Is this solution to Automotive Load Dump acceptable (LED headlight circuit)?

[Flyback]

Hello,
Is the below solution to Automotive Load Dump for a LED headlight acceptable?
It certainly works fine.....as soon as the battery bus voltage goes above 20V, the comparator switches off the in-line PFET, and the overvoltage doesn't get on to the electronics circuitry of the Headlamp's LED driver.
Would this solution be deemed acceptable by the authorites?
(LTspice simulation and schematic attached)

 

[jpanhalt]

Would this solution be deemed acceptable by the authorites?

What standards are you trying to meet? Can you give a link? Is the obvious answer, ask the authorities, out of the question? Why?

John

 

[Flyback]

The standards are just saying that the lamp should not be damaged by an automotive load dump, and should resume operation following the load dump...but they don't say if they think a comparator solution is a robust method. The standards don't give particulars about circuit techniques, and so it is open to interpretation..the only way to ultimately find out if its ok, is to pay a fortune to a test house who will then give the "yay or nay". But this is going to cost a lot of bucks.

 

[jpanhalt]

How will our opinion of the circuit affect your cost in getting it certified?

John

 

[Flyback]

If you can see something that I have missed? I mean, I am amazed that this circuit wasn't used at a headlight co that i worked in.....I mean, I would have thought an in-line fet, which turns off the supply, would be the standard method for dealing with load dump. Id expect to see this circuit in app notes for headlight drivers...but I do not see it...i wonder why?

 

[alec_t]

Do the standards allow switching off the supply during a load dump (which might cause unacceptable flicker)? Or do they require the light level to be maintained during the dump?

 

[Flyback]

**broken link removed**
..according to this, load dump can be up to 120V.......rating the entire led driver circuitry to be able to run off that seems unreasonable since the dump is only 400ms max...i'd say, switch it out with pfet, and if necessary have the led driver running off a big storage cap downstream of the in-line fet switch.
However, big caps mean electrolytic caps which means low lifetime.

The above link shows this kind of fet switch solution, but if it is acceptable here, then why do they not do it with rear led lamps for cars I wonder?......I saw a rear car led lamp pcb and it had about 15 SMC , 26V transorbs on the pcb......why on earth they didn't just use the fet switch I'll never know.
Also, they say they have to use the diode bridge for reverse polarity connection, but why didn't they just use another pfet for reverse polarity protection.

 

[ronv]

I din't run the simulation, but I would worry about the break down volyahe on the comparator output and the 3904 Vce & Vcb as well as perhaps the FET gate to source voltage. Those spikes can be fast I think.

 

[picbits]

Rather than use the FET to turn off the protected item, use it as a linear regulator to temporarily absorb the excess power generated.

There is a schematic online written by a manufacturer which shows how to do this - pretty simply and can be made for a few quid.

Unfortunately I don't have a link but it is easily found through searching.

 

[Flyback]

load dump can be up to 90V for 400ms....i dont fancy absorbing that.
I reckon have the switch, and if necessary ,a reduced power ride-through..reckon?