PS limiting current for no reason?

[AngraMelo]

Ok, so I have been in this project for a while. You guys already helped me with the protection circuit (the crowbar is not there anymore, Im using now an opamp that controls a transistor that controls a beefy relay. When the regulated voltage that comes from the output of the PS goes into the non inverting input of the op amp is within the parameters 13-14V, it turns on the transistor which turns on the relay and the binding posts of the PS are "on" if something shorts out and the output voltage goes higher than 14V the opamp output goes to ground which wont turn on the transistor which wont turn on the relay so the binding post are "off") It is working so thank you all for helping me!

That is not the issue anymore.

The issue is, it is limiting the current to around 7.5A. The voltage on pin 10 of the 723 is slowly dropping (around 30mV per second) so what happens is: if I connect a load that requires 10A, it provides only 9A and slowly drifts down to 7.5. What I think it is happening is that the 723 cannot drive Q1 which is an 3055, so when pin 10 drops the voltage given that it cant provide enough base current to Q1, that drops the voltage on the base of the power transistors limiting the current.
Am I right? What do you guys think?

UPDATED THE SCHEMATICS

[AngraMelo]

So I measured the current on the emitter of the pass transistor Q1 and it goes as high as 100mA and starts to go down. So something is not right. the 723 should be able to drive that Q1.
The 723 can output 150mA so if we consider a low gain of 20 for the 3055, if the 723 outputs 100mA Q1 would be able to provide 2A, which is way more than the 4 transistors will need on their bases to let a 10A current to pass.
Am I crazy here?

[rstofer]

I know the first line in the datasheet says 150 mA output current but I'll be darned if there is any other number anything like that anywhere in the datasheet (other than the marketing blurb in the upper right corner).  There are power dissipation curves and how much the regulator dissipates depends entirely on the operating voltages and the output current.

You didn't provide the PS output voltage (or I missed it) and, near as I can tell, you're going to have to go back and calculate the power dissipation.  This will be critical when outputting low voltage because there is more drop across the regulator.  You would still need to output high current into Q1 if you are driving high current from the PS and you would have a maximum voltage drop across the regulator.

I would measure the base voltage droop on Q1 and maybe put a finger on the regulator.  If the regulator is protecting itself, the output voltage will decrease.  There may also be a temperature rise.

[ArthurDent]

Just for the heck of it put another 0.1 ohm resistor in parallel with R7-R10 and let us know what the output current is.

Also replace Q1 with a 2N6576 or similar.

[AngraMelo]

I tried putting another 0r1ohm resistor and no cigar.
Im putting 28V and the output is supposed to be 13.8V.
When I connect any significant load, like for example 2A, the output drops about 0.5V and the current never exceeds 9amps.
I tried putting a 2222 in series with a bd139 and then to the 3055 (Q1)to minimize the current at the output of the 723 and the result was the same.
Im completely lost

[David Hess]

It might be oscillating.  C4 should not be on the output and the 723 and the output transistors should be decoupled from their supply inputs.

The output voltage should only be changing by millivolts at most when a full load it applied.

[AngraMelo]

So I should move c4 to close to the 723, and add another 100nf to the input of the 723 and the base of Q1and the collectors of the power transistors?

[spec]

+ AngraMelo

The LM723 is a great chip and still going strong after all these years.

By the way, neat schematic: nice and easy to follow.

Here are a few points about the PSU design and the early current limiting fault:

According to your schematic, the PSU should limit at 0.65/(0.1/4) = 26 amps. Is that your intention?

There is an issue with the four current sensing resistors. Sufficient to say at this stage that connecting four low value, high current resistors in parallel is problematic, but that is unlikely to be the cause of the early current limiting  problem.

There are no base emitter resistors on any of the five transistors this means that they increase current fast but decrease current slow. It would be wise to connect a 560R across the E/B of Q1 and 220R across the E/B of Q2 to Q5.

PSUs, of all types are notorious for oscillating and the other members have touched on this point. As you probably know, there are two control loops on an LM723 PSU; voltage and current.  I would suggest, at this point of the investigation, to remove both connections to the current limit transistor so that any issues with the current limit loop are eliminated.

The next thing to do is to disconnect the LM723 and transistors from the 28V raw supply line. Then draw 10A from the raw 28V supply line and monitor the voltage directly across the reservoir capacitors with a scope. Then if you could post the exact absolute voltage levels at the peaks and troughs of the ripple, that would assist in the investigation. Is it correct that the mains supply in your local is 220V RMS, 60Hz?

You have a reservoir capacitance of 2 x 22mF = 44mF. Can you state what tolerance the reservoir capacitors are? -40% +100% would be typical. If you have the type number for the reservoir capacitors that would be ideal. Can you also post the bridge rectifier part number?

You say that under maximum PSU output current (7A) the base current into Q1 is 150mA, but I calculate that for 10A PSU output Q1 base current should not exceed 25mA. The gross over current is odd. Two reasons for this fault condition spring to mind: either the circuit is oscillating, or the five output transistors have much reduced current gain.
 
As you most probably know, the physical layout of power circuits is critical to maximize frequency stability and accuracy. In the case of your power supply, star point techniques are essential. And on a similar theme, adequate decoupling is also essential, as has already been pointed out.

One last question: is it true that this PSU has always had the problem with early current limiting?

I do have an idea what the problem may be, but I will need the requested data above to progress any further.

[spec]

Just had an thought for a simple test! Remove R2 so that the PSU regulated output voltage drops down to 7.15V. Then see if the PSU behaves normally up to an output current of 10A.

