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Sorry if this question is not perfectly on topic, but I could not locate a better forum:

I have a USB memory stick ("SanDisk Ultra Fit (0781:5583), but actually many sticks behave similar) that is usually connected via a USB-2 cable to my PC. I noticed that the tiny stick gets quite warm, even when sitting idle.

Today I had connected the same stick to a USB-3.2 docking station, and when pulling it out I had realized that it was almost so hot that I could not touch it.
Again the stick was idle most of the time (I know that SSDs become quite hot when writing data, but not when "sitting idle").

I'm by no means a USB specialist, but my thought are like this:

  • Both, USB-2 and USB-3 power the stick with 5V, and the controller when being idle should do the same things, whether connected to USB-2 or USB-3 when idle.
    So why is it getting much hotter when connected to USB-3?
  • What does such a controller do when "sitting idle"?
  • Don't high temperatures increase the leakage current, shortening the time the bits are safely stored in the flash cells?

Windows (using USB-2 cable) lists these device properties:

Aktueller Energiestatus:
D0

Energiekapazität:
00000019
PDCAP_D0_SUPPORTED
PDCAP_D3_SUPPORTED
PDCAP_WAKE_FROM_D0_SUPPORTED

Energiestatuszuordnungen:
S0 -> D0
S1 -> D2
S2 -> D2
S3 -> D2
S4 -> D2
S5 -> D3

Information I found was not really helpful:

Updates:

As it had been suspected that the convection via cable makes a difference, I connected the stick to the docking station via an USB-3 cable. The temperature measured was about 60°C of the surface (using a contact-less infraread sensor).

Now a really curious fact:

As I had experienced a rather hot USB stick (Kingston Data Traveler 16GB) on my Fritz Box 7490 using USB-3, I had switched the port to USB-2. Now I wanted to switch it back to USB-3 to see a temperature increase of that stick, too (to prove the theory that USB-3 makes sticks becoming hotter). The temperature with USB-2 was about 35°C before, but surprisingly about 10 minutes after having switched the port to USB-3 the temperature descreased to about 29°C. However I had turned on the option "USB-Festplatte bei Inaktivität in den Energiesparmodus versetzen" (Put the USB hard disk to energy-saving mode when inactive) to 10 minutes.

Final Update

I wouldn't believe if I hadn't experienced myself: After some Windows updates the same stick on the same hardware doesn't get hot any more! I can only assume that some power-saving methods are used now (while a file is open on the stick, so it was not "ejected").

The energy data now look like this:

Aktueller Energiestatus:
D3

Energiekapazität:
00000019
PDCAP_D0_SUPPORTED
PDCAP_D3_SUPPORTED
PDCAP_WAKE_FROM_D0_SUPPORTED

Energiestatuszuordnungen:
S0 -> D0
S1 -> D3
S2 -> D3
S3 -> D3
S4 -> D3
S5 -> D3
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    \$\begingroup\$ Presumably keeping the 10x faster interface running requires considerably more power and so runs hotter. It may also keep the internal controller clicked higher (even when idle) to support the 10x faster data transfer. Especially for such a tiny USB device it doesn't take a lot of extra energy to make it a lot hotter. \$\endgroup\$
    – user1850479
    Commented Sep 3 at 23:23
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    \$\begingroup\$ Is the docking station itself hot too? I had a few devices that were so hot inside that they heated all the usb perpherials too \$\endgroup\$
    – bb1950328
    Commented Sep 4 at 6:27
  • \$\begingroup\$ Nice question. I wondered why my thumb-drives were getting hot even when not being interacted with, but never thought to ask the question. \$\endgroup\$
    – Hermann
    Commented Sep 4 at 10:01
  • \$\begingroup\$ AFAIK high temperatures do indeed make SSD data decay faster, but the same effect causes data that is written at a high temperature to be written more 'firmly' as the cells change state more easily when hot, so if you want to micro-optimize your SSD temperature, try keeping it hot when writing and cold when idle :) \$\endgroup\$
    – Criticizing Israel not allowed
    Commented Sep 4 at 20:13

3 Answers 3

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USB3 interface contains two nearly-independent buses and has to have two controllers in one chip. One part operates as USB3, and another operates as USB2. The signals run over separate sets of wires. For a flash drive either the USB3 or USB2 part of the controller IC is activated, depending on the host port capability. As "user1850479" commented above, USB3 controller has to operate at much higher clock rate (~10x, at 2-4GHz) internally, and must support much more complex USB3 (full-duplex) interface driving both transmitter and receiver simultaneously.

