Reducing diode Pd on ATX PSU - Page 1
Forum section: electronics / projects

author: GelatinCapacitor
author_rank: Contributor
timestamp: August 19, 2023, 02:41:29 am
content: I got a Delta PSU model GPS-350AB.I wanted to find a schematic diagram of it, but couldn't. I did find a few ones though, for similar models, that resemble mine very closely, but definitely are not the same. Still useful.They all feature this circuit as the secondary:It made sense to me.Then I looked inside my PSU and found that the secondary looks like this instead:Made sense to me that it works, and my conclusion was that the aim is to put away unnecessary stress on three diodes when only two are necessary.But then I thought, then why didn't they do it this way to begin with?:So I made that change on my PSU and it works!I later got and looked inside another ATX PSU: HP-D3057F3R.It's secondary is like this:What with all of these inconsistencies?Don't these designs put unnecessary stress on the +5V diodes?My guess is I'm missing something.

author: Konkedout
author_rank: Frequent Contributor
timestamp: August 19, 2023, 04:19:04 am
content: I don't want to take the time to fully understand this design, but I have been designing power supplies (more or less) for 47 years, and I did it for Delta for a couple of those years. If I understand correctly, one of the main variables is whether the 12V (really a 7V?) winding is connected in series with the 5V winding, or if it is connected in series with the 5V forward rectifier diode. The latter seems to waste power by rectifying twice. The +12 current passes through two diodes instead of just one.But it can improve load cross regulation. Typically the +5 is the main regulated output, while the +12 is not directly regulated. In the situation where +5 load current is low and +12 load current is high, there will be a tendency for the control loop to throttle back (so to speak) and the +12 voltage will sag. But by putting all +12 current through the +5 diode, the control loop is actually increasing Vout in order to compensate for increased loss in the +5 diode. The result is that the +12 output can end up higher, even though it is now passing through two diodes. I have designed a lot of flyback converters that way, and it is a good technique.Another less likely issue is that you get less peak-peak swing driving the +12 diode. So it may be possible to use a lower voltage schottky diode with lower forward voltage drop.

author: amyk
author_rank: Super Contributor
timestamp: August 19, 2023, 04:20:30 am
content: When you said it "works", did you try testing at full load?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: August 19, 2023, 05:56:01 am
content: "Works", perhaps, but did you check D2 power dissipation?If not, try it in a simulator: see what I mean by this.Tim

author: GelatinCapacitor
author_rank: Contributor
timestamp: August 19, 2023, 07:10:46 am
content: Thanks guys.@KonkedoutI did consider it had to do something about sensing and regulation, I wonder exactly how much it actually differs.@amykSadly I did not try full load. My load only consisted of 1Ω on each output.@T3sl4co1lSorry, I couldn't see your point.How flawed would this simulation be? (attached falstad file):After adding the Pd averages for every diode on every circuit, top to bottom respectively:13.6 W13.2 W9.38 W9.41 W

author: T3sl4co1l
author_rank: Super Contributor
timestamp: August 19, 2023, 04:31:40 pm
content: Is that 40Hz and 1mH? These are not realistic values.Tim

author: GelatinCapacitor
author_rank: Contributor
timestamp: August 19, 2023, 06:07:49 pm
content: Hi Tim.I tried some variations on the values. Changing the inductances made no difference once the current was stable. Changing the frequency changed the Pds but the overall average pattern across all circuits remained the same.Here with the generators having negative peaks, at 40kHz, the total averages (attached file):28.4 W24.5 W19.6 W19.8 WThis is not a proper study. I'll keep looking online for proper studies on this.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: August 19, 2023, 06:12:31 pm
content: Oh, of course; there's no coupling. You have to use a transformer in Falstad.Tim

author: GelatinCapacitor
author_rank: Contributor
timestamp: August 19, 2023, 06:29:47 pm
content: Thanks Tim.I tried again with transformer (attached file).28.5 W24.1 W18.8 W18.9 WI was also having fun reading the duscussion on this forum about diodes in parallel, since the last PSU I mentioned here, uses that.

author: magic
author_rank: Super Contributor
timestamp: August 19, 2023, 07:43:23 pm
content: Quote from: Konkedout on August 19, 2023, 04:19:04 amThat's neat.I think I can see how it works with flyback - the 5V winding voltage will increase by one diode drop, while the 7V winding proportionally by more than one drop, which fully compensates for the insertion of D1 into the circuit and then partly for D3. And if the second winding is more than 10V then it seems we could even get negative output resistance - fun

author: T3sl4co1l
author_rank: Super Contributor
timestamp: August 19, 2023, 09:56:23 pm
content: No screenshot so I guess I have to actually open the thing now--WTF the sources are all DC offset? How the hell ya gonna get that off the transformer?!You don't have the right ratio on the choke either. For the 5, 7 and 12V flyback cases you need the ratios specific for each one. Or the diodes don't share. But the ratio is fixed by the forward voltage, so imbalance is (I think, offhand) impossible to eliminate in such asymmetrical configurations.Tim

author: T3sl4co1l
author_rank: Super Contributor
timestamp: August 19, 2023, 10:26:05 pm
content: There, this makes a lot more sense.https://tinyurl.com/2dnbgkzlTim

author: mzzj
author_rank: Super Contributor
timestamp: August 19, 2023, 10:30:51 pm
content: Quote from: Konkedout on August 19, 2023, 04:19:04 amThat was long time ago.+12v has been the "main" regulated output on ATX power supplies for a while (Since ATX 2.x specification in 2003 or so)At least OP's last schematic appears to be from similarly ancient history based on weak 12v output.