30v 10A bench supply schematic. Is it any good???

[Audioguru]

The original supply was linear, not switching and used emitter-follower 2N3055 output transistors that needed a base voltage of 31.5V max to produce an output of 30.0V at 4A. The question was if a Mosfet would increase efficiency. I replied that the 10V gate-source voltage loss (of a source-follower) would require a minimum gate voltage of 40V then the opamp driving it would need a very high supply voltage that might destroy it. Even a logic level Mosfet would produce a gate-source voltage loss of 4.5V and would push the opamp to its maximum allowed supply voltage limits and maybe destroy the opamp when the load current is low then the transformer voltage is a little higher than its spec.

[Kleinstein]

Using MOSFETs can reduce the voltage lost in the high current path, but as a source follower it needs an extra higher supply voltage. MOSFETs also have difficulties having them in parallel. There are cases where MOSFETs are a good idea, but there are also cases where they are not.

The source follower has another problem: with BJTs the current gain tends to go down at high current. This tends to limit the peak current on a short. With MOSFETs the trans-conductance usually is going up with high current. So peak currents can be rather high. Related to this MOSFET speed also depends a lot on the current: at higher currents they a rather fast, but at low currents they are slow. This can make the design of the regulator more difficult. So often one needs a relatively high bias current to avoid the slow range. 

The circuit with the power stage as an emitter follower is usually chosen to have a low output impedance and this way allow for a smaller output capacitor and simpler regulator. A source follower (especially with those FETs with a good FBSOA) tends to have a higher output impedance than the emitter follower and this way might not work well in the similar simple circuit. It might take a rather high bias current to make it acceptable.

[ogden]

i don't see how a zener used to protect the gate of as mosfet  is a crime 

Protection is not a crime but assumption that mosfet gate voltage can be exceeded in linear supply is sign that you do not understand how linear supplies/regulators work. Basically you accused Audioguru of spreading BS w/o proper understanding what actually he is talking about.

low current switcher can be easily filtered down to virtually no noise , so i can't see a crime there either.

Well well. Whole essence of linear supplies and big, heavy mains AC transformers used is to avoid switching noise, bring regulation into low, AC mains 50/60hz frequency domain. If noise can be easily filtered in the regulation circuit, then it can be easily filtered in the power circuit as well - using much smaller inductors than that big heavy transformer. Think about it. Do you even know what is inductor parasitic capacitance and how it affects ability of filter to do actual filtering? Of course it is crime to introduce switching regulator into pure linear supply unless it is not a part of output regulation.

but after your last post i'm pretty sure you have a major gap in understanding of a mosfet as a series pass element...

No a lot of understanding is actually required to comprehend that MOSFET in linear supply is not used as a switch. Power transistors in linear regulators are never widely/fully open because there always  shall be some voltage headroom for regulation - in case mains (input) voltage drop. So your "voltage drop Vds of few millivolts" when we talk about linear regulator is BS instead.

[FotatoPotato]

So I just finished building the 0-20v 0-10A linear power supply. Everything works flawlessly and it is almost perfect. If you look at the schematic that I linked you can see that the main transistor is a TIP142 Darlington BJT. The original schematic is only rated for 1A but by adding 4 of them in parallel you can obtain a 10A output. So I did that, the only problem is that only 2 of the BJT's work at any given time. Even weirder is that its random. If you look at a pic of the PSU you can see that they are in groups of 2 (2 on the left and 2 on the right) and randomly only one of those pairs of 2 will work. They are all linked together identically and all their bases are connected as well.

So I have no idea why this is happening, maybe you do. If you have any advice on changes to make for all 4 BJT's to work please let me know! 

[ogden]

the only problem is that only 2 of the BJT's work at any given time. Even weirder is that its random.
So I have no idea why this is happening, maybe you do. If you have any advice on changes to make for all 4 BJT's to work please let me know! 

