Hi James, I have a few 12/24V automotive designs under my belt, and from what I've se= en, it's still installer error that causes the most problems. Vehicles nowa= days are pretty well snubbed unless you pick your power from some bizarre p= lace. The main thing to prepare for is your ground wire to be connected to +24V w= hile the device is solidly bolted to chassis. Everything I design now has the case isolated from vehicle supply/ground ex= cept for 1 Megohm and a small (33nF or thereabouts) cap. No more melted gro= und wires :) Cheerful regards, Bob ________________________________________ From: piclist-bounces@mit.edu on behalf of James = Wages Sent: Sunday, February 11, 2018 5:03 PM To: piclist@mit.edu Subject: Re: [PIC] Static damage prevention 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 wher= e 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 hi= gh as 125V. Static Discharge is similar yet not quite the same as a vehicl= e Load Dump. Here's a 24V vehicle PSU example: https://d2ffutrenqvap3.cloudfront.net/items/2B1T2F322u2i2L280a1U/24Vin_Vehi= clePowerSupply_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 Schot= tky type was chosen to lower the voltage drop. It comes after the TVS sinc= e the Schottky is rated only for 70V. A bidirectional TVS was therefore ch= osen 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 d= own to what wattage rating the TVS should be. I often see 600W rated TVS d= iodes used in applications like this, and there are not a lot of reported f= ailures. My guess is because not a lot of people removing the battery whil= e 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 wou= ld 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 di= odes in parallel, but such adds cost and requires more board space. The SM= D 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 t= o 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 experie= nced in use of ferrite beads, but I have read it is best to only use them o= n the high voltage side and not on the Ground (even though I've seen design= s that have 1 bead on the voltage input and 1 other bead on the ground inpu= t). I would also appreciate your thoughts on the Input line capacitance, k= eeping in mind the voltage ratings of the caps that would be needed there. Thank you, --James Wages --=20 http://www.piclist.com/techref/piclist PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist .