May I ask why the voltage of the B pole of the Q7 circuit is 23.5. When his B pole and the C pole of the Q8 are disconnected, the C pole of the Q7 is 24V. Please help explain this circuit. The following is my simulation results. I don’t know if Q7 is How to turn on, why does the B pole have voltage?
Kasper Posté sur July 2, 2020
From +24v down through a P/N junction voltage drop. Although there is no current, it is normal. If it is not 23.5V, there is a problem. Q7 B-E indirect 10M ohm resistor can be easily verified by simulation. In addition, Q7 is not turned on, and Q8 emitter can be easily verified by connecting a 1K resistor to ground.
Bjorn Posté sur July 2, 2020
The voltage drop of PN junction is 0.4-0.7, the parameters of the picture are not right, input 24 output 23.5 can not use this circuit.
Ozzy Posté sur July 2, 2020
How do I think it is right? The first picture, Q8, is on, so the Q7 B pole is initially low, so Q7 is also on. Does the output go higher and the B pole also goes higher;
The second picture, if left unconnected, will also leak out.
Sultan Posté sur July 2, 2020
Appropriately reduce the LDO input voltage, share power consumption, avoid excessive temperature rise of a single device, and avoid the use of heat sinks.
Rina Posté sur July 2, 2020
In Figure 2, the collector current of Q8 is the base current of Q7, so Q7 is always on. When turned on, the output voltage of the circuit (ie Q8 emitter) is equal to the base potential of Q8 minus a P/N junction voltage. What is the base potential of Q8? Obviously, the two 18V series voltage regulators at its base are in the cut-off state, so the base potential of Q8 is equal to +24V minus Ib * (R13 + R4), Ib is extremely small and can be ignored, so the output voltage is +24V-Vbe, About 23.5V.
This circuit can be regarded as an emitter follower with a very high current gain connected by two complementary transistors. The current gain is about the product of the β values of the two transistors. Therefore, in a large load current range, you can choose the Q8 base The voltage can get the required output voltage. For example, if you short-circuit an 18V regulator in the figure 201, you will get a regulated power supply of about 17.5V.
