LM25037EVAL: Hacking this to test for a wide Vin and large boost output.

Peter;  That helps my understanding.

Vehicles have different nominal supplies so 32V would not be transient. 

What do you think of a tapped winding on the transformer with an input voltage monitor that automatically directs the FET drives to an alternate set of FETs connected to the taps.  Essentially, changing gears on the transformer for a high input voltage. 

I understand the transformer change is needed. Thanks for the recommendations for that.

Thanks so much for your response.

Hi Keith,

The challenge with a tapped transformer is there will be step change in the operating point of the power stage and that transient will travel dierctly to the output where the output filter cap will take up some of the energy but the control loop will have to catch up. You will see an output voltage transient during the step. You will also need to add hysteresis on the step threshold to prevent chattering. Its the kind of approach that quickly grows arms and legs....

An alternative is to do a two stage approach, boost the input to 40V and run the dc/dc isolation and close to Dmax all the time for maximum efficiency.

The 4:1 input voltage range is not untypical and is seen in telecoms systems dc/dc converters, in general their efficiency is lower that design that have to cope with more moderate input voltage ranges, like 2:1. So It can be done but at a cost of efficiency.

Regards

Peter

Arms and legs indeed! Exactly my concern and your two stage gambit is what I was going to ask about next. I was just looking at an LM5022 42V output design in the Webench thinking 42V might be more likely than 40V but the same thing essentially. I think I'll give the two-stage a try.
Thanks again.