. They are getting harder and harder to find too.
On Sun, 11 Oct 2015 14:40:10 -0400, DecadentLinuxUserNumeroUno
<DLU1@DecadentLinuxUser.org> wrote:

>On Sun, 11 Oct 2015 11:26:34 -0700, John Larkin ><jjlarkin@highlandtechnology.com> Gave us: > >>On Sun, 11 Oct 2015 09:35:54 -0700, Jim Thompson >><To-Email-Use-The-Envelope-Icon@On-My-Web-Site.com> wrote: >> >>>HV OpAmp... >>> >>><http://www.analog-innovations.com/SED/HV_OpAmp_2015-10-11.png> >>> >>>Can be built to arbitrary HV supplies, just change Q1 and Q2 to higher >>>voltage devices. >>> >>> ...Jim Thompson >> >>You might need some loop compensation to drive real loads. What's the >>sinewave output look like with a load? HV things often have to drive >>capacitive loads, and arcs happen, so I generally have a cap of my own >>across the output. > > HV applications all differ from application to application.
All applications differ from application to application!
> > The proper HV supply ONLY supplies what is need to perform the >required HV function. Extra storage on the output is typically a bad >thing.
If I'm already expecting a capacitive load, adding an output cap of my own here has nice effects on loop stability. The dominant pole becomes my cap in parallel with the load. And the cap helps absorb spikes kicked back by arcs. The energy stored in my cap is tiny, not dangerous.
> > HV Arc suppression is properly managed by a series bulk form resistive >current limiting element (read carbon composition). It doesn't actually >stop the arc, it merely stops the upstream circuit element damage from >occurring. > > Your cap on the output will not stop all of your multiplier caps and >diodes from fusing.
Why would they ever fuse?
>Only an arc suppression current limiting series resistor of bulk form >media will do that job. They are getting harder and harder to find too.
If my customer wants to arc across his own capacitive load, let him go for it! But I don't want that to damage my box.
On Sun, 11 Oct 2015 11:56:24 -0700, John Larkin
<jjlarkin@highlandtechnology.com> Gave us:

