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One 、MOS Management knowledge

MOS tube , Namely metal （Metal）— oxide （Oxide）— semiconductor （Semiconductor） Field effect transistor , It is a kind of semiconductor device that applies the principle of field effect .

Compared with ordinary bipolar transistors ,MOS The tube has high input impedance 、 Low noise 、 Large dynamic range 、 Low power consumption 、 Easy integration and other advantages , In switching power supply 、 Ballast 、 High frequency induction heating 、 High frequency inverter welder 、 Communication power supply and other high-frequency power supply fields have been more and more widely used .

MOS Type and structure of pipe

MOS Tube is FET A kind of （ The other is JFET Junction FET ）, There are two main structural forms ：N Channel type and P Channel type ; According to different field effect principles , It is divided into depletion type （ When the gate voltage is zero, there is a large drain current ） And enhanced （ When the grid voltage is zero , The drain current is also zero , A certain gate voltage must be added before the drain current ） Two kinds of . therefore ,MOS Tubes can be made into P Channel enhanced 、P Channel depletion type 、N Channel enhanced 、N Channel depletion type 4 Types of products .

Chart 1  MOS Tubular 4 Types

every last MOS The tubes are provided with three electrodes ：Gate Grid （ Expressed as “G”）、Source Source pole （ Expressed as “S”）、Drain Drain electrode （ Expressed as “D”）. When wiring , about N The power input of the channel is D, Output is S;P The power input of the channel is S, Output is D; And enhanced 、 The connection method of depletion type is basically the same .

Chart 2  MOS Pipe internal structure diagram

It can be found from the structure diagram ,N The source and drain of the channel fet are connected to N On type a semiconductors , and P The source and drain of the channel fet are connected to P On type a semiconductors . The output current of the FET is determined by the input voltage （ Or field voltage ） control , The input current is very small or there is no current input , This makes the device have high input impedance , This is also MOS The important reason why tubes are called field effect tubes .

MOS How the tube works

1、N Principle of channel enhanced FET

N Channel enhanced MOS Tube in P A layer is formed on the type a semiconductor SiO2 Film insulation , Then, two highly doped..., are diffused by photolithography N Type area , from N Type area lead out electrode （ Drain electrode D、 Source pole S）; Between source and drain SiO2 The insulating layer is plated with metal aluminum as the grid G;P Type semiconductor is called substrate , Use symbols B Express . Because the grid and other electrodes are mutually insulated , therefore NMOS Also known as insulated gate FET .

When the grid G Heyuanji S No voltage is applied between , namely VGS=0 when , Because of the drain and source N+ There are P Type substrate , Equivalent to two back-to-back connected PN junction , The resistance between them is as high as 1012Ω, namely D、S There is no conductive channel between , So no matter in leakage 、 What polarity voltage is applied between the sources , Will not produce drain current ID.

Chart 3  N Channel enhanced MOS Schematic diagram of pipe structure

When the substrate B And source S Short circuit , At the gate G Heyuanji S Apply positive voltage between , namely VGS＞0 when , As shown in the chart 3（a） Shown , Then an electric field is generated between the grid and the substrate from the grid to the substrate . Under the action of this electric field ,P The holes near the surface of the substrate will move downward when they are repelled , Electrons are attracted by the electric field and move towards the surface of the substrate , Compound with holes on the surface of the substrate , A depletion layer is formed .

If further improved VGS voltage , send VGS Reach a certain voltage VT when ,P All the holes in the surface layer of the substrate are repelled and exhausted , Free electrons are largely attracted to the surface layer , From quantity to quality , Make the surface layer become multi electron free N Type layer , be called “ Inversion layer ”, As shown in the chart 3（b） Shown .

The inversion layer will drain D Heyuanji S Two N+ The type areas are connected , Constitute a leak 、 Between the sources N Type conductive channel . Start to form conductive channels VGS The value is called threshold voltage or on voltage , use VGS（th） Express . obviously , Only VGS＞VGS（th） Only when there is a channel , and VGS The bigger it is , The thicker the ditch , The smaller the on resistance of the channel , The stronger the conductivity ;“ Enhanced ” From this comes the word .

Chart 4   Structural diagram of depletion layer and inversion layer

stay VGS＞VGS（th） Under the condition of , If at the drain D Heyuanji S Add positive voltage between VDS, There will be current flow in the conductive channel . The drain current flows from the drain region to the source region , Because the channel has a certain resistance , So the voltage drop along the channel , The potential at each point of the channel decreases gradually from the drain region to the source region along the channel , Voltage near one end of the drain area VGD Minimum , Its value is VGD=VGS－VDS, The corresponding channel is the thinnest ; The voltage at one end near the source region is the largest , be equal to VGS, The corresponding channel is the thickest .

In this way, the thickness of the channel is no longer uniform , The whole channel is inclined . With VDS The increase of , The channel near one end of the leakage area is getting thinner .

When VDS Increase to a critical value , send VGD≤VGS（th） when , The channel at the leakage end disappears , Only the depletion layer is left , This situation is called channeling “ Pre pinch off ”, As shown in the chart 4（a） Shown . Keep growing VDS[ namely VDS＞VGS－VGS（th）], The pinch point moves towards the source , As shown in the chart 4（b） Shown .

