Switch mode PSU with linear output stage - Page 1
Forum section: electronics / testgear

author: PinheadBE
author_rank: Regular Contributor
timestamp: September 11, 2023, 11:49:45 am
content: Hi,I vaguely remember having seen a design of a PSU combining the best of both worlds, with a first switch mode stage for efficiency, followed by a linear LDO regulation stage for better noise performancesThe two stages were kind of syncronized, so that the drop out voltage on the linear stage always remains minimized.Does this ring a bell to anyone ? Anyone remembers having seen this ?Thanks a lot.

author: nctnico
author_rank: Super Contributor
timestamp: September 11, 2023, 11:53:49 am
content: IIRC R&S has these kind of PSUs

author: PinheadBE
author_rank: Regular Contributor
timestamp: September 11, 2023, 12:01:44 pm
content: I was looking for a design so that I could add a linear low-loss stage to a noisy switch mode PSU

author: Kean
author_rank: Unknown
timestamp: September 11, 2023, 12:07:18 pm
content: There are a bunch that work like this. The switching section is set to be a few volts above the linear output stage.I believe the R&S HMC804x PSU is considered a "linear switching" power supply, so high efficiency, low weight, and low-ish noise. There are probably others they have like this.The EEZ BB3 DCP405 module is an open design using this concept so you can check the schematics and I think they published some design notes on their website. Seehttps://www.envox.eu/dc-power-modules/dcp405-power-module/

author: coromonadalix
author_rank: Super Contributor
timestamp: September 11, 2023, 12:42:59 pm
content: some Kikusui has this topologyphase controlled / buck / and end with some linear section

author: sonpul
author_rank: Regular Contributor
timestamp: September 11, 2023, 01:56:58 pm
content: Riden has this topology with the letter P in the marking. For example Riden RD6006P. This is a great option if you need an inexpensive solution.

author: robert.rozee
author_rank: Frequent Contributor
timestamp: September 11, 2023, 03:15:53 pm
content: see:https://sound-au.com/articles/preregulators.htm#s5cheers,rob :-)

author: PinheadBE
author_rank: Regular Contributor
timestamp: September 13, 2023, 01:13:17 pm
content: Quote from: sonpul on September 11, 2023, 01:56:58 pmI knew Riden made good modules lately, but I wasn't aware it was *THAT* complex. Thanks for the schematics !Quote from: robert.rozee on September 11, 2023, 03:15:53 pmVery interesting in my case where I already have the Switching part ! I surely will experiment something this way ! Thanks !

author: sonpul
author_rank: Regular Contributor
timestamp: September 13, 2023, 02:25:40 pm
content: Quotekeep in mind, the Riden RD6006P regulation is made relative to"+". This means that the output of the main power supply " - " should not connect with ground, as sometimes happens. Because then connected to the oscilloscope scheme with grounding will make a short circuit.

author: PinheadBE
author_rank: Regular Contributor
timestamp: September 13, 2023, 10:10:46 pm
content: Yes, thanks. I will keep that in mind.Part of my lab bench is equipped with an isolation transformer. That shoud help keeping the magic smoke within my scope.

author: DrGeoff
author_rank: Frequent Contributor
timestamp: September 13, 2023, 10:21:31 pm
content: This was usually done in SMPS that had a primary rail and several auxilliary rails. Back in the 80's we had SMPS that had a beefy 100A 5V rail and a couple of 1A +/-12V rails. The SMPS regulation was performed using feedback from the 5V rail and the transformer windings for +/-12V did not have the same load regulation as the 5V rail so were fed through a the 7812/7912 regulators to maintain good regulation. Linear regulators won't help you with noise, it passes straight through them as the frequencies are usually very high (>100kHz). Usually good ferrites and chokes were used to keep the noise down.

author: tridac
author_rank: Regular Contributor
timestamp: September 13, 2023, 10:35:22 pm
content: There were several of that type, even decades ago. Some even used a thyristor as a preregulator. The key thing is that the pre and post regulators track, or are in sync. Maintaining a constant voltage drop across the linear part to minimise power dissipation and improve efficiency. The main reasons for such a design approach, while providing a low noise at the output...