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Old 19th March 2012, 11:42 PM   #21
tomchr is offline tomchr  United States
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Originally Posted by tubelab.com View Post
I have been experimenting with solid state regulators for use in tube amps for some time. I want to make one that's suitable for experimentors which imposes a new set of constraints. You need an adjustable output over a fairly wide range, and it needs to be stable for all loads including a short.
That's a really tall order. I'd take a look at how it's done in the HP6209B. The first stage of regulation is done in the rectifier (SCR-based). That's then followed up with a linear regulator. This minimizes the amount of power dissipated in the linear regulator. Clever design... I have one. It's a rock solid supply. It does put out a bit of 120 Hz crap resulting from the synchronous rectification, but otherwise, it's a really nice supply.

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Originally Posted by tubelab.com View Post
The one that I could never make right with the Maida design is the ability to handle an accidental (or in the Tubelab sense deliberate) short circuit on the output when the regulator is powered up and operational. Have you tried this. If not, and you do try it, shield yourself from mosfet schrapnel.
No kidding!! I have not tried this yet. I figured I'd do all the non-destructive testing first... But I haven't been able to build a regulator that would survive a short circuit either -- certainly not an indefinite short circuit. My experience with my previous Maida regulator designs have been similar to yours. My expectations of this "21st Century Maida" are no different. I'm hoping that it will at least survive a momentary short circuit (hence the addition of the Tranzorb across the regulator), but I'll be happy if it survives start-up into my 47 uF, 630 V Solen polypropylene caps (5 mOhm ESR!).

~Tom
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Last edited by tomchr; 19th March 2012 at 11:54 PM.
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Old 20th March 2012, 12:01 AM   #22
tomchr is offline tomchr  United States
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Originally Posted by jackinnj View Post
My guess is that below -80dB ripple rejection is going to become less relevant than Zout.
I guess I should complete my sentence... My goal is to have the best ripple rejection I can get while maintaining a low output impedance. How's that?

You're right. Zout does need to be low across the audio band.

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I would suggest looking at some of the fast transient response regulators from LLTC and not overly damping the device for low noise. Some of the LLTC regs allow for remote load sensing and this is a MAJOR IMPROVEMENT!
Remote sensing makes a huge improvement in Zout, that's for sure.

Before picking the LT3080, I did glance through a couple of other ICs. But I don't recall seeing a floating regulator with remote sensing capability. Am I mistaken?

~Tom
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Old 20th March 2012, 12:18 AM   #23
grufti is offline grufti  United States
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Are you thinking of something like the LT3070? I haven't been able to find any others with sense inputs.


Quote:
Originally Posted by jackinnj View Post
I would suggest looking at some of the fast transient response regulators from LLTC and not overly damping the device for low noise. Some of the LLTC regs allow for remote load sensing and this is a MAJOR IMPROVEMENT!
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Old 20th March 2012, 01:18 AM   #24
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That's a really tall order.
Tell me about it. I had about half a coffee cup full of blasted mosfets from my attempts. Place the coffee cup upside down over the mosfet before testing.

I have been using an old HP6448B for my current high powered amp experiments. It is a 60 Hz switcher with current limiting....but the reaction time of the limiter is hundreds of milliseconds AND there is a large electrolytic inside the unit across the output terminals. It is rated for 600 volts and 1.5 amps, and turned up all the way it makes 650 volts and 1.7 amps. I am sure that a tube short would set an OPT on fire before the limiter could work, and it isn't big enough any more! I have reached the end of the power supply extracting 525 watts from Pete Millett's 18 WPC big red board.

I am working on an even bigger amp so I am currently using an unregulated setup with some big transformers, a big variac, diodes and caps, big caps. Like 2 600uF 500 V polyprops in series, just a few milliohms ESR. It makes 750 volts and trips the bench breaker at just over 2 amps. Now, I want to make a mosfet regulator with a fail safe current limiter. Fuses explode like mosfets at this power level, so the coffee mug is back.

I experimented with the mosfet Maida and it kinda works but doesn't like a big cap across the output, but that is not wanted for safety reasons. I have been experimenting with other circuits, but I think the key is the CCS property of pentodes and mosfets. If you fix the grid or gate voltage you get a CCS. For the Fuji mosfets I am using (for now, I have 18 left out of 50) a 5.1 volt zener from source to gate limits the drain current to 2 amps. I never got it to work right in the Maida circuit, but that was 2 years ago. You still need a method of limiting dissipation. For now I am just setting the variac to limit the voltage across the fet to 100 volts max during normal operation. There is a .05 ohm resistor in the return lead for current sensing and fault detection. I am still working on a better solution.
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Old 20th March 2012, 01:57 AM   #25
tomchr is offline tomchr  United States
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Originally Posted by tubelab.com View Post
I have reached the end of the power supply extracting 525 watts from Pete Millett's 18 WPC big red board.
Nice...

