VOGONS


First post, by adalbert

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It would be really helpful if someone with actual engineering knowledge could take a look at this design 😀 I want to include some overvoltage protection.

I am trying to design something like pico PSU but for AT mainboards with better +5V line and negative rails. I already made a power supply for T3200SXC (DIY AT DC-DC power supply (replacement for Toshiba T3200SXC and others)), but it was non-standard and here I try to make generic AT power supply.

Pololu step down converters would be used for +12V,+5V and MC34063 + LM2662 for -12V,-5V.
Pololu converters claim to have internal overvoltage protection, but I would like to add something extra because I don't know about its reliability. If a reliable and reasonably cheap external overvoltage protection could be developed, then maybe this power supply could just use $7 buck converters like this one https://www.ebay.com/itm/DC-DC-15A-Buck-Adjus … le/113797338354 and be really inexpensive and safe. Pololu 15A buck converter costs $40.

Here is the schematic of current version (i didn't build/test this one):

converter3.jpg
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Besides buck converters, it uses a microcontroller, relay, TVS diodes and crowbar circuit.

Microcontroller has separate voltage stabilizer and diode for reverse input protection. If input voltage is OK it would engage the relay and power-up voltage converters. It measures positive voltages on analog pins and generates power good signal. If something goes bad, it disengages the relay. It also triggers crowbar circuit. Crowbar circuit is also separately triggered by zener diodes. It may blow the fuse or only clamp the voltage, depending on how quickly the relay is disengaged.
On negative rails there are only 125mA fuses and TVS diodes.

Does it make sense? Is the crowbar circuit wired properly? 😀

Repair/electronic stuff videos: https://www.youtube.com/c/adalbertfix

Reply 1 of 7, by pentiumspeed

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Best ones is analog circuit and you can measure current via current shunts. Also all in one DC to DC ICs also have over current built in, some are adjustable by setting a resistance value according to the datasheet.

Omit the microcontroller, this is central source of crashes and failures.

Cheers,

Great Northern aka Canada.

Reply 2 of 7, by Horun

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Wow brings some memories of the early 80's using SCR's and TRIAC's for overvoltage protection. The use of a SCR as a crowbar works well and think your use of the Zener's back to source of each could work. Has been a long time but theoretically each output should have it's own OV protection so you would build two, one for 5v and one for 12v. Of course am basing that on long forgotten memory and a standard circuit like below.
After a longer look: Using the micro controller to shut the relays off if either goes OV is a good idea if I follow your diagram proper. The crowbar should work ok even if the relay does not disengage, though would use two with big heatsinks just in case they have to pass a bunch of current for a while. Just a thought.

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Hate posting a reply and then have to edit it because it made no sense 😁 First computer was an IBM 3270 workstation with CGA monitor.

Reply 3 of 7, by adalbert

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Thanks for the sugesstions,

pentiumspeed wrote on 2020-08-15, 01:21:

Omit the microcontroller, this is central source of crashes and failures.
Cheers,

I can look for some ICs which are designed to monitor voltages, I've seen some in old power supplies ("housekeeping ICs") but I would need to find something which is easy to obtain. I also understand that it would be possible to use discrete components for everything, but I'm afraid that I would miscalculate something and it would be painful to get everything working.

Horun wrote on 2020-08-15, 01:53:

Has been a long time but theoretically each output should have it's own OV protection so you would build two, one for 5v and one for 12v. Of course am basing that on long forgotten memory and a standard circuit like below.

I read some opinions on that and sometimes it was recommended to put the crowbar next to the input, because if it was on the output the current would be limited by the converter circuitry or even by longer PCB traces, and the current passed through the crowbar could be then too small to blow the fuse. But other opinions stated that not having crowbar on the output might allow some overvoltage to pass, because it won't discharge output capacitors. I wonder if I could just use some TVS diodes or high power zeners to clamp 5V and 12V rails for that brief moment. I should probably just test different variants.

Repair/electronic stuff videos: https://www.youtube.com/c/adalbertfix

Reply 4 of 7, by Horun

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Yes you would want some fat traces to the crowbar. In some older ATX supplies the TI TPS3510P was used for OV protection and are still available for cheap, an AT equivalent would better suit your project though if looking into other methods of OV protection.

Hate posting a reply and then have to edit it because it made no sense 😁 First computer was an IBM 3270 workstation with CGA monitor.

Reply 5 of 7, by adalbert

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Horun wrote on 2020-08-16, 15:28:

TI TPS3510P

thanks, maybe I will use it and omit the microcontroller 😀 i will look for 5V/12V-only equivalent, but I guess that I could just use a zener, resistor divider or small stabilizer to create a "fake" 3.3V line required for that chip to work. Or maybe even an LED in series.

Repair/electronic stuff videos: https://www.youtube.com/c/adalbertfix

Reply 6 of 7, by gdjacobs

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Horun wrote on 2020-08-16, 15:28:

Yes you would want some fat traces to the crowbar. In some older ATX supplies the TI TPS3510P was used for OV protection and are still available for cheap, an AT equivalent would better suit your project though if looking into other methods of OV protection.

The TI TPS3510P doesn't provide OCP directly, only indirectly via UVP. More sophisticated ICs (WT7505, for example) provide OCP as well. Most of the more recent ICs used in power supplies are produced and sold out of Taiwan and you'd likely have to order them by the truck load.

Where were you thinking of sourcing your components? Digikey, Element14, Mouser, etc. all have tech support people that should be able to give you a hand if you outline your requirements.

All hail the Great Capacitor Brand Finder

Reply 7 of 7, by Horun

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gdjacobs wrote on 2020-08-21, 02:36:
Horun wrote on 2020-08-16, 15:28:

Yes you would want some fat traces to the crowbar. In some older ATX supplies the TI TPS3510P was used for OV protection and are still available for cheap, an AT equivalent would better suit your project though if looking into other methods of OV protection.

The TI TPS3510P doesn't provide OCP directly, only indirectly via UVP. More sophisticated ICs (WT7505, for example) provide OCP as well. Most of the more recent ICs used in power supplies are produced and sold out of Taiwan and you'd likely have to order them by the truck load.

Where were you thinking of sourcing your components? Digikey, Element14, Mouser, etc. all have tech support people that should be able to give you a hand if you outline your requirements.

Not going to argue that other IC's have more OVP, UVP, OCP built in but the TI TPS3510P datasheet is quite clear that the OVP is not UVP dependent on each other from what the data sheet says.. just my opinion. Added: Yes most suppliers do have fairly good tech support, Digkey and Mouser seem too from my little experience with them.

Hate posting a reply and then have to edit it because it made no sense 😁 First computer was an IBM 3270 workstation with CGA monitor.