OPA552 as power regulator

"John Larkin"  wrote in message 
news:akhl7el6gkiggo0teok51353l184e5jst1@4ax.com...
>
>
>
>I need some programmable power supply rails, maybe -5 to +40 volts or
>so. We have some OPA552s in stock, a nice 60 volt power opamp. It's a
>decomp, and it will be driving a big capacitive load, so there is an
>oscillation hazard. One of my guys got the model from TI and kluged it
>into LT Spice. The symbol is ugly but it seems to work. We don't know
>how to make it look like a real opamp.
>
>https://www.dropbox.com/s/0qkphdgnpv8y0xv/ADG_Vhi_1.jpg?dl=0
>
>This looks fine, even without C1. Maybe I'll include C1 on the pcb
>layout just to have another knob to turn.
>
>I'd like to use the OPA552 thermal cutoff as a secondary, slowish
>current limit, but I'd have to tune the pcb thermal resistance somehow
>to control that. Maybe a bunch of thermal-zero-ohm jumpers? Maybe a
>tweakable amount of gap-pad under the board, or above the amp? I
>suppose I could selectively drill out vias on the rev A board, but
>that's really ugly.
>
>I want a thermal conductivity trimpot. Or DAC.

Put a resistor right next to the OPA552 (or two, one on each side for better 
uniformity) as a heater and vary the power to control the "ambient" 
temperature the 552 is dumping it's heat to.

-- 
Regards,
Carl Ijames 

On Saturday, 2 March 2019 20:14:47 UTC, John Larkin  wrote:
> On Sat, 2 Mar 2019 11:50:26 -0800 (PST), tabbypurr wrote:
> >On Saturday, 2 March 2019 18:25:29 UTC, John Larkin  wrote:

> >> I need some programmable power supply rails, maybe -5 to +40 volts or
> >> so. We have some OPA552s in stock, a nice 60 volt power opamp. It's a
> >> decomp, and it will be driving a big capacitive load, so there is an
> >> oscillation hazard. One of my guys got the model from TI and kluged it
> >> into LT Spice. The symbol is ugly but it seems to work. We don't know
> >> how to make it look like a real opamp.
> >> 
> >> https://www.dropbox.com/s/0qkphdgnpv8y0xv/ADG_Vhi_1.jpg?dl=0
> >> 
> >> This looks fine, even without C1. Maybe I'll include C1 on the pcb
> >> layout just to have another knob to turn.
> >> 
> >> I'd like to use the OPA552 thermal cutoff as a secondary, slowish
> >> current limit, but I'd have to tune the pcb thermal resistance somehow
> >> to control that. Maybe a bunch of thermal-zero-ohm jumpers? Maybe a
> >> tweakable amount of gap-pad under the board, or above the amp? I
> >> suppose I could selectively drill out vias on the rev A board, but
> >> that's really ugly.
> >> 
> >> I want a thermal conductivity trimpot. Or DAC.
> >
> >Copper area fanning out round the chip, with the area split into lots of sections like a fanblade. Each fanblade narrows where it gets close to the chip: you can cut through those to make a fair difference to heat loss. I do that if desoldering something from an unwanted board.
> >
> >
> >NT
> 
> Production wouldn't like doing that. And we don't prototype.

I thought you wanted a proto were you could cut off the islands 1 by 1 to see how much heatsinking got you to the right place.

If cutting islands in production is out, which I assumed it would be, obvious other ways are
a) solder in links in production, eg 0 ohm Rs to connect thermal islands
b) solder in (or not) additional heatsinking items, eg bits of copper, bars for heatspread etc.

If you really want it to be fully variable rather than preset, I guess a bolt in a nut (soldered or crimped) could provide variable conduction. Not a very likeable arrangement though.


