Another Noob PSU Design Needing Review, Please

[iMo]

Open wiper: some time back I recommended to wire an higher value resistor Rx to the wiper against "something" thus when the contact is lost the Rx will pull/push the input of the opamp such the output voltage jumps towards MIN voltage.

[t1d]

I would recommend to put the new schematics into the LTspice.. All parts are available.
Soon or later you will start to deal with questions around dynamic parameters (ie stability under various loads/settings, gains, phase margins, overshoots) and the simulation works pretty good as we know from past.

That's a good suggestion, but LTspice is another h-u-g-e learning curve. I already have electrical engineering, KiCad, C Code, PIC MCUs/MPLab X, Lab Equipment Operations, etc. on my plate. So, simulation programs have had to wait. lol...

[xavier60]

TL071 exhibits phase-reversal and non-linear gain, you don't want those surprises in a power supply lol.

The history of this design is that it comes from an old National Instruments LM317 data sheet. The attached picture shows they originally used a LM301A op amp. IIRC, it is still included in the TI LM317 data sheet.

I listed some op amps that I have in stock that look like they can handle 36 volts. With reducing the goal to 30V output, maybe one will work. But, I need to research them fully. I am still working on that.

The first step is to learn how to determine their Safe Operation Area. I know what the term means, but not how it is determined for a specific component.

Notice that the original did not bother with the 0 volt goal/-5V rail.

It's not clear to me where LM301's V+ supply comes from. A suitable rail-to-rail, single-rail  opamp could work being powered from the PSU's output.

Extra: Actually only 2 important things are needed. the opamp needs to be able to safely run on 36V and the input common mode range needs to include V+.

[t1d]

If powered from a transformer, and this with some variations and hum on the main supply, it would be a good idea to have a seprate limit for the OPs supply. This could be a simple resistor +33 (or 35) V zener + emitter follower type circuit. So the OP would not see the variations and hum on top the main supply.

I understand the what and the why of what you are suggesting, but I will have to work on the how of how to accomplish it. Meaning, designing the needed circuitry.

[iMo]

I would recommend to put the new schematics into the LTspice.. All parts are available.
Soon or later you will start to deal with questions around dynamic parameters (ie stability under various loads/settings, gains, phase margins, overshoots) and the simulation works pretty good as we know from past.

That's a good suggestion, but LTspice is another h-u-g-e learning curve. I already have electrical engineering, KiCad, C Code, PIC MCUs/MPLab X, Lab Equipment Operations, etc. on my plate. So, simulation programs have had to wait. lol...

Sure, you have to master many tools to become an experienced Maker
But the simulation with LTspice has to have the Priority, as messing with PCBs, MCUs has no sense when the circuit you are designing does not work, imho
With mastering the LTspice your time spent with soldering jobs and endless debugging will drop down significantly, indeed..

[t1d]

Open wiper: some time back I recommended to wire an higher value resistor Rx to the wiper against "something" thus when the contact is lost the Rx will pull/push the input of the opamp such the output voltage jumps towards MIN voltage.

Do you have the link where you discussed this trick?

[iMo]

Open wiper: some time back I recommended to wire an higher value resistor Rx to the wiper against "something" thus when the contact is lost the Rx will pull/push the input of the opamp such the output voltage jumps towards MIN voltage.

Do you have the link where you discussed this trick?

For example here in this thread. There is also schematics with the resistor somewhere..
https://www.eevblog.com/forum/beginners/lm324-power-supply-with-variable-voltage-and-current/

[t1d]

It's not clear to me where LM301's V+ supply comes from. A suitable rail-to-rail, single-rail  opamp could work being powered from the PSU's output.

Ahh... And, I see that the op amp negative supply is tied to a mystery -6 volt rail. I had missed that...

[t1d]

Open wiper: some time back I recommended to wire an higher value resistor Rx to the wiper against "something" thus when the contact is lost the Rx will pull/push the input of the opamp such the output voltage jumps towards MIN voltage.

Do you have the link where you discussed this trick?

For example here in this thread. There is also schematics with the resistor somewhere..
https://www.eevblog.com/forum/beginners/lm324-power-supply-with-variable-voltage-and-current/

Thanks!

[iMo]

Here for example.. The R2 and R10 1Meg resistors (at CC and CV potentiometers) will not affect the setting linearity "too much" and in case of an open wiper the resistors pull the opmap's inputs to gnd.
Your design is different, so you have to think twice where to wire the cold sides of those Rwipers to..

[xavier60]

The TL071 is suitable "Common-mode input voltage range includes VCC+"
It could be given some Zener protection, but put the dropper resistor in series with the V- supply.

[t1d]

While I am learning about the needed op amp, maybe you good folks could give consideration to my options for the power transistor... My options are to use what I have, or buy something else. I would rather use what I have, of course.

