Problem: How can the attached circuit be optimized so that, over a temperature range of -50°C to 150°C, the short circuit detection threshold lies between 5 V and 9 V?
Requirements:
- The circuit is designed for short circuit detection.
- At device start-up, if the input voltage EX_IN is greater than or equal to 9 V and U_IN is not present, the circuit should not detect a short circuit.
- However, if U_IN is present and EX_IN exceeds 9 V, the circuit must detect a short circuit condition.
Circuit Description:
- The circuit includes a complementary pair of transistors: NPN (Q1, Q2) and PNP (Q3, Q4).
- Q1 and Q2 are NPN transistors; Q3 and Q4 are PNP transistors. BC846BPN-QX is used, cannot add it in schematic, please edit if needed.
Clarifications and Issues:
- Although BJT is Current controlled device, I am trying to understand why Q4 (PNP) is turning ON. Since its emitter and base are connected to U_IN, it should theoretically be OFF.
- The fact that Q4 is ON causes U_BE_Q2 break the threshold voltage and give Short circuit Failure, which creates major issue at high Temperature as threshold of BJT is lower around 200 - 300 mV.
- My simulation and optimization have yielded a detection range between 3 V and 9 V, but I do not fully understand the influence of resistors R7, R10, R8, and R9 on this behavior.
I assume, the NP (Base collector) Junction of Q4(PNP) is reverse biased and due to this current the voltage at base of Q2 is increased and forward biases the NPN. As R5 is also at high impedance in comparison to R7, the collector voltage has higher influence. Thus the forward biased is not only dependent on the voltage divider (R1, R2) and Q1 but also on Q4. Am i right? can somone please provide a theoretical understanding or better analysis, as from datasheet the leakage current can be 15nA and that is not enough to cause 200 -300 mV difference which can turn NPN ON.
Thank you in advance for your insights and assistance!
simulate this circuit – Schematic created using CircuitLab