NT

On 2 Mar 2019 14:57:55 -0800, Winfield Hill <hill@rowland.harvard.edu>
wrote:

>John Larkin wrote...
>>
>>> opa547 ?
>>
>> That's nice, with the programmable current limit. Probably
>> worth adding to stock. It swings reasonably close to the rails,
>> so with a 48v supply we could make 44 or 45 volt output pulses.
>
> For programmable pulse generators, how about using
> programmable rail voltages, with MOSFET switches?

That's what I'm trying to do, generate programmable 4-quadrant Vh and
Vl rails. Then I'm thinking about using GaN switches.

I've done some nice fast pulse gen output stages with mesfets and then
phemts, but the parts keep going EOL. I've never had much luck using
mosfets; high Cg-d makes the pulses ugly.




-- 

John Larkin         Highland Technology, Inc
picosecond timing   precision measurement 

jlarkin att highlandtechnology dott com
http://www.highlandtechnology.com

John Larkin wrote...
>
> I've never had much luck using mosfets;
> high Cg-d makes the pulses ugly.

 I'm surprised at that, I've had excellent results.
 You need to avoid using a too powerful part, keep
 capacitances down.  If it's sub 15ns tr tf you're
 seeking, go ahead and use SiC.  They're fine at
 low voltages too.  For good pulse waveforms into
 coax, you have to include 50-ohm back termination.


-- 
 Thanks,
    - Win

On 2 Mar 2019 16:12:19 -0800, Winfield Hill <hill@rowland.harvard.edu>
wrote:

>John Larkin wrote...
>>
>> I've never had much luck using mosfets;
>> high Cg-d makes the pulses ugly.
>
> I'm surprised at that, I've had excellent results.

The problems happen when Vh gets close to Vl, 1 volt say. The volts of
gate drive leak into the output and the pulse gets ugly, like a big
precursor in the wrong direction.

> You need to avoid using a too powerful part, keep
> capacitances down.  If it's sub 15ns tr tf you're
> seeking, go ahead and use SiC.  They're fine at
> low voltages too.  For good pulse waveforms into
> coax, you have to include 50-ohm back termination.

I'd like to get below 1 ns. SiC needs tens of volts of gate drive,
which is awkward. GaN needs roughly 4, which is bad enough. I really
miss phemts; 0.7 volt swing would switch them, and Cgd was close to
zero.

Our little time-domain biz is whiplashed by the cell phone/telecom
market. If they can't sell a billion parts a year, they shut down the
fab. Broadcom bought Avago and killed the good stuff.






-- 

John Larkin         Highland Technology, Inc
picosecond timing   precision measurement 

jlarkin att highlandtechnology dott com
http://www.highlandtechnology.com

On Sunday, 3 March 2019 00:58:48 UTC, John Larkin  wrote:
> On 2 Mar 2019 16:12:19 -0800, Winfield Hill <hill@rowland.harvard.edu>
> wrote:
> >John Larkin wrote...
> >>
> >> I've never had much luck using mosfets;
> >> high Cg-d makes the pulses ugly.
> >
> > I'm surprised at that, I've had excellent results.
> 
> The problems happen when Vh gets close to Vl, 1 volt say. The volts of
> gate drive leak into the output and the pulse gets ugly, like a big
> precursor in the wrong direction.
> 
> > You need to avoid using a too powerful part, keep
> > capacitances down.  If it's sub 15ns tr tf you're
> > seeking, go ahead and use SiC.  They're fine at
> > low voltages too.  For good pulse waveforms into
> > coax, you have to include 50-ohm back termination.
> 
> I'd like to get below 1 ns. SiC needs tens of volts of gate drive,
> which is awkward. GaN needs roughly 4, which is bad enough. I really
> miss phemts; 0.7 volt swing would switch them, and Cgd was close to
> zero.
> 
> Our little time-domain biz is whiplashed by the cell phone/telecom
> market. If they can't sell a billion parts a year, they shut down the
> fab. Broadcom bought Avago and killed the good stuff.

Maybe there's room for a biz that buys untraced parts & tests & certifies them.


