Electronics > Beginners

Linear PSU with modern components, will it matter ?

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BravoV:
Seeing lots of threads on building linear lab psu, and most are based on old proven design, like 723 and etc.

I've been wondering, if using current/latest technology advancements at today components on building it, will it make any significant differences ? And at which parts of PS performance that stand out ?

New components I'm talking here are at all fronts, say like starting like much improved op-amps, better performance as using modern process on the semiconductor or new material at discrete components e.g.: BJT, mosfet, better temp-co or polymer cap, cheap & fast MCU and etc, that practically not existed decades ago.

Ignore the cost part in this discussion please.

Love to hear from experienced EEs thought/idea/insight on this matter.

Kleinstein:
The main part of a linear power supply does not need modern parts.  The usual way to do it is using reference OPs and power transistors (either BJT or MOSFETs).  The modern OPs might give a little better precision.  Newer DACs and ADCs makes it more practical to use digital set points and digital display instead of the old way ten turn pot and analog meter. However good ADCs where already available in the 1980s.

Today one might chose to use a SMPS instead of the classical heavy iron transformer. This also make pre-regulation more practical.  However it still depends on the power level and required noise.

The LM723 was never intendent for a lab supply. The more lab supply like chip was the long obsolete MC1466.

An advantage one has today is that one can use a simulation to optimize a circuit that can be still quite conventional like build 30 years ago. So less real world tweaks needed.

tszaboo:
Well, for one, transistor packaging has come a long way, so while a an IRF540 in TO220 has something like 1.5K/W temperature coefficient, a modern FET in the same package could be 0.5K/W which means you need les transistors if you have the cooling for it. You have higher precision ADCs and DACs, so faster control loop can be built around it, and more accurate voltage.
In the end, you get something like a Keithley 2280

David Hess:
I would say that error amplifiers peaked not long after the OP-07 and similar parts.  Once  you get to 100nV/C of offset drift, chopper stabilized amplifiers do not help much in a large design because external thermocouple effects.  Even in the 1970s there were parts almost that good.  I have made some incredibly accurate power supplies with parts like the LT1007 and OP-27 taking advantage of their low input noise and high open loop gain.

Ring emitter bipolar transistors for low noise and fast response have been around almost as long.  Power MOSFETs are faster if you drive them hard enough but also higher noise.

There are more precision references available now but I think the availability of precision resistors has had a greater effect.  I think there was a better selection of good potentiometers in the past though.

So to sum up my thoughts, I think the design and implementation has been more important than the parts since the 1970s.

David Hess:

--- Quote from: blueskull on December 15, 2018, 02:49:45 am ---Faster opamps may introduce instabilities. You need to slow it down for stability. But low noise opamps are very appreciated.
--- End quote ---

The good news is that slowing them down does not increase their noise.  Trying to lower the noise too much results in such a low impedance for the feedback network that self heating ruins the gain stability if not the DC stability; this could matter for a programmable power supply.


--- Quote ---Polymer caps/MLCCs may also introduce instabilities. You need some feedback trick to introduce more output ripple into the loop for stability.
--- End quote ---

Power supplies designed for the best constant current performance minimize output capacitance which means like a 0.22 microfarad film capacitor per amp with a series resistor instead of the more typical 47 to 100 microfarads per amp.

But the trick I like now is to use high side current sensing with the current shunt on the output where it contributes to the output capacitor's ESR if AC feedback is taken before the current shunt.

There is another trick I have not tried where the error amplifier is compensated with *less than* 90 degress of phase shift increasing the phase margin so that a zero ESR output capacitor does not cause oscillation.  I read about this in connection with ATE drivers which have to deal with unpredictable loads.

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