0-30V Stabilized Power Supply

[paul18fr]

Very interesting project ; nevertheless I'm trying to find the transformer (primary = 230V for EU & secondary = 28V - 120 W at least) and the only ones I've found are too expensive (close to the price of a 30V - 5A power supply of the market), even if I degrade the output voltage to 24V, and thus the features of the power supply

Last chance : does somebody have ever find it  (even in CN, HK, etc. ..)

Thanks

Paul

 

[liquibyte]

Very interesting project ; nevertheless I'm trying to find the transformer (primary = 230V for EU & secondary = 28V - 120 W at least) and the only ones I've found are too expensive (close to the price of a 30V - 5A power supply of the market), even if I degrade the output voltage to 24V, and thus the features of the power supply

Last chance : does somebody have ever find it  (even in CN, HK, etc. ..)

Thanks

Paul

I used this one.  I don't know if you can get it somewhere other than Digikey but it's a Triad Magnetics model VPS28-4600.  Try a search with that and see if something local comes up.

 

[liquibyte]

I tried to do my best including all of the projects from both threads that had a semblance of completeness to them in the data sticky.

Some are woefully incomplete as they are just PDF's and would require work to be printed out and used to make boards.  PicMaster's version is there several times, sometimes as just the schematic and finally in his full version.  He only used PDF format so there is that.  Some are just image files but their authors reported working versions based on those.  I did include my version as well and can report that it's the only one that has the gerbers that can be sent out and made with no extra work (they are Itead gerbers).  I included a through hole version and a surface mount version that redwire did.  These are complete enough to generate gerbers from and sent out.

I unlocked the topic so that those wishing to add their completed projects can do so.  This is NOT a discussion thread, it's a completed thread.  I don't think it needs to be a requirement to have generated gerbers but I do think that at the very minimum a pic of the schematic used and parts list should be included as well as completed project pics.  Not everyone uses Eagle but I think that adding those files would be nice.  The same goes for Kicad or Diptrace.  If you use Altium or some unrealistically expensive commercial product, at least have the courtesy to attach screenshots/exports in addition to the programs default generated files.  Remember also that not everyone uses windows and not doing so excludes Linux and BSD users from even wanting to try your version.

If I missed something, let me know.  I can assure you it wasn't on purpose, those are both really, really long threads.

 

[liquibyte]

Why was Q1 in the original schematic removed?  After researching the voltage spike issue a bit after participating in a thread over at eevblog and one of the members pointing me to a solution that was talked about recently and directing me to the Tektronics PS-503A manual that suggested this situation happening and showed this exact workaround, it seems that this solution was already designed into the original project but then removed with the update.

 

[audioguru2]

Why was Q1 in the original schematic removed?  After researching the voltage spike issue a bit after participating in a thread over at eevblog and one of the members pointing me to a solution that was talked about recently and directing me to the Tektronics PS-503A manual that suggested this situation happening and showed this exact workaround, it seems that this solution was already designed into the original project but then removed with the update.

On the Tektronics circuit it looks like transistor Q15 shorts the output of the voltage error amplifier opamp U45 to ground when the main power is turned off. I did not see their discussion of it.
I do not know which opamp they use since they do not use standard part numbers.

The original circuit of this 0V-30V power supply here used the TL081 opamp that has the problem called "Opamp Phase Inversion" which causes its output to suddenly go high when an input voltage goes outside its allowed negative input common mode voltage range. The allowed negative input voltage range for the TL081 is a maximum of 4V more positive than its negative power supply voltage which was -5.6V. Then when the main power is turned off the negative -5.6V supply disappears quickly and causes the input of the voltage error amplifier opamp U2 to have a voltage outside its allowed input common mode voltage range. Then it would cause the output voltage of the project to go as positive as it can while the huge main positive filter capacitor was discharging. They added Q1 to short the output of U2 to ground when the main power was turned off.

The new MC33071 and TLE2141 opamps do not have this problem so transistor Q1 was removed.

