power supply circuit based on PWM?

HI TO EVERYONE, I HAVE TO FIND A POWER SUPPLY CIRCUIT (2-12V/1A)
BASED ON PULSE WIDTH MODULATION (PWM).DOES ANYONE KNOW SOMETHING ABOUT IT, OR EVEN SITES THAT MIGHT HAVE USEFUL INFORMATIONS ABOUT PWM? ANY GOOD... :idea:???

National Semiconductor Application Notes will give some good info. www.nsc.com. Check out AN-21 and AN-343.