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tomchr 18th March 2012 03:49 AM

21st Century Maida Regulator
 
10 Attachment(s)
If you want the ultimate in ripple rejection and ease of use, this regulator is for you.

Key features:
  • True floating regulator design
  • Phenomenal ripple rejection (20 uV output ripple in my setup!)
  • Soft start
  • Stable with capacitive load
  • No need for expensive and bulky high-power resistors
  • 2x2 inch (50x50 mm) board footprint

It's now 32 years since Mike Maida authored National Semiconductor Linear Brief 47, describing a high voltage regulator based on the LM317. Lots have happened since then. National Semiconductor was acquired by Texas Instruments for one... And semiconductors have improved tremendously since the 1970'ies. So given that it's been 32 years to the month since LB-47 was published, I figured I'd do an update of Mike Maida's original regulator.

The main drawback of the original Maida regulator is that it requires at least 5 mA (10 mA worst case) to flow in the regulator for it to regulate properly. Typically, this current flows in the feedback network. For lower output voltages, this is no big deal. But for higher output voltages - such as the ones typically used in tube circuits - the power dissipated in the feedback network becomes quite significant, necessitating the use of 5~10 W rated resistors.
In addition, implementing soft start on the original Maida regulator is actually a bit of a challenge as it requires the use of high-voltage PNP or PMOS devices. These are becoming increasingly hard to source.
Modern voltage regulators also have much lower drop-out voltages than the LM317, hence, less power is dissipated in the regulator. As a result, the only heatsink needed is for the cascode device.

My "21st Century Maida Regulator" is based on the same topology as the original Maida Regulator; a low voltage regulator with a cascode in front to drop the voltage. I chose the LT3080 as it has a low drop-out voltage and needs only 300 uA (typ; 500 uA worst case) to operate. As described above, this minimizes the amount of power dissipated in the feedback network. Hence, only 2~3 W rated resistor types are needed.
The LT3080 is a low dropout regulator and only needs 1.4 V (worst case) across it to regulate. This minimizes the power dissipated in the LT3080. It doesn't even need a heat sink.
For the cascode I use a beefy NMOS - STW12NK95 (10 A, 950 V). I've used these in my other regulators and they work well. They're also capable of surviving the conditions present at regulator start-up without running into SOA limits.

My prototype regulator was adjusted to 420 V out @ 200 mA. There is no measurable ripple on its output. With 16 V RMS (50 Vpp) ripple in, I measure 20 uV (yes, micro volts) RMS of ripple and noise on the output of the regulator. Attached pictures show the transient response as function of load current and load capacitance. It looks rock solid to me...
The start-up time comes in at about 10 seconds. This does, however, require a resistive load. Without load, the start-up time is about one second as the output capacitor is charged through zener diode D2. The start-up is smooth without tendency to overshoot.

Using the values in the schematic, I only get about 1 mA running in the feedback network. In order for the LT3080 to regulate properly, at least 300 uA must flow in the LT3080. Hence, with Iout = 0 A, the current flowing in zener diode D2 must not exceed 700 uA. With R1 = 68 kOhm, I get 700 uA when Vin-Vout > 48 V. This isn't enough to guarantee reliable start-up across worst case mains variation, hence, the 330 kOhm "minimum load" seen across the output terminals in the image of the regulator prototype. I'll probably end up burning 1.5~2 mA in the feedback path to ensure that the regulator will start up and regulate properly. I'll also increase R1 from the value in the schematic.

After running for a few hours feeding 200~210 mA into my 300B amplifier, the LT3080 has reached 35 deg C. Clearly, no heat sink is needed for the LT3080 - the cascode still needs a heat sink, obviously. I have no audible hum in the speakers and the amp is dead quiet. I like it....

I plan to offer boards for sale on my website. I haven't done the cost calculations yet, but I figure I'll land around $10.

~Tom

Globulator 18th March 2012 11:07 AM

Hmm, decent regulators are hard to come by ;)

Surely you don't know what the ripple will be until you play music through the amp it is connected to?

Compared to music signal ripple surely mains ripple is almost irrelevant?

What is the dynamic impedance (in the audio band 20-20kHz) of the output?

ETA: What was the Vin for your testing at 420V 0.2A output?
The board looks very neat BTW.

pauldune 18th March 2012 11:46 AM

I like it! So it reduces 50v pp ripple to 20uV? At 200mA/420V output?
Looks like bye bye for chokes....

I don't see the 10u and 100uF electrolitics on the board?

greetings, Paul

tomchr 18th March 2012 07:11 PM

Quote:

Originally Posted by Globulator (Post 2950687)
Surely you don't know what the ripple will be until you play music through the amp it is connected to?