Adding Q1 back might not fix the existing voltage spike because it might not activate quickly enough.
Liquibyte, can you try it please?  View attachment 42213

 

[liquibyte]

I'm starting to have trouble keeping up with the changes between how the original circuit was tied to ground vs. the revised version.  In the original schematic, U2 was at the same ground potential as U3 with Q1's voltage divider being tied to that point.  In the revised version, U2 grounds out to the common ground.  I tried to tie in Q1 like in the original but left U2 alone.  I also didn't add R15 back in as I wasn't sure of its purpose.  After doing all that, I was only able to get 200-250mV out regardless of what I had the voltage pot set at so I suspect that Q1 was shunting to ground regardless.  I only have a few basic NPN transistors such as the 2N2222A so I'm not sure if the differences between that and the original BC548 is an issue or not.  I'm also not sure if I'd need to modify the values of R14 and R15 from 1.5K and 10K respectively.  I've also been told that D10 may need to be added back to ensure reverse Vbe protection.

I was also reading another thread where a user had an inductance issue and this app note was posted as a solution.  His issue was with inductance on long battery leads being connected to the device versus a power supply but I'm starting to think that several things could be at play here and I found this app note interesting though it goes above my pay grade quite a bit.  I don't have a scope that can sample and hold so I can only go by the pics redwire posted and the brief glance I can get from the old one I have and my meter but there seems to be a correlation here.  I'm especially seeing what you mentioned with "the output voltage of the project to go as positive as it can while the huge main positive filter capacitor was discharging" on redwire's version that I've been testing so I have a feeling that the inversion is happening there but not necessarily with my versions where I just get the spikes.

 

[audioguru2]

The original circuit used R13 and R14 to keep Q1 turned off when the positive unregulated supply was about +28V and the negative supply was -5.6V.
When the mains power was turned off then the negative supply disappeared first allowing R13 to turn on Q1.

To add Q1 to the latest circuit then the value of R14 must be decreased a lot so that the base voltage of Q1 is at a negative voltage when the circuit is working.

The 0V (ground) of both circuits is the output 0V, except the adjustable reference voltage for U3 from R17. The original circuit had -5.6V for the same negative supply for U2 and U3 (R10 was connected wrongly to 0V instead of to the negative supply).

The latest circuit uses an opamp for U2 that works perfectly without a negative supply so it does not have a supply voltage more than its maximum rating of 44V.
U3 needs to have its output go negative one diode voltage drop so that D9 can pull down the drive voltage during current regulation. Then the negative supply for U3 was made -1.3V.

R15 was removed because it didn't do anything useful and it wasted valuable output drive voltage. D10 also never did anything and still doesn't do anything. 

 

[liquibyte]

To add Q1 to the latest circuit then the value of R14 must be decreased a lot so that the base voltage of Q1 is at a negative voltage when the circuit is working.

Something like a standard 270R or so or am I way off here?

 

[audioguru2]

To add Q1 back in I calculated the voltage divider values like this:

View attachment 42216

 

[liquibyte]

There's good news and bad news.  Adding Q1 back in with a 12K 160R voltage divider and moving U2's pin 4 to the D7/R3 junction stopped the spikes.  The bad news is that I can now only get up to around 7.6V at the output.  More than half of the voltage pots travel on the upper end does nothing.  I think we're getting close, but I wonder what may have caused this.

Just a note, I replaced redwire's original single turn pots with 10 turn versions so I could have greater resolution while testing but that shouldn't make a difference.

Edit:  Hang on, I just realized I didn't move R10 from where it is to where it needs to be.

 

[audioguru2]

Adding Q1 back in with a 12K 160R voltage divider and moving U2's pin 4 to the D7/R3 junction stopped the spikes.  The bad news is that I can now only get up to around 7.6V at the output.

Sorry, I made a mistake converting the original schematic to the fixed and improved one.

You do not want the negative supply pin 4 of U2 to be negative -1.3V because then U2 will have its total supply voltage very close to its maximum rating of 44V (+42.6V plus -1.3V= 43.9V).
U2 does not need a negative supply so connect its pin 4 and R10 to the 0V from the unregulated power source.

Most little NPN silicon transistors have avalanche breakdown (like a zener diode) of their base-emitter junction when it is reverse biased more than about 6V so maybe you have the emitter and collector pins of Q1 reversed?

Here is my corrected schematic:

View attachment 42217

 

[liquibyte]

Sorry, I made a mistake converting the original schematic to the fixed and improved one.

You do not want the negative supply pin 4 of U2 to be negative -1.3V because then U2 will have its total supply voltage very close to its maximum rating of 44V (+42.6V plus -1.3V= 43.9V).
U2 does not need a negative supply so connect its pin 4 and R10 to the 0V from the unregulated power source.

