I would like to know how to use irf644 as a replacement of irf630.Whether they are on circuits that are not demanding, the two can still be used interchangeably？Hope please, thanks.
If circuits works with IRF630, in most cases will work with IRF644.
IRF630: 200V 9A N-Channel
IRF644B: 250V 14A N-Channel
Looks like a fair replacement.
The 644 has way more gate charge(*) than the 630, so it will need 6 times the gate current in high speed PWM circuit, but for simple on-off switching this isn't relevant.
(*) 68nC v. 11.6nC