High power opamps as lab supply base - Page 1
Forum section: electronics / projects

author: Spikee
author_rank: Frequent Contributor
timestamp: July 11, 2013, 12:34:37 pm
content: So there are these high power opamps like the TI/BB OPA541.http://www.ti.com/product/opa541Woulden't those be suited for an easy (digital) power supply?Because they are opamps they get pretty close to the 0V en the max output is pretty close to the rail voltage.And they are available in those big ass audio amp packages so they can handle high wattage's.It even has a adjustable current limit so it kinda is a one package high power voltage regulator.I know that they are a bit expensive (~ 10USD ) but the part count will be less so it kinda evens out.Or am i missing something pretty obvious ?

author: madires
author_rank: Super Contributor
timestamp: July 11, 2013, 12:56:24 pm
content: The datasheet claims programmable power supplies as application and there's an circuit example too.OUTPUTVoltage SwingIout = 5A, Continuous ±(|VS|–4.5)VIout = 2A ±(|VS|–3.6)VIout = 0.8A ±(|VS|–3.2)VI wouldn't call that pretty close to the rails but with ±40V or +80V supported it should be good enough for a 30V PSU. And for the current limiter toss the fixed resistor (for the 0.6V threshold) and add a low ohms current shunt plus OPamp and a pot (or another DAC plus comparator).

author: Spikee
author_rank: Frequent Contributor
timestamp: July 11, 2013, 01:07:16 pm
content: Yeah i also discovered the offset XD , i'm now simulating a simple design using TI spice and i will report my progress here.

author: trophosphere
author_rank: Frequent Contributor
timestamp: July 11, 2013, 04:08:09 pm
content: What would happen if you had a large amount of capacitance on its output?

author: madires
author_rank: Super Contributor
timestamp: July 11, 2013, 08:34:37 pm
content: Quote from: trophosphere on July 11, 2013, 04:08:09 pmWhile setting a lower output voltage? Or did you mean something else?

author: trophosphere
author_rank: Frequent Contributor
timestamp: July 11, 2013, 10:11:41 pm
content: Quote from: madires on July 11, 2013, 08:34:37 pmYes, in general, if the voltage were to be varied with or without load. I would think it would oscillate but I haven't read much into the datasheet.Would you need to drop in a resistor or ferrite in series with its output? Even then, there would be that voltage drop at higher loads which could be compensated by placing it in the feedback loop but the power loss would still be there. Other compensation techniques would be to put a capacitor in the feedback loop or raising the gain.

author: Spikee
author_rank: Frequent Contributor
timestamp: July 12, 2013, 12:12:57 am
content: In my design i will be controlling it digitally. Using the internal 12 bit adc of the Cypress psoc 4 and a Microchip MCP4728 12b dac.Tomorrow I will post some more info about the design.Test will tell if it is stable

author: Psi
author_rank: Super Contributor
timestamp: July 12, 2013, 12:34:16 am
content: This is something i wanted to build also. I've got a tube of OPA549 sitting around here somewhere.Ya can make quite a nice powersupply that can source and sink current as well produce waveforms at high current.

author: Spikee
author_rank: Frequent Contributor
timestamp: July 12, 2013, 01:15:11 am
content: Quote from: Psi on July 12, 2013, 12:34:16 amTrow it on a breadboard or experiment pcb board and have some fun!

author: M. András
author_rank: Super Contributor
timestamp: July 12, 2013, 08:53:42 am
content: Quote from: Psi on July 12, 2013, 12:34:16 ampossbilities of making a high power very low ripple psu is ahead youwish i could affford a tube of them right now with some other goodies to start experimenting with my own idea of using them as power supply

author: Psi
author_rank: Super Contributor
timestamp: July 12, 2013, 08:56:59 am
content: I didn't buy them.A friend went "free sample" crazy while at university and when he went flatting there was no room for it all so i inherited boxes of component tubes.(i don't condone this)

author: madires
author_rank: Super Contributor
timestamp: July 12, 2013, 01:16:43 pm
content: Quote from: trophosphere on July 11, 2013, 10:11:41 pmIf you've set the output voltage to 20V and then set it down to 10V the OPA541 needs to sink current until the voltage at the output (connected to the large capacitors) reaches 10V. It's part of the regulation loop. I don't know how much it would take to make the regulation loop oscillate. Maybe it would require a load with a regular fast varying current consumption and regular output voltage changes for the OPA541. But I think it wouldn't be much different from a classic PSU design (OPamps and a pass power transistor), the only difference it that it also sinks current (could be very useful for some situations).QuoteAn in-series inductor would soften the current changes (both directions). But based on the datasheet the current limiter should be able to cope with high currents too. For high capacitive loads that would decrease the voltage slew rate as the current is limited.

