4 ms·

> Is the design solution to use a FET with a lower Rds at 5V Vds for smaller heat dissipation? Honestly - Its to not cheap out on the design and use a proper h

by mbell 14y ago

> Is the design solution to use a FET with a lower Rds at 5V Vds for smaller heat dissipation? Honestly - Its to not cheap out on the design and use a proper hot-swap controller. Examples (many companies make these): http://www.ti.com/ww/en/analog/power_management/system-protection-hot-swap.html http://www.ti.com/ww/en/analog/power_management/system-prote... You can rig up something slightly better by designing a current sense circuit with feedback into the FET gate if your adventurous. In either case the goal is to limit and control inrush current during power on or disk insertion. Based on what you've said I doubt the steady state current of the disks is the problem at all. It just sounds like the 3TB drives have a higher inrush current during power up and its either high enough or lasts long enough to blow the FET junction.

cyanoacry 14y ago
The funny thing is, I'm not sure that the hot-swap circuitry is even necessary in the first place due to the connector design. The SATA spec defines "pre-charge" voltage pins that are connected after ground, but before any of the other voltage pins are connected. The idea is that by inserting a small (10 ohm) resistor, you can limit inrush current to a tolerable value while capacitors charge and regulators start up, and then when the other pins connect a couple of milliseconds later, the drive gets the proper low-impedance power connection. Do you know if implementing pre-charge via connector mechanics obviates hot-swap protection circuitry? Supermicro seems to have hot-swap protection on their backplanes as well, but I haven't had a chance to closely inspect it.
- mbell 14y ago
  It can partially solve the inrush current issue. There are still timing issues, if you slam a drive in you can shorten the timing of pin contact so much that its not effective. You also gain a real benefit in that one drive failing by something major, like a straight short, won't take out your entire system as the controller will detect the current spike and cut off the drive. Most hot-swap controllers also provide additional protection against things like pin reversal, ESD and accidental shorts on insert that you can't solve with just the connector. It all comes down to how robust you want the design really, how many failure modes you wish the system to survive. EDIT: Those connector pins aren't normally solid gold. They are deposited metal (copper or tin) with electroplating of a few microns of gold on the surface. Another reason to control insertion spikes is that when the first power pin hits you'll get a little arc (spark). This can cause small damage to the pins in the form of small chipping of the coating and/or carbon deposits(or oxidation of some metals). This contributes to a reduction in the number of insertions cycles the connectors will survive.

by mbell 14y ago

cyanoacry 14y ago

The funny thing is, I'm not sure that the hot-swap circuitry is even necessary in the first place due to the connector design. The SATA spec defines "pre-charge" voltage pins that are connected after ground, but before any of the other voltage pins are connected. The idea is that by inserting a small (10 ohm) resistor, you can limit inrush current to a tolerable value while capacitors charge and regulators start up, and then when the other pins connect a couple of milliseconds later, the drive gets the proper low-impedance power connection. Do you know if implementing pre-charge via connector mechanics obviates hot-swap protection circuitry? Supermicro seems to have hot-swap protection on their backplanes as well, but I haven't had a chance to closely inspect it.

mbell 14y ago
It can partially solve the inrush current issue. There are still timing issues, if you slam a drive in you can shorten the timing of pin contact so much that its not effective. You also gain a real benefit in that one drive failing by something major, like a straight short, won't take out your entire system as the controller will detect the current spike and cut off the drive. Most hot-swap controllers also provide additional protection against things like pin reversal, ESD and accidental shorts on insert that you can't solve with just the connector. It all comes down to how robust you want the design really, how many failure modes you wish the system to survive. EDIT: Those connector pins aren't normally solid gold. They are deposited metal (copper or tin) with electroplating of a few microns of gold on the surface. Another reason to control insertion spikes is that when the first power pin hits you'll get a little arc (spark). This can cause small damage to the pins in the form of small chipping of the coating and/or carbon deposits(or oxidation of some metals). This contributes to a reduction in the number of insertions cycles the connectors will survive.