I'm no computer expert, and may be wrong here, but having had the 'burning-out' problem myself I suspect I at least know the reason, and therefore the remedy, provided you catch the problem early enough.
The 'male' part of the MagSafe port, or the end which plugs into the computer has five metal contacts. These each have a tiny spring under them to ensure a contact is made. If, for any reason, these become stuck, the contact may not reach its corresponding female part in the computer, causing it to fail to connect, or to arc, and it will be this arc-ing which causes the burning. If the contacts in your computer are not beyond redemption, they can be scraped clean (I used a jewellers screwdriver) and a new charging lead purchased. In my case I managed to free-up the two stuck male pins, but suspect this will be a temporary fix. I'll keep checking my new lead when I get it!
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