Hi if your are fast charging your device this can cause heat
I only use when in need of power as I believe it can wear the battery a little quicker than using normal charge
Do you have a case on the phone @Graco-s7 ?
A phone will warm up when its being charged.
Are you using an official Samsung Charger or a 3rd party option ?
If your concerned then a Samsung Service Location can look at the phone for you.
Current Phone ~ S8 Model: SM-G950F. CsC:BTU.