The invention relates to a carboxylic acid functional branched polyester for thermosetting powder coating composition, wherein said polyester is prepared from at least the following monomers: a difunctional aromatic acid, the difunctional aromatic acid being a combination of terephthalic acid (TPA), isophthalic acid (IPA), an at least trifunctional monomer and a difunctional alcohol, the difunctional alcohol being a combination of at least ethylene glycol (EG) and/or diethylene glycol (DEG) and neopentyl glycol (NPG), the molar ratio of TPA to IPA in the carboxylic acid functional branched polyester is at most 12.4, and the functionality of the carboxylic acid functional branched polyester is from 2.15 to 4.0, and the carboxylic acid functional branched polyester has an acid value of from 14 to 80 mg KOH/g resin, and the carboxylic acid functional branched polyester has a glass transition temperature of at least 40C. The powder coating compositions of the present invention derived upon curing of the thermosetting powder coating compositions which comprise the polyester according to the invention together with a crosslinker with oxirane groups and an epoxy cure catalyst present good corrosion resistance to steel substrates. Furthermore, said low cost and corrosion resistant powder coatings may present good reverse impact resistance (RIR) and/or good smoothness.