Although the pinch point is moving , But the channel area （ Source pole S To pinch point ） The voltage drop remains unchanged , Still equal to VGS－VGS（th）. therefore ,VDS Excess voltage [VDS－（VGS－VGS（th））] All down to the pinch off area , A strong electric field is formed in the pinch off region . At this time, electrons flow along the channel from the source to the pinch off region , When the electron reaches the edge of the pinch off region , Under the action of strong electric field in the pinch off area , It will drift to the drain very quickly .

Chart 5   Schematic diagram of pre pinch off and pinch off area formation

2、P Principle of channel enhanced FET

P Channel enhanced MOS The reason is N Type substrate P Type inversion layer , It is through lithography 、 Diffusion method or other means , stay N Type substrate （ Substrate ） Two doped P District , Lead out the electrodes respectively （ Source pole S And the drain D）, At the same time, between the drain and the source SiO2 A metal grid is made on the insulating layer G. Its structure and working principle are the same as N Ditch MOS Tube similar ; Just use the grid - Source and drain - Source voltage polarity and N Ditch MOS Tube opposite .

In normal operation ,P Channel enhanced MOS The substrate of the tube must be connected to the source , And the voltage of drain to source VDS Expected negative value , To ensure two P Between the region and the substrate PN Are all negative , At the same time, in order to form a conductive channel near the top surface of the substrate , The grid to source voltage should also be negative .

Chart 6  P Channel enhanced MOS Schematic diagram of pipe structure

When VDS=0 when . Add negative voltage ratio between grid and source , Due to the existence of insulating layer , So there is no current , But the metal grid is recharged and accumulates negative charges ,N The multiproton electrons in type a semiconductors are repulsed by negative charges and move into the body , The surface leaves positively charged ions , Form depletion layer .

With G、S The increase of negative voltage between , Depletion layer widening , When VDS When it reaches a certain value , Holes in the substrate （ Young son ） Attracted to the surface by the negative charge in the gate , A gap is formed between the depletion layer and the insulating layer P Type thin layer , Called inversion layer , As shown in the chart 6（2） Shown .

This inversion layer forms a conductive channel between the drain and the source , At this moment VGS This is called the on voltage VGS（th）, achieve VGS（th） Then add , The more holes induced on the substrate surface , The inversion layer is widened , The width of the depletion layer does not change , So we can use VGS The size of controls the width of the conductive channel .

Chart 7  P Channel enhanced MOS Schematic diagram of tube depletion layer and inversion layer formation

When VDS≠0 when . After the conductive channel is formed ,D、S When negative voltage is applied between , Then there will be a drain current between the source and drain ID circulation , and ID along with VDS And increase ,ID The voltage drop along the channel makes the voltage between each point on the channel and the grid no longer equal , This voltage weakens the effect of the negative charge electric field in the grid , Gradually narrow the channel from drain to source , As shown in the chart 7（1） Shown .

When VDS Increase to make VGD=VGS（ namely VDS=VGS-VGS（TH））, The channel is pre clamped near the drain , As shown in the chart 7（2） Shown . Continue to increase VDS, The pinch off area is only slightly longer , The channel current basically maintains the value at pre pinch off , The reason is that it will continue to increase when there is pre pinch fracture VDS,VDS The excess part of the is all added to the pinch off area near the drain , Therefore, the drain current formed ID Approximate to VDS irrelevant .

Chart 8  P Channel enhanced MOS Schematic diagram of pipe pre pinch off and pinch off area formation

3、N Principle of channel depletion FET

N Channel depletion type MOS Tube structure and reinforced MOS The tube structure is similar , It's just a little different , Namely N Channel depletion type MOS Tube at grid voltage VGS=0 when , Channel already exists . This is because N The channel is pre formed in the manufacturing process by ion implantation D、S Between the surface of the substrate 、 Below the grid SiO2 A large number of metal positive ions are mixed into the insulating layer , This channel is also called the initial channel .

When VGS＝0 when , These positive ions have induced an inversion layer , Formed a channel , So as long as there is leakage source voltage , There is a drain current ; When VGS＞0 when , Will make ID Further increase ;VGS＜0 when , With VGS Reduction of , The drain current gradually decreases , until ID＝0. Corresponding ID＝0 Of VGS It is called pinch off voltage or threshold voltage , Use symbols VGS（off） or Up Express .

Due to depletion MOSFET stay VGS=0 when , The channel between the drain and the source already exists , So just add VDS, There is ID circulation . If you increase the positive grid voltage VGS, The electric field between the grid and the substrate will induce more electrons in the channel , The ditch thickens , The conductance of the channel increases .

If a negative voltage is applied to the grid （ namely VGS＜0）, A positive charge will be induced on the surface of the corresponding substrate , These positive charges cancel N Electrons in the channel , Thus, a depletion layer is generated on the surface of the substrate , Narrow the channel , Channel conductance decreases . When the negative gate voltage increases to a certain voltage VGS（off） when , The depletion region extends to the whole channel , The channel is completely clamped off （ Run out of ）, Even if VDS There is still , There will be no drain current , namely ID=0.

Chart 9  N Channel depletion type MOS Tube structure （ Left ） And transfer characteristics （ Right ） Sketch Map

4、P Principle of channel depletion FET

P Channel depletion type MOS The working principle of the tube is similar to N Channel depletion type MOS The tubes are exactly the same , Only the conductive carriers are different , The polarity of the supply voltage is also different .

