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Power Mosfet Tutorial with Video

Author: Apogeeweb Date: 23 Apr 2018  3651

Power mosfet tutorial

Summary

This paper is equal to a complete tutorial of power mosfet,the contents include the structure, working principle,characteristics and basic working circuit of power MOSFET and how to select the right power mosfet and etc. Since the development of power MOSFET in 1976, because of the development of semiconductor technology, its performance has been improved continuously: such as high voltage power MOSFET its working voltage can reach 1000V; low conduction resistance MOSFET its resistance is only lOm omega; working frequency range from DC to MHz; protection measures are more and more perfect; and develop a variety of patch type. Power MOSFET (such as Siliconix recently developed thickness of 1.5mm "Little Foot series"). In addition, the price is also decreasing, making the application more and more extensive, and many places are replacing bipolar transistors. Power MOSFET is mainly used in computer peripherals (soft and hard drives, printers, drafting machines), power supply (AC / DC converter, DC / DC converter), automotive electronics, audio circuits and instruments, instruments and other fields.

Catalog

Summary

Catalog

I Power Mosfet Basics

 1.1 What is Mosfet and Power Mosfet

 1.2 Types of Power Mosfet

II The Construction of Mosfet 

 2.1 The Structure of Power Mosfet

 2.2 The Working Principle of Power Mosfet

III Basic Characteristics of Power MOSFET

 3.1 Static Characteristic

 3.2 Dynamic Characteristics

 3.3 The Switching Speed of MOSFET

 3.4 How to Improve the Dynamic Performance of Power MOSFET

IV Compare Bipolar Power Transistor(BPT) and Power Mosfet

V The Selection Criteria of Power MOSFET

Book Recommendation

I Power Mosfet Basics

1.1 What is Mosfet and Power Mosfet

Bases on the questions that more users asked me is about mosfet basics,power mosfet basics,IGBT basics,after I updated the article "What is the Difference Between MOSFET and IGBT",I decided to make a series of mosfet to provide users more valuable knowledge.This article is about power mosfet.

Mosfet: MOS( Metal Oxide Semiconductor),FET (Field Effect Transistor), means that mosfet is to control semiconductor(S) field-effect transistor based on the effect of electric field across the oxide layer(O) of the metal layer(M) gate.

Power Mosfet: Power field effect transistors ( short for : POWER MOSFET) are also divided into two types: Junction Field Effect Transistor and Insulated Gate Field Effect Transistor, but usually mainly refer to the MOS (Metal Oxide Semiconductor FET) in the insulated gate type. Junction type power field effect transistor is generally called Static Induction Transistor (SIT).Its characteristic is that the gate voltage is used to control the leakage current, the driving circuit is simple, the driving power is small, the switching speed is fast, the working frequency is high, the thermal stability is better than GTR, but its current capacity is small and the voltage is low, and it is generally only suitable for power electronic devices with no more than 10kW power. Please video the following video to help you learn power mosfey more intuitively:

Tutorial on MOSFETs

1.2 Types of Power Mosfet

The type of power MOSFET: according to the conductive channel, it can be divided into P channel and N channel. When the gate voltage is zero, there is a conductive channel and an enhanced type. For N (P) channel devices, the gate voltage is greater than (less than) zero, and the power MOSFET is mainly the N channel enhancement.

Enhancement type: Normally switched “off”

Enhancement type of Power Mosfet

Enhancement type of Power Mosfet

Depletion type: Normally switched”ON”,without a gate bias voltage but requires a gate to source voltage(Vgs) to switch the device “OFF”

Depletion type of Power Mosfet

Depletion type of Power Mosfet

II The Construction of Mosfet 

2.1 The Structure of Power Mosfet

Power MOSFETs can be broadly categorized according to their gate and drift structures. 

following picture illustrates the three common structures currently used.

following picture (a) shows a double diffusion MOS (D-MOS) structure.

For the fabrication of D-MOS devices, channels are formed in a double diffusion process that provides high withstand voltage. The D-MOS process is well suited to increasing device density, making it possible to realize high performance power MOSFETs with low on-state resistance and low power loss. 

following picture (b) shows a trench gate structure.

The trench-gate process forms a vertical gate channel in the shape of a U groove in order to increase device density and thereby further reduce on-state resistance. The trench gate structure is employed to fabricate power MOSFETs with relatively low voltage.

following picture (c) shows a superjunction (SJ) structure.

