With the TOPO kit, self ligation is very low because the vector has a 5'  overhanging T nucleotide on both ends.  The PCR products have an added A which will stick to the T overhang.   But unlike a restriction digest, the two overhangs in the vector are not complementary and thus self ligation is very unlikely. 

probability of self ligation is generally quite low in TOPO TA kit

Thank you Sinha. Thanks Gregory