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Originally Posted by tubelab.com View Post
If you fix the grid or gate voltage you get a CCS. For the Fuji mosfets I am using (for now, I have 18 left out of 50) a 5.1 volt zener from source to gate limits the drain current to 2 amps. I never got it to work right in the Maida circuit, but that was 2 years ago. You still need a method of limiting dissipation.
For my circuit at relatively low energy levels (compared to the ones you're dealing with anyway), the soft start of the regulator will actually save the cascode device when starting up into a capacitive load. In Mike Maida's original circuit, implementing softstart for higher output voltages was a royal pain. And without the softstart, you're practically guaranteed to hit the SOA limit on the cascode during start-up. However, with the LT3080, it's just a matter of adding a low voltage cap. Yay. This is how my regulator is capable of starting up with 47 uF on the output as long as there is enough resistive load to take up the current that flows through the zener diode (D2).

Currently, I'm only running 1 mA in the resistive divider forming the feedback network. That's a bit on the light side. I need at least 3~500 uA plus the worst case current through the zener diode, D2. This way the regulator should be able to start up into a purely capacitive load. But I still need to get some more resistors and verify this in the lab...

I may try your trick of limiting the Vgs of the cascode. But given that the drain current is some exponential function of Vgs, I doubt it can be used as a reliable protection device. It may work as a one-off deal, but I'm afraid repeatability will be an issue due to part-to-part variation. Then again, it might just work well enough...

~Tom
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Old 20th March 2012, 02:31 AM   #26
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Originally Posted by grufti View Post
Are you thinking of something like the LT3070? I haven't been able to find any others with sense inputs.
LT3070 has a "physical" sense input, LT1963 has a "meta-physical" sense input if you look into the datasheet. It works!!!

I'm not kiddin'
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Old 20th March 2012, 02:38 AM   #27
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It may work as a one-off deal, but I'm afraid repeatability will be an issue due to part-to-part variation.
I have seen anywhere from 1.5 to 3 amps on different fets from the same batch. The worse part is the temp variation. So far it works to save a fet rated at 7 amps continuous, 25 amps peak. Without the diode the fet just explodes. It also lets the fuse blow, not explode.

It's all a temporary solution for now. I am working on a big amp breadboard for learning purposes. Then I'll get back to the power supply. I am thinking buck converter....with a zener across the gate of the switch fet. Linear after that. Otherwize I'll need a really big heat sink, which I do have.
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Old 20th March 2012, 02:50 AM   #28
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Originally Posted by tubelab.com View Post
I am thinking buck converter....with a zener across the gate of the switch fet. Linear after that. Otherwize I'll need a really big heat sink, which I do have.
SMPS on breadboard are recipes for disaster.
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Old 20th March 2012, 01:18 PM   #29
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SMPS on breadboard are recipes for disaster.
SMPS on breadboard at 750 volts are called fireballs.

I am making a "breadboard" amplifier. It is a two layer monster that uses several PC boards and two slabs of plywood. The power supply is currently all unregulated supplies with transformers, diodes, caps, and variacs. It takes up the entire lower level and I can barely lift it. I needed something simple and effective so I could get started with the fun stuff.

The variable regulated supply is an entirely different project and will be all PC board construction. The HV supply may use a buck converter for the first stage just to lower the dissipation. The second stage will be a linear regulator.
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Old 20th March 2012, 06:12 PM   #30
tomchr is offline tomchr  United States
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Originally Posted by jackinnj View Post
LT3070 has a "physical" sense input, LT1963 has a "meta-physical" sense input if you look into the datasheet.
Do you actually have this working in a high-voltage setup?

The LT1963 requires at least 1 mA (1.5 mA worst case) to regulate. Add the zener current for the cascode bias, some margin, and you'll have 2.5~3.5 mA flowing in the feedback resistors. Still better than the original Maida with its 10 mA, but getting kinda high...

But after our conversation about output impedance the other day, I got to thinking... How low output impedance is actually needed? Given that the DC resistance of a typical OPT is around 100 ohm, having Zout of 1 ohm or even 10 ohm is probably OK.

I'm currently working to build an output impedance test rig. I should have measurements by the weekend, assuming the rig works.

~Tom
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Last edited by tomchr; 20th March 2012 at 06:14 PM.
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