5、 Depletion and enhancement MOS The difference between tubes

The main difference between the depletion type and the enhanced type is the depletion type MOS Tube in G End （Gate） There are conductive channels when no voltage is applied , And enhanced MOS The tube can only be opened , There will be conductive channels ; The control methods of the two are also different , Depletion type MOS Tubular VGS（ Grid voltage ） You can use positive 、 zero 、 Negative voltage control on , And enhanced MOS The tube must make VGS>VGS（th）（ gate threshold voltage ） Talent .

Due to depletion N Ditch MOS Tube in SiO2 A large amount of Na+ or K+ Positive ions （ manufacture P Channel depletion type MOS When the tube is doped with negative ions ）, When VGS=0 when , The electric field produced by these positive ions can be in P Enough electrons are induced in the substrate , formation N Type conductive channel ; When VGS>0 when , Will produce a larger ID（ Drain current ）; If so VGS<0, Then it will weaken the electric field formed by positive ions , send N The channel narrows , So that ID Reduce .

These characteristics make depletion MOS Tube in practical application , When the device is powered on, it may be triggered by mistake MOS tube , Cause the whole machine to fail ; Not easily controlled , Make its application very few .

therefore , What we see everyday NMOS、PMOS Mostly enhanced MOS tube ; among ,PMOS It can be easily used as a high-end driver . however PMOS Due to the existence of high on resistance 、 Expensive price 、 Problems such as few replacement types , In the high-end drive , Usually it's still used NMOS replace , This is also the application or product category on the market , Enhanced NMOS Tube is the most common and important reason , Especially in the application of switching power supply and motor drive , Commonly used NMOS tube .

MOS Important characteristics of pipe

1、 Conduction characteristics

The meaning of conduction is to act as a switch , It's like the switch is closed .NMOS Characteristics of ,VGS Greater than a certain value will turn on , It is applicable to the case when the source is grounded （ Low end drive ）, Only the grid voltage reaches 4V or 10V That's all right. .PMOS Its characteristic is ,VGS Less than a certain value will turn on , Suitable for source connection VCC The case when （ High end drive ）.

2、 Loss characteristics

Whether it's NMOS still PMOS, After conduction, there is conduction resistance , The current will be consumed by the resistance , This part of the energy consumed is called conduction loss . Small power MOS The on resistance of the tube is generally about a few milliohms to tens of milliohms , Select the one with low on resistance MOS The tube will reduce the conduction loss .

MOS When the tube is conducting and cutting off , The voltage at both ends has a falling process , The current flowing through has a rising process , In the meantime ,MOS The loss of the tube is the product of voltage and current , This is called switching loss . Usually, the switching loss is much greater than the conduction loss , And the faster the switching frequency , The greater the loss .

The greater the product of voltage and current at the moment of conduction , The greater the loss . Reduce switching time , It can reduce the loss of each conduction ; Reduce the switching frequency , It can reduce the number of switches per unit time . Both methods can reduce the switching loss .

3、 Parasitic capacitance drive characteristics

Compared with bipolar transistors ,MOS Tube needs GS Only when the voltage is higher than a certain value can it be turned on , And we also need a faster conduction speed . stay MOS The structure of the tube can be seen , stay GS、GD There is parasitic capacitance between , and MOS Tube drive , Theoretically, it is the charge and discharge of capacitors .

Charging a capacitor requires a current , Because the capacitor can be regarded as a short circuit at the moment of charging , So the instantaneous current will be relatively large . choice / Design MOS The first thing to pay attention to when driving the tube is the size of the instantaneous short-circuit current ; The second thing to pay attention to is , Widely used for high-end drive NMOS, When conducting, the grid voltage is required to be greater than the source voltage .

And high-end drive MOS Source voltage and drain voltage when the tube is on （VCC） identical , So at this time, the grid on voltage is higher than VCC high 4V or 10V, And the higher the voltage , The faster the conduction speed , The smaller the on resistance .

Chart 10  4 Kind of MOS Schematic diagram of pipe characteristics comparison

4、 Parasitic diode

There is a parasitic diode between the drain and the source , namely “ Body diode ”, When driving inductive load （ Like a motor 、 Relay ） Application , Mainly used for protection circuit . However, the bulk diode is only in a single MOS There is... In the pipe , There is usually nothing inside the integrated circuit chip .

Chart 11   Schematic diagram of parasitic diode position

5、 Different withstand voltage MOS Tube characteristics

Different withstand voltage MOS tube , The resistance proportion distribution of each part of the on resistance is different . Such as withstand voltage 30V Of MOS tube , The resistance of the epitaxial layer is only... Of the total on resistance 29%, Withstand pressure 600V Of MOS The epitaxial layer resistance of the tube is of the total on resistance 96.5%.

Different withstand voltage MOS The main difference between tubes is , High pressure resistant MOS Although its reaction speed is faster than that of low-pressure resistant MOS Be slow , therefore , Their characteristics also show differences in practical applications , Such as resistance to medium and low voltage MOS The tube only needs a very low gate charge to meet the strong current and high power processing capacity , In addition to the fast switching speed , It also has the characteristics of low switching loss , Especially adapted to PWM Output mode application ; And resistant to high pressure MOS The tube has the characteristic of high input impedance , In electronic ballast 、 Electronic transformers 、 Switching power supply is widely used .

