Let’s talk a bit about RB power voltage requirements.
I made some study about poe power handling inside routerboards in order to verify that they can be safely powered with meanwell AD-155B “24V” UPS power supply.
The AD-155B gives 28Vdc power output when it’s adjusted to provide correct 27.2V end-charge to 24V battery (13.6V x 2).
So, I examined RB912-2HPND, RB750UP, RB750, SXT lite5.
RB912-2HPND is rated 30V max by Mikrotik, but it’s LSP5523 power chip is rated 27V, so max safe voltage will be 27.5V (with poe diode drop voltage).
RB750UP is rated 30V max by Mikrotik, but it’s LSP5502 power chip is rated 27V, so max safe voltage will be 27.5V (with poe diode drop voltage).
RB750 is rated 28V max by Mikrotik, but it’s LSP5526 power chip is rated 23V, so max safe voltage will be 23.5V (with poe diode drop voltage).
SXT lite5 is rated 30V max by Mikrotik, but it’s LSP5526 power chip is rated 23V, so max safe voltage will be 23.5V (with poe diode drop voltage)!!!
So, Mikrotik support, why are you over-rating poe requirements of your devices?
Do you think it’s safe to play h24 @ 28Vdc an SXT lite5 which power chip is rated 23V and absolute maximum 25V ?
Give me some explanation about this.
Thanks
Manufacturer guarantees normal operation of the parts at voltages we list. They are tested to withstand these loads in our implementation.
As you list that your voltage input will be 27.2V - that is absolutely fine and will not impact longevity of the router.
Also, there is overvoltage protection in place to ensure that router is not damaged with voltages that are too high (of course if you do not reach over maximum what protection can withstand). Cutoff usually is at 30V for low voltage products.
It’s not completely right. Costant voltage from AD-155B would be 28V.
Does RBSXT5HPnD with its LSP5526 power chip (23V, max 25V) would survive long time @ 28Vdc?
Why didn’t you use at least the same chip of RB750UP (LSP5502 step-down, rated 27Vdc) ?
due to lower current requirements.
Yes, pheraps you’re right. But I don’t know any relation between current requirements and mosfet’s breakdown voltage. High voltage near breakdown (which limits the max voltage rating of switching devices) occurs during “OFF” cycles of switching square wave, so it isn’t related to current flowing during “ON” cycles.
Working near breakdown voltage means that possible spikes would kill switching devices, reducing their life expectancy…
And what about TVS diode in parallel with poe input? What kind of overvoltage and spikes protection do you use?
TVS diodes did not do their job!
http://i.imgur.com/xfBYZxl.jpg