car battery & DC/DC converter

[samsuffy]

I'd like to design a 90W DC/DC converter to use a laptop in my car.
It should convert 12v of my car to 20v.

I know that the car battery do not provide always 12v... but I've no experience with it.

Is someone know :

- what is lower DC voltage (voltage drop ) that my converter must cope with ?
during car use ? during engine start-up ?

- what is highest transient voltage that my converter must stand ?

- is there a typical circuit to protect my converter against transient ?

- is there a better choice than the "boost" configuration ?

Thanks for your help

 

[kileen]

Check this link:

**broken link removed**

and also see the attached file .....hope it helps

 

[samsuffy]

... the comments of your doc is in Russian (or some language that I don't understand...)

in the design from the website ,what is the use of the input 12uH coil ?

 

[Sputnik]

The 12µH is (to the best of my logical knowledge of electronics and mechanics) there to suppress noise. Noise? Yes, from the alternator. The alternator produces AC that passes through a rectifier before going directly to the battery where all other devices acquire their power from. The radio will also have something similar in series with the battery, otherwise you will have a constant "whine" in the sound that goes up and down with engine speed.

I hope my answer is correct, or least on track.
Sputnik :idea:

 

[samsuffy]

ok... so, no real constraints on that coil.
The value can be roughly guessed as we don't know about the noise from the car ?...

Actually, I found strange to add a coil here because the switching regulator is precisely supposed to regulate the output whatever the input, isn't it...

 

[IanP]

There is no clue why did the author choose the value of 12µH. To provide good "separation" between car supply and this switch mode PS I think it can be anything between 10-100µH - this value is not critical. Its use is just precaution and it is quite possible that you may not need it at all..

 

[Sputnik]

The alternator produces 3 sine waves 120 degrees apart per revolution. The alternator spins at roughly 1.5 to twice the speed of the engine so, if the engine is idling (at 1 000 RPM), the frequency is roughly:

(1 000 / 60) * 1.5 * 3 = 75 Hz

Engine at full speed (5 000 RPM)

(5 000 / 60) * 1.5 * 3 = 375 Hz

Sputnik :idea:

 

[rongli]

i think it will be easy.
use the pwm ic.