Electronics > Beginners

TL431 linear power supply

<< < (26/32) > >>

blackdog:
Hi Kleinstein  :)

Thank you for finding the errors in de links en telling me!

Almost all links were broken, sometimes even a double address in one line and even one line whas started with http:www.eevblog.com in front of it...

And when I clicked on a link, the pdf was downloaded... spooky  :-DD

I hope it is now OK

Kind regards,
Bram

David Hess:

--- Quote from: blackdog on December 21, 2018, 02:58:10 pm ---The LM358 is a slow opamp and with a well-designed circuit it should provide a reasonable power supply with the 2N3055 transistors.
--- End quote ---

I think there is some benefit up to a 3MHz operational amplifier (except for likely needing extra frequency compensation) but the old JFET input parts were 3MHz and had much higher slew rates which is even more important for designs where the voltage or current error control loop sits in saturation.  The old RC4136 was a weird 741 class operational amplifier which was 3MHz so this was not universal.

Old slow LM358/LM324s, 741s, and 310As could almost always be used without any extra frequency compensation and perform just fine.


--- Quote ---The TIP41 and the 2N3055 are both "slow" transistors and they do not help to make the circuit stable.
Hint, make the base emittor resistor a little lower say 56 or 68 Ohm, which helps with the paracit capacitors of the 2N3055.

And also this, there are so may different 2N3055 transitoren, Ft 0f 0,8Mhz, Ft of 3Mhz etc,
So if someone present here a schematic with 2N3055 transistors, then you will actually have to take into account the worst version when it comes to loop compensation.
--- End quote ---

I do not think anybody has produced the slowest 2N3055s for a long time now.  If you want one of those, use the 2N3771G or 2N3772G.  See below.


--- Quote from: Kleinstein on December 21, 2018, 08:36:18 pm ---The slow 2N3055 were the more robust old style transistors. One hardly finds them any more. However the once popular 2N3773 are usually still slow. For a high performance power supply it really helps if the transistors are fast, but for a simple low cost supply one can still use the normal 2N3055 or TIP35. It still helps if the second transistor in the Darlington configuration is fast.
--- End quote ---

Be real careful about the 2N3773; I thought they all used the 0.8MHz process but apparently all new production at least from On Semiconductor is 4MHz.  On the other hand, the 2N3773G has the highest secondary breakdown voltage of parts listed below at 100 volts.  The MJ15015 and MJ15016 are not far behind at 90 volts.

2N3771G NPN 40V 30A 150W 0.2MHz
2N3772G NPN 60V 20A 150W 0.2MHz

2N3055AG NPN 60V 15A 115W 0.8MHz
MJ15015 NPN 120V 15A 180W 0.8MHz

MJ15016 NPN 120V 15A 180W 2.2MHz

2N3773G NPN 140V 16A 150W 4MHz
MJ15022 NPN 200V 16A 250W 4MHz
MJ15023 PNP 200V 16A 250W 4MHz
MJ15024 NPN 250V 16A 250W 4MHz
MJ15025 PNP 250V 16A 250W 4MHz


--- Quote ---A low power the BD139 is a good choice, at higher power something like D44H11.  A slower supply would need a larger output capacitor though.
--- End quote ---

The D44H11 is a RET (ring emitter transistor) style so it may have other advantages as well.  I commonly see them used in low noise and fast response regulators and of course audio power amplifiers.  Oddly enough, the final design of the problematical single ended high voltage inverter Tektronix used in their 465 series oscilloscopes changed from a selected 2N3055 to a much faster RET D44H/D45H series.

spec:
Thank you all for the interesting information in the last posts #122 to #127 above. But, I am afraid that much of the good stuff cannot be embodied in this PSU because the components are a given by the OP, who probably has limited access to components in his local. As stated a few times now, given a free hand, I would not have have used the components or architecture in the PSU of post #89, especially the opamp, or the extra transistor stage, or at least not in the common emitter configuration.

I agree that the additional transistor stage is a PITA from a frequency compensation point of view, but it is the only way I could provide adequate output voltage swing and current sinking, while providing sufficient base drive for the output driver transistor. My scheme on the original design was, in the interests of frequency stability, to get the voltage gain of the transistor stage down to 1 at as low a frequency as practicable.

It may be possible to drive the output stage directly from the LM358 if another emitter follower were added to drive the output stage. There is another aspect though, that concerns the relationship between the constant voltage and constant current modes. This is always a difficult area and IMO the safest way to do it is by the 'decreasing gm' method which precludes the use of a voltage drive of the output transistors directly by the opamp.

The other aspect is that the OP wants to use an LM317 regulator and with its dropout voltage and 40V input limit that makes things difficult.

About the 24:1 voltage divider. While I agree that it may increase the noise and voltage drift, I am not sure that it is that significant. But the attenuation does reduce the feedback and should help with the frequency stability, at least that was my thinking. It also allows some flexibility in tailoring the open loop response, as has already been done.

If anyone has an improved complete, practical circuit, that meets all of the OPs requirements and is easy to read and that the OP can use to build a PSU, please post it (this is a genuine request).

Also if anyone has an approach, in addition to not1xor1's work, for compensating the reply #89 circuit please post a complete practical circuit (of the compensation that is). That would not only be interesting, to me anyway, but also very useful.

 

Kleinstein:
The initial circuit right from the start is not that bad - it is mainly missing the frequency compensation. So with the first response one could call it problem solved. There are still some points to optimize and the selection of parts could be better, but nothing serious.

For just a simple power supply the "normal" modern (e.g. 3-4 MHt Ft) 2N3055, TIP35 or similar should be sufficient in speed. It helps if in the Darlington circuit the smaller transistor is a little faster. But this is easy, as the BD139 is nothing special. Even the D44H11 is not that exotic.
The voltage regulator with the Darlington emitter follower output and thus a low output impedance output stage normally does not need a fast OP for the regulation, unless one has high demands. So for normal use the LM358 is sufficient, at least if one does not divide down the feedback signal very much (e.g .down to 1.2 V), as this would need more gain.

If there is a second loop for current regulation (and not just the transistor for a crude current limit), than one might want a faster OP in the simple circuit, as the slew rate of the OP would limit how fast the current limit can engage.  The 0.3 V/µs of the LM358 would require some 100 µs to come down from 30 to near 0 in case of a short. Even with a faster OP, the current regulation is a little harder in this type of circuit.

With only up to 25 V the simple circuit type is perfectly fine. Going up to the usual 30 V starts to get tricky as the supply for the OPs is limited.

The LM317 was an idea for the OPs supply. Here the 40 V limit may indeed be a problem - though it only applies to the in to out difference, not the absolute voltage that is never seen by the LM317. Still I would prefer the simple Zener solution for the rather low current (e.g. 2 mA) needed for the OP.


iMo:
This works nice, imho..
It oscillates without the C4 capacitor.
Responses I_Load=10mA/5A for a list of C_Load capacitors.

Navigation

[0] Message Index

[#] Next page

[*] Previous page

There was an error while thanking
Thanking...
Go to full version
Powered by SMFPacks Advanced Attachments Uploader Mod