MOSFET will not work if the gate to source voltage is not satisfied. VGS threshold is one of the specifications in the datasheet. However, it needs little understanding on how to determine the right VGS threshold. This article will discuss how to determine the correct MOSFET VGS threshold.
How to Determine the Correct MOSFET VGS Threshold from the Datasheet
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Under “MAXIMUM RATINGS” table, there is a gate to source (VGS) specification (see below). Is this the VGS threshold? This is not! This is the maximum voltage limit that the gate to source terminals can tolerate. Exceeding this value will damage the MOSFET.
Under ON-STATE CHARACTERISTICS table, there is a specification of gate threshold voltage, VGS(TH). Is this the one we are after for? Yes, this is the VGS threshold. However, this value is only for a specific condition which is a drain current of 37mA. This value may not suits your application. Referring to this value is not yet conclusive if your design is a rock solid.
In Figure 5, there is a Transfer Characteristics of VGS versus DRAIN CURRENT (ID) with available curves for junction temperature of 25’C, -55’C and 175’C. This is the most appropriate to use as a reference. To ensure MOSFET turn-on at extreme condition, consider the worst-case profile. From the transfer characteristics, the MOSFET will require a bigger voltage to the gate to source terminals when at lower temperatures, particularly at -55’C. If you design in this temperature, the MOSFET will turn-on to other temperatures guaranteed.
Procedure on How to Determine the Correct MOSFET VGS Threshold based on the Transfer Characteristics
1. Compute the drain current of your circuit. You can use engineering methods like ohms law or KVL/KCL and substitution method. If you find it difficult, you can assume the MOSFET voltage drop as zero. So, for a simple relay driver with a MOSFET and relay, the drain current is just the supply voltage divided by the relay coil resistance.
2. With the actual drain current, project it to the -55’C curve.
3. Look for the equivalent VGS on the point where the projected drain current intersects to the -55’C curve.
4. Add a design margin.
For example, the circuit drain current is 10A. From the -55’C curve, the need VGS is around 7.5V. Refer to below projection. Then, how much design margin to add?
Under the MAXIMUM RATINGS table, the maximum voltage that the gate to source could stand is +22V. This is your guide on up to which level you should set the applied gate to source voltage. For a 7.5V requirement, you can use a gate to source voltage of 12V. This is already a safe value for both the VGS threshold requirement and for the maximum limit.
Now, it may be clear to you on how to determine the correct MOSFET VGS threshold based on the explanation above. However, aside from the Transfer Characteristics, when the MOSFET is switching between on and off continuously, the total gate charge versus VGS must be considered also. Set the circuit VGS above the plateau (this is called the Miller Plateau). So, for the example MOSFET here, a 12V in the gate to source terminals will still be very sufficient to satisfy the Transfer Characteristic and the Miller Plateau.
Summary
Everything needed on how to determine the correct MOSFET VGS threshold is indicated in the datasheet of the MOSFET used. These are the key takeaways of this article:
1. Always use the Transfer Characteristics to get the right gate to source voltage.
2. The transfer characteristic is a graph of VGS versus drain current (ID) wherein there are options for the extreme operating temperatures.
3. When the MOSFET is used in switching applications (such that it will change state between on and off continuously), also consider setting the applied VGS above the Miller Plateau.
4. A design margin of at least 20% from the maximum required VGS is a good rule of thumb. For instance, the required VGS based from the Miller Plateau is 10V, then the minimum level of the applied VGS must be 12V [ (12 – 10)/10 = 20% ].
5. For non-switching applications (such as the MOSFET will not change state between on and off continuously), the circuit VGS margin could be set to higher values, as long as the maximum gate to source voltage limit is still far. For instance, the limit is 22V and the required VGS is only 7.5V from the Transfer Characteristics, the applied VGS could be set to 15V. This will give 68.2% (15/22) stress, which is still very good value. But try to limit the stress level to 70-80% to prolong the MOSFET life.
6. For MOSFET operating in continuous switching, the circuit VGS margin could not be exaggerated because the gate losses (switching loss) will increase. The applied VGS must be a trade off between a guaranteed value to turn on the MOSFET and the gate loss. Read the article How to Compute MOSFET Switching Losses to understand further the switching losses involved in MOSFET circuits.
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