Important MOSFET Design Parameter

Important MOSFET Design Parameter to Consider

The important MOSFET design parameter to consider during circuit design are gate threshold voltage, on-state resistance, current stress, voltage stress, power stress and thermal stress.

Threshold voltage refers to the gate to source voltage requirement for the MOSFET to turn on. On-state resistance is the equivalent resistance between drain to source when the MOSFET is on. Current stress is related to the actual current and current rating. Voltage stress is related to the actual voltage and voltage rating. Power stress and thermal stress relate to each other wherein power stress is related to actual power dissipation and the power rating while thermal stress is related to actual temperature with respect to the temperature rating.

Important MOSFET Design Parameter Explained

Gate to Source Threshold Voltage, VGSth

VGSth is a MOSFET requirement for turn on. The circuit gate to source voltage must higher than VGSth. Otherwise, the MOSFET will not turn on. Each datasheet has its own way of defining the VGSth. For instance IPP040N06NF2SAKMA1 MOSFET from Infineon Technologies defined the VGSth as below table.

Important MOSFET Design Parameter -VGSthreshold

The VGSth in the table is very specific only to a certain junction temperature, drain voltage and drain current. Your application may not fall to this scenario. To know how to get the right VGSth, read How to Determine the Correct MOSFET VGS Threshold.

Once you know the right VGSth, you can size the level of the applied VGS. In my designs, I always put sufficient design margin. For instance, a VGSth of 3.3V maximum, I set the applied VGS to at least 30% more if there is no limiting factor. Which means for a 3.3V VGSth, I will choose to apply voltage across gate to source of 4.29V [ (4.29/3.3)- 1].

Drain to Source On-state Resistance, RDS(on)

Another important MOSFET design parameter to consider is the drain to source on-state resistance. This is the equivalent resistance from drain to source when the MOSFET is on-state. For power applications, this parameter must be very low as this will dictate the conduction loss. Higher conduction loss will result to higher power dissipation, and this will translate to a hotter temperature on the MOSFET and will jeopardize the system efficiency.

Using the same MOSFET as above example, RDS(on) is defined as below table. It is specified together with some conditions like VGS value and drain current value (ID). You can choose from these conditions which suits your application.

Important MOSFET Design Parameter -RDSon

Furthermore, there is a graph of Tj (‘C) versus RDS(on) specification in the datasheet. See below. You can project from the graph on which points your application falls. For instance, you expect that you reach at 100’C junction temperature in your actual design, then the corresponding RDS(on) is around 5.5 milliohm using the maximum curve.

Why Current Stress is Very Important MOSFET Design Parameter to Look at?

Another important MOSFET design parameter is the current stress. Current stress is defined as

Current Stress = ( Actual Current / Current Rating ) x 100%

Actual current is the current flowing to the circuit. You may compute or measure it. On the other hand, the current rating is the current specified in the datasheet. Using the same MOSFET as example, below is the specified continuous drain current. Like the RDS(on), it is specified with some conditions.

Important MOSFET Design Parameter -drain current

Assuming that your actual application falls to VGS = 10V and Tc = 100’C, then you can use 80A as the current rating.

If the actual current flowing to the MOSFET is 60A, then the current stress is

Current Stress = ( Actual Current / Current Rating ) x 100%

Current Stress = ( 60A / 80A ) x 100% = 75%

To prolong the MOSFET life and ensure high reliability, limit the current stress to 70%.

Another equally important MOSFET Design parameter specification is the pulsed drain current. This is the current the MOSFET that can endure within a very short period. The sample MOSFET has pulsed drain current limit in the table below. Simply do not exceed this limit to avoid device failure.

How to Get the Actual Current

In actual application, to get the current, use an ammeter and connect it in series to the MOSFET drain. Ensure the ammeter fuse can handle the current level. Another way is to use an oscilloscope with a current probe. Simply clamp the current probe to the wire that connects to the MOSFET drain. You can set the measuring equipment to either record the average (DC) or the rms value. For oscilloscope, both parameters could be displayed at the same time. The pulsed drain current cannot be measured using an ammeter. The most appropriate equipment is an oscilloscope.

