MOSFET and transistor, in the ON state, MOSFET usually uses Rds, and transistor usually uses saturated Vce. So is there a situation where the reverse is possible, where the transistor uses saturated Rce and the MOSFET uses saturated Vds?
When the transistor is in the ON state, it works in the saturation zone. The conduction current Ice is mainly determined by Ib and Vce. Since the base driving current Ib of the transistor generally cannot remain constant, Ice cannot be simply determined by Vce alone, that is, the saturated Rce cannot be used. to express (because Rce will change). Since Vce is small in the saturated state, transistors are generally represented by saturated Vce.
When the MOS tube is in the ON state, it works in the linear region (equivalent to the saturation region of the triode). Similar to the triode, the current Ids is determined by Vgs and Vds, but the driving voltage Vgs of the MOS tube can generally remain unchanged, so Ids can only be affected by Vds. The impact is that when Vgs is fixed, the on-resistance Rds remains basically unchanged, so the MOS tube adopts the Rds method.
Current can flow through D and S of MOSFET in both directions. It is this outstanding advantage of MOSFET that makes there is no concept of DCM in synchronous rectification. Energy can be transferred from input to output, and energy can also be returned from output to input. Able to achieve two-way flow of energy.
Next, let’s discuss it in depth. Since D and S of point and MOS are interchangeable, why do we define DS?
For the MOS tube inside the IC, it must be completely symmetrical during manufacturing. The purpose of defining D and S is to facilitate the discussion of current flow and calculation.
Second point, now that D and S are defined, what is the difference between them?
For power MOS, sometimes due to special applications, such as withstand voltage or other purposes, a lightly doped region withstand voltage is made at the D end of the NMOS. In this case, D and S will be different.
Third point: After D and S are exchanged, how are the characteristics of MOS different from the original ones? Such as Vth, Miller effect, parasitic capacitance, on-resistance, breakdown voltage Vds.
After the DS is exchanged, when Vgs=0, the tube can still conduct as long as Vds>0.7V, but it could not be conducted before the exchange. When Vgs>Vth, the inversion layer channel has been formed, and the two have the same characteristics after interchange.
Determination of D and S
We just say that the current can be from D--to--S, or it can be from S----to---D. But it does not mean that the names of the two terminals D and S are interchangeable.
The width of the DS channel is controlled by the GS voltage. When G is fixed, who is S is determined.
If the above is determined to be the S end, it is considered to be D.
Think of what was originally D as S, and apply voltage to G and this S. As a result, the channel does not change and is still closed.
When Vgs does not reach Vth, Cgs is charged through the driving resistor R. The model at this stage is a simple RC charging process.
When Vgs charges to Vth, the DS conductive channel begins to open, and Vd begins to decrease sharply. According to I=C*dV/dt, the parasitic capacitance Cgd has current flowing in the direction: G --》D. According to the G contact KCL, the Igd current will shunt IR, and most of the driving current will be diverted to Igd, leaving a small part to continue flowing to Cgs. Therefore, Vgs appears a short period of flatter change. This is the miler platform.
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