This structure has a drift region that consists of alternating p- and n-type semiconductor layers. This process overcomes the inherent limitations of the vertical silicon process used with conventional power MOSFETs and delivers extremely low on-state resistance.Compared to conventional power MOSFETs, the superjunction process provides significant improvement in the trade-off between VDSS (maximum drain-source voltage) and Ron∙A (normalized on-state resistance per specific area), and therefore helps to considerably reduce conduction loss.

Power MOSFET Structures

Power MOSFET Structures

The internal structure and electrical symbols of the power MOSFET are shown in Figure 1; in its conduction, only one polar carrier (multichild) is involved in conducting and is a unipolar transistor. The electrical conduction mechanism is the same as the small power MOS tube, but the structure is different. The small power MOS tube is the transverse conductive device. The power MOSFET mostly uses the vertical conductive structure, also known as the VMOSFET (Vertical MOSFET), which greatly improves the voltage and current resistance of the MOSFET device.

The internal structure and electrical symbols of the power MOSFET

The internal structure and electrical symbols of the power MOSFET

According to the difference of vertical conductive structure, it is divided into VVMOSFET and VDMOSFET (Vertical Double-diffused MOSFET) with vertical conducting double diffusion MOS structure by using V type groove. This paper is mainly discussed with VDMOS device as an example.

The power MOSFET is a multicomponent integration structure, such as the HEXFET of the International Rectifier company (International Rectifier) with hexagonal units; the SIPMOSFET of SIEMENS (Siemens) uses a square unit; and the TMOS of Motorola (Motorola) uses a rectangular unit to be arranged in a "product" form.

2.2 The Working Principle of Power Mosfet

Deadline: drain source and positive power supply, gate voltage between zero source. The PN junction J1 formed between P base and N drift region is reverse biased, and there is no current flowing between the drain source.

Conduction: the positive voltage UGS is between the gate source and the grid is insulated, so no gate current will flow. However, the positive voltage of the grid pushes the hole in the P area below, and draws the minority electron in the P region to the P surface below the gate.

When the UGS is greater than the UT (opening voltage or threshold voltage), the electron concentration on the surface of the P area under the gate will exceed the hole concentration, making the P type semiconductor reverse form to the N type and become the reverse layer. The reverse layer forms the N channel to make the PN junction J1 disappear, the leakage and the source conduction.

III Basic Characteristics of Power MOSFET

3.1 Static Characteristic

The transfer characteristics and output characteristics of static characteristics are shown in the figure.(a) is transfer characteristics (b) is output characteristics

static characteristic of power mosfet

static characteristic of power mosfet

The relationship between the leakage current ID and the gate voltage UGS is called the transfer characteristic of MOSFET. When the ID is larger, the relationship between ID and UGS is approximately linear, and the slope of the curve is defined as the transconductance Gfs.

MOSFET's leakage volt ampere characteristics (output characteristics): cut-off zone (corresponding to the cut-off area of GTR); saturated zone (corresponding to the amplification area of GTR); unsaturated zone (corresponding to the saturated zone of GTR). The power MOSFET operates in the switch state, that is, the conversion between the cut-off area and the unsaturated zone. There is a parasitic diode between the drain poles of the power MOSFET, and the device is connected when the drain source is connected with the reverse voltage. The on state resistance of power MOSFET has positive temperature coefficient, which is beneficial for current sharing when parallel devices are connected.

3.2 Dynamic Characteristics

The test circuit and switching processing waveform of dynamic characteristics of power mosfet are shown in the figure.(a) is test circuit while (b) is switching processing waveform.

206a.png

dynamic characteristics of power characteristics

The opening process is: opening delay time TD (on) - up forward time to uGS=UT and starting the time interval between iD.

The rise time tr - uGS rises from uT to MOSFET to enter the time of gate pressure UGSP in unsaturated area.

The steady-state value of iD is determined by drain power supply voltage UE and drain load resistance. The size of UGSP is related to the steady-state value of iD. When UGS reaches UGSP, it continues to rise under up until it reaches steady state, but iD has not changed.

The opening time is ton -- the sum of the opening delay time and the rise time.

Turn off delay time TD (off) - up drops to zero, Cin passes through Rs and RG discharge, uGS decreases exponentially to UGSP, and iD begins to decrease to zero.

