2
\$\begingroup\$

I am working on a small project in which I use a CH340 to communicate with an MCU, but during prototyping I probably should have a way to protect the controlling computer's USB port from shorts. I designed this circuit to handle the task at hand:

The USB Circuit

I have two questions regarding this design:

  1. I am faced with the decision to use the NUF2042XV6T1G for ESD protection, which is surprisingly the only ESD protection IC my supplier has. Checking the datasheet, I noticed there were some capacitors and resistors shown in the circuit description - which I think are not going to cause any issues since the datasheet clearly states that the IC can be used in USB hubs. I do not want to use discrete TVS diodes to keep the circuit a lot more compact (and also because even 2 of them costs around the same as the IC here). Would there a better way to handle the ESD protection, or would this be sufficient?

  2. I am planning on using a PPTC fuse (FSMD025-1206-R) which has a tripping current of 500mA to prevent damage to the USB port of the controlling PC/laptop in case of a short somewhere on the board. I know most computers have internal means of protecting the ports, but I want this project to be something anyone can use easily without the risk of damaging their devices. My question here is I am unsure of what hold/trip currents I should pick, and if a PPTC in this configuration is enough to actually protect the USB port from overcurrent.

I am also open to any criticism, since I want to learn more about proper circuit design.

Thanks in advance!

\$\endgroup\$
4
  • 1
    \$\begingroup\$ Which ESD protector you need to select depends on the level of ESD protection you want to handle. If you only have this one type available then either use it or none. But it might be better to focus on getting the basics right first, the 47 uF cap exceeds USB limits. And you may not really want to connect the metal connector shell directly to GND, but it depends on many factors. \$\endgroup\$
    – Justme
    Aug 15, 2023 at 19:43
  • \$\begingroup\$ the 47 uF cap exceeds USB limits I was thinking of using it to stabilise the input voltage - should I pick a smaller value for that? And you may not really want to connect the metal connector shell directly to GND, but it depends on many factors I have seen some people connecting them together through a resistor, or sometimes not even connecting the two - but is there a standard way of handling this, or maybe some guides? \$\endgroup\$
    – Kitsuinox
    Aug 15, 2023 at 20:08
  • \$\begingroup\$ The device will only be powered by the USB, so wouldn't it be fine to connect the ground together with the shell? (sorry for commenting separately, missed the 5 minutes to edit the first comment) \$\endgroup\$
    – Kitsuinox
    Aug 15, 2023 at 20:14
  • 1
    \$\begingroup\$ The 47uF is rather useless stabilizing anything if it is an electrolytic capacitor. A 1uF to 4.7 uF ceramic would be much better at high frequencies. 10uF is the total capacitance limit (unless you limit the inrush energy). And for the shell, usually, the host already connects the cable shield to ground, so connecting cable shield to ground also on device end makes it a conductor and path for DC ground return currents, and the shield is not supposed to be used as a current carrying conductor, but only as a shield. There is no simple one solution how to connect it without more info. \$\endgroup\$
    – Justme
    Aug 15, 2023 at 20:52

1 Answer 1

2
\$\begingroup\$

I'd probably just use an 0805 or 1206 size 10uF (25V) ceramic cap on VBUS (but I can't remember what the USB spec is). 0805 or 1206 size will allow a wide variety of capacitance values if you need to change it.

Polymer fuses work great, but with limitations. They don't trip very fast and the tripping current is rated at 20C or 25C. So, the trip current will be less as the temperature goes up (look at the datasheet for the I vs. Temp curves). I think you probably don't need a fuse on your device. The upstream port should have have overcurrent protection built in.

If you want overcurrent protection, set the limit for how much current your device will use, (plus some margin), not how much power the upstream port can supply. If you device only uses 50mA, a 100mA or 200mA fuse is fine - a 500mA fuse is overkill.

I'd go with one ground for the whole design - like you have. Everyone always tries to overcomplicate grounding. I've always had very good results with 1 ground for everything.

\$\endgroup\$
2
  • \$\begingroup\$ Is there an exact reason not to use an electrolytic capacitor in this case, other than the fact that ceramic 0805s have a wider range and are cheaper - with a smaller footprint? Also thanks for the tips on the polymer fuses - I wanted to use them to prevent user caused damage if I lend the device to other people. The reason I picked 500mA was because my supplier only sells fuses with 390, 400 and 500mA trip currents - but I can change the fuse for one with a lower value if I find more options somewhere else. \$\endgroup\$
    – Kitsuinox
    Aug 15, 2023 at 21:49
  • 1
    \$\begingroup\$ It depends on the use of the cap. Electrolytic caps have high ESR (hundreds of milliohms). Ceramic caps have very low ESR in comparison (tens of milliohms or less) and perform better. I usually only use electrolytics in a couple of situations: - when I need > 22uF - large bulk caps; - audio applications where ceramic caps have microphonic action. Especially in high-gain microphone amps, you can tap on some ceramic caps and hear it in the audio. \$\endgroup\$ Aug 24, 2023 at 18:59

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.