I'm repeating ...
You should design your power supply only with the standards requirements.
A power supply has not only to be operational, but it has to be compliant to the standards used.
Your device should have an equipment class which gives many constraints according to the area where it's located (engine, cab, ...)
For 12V truck application, the voltage range is 6-16V (3.5V during 100ms for cranking phase). For 24V application it's worst. Sometimes a capacitor is enough to keep the electronic working.

Next, do not think that your device has only to be operationnal in the normal voltage range. This is not enough. It has also to accept the load dumps (pulse more than 100V and 400ms duration), it has to be protected against open battery (alternator voltage), open inductive loads (in trucks it's terrible), ....

There is also EMC issues to take care, Electrical discharges (ESD), dielectric strengh, ... and I've not listed the mechanical constraints like temperature which makes all components to have worts performances when temeprature increases. Sometimes, your device is well qualified for normal temperature/normal voltage, but it could be under the requirements when voltage and temperature are near the limits.

For example you can refer to SAE J1113 or SAE J1455 for further informations.

And most important, do not forget to conduct a FMEA (Failure Mode Effect Analysis). This is the first step of your design. You can refer to my website for information on this.

I've planned to add in my website a pratical approach for the standard use in the designs.

Best regards
Stephane REY
http://perso.wanadoo.fr/rs-rey/el...essources/