Overvoltage cut off not quick enough? - Page 1
Forum section: electronics / beginners

author: Heisen
author_rank: Contributor
timestamp: June 26, 2023, 12:18:32 pm
content: Greeting everyone,This is continuation of my previous thread, but with different problem.The following circuit is based on this app note from TI.https://www.ti.com/lit/an/snva717/snva717.pdfThe cut off voltage in this circuit is set to 15v + two diode drops, during the overvoltage condition the output turns off but with a huge voltage spike at the load. The P-FET used here is SSM3J356R, I have included the model in LTSPICE simulation.The transient rise time is not even that fast only 350us, the load is resistive,my goal is to only protect up to 42V.Any idea what can be done here?Thanks.

author: Heisen
author_rank: Contributor
timestamp: July 03, 2023, 07:34:31 pm
content: UPDATE:I was able to increase(slow)the rise time of the transient by adding a 22uF cap after the TVS, this gives enough time to the overvoltage disconnect to react. The load sees nothing beyond 16v.The circuit is suppose to handle reverse battery, 24V jump start, all 12v automotive transients including the most nasty ones which I think are :-ISO 7637-2 Pulse1 (-150V)ISO 7637-2 Pulse2a (+112V)ISO 16750-2:2012- 4.6.4 Load dump Without Centralized SuppressionCircuit works in the LTSpice which has these waveforms by default, but no idea if it will work in real life.(which is always the case in my hobbyist practices)

author: radiolistener
author_rank: Super Contributor
timestamp: July 03, 2023, 08:01:53 pm
content: here is more safe and reliable overvoltage and reverse polarity protection, it doesn't have 0.5-1V voltage drop on diode, because there is no diode:Such protection circuit is mandatory to use with high power power supply due to very high possible current (20-60 Amps and more), where overvoltage or reverse polarity can destroy very expensive equipment and make fire.

author: Zero999
author_rank: Super Contributor
timestamp: July 03, 2023, 09:35:31 pm
content: Add a zener diode, with a breakdown voltage a little higher than the cut-off, across the load.

author: Tomorokoshi
author_rank: Super Contributor
timestamp: July 05, 2023, 12:22:56 am
content: There is no impedance between the source and D1. Look at the datasheet for D1 and note the clamping response. What does LTspice show for the current from U1?

author: Heisen
author_rank: Contributor
timestamp: July 05, 2023, 02:22:48 am
content: Quote from: Tomorokoshi on July 05, 2023, 12:22:56 amThe source impedance is built into all the automotive transient waveforms in LTspice by default shown herehttps://www.analog.com/en/technical-articles/ltspice-models-of-iso-7637-2-iso-16750-2-transients.htmlthe ISO 7637-2 Pulse1 has10 ohmsof source impedance,the ISO 7637-2 Pulse 2a has2 ohmsandthe ISO16750-2: 4-6-4 12V has0.5 ohmsThe current through the D1 under load dump is110Ain LTspice. Dissipating about 4 KW. The TVS is rated for 5 KW so I guess it's okay?

author: Tomorokoshi
author_rank: Super Contributor
timestamp: July 05, 2023, 03:00:10 am
content: Quote from: Heisen on July 05, 2023, 02:22:48 amUsing the Littelfuse part:https://www.littelfuse.com/~/media/electronics/datasheets/tvs_diodes/littelfuse_tvs_diode_5_0smdj_datasheet.pdf.pdfA few things to check:1. Determine maximum ambient temperature conditions and apply the power derating curve from Figure 3.2. Check Note 1 and Note 2 referenced from the Maximum Ratings table for PPPM.3. What happens if the actual waveform is different from the three test waveforms?

author: Heisen
author_rank: Contributor
timestamp: July 05, 2023, 04:09:21 am
content: Interesting.Quote from: Tomorokoshi on July 05, 2023, 03:00:10 amSo, according to that datasheet, If I assume 75 degree ambient temperature, which makes the junction temperature also 75 degree since no power is dissipating initially, it derates the power dissipation to 4000W, I am at 3800W with the load dump transient which is the worse one.Quote from: Tomorokoshi on July 05, 2023, 03:00:10 amWill keep that in mind.Quote from: Tomorokoshi on July 05, 2023, 03:00:10 amNow this is what I am confused about, I thought an automotive gadget protection is designed on the bases of these ISO defined waveforms, which I think are the worse ones(anything happening beyond this seems highly unlikely)so if the product passes these then it's probably okay. Not sure though.