author: trophosphere
author_rank: Frequent Contributor
timestamp: July 12, 2013, 01:59:11 pm
content: @madiresThanks for the clarification!

author: M. András
author_rank: Super Contributor
timestamp: July 12, 2013, 07:59:45 pm
content: Quote from: Psi on July 12, 2013, 08:56:59 amhmm ti had the opa549 in the sample program and even a whole tube? or it was before they aquired Burr Brown?

author: Spikee
author_rank: Frequent Contributor
timestamp: July 12, 2013, 09:28:44 pm
content: You can sample like 3 pieces of one item and 2-3 items in total?So you can order different versions of the same chip (lead free etc...) and you will have ~ 9 pieces of the OPA541 or similar from ti.

author: ddavidebor
author_rank: Super Contributor
timestamp: July 12, 2013, 09:50:23 pm
content: Aaaaah free sample <3

author: M. András
author_rank: Super Contributor
timestamp: July 12, 2013, 09:56:24 pm
content: naah you cant freely, neither the opa 549. on the 541 they list contact sales office or distributor. but you cant order them like some of their 4 channel 24bit delta sigmas. and btw it lets you choose 5 different product and after that you cant order for 1-2 month. btw do the idiot customs office charge anything for samples? like if they cant read the big label engineering samples etc? just the stated value?

author: Spikee
author_rank: Frequent Contributor
timestamp: July 13, 2013, 12:32:42 am
content: TI pays the tax for it. You never have to pay anything.

author: M. András
author_rank: Super Contributor
timestamp: July 13, 2013, 08:03:41 am
content: Quote from: Spikee on July 13, 2013, 12:32:42 amthe odd and in the same time reasonable things i found when looking for some cheaps and a 1-1 free sample for it from different manufacturers only ti has no problem with free email accounts google, yahoo, etc the rest asked for corporate email accs or similar, which i cant get and for a 1 chip that cost 3 times as much from farnell in single quantity then at 10-100 thats the way for me when i need to decide on test that chip is good for me for that purpose or should look at a better or similar.offf topic ended

author: Psi
author_rank: Super Contributor
timestamp: July 13, 2013, 09:11:47 am
content: Quote from: M. András on July 12, 2013, 07:59:45 pmI cant be sure where he got them from, they were just in the boxes of free samples he gave me, so i assumed they were free samples also.Other opamps in the boxes were tubes of 10+ OPA627 OPA2132 LM3900 TL072 RC4580 NE5534 NE5532 LMX324.There are 4 types of OPA549 in the tube, straight legs vs alternating bent legs and metal tab vs insulated plastic tab.So that would make sense if he got samples of each type they stocked.This was maybe 10 years ago.

author: M. András
author_rank: Super Contributor
timestamp: July 13, 2013, 10:22:39 am
content: can you post a well oriented photo on the bent legged one next to a straight? cant decide which one to order or would be easier to route the board for it

