What is the SOA or ASO of MOSFET?

Safe Operating Area（SOA）

Some manufacturers call it “Area of Safe Operation (ASO)”

Safe work area ：SOA(Safe operating area) A series of ( voltage , electric current ) A two-dimensional area formed by coordinate points , When the switching device works normally, the voltage and current will not exceed this region . Simply speak , As long as the device works SOA The area is safe , Beyond this area there is danger .

SOA（Safe operating area） Means a safe working area , A drain source voltage consisting of a series of constraints VDS And drain current ID Two dimensional coordinate diagram of , The voltage and current of the switching device during normal operation shall not exceed the limit .

SOA The area is divided into the following 5 Regions .

A Line    Is caused by the on resistance RDS（on）max[ID=VDS/RDS（on）] Restricted area . This area is generally ASO Discuss separately .

Because in the fixed VGS Voltage and ambient conditions , power MOSFET Of RDS(ON) Is constant , So the slope of this slash is 1/R(DS(ON)).

B Line   Is the maximum rated current IDC（ Steady state DC ）、ID（pulse）（ pulse ）max Restricted area . namely ：Ids A line capable of carrying the maximum current limit .

It should be noted that IDM Is the maximum current in the pulse operating state IDC, Usually the maximum drain pulse current IDM Is the continuous drain current ID Of 3 To 4 times , Therefore, the pulse current is much higher than the continuous DC current .

C Line   It is caused by channel loss （Channel dissipation perhaps Channel loss,ID Flow through DS This is the power consumption generated by the main power ） Restricted area , The maximum value of the product of current and voltage , That is, the circuit with rated power consumption limit .

As we have noticed , This line has a constant slope , But it's a negative slope . It's constant , Because of this SOA Each point on the power limit line carries the same constant power , By formula P = IV Express .

therefore , In this SOA In logarithmic curve , This will produce -1 The slope of . The minus sign is because MOSFET The current decreases with the increase of drain source voltage .

This phenomenon is mainly due to MOSFET The negative coefficient characteristic of will limit the current through the device when the junction temperature increases .

D Line   yes Vds Is related to the rated voltage , Withstand voltage VDSsmax Restricted area .

Leakage source breakdown voltage BVDSS It limits the maximum voltage range of the device , At power MOSFET In normal operation , If the voltage between the drain and the source increases excessively ,PN Avalanche breakdown occurs when the junction is reversed , To ensure device safety , The maximum voltage between drain and source that must be withstood during the shutdown process and its steady state shall be lower than the drain source breakdown voltage BVDSS.

V(BR)DSS： leak - Source breakdown voltage ( Breaking the voltage )

V(BR)DSS（ Sometimes it's called BVDSS） 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 , When the temperature is low V(BR)DSS Less than 25℃ Maximum rated drain source voltage at . stay -50℃, V(BR)DSS It's about 25℃ Of the maximum drain source rated voltage 90%.

E Line   Is the secondary breakdown limit ,  It is the same as the type of secondary breakdown region in bipolar transistors , This area operates continuously or with relatively long pulse width （ A few milliseconds or more ） Appears under open conditions . This is because , When the working voltage rises at the same external power line , The working current naturally decreases , But in this low current region , Output transmission characteristics （Vgs Id characteristic ） Is a negative temperature characteristic . When this area becomes a high current area , When it is necessary to change the positive temperature characteristic , The phenomenon disappears . The current value whose temperature characteristic changes from negative to positive is different from product to product , And a few amperes or less , This is unlikely to happen , This can be ensured by the so-called constant power line without secondary breakdown .

ABCDE The line corresponds to datasheet It's marked in 2、1、3、5、4

Due to the power MOS FET Usually used in switching applications , In normal operation , They are usually used in limited areas （2）. One thing to pay attention to in circuit design is the control system sequence .

chart 1.12 Shows when the source power of the system is cut off , Examples of power supply voltage and gate drive voltage sequences for electronic equipment .

As shown by the solid line in the figure , If until the supply voltage VDD The down time of the shutdown is longer than the grid drive voltage VGS Falling time of , be VGS The period in the graph t1 Under actuated , And enter ASO Restricted area （4） or （5）, Therefore, it is necessary to confirm whether it is in the safe operation area .

An effective way to avoid such operating areas is to perform sequence control , So that the grid drive voltage VG The falling time of is delayed to the supply voltage VDD After the descent , Such as Vgs The dotted line of that figure shows .

official MOSFET Of SOA And the content , You are welcome to add .

This part of the materials is excerpted from Reza datasheet And technical documentation .