1. Set each pineapple ring on a plate. Top each with a scoop (1/2 cup) of frozen yogurt and 1/4 cup raspberries.
2. To make the caramel, pour sugar into an 8- to 10-inch frying pan over medium-high heat; shake and tilt pan often until sugar is melted and amber-colored, 2 to 3 minutes. At once, pour slowly, in thin streaks, over desserts.