Chart 12   Different withstand voltage MOS List of pipe characteristics

MOS Tube and triode 、IBGT The difference between

1、MOS The difference between transistor and triode

Triode is called semiconductor triode , Its main function is to stop amplifying small signals .MOS There are many similarities between transistor and triode , There are also many differences .

The first is the difference in switching speed . When the triode works , Two PN The junction will induce an electric charge , When the switch tube is in the on state , The triode is saturated , Suppose at this time, the triode stops ,PN The charge induced by the junction should be restored to equilibrium , This process takes time . and MOS Due to different working methods , No recovery time is required , Therefore, it can be used as a high-speed switch tube .

Secondly, the control methods are different .MOS Tubes are voltage control elements , The triode is a current control element . In the case that only less current is allowed to be taken from the signal source , Should be selected MOS tube ; And when the signal voltage is low , It is also allowed to take more current from the signal source , Triode shall be selected .

Then there are different kinds and numbers of carriers . Monopole devices mentioned in power electronics technology refer to devices that rely on only one carrier to conduct electricity , Bipolar devices are devices that conduct electricity by two kinds of carriers .MOS Only one type of majority carrier conduction is applied to the tube , So it is also called unipolar device ; The triode has most carriers , Minority carrier conduction is also used ; It is a bipolar device .

The third is different flexibility . There are some MOS The source and drain of the tube can be used interchangeably , The gate voltage can also be positive or negative , More flexible than a triode .

Fourth, different integration capabilities .MOS The tube can work under the condition of very small current and very low voltage , And its manufacturing process can easily put a lot of MOS The tube is integrated on a silicon wafer , therefore MOS Transistor has been widely used in large-scale integrated circuits .

Fifth, the input impedance and noise capacity are different .MOS The transistor has the advantages of high input impedance and low noise , It is widely used in various electronic devices , Special use MOS The tube is the input stage of the whole electronic equipment , It can obtain the performance that ordinary triodes are difficult to achieve .

Finally, the power consumption is different . In the same situation , use MOS Tube time , Low power consumption ; When choosing triode , The power consumption is much higher .

Of course , In terms of use cost ,MOS The tube should be higher than the triode , Therefore, according to the characteristics of the two components ,MOS Tubes are often used in high-frequency and high-speed circuits 、 Places with high current , And the central area sensitive to the base or drain control current ; And triode is used in low-cost places , Only when the effect is not achieved will the replacement be considered MOS tube .

surface 13  MOS A list of the main differences between transistor and triode

2、MOS Management and IBGT The difference between

IGBT（Insulated Gate Bipolar Transistor）, Insulated gate bipolar transistor , By BJT（ Bipolar triode ） and MOS A composite fully controlled voltage driven power semiconductor device composed of insulated gate FET , Both MOSFET High input impedance and power transistor （GTR） Low conduction voltage drop .

GTR The saturation pressure decreases , High current carrying density , But the driving current is large ;MOSFET The driving power is very small , Fast switching speed , But the conduction voltage drop is large , Low current carrying density .IGBT It combines the advantages of the above two devices , The driving power is small and the saturation voltage is reduced . common IGBT It is also divided into single tube and module , Appearance and MOS Although a little similar , Common manufacturers include Fuji Electric 、 Fairchild Semiconductor, etc , Module products are generally internally packaged with several individual IGBT, It is internally connected into a suitable circuit .

because IGBT The principle is to open first MOS tube , Then drive the triode to open , This principle determines IGBT Switching speed ratio MOS Slow down , But faster than triode .

Manufacturing costs ,IGBT than MOS The tube is much higher , This is because IGBT The production of more thin back ion implantation 、 Low temperature annealing of sheet （ Such as laser annealing ） working procedure , And these two processes need expensive machines specially for the sheet process .

Under low pressure , low pressure MOS The conduction pressure drop of the tube is usually controlled at 0.5V following （ Basically not more than 1V Of ）, such as IR4110 low pressure MOS tube , Its internal resistance is 4mΩ, Give it 100A The conduction current , The conduction voltage drop is 0.4V about . Current conduction voltage decreases , It means that the conduction loss is small , At the same time, it has the characteristics of low switching loss , therefore ,IGBT relative MOS Tube has no advantage in electrical performance , Plus, in terms of cost performance MOS Tube has more advantages , So basically, there is no low pressure IGBT.

MOS The biggest disadvantage of the tube is that as the pressure increases , The internal resistance increases rapidly , So the internal resistance is very large under high pressure , the MOS Tubes cannot be used in high-power applications .

In the high-voltage field ,MOS The switching speed of the tube is still the fastest , But under high pressure MOS The conduction pressure drop of the tube is very large （ The internal resistance increases rapidly with the increase of withstand voltage ）, Even under pressure 600V Of COOLMOS tube , The on resistance can be as high as several ohms , Resulting in little current resistance .

and IGBT Under high withstand voltage , The conduction voltage drop hardly increases significantly （IGBT The conduction current of is processed by triode ）, So under high pressure IGBT Obvious advantages , Existing high switching speed , It also has the high current characteristic of triode ; in addition , In the new generation IGBT In the product , High switching speed （ Nanosecond ）, Conduction voltage drop 、 Switching losses have also made great progress , bring IGBT Stronger impulse current resistance , And high pressure resistance 、 The advantages of small driving power are more prominent .