The descending time TF - uGS decreases from UGSP, and iD decreases to uGS.

Turn off time toff - turn off delay time and fall time.

3.3 The Switching Speed of MOSFET

The switching speed of MOSFET has a great relationship with the charge and discharge of Cin. The user can not reduce “Cin”, but it can reduce the drive circuit's internal resistance Rs to reduce the time constant and speed up the switching speed. MOSFET only depends on the multison conduction, and there is no small son storage effect, so the turn off process is very fast, the switching time is between 10 and 100ns, the working frequency can be reached. Above 100kHz is the highest in the main power electronic devices.

The field control device is static with almost no input current. But in the process of switching, it is necessary to charge and discharge the input capacitor, and still needs a certain driving power. The higher the switching frequency, the greater the required driving power.

3.4 How to Improve the Dynamic Performance of Power MOSFET

In addition to the device's voltage, current and frequency, the device must be used to protect the device in the application, and the device is not damaged in the transient change. Of course, the thyristor is a combination of two bipolar transistors, plus the large capacitance brought by large area, so its dv/dt capability is relatively fragile. For di/dt, there is still a problem of expansion of the conduction area, so it also brings quite strict restrictions.

The situation of power MOSFET is very different. Its ability of dv/dt and di/dt is usually estimated by the ability of every nanosecond, rather than every microsecond. Nevertheless, it also has limitations in dynamic performance. These can be understood from the basic structure of power MOSFET.

The following picture is the structure and equivalent circuit of power MOSFET. In addition to capacitance in almost every part of the device, we must consider that MOSFET is connected with a diode in parallel. At the same time, from a certain perspective, it still has a parasitic transistor. (like IGBT is also a parasitic thyristor). These aspects are very important factors to study the dynamic characteristics of MOSFET.

the structure and equivalent circuit of power MOSFET

the structure and equivalent circuit of power MOSFET

IV Compare Bipolar Power Transistor(BPT) and Power Mosfet

  • Drive circuit

Bipolar: Drive conditions are difficult to determine because switching time varies with drive current conditions. Also, the drive circuit suffers high power loss.

Power Mosfet: The drive circuit for the voltage control of a power MOSFETs is simpler and offers lower power loss than that of a bipolar resistor.

  • On-state voltage

Bipolar: Even high voltage bipolar power transistors have very low on-state voltage and generally have a negative temperature coefficient.

Power Mosfet: Low-voltage power MOSFETs have an extremely low on-state voltage. High voltage devices have a slightly higher on-state voltage. Power MOSFETs have a positive temperature coefficient, which is beneficial in connecting multiple devices in parallel.

  • Switching time

Bipolar: Due their structure, bipolar transistors have a storage time tstg and therefore a longer switching time than MOSFETs.

Power Mosfet:Power MOSFETs are much faster than bipolar power transistors. Power MOSFETs have no storage time and are less affected by

temperature.

  • Temperature stability

Bipolar: A certain amount of care is required because an increase in temperature causes hFE to increase and VBE to decrease.

Power Mosfet: Various characteristics exhibit outstanding temperature stability.

5. Breakdown voltage(Collector-emitter,drain-source)

Bipolar: Bipolar power transistors are often used with a reverse current between the base and emitter. Sometimes, both VCES and VCEX (VCBO) are rated.

Power Mosfet: except for trench MOSFETs operating in a reverse gate bias condition (during which the withstand voltage is restricted by VDSX).

V The Selection Criteria of Power MOSFET

How to select products that meet your needs according to the manufacturer's manual sheet? The following four steps can be taken to select the right MOSFET.

  • Channel selection

The first step in selecting the correct devices is to decide that N channel or P channel MOSFET. is used in typical power applications. When a MOSFET is grounded and the load is connected to the main line voltage, the MOSFET forms a low-voltage side switch. In the low voltage side opening, N channel MOSFET should be adopted, which is due to the consideration of the voltage needed for closing or conduction devices. When MOSFET is connected to the bus and the load grounding, the high voltage side switch is used. Usually, the P channel MOSFET is used in this topology, which is also due to the consideration of voltage drive.

