Proper way of designing a power supply section - Page 1
Forum section: electronics / projects

author: RawCode
author_rank: Regular Contributor
timestamp: May 21, 2023, 09:48:29 am
content: HelloI'm designing a DC to 10 MHz low noise amplifier (in simulation less than 2nV/sqrt(Hz)), and I was going to design the power supply section.I always built power supply sections according to this scheme: (toroidal) transformer -> full-bridge rectifier -> LM7815/LM7915 -> LM7805/LM7905.Everything always worked fine, but in this case, I have the doubt that I can do better. To make everything a bit fancier, since I want a quieter power supply, I put between the +/-15V rail and the +/-5V rail a capacitance multiplier.I also always do the layout in a very broad way, avoiding the capacitors to stay close to the heat generating components.Nonetheless, I was wondering if there is a better way of building a power supply section.Should I use different ICs? Should I design a full discrete power supply?How do you design your power supplies?

author: Kleinstein
author_rank: Super Contributor
timestamp: May 21, 2023, 01:42:40 pm
content: A good amplifier should have good PSRR and thus would usually not need a super low noise supply. So the old 7815 and so on are still OK. For the more standard range a discrete build supply is usually not a good idea.One may rethink the toroidial transformer and prefer a EI type core with 2 chamber windings to get better isolation. A low noise amplifier usually does not need that much power to really care about ultimate efficiency.

author: Vovk_Z
author_rank: Super Contributor
timestamp: May 21, 2023, 03:05:22 pm
content: Low-noise demon hides in small details. My way to get a simple and cheap but still low-noise regulated power supply for small signal circuits is:1. Non-toroidal transformer to have better isolation from mains.2. RC-snubber after a secondary, together with Schottky diodes in the bridge. The large the load current - the more sense in using Schottky diodes. Schottky may not help much if a load current is very small (milliamperes) so general-use diodes may work fine too. It may need a separate investigation.3. CRC filter after a rectifier instead of a C-filter combined with a good wiring of filter caps wires.4. Good filter caps wiring is when wires go straight through caps pins.5. Possibly a transformer with separated secondary windings (and separated bridges and filters) to use two identical LM317/317L. 'Positive' IC regulators have better PSRR than 'negative' IC regulators. And with separate secondary windings, it is much easier to meet the previous requirement of good filter caps wiring.6. LM317 with an Adj cap has a bit less output noise than LM7812/7805.All this has a good synergy together.I can't remember the exact numbers now, but such described design gave me a simple small-signal power supply with something like tenths of millivolts of noise (in the 5Hz - 5 MHz range, according to my true thermal 'TrueRMS' microvoltmeter V3-57).

author: CosteC
author_rank: Regular Contributor
timestamp: May 24, 2023, 05:22:40 am
content: Nobody mention shielding. Low noise regulators, transformers, proper capacitor wiring - it is all good, yet noise, especially today, with tons of RF sources can couple over the air. Consider separate shield for power section and separate sections for device itself.Good filter on mains side will help too, there are of-the-shelf modules worth consideration.

author: srb1954
author_rank: Super Contributor
timestamp: May 24, 2023, 07:39:03 am
content: Quote from: RawCode on May 21, 2023, 09:48:29 amThere are some fantastic regulator ICs available these days which have previously unheard of power supply rejection ratios and extraordinarily low output noise levels to match. You might want to look at the Analog Devices LT3045 and LT3090 for example.These devices are not cheap but you would be very hard pressed to achieve that level of performance by any other means and for any reasonable price. However, you have to take considerable care with component selection and your PCB layout to get the ultimate performance out of these chips.

author: srb1954
author_rank: Super Contributor
timestamp: May 24, 2023, 07:47:06 am
content: Quote from: CosteC on May 24, 2023, 05:22:40 amIf you have the space consider a larger double-section mains filter with larger inductors to provide better noise rejection at the lower end of the spectrum.Many of the commodity power inlet module filters have too little inductance to provide much useful filtering below 100kHz. Although, choosing the lower current rating devices helps as the inductors in those can be higher values without saturating at the rated load current.