>It needs to handle an input voltage between 20 and 50 volts, supply a
>current of between 30mA and 500mA with an output voltage of 5 volts.

Personally I like the Linear technology range, but they do not have many 
that would handle that input voltage.

So I would also recommend the MC34063 that Jinx suggested, as a rugged, well 
proven, and available from several sources device. 

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