Sean,
Have you designed circuits for 12V and 24V automotive use? I am currently reviewing such PSU designs, one in particular for use in a 24V vehicle where a worst case 202V Load Dump pulse could be present (on vehicle's lacking suppression within the alternator). A 12V vehicle Load Dump could be as high as 125V. Static Discharge is similar yet not quite the same as a vehicle Load Dump.
Here's a 24V vehicle PSU example:
https://d2ffutrenqvap3.cloudfront.net/items/2B1T2F322u2i2L280a1U/24Vin_VehiclePowerSupply_12Vout-1A_v1.5.pdf
And here's a 12V vehicle PSU example, also showing expected current flow to the Load:
https://d2ffutrenqvap3.cloudfront.net/items/123B2f3b1M1t122D2c2t/12Vin_PSU_12Vo_and_3Vo.pdf
Note that the Schottky diode protects against reverse polarity, and a Schottky type was chosen to lower the voltage drop. It comes after the TVS since the Schottky is rated only for 70V. A bidirectional TVS was therefore chosen so it will not overload in the event of reverse polarity. The Voltage input is externally fused because TVS diodes fail shorted. There is not a lot of capacitance on the Voltage input because the caps need to be rated for high voltage there, and because the Buck regulator really only requires the shown capacitance to operate adequately. So the question then comes down to what wattage rating the TVS should be. I often see 600W rated TVS diodes used in applications like this, and there are not a lot of reported failures. My guess is because not a lot of people removing the battery while the engine is running (which is the only time one would experience a Load Dump), and because most modern vehicles have suppression in the alternator, which often will cap a Load Dump spike to 80V or less. Even so, if one were to design the circuit without reliance on such alternator based suppression, one would need to give though to how effective a given TVS diode would be. Sure, it would be "safer" to just use a 2200W TVS, or hey, even two 5000W TVS diodes in parallel, but such adds cost and requires more board space. The SMD type TVS diodes seem to be better for suppression than the leaded types, yet the SMD types only go up to 1500W. So I am unsure if it is best just to stick with the 600W SMD TVS or move up to something higher wattage.
If after reviewing the above 2 schematics you would recommend the addition of a ferrite bead, what specific value would you propose? I am not experienced in use of ferrite beads, but I have read it is best to only use them on the high voltage side and not on the Ground (even though I've seen designs that have 1 bead on the voltage input and 1 other bead on the ground input). I would also appreciate your thoughts on the Input line capacitance, keeping in mind the voltage ratings of the caps that would be needed there.
Thank you,
--James Wages
Sat, 10 Feb 2018, Sean Breheny <***@cornell.edu>:
It is really quite important to consider how
static discharge could affect your product through any of its i/o or power
inputs or even switches or other openings in the case. I usually employ a
combination of a ferrite bead, ceramic capacitor, and tvs diode on every
wire which leaves or enters the pub. Sometimes this scheme must be modified
because the signal on said wire is too high frequency or carries too much
current. But the general strategy is absorb-block-clamp.
The capacitor helps to convert a very short high voltage pulse into a
longer duration but much smaller amplitude pulse (absorb). The ferrite bead
limits the current in the fast pulse so that parasitic inductance in the
ground path and in the capacitor do not develop a high voltage drop. (Block)
Finally, the tvs diode clamps the remaining pulse to no greater than the
max rated voltage of the i/o pin. (Clamp)
So the ferrite bead would be located nearest the outside world, and then
the cap and tvs diode would be in parallel between the signal line and gnd
after the ferrite.
I also like to maintain a separate ground "ring" around the perimeter of
the board which connects to the rest of the ground plane at only one point.
The tvs diodes and esd-related caps connect to this ground and this ground
is also the only connection to chassis (not always feasible when you have
RF signals)
This ground ensures that transient high voltage drops due to fast was
pulses do not develop between locations on the main ground plane.
In the end, all of this must be tested and any remaining vulnerabilities
corrected.
I have designed 4 or 5 high volume (10k to 200k) industrial products using
variations of this strategy with no problems passing esd tests and very few
known field esd failures.
Sean
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