Here's what I have in stock:
MJ4502G Power P-Transistor - 100V/30A/200W - Qty(1)
IRF9540NPBF P-MFET - 100V/23A/140W - Qty(2). Curiously, the data sheet description says this component is typically used for 50 watt applications?
SPP18P06PH P-MFET - 60V/18.7A/81.1W - Qty(2).
Data Sheets below.

It appears that two of the available P-MFETs would be needed to manage the watts with sufficient room to spare. It is (I think) within my skills to design the extra circuitry, if using two MFETs is our consensus... Other than there seems to be some debate as to whether the small balancing resistors go in the Drain, or the Source, of the MFETs.

To be honest, it is clear to me that it would require a full college course to learn all the ins and outs surrounding the choice of the op amp. Kleinstein has been kind enough to rule out the ADAD8675 and to suggest the TL071H or OPA991, if having to purchase it. But, I wonder if any of the other op amps that I listed as having on hand might do?

[t1d]

Here for example.. The R2 and R10 1Meg resistors (at CC and CV potentiometers) will not affect the setting linearity "too much" and in case of an open wiper the resistors pull the opmap's inputs to gnd.
Your design is different, so you have to think twice where to wire the cold sides of those Rwipers to..

Thank you so much for the extra effort to post this. It is very helpful!

[xavier60]

Just in case it was missed. Powering the opamp from the PSU's own output will reduce the voltage stress to the opamp.

[iMo]

The power loss (the heat created in Watts) at your power transistor in your PSU design does not depend on the type of your power transistor, but on the equation (aprox):

P[Watt] = (Vin - Vout) * Iout   [V, A]

In your case Vin = 33V and Vout = 0..30V, and Iout = 0..3A
For example with Vout = 1.5V and Iout = 3A your P = 94.5 Watt.

Therefore you have to use a transistor(s) which can deal with a) current, b) voltage difference Vin-Vout, c) power loss in the entire operating area of your PSU.

The max power ratings of your transistors (like 100W or 200W) are valid only when you keep the temperature of the transistor's case at 25degC. Could you do it? That is the big question..

[Kleinstein]

If the OP is powered from an extra supply, it does not need to work all the way to it's upper supply at the input.
The OPs listed are pretty similar low noise BJTs and they work up to some 2-3 V to the positive rail only. With the positive supply at some 33 V they would work. They may have a lower limit to the supply and thus powering from the output does not work.

Current sharing with MOSFETs would be with a resistor at the source. It needs relatively large resistors or some additional banancing circuit. As a downside the MOSFET would only start to turn on with more voltage at the gate. So it needs an input voltage that is some 2-4 V higher than with the current circuit.
So the P BJT is already a reasonable good choice.
 I don't know why the NS datasheet claims 5 A for the shown circuit - with 35 V at the input the transistor SOA is only good for some 3 A.

To get 30 V DC out, one would need some 33 V as the minimum at the input. With some ripply this would be more like 36 to 40 V needed at the input.

[xavier60]

I should have mention that the the opamp would be powered from between the PSU's output and the -6V rail. The TL071 should work.

[iMo]

TL071's absolute max power voltage rating is 36V.. (TI's datasheet RevS).
PS: I would be  having my TL071 powered from 36V..

[xavier60]

This data sheet says 40V.  Anyways, designs that stretch things to their limits should be avoided.


https://www.ti.com/product/TL072?utm_source=google&utm_medium=cpc&utm_campaign=asc-null-null-GPN_EN-cpc-pf-google-sa&utm_content=TL072&ds_k=TL072+Datasheet&DCM=yes&gclid=CjwKCAjwhaaKBhBcEiwA8acsHKgsVQwyEYIaCTSuLtymSBBGl8DBvwAMnrsrspMm3qQ7u_7eziXOLRoCeRgQAvD_BwE&gclsrc=aw.ds

[iMo]

I added above the link for 2021 DS of the TL071.
TL071  36V
TL071H  42V

[xavier60]

The opamp's VCC- pin only needs to be say 10V below its VCC+ pin to drive the LED and have full control of the LM317.
But achieving this could get complicated. The can of worms is opening.

[iMo]

In my reply #14 there is a link to ADI's high voltage opamps.
For example the in Metrology section pretty popular 2057 - its high voltage version LTC2057HV is 60V
(slow for this app, imho).

ADI lists 103 opamps with Vcc abs max > 40V (15 opamps from 50V up to 220V)

TI lists 127 opamps with Vcc abs max >40V (17 opamps from 50V up to 180V)..

[xavier60]

The  LTC2057HV is a bit expensive but no where near so as most of the prices I just seen for high voltage opamps.

[iMo]

Any brave worms collector here?
Below the LTSpice file..

[rstofer]

How is the pass transistor going to survive an output of 1V at 10A (specified in the original post).  It can't really handle 320W, it is only rated for 200W and that's probably when the moon is low on the horizon.

If you back off on the 10A requirement, maybe... 

As drawn in the original sketch, there is a missing joint in the current adjust circuit.