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Ask for help about the switching control of the transistor.

Hardware design
8월 04, 2020 by Camille 1434

As shown in the figure, 5vp_ctl is connected to the single-chip microcomputer control, and DC_5VP outputs a controllable power supply. When the R72 uses 51K, the back-end power supply may not be available in some cases. intermittent. But when I change to 100K, this problem will be eliminated. Please ask everyone. In my own understanding, when using 100K, VBE will be relatively smaller and Ib will be relatively smaller. On the contrary, it can eliminate the situation that the back-end power supply cannot be supplied. Very strange, thank you all for answering

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Dustin 게시 날짜 August 4, 2020

It feels like a problem with bias voltage.

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Erika 게시 날짜 August 4, 2020

VBE will not change, it has been around 0.7V.

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Allie 게시 날짜 August 4, 2020

According to your statement, we use VBE=0.7V to calculate, Ir76=0.7/33k=0.021mA, Ir72=4.3/51k=0.084mA, ib=0.063mA, ic=5/51k=0.098mA when the transistor is turned on (ignoring MOS Tube current), as long as the magnification of the transistor is greater than a few points, it can actually drive the load. Insufficient back-end power supply may be related to the selection of the MOS tube. I checked the VGS turn-on voltage of this MOS tube is 8V, and your input VGS is 5V. As the load current of the MOS tube increases, the voltage of the MOS tube VGS is What can't be driven. As for what you said, changing the resistance is just fine. It is just a coincidence caused by the GS junction capacitance of the MOS tube. If you continue to increase the load current, this phenomenon should still occur.

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