Delay effect when transistor turns on

乱世煮酒论天下

Delay effect when transistor turns on [Copy link]

Delay problems that occur when transistors are switched in saturation:

1. In order to make the field effect transistor switch, the base current added to the transistor is larger than the collector current/amplification factor, so as to reduce the loss by making the transistor saturated with a smaller Vce voltage. In this case, even if the base current is zero, the transistor cannot be turned off immediately, and the collector current will not become 0 until the accumulation time and rise time have passed. Then the voltage output by the collector changes at the same time as the collector current. What is the reason for this phenomenon? Is it related to the internal structure of the conductor?

2. In this working state, it is problematic to use it as a high-speed switching circuit. To make the transistor turn on more quickly, a capacitor is usually connected in parallel to the base resistor. This method will cause overdrive in the form of reverse current by improving the stored charge in the transistor, and the base current will have a downward spike. Is there any way to reduce this downward spike? In addition to connecting a capacitor in parallel to the base resistor, are there other ways to reduce the shutdown delay?

maychang

The title is "Delay effect when transistor is turned on", which should be changed to "Delay effect when transistor is turned off" because what needs to be discussed is: "In this case, even if the base current is zero, the transistor cannot be turned off immediately."

maychang

[Apart from adding a capacitor in parallel with the base resistor, are there any other methods to reduce the turn-off delay? ]

In principle, reverse current should be applied to the base when turned off.

led2015

By adjusting the value of the parallel capacitor, a balance can be found between reducing the turn-off delay and avoiding excessive spikes.

乱世煮酒论天下

maychang published on 2024-9-9 17:58 The title is "Delay effect when transistor is turned on", it should be changed to "Delay effect when transistor is turned off". Because the topic to be discussed...

This is a bit like why the CE voltage did not immediately rise to half of the bus voltage when a short circuit occurred when I measured it before, but there is a difference. The CE voltage did not recover after hundreds of us after the short circuit, but this one should recover to the power supply voltage in just a few us.

And generally this phenomenon should be more common when the encoder signal output collector is output.

吾妻思萌

The turning on and off of a transistor is the physics of a PN junction. Let's take a look at its physical effects.

吾妻思萌

When the PN junction is biased by a forward voltage, the diffusion current of the carriers increases, forming a forward current; when the PN junction is biased by a reverse voltage, a reverse saturation current is formed.

Teacher May said that adding a reverse voltage/current is the correct answer.