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Application of Field Effect Transistor in Switching Circuit

Warm hints: The word in this article is about 2050 and  reading time is about 11 minutes. 
Guidance: In mpn, the field effect transistor looks like the triode we talked about earlier, so many friends who repaired MPN have not been able to distinguish for a long time. Unified these same-looking triodes, field effect transistors, double diodes, and all kinds of voltage-stabilized IC are collectively referred to as "three-foot tube", ha, if so numb. I'm afraid it's very difficult for you to improve your maintenance skills quickly.

In mpn, the field effect transistor looks like the triode we talked about earlier, so many friends who repaired MPN have not been able to distinguish for a long time. Unified these same-looking triodes, field effect transistors, double diodes, and all kinds of voltage-stabilized IC are collectively referred to as “three-foot tube”, ha, if so numb. I’m afraid it’s very difficult for you to improve your maintenance skills quickly. Well, I’m afraid I don’t need to map the appearance of FETs here. It’s usually shown in the following figure in the circuit diagram.

field effect transistor
field effect transistor

Because its construction principle is abstract, we talk about its use in a popular way, so let’s not talk about it more. Because there are many kinds and different characteristics according to the requirements of the use occasion, we usually use the switch as power supply in mpn, so we need to pass through the current. Because of its large size, an enhanced field effect transistor (MOS) has been fabricated by a special manufacturing method. Its circuit diagram symbols are as follows:

circuit diagram of power mosfet
circuit diagram of power mosfet

Looking closely, you can see that there seems to be a difference between the two diagrams. By the way, there are actually two different kinds of enhanced FETs. The first one is called N-channel enhanced FETs, and the second one is called P-channel enhanced FETs. Their functions are just the opposite. As mentioned earlier, FETs are electrically controlled switches, so let’s first talk about how to use them as switches. From the figure we can see that they also have three feet like transistors. These three feet are called gate (G), source (S) and drain (D). The schematic diagram of patch elements in MPN is as follows:

schematic diagram of patch elements in MPN
schematic diagram of patch elements in MPN

‍According to the picture, G1 is the gate. This gate is the control pole. In the gate, voltage and no voltage are added to control the connection and disconnection of foot2 and foot3. In the N channel, foot2 and foot3 of voltage are added to the gate and the power is turned on. When the voltage is removed, the voltage is turned off. On the contrary, in the P channel, the voltage is turned off (high potential). Voltage drop (low potential) will be connected!

P-channel MOS transistors are often encountered in the open circuit of power supply in our common 2606 main control circuit diagram.

2606 Main Control Circuit Diagram
2606 Main Control Circuit Diagram

SI2305 in this picture is a P-channel MOS transistor. Because many friends are confused about checking this part of the fault, it is necessary to explain its working principle here to deepen your impression.

The positive current of the battery is connected to the two-foot source of the field effect transistor Q1 through the switch S1. Because Q1 is a P-channel transistor, its one-foot grid provides a positive potential voltage through the resistance R20, so it can’t turn on, the voltage can’t continue to pass, and the input foot of the 3V voltage stabilized IC can’t get a voltage, so it can’t turn on! At this time, if we press the SW1 boot button, the positive current is added to the base of the transistor Q2 through the button, R11, R23, D4. The base of the transistor Q2 obtains a positive potential. The transistor is on (as mentioned earlier when the transistor is on). Because the emitter of the transistor is directly grounded, the transistor Q2 is equivalent to Q1. The gate is directly grounded, and the voltage added to it through R20 resistor goes directly to the ground. The gate of Q1 changes from high potential to low potential. The Q1 conducts electricity from Q1 to the input pin of the 3V voltage stabilized IC. The 3V voltage stabilized IC is the Vcc of the U1 output 3V voltage, which is supplied to the main control. The main control is detected by resetting and clearing 0, reading firmware program. After a series of actions, a control voltage is transferred to PWR_ON and then sent to the base of Q2 through partial voltage R24 and R13, keeping Q2 in the on-state. Even if you open the key and disconnect the base voltage of Q1, when the control voltage from the main control is maintained, Q2 will always be on-state, and Q1 will be continuously on-state. Provide working voltage for 3V voltage regulator IC! SW1 also sends control signals of different time and times to the main control PLAY ON foot through the voltage divider of R11 and R30 resistors. The main control outputs different results to the corresponding control points through firmware identification, such as playback, pause, startup and shutdown, in order to achieve different working conditions.

Working Principle of junction field effect transistor (N-channel JFET)

N-channel JFET can be regarded as a faucet with “artificial intelligence switch”. There are three parts: inflow, artificial intelligence switch and effluent, which can be regarded as d, g and S poles of JFET respectively.

“Manual” embodies the “control” function of the switch, that is, vGS. When JFET works, a negative voltage (vGS < 0) is added between the gate and the source to reverse the PN junction between the gate and the channel. The gate current is iG 0, and the FET presents an input resistance of up to 107. By adding a positive voltage (vDS > 0) between the drain and the source, most carriers (electrons) in the N channel move from the source to the drain under the action of electric field, forming current iD. The size of iD is controlled by “manual switch” vGS. When vGS increases from zero to negative, the depletion layer of PN junction will be widened and the conductive channel narrowed. The greater the absolute value of vGS, the closer the manual switch is to turn off. The outflow of water (iD) must be smaller and smaller. When you turn off the switch to a certain extent, the water will not flow.

