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Old 16th September 2013, 10:08 PM   #1
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Default CCS + Zeners or resistors for 450V reference

I need a 450V voltage reference for the feedback circuit of a series regulator. I know of two simple methods; zeners and resistors, loaded with a CCS to get the proper operating point or developed voltage respectively. What method is regarded best?

I understand both have noise, but bypassing should take care of that for the most part. If there are better alternatives, I'm all ears.
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Old 17th September 2013, 10:19 AM   #2
Elvee is offline Elvee  Belgium
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Why do you use such a high reference voltage: in general, the error amplifier is made to operate at a conveniently low voltage, where optimization of noise, parasitic effects, etc can be carried out easily.
At such a high voltage, both solutions will be inconvenient: you will need a stack of multiple high voltage zeners (avalanche), having relatively poor performance, and the CCS option will be sensitive to the tiniest parasitic conductance anywhere near.
Plus, the signal part of the regulator will have to be 450V-rated, and high voltage devices tend to be a tradeoff between voltage and all the rest, meaning their performances will be rather poor
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Old 17th September 2013, 11:08 AM   #3
pergo is offline pergo  Italy
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http://www.supertex.com/pdf/datasheets/LR8.pdf

but is up to 450V
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Old 17th September 2013, 11:25 AM   #4
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Thanks for the responses. The schematic below might shed some light as to why I want such a high reference. I'm still pretty new to these kind of PSU's so it;s all a learning experience. The only way I know of to make the error amp work at lower voltages, is take the output voltage and lower it by means of a voltage divider and bypass the upper part of the resistor network. Or are there 'better' schematics to achieve the same thing?

@Pergo: I didn't know this device. I'll look into it.

psu.png
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Old 17th September 2013, 11:38 AM   #5
Mooly is offline Mooly  United Kingdom
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I would have thought a more conventional discrete arrangement would be simpler. The error amp is ground referenced and uses a low voltage zener as a reference. Floating a 741 like that looks a recipe for problems tbh, its so easy to kill the IC with transients etc.

This shows the basic idea. Its a common and standard topology thats easily scaleable and tweakable.
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Old 17th September 2013, 12:07 PM   #6
pergo is offline pergo  Italy
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if you need current, the Maida regulator is OK
21st Century Maida Regulator
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Old 17th September 2013, 02:16 PM   #7
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Quote:
Originally Posted by Mooly View Post
I would have thought a more conventional discrete arrangement would be simpler. The error amp is ground referenced and uses a low voltage zener as a reference. Floating a 741 like that looks a recipe for problems tbh, its so easy to kill the IC with transients etc.

This shows the basic idea. Its a common and standard topology thats easily scaleable and tweakable.
Thanks, I'll look into the schematic. As far as the 741 is concerned, the opamp's powerrails are held at a 20V differential by the zener, and the four diodes at the input prevent the inputs from ever going past the powerrails. How would it be able to get damaged?

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if you need current, the Maida regulator is OK
21st Century Maida Regulator
I've worked with the original Maida circuit before in a preamp project with great results. But this puppy needs to go into my power-amp, with peaks up to 800mA + the 60mA idle current (parallel-push-pull). Both Maida configurations would exploded AFAIK, since it's not only the pass MOSFET that has to handle the current.
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Old 17th September 2013, 02:30 PM   #8
Elvee is offline Elvee  Belgium
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The awkward part of your schematic lies in the supply/reference via the zener string and CCS.

You can rearrange it somewhat differently and eliminate practically all the issues: you can reference the + opamp supply to the output, and generate the zener current with a simple resistor: this will work, because the output voltage is constant, and will therefore generate a constant current.
You can then sample the output voltage (negative side) with a conventional voltage divider.
It looks somewhat incestuous, but it actually works because of the gain of the error amplifier.
There may be some start-up issues, but they can be solved with a diode and a few additional components

That is basically what I do in the Simple Series Reg, except it is discrete.
Doing the same with an opamp is certainly possible.

The big advantage of such an approach is that the only components in the control part actually seing the full input voltage are just resistors, which means it very safe
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Old 17th September 2013, 02:59 PM   #9
Mooly is offline Mooly  United Kingdom
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Quote:
Originally Posted by funk1980 View Post
Thanks, I'll look into the schematic. As far as the 741 is concerned, the opamp's powerrails are held at a 20V differential by the zener, and the four diodes at the input prevent the inputs from ever going past the powerrails. How would it be able to get damaged?
Would the diodes protect in all circumstances and conditions ? What if there were a glitch on the output, something pulling it down for even a few microseconds. Pin 2 of the opamp would be heading toward 0 volts and the nature of transients are that rise and fall times can be very quick indeed. What would the differential be between pin 2 and all the other pins during that instant ? Its conditions like that which cause unexplained failures. Similarly the junction capacitance of semiconductors (diodes !) can couple significant energy if a transient glitch appears. It may happen only very infrequently but if its a possible source of failure then its no good. The FET has very high junction capacitance and could be a prime suspect under adverse conditions.

The circuit just has that "look" about it. That its to fussy, could be prone to problems.
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Old 17th September 2013, 04:00 PM   #10
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Quote:
Originally Posted by Elvee View Post
The awkward part of your schematic lies in the supply/reference via the zener string and CCS.

You can rearrange it somewhat differently and eliminate practically all the issues: you can reference the + opamp supply to the output, and generate the zener current with a simple resistor: this will work, because the output voltage is constant, and will therefore generate a constant current.
You can then sample the output voltage (negative side) with a conventional voltage divider.
It looks somewhat incestuous, but it actually works because of the gain of the error amplifier.
There may be some start-up issues, but they can be solved with a diode and a few additional components

That is basically what I do in the Simple Series Reg, except it is discrete.
Doing the same with an opamp is certainly possible.

The big advantage of such an approach is that the only components in the control part actually seing the full input voltage are just resistors, which means it very safe
I don't quite follow. I get the part about the constant zener current, which would negate the use of a CCS, but you lost me with the opamp. Do you mean it'still floats on top of the high voltage zeners?
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