Overvoltage cut off not quick enough?

[Heisen]

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.pdf



The 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.

[Heisen]

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 Suppression

Circuit 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)

[radiolistener]

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.

[Zero999]

Add a zener diode, with a breakdown voltage a little higher than the cut-off, across the load.

[Tomorokoshi]

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?

[Heisen]

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?

The source impedance is built into all the automotive transient waveforms in LTspice by default shown here https://www.analog.com/en/technical-articles/ltspice-models-of-iso-7637-2-iso-16750-2-transients.html

the ISO 7637-2 Pulse1 has 10 ohms of source impedance,
the ISO 7637-2 Pulse 2a has 2 ohms and
the ISO16750-2: 4-6-4 12V has 0.5 ohms

The current through the D1 under load dump is 110A in LTspice. Dissipating about 4 KW. The TVS is rated for 5 KW so I guess it's okay?

[Tomorokoshi]

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?

The source impedance is built into all the automotive transient waveforms in LTspice by default shown here https://www.analog.com/en/technical-articles/ltspice-models-of-iso-7637-2-iso-16750-2-transients.html

the ISO 7637-2 Pulse1 has 10 ohms of source impedance,
the ISO 7637-2 Pulse 2a has 2 ohms and
the ISO16750-2: 4-6-4 12V has 0.5 ohms

The current through the D1 under load dump is 110A in LTspice. Dissipating about 4 KW. The TVS is rated for 5 KW so I guess it's okay?

Using the Littelfuse part:
https://www.littelfuse.com/~/media/electronics/datasheets/tvs_diodes/littelfuse_tvs_diode_5_0smdj_datasheet.pdf.pdf

A 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 P_PPM.
3. What happens if the actual waveform is different from the three test waveforms?

[Heisen]

Interesting.

1. Determine maximum ambient temperature conditions and apply the power derating curve from Figure 3.

So, 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.

2. Check Note 1 and Note 2 referenced from the Maximum Ratings table for P_PPM.

Will keep that in mind.

3. What happens if the actual waveform is different from the three test waveforms?

Now 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.