The important is to limit the output voltage to 28V and have maximum
output voltage when input drop under ripple
One solution I can figure out is
http://www.noding.com/la8ak/12345/images/28vReg-4.gif

another more commonly found solution is
http://www.noding.com/la8ak/12345/images/28vReg-3.gif

the advantage of the first over the second is that the it needs single
supply voltage and the power fet can be mounted directly in chassis

73
Jan-Martin, LA8AK
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J. M. Noeding, LA8AK, N-4623 Kristiansand
http://home.online.no/~la8ak/91n.htm