“Intelligence” reflects the “influence” of the switch. When the pressure difference between the two ends of the tap (vDS) is greater, the artificial switch will automatically “grow” intelligently. The larger the value of vDS, the faster the artificial switch grows, the closer the flow channel closes, the smaller the outflow water (iD), and when the artificial switch grows to a certain extent, the water will not flow. In theory, with the gradual increase of vDS, on the one hand, the channel electric field intensity increases, which is conducive to the increase of drain current iD; on the other hand, with vDS, a potential gradient along the channel is generated in the N-type semiconductor region composed of the source through the channel to the drain. Because the potential of N-channel increases gradually from the source to the drain, the potential difference between the drain and the drain is unequal at different positions from the source to the drain. The farther away from the source, the larger the potential difference is. The larger the reverse voltage of PN junction is added to the drain, and the more the depletion layer extends to the center of N-type semiconductor, making it close to the drain. The conductive channel at the pole is narrower than that near the source, and the conductive channel is wedge-shaped. So it is figuratively analogized that when the pressure difference between the two ends of the faucet (vDS) is greater, the artificial switch will automatically and intelligently “grow”.

When the switch first collides, it is in the pre-clamping state. After the pre-clamping, the ID tends to be saturated.

When vGS > 0, the PN junction will be positively biased to produce a larger grid current, which destroys its control effect on drain current iD. The manual switch will be pulled out, and a water inlet pipe will be added at the switch, which will have no control effect on the tap.

Working Principle of junction field effect transistor (N-channel JFET)
Working Principle of junction field effect transistor (N-channel JFET)

Working Principle of Insulated Gate Field Effect Transistor (N-Channel Enhanced MOSFET)

N-channel MOSFET can be regarded as a faucet with “artificial intelligence switch”. The corresponding situation is the same as JFET. Unlike JFET, the MOSFET was initially turned off manually and the water could not flow out. When vGS > 0 is added between grid sources and N-type induced channel (inversion layer) is generated, the manual switch is gradually turned on and the flow (iD) becomes larger and larger. The size of iD is controlled by the “manual switch” vGS. When vGS increases from zero to positive, the gate and P-type silicon wafer are equivalent to the planar capacitor with silicon dioxide as the medium. Under the positive gate source voltage, an electric field perpendicular to the surface of semiconductor, directed by the gate to the P-type substrate, is generated in the medium. This electric field repels the void. The minority electrons in the P-type substrate are attracted to the surface of the substrate. These electrons form a N-type thin layer on the P-type silicon near the gate, i.e. the N-type conductive channel between the conducting source and drain. The larger the gate voltage vGS is, the stronger the electric field on the semiconductor surface will be. The more electrons will be attracted to the P-type silicon surface. The thicker the induced channel, the smaller the channel resistance will be. The closer the manual switch is to be turned on, the more water (iD) will surely flow out. When you turn the switch on to a certain extent, the water flow will reach its maximum. The “intelligence” of MOSFET is the same as that of JFET.

Working Principle of Insulated Gate Field Effect Transistor (N-Channel Enhanced MOSFET)
Working Principle of Insulated Gate Field Effect Transistor (N-Channel Enhanced MOSFET)

Working Principle of Insulated Gate Field Effect Transistor (N-Channel Depletion MOSFET)

Basically the same as N-channel JFET, only when vGS > 0, N-channel depleted MOSFET does not produce positive current of PN junction due to the existence of insulation layer, but induces more negative charges in the channel, which makes the control effect of AI switch more obvious. There are only two states on and off for the switch. There are three states for the triode and FET to work: 1, cut-off, 2, linear amplification, 3, saturation (base current continues to increase while collector current does not increase). The circuit that makes the transistor work only in states 1 and 3 is called switching circuit, which is usually expressed as transistor cut-off and collector does not absorb current; when the transistor is saturated, the voltage difference between emitter and collector is close to 0 V. When switching circuit is used in digital circuit, the output potential is 0 when it is close to 0V and 1 when it is close to power supply voltage. So the transistors inside the digital integrated circuits all work in the switching state.

The saturation condition of transistor, V (working voltage) / Rc (load resistance value) = Ic, Ic/beta < Ib.

Transistor cut-off conditions, Ic_0; Ib < 0 (base pole can not be suspended with at least resistance grounding, if necessary can be used reverse bias, N-channel field effect transistor NFET, DS plus forward voltage, GS plus voltage Vgs, such as Vgs-Vdson=5v, then NFET is on, equivalent to the saturated conduction state of triode. Make voltage-controlled linear resistance and contactless, closed state electronic switch. NFET cuts off when Vgs is less than the pinch-off voltage. Make contactless, on-state electronic switch:

switch circuit of power mosfet(1)
switch circuit of power mosfet(1)
switch circuit of power mosfet(2)
switch circuit of power mosfet(2)

Relevant article: Working Principle of Power Mosfet

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