When it is necessary to withstand voltage more than 150V Under the conditions of use ,MOS Tube has basically no advantage . With typical IRFS4115 With the fourth generation IGBT type SKW30N60 In contrast , stay 150V、20A Operate under continuous conditions , The switching loss of the former is 6mJ/pulse, The latter only 1.15mJ/pulse, Less than the former 1/5; If extreme working conditions are used , The power load difference between the two will be even greater ！

at present , Such as metallurgy 、 Iron and steel 、 high-speed railway 、 Ships and other fields with high power demand have been rarely seen MOS tube , But widely used IGBT components and parts .

in general ,IGBT More suitable for high pressure 、 High current 、 Low frequency （20KHZ about ） place , The higher the voltage ,IGBT The more advantages , stay 600v above ,IGBT The advantages are obvious ; and MOSFET It is more suitable for low voltage 、 Small current 、 Low frequency （ Dozens of KHz~ A few MHz） field , The lower the voltage ,MOS Tube has more advantages .

MOS Main parameters of pipe

There are many parameters of FET , Including limit parameters 、 Dynamic electrical characteristic parameters and static electrical characteristic parameters , The important parameters are ： Saturated drain source current IDSS、 Pinch off voltage Up、 Turn on the voltage VT（ Reinforced insulated grid tube ）、 transconductance gM、 Leakage source breakdown voltage BVDS、 Maximum dissipation power PDSM And the maximum leakage source current IDSM etc. .

1、 Maximum rated parameters

Maximum rated parameters , All values are required to be obtained under Ta=25℃.

Chart 14  MOS Example of absolute maximum rating of tubes

VDS/VDSS Maximum leakage source voltage

Short circuit the gate source , Drain source rated voltage VDSS[ Or writing V(BR)DSS] It refers to leakage - The maximum voltage that can be applied before avalanche breakdown of the source occurs . Depending on the temperature , The actual avalanche breakdown voltage may be lower than the rated VDSS.

VGS/ VGSS Maximum gate source voltage

VGS[ Or writing V(BR)GSS] The rated voltage is the maximum voltage that can be applied between the two poles of the grid source . The main purpose of setting the rated voltage is to prevent damage to the gate oxide layer caused by high voltage . The voltage that the actual gate oxide can withstand is much higher than the rated voltage , But it will change with different manufacturing processes , So keep VGS The application reliability can be guaranteed within the rated voltage .

ID Continuous leakage current

ID Defined as the chip at the maximum rated junction temperature TJ(max) Next , The tube surface temperature is 25℃ Or at higher temperatures , Maximum allowable continuous DC current . This parameter is the rated thermal resistance between the junction and the shell RθJC And shell temperature ：

ID Switching losses are not included , And keep the surface temperature of the tube at 25℃（Tcase） It's also hard. . therefore , In hard switching applications, the actual switching current is usually less than ID Rating @ TC=25℃ Half of , Usually in 1/3～1/4.

notes ： Use thermal resistance JA It is possible to estimate ID, This value is more realistic .

IDM/IDSM Pulse drain current / Maximum leakage current

This parameter reflects the pulse current that the device can handle , The pulse current is much higher than the continuous DC current . Definition IDM The purpose is to ： Ohmic region of line . For a certain grid - Source voltage ,MOSFET After conduction , There is maximum drain current , As shown in the chart 15 Shown , For a given grid - Source voltage , If the working point is in a linear region , The increase of drain current will increase the leakage - Source voltage , This increases the conduction loss . Work under high power for a long time , Will cause device failure . therefore , At a typical grid drive voltage , It needs to be rated IDM Set under area , The dividing point of the area is VGS Intersection point with curve .

Chart 15  MOSFET After conduction , There is maximum drain current

Therefore, it is necessary to set the upper limit of current density , Prevent the chip from burning due to high temperature . This is essentially to prevent excessive current from flowing through the package leads , Because in some cases , The most on the whole chip “ Weak connections ” Not a chip , Instead, it encapsulates the leads .

Considering the thermal effect on IDM The limitation of , The rise of temperature depends on the pulse width , Time interval between pulses , Heat dissipation condition ,RDS(on) And the waveform and amplitude of the pulse current . The pulse current shall not exceed IDM The upper limit does not guarantee that the junction temperature will not exceed the maximum allowable value . Refer to the discussion on instantaneous thermal resistance in thermal and mechanical properties , To estimate the junction temperature under pulse current .

PDSM Maximum dissipation power

That is, the total allowable channel power consumption , The maximum power dissipation of the device is calibrated , It can be expressed as the maximum junction temperature and shell temperature 25℃ As a function of thermal resistance .

TJ、TSTG Range of operating temperature and storage ambient temperature

These two parameters calibrate the junction temperature range allowed by the device working and storage environment . This temperature range is set to meet the requirements of the shortest working life of the device . If you ensure that the device operates within this temperature range , Will greatly extend its working life .

EAS Single pulse avalanche breakdown energy

If the voltage overshoot value ( It is usually caused by leakage current and stray inductance ) The breakdown voltage is not exceeded , Then the device will not have avalanche breakdown , Therefore, there is no need to dissipate the ability of avalanche breakdown . The avalanche breakdown energy calibrates the safety value of the instantaneous overshoot voltage that the device can tolerate , It depends on the energy dissipated by avalanche breakdown .

