Deep understanding of the various parameters in the power MOSFET data sheet

In various applications of driving loads in automotive electronics, the most common semiconductor component is the power MOSFET. This article is not intended to be a technical encyclopedia introducing power MOSFETs, but to let readers understand how to correctly understand the commonly used main parameters in the power MOSFET data sheet to help designers better use power MOSFETs for design.

The parameters in the data sheet are divided into two categories: maximum ratings and electrical characteristic values. For the former, they cannot be exceeded under any circumstances, otherwise the device will be permanently damaged; for the latter, they are generally given in the form of minimum, maximum, and typical values, and their values ​​are closely related to the test method and application conditions. In actual applications, if the electrical characteristic values ​​are exceeded, the device itself is not necessarily damaged, but if the design margin is insufficient, it may cause the circuit to malfunction.

Among the parameters given in the data sheet of power MOSFET, the most basic parameters that are usually of concern are I , Qgs _, and Vgs _. More advanced parameters, such as I _, Rthjc, SOA, Transfer Curve, EAs _, etc., will be introduced in the next part of this article.

In order to make the description of each parameter more intuitive and easy to understand, the power MOSFET of Infineon Technologies, model IPD90N06S4-04 (http://www.infineon.com/optimos-T) was selected. All the tables and charts in this article are also extracted from IPD90N06S4-04. The following is an introduction to these parameters one by one.

: On-state resistance. It is a parameter related to temperature and V _gs , and is one of the important parameters of MOSFET. In the data sheet, the typical and maximum values ​​at room temperature are given, and the test conditions for obtaining this value are given, see the table below for details.

In addition to the table, the data sheet also provides a data graph of the on-resistance changing with junction temperature. As can be seen from the graph, the higher the junction temperature, the higher the on-resistance. It is precisely because of this characteristic that when the current capacity of a single power MOSFET is insufficient, multiple power MOSFETs of the same type can be connected in parallel to expand the current.

If you need to calculate at a specified temperature , you can use the following calculation formula.

The above formula is a constant related to process technology. For this type of power MOSFET from Infineon, 0.4 can be used as the constant value. If a quick estimate is needed, it can be roughly assumed that the on-resistance at the highest junction temperature is twice the on-resistance at room temperature. The curve in the table below shows the relationship with ambient temperature.

: Defines the maximum DC voltage that the source and drain of the MOSFET can withstand. In the datasheet, this parameter is given on the front page of the datasheet. Note that the values ​​given are at room temperature.

In addition, the data sheet will also give a curve of temperature change over the full temperature range (-55 C…+175 C).

As can be seen from the table above, it changes with temperature, so in the design, it should be noted that at the extreme temperature, can still meet the system power supply requirements for .

Q _gs : The datasheet shows the curve of gate charge change when Q _gs changes at a given V _{ds voltage in order to turn on the power MOSFET. From the graph, it can be seen that in order to fully turn on the MOSFET, the typical value of Q gs} is approximately equal to 10V. Since the device is fully turned on, the static loss of the device can be reduced.

V _gs : describes the gate-source voltage required for a specified drain current. The datasheet gives the V _{gs voltage at room temperature when V ds} = V _gs and the drain current is in the microampere range . The datasheet gives the minimum, typical, and maximum values.

It should be noted that at the same drain current, the V _gs voltage will decrease as the junction temperature increases. At high junction temperatures, the drain current is close to I _dss (drain current). For this reason, the data sheet will also provide a drain current curve that is 10 times larger than the specified current at room temperature as a design reference. As shown in the figure below.

The above introduces the basic parameters , , Q _gs , and V _gs that designers are most concerned about in the power MOSFET data sheet .

In order to have a deeper understanding of other parameters of power MOSFET, this article still uses Infineon's power MOSFET as an example, model IPD90N06S4-04 ( http://www.infineon.com/optimos-T ). In order to make the description of each parameter more intuitive and easy to understand, all tables and charts are also extracted from IPD90N06S4-04. The following is an introduction to these parameters one by one.

