Patent ID: 7495499

Claim:
A power transistor circuit, comprising: a first power transistor having a drain, a source and a gate, wherein the first power transistor has a first voltage limit and a first inner resistance; a second power transistor having a drain, a source and a gate, wherein the first power transistor has a second voltage limit and a second inner resistance, and the drain of the second power transistor connects to the drain of the first power transistor; a start-up circuit including a comparator, the comparison voltage of the comparator corresponding to the first voltage limit, one input end of the comparator connected to the drain of the first power transistor, and the output end of the start-up circuit connected to the gate of the first power transistor; wherein the first power transistor and the second power transistor are manufactured by different processes, the first voltage limit is smaller than the second voltage limit, the first inner resistance is smaller than the second inner resistance, and the conducting time of the first power transistor is later than that of the second power transistor.