Variable Bipolar Power Supply

[audioguru]

The LM317 has a high gain compensated opamp inside. The compensation causes 90 degrees of phase shift. If it is in the negative feedback loop of a similar opamp like a 741 then the phase shift adds to 180 degrees at high frequencies and the combination will oscillate.

 

[Hero999]

With the 741's common mode input level at zero volts, you need a negative supply for it.

Of course I do. I knew that! I've made typical shoolboy error there!

Try a pulsed current sink for the load. I ran basically this same sim. See below.
First waveform, Ccomp=0.
Second waveform, Ccomp=100nF.

I'll try that.

It5 still works according to the simulation, there's some spikes when the switch turns off and dips when it turns on but there isn't any ringing.

Anyway I don't trust simulators. I've brought it home from work now and I'll test it properly tomorrow.

 

[Hero999]

Now my scope's broken!

I just got round to testing it when I turned the scope on, it made a buzzing sound, a gree line appeared on the screen briefly then it stopped working.

I'll start a new thread about this .

 

[Hero999]

Well I still haven't got round to looking at the scope but I took my power supply into work and tested using a scope that works.

I couldn't see any oscillation on either the -V or +V supply but both behaved pretty poorly under transient conditions. I connected a MOSFET to a 12hm: resistor on the output and drove the gate with a 50kHz waveform. I treid to make the leads as short as possible but there was significant ringing on the output when the load was turned on and a big glitch when it turned on. I would say that the noise at worst was about 1Vp-p. I performed this test on both the positive and negitive lines and got similar results, as expected noise on the +V line gets transferred to the -V line but not the other way round.

I was impressed with the current output though, I managed to set the supply upto +-12V with 12hm: connected from + to - and the output didn't drop at all. 2A isn't bad at all for a pair of regulators only rated for 1.5A.

 

[Roff]

So did you try adding some compensation? It might get rid of the overshoot and ringing.

 

[Hero999]

No I haven't. I reasoned that it probably wouln't help because the positive line is just as bad as the negitive.

I don't know how I can improve it anymore. I've tried adding 100nf to the output it it didn't get much better to I increased this to 1:mu:f without seeing a significant improvement. I suppose I might be able make the internal leads connecting the PCB to the output shorter and thicker to reduce their inductance and skin effect but I don't know what else can be done.

 

[Roff]

I haven't read **broken link removed** carefully, but I have seen ringing on LM317 sims. I have not made hardware comparisons to verify the sim results. I thought you might find this interesting.

 

[eng1]

2A isn't bad at all for a pair of regulators only rated for 1.5A.

Even if the LM317 is rated at 1.5 A output current, it can source up to 2.2 A. Beyond this limit the internal protection circuit limits the current. Also.. the chip may be turned off by the thermal protection circuitry, if its temperature rises above 170°C.

I haven't read "Spice uncovers regulator-stability problems" carefully, but I have seen ringing on LM317 sims. I have not made hardware comparisons to verify the sim results. I thought you might find this interesting.

I remember that article... If the model of the LM317 doesn't cover stability correctly, you may see ringing on a scope, even if you didn't on the simulator.
But if you see ringing on the simulator...!?!

 

[Roff]

Even if the LM317 is rated at 1.5 A output current, it can source up to 2.2 A. Beyond this limit the internal protection circuit limits the current. Also.. the chip may be turned off by the thermal protection circuitry, if its temperature rises above 170°C.



I remember that article... If the model of the LM317 doesn't cover stability correctly, you may see ringing on a scope, even if you didn't on the simulator.
But if you see ringing on the simulator...!?!

Well, it's possible that the spice model could be wrong about phase margin (stability) in either direction. You're probably right in thinking that it would more likely err on the side of being more stable vs less.

 

[Hero999]

The datasheet says the LM317 will current limit between 1.5A and 3.4A with 2.2A being typical so I suppose I have a pretty average LM317.

It seems silly that the spice models say it's more stable than it is. I would make it the other way round so when engineers actually use it they know it'll work when the simulation says it will.

I'll try using a 4.7nf capacitor on the adj pin and it the, then I'll see what I can do about the negitive supply.

 

[eng1]

Anyway the LM317 wouldn't be a problem without the op-amp.
You have to see if an RC network can be found to shape the loop gain (you need a zero!).
If you take my advice, you may try to re-design the PSU with two op-amps and a voltage reference. I gave this direction to my ideas and I'm designing the circuit. But I need a new transformer and it will take some time.

