HP Common Slot PSU reverse voltage protection - Page 1
Forum section: electronics / general

author: jsilva
author_rank: Newbie
timestamp: October 06, 2020, 05:53:33 pm
content: Hello everyone,I am planning to use two HP Common Slot PSUs (HSTNS-PL18) in series for 24V by floating one of the units, like many people have done in the past.I will be monitoring them via I2C and turning the main output on/off as needed (using a circuit powered by the 12VSB of the bottom unit).The problem is that there can and will be a delay between both units turning on. Although short, this delay might lead to a reverse voltage being present at the terminals of the last supply to turn on. I do not have a diode that can handle 60A, and connecting diodes in parallel is not a great idea, so I wanted to know if the supplies have internal reverse voltage protection.Has anyone tested this (either intentionally or by accident - lol)?Best regards,João

author: tom66
author_rank: Super Contributor
timestamp: October 06, 2020, 07:30:04 pm
content: Almost no commodity SMPS has reverse voltage protection on the output. What is present on the output is roughly equivalent to a large electrolytic capacitor, plus feedback logic and any ancillary loads (e.g. fans, DC-DC converters for other rails)Generally for a typical converter (forward, LLC resonant, a few others), where the output stage resembles a non-synchronous buck converter, the transformer winding itself will be protected from reverse polarity. This is necessary due to how the secondary side operates. However many newer power supplies are using synchronous rectification featuring a MOSFET and intelligent logic, which really will not like reverse polarity at all.Now I do have to ask a few questions...- Are you isolating the ground from one PSU? This includes the case. Often the -ve is connected to mains earth which is also connected to the case. So you need to physically isolate the supply too.- Why do you think the other PSU will experience reverse polarity? Unless your load injects a huge amount of current into the other rail, this should not happen. So, can you delay the start up of your load?You could possibly use a pair of MOSFETs to isolate the power supply outputs until they are ready then turn both on via a ramp up circuit (a few milliseconds to charge a capacitor on the output)

author: jsilva
author_rank: Newbie
timestamp: October 06, 2020, 08:52:17 pm
content: Quote from: tom66 on October 06, 2020, 07:30:04 pmThanks.Like I said, I will be floating one of the supplies, this means disconnecting the output negative terminal from the case, but keeping the mains earth ground connected to the case.From what I interpret from this drawing, if PSU1 is off (i.e. not forcing any voltage at its terminals), but still connected to the circuit, and the load is still connected, there will be the whole voltage of PSU2 present at PSU1 terminals, but in reverse.At this point the load is the only thing that limits the current, and if it does not limit the current enough, there will some potential to blow the polarized caps at the output of PSU1, etc...Am I missing something?I could place a lower rated diode at the output of both supplies, in reverse, and use a relay to switch the load (cheaper solution IMO, if we consider I will be pulling about 30A).This way the diode only has to deal with low currents, granted that I only switch the relay on once I am sure both supplies are on. Right?EDIT: I just took a measure, I need advice about my interpretation of the measure tho.I put my multimeter in diode mode and measured across the supply terminals, plus to plus, minus to minus. I can see the multimeter voltage rising. I assume this means the capacitors on the output of the supply are charging.If I reverse the polarity, I only see the voltage climbing up to about 0.5V, which is clearly a diode drop. Now, what would happen if I there wasn't a diode at the output of the supply? Would the caps charge negatively? Up to what voltage level? Is it possible that a capacitor reverse polarized only charges up to a certain level that resembles a diode drop?If I measure an electrolytic cap in reverse with the meter in the same mode, the voltage quickly climbs up to out of the range of my meter.Does this mean there is in fact a diode in reverse at the output of the supply?

author: tom66
author_rank: Super Contributor
timestamp: October 07, 2020, 07:17:19 am
content: If you push current that way through the power supply, all you will be doing is charging the output capacitors normally. So no reverse polarity should occur. That would be my assessment from what I can see.P.S. Many electrolytic capacitors have a diode-like tendency in reverse. 0.5V or so sounds about right for the maximum reverse bias. Many capacitor manufacturers state somewhere between 0V and 1V as the acceptable limit for bipolar caps.