  • Voltage and current

The greater the rated voltage, the higher the cost of the device. According to practical experience, rated voltage should be greater than trunk voltage or bus voltage. Only in this way can enough protection be provided to ensure that MOSFET will not fail. For the choice of MOSFET, it is necessary to determine the maximum potential voltage that may be taken between the drain pole to the source, that is, other security factors to be considered by the largest VDS. Design Engineer include voltage transients induced by switched electronic devices such as motors or transformers. The rated voltage of different applications is also different; usually, the portable device is 20V, the FPGA power supply is 20~30V, the 85~220VAC application is 450~600V. in the continuous conduction mode, the MOSFET is in the steady state, and the current passes through the device continuously. Pulse spike refers to a large number of surge (or spike current) flowing through the device. Once the maximum current under these conditions is determined, it is necessary to directly select the device that can withstand the maximum current.

  • Calculation of conduction loss

The power loss of MOSFET devices can be calculated by Iload2 * RDS (ON). As the conduction resistance changes with temperature, the power consumption will also change proportionately. For portable design, it is easier to use lower voltage, but for industrial design, higher voltage can be adopted. Note that the resistance of RDS (ON) will increase slightly with the current. A variety of electrical parameters on RDS (ON) resistance can be found in the technical data sheet provided by the manufacturer.

  • Heat dissipation requirements of the computing system

Designers must consider two different situations, that is, the worst case and the real situation. It is recommended to use the worst case calculation results, because this result provides greater margin of safety to ensure that the system will not fail. There are also some measured data on the MOSFET table, such as the thermal resistance between the semiconductor junction and the environment, as well as the maximum junction temperature.

Switching loss is also a very important index. The product of voltage and current is quite large at the instant of conduction, which determines the switching performance of the device to some extent. However, if the system requires a relatively high switch performance, it can choose a relatively small gate power QG MOSFET.


Book Recommendation

  • Power MOSFETs: Theory and Applications

Details the theory of power MOSFETs and their applications. Explains the basis of MOSFET characteristics, and the features that determine MOSFET behavior. Examines the interaction of the MOSFET device with other elements in the circuit, and how device characteristics influence circuit design. Describes several circuits at length to highlight the practical details of power MOSFET use.

--Duncan A. Grant(Author), John Gowar(Author)

  • Advanced Power MOSFET Concepts

During the last decade many new concepts have been proposed for improving the performance of power MOSFETs. The results of this research are dispersed in the technical literature among journal articles and abstracts of conferences. Consequently, the information is not readily available to researchers and practicing engineers in the power device community. There is no cohesive treatment of the ideas to provide an assessment of the relative merits of the ideas.

"Advanced Power MOSFET Concepts" provides an in-depth treatment of the physics of operation of advanced power MOSFETs. Analytical models for explaining the operation of all the advanced power MOSFETs will be developed. The results of numerical simulations will be provided to give additional insight into the device physics and validate the analytical models. The results of two-dimensional simulations will be provided to corroborate the analytical models and give greater insight into the device operation.

--B. Jayant Baliga  (Author)

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pinglun 4 comments

    • pingluntus
    • kur on 2018/4/23 15:05:28

    Thank you sir the knowledge about power mosfet is clearly

    • pingluntus
    • sarumdo on 2018/4/23 15:17:49

    The switching speed of MOSFET has a great relationship with the charge and discharge of Cin. The user can not reduce the Cin, but it can reduce the drive circuit's internal resistance Rs to reduce the time constant and speed up the switching speed. The MOSFET only depends on the multison conduction, and there is no small son storage effect, so the turn off process is very fast, the switching time is between 10 and 100ns, and the working frequency can reach 10. Above 0kHz is the highest among the main power electronic devices.

    • pingluntus
    • lana on 2018/4/23 15:22:21

    What's the difference between P channel and N channel?

      • pingluntu
      • author on 2018/4/23 15:29:11
        author

      Re:

       Power field effect transistors ( short for : POWER MOSFET) are also divided into two types: Junction Field Effect Transistor and Insulated Gate Field Effect Transistor.

    • pingluntus
    • tomy on 2018/4/23 16:17:10

    I am trying to control a 12V and 24V LED inside a push button by using mosfet.However I can't make the LED fully turn OFF and ON as it would when applying directly the 12V or 24V power source to the LED directly.I guess no current flows when joining LED + mosfet? How could I do?

      • pingluntu
      • author on 2018/4/23 16:20:59
        author

      Re:

      sir,you need to give me more detail about it,I guess that:

      maybe you mosfet is broken?

      maybe your mosfet is not suitable for you circuit?

      maybe your code is wrong?

      Just check it clearly

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