The MCCB electric trip unit (ETU) is replacing thermal magnetic tripping device in low voltage power distribution, and the further trend is developing intelligentization in future. So as to achieve electronization and intelligentization, this paper propose a new solution based on the thermal diode, it’s using the relationship between PN Junction voltage and temperature achieve long delay time protection. At the same time using the saturable current transformer for power harvesting and instantaneous protection for over load current. The result shows the electronic trip unit could achieve long delay trip and over instantaneous trip function, the response time and accuracy meet breaker requirements.