Explain the difference between PMOS and NMOS
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2017-09-18
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MOS tubes are divided into two types: N channel and P channel. In practical applications, we are basically enhanced MOS tubes.
We usually use NMOS because of its small resistance and easy fabrication. As shown on the MOS tube diagram, there is a parasitic diode between the drain and the source. This is called body diode, which is important in driving inductive loads (such as motors). By the way, the body diode only exists in a single MOS tube, usually not inside the integrated circuit chip.
1. pass characteristic
NMOS characteristics, Vgs greater than a certain value will be on, suitable for source grounding (low driven), as long as the gate voltage reached 4V or 10V on it.
PMOS features, Vgs is smaller than a certain value will be on, suitable for source access to VCC (high-end driver). However, although PMOS can be easily used as a high-end driver, due to the high resistance, high cost and few types of substitution, NMOS is usually used in high-end drives.
2.MOS switching loss
Whether it is NMOS or PMOS, the conduction resistance is present after conduction, so that the current will consume energy on this resistor, and this part of the energy consumed is called conduction loss. Selecting a small MOS tube with a small resistance will reduce conduction losses. Now the small power MOS tube conduction resistance is generally in the tens of cents or so, a few cents Europe also has.
MOS must not be done in a flash at the time of turn-on and stop. The voltage at both ends of the MOS has a descent process, and the current flowing through it has a rising process. During this time, the loss of the MOS tube is the product of voltage and current, called switching loss. In general, switching losses are much larger than conduction losses, and the higher the switching frequency, the greater the loss.
The product of the turn-on of the instantaneous voltage and current is very large, and the losses are great. Shortening the switching time can reduce the loss per turn-on, and reduce the switching frequency, which can reduce the number of switches per unit time. Both of these two methods can reduce switching losses.
3.MOS tube drive
Compared with bipolar transistors, it is generally assumed that the MOS tube does not require current conduction, as long as the GS voltage is higher than a certain value. It's easy to do, but we still need speed.
In the structure of the MOS tube, it can be seen that there is a parasitic capacitance between the GS and the GD, and the driving of the MOS tube is actually charging and discharging of the capacitor. Charging a capacitor requires an electric current, because the capacitor can be charged as a short circuit, so the instantaneous current will be larger. The first thing to note in choosing / designing a MOS tube driver is the ability to provide instantaneous short-circuit current.
Second note that the NMOS is commonly used in high-end drives where the gate voltage is greater than the source voltage. When the MOS transistor is driven by the high side, the source voltage is the same as the drain voltage (VCC), so the gate voltage is greater than VCC, 4V or 10V. If you want to get a larger voltage than the VCC in the same system, you need a special boost circuit. Many motor drives are integrated with charge pumps, and it should be noted that a suitable external capacitor should be selected to obtain sufficient short circuit currents to drive the MOS tube.
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