author: Spikee
author_rank: Frequent Contributor
timestamp: July 13, 2013, 10:57:52 pm
content: This is how i am designing it now:https://dl.dropboxusercontent.com/u/100819959/lab_supply.pngThe voltage regulation part of the TI OPA541 works good (needed to do some negative rail trimming on the 8x gain opamp to get ~0.00x volt).But the current regulation using the dac or even the resistor does not work that good ... (Using NI Multisim).The lower voltage equivalent OPA549 60V 10A peak power opamp works much better (current limiting using dac) :https://dl.dropboxusercontent.com/u/100819959/OPA549.png-Vs also show as Ref in the picture above will be -8V to get 0 -> +Vs out. Therefore the current dac circuitry has to output -3.25V of 0A to -8V for 10A limit. This is easy achievable by simple op-amp circuitry.In my simulated design i made sure that the pcb is very versatile , if you want-10V- 0 -10Vout or-80and+80V you don't need to change any parts (except the power opamp and maybe some passives). I made sure while i made the design that it can support all kind of voltage ranges.In the pcb design i will make sure that i have enough track isolation so high voltage is possible but also tracks for 10-20-30A so low voltage high current is also possible. Ti has many high power pin compatible opamps so you can chose for diffrent voltages , power and you can even parallel them.In my final design i will incorperate a dual channel voltage/current regulation.The end resolution i want to achieve is 10mV / 10mA and that seems plausible in the current simulation. In incorporate a lot of trimming pots so most of the offsets are easy to compensate.The power dissipation part is easy when using a good quality heat-piped cpu cooler as i have earlyer experimented in my constant current load project (some vids are on my Youtube channel ).Too bad i can't get OPA549 samples =/ guess i'll go to ebay.OPA541 datasheet:http://www.ti.com/lit/ds/symlink/opa541.pdfOPA548 page:http://www.ti.com/product/opa549

author: C
author_rank: Super Contributor
timestamp: July 14, 2013, 12:35:10 am
content: I would suggest that you change from single ended design to a differential version.The added cost can be very little while the benefits are great.If you look at the better ADC's & DAC's they use a differential connection to assist in the removal of noise and common ground problems.Converting from single ended to differential can be done very easy.C

author: Spikee
author_rank: Frequent Contributor
timestamp: July 14, 2013, 12:41:07 am
content: The adc in the Psoc 4 can be configured as differential and i was planning to use that. Are you also talking about a differential dac ?I have tested the 12bit adc on differential mode and 256 times oversampling (low sps) and it was pretty much spot on when testing it whit my Siemens voltage calibrator.As for a dac i looked at AD before but they were generally pretty expensive. Do you have a suggesting for a reasonable priced 4 channel differential dac that uses I2C / SPI ?

author: C
author_rank: Super Contributor
timestamp: July 14, 2013, 01:34:35 am
content: With single ended design your return path is one of the power rails which has a lot of problems.QuoteYES.A single ended DAC or ADC works with a reference to +- analog references.For a DAC you just add a reference based on the +- analog references. This can be as simple as a pot connected between these two analog references and the wiper used as the second wire of the differential signal.While true differential has both wires effected in opposite directions this still looks like a differential signal to a differential input while removing it from the mess on the rails.With differential signals you remove a lot of problems.Note in the data sheet should be a section about PC board layout.The output has two high current return paths back to the two power pins on the chip. You have three or more things that need a single point connection to the AMP's power pins, The Output, The Input , The Power Source. Differential makes this easer.The OPA541 can be many things with just an input change.Fixed input = a power supplyWith Input based on (fixed input & output) = a LoadA power Amp up to the bandwidth limits of the OPA541.A differential connection just makes it a lot easer to use.C

author: Psi
author_rank: Super Contributor
timestamp: July 14, 2013, 04:08:48 am
content: Quote from: M. András on July 13, 2013, 10:22:39 amHere's the OPA549 bent vs straight leg.Personally i prefer bent legs, there's more room to get high current traces all around the pins.

author: M. András
author_rank: Super Contributor
timestamp: July 14, 2013, 07:48:35 am
content: thanks for the pictures, it would be simplier if the bent out pins would be the input output high current paths and the rest is the control signals but its still good

author: Spikee
author_rank: Frequent Contributor
timestamp: July 14, 2013, 04:43:21 pm
content: The bent legs version seems much more capable to handle bigger currents and it will be easier when designing the pcb.I have also chosen to use the OPA549 now and I'm currently simulating the circuit again.

author: KuchateK
author_rank: Regular Contributor
timestamp: July 29, 2013, 03:39:38 pm
content: I will be watching this thread closely and I will join your efforts. I bought few OPA549 amps few years ago for power supply, but I still didn't try to build anything.

author: LEECH666
author_rank: Frequent Contributor
timestamp: August 26, 2013, 11:57:54 am
content: Any update, Spikee?

author: Spikee
author_rank: Frequent Contributor
timestamp: August 26, 2013, 01:03:51 pm
content: School starts soon and then i will have time to fix it =)