Three times and five times three-stage tube
Into the wood three points to talk
Teacher Zhang Fei takes you to answer questions
Question one
Problem 1: In the push-pull output driver circuit, the EB terminal of the transistor can withstand the maximum reverse voltage Veb=6V (NPN). When the switching signal is low, the gate voltage is 12V when discharging, The pressure drop is greater than 6V, isn't it burning the tube?
Answer: This is from the process analysis, PWM rising edge and falling edge are time-consuming, and the diode between be has a 0.7V voltage drop clamp function. During normal operation, during the process of driving PWM from 0 to 12V, the N tube can be turned on as long as it is greater than 0.7V, the emitter follows, and the P tube is completely turned off due to the negative voltage of 0.7V; when the PWM is driven from 12V to 0V In the middle, when the voltage drop of the P-tube reaches 0.7V, the conduction starts, and the e-pole decreases with the decrease of the b-pole voltage, and the negative pressure of the N-tube be 0.7V is completely cut off, and does not exceed 6V, so it is safe.
Question two
What is the understanding of the overvoltage protection circuit (partial circuit) in the figure below?
Answer: Assume that the normal output voltage is 12V. When the output is overvoltage, the Zener has a current flowing through it, the transistor is turned on, and the output is low to turn off the PWM signal. Among them, the overvoltage threshold is mainly determined by the Zener diode. The Zener diode generally needs to pass 2mA to achieve voltage regulation. By changing the Zener diode to achieve large-scale adjustment; R20 and R21 achieve resistance voltage division, which can affect the range within a certain range. The magnitude of the pressure threshold, another role of R20 is the current limit; the purpose of capacitor C9 is to absorb the peak voltage on the one hand, and to stabilize the voltage on the other hand, and also play a certain delay.
Question three
When the triode works in the switching state, the purpose of the pull-up resistor and the pull-down resistor?
answer:
1. Prevent high resistance state, affect the normal operation of the circuit, and cause safety accidents;
2. Providing a discharge circuit for the triode interelectrode capacitance;
3. Provide a bleed circuit for static electricity and lightning, and protect the device;
According to engineering experience, it is generally appropriate to take 2K.
Question four
As shown in Figure 4, how to use the small signal of the microcontroller to control large signals?
Answer: Since Q1 is connected to a large voltage of 50V, if you want to directly control it, the driving voltage needs to be high, and the microcontroller cannot do it. So you can change the angle, add a switch on the Q1 control end, which is equivalent to a large resistance. When the switch is open, R tends to infinity, then the drive terminal voltage is 50V, Q1 is open; when the current switch is closed, the resistance is approximately 0. Q1 can be turned on, so that the circuit function can be realized, so the front switch can be realized with an NPN. The R3 resistor is used for current limiting.
Question five
Regarding the Zener diode, if it does not reach the regulation value (such as 5.6V), is there no current flowing?
answer:
Regarding these semiconductor devices, there is a leakage current, but since the leakage current is small, it is negligible, and no current flows. Now analyze the circuit of Figure 5:
In the figure, C1 is the bootstrap capacitor. The purpose of this circuit is to charge C1 through R1 for 30V input, charge to 12V and then supply power to the back circuit. Here is the use of the characteristics of the Zener diode, does not reach the voltage of 11V, there is no current flowing. However, this circuit is only suitable for theoretical analysis, because the triode works in an amplified state, and the voltage drop between CEs is large (approximately 12V voltage drop), and the loss is large. There is also the R3 resistor in the figure, which can be omitted here .
UV Screen Protector,UV Curing Machine,UV Curing Screen Protector
Shenzhen Jianjiantong Technology Co., Ltd. , https://www.jjthydrogelprotector.com