Devices that define the rated avalanche breakdown energy usually also define the rated EAS. Rated avalanche breakdown energy and rated UIS Have a similar meaning .EAS The device can safely absorb the reverse avalanche breakdown energy .

L Is the inductance value ,ID Is the peak current flowing on the inductance , It will suddenly convert to the drain current of the measuring device . The voltage generated on the inductor exceeds MOSFET After breakdown voltage , Will cause avalanche breakdown . When avalanche breakdown occurs , Even if MOSFET In off state , The current on the inductance will also flow MOSFET device . The energy stored on the inductor is stored on the stray inductor , from MOSFET Dissipated energy is similar .

MOSFET After parallel connection , The breakdown voltage between different devices is difficult to be exactly the same . Usually it's ： Avalanche breakdown first occurred in a certain device , Then all avalanche breakdown currents ( energy ) All flow through the device .

EAR Repeat avalanche energy

Repeated avalanche energy has become “ Industrial standards ”, But there is no set frequency 、 Other losses and cooling capacity , This parameter has no meaning . Dissipate heat ( cooling ) The situation often restricts the repeated avalanche energy . It is also difficult to predict the energy level produced by avalanche breakdown .

rated EAR The real significance of the device is to calibrate the repeated avalanche breakdown energy that the device can withstand . The prerequisite for this definition is ： No restrictions on frequency , So that the device will not overheat , This is realistic for any device that may have avalanche breakdown . In the process of verifying the device design , It is best to measure the temperature of the device or heat sink in working state , To observe MOSFET Whether the device is overheated , Especially for devices that may have avalanche breakdown .

IAR Avalanche breakdown current

For some devices , The tendency of current pooling on the chip during avalanche breakdown requires that avalanche current IAR Limit . such , Avalanche current becomes avalanche breakdown energy specification “ Elaborate ”; It reveals the true capabilities of the device .

Chart 16  Avalanche damage tolerance measurement circuit and waveform

SOA Safe work area

Each of these MOS Tube will give its safe working area , power MOS The tube will not show secondary breakdown , Therefore, the safe operation area is simply defined from the dissipated power when the junction temperature reaches the maximum allowable value .whaosoft aiot http://143ai.com

2、 Static electrical characteristics

Chart 17  List of static electrical characteristics and parameters

V(BR)DSS/VBDSS Leakage source breakdown voltage ( Breaking the voltage )

Or call BVDS, It refers to the specific temperature and grid source short circuit , The drain source voltage when the drain current reaches a specific value . In this case, the drain source voltage is avalanche breakdown voltage .

V(BR)DSS Is the positive temperature coefficient , The maximum rated value of the drain source voltage decreases as the temperature decreases , stay -50℃ when ,V(BR)DSS It's about 25℃ Of the maximum drain source rated voltage 90%.

BVGS Gate source breakdown voltage

In the process of increasing the gate source voltage , Make the grid current IG When it increases sharply from zero VGS.

VGS(th) Threshold voltage

Also used VT Express , It means that the applied gate source voltage can make the drain begin to have current , Or turn off MOSFET The voltage when the current disappears , Test conditions （ Drain current 、 Leakage source voltage 、 Junction temperature ） There are also specifications . Under normal circumstances , be-all MOS The threshold voltage of gate devices will vary . therefore ,VGS(th) The scope of change is well defined .VGS(th) Is the negative temperature coefficient , When the temperature rises ,MOSFET It will turn on at a relatively low gate source voltage .

VGS(off) Pinch off voltage

Also used Up Express , It refers to the junction type or depletion type insulated gate FET , Make the gate voltage when the drain source is just cut off .

RDS(on) On resistance

Refers to the specific leakage current （ Usually it is ID Half the current ）、 Grid source voltage and 25℃ Leakage measured in the case of - Source resistance .

RGS Gate source resistance

That is, in the grid 、 The ratio of the voltage applied between the sources to the grid current , This characteristic is sometimes represented by the gate current flowing through the gate MOS Tubular RGS Can easily surpass 1010Ω.

IDSS Zero gate voltage drain current

Also known as saturated drain source current , Refers to the current grid source voltage VGS＝0 when , Leakage current between drain and source under specific drain source voltage . Since the leakage current increases with the increase of temperature ,IDSS There are regulations at room temperature and high temperature . The power consumption caused by leakage current can be used IDSS Multiply by the voltage between the drain and the source , Usually, this part of power consumption can be ignored .

IGSS Gate source leakage current

Refers to the leakage current flowing through the grid under a specific grid source voltage .

3、 Dynamic electrical characteristics

Chart 18  List of dynamic electrical characteristics and parameters

Ciss Input capacitance

Short circuit the drain source , The capacitance between the grid and the source measured by AC signal is the input capacitance .Ciss It is made up of gate drain capacitance Cgd And gate source capacitance Cgs In parallel , perhaps Ciss=Cgs+Cgd. The device can only be turned on when the threshold voltage is caused by the charging of the input capacitor , Only when the discharge reaches a certain value can the device be turned off . Therefore, the driving circuit and Ciss It has a direct impact on the on and off delay of the device .

