Dual transistor improves current-sense circuit

In multiple-output power supplies in which a single supply powers circuitry of vastly different current draws, two perplexing steps are sensing the current that each output draws and deactivating the power supply in the event of an overload on that output. These issues are especially important in protecting the fragile PCB (printed-circuit-board) traces in low-level circuits. A typical circuit would use the base-emitter threshold voltage of approximately 0.6V of a bipolar transistor to trigger the power-supply-protection circuits. Although economical, the transistor’s threshold varies excessively over temperature; hence, the protection level is unstable.

figure shows.

Figure 1):

V_BA+(I_LOAD×R_SENSE)+(I_E×R₂)–V_BB=0.

[(V_BA–V_BB)+(I_LOAD×R_SENSE)]–I_ER₂=0.

I_C+I_B=I_E.

(V_BA–V_BB)+(I_LOAD×R_SENSE)–(I_C+I_B)R₂=0.

I_B=I_C/β.

V_BA–V_BB+I_LOAD×R_SENSE–(I_C+I_C/β)R₂=0.

V_BA–V_BB+I_LOAD×R_SENSE–[I_C×(β+1)/β]R₂=0.

V_OUT=I_CR₃.

I_C=V_OUT/R₃.

V_BA–V_BB+I_LOAD×R_SENSE–(V_OUT/R₃)(β+1/β)R₂=0.

If V_BA=V_BB, then V_BA–V_BB=0, and

I_LOAD×R_SENSE–(V_OUT/R₃)(β+1/β)R₂=0.

V_OUT=I_LOAD×R_SENSE[R₃/(β+1)](β/R₂).

If β is high, then β/(β+1)β)≈1, and V_OUT=(I_LOAD×R_SENSE×R₃)/R₂.