Circuit design is a combination of technical knowledge and understanding to each device parameters. Each device parameters are defined in the datasheet. Therefore, knowing how to read MOSFET datasheet is a must in circuits involving MOSFETs.
In this another article, in order to explain the ideas clearly, let us use an actual MOSFET part from Infineon. The parts number is IGLR60R190D1XUMA1 and it is a 600V CoolGaN enhancement-mode power transistor. Infineon’s MOSFET’s are very robust and competitive as proven in several applications. Visit them at https://www.infineon.com/. Keep reading below.
1. Product Highlights
The very first page of the datasheet is advertising the product highlights. It is not yet the exact value of the parameters that you must consider based on you application. Some manufacturers just want to show case in the first page the best numbers their product can give.
In the example MOSFET, the product highlight is tabulated in the table and called as “Key Performance Parameters at TJ=25’C”.
These values are not valid for TJ other than 25’C. The details are written in the datasheet further, so don’t get misled with this.
The values in the table are derived from a junction temperature of 25’C. Thus, it is not valid to use for a junction temperature above 25’C.
2. Maximum Ratings
Maximum ratings are device limits. There is no other meaning except the device will get destroyed when the maximum rating is exceeded.
Below table is how Infineon defined its MOSFET maximum ratings.
There are manufacturers who called this as absolute maximum rating. In actual application, the circuit value should be limited to a level wherein the MOSFET is not going to carry heavy stress. A good value is 70% maximum stress level. For instance, the maximum drain to source voltage is specified at 750V, thus the maximum level of the circuit voltage must only 0.7 X 750V = 525V.
3. Thermal Characteristics
Thermal characteristics is one of the very important thing to deeply understand when learning how to read MOSFET datasheet. Thermal characteristics includes the thermal resistance from junction to ambient (Rthja), thermal resistance from junction to case (Rthjc), soldering temperature and others.
Understanding Thermal Resistance
Thermal resistance is either measured from junction to ambient, junction to case or junction to lead or junction to PCB for other manufacturers. All of them has individual consideration but all of them are used to get the de-rated power of the MOSFET. They are also useful to compute the equivalent thermal resistance when the actual power dissipation is given. The unit of thermal resistance is ‘C/W or K/W. Do not be confused as these units are interchangeable.
4. How to Read MOSFET Datasheet Electrical Characteristics
The values and numbers under the electrical characteristics are not the limiting values but these are the numbers derived from a particular test condition. For example, the gate threshold voltage, the values in the first row are derived when the IDS=0.96 mA; VDS=10 V; Tj=25’C. The values in the second row is also derived from the corresponding test conditions.
Static Characteristics
From the term itself, static means steady state. Static characteristics define the MOSFET parameters when it is running at steady state.
Definitions:
Gate threshold voltage or VGSth – is the voltage requirement of the gate to source. MOSFET will only start to operate correctly when this is complied. Always consider the highest value specified in the datasheet as the reference during the design. For instance, from the table above, the maximum value is 1.6V. Thus, ensure the applied voltage to the gate to source is much higher than 1.6V.
Drain to Source On-state Resistance or RDSon– is the resistance value measured across the drain to source when the MOSFET is conducting or on. This resistance value is the one responsible to the power dissipation of the MOSFET during conduction. Some are calling the power dissipation due to this resistance as conduction loss.
There are still other parameters defined in the above table. However, what we discussed are the most important items so that the MOSFET will work properly and will not damage. For sure, other parameters too have uses but we will not cover this anymore in this topic.
If your actual operating conditions fall to the test conditions in the table above, you can use the values given. If ever will not fall to the test conditions in the table, you can refer to the other tables in the datasheet of to the graph. Should there is no other information, a designer can judge it through experience and analysis and go for the worst-case scenario.
Dynamic Characteristics
Dynamic characteristics on the other hand is the characteristics of the MOSFET during transient or continuous switching actions.
Definitions:
Input Capacitance, Ciss – this is the equivalent capacitance in the gate to source. There is no literal capacitor installed but this is a parasitic capacitor as part of the MOSFET construction. This will be used to determine the MOSFET gate loss and the switching speed.
Output Capacitance, Coss – is the equivalent output capacitance particularly in drain to source. There is no physical capacitor as well but this is formed due to the MOSFET construction. This parasitic capacitance is dependent on the drain to source voltage.
Rise time and Fall time – These are values that used to calculate the switching loss of the MOSFET due to rise and fall times.
Total Gate Charge – some datasheet includes the total gate charge parameter into the dynamic characteristics. This parameter is also can be used in determining the switching loss in the gate.
Other parameters in the table above have uses but the most commonly used are the ones defined above.
The total MOSFET switching losses is composed of the input capacitance or gate charge loss, output capacitance loss and the loss due to rise and fall times. The detailed derivations about MOSFET switching losses is here.
5. How to Read MOSFET Datasheet Graphs and Curves
Power Dissipation
Power dissipation curve is a graphical specification of the MOSFET power dissipation capability with respect to the case temperature or junction temperature. The power dissipation capability is not 100% all though out but will start to degrade linearly after 25’C. in Semiconductor, 25’C is the adopted nominal or typical ambient temperature.
Transient Thermal Impedance
This is similar to the thermal resistance however this is intended for thermal conditions only. This means that the value provided in the graph cannot use to steady state. From the term transient, this specification could only be used during transient conditions wherein the design engineer want to check if the MOSFET can still handle the actual power dissipation.
Safe Operating Area (SOA)
Safe operating area is the ultimate curve showing the MOSFET boundaries. As long as you stay inside it, the MOSFET will not break. It is easy to use. The x-axis is the drain voltage while the y-axis is the drain current.
Drain to Source on-state Resistance Graphs
This is a curve that showing the relationship between the drain to source resistance and the junction temperature. Another variation of the curve is the drain to source resistance versus the drain currents.