NT

tabbypurr@gmail.com wrote...
>
> Maybe there's room for a biz that buys untraced parts
> & tests & certifies them.

 I've had surprising good luck finding hidden stock of
 un-obtainium parts in Asia, searching Alibaba sellers.
 Put up a part number and ask for quotes on 250 pieces.


-- 
 Thanks,
    - Win

On Sunday, 3 March 2019 02:55:54 UTC, Winfield Hill  wrote:
> tabbypurr wrote...

> > Maybe there's room for a biz that buys untraced parts
> > & tests & certifies them.
> 
>  I've had surprising good luck finding hidden stock of
>  un-obtainium parts in Asia, searching Alibaba sellers.
>  Put up a part number and ask for quotes on 250 pieces.

Yes, lots is gettable that way. Could test & certify replace manufacturer traceability sometimes? Not for milspec normally, but I remember a big military ask for any old 486 PCUs folk had lying around, so even mil can accept that in some circumstances.


NT

tabbypurr@gmail.com wrote...
>
>On Sunday, 3 March 2019 02:55:54 UTC, Winfield Hill  wrote:
>> tabbypurr wrote...
>
>> > Maybe there's room for a biz that buys untraced parts
>> > & tests & certifies them.
>> 
>>  I've had surprising good luck finding hidden stock of
>>  un-obtainium parts in Asia, searching Alibaba sellers.
>>  Put up a part number and ask for quotes on 250 pieces.
>
> Yes, lots is gettable that way. Could test & certify
> replace manufacturer traceability sometimes?  Not for
> milspec normally ...

 I'll give an example, I needed a certain 600V MOSFET
 that had been long been discontinued, placed a request
 for 250 pieces on AliExpress.  Got quote at $0.38 each,
 ordered with DHL shipping.  The parts quickly arrived.
 Right packaging, right labeling.  Sets of capacitance
 measurements showed the part's unique values.  Would
 a counterfeiter go to the relabeling trouble for a $95
 order?  Did he have the time?  How could he spoof the
 physical measurements?  One has to ask, how are the
 parts not going to be exactly what they appear to be?

 I'm amazed these parts hid for years, without Google
 revealing any sign that they existed, but they indeed
 had hidden, and the Alibaba RFQ system turned them up.

 As a followup, the instruments made with these parts
 worked perfectly.


-- 
 Thanks,
    - Win

Winfield Hill wrote...
>
>tabbypurr@gmail.com wrote...
>>
>>On Sunday, 3 March 2019 02:55:54 UTC, Winfield Hill  wrote:
>>> tabbypurr wrote...
>>
>>> > Maybe there's room for a biz that buys untraced parts
>>> > & tests & certifies them.
>>> 
>>>  I've had surprising good luck finding hidden stock of
>>>  un-obtainium parts in Asia, searching Alibaba sellers.
>>>  Put up a part number and ask for quotes on 250 pieces.
>>
>> Yes, lots is gettable that way. Could test & certify
>> replace manufacturer traceability sometimes?  Not for
>> milspec normally ...
>
> I'll give an example, I needed a certain 600V MOSFET
> that had been long been discontinued, placed a request
> for 250 pieces on AliExpress.  Got quote at $0.38 each,
> ordered with DHL shipping.  The parts quickly arrived.
> Right packaging, right labeling.  Sets of capacitance
> measurements showed the part's unique values.  Would
> a counterfeiter go to the relabeling trouble for a $95
> order?  Did he have the time?  How could he spoof the
> physical measurements?  One has to ask, how are the
> parts not going to be exactly what they appear to be?
>
> I'm amazed these parts hid for years, without Google
> revealing any sign that they existed, but they indeed
> had hidden, and the Alibaba RFQ system turned them up.
>
> As a followup, the instruments made with these parts
> worked perfectly.

 I had similar good results with an unobtainium 1.2kV
 MOSFET, off the market for years, only a few pieces
 available for about $10 each, got 250 at ~ $0.60 each.


-- 
 Thanks,
    - Win