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Old 5th February 2012, 11:02 PM   #1
BRSHiFi is offline BRSHiFi  United States
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Default High Current Maida

Hi guys,

I am experimenting with the Maida topology to create a regulator that puts out 500V and 600mA for a powerful amplifier I am building. So far the results have been funny, a breakdown may occur and other times it works fine. The regulator sometimes looses its ability to regulate effectively, meaning about 2 volts or so of 120Hz ripple. Replacing the regulator fixed it on my last test. I have had stability issues on previous tests too.

Something seems to be ruining the regulator, I suspect power ups and downs. I have protection diodes across the 317. The schematic for my setup is attached. Does anyone have any suggestions for improvement?
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Old 6th February 2012, 01:19 AM   #2
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Typically one puts back-to-back 15V or so zeners from the mosfet gate to source. I don't know if this is your problem but it is easy to kill mosfets without them.
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Old 6th February 2012, 01:32 AM   #3
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By the way, I have built 300mA Maida regulators before so I don't think 600mA will be a problem with good parts selection and design.
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Old 6th February 2012, 01:34 AM   #4
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Are you testing this loaded? If not, it doesn't seem to draw enough current through the output divider to regulate correctly. You need to set it up to draw 8-10mA for it to regulate correctly unloaded.
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Old 6th February 2012, 04:19 AM   #5
tomchr is offline tomchr  United States
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You'll need a beefier cascode than the 600 V device you've got in the schematic. I've mucked quite a bit with a Maida regulator that was designed to deliver 475 V, 200 mA. My problem was that my amp has a 47 uF cap in it that ends up across the power supply on start-up. This created enough in-rush current to pop the cascode -- even when using a 900 V, 12 A (I think it was) cascode. SOA violations...

I ended up designing a floating regulator. You can find it on my website (see link in my signature below).

But I have recently been tempted to take another look at the Maida regulator again. To survive startup into a capacitive load, one has to implement soft-start, though...

~Tom
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Old 6th February 2012, 11:10 AM   #6
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Even the original bipolar version (National LB-47) had some resistance between the pass transistor(s) and the regulator input to help limit the current when shorted or suddenly seeing a capacitive load. If this design were already under load (FET on), and then saw a sudden load spike, a tube arc or something, I'd wonder if the gate charge would momentarily keep the FET on even after the Zener was pulling down the gate drive, putting considerable voltage across the 317. I'd consider adding the limiting resistor, a MOV where D11 is (maybe one of the types used to protect 5V supply lines), and possibly a diode across the FET gate resistor, cathode to the Zener, to speed up gate discharge. A Zener from the output of the added current limit resistor back to the gate could help more clearly define the current limiting (since the threshold voltage and transconductance spec may have a wide spread) and provide forward and reverse gate voltage clamping. Note that the current limiting would only be a short term protection since the power dissipation would be quite high. To be really protected against a short, foldback current limiting seems a more effective solution. Somewhere there should be limiting to prevent your output transformers from being ruined by a shorting tube. Although it might only be needed if this where a bench supply and a charged cap got connected to the supply output with it turned off, I'd plan for some additional clamping to cope with any possible spike to the 317 from C8 charging. I'd put an MOV across the transformer primary as protection against externally and internally generated transients. MOVs are handy in multiple places. Many transformers designed for heavy loads don't have very high primary inductance, and that results in a significant no load current. An MOV on the primary can help absorb the transient as a power switch is turned off. Also, the 150L20 MOV looks like a great 280 V (either direction) Zener on a curve tracer, and can dissipate about 5 Watts continuous at 85C. And they can take huge surges that would fry a Zener. Also being about 1.5 nF capacitive, they're a relatively quiet dead simple shunt regulator for screens, preamp/driver stages and perhaps a reference for a regulator. If this supply proved too unreliable, getting rid of everything past the FET and just putting two series 105L20's from the gate drive to ground, you'd have a supply of about 560V that would be extremely reliable. To any thinking of using MOVs or the fancy shunt regulator designs elsewhere on this site for screen regulation: If individual screen shunt regulators return to the cathodes instead of ground, and screens are fed with a current source setting worst-case input power (encountered at/beyond clipping with no load, prevent the rise when plate V drops to 50V or so) the total screen and shunt current at the cathode will just be clean DC, allowing the cathode resistor to be sampled as feedback to a driver. (its easier than using floating screen supplies that return to the cathodes) That linearizes the stage current and prevents the feedback from allowing the plate current to be deficient by the screen current, a separate source of distortion from current-dependent transconductance shifts. That means no more pentode screen robbing or kink induced distortions. (the return to cathodes is not used for triode mode since the screen current is part of the output in that case) I know this seems off topic, but I feel it is important to mention since it may help achieve the sonic and reliability improvements you're seeking when going to the trouble of more complex power regulation.
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Old 6th February 2012, 05:19 PM   #7
Mikha is offline Mikha  Russian Federation
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Hi all!

You will be really surprised if will try for high voltage regulator schematic like this: T-Reg - ELEKTOR.com | Electronics: Microcontrollers Embedded Audio Digital Analogue Test Measurement
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Old 6th February 2012, 06:00 PM   #8
BRSHiFi is offline BRSHiFi  United States
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What I am looking for is something to prevent ON/OFF spikes, capacitive loads, etc from ruining the regulator, which riccoruder's suggessions address. Should they be sufficient or should I add something else as well?

I also am looking for short circuit protection that protect the semiconductors until a slow blow fuse in the power transformer primart can react.
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Old 7th February 2012, 03:03 AM   #9
BRSHiFi is offline BRSHiFi  United States
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Has anyone had any experience with this type of circuit?

A New High-Voltage Regulator

I can easily simulate the circuit to see what tweaks are necessary to put out 500V at 600mA.
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Old 7th February 2012, 07:28 AM   #10
tomchr is offline tomchr  United States
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Wow... Tubecad really needs to learn how to draw schematics. Signal flow is from LEFT to RIGHT always. In a power supply the power is the signal. Hence the input voltage is on the LEFT and the output voltage on the RIGHT. Anyway...

It's not clear which one of their circuits you're referring to. There're 3~4 circuits on that page. The bottom one is fairly close to the High Voltage Regulator that I designed (see my website).

The "Simple Solid State" regulator on the Tubecad site should work as well.

You could also use a zener stack with an emitter follower. There are many options. Regardless of what you choose, I recommend implementing a soft-start so you don't blow the pass device of your regulator on start-up.

~Tom
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