My 2015 Mac Pro (15") was fine till last week when the green charge light disappeared...
replacement charger (Mag Safe 85W) which also was burnt after one hour of use due to overheating
The magsafe charger gets it's information from the DC-in board on the computer.
You can try running the Diagnostic/AHT http://support.apple.com/kb/HT1509
However, not definitive by any means, but may kick out an error code—and if it does you can post it here for interpreting.
reboot holding the option command D keys
Apple Portables: Troubleshooting MagSafe adapters - Apple Support
"The voltages at the power adapter output are unregulated, and circuitry inside the Mac regulates and conditions it to exactly what it wants for its battery charging.
You can always use a power adapter with more capacity with impunity. The computer only draws what it needs and is not "overpowered".
Using a smaller one can cause some issues with powering and running at the same time, and you may see sudden uncontrolled power-down is certain high-stress situations. This is because the computer freely "borrows" from the battery when the power adapter is connected, especially in high-stress situations.
But using a smaller one for overnight charging while powered off should not be an issue -- it will just charge slower." —Grant Bennet-Alder
