In general, when a current flows through a bipolar transistor, Joule's heat is produced and an electrical resistance of semiconductor device is reduced by the produced Joule's heat. Accordingly, larger currents flow through a bipolar transistor and more Joule's heat are produced. If this process is repeated, very large currents flow through a bipolar transistor and a power characteristic of bipolar transistor is degraded by the produced Joule's heat.
A bipolar transistor may be destroyed by more Joule's heat and this phenomenon is called a thermal runaway. A solution to prevent that thermal runaway is that a constant current flows through a bipolar transistor or a semiconductor transistor.
A well known constant current circuit essentially includes a comparator comparing a feedback current with a reference current in a load terminal. Since this constant current circuit should include a comparator, the constant current circuit has a complicated structure and is expensive. If large currents flow through the constant current circuit like that, a thermal runaway occurs in a semiconductor device itself. Thus, that constant current circuit is not suitable as a constant current circuit through which relatively large currents flow.
A constant current circuit in which a thermal runaway relatively does not occur and through which relatively large currents can flow is disclosed in K.R Patent No. 10-0964186-00-00, titled “heat control circuit of transistor using a metal-insulator transition (MIT) device and a heat control method” invented by multiple inventors besides Hyuntak kim.
As disclosed in claim 18 of the patent, a constant current circuit through which larger currents can flow may be developed using the MIT device and two electric power transistors.
However, when designing a constant current circuit using a bipolar transistor, a current may be inefficiently wasted. Thus, in a constant current circuit using a bipolar transistor, measures to increase an efficiency of transistor are required.