Forums

OPA552 as power regulator

Started by John Larkin March 2, 2019

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.


-- 

John Larkin         Highland Technology, Inc

lunatic fringe electronics 

On Saturday, March 2, 2019 at 1:25:29 PM UTC-5, 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.
What's wrong with using FLAG? Is that 1p for C1 right? You have stray that's 10x that.
> > > -- > > John Larkin Highland Technology, Inc > > lunatic fringe electronics
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
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.
There's a simple way to safely use decompensated opamps, often useful to get faster slewing, etc. I looked in AoE III, to give the page, but discovered I didn't get it in there! First, you can add a resistor between the opamp's two inputs (for an inverting amp this would be to ground), so it thinks it's working at a higher gain, while not interfering with the actual in-out function. But this adds extra output offset, multiplying the input offset by the additional DC gain. However, since you only care about high-frequency loop-compensation, just add a capacitor in series with the resistor. So that's it, add a series R + C to insure meeting the opamp's minimum gain spec. To combine this with the standard capacitive-load isolation trick, Figure 4.76, page 264, add a resistor in series with the new feedback cap. -- Thanks, - Win
On Sat, 2 Mar 2019 11:38:17 -0800 (PST),
bloggs.fredbloggs.fred@gmail.com wrote:

>On Saturday, March 2, 2019 at 1:25:29 PM UTC-5, 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. > >What's wrong with using FLAG?
For what? We'll probably not connect it in real life. I guess it could light up a red LED or something.
> >Is that 1p for C1 right? You have stray that's 10x that.
That's a placeholder for "maybe we don't need a cap there." I said that. Tweaking a bit more, it looks like C1 is a good idea: 1 uF. The opamp also (according to Spice) oscillates when it current limits. That will happen when the dac voltage steps and a lot of current is briefly forced into the load cap. That's probably not a big deal, but we can slew-rate limit the dac output, in hardware or software. -- John Larkin Highland Technology, Inc lunatic fringe electronics
On Sat, 2 Mar 2019 11:50:26 -0800 (PST), tabbypurr@gmail.com 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. -- John Larkin Highland Technology, Inc lunatic fringe electronics
On 2 Mar 2019 12:06:23 -0800, Winfield Hill <hill@rowland.harvard.edu>
wrote:

>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. > > There's a simple way to safely use decompensated opamps, > often useful to get faster slewing, etc. I looked in > AoE III, to give the page, but discovered I didn't get > it in there! > > First, you can add a resistor between the opamp's two > inputs (for an inverting amp this would be to ground), > so it thinks it's working at a higher gain, while not > interfering with the actual in-out function. But this > adds extra output offset, multiplying the input offset > by the additional DC gain. However, since you only > care about high-frequency loop-compensation, just add > a capacitor in series with the resistor. So that's > it, add a series R + C to insure meeting the opamp's > minimum gain spec. To combine this with the standard > capacitive-load isolation trick, Figure 4.76, page 264, > add a resistor in series with the new feedback cap.
I need gain from DAC to Vhi, so the gain divider, where it is, takes care of the decomp issue. This is a little better: https://www.dropbox.com/s/3ecb7qtn9ilgqfa/ADG_Vhi_2.jpg?dl=0 I've got to go clean some windows. -- John Larkin Highland Technology, Inc lunatic fringe electronics
l&oslash;rdag den 2. marts 2019 kl. 19.25.29 UTC+1 skrev John Larkin:
> 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. >
opa547 ?
On Sat, 2 Mar 2019 12:26:31 -0800 (PST), Lasse Langwadt Christensen
<langwadt@fonz.dk> wrote:

>l&#2013266168;rdag den 2. marts 2019 kl. 19.25.29 UTC+1 skrev John Larkin: >> 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. >> > >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. -- John Larkin Highland Technology, Inc picosecond timing precision measurement jlarkin att highlandtechnology dott com http://www.highlandtechnology.com
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? -- Thanks, - Win