Electronics > Beginners

TL431 linear power supply

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xavier60:

--- Quote from: mike_mike on December 13, 2018, 01:25:36 pm ---
And I want to be very stable and I want to NOT oscillate... this is the hardest thing for me.


--- End quote ---
Most power supply designs will oscillate without proper compensation, especially those that use op-amps.
The power supply circuit in your first post will work well  enough with compensation.

spec:
Hi mike-mike,

Attached below, is a schematic for your PSU as promised. Apologies for the delay, but it took longer to sort than expected, and other things got in the way too. >:(

Bear in mind that this is a paper design and I do not have a full specification for your mains transformer so I have made some assumptions. Also, right now, I do not have access to a lab so I cannot build and test a prototype for you. But I have analyzed the circuit and used a conservative approach whenever possible.

You may find the circuit odd, but it is not an outline circuit like you normally see, and shows the required wiring including star points (SPs), padding, and decoupling.

The two opamp options shown on the schematic are unique, and not any old opamp will do, although you could use other opamps with reduced performance. The OPA192 is the better of the two, but costs a bit more. But, in terms of your PSU there would be little difference in the performance. Mainly the accuracy of the 25V setting would be affected.

The frequency compensation is very heavy, but that is intentional. If necessary the frequency compensation can be optimized at a later date, by a few simple component changes. The important thing is to get the basic circuit working at this initial stage.

You will need very good heat-sinking for the output transistors and fan cooling would be advisable, but we can talk you through all that.

Finally, I cant emphasis how important it is to do a good layout with the connections as shown in the attached schematic. The thick lines on the schematic indicate where thick wires/PCB traces are required. It is also essential to keep wires and PCB traces short and compact. The 100nF capacitor across the supply pins of the opamp, must have short leads and be physically connected to the opamp pins, or as close as possible to the pins. Likewise the padding resistors and other decoupling capacitors must be directly on, or close to the opamp pins.

There is a physical layout that I would recommend, which will simplify construction a lot and optimize performance. Once you have had a look at the schematic, we can discuss the layout if you like.

Unless otherwise stated, all solid capacitors are ceramic X7R through hole types, the physically bigger the better (not surface mount).

The specification for the PSU is 0V to 25V (exactly), 0A to 5A output current.

spec:

--- Quote from: David Hess on December 13, 2018, 04:28:48 am ---The only reason I have not used the TL431 that way is why would I bother when better performing and easier to use operational amplifiers are plentiful?  It is routine in isolated switching power supplies however.

The problem is a little like using the 723 or most switching regulator controllers where stabilizing feedback to the inverting input comes from the transconductance output instead of the low impedance buffered output.  The TL431 has a transconductance output, despite what the datasheet specifications show, so gain varies considerably from device to device and with load.  If you build a series regulator with it, then the variable biasing conditions are altering the transconductance unless steps are taken to prevent it so the frequency compensation becomes difficult to control.



A better question might be *why* the TL431 operates so benignly when used as a simple shunt regulator.  It will normally have a relatively large load capacitance which rolls off the gain of its transconductance output for dominant pole compensation which is exactly how a transconductance amplifier is compensated.  But in a circuit where it controls a series regulator, that cannot be done if fast response is desired and instead feedback from the squirrely output to the input implements the frequency compensation because people are used to working with voltage feedback amplifiers.  It can work as the 723 shows but can also be hit or miss.

This brings up an alternate compensation scheme; place a series RC network from the TL431 output to ground.

--- End quote ---

I take it then that you yourself or anyone that you know has built a successful TL431 PSU.

not1xor1:

--- Quote from: spec on December 14, 2018, 06:43:48 am ---
--- Quote from: not1xor1 on December 13, 2018, 07:58:37 am ---
--- Quote from: spec on December 12, 2018, 05:21:36 pm ---No probs :)

I will have a look for a suitable PSU, but I am quite busy at the moment. A number of members are experienced in PSUs, perhaps they could recommend some suitable designs as well.

But I can recommend a complete lab PSU that will do your job for around £47UK, including shipping (the transformer for a DIY PSU would cost more than that). The lab PSU provides 0V to 30V and 0A to 10A, with digital readouts for both voltage and current. There are controls for coarse and fine voltage output and the same for current. I have three of these and use them for electronics, battery charging, you name it.

https://www.ebay.co.uk/p/Adjustable-DC-Power-Supply-30v-10a-Variable-Precision-Digital-Lab-PSU-Clip-Cable/23025566378

You can probably get this generic lab PSU delivered to Romania.

--- End quote ---

From the pictures (if you look through the case aeration slots it looks empty inside) I guess it is a switching PSU.
The linear ones are better as they can be easily improved by replacing the electrolytic caps (they are low quality and too low voltage) and adding a proper TO3 heatsink (as minimal cure).

--- End quote ---
It is a linear PSU, with a very low level noise output of 3mV, why do any guessing. :-//

--- End quote ---

There are lots of chinese PSUs looking the same and with small or large differences inside (some with a huge heatsink on the back some with just a piece of aluminium inside).
I bought a similar one (30V 5A) for about 45€ including shipping.
It has a huge transformer.
The one you linked is 30V 10A. So it should have a 500VA transformer, but since they are coarsely overrated a 300VA one would be more likely.
But if you zoom the pictures and look at the last one, you can clearly see that the case is empty.
So unless it uses a low profile toroid it can be just a switching PSU.
OK ...I found a youtube video... it is definitely a switching PSU.
https://youtu.be/1_ZK68_c3Ok?t=145

not1xor1:

--- Quote from: spec on December 14, 2018, 07:11:32 am ---Hi mike-mike,

Attached below, is a schematic for your PSU as promised. Apologies for the delay, but it took longer to sort than expected, and other things got in the way too. >:(


--- End quote ---

you must be kidding  ;D
a 20V/µs opamp with a huge 1µF compensation capacitor...
and what would be the advantage versus a proven and foolproof design using the traditional LM723 ?

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