When connected to USB2-only port, the flashdrive IC operates only in USB2 mode, and the entire logic of USB3 (and PHY) is in shutdown state. That's why there is a substantial difference in power dissipation between two modes.

Now, the question "What does such a controller do when "sitting idle"?" is an interesting topic. The USB3 interface is designed to support four levels of activity, U0 state (continuous active operation), and power saving states U1/U2 and U3 (suspend).

In U0 state the transmitter and receiver are continuously transmitting/receiving/decoding link management packets, and even the "idle" state of the USB3 bus is a continuous transmission of "logical idle" state, which is encoded and scrambled, and does not differ from any data or control packets, and has the same signal amplitude. The signal looks like plain noise. Maintaining this state requires a lot of power (~0.5-1W).

In U1 and U2 the interface is going up and down into active/inactive state with different enter and exit latencies. Inactive state is "electrical idle", and signals are at zero. However, to operate in this mode both host and device must support it. Entering and especially exiting the U1/U2 states needs to be coordinated. And every exit from low-power state involves link re-training, which includes LFPS wake-up handshake and exchanging thousands of clock synchronizing and symbol sync patterns. The process seems to be challenging for the entire industry, and for more than a decade the support for U1/U2 in mass storage devices can be found only in 20-30% of devices on the market. In the not so distant past, not every PC would support the USB3 link power management, and it was simply disabled in OS to avoid incompatibility with 80% of devices.

Bottom line: if a device or host does not support USB3 link power management, the USB3 device will run relatively hot.

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USB3 allows for more speeds and power. A device that gets connected via USB3 link can assume and negotiate more current to work faster, like in this case, actually read and write data as fast it can over a 5 Gbps link.

With USB2, it only has a 480 Mbps link, so it's enough to read and write data slower as it can only transfer it slower. The flash controller can run at reduced speed.

Also some of the heat may depend on where you plug it. If the docking station itself runs warm, then the result is that the USB thumbdrive will heat up to ambient docking station temperature plus whatever heat it produces itself to rise above ambient.

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I've got the exact same USB stick here and can confirm that it does get pretty warm even while "idling" at USB3; warmer than other USB 3 sticks I use.

Customer reviews found on various online shops that sell the SanDisk Ultra Fit seem to confirm this - lots of users comment about temperatures which indicates there must be something cooking (pun intended).

Two things might contribute to it:

  • The small form factor of the stick doesn't dissipate heat as good as a bigger device
  • The controller used in this device is not the most energy-efficient
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    \$\begingroup\$ The small form factor is same for both USB 2 and 3 so it can't be a factor why it runs hotter with USB 3 link. Or the energy efficiency, because a bidirectional 5 Gbps active link consumes more power anyway than a half-duplex 480 Mbps link which is allowed to idle between packets. \$\endgroup\$
    – Justme
    Commented Sep 5 at 19:04
  • \$\begingroup\$ @justme a smaller form factor could definitely cause a difference in cooling which is only noticeable above a certain power threshold which USB 2 doesn't hit. \$\endgroup\$
    – barbecue
    Commented Sep 6 at 15:51
  • \$\begingroup\$ @barbecue The same USB stick with same form factor runs at different temperature based on link speed. Of course it means it uses more power and heats up more. But the form factor is not a variable that causes it. Even a larger or smaller form factor would run at different temperatures. But there is only one form factor so not a variable. The question was about same stick running hotter or cooler. \$\endgroup\$
    – Justme
    Commented Sep 6 at 15:58
  • \$\begingroup\$ @justme the point raised in the answer was that the small form factor of this specific drive could be a factor in why it heats up more than other drives when using USB 3. \$\endgroup\$
    – barbecue
    Commented Sep 6 at 16:08
  • \$\begingroup\$ @barbecue It was also plugged into a dock that might heat it instead of hanging loose on a cable with convection cooling. The question also was not about how it compares to other drives. \$\endgroup\$
    – Justme
    Commented Sep 6 at 16:12

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