Electronics > Beginners
how does blackdog's PSU work?
<< < (9/14) > >>
T3sl4co1l:
Another way to look at it: a diffusion network.  This gives an approximately Z ~ 1/sqrt(F) characteristic, which has equal components of resistance and capacitance.  In a sense, it's the most lossy capacitor you can have, over a wide range of frequencies, and therefore at the least risk of underdamped loads above the controller's cutoff frequency.  (The purpose of the control is to set Zo very low, so within the passband, one should not expect damping to occur -- attaching an LC network to the output, such that the resonance falls in the passband, should indeed be poorly dampened, an indication of the low output impedance of the supply.)

Tim
Wortel:
Hoi Bram,
Ik zie dat je de LM317 current source als voorbelasting van een hele tijd geleden er weer ingezet hebt, en volgens mij, net als vroeger, met de uitgang aan de verkeerde pin. De constante stroom loopt bij zo'n 3-pins regelaar toch van de output pin,  *door* de weerstand, dus rechtstreeks aangesloten op de sense (of ground)-pin?

Ik heb je NA-01 blog op CO helemaal gelezen, groot respect! maar is dat nu doodgebloed?

Grtz,
Wortel
blackdog:
Hi Carrot,

No, the project was not dead, but when I was busy with the extended testing of the 230V net filtering, it went very badly with one of my family members.
So it is still a bit of a sensitive point...

But I had already decided to make a smaller finnishd version first (2.5A), so that I can place it better between my other equipment in my LAB.
And 2.5-Ampere is sufficient for most of my applications, i started building of the next test version, see the picture below.
The 5V Reference on this board is a MAX6350, also a fine part.



The basic schematic is ready, the reference section, Power Off Glitch Protection, and control loops and power section are extensively tested and ready for use.
Depending on the used opamps, the compensation capacitors can be slightly adjusted for optimization. (C16 and C16)
Because there is now a somewhat smaller transformer for me, the 230V grid filtering will also be adjusted a bit.

And if I have tested with the new Analog Devices opamps: ADA4625-1 then together with the lower 2.5-Ampere output current, also the 220uF capacitor over the output can be smaler.
This in turn will make the peak currents in the connected D.U.T smaler, in fault situations.
But these are no big changes.

Anyone who still wants to apply DAC control can connect the top of R32 to the U-DAC (0 to +5V).
You will have to calculate the voltage divider for the I-DAC yourself(0 to +5V).
The simplest method is to replace P1 with a 1K resistor and R31 on the top of this new 1K resistor.
If you want two current ranges, connect R27 or R28 to the output of the I-DAC.
You will have to do it with this information, I will not give any further advice about it, you are on your one  :)

Wortel, thanks for detecting the error in the schematic, it is now Ok (i hoop)

Kind regarts,
Bram

David Hess:

--- Quote from: blackdog on March 20, 2018, 10:12:24 pm ---Can you explane this?
The displayed load transient response is poor; power supplies should have a very tame transient response to handle difficult loads.  In production designs, I slightly overcompensate for the *worse* case of component and load variation.

David did you read everything?
Wat is bad about this test see the two pictures below, do you find this horrible?
--- End quote ---

The first test you showed indicates a problem in the small signal response.

The test conditions for the others are not clear enough to rely on but they do seem to show a large signal response problem which might be caused by the error amplifier not being able to pull charge out of Q3 and Q6 quickly enough.  I would usually expect a base-emitter shunt resistors on Q3 and Q6, capacitive bypassing to the base of Q3 and Q6, and much stiffer drive to Q3 and Q6 which goes along with unloading the outputs of the operational amplifiers to improve precision.

The precision design shown at the end of my reply for instance would nominally use twice the drive current to the pass element while having 2 orders of magnitude less loading on the error amplifiers.  This actually understates the difference because an integrated pass element has much higher current gain; idle current through the buffer is high to keep the buffer transistor characteristics consistent.


