LM317/337 ripple/noise/oscillation

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I do have some trouble understanding the lm317/337 datasheet. What is the V in max for them ?

http://www.ti.com/lit/ds/symlink/lm317.pdf

"1 Features
1• Output Voltage Range Adjustable
From 1.25 V to 37 V
• Output Current Greater Than 1.5 A
• Internal Short-Circuit Current Limiting
• Thermal Overload Protection
• Output Safe-Area Compensation"

"9 Power Supply Recommendations
The LM317 is designed to operate from an input voltage supply range between 1.25 V to 37 V greater than the
output voltage. If the device is more than six inches from the input filter capacitors, an input bypass capacitor, 0.1
μF or greater, of any type is needed for stability."

Am I wrong to think that the vin max is 37V max out +37max difference ?
 
asuslover, you are right. It is the relative voltage on the LM317/LM337 that counts. Max. 40 volt between input and output (and 41.25V between input and the control pin).
Your input voltage may be 200V and your output voltage 170V. The LM317 will not notice the high potential to ground because it is not connected to ground.
 
Here are my suggestions (appended).

First schematic: LM317 buffered with a TIP3055 (15A max). No fine regulation of the output so the output may sag about 0.4V from 2A to 10A loading. Simple circuit and no need to adjust a regulation loop.

Second schematic: LM317 buffered with a TIP3055 (15A max) and including a fine regulation loop. More complex and a need to adjust the regulation loop.

Third schematic: The traditional way to buffer a voltage regulator with a PNP-transistor (TIP2955/15A). The problem is to "ẗame" the loop if it oscillates.

Fourth schematic: LM317 for high voltage use with power zener diodes for start-up and shut-down protection.

Now to the more problematic part - your transformer:
When a transformer is loaded much below its capacity, the buffer capacitor(s) "rides" on top of the rectified sine-wave and we get the transformer RMS voltage multiplied with 1.4 (minus the diode voltage drops). The more we approach the nominal loading of the transformer, the more the transformer voltage drops (winding resistance) and the buffer capacitors start having important ripple. At the nominal loading of the transformer, the voltage at the buffer capacitors will be only little above the RMS voltage of the transformer and with important ripple.
The schematics I suggest will require around 3V (input-to-output) for regulation. If you want 35V output, the voltage at the buffer capacitors must not drop below 38V, including ripple.
I doubt a 30V transformer can maintain a rectified voltage above 38V (at any moment) when loaded with 10A. My guess is that you need a transformer with close to 40V RMS and quite some buffer capacitors.
 

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You said 10 Amps before! 10 Amp is the absolute maximum I would use for a 15 Amps transistor.

I would never use a transistor close to its maximum rating. The gain drops and the Vbe and Vsat increase. If you use more transistors in parallel, use an emitter resistor for each transistor. Typically 0.1 Ohm. It will cost you a bit more in voltage drop but they are needed to make the transistors share the current.

More transistors sharing the current makes cooling easier.
 
Linear regulators are great for signal circuits, for 100mA or less.

For 1A or more consider switching regulators. In switching power conversion applications a 15A transistor can be used to pass 15A or more, depending on max. duty cycle.

"Madam",

Linear regulators are great for low power applications because of simplicity and price. Linear regulators are great for low noise applications because they have no inherent switching noise (apart from the input rectifiers of course) and can be designed for very low ripple. Linear regulators are great when fast step response is needed.

SMPSs are great when power efficiency is important or excessive heating must be avoided. In the present case you may be right, depending on the actual purpose. SMPSs are great when the performance is less critical and you just need some power at an attractive price. SMPSs are great when you need a low weight system or a compact system.

Advantages and disadvantages for both.
 
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Thanks, a very informative post. Will it always be better to use an even more overdimensioned transformer?

In (technical) theory, the bigger the better. More stable output voltage(s), more power margins, less heating etc.
Evidently over-dimensioned transformer have disadvantages as well: costs, weight, size etc.

Alway think about what is the use of the transformer (continuous loading? an important crest factor? cooling possibilities? results of voltage drops? importance of price? etc.) and you find the right transformer for that need.
It is (once more) a question of balance.
 
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In my experience bench power supplies are rarely demanded more than +/-30V +/-1.5A for experiments where output ripple matters (signal).

For higher current and voltage a very clean (linear) power source is rarely required.

There is one winner technology for fine things and another for coarse things.
Bigger is not necessarily better. It takes space.
 
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