Current Limiting For High Voltage OpAmp

[Samuel Groner]

Hi

Inspired by some commercial gear, I played with a high voltage discrete opamp ([removed]).

Realizing that shorts get very quickly deadly to the output devices with these voltages, I checked output current limiting a bit more carefully and realized that it does not work well. As shown, it is very asymmetric (a few mA in one, some A in the other direction). With two diodes instead of Q11/Q12 (as in the 990) things are about the same.

How would you go about this? I think a reasonable goal were a few hundred mA in both directions. Any circuit ideas?

Oh yeah, I would like to make this fit a 2520 footprint, so no big heatsink...

BTW, apart from this, the design seems to behave very well.

As always, big thanks!
Samuel

 

[PRR]

> it is very asymmetric

Q7 needs current limiting.

 

[Samuel Groner]

Thanks, that helped.

Does this implementation ([removed]) look right? C2 is needed to maintain stability. A simple emitter resistor doesn't do it, as it needs to be very big.

Samuel

 

[tk@halmi]

You may be able to get away with just little larger R9 by itself. The JH990 does.

 

[Samuel Groner]

Unfortunately not, R9 needs to be 1k to give aprox. 250 mA current limit. I did not study the difference between the 990 (which simulates with one- or twohundred mA more IIRC) and my design. A first glance does not reveal much difference..

Samuel

 

[tk@halmi]

The major differece besides inverted polarity is that your amp runs on considerably higher rails resulting in more possible power. I use TO-126 (BD139/140 or MJE172/182) devices at +-22V with heatsinks on the output all the time and it takes some serious abuse to kill them in a circuit. They take momentay short, but cannot dissipate heat from prolonged shorts if you jumper the output directly to ground for minutes.

Adding one transistor for safety is not a big deal if the opamp still behaves and sounds as you expect it. Also consider if a power supply should allow you to pull many amperes of current and facilitate a melt down. The opamp will not be the only place in a product where shorts may occur.

Cheers,
Tamas

 

[Samuel Groner]

The major differece besides inverted polarity is that your amp runs on considerably higher rails resulting in more possible power.

Of course, but isn't the short-circuit current more ore less independet of supply voltage with the given architecture?

Samuel

 

[PRR]

> C2 is needed to maintain stability.

Should not worsen stability when not limiting.

Asking without thinking: why are you using a transistor current limiter to control Q7, when you (maybe) could use a couple diodes like the output stage uses?

The added transistor will tend to amplify and become unstable as it goes in and out of limiting.

I also suspect that Q7's limit point should be much more than twice its idle current (and yet less than its breakdown/melt current). 10mA or 20mA. Say 39-47 ohms in its emitter.

OH! In your second plan, when Q8 clamps Q7's base to limit current, Q6 current rises without limit. Or since it can only ever get 2.2mA drive, to about 200mA-600mA. I smell smoke.

The elaborate fix is to have Q9 clamp Q6 base, thus limiting current in both Q6 and Q7. But this 3-transistor local loop will love to oscillate. And if you clamp Q6 base to the negative supply rail, it will take forever to recover after saturation. Again I'd be thinking a 3-diode clamp, but now at Q6 base.

Don't assume an output overload will only melt the outputs. It often ripples far forward in the amp. Under feedback, the NBF will drive everything as far as it will go (or farther).

 

[Samuel Groner]

R9 should not worsen stability when not limiting.

Not sure whether it should or not, but it does :grin:. Even a small signal analysis shows some decrease in phase margin. Always wondered what's up with this.

why are you using a transistor current limiter to control Q7

A single diode didn't limit things enough, so I took the transistor.

In your second plan, when Q8 clamps Q7's base to limit current, Q6 current rises without limit.

Hm, nothing learned from the first lesson!

Again I'd be thinking a 3-diode clamp, but now at Q6 base.

Wouldn't this make ouput current limit Q7-beta-dependent?

Thanks so far for the help!
Samuel

 

[bcarso]

You could try moving Q8's collector to Q6's base.

 

[PRR]

> Wouldn't this make ouput current limit Q7-beta-dependent?

Clamp Q6 base at 3Vbe. Put a resistor under Q7 emitter. Both transistors are current limited.

Retain the clamps at Q9 and Q10.

EVERY transistor MUST have current limiting. In simple circuits, the circuit resistors are often enough. In this plan Q1 Q2 Q3 Q4 Q5 and Q9 have current limiting and only severe inputs could force large currents. But Q6 Q7 Q10 and Q11 can rise without limit (except their own parasitic R, which will make them overheat). Only rarely can you use one clamp to protect two transistors; here you can let Q6 Q7 share a clamp and resistor, and Q10 Q11 can share a clamp.

 

[Samuel Groner]

The three-diode-clamp seems to work. Some transistors still swing a bit over the power rating with a nasty square-wave input but I guess in practice this should work.

Thanks!
Samuel