[Sylvi]

Hi

10uF on the output is way too low.Also needs the BE turn-off resistors others described.

There should also be a protection diode across the relay coil or the Q driving it could be toast from the flyback voltage when the relay turns off.

Is 1nF suitable for the pot wiper? It might be wise to add a safety resistor from the wiper to ground since the contact integrity of the pot wiper might be irregular over time. Hopefully this is a real pot not a trimmer? Trimmers are good for 200-cycles; a cheap pot is 15k-cycles.

What is the love for 2N3055 everyone has? They are not great devices and lose gain at currents much below their rating. Why not use modern devices with sustained-beta?

[spec]

What is the love for 2N3055 everyone has? They are not great devices and lose gain at currents much below their rating. Why not use modern devices with sustained-beta?

They are dirt cheap, rugged, have a reasonably low thermal resistance, junction to case, and they have a maximum junction temperature of 200 deg C. They also have a TO3 case which is good for heatsinking.

What more could you ask for? Well, a better frequency response, more consistency from maker to maker and batch to batch, lower saturation and lower VBE. And, as you say a flatter HFE curve with current changes.

I feel the same way as you about the 2N3055, but they are not too bad for this application.

[IanMacdonald]

BDY57/58 are fantastic power devices, especially for amplifiers. They cost a fair bit though.  Modern devices, especially FETs, suffer from some dreadful SOA (secondary breakdown) limits. Another good and cheap muscle BJT is the 2N3773. This is used in many guitar amps. 

As for the 723 problems, if you check the IC schematic you'll see that your arrangement puts a quadruple darlington between the supply and output. That's gonna lose a fair bit of voltage. It may be just that this burden voltage is too great. If you replaced your driver transistor with a PNP, and fed its base from pin 7 (see the application notes fig 20) that would drastically reduce the burden voltage.  As mentioned you should have a turn-off resistor between base and emitter of the driver to stop stray currents or capacitance from turning it on. Say 470R. (Not critical)

http://www.ti.com/lit/ds/symlink/lm723.pdf

Other point, a crowbar which shorts the output can be dangerous if the PSU is used to charge a battery and there is no fuse after this point.

[spec]

BDY57/58 are fantastic power devices, especially for amplifiers.

I remember those  

Modern devices, especially FETs, suffer from some dreadful SOA (secondary breakdown) limits.

MOSFETs also suffer from huge parasitic capacitances, sometimes in the order of nano farads, which cause all kinds of problems

Another good and cheap muscle BJT is the 2N3773. This is used in many guitar amps. 

Know them well.

As for the 723 problems, if you check the IC schematic you'll see that your arrangement puts a quadruple darlington between the supply and output. That's gonna lose a fair bit of voltage. It may be just that this burden voltage is too great.

That is what my fag packet voltage budget calcs show, and what I was hinting at. The 723, itself has a 3V dropout. 

If you replaced your driver transistor with a PNP, and fed its base from pin 7 (see the application notes fig 20) that would drastically reduce the burden voltage.

Yes, that would improve the DC conditions, but it also tends to increase the open loop gain and can lead to frequency stability problems, especially when the constant current shunt transistor kicks in. Many LDO regulators use PNP transistors to get a low voltage overhead with the result that they are a touch critical. 

Back on to power BJTs, here are my favorites:

ON MJL3281A/MJL1302A (TO264) These are the blue blood of low distortion output transistors- notice the flat HFE and high frequency response. It is not unusual to greatly improve an audo amp just by fitting these beauties.

ON MJ15003G/MJ15004G (TO3) similar idea, with a bit more clout.

ON MJL4281/MJL4282 (TO247) more clout.

Must stop now.

 

[IanMacdonald]

"If you replaced your driver transistor with a PNP, and fed its base from pin 7 (see the application notes fig 20) that would drastically reduce the burden voltage.

Yes, that would improve the DC conditions, but it also tends to increase the open loop gain and can lead to frequency stability problems, especially when the constant current shunt transistor kicks in. Many LDO regulators use PNP transistors to get a low voltage overhead with the result that they are a touch critical. "

Indeed, when you add more loop gain and delay to any opamp arrangement you may need to increase the compensation to keep things stable. That is true anyway though. With a driver and output stage both using relatively slow transistors, the standard value of compensation cap will likely be a tad too small. Main thing is not to increase it beyond the point where overshoots start to show when the load is removed.

[spec]

Indeed, when you add more loop gain and delay to any opamp arrangement you may need to increase the compensation to keep things stable. That is true anyway though. With a driver and output stage both using relatively slow transistors, the standard value of compensation cap will likely be a tad too small. Main thing is not to increase it beyond the point where overshoots start to show when the load is removed.

You obviously have some experience with compensating PSUs. The problem is that with lab type PSUs you never know what will be connected to the output  terminals: it could be anything from light restive loads to heavy highly inductive or capacitive loads, and even loads with negative resistance.

It seems that the classic PSU architecture, as in the LM723 standard configuration with emitter follower output, is generally the best behaved, but as you imply, not the best for voltage overhead.

The other area that can cause problems is the point where both voltage and current feedback loops are active. Here a diode AND function seems the best behaved, but I am not sure what the arrangement actually is inside the LM723 (the circuit diagrams shown on datasheets are much simplified versions of the actual chip circuit).

One dodge is to decouple the PSU output to 0V over a wide frequency spectrum using low ESR capacitors, and fit a very low resistance high current inductor between the output transistors and the decoupling.

All good fun, and I have a mountain of dead power transistors to prove it.