>On Sun, 11 Oct 2015 14:40:10 -0400, DecadentLinuxUserNumeroUno ><DLU1@DecadentLinuxUser.org> wrote: > >>On Sun, 11 Oct 2015 11:26:34 -0700, John Larkin >><jjlarkin@highlandtechnology.com> Gave us: >> >>>On Sun, 11 Oct 2015 09:35:54 -0700, Jim Thompson >>><To-Email-Use-The-Envelope-Icon@On-My-Web-Site.com> wrote: >>> >>>>HV OpAmp... >>>> >>>><http://www.analog-innovations.com/SED/HV_OpAmp_2015-10-11.png> >>>> >>>>Can be built to arbitrary HV supplies, just change Q1 and Q2 to higher >>>>voltage devices. >>>> >>>> ...Jim Thompson >>> >>>You might need some loop compensation to drive real loads. What's the >>>sinewave output look like with a load? HV things often have to drive >>>capacitive loads, and arcs happen, so I generally have a cap of my own >>>across the output. >> >> HV applications all differ from application to application. > >All applications differ from application to application! > > >> >> The proper HV supply ONLY supplies what is need to perform the >>required HV function. Extra storage on the output is typically a bad >>thing. > >If I'm already expecting a capacitive load, adding an output cap of my >own here has nice effects on loop stability. The dominant pole becomes >my cap in parallel with the load. And the cap helps absorb spikes >kicked back by arcs. The energy stored in my cap is tiny, not >dangerous. > > >> >> HV Arc suppression is properly managed by a series bulk form resistive >>current limiting element (read carbon composition). It doesn't actually >>stop the arc, it merely stops the upstream circuit element damage from >>occurring. >> >> Your cap on the output will not stop all of your multiplier caps and >>diodes from fusing. > >Why would they ever fuse? > >>Only an arc suppression current limiting series resistor of bulk form >>media will do that job. They are getting harder and harder to find too. > >If my customer wants to arc across his own capacitive load, let him go >for it! But I don't want that to damage my box. >
HVPS design must ONLY provide the amount of energy the customer requirements calls for. And no, not all HV load circumstances are capacitive. Yes, arcs at the output can damage both the HV caps and the HV diodes in the multiplier if no arc suppression is implemented. This is assuming that most modern HV supplies are of the C-W multiplier design on the output stage, and in fact, most are.
Jim Thompson wrote:
> HV OpAmp... > > <http://www.analog-innovations.com/SED/HV_OpAmp_2015-10-11.png> > > Can be built to arbitrary HV supplies, just change Q1 and Q2 to higher > voltage devices. > > ...Jim Thompson
I do not remember all of the details of an alternate way; been 30+ years. Use HV PNP and HV NPN that are powered from the + supply pin of (say) a 741. The output of the op-amp had a load to gnd,and also went to a divided version of the output. Something like that. Maybe HV zeners used? Too long ago; sorry.
On Monday, 12 October 2015 05:40:29 UTC+11, DecadentLinuxUserNumeroUno  wrote:
> On Sun, 11 Oct 2015 11:26:34 -0700, John Larkin > <jjlarkin@highlandtechnology.com> Gave us: > > >On Sun, 11 Oct 2015 09:35:54 -0700, Jim Thompson > ><To-Email-Use-The-Envelope-Icon@On-My-Web-Site.com> wrote: > > > >>HV OpAmp... > >> > >><http://www.analog-innovations.com/SED/HV_OpAmp_2015-10-11.png> > >> > >>Can be built to arbitrary HV supplies, just change Q1 and Q2 to higher > >>voltage devices. > >> > >> ...Jim Thompson > > > >You might need some loop compensation to drive real loads. What's the > >sinewave output look like with a load? HV things often have to drive > >capacitive loads, and arcs happen, so I generally have a cap of my own > >across the output. > > HV applications all differ from application to application. > > The proper HV supply ONLY supplies what is need to perform the > required HV function. Extra storage on the output is typically a bad > thing. > > HV Arc suppression is properly managed by a series bulk form resistive > current limiting element (read carbon composition).
Carbon composition and carbon film resistors aren't a good choice for current limiting. Carbon has a negative coefficient of resistance with temperature, which means you can form low-resistance hot channels. I once saw a 10k carbon film resistor carrying 0.5amp at 0.5V for half an hour through such a channel - it was a contrived set-up, and a big capacitor charged to quite a high voltage had been used to establish the hot channel which was then sustained by a regular lab power supply. It still measured 10k after the test, though there was dark line on the resistor value colour rings over what had been the path of the hot channel. Metal oxide resistors are a much better bet, and I'd tend to pick one that was guaranteed to act as a fuse by blowing to open circuit under overload.
> It doesn't actually > stop the arc, it merely stops the upstream circuit element damage from > occurring.
Carbon film and carbon composition resistors don't reliably stop that.
> Your cap on the output will not stop all of your multiplier caps and > diodes from fusing. > Only an arc suppression current limiting series resistor of bulk form > media will do that job. They are getting harder and harder to find too.
Spreading the potential load over a few resistors will help there, but John Larkin doesn't like doing that. -- Bill Sloman, Sydney
On Sun, 11 Oct 2015 17:24:37 -0700 (PDT), Bill Sloman
<bill.sloman@gmail.com> Gave us:

> >Carbon composition and carbon film resistors aren't a good choice for current limiting.
Yes they are in HV circuit outputs (low power). If the circuit arcs too much, they need to get replaced because they begin to fused shorted within the resistance medium.
On Sun, 11 Oct 2015 17:24:37 -0700 (PDT), Bill Sloman
<bill.sloman@gmail.com> Gave us:

> Carbon has a negative coefficient of resistance with temperature, > which means you can form low-resistance hot channels.
Doesn't matter. The desired mechanism is the fact that they are of bulk medium construction.
On Sun, 11 Oct 2015 17:24:37 -0700 (PDT), Bill Sloman
<bill.sloman@gmail.com> Gave us:

> >Metal oxide resistors are a much better bet,
Not for HV arc suppression They fuse open with the first few arc events. You really know absolutely nothing about this. Whereas I engineered such supplies for years for the likes of NOAA and LANL and SAIC and LLNL, etc.