TVS/ESD protection on VBUS of USB for an industrial device

[electroro]

Hello folks,

I am designing a device which will be used in an industrial setting like factories, offices, etc. It has a LiPo battery charger circuit in it.  See attached image.

The device does not use the USB data lines, only the VBUS is used to charge the LiPo. I am stuck at selecting a TVS diode.

1) Do I need a TVS diode?
2) What specs should I be looking for? Especially the working voltage and clamping voltage.
- Can the working voltage (Vrwm) be 5V like in SMBJ5.0CA or ESD9101P2T5G or should it be >5V since some supplies may have 5.1V VBUS.
- What clamping voltage is appropriate? Does it strictly need to be below Absolute maximum ratings of the devices?

[T3sl4co1l]

I would only worry about inrush surge and possible ESD strikes.  No need for huge ratings or anything, not even an SMAJ should be needed I think.

Inrush surge is mainly a current thing, but it can become quite substantial voltage when combined with the nonlinear dielectric of type 2 capacitors (C5, C6).

The linear case, with no resistance and zero initial conditions, is an inrush current equal to Vin / sqrt(L / C), and a peak voltage of 2Vin; where L is the loop inductance (mostly the cable, ca. 0.5uH per m length), and C is the total input capacitance.  The voltage peak is somewhat lower with ESR of course, and further ringing is dampened.

The nonlinear case, where C decreases with increasing V, allows the peak to rise much higher, often over 3Vin.  How much, depends on the C(V) curve of your chosen parts; you'll have to go digging for the char sheets, if they provide them at all.  Also, hopefully you'll check for reasonably nominal value at operating voltage; 1uF 0603s being say -50% at 5V (regardless of voltage rating) is not uncommon.  0805+ should be fine, but do check as they can always skimp on the internals, the only thing you're really guaranteed is a too-small part won't hold enough charge.

So, for these cases, having a TVS to clamp the peak, and an LDO rated for the clamped peak, is a good idea.  Typically a TVS rated for nominal operating voltage (i.e. 5V) will clamp around 30% higher, or maybe worse I guess at lower voltages (I'd expect 8V here?).  So a 10V rating for those parts should be adequate.

Tim