If you want to better understand power MOSFET, you need to know more parameters, which are necessary and useful for design. These parameters are I _D , R _thjc , SOA, Transfer Curve, and E _AS .

I _D : defines the current that the drain can operate at for a long time at room temperature. It should be noted that this I _D current is the I _{D current value at V gs} at a given voltage and T _C = 25°C .

_{The size of ID} can be calculated by the following formula:

Taking IPD90N06S4-04 as an example, the calculated result is equal to 169A. Why is it only marked as 90A in the datasheet? This is because the maximum current is limited by the package pins and the wire diameter. A detailed explanation can be found in the datasheet's Note 1). As shown in the following table:

In addition, the datasheet also gives the curve relationship between _ID and junction temperature. As can be seen from the table below, when the ambient temperature rises, ID _{will change with temperature. In the worst case, it is necessary to consider that the current of ID} at the maximum ambient temperature still meets the normal current requirements of the circuit design.

R _thjc : Temperature resistance is very important for designers

If you need to calculate the power loss of a power MOSFET in a single pulse and at different duty cycles, you need to look at this data sheet to make design estimates. I will explain in detail how to use the data sheet to make design estimates.

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SOA: The overload capacity of power MOSFET is low. In order to ensure the safe operation of the device, with high stability and long life, there are certain restrictions on the current, voltage, and power that the device can withstand. This restriction is expressed by the U _ds -I _d coordinate plane, which constitutes the safe operating area (SOA) of the power MOSFET. For the same device, the size of its SOA is related to the bias voltage, cooling conditions, and switching mode. If you want to subdivide SOA, there are two ways to divide it. According to the gate bias, it is divided into forward bias SOA and reverse bias SOA; according to the signal duty cycle, it is divided into DC SOA, single pulse SOA, and repetitive pulse SOA.

The power MOSFET must maintain the correct gate bias during the turn-on process and stable conduction. The positive bias SOA is the allowable operating range of the device in the on state; on the contrary, when the device is turned off, in order to improve the turn-off speed and reliability, the gate needs to be reverse biased, so the reverse bias SOA is the allowable operating range when the device is turned off.

DC SOA is equivalent to the working condition when the duty cycle is -> 1; single pulse SOA corresponds to the working condition when the duty cycle is -> 0; repetitive pulse SOA corresponds to the working condition when the duty cycle is 0 < D < 1. From the data table, it can be seen that single pulse SOA is the largest, repetitive pulse SOA is the second, and DC SOA is the narrowest.

Transfer Curve: It is a graph that expresses the functional relationship between ID _and Vgs _. Manufacturers will provide three curves at different ambient temperatures. Usually these three curves will intersect at one point, which is called the temperature stability point.

_{If the V gs} applied to the MOSFET is lower than the temperature stability point (V _gs <6.2V in IPD90N06S4-04 ), the MOSFET has a positive temperature coefficient, that is, the current of _ID increases with the junction temperature. In design, when applied to high current designs, the power MOSFET should be avoided from operating in the positive temperature coefficient region.

When Vgs _{exceeds the temperature stability point ( Vgs} >6.2V in IPD90N06S4-04 ), MOSFET has a positive temperature coefficient, that is, the I _D current decreases as the junction temperature increases. This is a very good feature in practical applications, especially in high current design applications, this feature will help the power MOSFET reduce the increase in junction temperature by reducing the I _D current.

E _AS : In order to understand the working condition of power MOSFET under avalanche current, the data sheet gives a curve corresponding to avalanche current and time. From this curve, we can read the time that the power MOSFET can withstand without being damaged under the corresponding avalanche current. For the same avalanche energy, if the avalanche current is reduced, the time that can be withstood will be longer, and vice versa. The ambient temperature also affects the level of avalanche current. When the ambient temperature rises, the avalanche current that can be withstood will decrease due to the limitation of the maximum junction temperature.

The datasheet gives the value of the avalanche energy that the power MOSFET can withstand. In this example, EAS = 331mJ at _room temperature

_{The table above only gives the EAS} at room temperature . In the design, the _EAS at different ambient temperatures is also needed , which the manufacturer will also give in the data sheet, as shown in the figure below.