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Old 2nd February 2014, 03:33 PM   #151
audiostrat is offline audiostrat  Greece
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Quote:
Originally Posted by B&W_arthur View Post
How to boost the output current of the Super reg? Or, what is its limit?
Reduce R1 and choose a Q1 having higher hfe.
What else?
First of all, you need to remember that the opamp will be able to feed a certain amount of current. If you go near its actual limits, you need to consider heatsinking of the IC - it may not require a lot of it, but just don't forget ensuring that everything runs cool. Read the thread again, you will find a relevant post of Jan.

So, since your opamp's output current is limited and known, you can go as big as your pass transistor's hfe allows you to go, given the opamp's current. Then maybe you should look for an opamp able to feed more current.
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Old 2nd February 2014, 04:42 PM   #152
Mark Johnson is offline Mark Johnson  United States
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To increase the maximum output current, modify the design.

One possible design mod would be to substitute a high current depletion mode MOSFET + plenty big heatsink, for the series pass transistor. Perhaps something like the IXTP3N100.

Another possible design modification would be a 2-transistor replacement for the D44H11. You could consider a Complementary Feedback Pair ("Sziklai Pair"), a Darlington pair, or a depletion MOSFET-BJT pair. The Supertex DN4250 depletion MOSFET that Walt Jung advocates, might be an especially good choice.

A third possible design modification would be to use the Jung/Didden SuperRegulator with a conventional PNP (on big heatsink) current booster. The SuperReg supplies the first 75mA and the PNP supplies all the rest. Chose R1 = (VBE / 75mA) in the generalized schematic below.

In each case, your designer would want to verify that the modified circuit was unconditionally stable, and that its regulation characteristics (output impedance, line regulation, load regulation, noise, etc) remained excellent.

Click the image to open in full size.
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Old 3rd February 2014, 12:12 PM   #153
B&W_arthur is offline B&W_arthur  Hong Kong
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Audiostrat, I have noted that Jung/Didden mentioned about it in their articles.

Mark, thank you for the details explanation. Maybe, I can do an experiment of dropping in a Darlington transistor in lieu of D44H11 and see what happens. If it does not work, then I have to do study the recommended modifications in depth.

Forget to make it clear, the target current output is about ~3A.
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Old 3rd February 2014, 01:29 PM   #154
jackinnj is offline jackinnj  United States
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Quote:
Originally Posted by B&W_arthur View Post
Forget to make it clear, the target current output is about ~3A.
3A for a +/- 13.7 V supply? -- I am wondering where you're going here. Perhaps for a mixing board in a recording studio, in which case you'd be better off having multiple regulators as there will be plenty of reverse PSRR.
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Old 3rd February 2014, 04:08 PM   #155
jan.didden is online now jan.didden  Europe
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Quote:
Originally Posted by audiostrat View Post
First of all, you need to remember that the opamp will be able to feed a certain amount of current.
It is not the opamp that feeds the base current for the pass device - it's the current source that does that.

That said, it is of course the opamp that has to absorb that current if you unload the regulator. So if you want your reg to be working between zero load and max load, the effect will be that the opamp has to be able to absorb the full current source current.
In the end the effect will be the same, but I thought I'd mention the difference for better understanding.

Re: use of MOSFET pass device: for me the biggest disadvantage is the much lower device gain, which means less loop gain which means less corrective feedback which means lower perfomance in the audio band.

Jan
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Old 3rd February 2014, 06:15 PM   #156
Mark Johnson is offline Mark Johnson  United States
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Quote:
Originally Posted by jan.didden View Post
Re: use of MOSFET pass device: for me the biggest disadvantage is the much lower device gain, which means less loop gain which means less corrective feedback which means lower perfomance in the audio band.
Jan, I suspect you're speaking about the transconductance gm of the series pass transistor. Work out the transfer function of a voltage regulator consisting of a pass transistor with transconductance "gm", driving a load resistor "RL", with an error amplifier whose (finite) open loop voltage gain is "Av". From the voltage reference input to the regulator output, the transfer function is
Click the image to open in full size.
Please note that pass transistor transconductance gm is multiplied by open loop gain Av.

BJT transconductance is higher than MOSFET transconductance; it's true. But the ratio is less than ten to one.

Opamp open loop gain Av varies much more than ten to one; for the AD825 (76dB) versus the AD797 (146dB), the ratio is 3000 to 1. Yet the impact upon measured low-frequency Zout in Jack Walton's curves, is only 10 to 1 (see attachment). Conclusion: as long as (gm x Av) is big enough, you'll get world class performance. Want to use a MOSFET pass transistor? It'll be wonderful; just make sure to pick an opamp whose open loop gain Av is greater than 80 dB.
Attached Images
File Type: png voltage_regulator.png (4.8 KB, 699 views)
File Type: png Zout_Graph.png (110.0 KB, 706 views)
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Old 4th February 2014, 02:54 AM   #157
B&W_arthur is offline B&W_arthur  Hong Kong
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Quote:
Originally Posted by jan.didden View Post
It is not the opamp that feeds the base current for the pass device - it's the current source that does that.

That said, it is of course the opamp that has to absorb that current if you unload the regulator. So if you want your reg to be working between zero load and max load, the effect will be that the opamp has to be able to absorb the full current source current.
In the end the effect will be the same, but I thought I'd mention the difference for better understanding.

Re: use of MOSFET pass device: for me the biggest disadvantage is the much lower device gain, which means less loop gain which means less corrective feedback which means lower perfomance in the audio band.

Jan
Yup, it was mentioned in the article that large amount of unload current makes the opamp heat. The schematic tells me the same story.

Thank you for making it clear here again.
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Old 4th February 2014, 02:55 AM   #158
B&W_arthur is offline B&W_arthur  Hong Kong
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Quote:
Originally Posted by Mark Johnson View Post
Jan, I suspect you're speaking about the transconductance gm of the series pass transistor. Work out the transfer function of a voltage regulator consisting of a pass transistor with transconductance "gm", driving a load resistor "RL", with an error amplifier whose (finite) open loop voltage gain is "Av". From the voltage reference input to the regulator output, the transfer function is
Click the image to open in full size.
Please note that pass transistor transconductance gm is multiplied by open loop gain Av.

BJT transconductance is higher than MOSFET transconductance; it's true. But the ratio is less than ten to one.

Opamp open loop gain Av varies much more than ten to one; for the AD825 (76dB) versus the AD797 (146dB), the ratio is 3000 to 1. Yet the impact upon measured low-frequency Zout in Jack Walton's curves, is only 10 to 1 (see attachment). Conclusion: as long as (gm x Av) is big enough, you'll get world class performance. Want to use a MOSFET pass transistor? It'll be wonderful; just make sure to pick an opamp whose open loop gain Av is greater than 80 dB.
Thank you for the hint.
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Old 4th February 2014, 02:58 AM   #159
B&W_arthur is offline B&W_arthur  Hong Kong
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Quote:
Originally Posted by jackinnj View Post
3A for a +/- 13.7 V supply? -- I am wondering where you're going here. Perhaps for a mixing board in a recording studio, in which case you'd be better off having multiple regulators as there will be plenty of reverse PSRR.
My friend request a single 12V for 2.4~3A supply.
I am exploring the possibility of pushing the current output to meet his need.
It may work or it may not. But, why not give it a try?
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Old 4th February 2014, 03:02 AM   #160
Jason is offline Jason  Australia
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The new Super Regulator V2.2 boards are now in stock and shipping. Thanks, Jan!
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