20v 10A PSU Weirdness

[FotatoPotato]

Hello everyone!

So I just finished building a 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 when looking at a similar one I saw that 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! 

[glarsson]

They are all linked together identically and all their bases are connected as well.

This is not the way to add more pass transistors. They will not share the current equal.

Continue looking at other designs and you will find that they use, for example, small value emitter resistors to balance the currents.

[not1xor1]

Each transistor has slightly different characteristics, vbe, hfe, etc.

You cannot just connect each terminal in parallel 

you have to put a power resistor (0.22-033\$\Omega\$) on the emitter of each transistor and then connect the other terminals of the resistors to a common point that has then to be connected to the current sense resistor.

The resistors will provide some feedback so that if a transistor has a lower Vbe and let pass more current the voltage through its emitter resistor will increase and will so reduce Vbe so that all transistor at the end share about the same current.

BTW I hope you changed the current sense resistor, 10A squared * 1\$\Omega\$ are 100W. 

[FotatoPotato]

Ok, thanks for the reply. I will make the modifications you pointed out and ill come back to you if it works. And yes I did change the sense resistor, it is 0.1 now and it is in a TO-220 package so that I could mount it directly to the heat sink. Also will I need to get a large power resistor for the 0.22 resistor or will it not pass much current.

lol I feel so stupid for not realizing that each transistor would be a bit different 

[not1xor1]

I think that in the worst case the current difference among the darlingtons would be less than 1A... so even in the case of 4A of current that would be 4^2*.22 = 3.52W so you should use 5-10W resistors.

You might even make them yourself with some thin iron wire that also has a positive temperature coefficient and would so improve the current sharing.

[drussell]

Yup...  Your forgot your four emitter resistors... 

[Twoflower]

...and possibly overload the poor little 723 with four TIP142s. The max current the V_Z can provide is 25mA. A single TIP142 can draw, depending on the CE current up to 50mA on its basis.

[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)

[Zero999]

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)

All the pass transistors should have a 0.22R emitter resistor.

[glarsson]

And I do have a 220 resistor on the output signal to limit the current, is that ennough or should it be lower?

The resistor on the 723 output?
If you add more pass transistors you need more base current to drive them. If the available current from the 723 is lower than what is required to drive the bases of the pass transistors then you need an extra transistor between the 723 and pass transistors to amplify the current. Only changing the 220 ohm resistor is not enough.

[FotatoPotato]

Ok so would I add a 0.22 resistor, a 2N3904 and its 470 resistor to each or the 4 power transistors?  And for the extra base current would I use something like a 2N2222 NPN transistor or to amplify the 723's output for the 4 power transistors or would I use a PNP transistor like the 2N3906?

Also will the 0.22 resistor be passing a lot of current or can I get away with using something like a 1W one?

[Zero999]

Ok so would I add a 0.22 resistor, a 2N3904 and its 470 resistor to each or the 4 power transistors?

Yes, I forgot about the 470R, but the 2N3904 is only required for one transistor. It performs the current limiting function, by cutting off the base current to the drive transistors, when the current through the 0.22R resistor, causes the voltage drop across it to exceed around 0.7V.

And for the extra base current would I use something like a 2N2222 NPN transistor or to amplify the 723's output for the 4 power transistors or would I use a PNP transistor like the 2N3906?

An emitter follower is better, so it needs to be NPN.

Also will the 0.22 resistor be passing a lot of current or can I get away with using something like a 1W one?

What do you think? Look at the current path and calculating the power dissipation in the resistors, using Ohm's law.

[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!