Compared to music signal ripple surely mains ripple is almost irrelevant?

What is the dynamic impedance (in the audio band 20-20kHz) of the output?

I think you're intermingling questions here.... :)

The regulator needs to provide good attenuation of mains ripple AND low output impedance.

Mains ripple: The ripple attenuation is important even (or perhaps especially) without music playing as any ripple on the regulator output will result in hum on the amplifier output. With music (or tones) playing, it may also result in IMD. Those who have followed my threads over the past couple of years will know that I've tried many regulator topologies and as a result have had the opportunity to listen to my amp with varying levels of ripple on B+. I find that even with my relatively inefficient speakers (87 dB @ 1W, 1m) I can hear the hum from my listening position if the ripple on B+ exceeds 2~3 mV. Ripple below 1 mV is acceptable and below 2~300 uV not audible in my setup.

Output impedance: The output impedance can be viewed as a measure of how much voltage sag should be expected for a given change in load current. This is relevant when the amp is reproducing music through speakers. For accurate reproduction, the supply impedance should be as low as possible.
I have not measured the output impedance yet. It's on my list... Also, getting a setup going that can measure the output impedance as function of frequency is on my list as well. I've looked at the setups used by several people, but question if they will actually provide accurate results.

Quote:

Originally Posted by Globulator (Post 2950687)
ETA: What was the Vin for your testing at 420V 0.2A output?

About 475 V (with 5 Vpp ripple). For the test with 50 Vpp ripple in, I reduced the supply capacitance (external to the board) by a factor of 10 and raised the input voltage to a bit over 500 V to ensure that the regulator had enough drop-out voltage to regulate properly.

I measured the drop-out voltage to be about 15 V. I'd probably use 25 V as a minimum in my designs.

Quote:

Originally Posted by Globulator (Post 2950687)
The board looks very neat BTW.

Thanks. I took my time placing the components. It came together pretty nicely.

~Tom

tomchr 18th March 2012 07:22 PM

Quote:

Originally Posted by pauldune (Post 2950735)
I like it! So it reduces 50v pp ripple to 20uV? At 200mA/420V output?
Looks like bye bye for chokes....

That's how I see it...

I don't think I've actually found the actual ripple rejection yet. Even with 50 Vpp ripple in, the measurement accuracy is limited by that of my 6.5 digit AC voltmeter... It may just be the noise floor of the regulator I'm measuring. The voltmeter has about 1 MHz bandwidth, so 20 uV is quite good, actually.
At 20 mV/div (highest sensitivity with a 10x probe) on my oscilloscope, I get a straight line when measuring the AC voltage.
I think I'll try to measure it with my HP3581A wave analyzer (frequency selective voltmeter).

Quote:

Originally Posted by pauldune (Post 2950735)
I don't see the 10u and 100uF electrolitics on the board?

There aren't any. Anything labeled "load" in my charts is applied externally. I.e. the Cload = 47 uF, for example means that I attached a 47 uF capacitor to the output of the regulator. I'm just verifying that the regulator can tolerate it if someone decides to add more capacitance to its output, that's all.

~Tom

grufti 18th March 2012 07:46 PM

I believe Paul was refering to C1b and C3 in your schematic.


Quote:

Originally Posted by tomchr (Post 2951214)

Originally Posted by pauldune http://cdn2.dastatic.com/forums/imag...s/viewpost.gif
I don't see the 10u and 100uF electrolitics on the board?


There aren't any. Anything labeled "load" in my charts is applied externally. I.e. the Cload = 47 uF, for example means that I attached a 47 uF capacitor to the output of the regulator. I'm just verifying that the regulator can tolerate it if someone decides to add more capacitance to its output, that's all.

~Tom


Globulator 18th March 2012 08:06 PM

Thanks for the answers, I'll be keeping a eye on this thread and when you release the PCBs.

Have you got dimensions of the populated final module?
TIA!

pauldune 18th March 2012 09:15 PM

Quote:

Originally Posted by grufti (Post 2951238)
I believe Paul was refering to C1b and C3 in your schematic.

Correct! Also, are there any equivalents for the nmos? I can't find them anywhere.(in the netherlands/ europe)

FLT 18th March 2012 10:09 PM

Quote:

Originally Posted by pauldune (Post 2951355)
Correct! Also, are there any equivalents for the nmos? I can't find them anywhere.(in the netherlands/ europe)

Go to the distrelec.com website and search for "stw".

Hope that helps

regards

jackinnj 19th March 2012 12:53 AM

before you sell it, you'd best run a phase-gain plot to make sure it's stable -- this is, after, an LDO


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