Why do automotive led drivers only have 5V fet gate drive outputs?

[treez]

A car battery is at least 12V, meaning you have a decent voltage to drive a fet on with.....so why do pwm controllers like TLE8386 only have 5V fet drive.?....fet switching losses are far worse with such low voltage gate drive.....I know car batteries go down to 6v when cold cranking, but thats just a short transient condition.

Surely the way to drive led headlamps is with a 10v gate drive at least?

TLE8386 PWM CONTROLLER
**broken link removed**

 

[mayd85]

Not a direct answer to your question, but as per my understanding LED drivers are mostly boost converters which use NMOS. Even Infineon recommends using 5V gate voltage compatible MOSFETs in their application notes.

Also, during worst case cold cranking the input voltage can go as low as 4.5V, which is also the lower end of operating voltage of LED drivers. I guess that is the reason the why the gate drive is 4.5-5.5V.

 

[treez]

Thanks Mayd85, I see your point, but quite frankly, I think having a car headlight which is able to run on 6v is absurd, the batt voltage is down at that level for such short intervals that Its not worth the bother to have it working at such low voltage.

A 10V gate drive will always give less switching losses.

 

[mayd85]

Thanks Mayd85, I see your point, but quite frankly, I think having a car headlight which is able to run on 6v is absurd, the batt voltage is down at that level for such short intervals that Its not worth the bother to have it working at such low voltage.

I believe that has got something to do with the automotive EMC standards. The way they define performance criteria for the ECUs especially for functions like headlamps, DRL etc. does make it mandatory that the lamp should remain ON even during disturbance.

A 10V gate drive will always give less switching losses.

Agreed. But voltages less than 10V, even if momentary, is a common occurrence in an automotive system.

 

[treez]

sorry to hanker it, I did work at a car headlight place once, and they had a 40w led bulb still running at 40w even when vin was down at 6v........I proposed that we make it operable at that voltage (6v), but that its power should be staunched (ie the current sense resistor in the source of the fet not reduced to allow full power operation at 6v)...but they wouldn't have it......it was unbelievable, the fet was frying at 6v=vin and 40w.