if we add a smartcard!

Hi everybody and experts in freeradius; :) I'm a new user of the server.. I succeeded to connect a client (supplicant) over wifi to the radius server using eap method.. now, i have to secure the server private key in a smart card.. have you an idea what can I add in eap.conf and in this attribute: certificate_file = ?? I'll be grateful for your help! thanks!