For 2 or more transistors to work in parallel - they shall be identical, but usually they are not. To have better chances of equalizing current between transistors, you shall add small value (0.2 to 0.5 ohm resistor) to emitter of each transistor as shown in article below. Make sure you calculate worst case dissipation and size those resistors accordingly. IMO each transistor shall have it's own base resistor as well (or maybe not). Try just emitter/ballast resistors first:



Article:
https://www.allaboutcircuits.com/textbook/semiconductors/chpt-4/bjt-quirks/

[Twoflower]

Make sure you don't exceed the 25mA the V_Z pin of the 723 can provide. The TIP142 states for the ON-Characteristics I_B = 10mA or even 50mA depending on the I_C. That's per transistor not for the the four you have.

[FotatoPotato]

Would something like this work to balance the current between all four transistors?  And I do have a 220 resistor on the output signal to limit the current, is that ennough or should it be lower?

The 5th transistor is a 2N3904 if you couldnt read it (sorry the schematic immage is a bit blurry)

[ogden]

What exactly 2n3904 transistor is doing there?! Please explain your intention.

This 4-transistor power stage operates as emitter follower, no any additional trickery/transisors needed - connect bases of power transistors together, add individual ballast resistor to emitter of each transistor.

220Ohm is fine. For starters. The higher value resistors - the better balancing of the power, but higher dissipation/losses. You can use even smaller ballast values if transistors are well matched, but only measurements will tell. Precisely measure resistance and voltage drop on each ballast to estimate current flowing, share your results

[FotatoPotato]

OK, so I added a 0.22 resistor to the emitter of each transistor to the supply rail. This seems like it has worked to an extent because when I put a large load on the supply, all 4 transistors begin to get hot which to me indicates that all 4 of them are working. This is all fine and well but there is one problem. On larger loads over 1A one of the pairs of 2 transistors still gets WAY too hot. They are hitting 180c at 4A of current draw while the others are sitting at about 65c. I know they should get hot but not that hot! Is this just a case of crappy thermal pads or is something else wrong here?

Also it seems like I’m well within the current limit of the 723's Vz pin because I have pulled 8A so far with all 4 transistors and it hasn’t popped. Let’s hope it stays that way!

[glarsson]

Measure the voltage drop over each 0.22 ohm resistor. If the transistors share the load equally the voltages should be close. If the voltages are close (i.e. same current, same voltage drop over the transistor, same power dissipation in the transistors) then check if there are differences in cooling, e.g. thermal pads, paste, etc.

[FotatoPotato]

Huh, this doesn’t seem right.... when I pull 500mA at 20V there is a different drop across each resistor. On the first transistor there is a drop of 60mv with a resistance of 0.226 , on the second transistor there is a drop of 7mv with a resistance of 0.226 again, on the third transistor there is a drop of 3mv with a resistance of 0.227 , again, and on the final transistor there is a drop of 27mv with a resistance of 0.229 ..... I don’t think that is what is supposed to happen. Any advice?

[ogden]

Huh, this doesn’t seem right.... when I pull 500mA at 20V there is a different drop across each resistor. On the first transistor there is a drop of 60mv with a resistance of 0.226 , on the second transistor there is a drop of 7mv with a resistance of 0.226 again, on the third transistor there is a drop of 3mv with a resistance of 0.227 , again, and on the final transistor there is a drop of 27mv with a resistance of 0.229 ..... I don’t think that is what is supposed to happen. Any advice?

Perhaps some or all power transistors are already damaged because you said that transistors (their cases?) approached 180oC temp, but at 150oC junction temp manufacturer says they are basically dead. I would never let transistors to approach more than 90oC or so.

I would derate/downgrade my power supply power plans, leave two most similar transistors and debug circuit with just two, obviously without overcooking them.

[FotatoPotato]

After some more testing I have concluded that it was shitty thermal pads I took them off and replaced it with some nice thermal paste, now all 4 transistors sit comfortably at 50c while pulling 6.5A at 13v. Welp, problem solved!

It also seems like the 25W 0.22 resistos that I'm using are a bit overkill as they dont even get mildly warm under a heavy load.

[ogden]

It also seems like the 25W 0.22 resistos that I'm using are a bit overkill

It is. You really shall use Ohms law *before* you pick resistors.

Power_dissipated=Current*Current*Resistance