50ms Power Interrupt in DCDC Power Supply (18-32v)
Your input power can be as high as (32 V)(200 mA) = 6.4 W.  You want to ride out a 50 ms outage.  One way or another, something has to come up with (6.4 W)(50 ms) = 320 mJ.  That's basic physics.

So now you look around at what can store 320 mJ and be able to cough it up again over 50 ms.  The only realistic choices are capacitors or batteries.

Your spec of 18-32 V is a bit unclear, but let's say you arrange to keep a cap charged to 32 V when input power is present.  When input power goes away, the cap can't go below 18 V (drop by more than 14 V) within 50 ms.  (200 mA)(50 ms)/(14 V) = 720 &micro;F.

That's the simple system, but uses only 68% of the cap's capacity due to it still containing some energy when charged to 18 V.  Draining a cap more fully is possible, but requires more sophisticated electronics.