Difference And Selection Analysis Of Triode And MOS Tube In Switching Power Supply Circuit Design

- Dec 29, 2019-

How do triodes and MOS transistors distinguish and choose in the design of switching power supply circuits:

The working principle of the triode: the triode is a current amplifier device with three poles, which are called collector C, base B, and emitter E. Divided into NPN and PNP. We take the common emitter amplifier circuit of the NPN transistor as an example to explain the basic principle of the transistor amplifier circuit.

We call the current flowing from base B to emitter E as base current Ib; the current flowing from collector C to emitter E is called collector current Ic. These two currents flow out of the emitter, so an arrow is used on the emitter E to indicate the direction of the current. The amplification effect of the triode is that the collector current is controlled by the base current (assuming that the power supply can provide a large enough current to the collector), and a small change in the base current will cause a large change in the collector current, and The change satisfies a certain proportional relationship: the change amount of the collector current is β times the change amount of the base current, that is, the current change is amplified by β times, so we call β the magnification of the triode (β is generally much larger than 1, for example, a few Ten, hundreds). If we add a changing small signal between the base and the emitter, this will cause the base current Ib to change. After the change in Ib is amplified, it will cause a large change in Ic. The transistor is a current-controlled device.

Mos tubes are metal-oxide-semiconductor field effect transistors. It is also called metal-insulator-semiconductor. The source and drain of the MOS tube can be reversed. They are both N-type regions formed in the P-type backgate. In most cases, these two areas are the same, even if the two ends are swapped, it will not affect the performance of the device. Such devices are considered symmetrical.

What happens when the gate of the MOS capacitor is positively biased relative to the substrate. The electric field passing through GATEDIELECTRIC strengthened, and more electrons were pulled up from the substrate. At the same time, the holes are repelled from the surface. As the GATE voltage increases, there will be more electrons on the surface than holes. Due to the excess electrons, the silicon surface looks like N-type silicon. The inversion of the doping polarity is called inversion, and the inverted silicon layer is called a channel. As the GATE voltage continues to increase, more and more electrons accumulate on the surface, and the channel becomes a strong inversion. The voltage when the channel is formed is called the threshold voltage Vt. When the voltage difference between GATE and BACKGROUND is less than the threshold voltage, no channel is formed. So MOS is a voltage-controlled device.

(1) The field effect transistor is a voltage control element, and the transistor is a current control element. In the case where only less current is allowed to be taken from the signal source, a field effect transistor should be used; while in the condition that the signal voltage is lower and more current is allowed to be taken from the signal source, a transistor should be used.

(2) The field-effect transistor conducts electricity by using majority carriers, so it is called a unipolar device, and the transistor has both majority carriers and conduction by minority carriers. It is called a bipolar device.

(3) The source and drain of some FETs can be used interchangeably, the gate voltage can also be positive or negative, and the flexibility is better than that of a transistor.

(4) FETs can work under very low current and low voltage conditions, and its manufacturing process can easily integrate many FETs on a silicon chip, so FETs are used in large-scale integrated circuits Has been widely used.

(5) Field-effect transistors have advantages such as higher input impedance and low noise, so they are also widely used in switching power supplies and various electronic devices. In particular, the field-effect transistor is used as the power driver of the switching power supply, and performance that is difficult to achieve by ordinary transistors can be obtained.

(6) The field effect tube is divided into two types: junction type and insulated gate type, and their control principles are the same.

There are many differences between the transistor BJT and the field effect transistor FET. A few are listed briefly:

1. Current control for triode, MOS tube is voltage control, BJT amplifies current, FET converts gate voltage to drain current. The first parameter of BJT is the value of the current amplification factor β, and the first parameter of the FET is the transconductance gm;

2. Driving ability: MOS tubes are commonly used for power switching tubes and high-current local switching circuits;

3. Cost issues: Triodes are cheap, MOS tubes are expensive;

4.BJT has poor linearity and FET has good linearity;

5.BJT noise is larger, FET noise is smaller;

6. BJT polarity has only two types of NPN and PNP. FET polarity has N-channel, P-channel, and depletion type and enhancement type, so FET selection and use are more complicated;

7. Power consumption problem: BJT input resistance is small, high current consumption, FET input resistance is large, almost no current consumption;

In fact, the triode is relatively cheap and convenient to use, and is often used in digital circuit switch control; MOS tubes are used in high-frequency and high-speed circuits, high current occasions, and places that are more sensitive to base or drain control current.

In general, whether in discrete component circuits or integrated circuits, FETs replacing BJTs are a big trend.