Coss Output capacitance

Short circuit the gate source , The capacitance between drain and source measured by AC signal is the output capacitance .Coss It is caused by drain source capacitance Cds And gate drain capacitance Cgd In parallel , perhaps Coss=Cds+Cgd, For the application of soft switch ,Coss It's very important , Because it may cause the resonance of the circuit

Crss Reverse transfer capacitor

When the source is grounded , Measured capacitance between drain and grid is reverse transmission capacitance . The reverse transmission capacitance is equivalent to the gate drain capacitance .Cres=Cgd, Reverse transmission capacitance is also often called Miller capacitance , It is one of the important parameters for the rise and fall time of the switch , He also affects the shutdown delay time . The capacitance decreases with the increase of drain source voltage , Especially output capacitance and reverse transmission capacitance .

Eoss The output capacitor stores energy

Indicates the output capacitance Coss stay MOS The amount of energy stored in the tube . because MOS The output capacitance of the tube Coss It has very obvious nonlinear characteristics , along with VDS The voltage changes . So if Datasheet This parameter is provided , For evaluation MOS The switching loss of the tube is very helpful . Not all MOS This parameter will be provided in the management manual , In fact, most Datasheet It does not provide .

di/dt Current rise rate

This parameter is responding MOSFET Reverse recovery characteristics of bulk diodes . Because diodes are bipolar devices , Affected by charge storage , When the diode is reverse biased ,PN The charge stored in the junction must be removed , The above parameters reflect this characteristic .

Chart 19  Parasitic capacitance structure and circuit diagram

Qgs、Qgd and Qg（ Grid charge value ）

Qg Grid charge value , Also known as total grid charge , Reflect the charge stored in the capacitance between terminals , Since the moment of switching , The charge on the capacitor changes with the voltage , Therefore, the influence of gate charge should be considered when designing gate drive circuits .

Qgs For from 0 The charge starts at the first inflection point ,Qgd It is the part between the first inflection point and the second inflection point （ It's also called “ miller ” Electric charge ）,Qg It's from 0 Point to VGS Part equal to a specific driving voltage .

Chart 20  Qgs、Qgd and Qg Schematic diagram of parameter meaning

The change of leakage current and drain source voltage has little effect on the value of gate charge , And the gate charge does not change with temperature . The test conditions are specified . The graph of gate charge is shown in the data sheet , Including the change curve of gate charge under the condition of fixed leakage current and changing drain source voltage . In the diagram above , Platform voltage VGS(pl) With the increase of current, the increase is relatively small （ As the current decreases ）. The platform voltage is also proportional to the threshold voltage , So different threshold voltage will produce different platform voltage . See the following figure for details ：

Chart 21  Qgs、Qgd and Qg Parameter meaning decomposition

td(on) Conduction delay time

When the gate source voltage rises to 10% When the grid driving voltage rises, the leakage current rises to the specified current 90% The time it takes .

td(off) Turn off delay time

When the gate source voltage drops to 90% When the grid driving voltage is reached, the leakage current drops to the specified current 10% The time it takes . This shows the delay experienced before the current is transmitted to the load .

Tr Rise time

The rise time is the drain current from 10% Rise to 90% The time it took .

Tf Descent time

The drop time is the drain current from 90% Down to 10% The time it took .

NF Low frequency noise figure

In decibels （dB）, The noise is caused by the irregular movement of carriers inside the tube , Because of its existence , It can make the amplifier even when there is no signal input , There will also be irregular voltage or current changes at the output . Noise factor NF The smaller the numerical , It means that the less noise the pipe produces , The noise figure of FET is about several decibels , Smaller than bipolar triode .

gM transconductance

Is the gate source voltage VGS Drain current ID The ability to control , Drain current ID Variation and gate source voltage VGS The ratio of the amount of change , It is an important parameter to weigh the amplification ability of FET .

4、 Other important parameters

In addition to the parameters described above ,MOS Although there are many important parameters , The details are as follows ：

surface 22  MOS List of other important parameters -  Yanqing Chuanbei community 45 Miss sun Collect and sell waste products and junk for stock speculation Waste sun Recycle

Two 、 How to get started with SCM

As an embedded old driver , For many years, I have been working with SCM 、Keil、C Language 、AD、 Soldering iron 、 Wind gun 、 Oscilloscope 、 Electronic components are tangled . During this period, I experienced a lot of training , Through many nights , Also lost a lot of hair .

     Staying up late lit up a LED, Such a small sense of achievement makes me forge ahead .

    At first, I was not optimistic about this industry , More work, less pay , Need to keep learning new knowledge . A few years ago, I stayed up late to weld boards , Curse while working .

    Many years later , Glad to have this skill , Let me eat enough 、 Get in the car , There is a family .

Just graduated and changed careers

     I changed my profession by self-study , Major in elevator , Near graduation 20 Year old , There is no place to be energetic , A piece of 51 The development board is equipped with a video tutorial , Besides eating and going to the bathroom , Never leave the room .

     At the beginning , Like many novices , Do not understand the circuit , I don't know how to program , Do not understand SCM , There are many problems in every aspect of SCM development .

     Fortunately, the learning ability is ok , And patient , Follow the video step by step , Read a chapter and I'll try it on the development board .

     Soon I will be able to turn from lighting to making an electronic clock , In fact, SCM programming is much simpler than pure software , There's a lot less code , So it's good to learn , As long as your learning path is correct . If you are not interested in doing this, bless , It's still hard .

