Patent ID: 8089742

Claim:
An overcurrent protection apparatus, comprising: a semiconductor element (T 1 ) which is provided between a load (RL) and a power supply (VB); a load circuit which controls turning-on and turning-off of the semiconductor element to control driving and stop of the load; a comparing unit (CMP 1 ) which compares a voltage (Vds) generated across both terminals of the semiconductor element when the semiconductor element is in a turned-on state with a determining voltage (VB−V 1 ) set in advance; and a semiconductor element control unit which turns the semiconductor element off to protect the load circuit when the comparing unit determines that the voltage (Vds) is larger than the determining voltage, wherein a temperature increasing amount of the semiconductor element caused when the semiconductor element (T 1 ) is turned on is ΔTch, an on-resistance value of the semiconductor element (T 1 ) is Ron, and a thermal resistance value of the semiconductor element (T 1 ) is Rth_f; wherein a temperature increasing amount of a connecting wiring is ΔTw when a current flows into the connecting wiring (WL) for connecting between the load and the semiconductor element, a resistance value per unit length of the connecting wiring is Rw, and a thermal resistance value per unit length of the connecting wiring is Rth_w; and wherein ΔTch is obtained based on an expression of ΔTw/ΔTch=Rth_w/Rth_f*Rw/Ron with respect to the temperature increasing amount ΔTw not exceeding a difference between an upper limit of a permissible temperature of the connecting wiring (WL) and an upper limit of an operational peripheral temperature, then the voltage Vds is obtained based on an expression of ΔTch=Rth_f*Vds 2 /Ron with respect to the ΔTch, and the obtained voltage Vds is set as the determination voltage.