3 ms·

The FETs blew as soon as we turned on the case, oddly enough, and even if we had a single drive plugged in, that FET would blow by itself. This is even the case

The FETs blew as soon as we turned on the case, oddly enough, and even if we had a single drive plugged in, that FET would blow by itself. This is even the case when we took the backplanes out and started testing them individually in free air in the server room, where ambient is more like 18C instead of 25. Is the design solution to use a FET with a lower Rds at 5V Vds for smaller heat dissipation? But more importantly, is there a standard current that most chassis are tested to? I would expect all SATA hot-swap bays would support all SATA drives on the market, since nobody ever gives power dissipation or consumption figures.

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.
digikata 14y ago
My thought was that even though it was the Norco hardware that was blowing out, it could be the 3T drives are the thing that are somehow exceptional. Their rated current is 0.75A, but what do they draw and for how long in the initial power on surge?
- mbell 14y ago
  Well, there are a couple of surges not dealt with here. The first and likely highest current spike is the charging of all the capacitance in the hard drive. I've never looked but I would imagine the motor driver circuit and the electronics have fairly large capacitors on the input power rails. This should be relatively short lived but higher Rds from a crappy MOSFET would increase the charging time. The second is everything powering on, there are likely secondary regulators for the electronics (probably needs 1.8 or something not 5 or 3.3) and they could be regulating the 12V rail to something more controlled for the spindle motor as well. Those switchers coming online and charging capacitors on the output side will cause a bit of a spike also. Then there is the spike from firing up the spindle motor and the head servo, likely not insignificant. I have no idea if there is an inrush current / duration limit in the SATA/SAS spec or not, I would think so but I've never seen a drive manufacturer quote these numbers.
michaelt 14y ago
I've done some work with high current motor control boards - one of the problems we've had is '100 Amp' boards with capacitors on - when first plugged in, they draw more than 100 Amps as the capacitors charge. If you have spare boards and a hankering for destruction, you could repeat your test with resistors instead of hard disks, to verify the current drawn by the hard disk is as advertised.

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.

digikata 14y ago

My thought was that even though it was the Norco hardware that was blowing out, it could be the 3T drives are the thing that are somehow exceptional. Their rated current is 0.75A, but what do they draw and for how long in the initial power on surge?

mbell 14y ago
Well, there are a couple of surges not dealt with here. The first and likely highest current spike is the charging of all the capacitance in the hard drive. I've never looked but I would imagine the motor driver circuit and the electronics have fairly large capacitors on the input power rails. This should be relatively short lived but higher Rds from a crappy MOSFET would increase the charging time. The second is everything powering on, there are likely secondary regulators for the electronics (probably needs 1.8 or something not 5 or 3.3) and they could be regulating the 12V rail to something more controlled for the spindle motor as well. Those switchers coming online and charging capacitors on the output side will cause a bit of a spike also. Then there is the spike from firing up the spindle motor and the head servo, likely not insignificant. I have no idea if there is an inrush current / duration limit in the SATA/SAS spec or not, I would think so but I've never seen a drive manufacturer quote these numbers.

michaelt 14y ago

I've done some work with high current motor control boards - one of the problems we've had is '100 Amp' boards with capacitors on - when first plugged in, they draw more than 100 Amps as the capacitors charge. If you have spare boards and a hankering for destruction, you could repeat your test with resistors instead of hard disks, to verify the current drawn by the hard disk is as advertised.