Personal positioning , Software or hardware ？

     Generally, when developing a product in actual work , It is absolutely impossible for one person to make the product from beginning to end . Generally, some people are responsible for the hardware part 、 Some people are responsible for the software part , These are also the two major directions of MCU development .

     Carry out single chip microcomputer development , Skills should be inclined , Will you be a hardware engineer or a software engineer in the future ？

     Why should there be software and hardware ？ In fact, big factories have a fine division of labor , This makes the work flow 、 simple , Improve work efficiency . In addition, it also improves the stability of the company's business , Even if someone leaves suddenly, it won't have much impact .

     For some products, the program alone is enough for you to toss about for a month , If you do shell design again , And do hardware design , Make software again . When the product comes out , The customer may be gone .

     therefore , A product , The minimum configuration is also a hardware engineer and a software engineer .

     An all powerful person , Unless you have been in the industry for many years , Otherwise, it's hard to learn everything well , So most of them only know the surface , Can make things , But it is difficult to optimize to a professional level .

     After reading the above, do you already know , No matter what industry you want to engage in , First of all, positioning is very important .

     Before positioning, you must understand the current situation of this industry , What positions are there ？

     The career orientation of SCM development includes hardware engineer and SCM software engineer .

     All my apprentices , I would suggest that they start with SCM software , Best value for money ！ You can search the salary on the recruitment platform .

     Of course, the hardware is ready , Revenue every minute kills software , But it is still difficult to do a good job of hardware , At least not what you can do by watching videos and reading books , But suffer from countless actual products .

     And now the integration of circuits is very high , Novice hardware is difficult to access the core technology .

     It's better to start with software , Wages are also high. , Anyway, do SCM development , Even if you make software , It is also adjusting the hardware every day , Too much , You'll get it .

     Hardware is different , It's generally difficult to touch procedural things , Even if contact does not form a system .

Learning content

     With clear positioning , Next, we need to formulate the learning path of our system according to this positioning . The purpose of formulating the learning path is 2 individual ：

• Avoid detours

• Plan your time , Urge you to execute

     Combined with my experience, I will directly summarize the most important keywords engaged in MCU development ：

• C51 Single chip microcomputer /STM32 Single chip microcomputer

• C Language

• Can understand the schematic diagram

• Basic welding ability

     Let's talk about why many people have learned every peripheral routine on the development board , But I still feel like I can't do anything ？

     What you lack is a certain product thinking , Is the idea of realizing a function . To solve this problem is to do projects , There's no other shortcut .

     As the old saying goes, half a mile is worth a mile , MCU peripherals 、C Language 、 Circuit principle 、 Welding ability , In fact, these are just paving the way for the starting project .

     The learning of MCU peripherals is relatively simple , Buy a development board , After running the routine of each peripheral several times, I have a general understanding . Write the actual application scenarios of each peripheral of the single chip microcomputer by yourself , The next time you encounter the same functional requirements, you will know how to solve them .

3、 ... and 、 Understand the triode with a few pictures

The current amplification function of triode should be regarded as a difficult content in analog circuit , I want to use these animations to simply explain why there is a small current Ib Can control large current Ic Size , And the principle of amplification circuit .

     The triode here is also called bipolar transistor , The amplification circuit of analog electricity and the simple logic circuit of digital electricity will be used . There's a collector c、 The base b、 The emitter e、 And two PN junction ： Collector junction and emitter junction . The collector area is relatively large , The base is thin and the carrier concentration is low . The picture below is a NPN Type a transistor ：

     When the emitter junction is positive , The charge distribution changes , The width of the emitter junction will narrow ; It's like opening a window for electrons e To b When the gate collector junction is reversed , The charge distribution will also change , The width of the collector junction will become wider . It's like opening a barrier to electrons from c The gate out of the class , The animation is shown below ：

    b The stage will be connected to a large resistor RB Limit the current Ib Size , Run to b The extra electrons of the pole have to pass through the collector junction , Form an electric current Ic, The animation is shown below ：

     If the base voltage doubles , The charge distribution will continue to change , The width of the emitter junction will become narrower , The gate has become wider , There will be more electrons running to b level . The animation is shown below ：

     because RB It's a big resistance ,Ib Even if it doubles, it's still very small , So more electrons will pass through the collector junction , Give Way Ic It's doubled . The animation is shown below ：

     Two DC power supplies can be combined , Plus the small signal ui And two capacitors , You get the amplification circuit , As shown in the figure below . Recommended articles ： Design skills of triode amplification circuit .

     If the resistance is appropriate , This amplifying circuit can transmit small signals ui Amplify into a large signal with opposite phase uCE, The animation is shown below ：

     Red is the input ,ui Changes in UBE, Think of the emission junction as a small resistor , Red Q The point will move along the black line , And then draw iB Image ; according to iC=βiB, Draw iC Image , Ordinate from μA Turned into mA; And the output end has UCE=UCC-ICRC, When UCC、RC Constant time ,UCE And IC Reverse phase .

     Finally, let's talk about the shortcomings of these animations ：

• The bell shaped triode is not my original creation , But the metaphor of water tank is easy to cause a misunderstanding , Think IC Maximum , Actually IE Is the maximum current .

• The thermal velocity of electrons is completely ignored in the animation , That speed is much higher than the drift speed of electrons under voltage .

• There is no energy level in the animation 、 energy band 、 Fermi distribution and so on .