Fusing, ESD TVS, Reverse Polarity Protection of a Device supplied wit 24VDC - Page 1
Forum section: electronics / beginners

author: zero0d
author_rank: Contributor
timestamp: January 26, 2023, 12:28:41 pm
content: Hi All,I would like to discuss a 24VDC powered device protection scheme.Assuming the device is powered with 24VDC and can consume up to 60W, and it needs fusing, esd protection and reverse polarity protection ( LED should signal the reverse polarity event). The idea is to use standard components opposed to integrated solution.Here is my schematic attached, either using D1 or D2 for ESD.1) Would you suggest using D1 or D2 when there is a PMOS reverse polarity protection scheme ?2) What is the drawback if for D1 instead of bidirectional TVS and unidirectional TVS would be used ?3)in some app notes instead of bidirectional TVS, 2 back to back unidirectional are used, what is the advantage of this regarding to bidirectional tvs?4) Would you add a Zener diode parallel to R2, is there any drawback if we don't use the Zener diode on the gate of p-mos?5) Can you propose a simple LED circuit that will lit up when there is a reverse polarity event ?Br.

author: wasedadoc
author_rank: Super Contributor
timestamp: January 26, 2023, 12:57:56 pm
content: 5) Diode, LED and series resistor directly across the supply input, ie before the fuse. LED lights on reversed input and stays lit after fuse blows.As to the rest I think you are overdoing it. A reverse biased power diode between fuse output and negative. No MOSFET. One TVS across the output.

author: TimNJ
author_rank: Super Contributor
timestamp: January 26, 2023, 01:13:44 pm
content: What kind of 24VDC system? Automotive?4. As is, the gate sees 1/2 the supply voltage. If there is a transient (let’s say the supply voltage jumps up to 50V for a moment), then the gate will see 25V. With a zener, the gate is always clamped to the zener voltage.5. To serve as a visual aid, try re-labeling your schematic so that the (+) and (-) terminals are swapped. (This is what happens when the polarity is wrong, after all.)Just need an LED, diode, and resistor in series. Cathodes (Ks) facing the positive terminal. Reverse breakdown voltage of an LED might be only 5V so in normal operation, a 24V reverse bias would be too much. So add a diode in series to block the LED frim seeing this high reverse voltage. You can use a 75V/100V diode, 1N400x, 1N4148, etc.(-) — A(LED)K — A(diode)K — resistor — (+)

author: TimNJ
author_rank: Super Contributor
timestamp: January 26, 2023, 01:21:28 pm
content: Quote from: wasedadoc on January 26, 2023, 12:57:56 pmI think “overdoing it” depends strongly on the desired behavior of the system during/after the reverse polarity fault condition. Usability wise, very few userswantthe fuse to blow and system to be disabled, but often it is used because it is a simple technical solution.OP, if you do not need to maintain ground continuity between the device being powered and the 24V supply, you can use and NMOS in the low-side. They’re cheaper for a given Rds(on).

author: jonpaul
author_rank: Super Contributor
timestamp: January 26, 2023, 02:24:21 pm
content: DC bus nature ?application eg car, plane, sea?threats of transients eg lightning alternators load dump, EMP ?Usually just a TAZ is sufficientj

author: zero0d
author_rank: Contributor
timestamp: January 27, 2023, 07:18:50 am
content: Hi All,The system is for an Industrial device ( Automotive factory ). Also we need GND continuity and the option to use PMOS or some Ideal diode controller with NMOS, but i would like to stick to PMOS.If you take that the max current is 2.5A then the max dissipation on P-mos is ~2.5*2.5*4.8mOhm = 30mW.Currently i'm using the attached schematic 1. but i would like also to ESD protect the PMOS.1)So i would like to that you look at schematic 2,3,4,5 and comment which one would you preferer and why?Schematic 2) Bipolar TVS before PMOSSchematic 3) Unipolar TVS before PMOSSchematic 4) Back-to-Back TVS before PMOS - is there any advantage over Bipolar TVS before PMOS?Schematic 5) Bipolar TVS before PMOS and Unipolar After PMOS2)Also the Bipolar (SMCJ24CA) and Unipolar (SMCJ24A) TVS from Bournshttps://hr.mouser.com/datasheet/2/54/SMCJ-778393.pdfhave max clamping voltage of 50V ( for 8/20 pulse) and 39V (for 10/ 1000 pulse) , is the selection of PMOS ok regarding to its VDS maximum ? I'm assuming Vd and Vs are on same potential in a ESD event or am i wrong?3)Regarding the LED ofr reverse polarity indication, what do you think of the schematic below ( LED Schematic), the connection is after the fuse.Br,d.