Most little NPN silicon transistors have avalanche breakdown (like a zener diode) of their base-emitter junction when it is reverse biased more than about 6V so maybe you have the emitter and collector pins of Q1 reversed?

Here is my corrected schematic:

Moved pin 4 of U2 and R10 back to the original locations and we're good.  You were right about the transistor being in backwards.  I think the datasheet I looked up must have had the pins backwards from the transistor I was using because it was in right according to the datasheet but as soon as I swapped the collector and emitter pins it started working beautifully.

With your help, we have just solved the transient issue.  I'm now getting 30V with no spikes.  I'm going to redesign my boards to reflect this change and redo redwire's version for him as well.  His outputs are the smoothest thing I've seen on my scope so there must be some real value to the voltage regulator he added in to replace U1.  I think a rev. 7 is in order.  Would you like to do it or should I?

 

[audioguru2]

Hi Liquibyte,
I am glad it is fixed.
I didn't build this project so please can you show a 'scope photo of the output noise from U1 and a comparison to the output noise of a voltage regulator? I think the old zener diode is a noise source.

Can you also show a 'scope photo of a comparision in time of when the power is turned off and when the collector of Q1 drags down the output voltage? I think there will be a delay while C3 discharges.

Please update the schematic to Rev.7 showing the new voltage regulator as the reference voltage. My copy of Redwire's schematic is 2008 with at least one error so I do not know what he has done lately. Please post his updated schematic too.

 

 

[liquibyte]

Hi Liquibyte,
I am glad it is fixed.
I didn't build this project so please can you show a 'scope photo of the output noise from U1 and a comparison to the output noise of a voltage regulator? I think the old zener diode is a noise source.

Can you also show a 'scope photo of a comparision in time of when the power is turned off and when the collector of Q1 drags down the output voltage? I think there will be a delay while C3 discharges.

Please update the schematic to Rev.7 showing the new voltage regulator as the reference voltage. My copy of Redwire's schematic is 2008 with at least one error so I do not know what he has done lately. Please post his updated schematic too.

We'll have to see if redwire can add these mods to one of his boards and post screenshots of his scope.  I only have an old Conar 255 from way back in the day and therefore has no hold function or screenshot export.  I'd have to use my wife's phone to do a movie and try and grab frames to post which wouldn't be very easy or fun to try.  He has a new Rigol scope that can do this better than I ever could.

I'll hack this mod into my boards and do a full load test at various voltages and currents watching the scope for issues but I think it should work just like it did on redwire's board.  I'll redo redwire's schematic later but I have redone rev. 6 to a rev. 7 with Q1 and R13 and R14 added back in.  I'll do a proper schematic and board of this version at a later date as well with proper screenshots.

Quick edit:  Some other engineers over at eevblog forums seem to think that D10 and R15 may be important in the grand scheme of things.

Dont forget D10 and that R15 (or some scheme) for your Q2 Veb(max)protection.

I'm not sure how to even go about testing this as a valid assumption and I'm sure I'd destroy something in the process if I started mucking around with it.

 

[audioguru2]

Hi Liquibyte,
Your Rev7 looks good. You have R13 as 10k but I calculated with 12k but it doesn't make much difference. Leave it as 10k.

When Q1 shorts the output of U2 and the base of Q2 to ground then R15 and D10 will prevent the charge on C7 (and on an external load capacitor) from causing excessive reverse voltage on the base-emitter junction of Q2.

But Q1 causes a nightmare:
1) Q1 shorts the output of opamp U2 to ground.
2) When the output of U2 is externally forced to ground then its feedback forces its output to go high with all the current the opamp can provide.
A nice fight.
R15 reduces the current in the fight to a safe amount but causes a voltage drop so the output of the project might not go as high as 30V when it is loaded with 3A.

It might be better if Q1 shorted the input pin 3 of U2 to ground then there will be no fight and R15 is not needed.
Test it with Q1 shorting the input of U2 to ground to see if there is still no spikes. 

 

[liquibyte]

Hi Liquibyte,
Your Rev7 looks good. You have R13 as 10k but I calculated with 12k but it doesn't make much difference. Leave it as 10k.

I went ahead and changed it.  I used a 12K 160R but forgot to modify that when I redid the picture.

When Q1 shorts the output of U2 and the base of Q2 to ground then R15 and D10 will prevent the charge on C7 (and on an external load capacitor) from causing excessive reverse voltage on the base-emitter junction of Q2.

But Q1 causes a nightmare:
1) Q1 shorts the output of opamp U2 to ground.
2) When the output of U2 is externally forced to ground then its feedback forces its output to go high with all the current the opamp can provide.
A nice fight.
R15 reduces the current in the fight to a safe amount but causes a voltage drop so the output of the project might not go as high as 30V when it is loaded with 3A.

It might be better if Q1 shorted the input pin 3 of U2 to ground then there will be no fight and R15 is not needed.
Test it with Q1 shorting the input of U2 to ground to see if there is still no spikes. 

I'll change it tomorrow and do a test and let you know what I find out.

I have noticed a bit of a spike that shows up from about 12V to up around 30V once the filter caps are bled off below a volt and then the supply is turned back on.  It doesn't happen on power off however.  It's not near as bad as it was but still kind of annoying.  My meter is showing about a 6V spike or so when the supply is set at 30V.  I figure any project that's at 30V should be able to handle a 6V spike but it's still annoying.  I tested from some arbitrary points, 30V, 25V, 20V, 12V and 10V.  I didn't really write anything down and I just remember how much it was when set at 30V.  Tomorrow, I'll document things a bit better.  Once around 10V the problem seems to disappear or is small enough that my meter can't see it fast enough.  I also tested lower at 5V, 3.3V and 1V and at these levels there is no noticeable spike.  One of these days I'll have a proper scope to do better testing with.

 

[audioguru2]

I do not know how a meter can show a spike. It can show a good long pulse but not a short duration spike.
Didn't Redwire discover and measure the spike with his 'scope? Where is the post?

 

[liquibyte]

I guess it's doing it over a long enough time to show up on the meter because I can definitely see the voltage rise and then settle.  I can see it on the scope too.  It's more pronounced on startup if the caps have drained fairly low, say below 200mV or so.  If I just flip the switch off and back on it doesn't happen at all.  Like I said, if I had a better scope I could probably provide a more detailed description.  I'm really hoping redwire could provide more insight with a better scope analysis with pics.

His original scope pics are in the following posts:
http://www.electronics-lab.com/forum/index.php?topic=19066.msg1016770#msg1016770
http://www.electronics-lab.com/forum/index.php?topic=19066.msg1016974#msg1016974
http://www.electronics-lab.com/forum/index.php?topic=19066.msg1016986#msg1016986

 

[audioguru2]

I found Redwire's 'scope photos of the 24MHz output ringing at turnon and shutdown on page 141 on May29/2014. The output goes -30V then +30V back and forth many times.
I cannot read the settings on his 'scope so I don't know how much delay occurs from when shutdown occurs to when the ringing begins.

What causes the ringing?
The 2N3055 output transistors are slow with an fT (no gain) at about 3MHz but the BD139 driver is fast with an fT at 190MHz. The TLE2141 opamp has an fT at 6MHz but its output cannot slew faster than 700kHz.

 

[liquibyte]

I've tested with the collector of Q1 at pin 3 and pin 6 of U2.  Given my meters error rate (and that of my eyes), the results are roughly the same.  There is definitely a voltage rise happening on power on.  Once I get above 1V the auto resolution of the meter probably hides the rise I get on power off but starting at 800mV and under I can see a definitive rise to around 1.25V - 1.5V as the caps drain out over many seconds.  I wouldn't want to keep something that required very low power levels hooked up on power off because all control seems to be lost.

To test consistently, I waited to power on once my meter settled to under 200mV as the caps drained and then turned on.  I did this several times to make sure it was consistent and to compensate for my brain not registering the number the first go around.

[pre]   Pin 3 Pin 6

30V 36.75 36.6
25V 33.06 32
20V 29 28
15V 23.56 23
10V 16.23 16.69
5V 7.95 8.56
3.3V   5.49 5.6
2V     3.6   3.9
1.5V   3.2   3.4
1.2V   3 2.8
1V     2.7   2.6
.8V 2.2   2.2
.5V 1.6   1.5
.2V .9   .3
.1V .36   .3
0V Too small and quick to see (39.2mV == 0)[/pre]