switching transistor

i am an aussie frustrated by supplier of a 20A switching battery charger who will not supply a shot transistor simply marked doy217 . i cant find an equivalent in australia!! it is in series with the primary of a transformer and is controlled by logic circuit which alters the duty cycle of its switching... capacity has to be 250W .. can any one help with a substitute??[/b]

If this semiconductor in primary side of transformer, maybe a TRIAC.
When You attach the sematic the help is easyer...