--- Quote ---Why use a Sziklai pair instead of a Darlington pair when not required?  This is especially odd since the output power transistor is a PNP instead of NPN although the difference in price probably does not matter anymore; PNP power transistors used to be much more expensive than NPN power transistors.  I suspect the lack of local feedback around the Sziklai pair is causing problems.  Designs like this using much slower transistors often include local feedback.  Even fast Darlington designs often use local feedback.

My measurements tell me different.
The rightmost line (F) with the text Double Compound transistor indicates the output impedance of this power supply.
And it is measured with the NE5532A as used opamp, can you point my to a powersupply that has a lower Ri over this frequency range?
--- End quote ---

It is not a major factor that I have ever noticed other than more care being needed in controlling the local frequency compensation.  I always found transient response to be dominated by the overall frequency response which is the point here; your first test indicates a problem in this area so a low output impedance from the pass element is more important than it otherwise should be.

Some audio amplifiers use a couple of small signal transistors to add local feedback to the output transistors and reduce output impedance even further.


--- Quote ---The ultimate performance of the suggested ADA4077 precision operational amplifier is completely wasted here...
Nop, The DC stability is fine with this opamp in use, but the hf performance of the ADA4077 is ofcoure less then with a faster opamp.
Some DC measurements: With the NE5532A at 15V output the DC drift was < 0.2mV, thats less than 0.0015% within a time of 6 hours.
--- End quote ---

How good was the load regulation though?  Using remote sense, I would expect the load regulation using the ADA4077 to be almost unmeasurable.  An ADA4077 is not required to deliver a DC drift of less than 0.2mV.


--- Quote ---Some attention
The only thing I think about is the opamp of the U control loop, in difficult situations there is quite a lot of energy going to the + input of this opamp.
This can probably be solved with two diodes that are anti-parallel from the + output to the + input of the opamp.
--- End quote ---

Internally they already have anti-parallel diodes (which is one reason I don't like AD's datasheet; it does not show them) but this is shorting the inputs during large signal conditions and may be upsetting the filter networks for the current and voltage setpoints.  If this is happening, then it is going to show up as long settling time and poorer large signal response which does appear to be a problem.  Low differential input voltage operational amplifiers are almost always a poor choice where open loop operation is possible but in this case even with the feedback clamp network, this may be causing a different problem.


--- Quote ---Some remarks
Let me make it clear again, this is a power supply that has very low noise, good DC stability, and good dynamic performance.
He must meet my requirements, for my purpose.
--- End quote ---

The performance is good.  I just think it should be much better given the complexity and parts selection.


--- Quote ---Let me make it clear again, this is a power supply that has very low noise, good DC stability, and good dynamic performance.
He must meet my requirements, for my purpose.

...

This design is not meant for production, but only for my LAB.
And if I want to use more expensive parts, I do that because I think this is necessary for my application.
--- End quote ---

If it is good enough, then it is good enough and best is the enemy of good.

I have designed and built my fair share of special purpose power supplies for internal production and testing.  My favorite involved the design shown below but modified for extreme precision and low noise using an LT1007 precision operational amplifier, precision reference, and lead-lag frequency compensation which involved 2 more resistors and 2 more capacitors not shown in both the feedback and input networks to optimize dynamic performance.  Load and line regulation from 0 to 1 amp ended up being too good to accurately measure at better at 1 ppm or better.

Note that I am not suggesting this design to you or anybody else.  I just used it as a handy example.

I have been considering doing a precision floating variable output tracking implementation but at lower maximum current but the problem is handling power dissipation at low output voltages without excessive power dissipation.
blackdog:
Hi DAvid,

Thank you for your remarks, it is late now here in Amsterdam and tomorrow i wil explane some more things.
Like the photo you probably saw first, that is the photo with some ringing, that is from my "torture test" for power supply's :-)

Talk to you later.

Kind regards,
Bram
Navigation
Message Index
Next page
Previous page
There was an error while thanking
Thanking...

Go to full version
Powered by SMFPacks Advanced Attachments Uploader Mod