______
See also AN-1148 (National): nice app note about linear regulators.

 

[Roff]

Anyway the LM317 wouldn't be a problem without the op-amp.
You have to see if an RC network can be found to shape the loop gain (you need a zero!).

I didn't put a zero in the loop, and it worked fine. The pole just needs to be low enough frequency to ensure that the loop gain is below unity when the second pole in the loop (from the regulator) cuts in. The regulator/follower has very low output resistance, so it takes care of the transient response. The op amp just provides low frequency tracking. In fact, the feedback loop should be slow enough that it does not follow transients on the "master" supply.

If you take my advice, you may try to re-design the PSU with two op-amps and a voltage reference. I gave this direction to my ideas and I'm designing the circuit. But I need a new transformer and it will take some time.

______
See also AN-1148 (National): nice app note about linear regulators.

The big advantage of Hero's scheme is the use of the integrated regulator as the series pass element in the feedback loop. It is virtually bullet-proof. Duplicating this robustness with discrete parts requires a LOT of parts and a lot of smarts.

 

[eng1]

Duplicating this robustness with discrete parts requires a LOT of parts and a lot of smarts.

Of course the regulators will be part of the PSU, but the Vadj can be generated by two op-amps used in a close loop configuration (offsets, mismatches,... must be properly evaluated / compensated).
This is a valid method and ensures good tracking.

 

[eng1]

In my opinion there are two l.f. poles: the first is set by the internal compensation (Cc); the second is from the op-amp. The loop gain should roll off with -40 dB/decade slope in the mid freq. range. Unfortunately I can't simulate the circuit.

 

[Hero999]

Anyway the LM317 wouldn't be a problem without the op-amp.

But I am having the problem with the LM217 even though it isn't connected in the feedback loop of an op-amp.

What you are forgetting is that if there are two low frequency poles there is double the phase shift, therefore 180 degrees will occur well before the op-amp reaches unity gain causing oscillation.

 

[eng1]

What you are forgetting is that if there are two low frequency poles there is double the phase shift, therefore 180 degrees will occur well before the op-amp reaches unity gain causing oscillation.

Hi Hero999. If I was forgetting that, why did I write that you need a zero then?

But I am having the problem with the LM217 even though it isn't connected in the feedback loop of an op-amp.

What do you mean exactly?

 

[Hero999]

The transient response it just as poor on the positive supply as the negitive, even though it isn't using an op-amp.

 

[Roff]

The transient response it just as poor on the positive supply as the negitive, even though it isn't using an op-amp.

Your schematic doesn't have 100nF caps from the input pins to GND. These are recommended by some (all?) manufacturers for stability. Your big polarized caps don't satisfy this requirement.

 

[FMadi]

Hi,

As Ron H pointed out you really should add those input capacitors, last year I was working in the tech department of a medical device company here in Brazil, and we were having similar problems with a power supply that used 7805, and adding those small tantalum caps, (before and after) the regulators really helped with the ripple figures, here is a quote from the application notes on the datasheet from national:
“An input bypass capacitor is recommended. A 0.1µF disc or
1µF solid tantalum on the input is suitable input bypassing
for almost all applications. The device is more sensitive to
the absence of input bypassing when adjustment or output
capacitors are used but the above values will eliminate the
possibility of problems.”

What type of capacitor did you use for the adjustment pin bypass?

Can you measure the ripple frequency at the output?

This is my first post here, hope i could help, and sorry for any grammar mistakes, English is not my native language.

 

[Roff]

Hi,

As Ron H pointed out you really should add those input capacitors, last year I was working in the tech department of a medical device company here in Brazil, and we were having similar problems with a power supply that used 7805, and adding those small tantalum caps, (before and after) the regulators really helped with the ripple figures, here is a quote from the application notes on the datasheet from national:
“An input bypass capacitor is recommended. A 0.1µF disc or
1µF solid tantalum on the input is suitable input bypassing
for almost all applications. The device is more sensitive to
the absence of input bypassing when adjustment or output
capacitors are used but the above values will eliminate the
possibility of problems.”

What type of capacitor did you use for the adjustment pin bypass?

Can you measure the ripple frequency at the output?

This is my first post here, hope i could help, and sorry for any grammar mistakes, English is not my native language.

Wow, Felipe! Your English is better than most of our English-as-first-language contributors.