Voltage Stress is the important MOSFET design parameter to watch out in order to prevent damage from over voltage

Voltage stress is defined as

Voltage Stress = ( Actual Voltage / Voltage Rating ) x 100%

Actual voltage is the voltage that the MOSFET will experience while voltage rating is the voltage specified in the datasheet. For MOSFET, voltage stress is applicable to both drain to source (VDS) and gate to source (VGS).

IPP040N06NF2SAKMA1 MOSFET is specifying below numbers for the VGS and VDS.

These values must not be exceeded to ensure longer life for MOSFET. A good design margin is to limit the stress level from 70% to 80%. So, for VGS rating of 20V, the maximum voltage to be applied to the gate to source must only be

Voltage Stress = ( Actual Voltage / Voltage Rating ) x 100%

Actual Voltage = ( Voltage Stress / 100% ) x Voltage Rating = ( 80% / 100% ) x 20V = 16V

In the manner, the maximum allowable drain to source voltage must only be

Actual Voltage = ( Voltage Stress / 100% ) x Voltage Rating = ( 80% / 100% ) x 60V = 48V

How to Get the Actual Drain Voltage

Actual drain voltage could be measured using a voltmeter. Connect the voltmeter across the MOSFET. If you know the MOSFET drain, connect the voltmeter positive probe to it. Then the voltmeter negative probe to the MOSFET source.

For applications wherein the MOSFET is continuously switching between on and off, it is difficult to capture the drain voltage using a voltmeter. The most appropriate measuring equipment to use is an oscilloscope. The probe connection is the same as the voltmeter.

How to Get the Actual Gate to Source Voltage

Get a voltmeter, put the positive probe to the gate while the negative probe to the source of the MOSFET. You can also use an oscilloscope to do the job.

Power Stress

Power stress is related to thermal stress as pointed out earlier. Power stress is mathematically defined as:

Power Stress = ( Actual Power Dissipation / Power Rating ) x 100%

Actual power dissipation is the sum of the conduction loss and switching loss. For non-switching operation, only the conduction loss to be considered. Conduction loss is the power dissipation related to the channel on state resistance and drain current. To know how to compute the conduction loss, read the article ” How to Compute MOSFET Conduction Loss”. To know how to compute the switching losses, read the article “How to Compute MOSFET Switching Loss”. To prolong the life of MOSFET, limit the power stress to 70%-80%.

The power rating of the MOSFET will go with the operating temperature. At lower temperatures, the power rating is at 100% as rated. However, when temperatures rise above the rated value (most of the times at 25’C), the power rating starts to de-rate. For instance, below is how the sample MOSFET defined on its datasheet. There are two power dissipation ratings. One is considering case temperature while another one is considering ambient temperature. In both cases, the reference temperature is 25’C.

The rating TC is to be used when the MOSFET is to be mounted in a heatsink. On the other hand, when the MOSFET is mounted in free air, the TA rating will be used. TC means case temperature while TA means ambient temperature.

How to Compute the De-rated Power Rating

The de-rated power can be computed using the equations below.

Power Rating (de-rated) = ( TJmax – TC ) / RthJC

Or

Power Rating (de-rated) = ( TJmax – TA ) / RthJA

Supposing the desired maximum case temperature should not exceed 80’C, the de-rated power is then

Power Rating (de-rated) = ( TJmax – TC ) / RthJC

Power Rating (de-rated) = ( 175’C – 80’C ) / 1.4 K/W = 67.85W

You should not confuse with the K/W unit of RthJC and RthJA. It is directly same to ‘C/W. The TJmax, RthJC and RthJA are defined below.

If the target maximum ambient temperature is only 50’C, the de-rated power will be

Power Rating (de-rated) = ( TJmax – TA ) / RthJA

Power Rating (de-rated) = ( 175’C – 50’C ) / 62 K/W = 2.016W

For example, the actual power dissipation computed is 1.5W (with the MOSFET in free air) at 50’C ambient, the power stress would be

Power Stress = ( Actual Power Dissipation / Power Rating ) x 100%

Power Stress = ( 1.5W / 2.016W ) x 100% = 74.4%

Thermal Stress

Thermal stress mathematical definition is

Thermal Stress = ( Actual Junction Temperature / Maximum Junction Temperature ) x 100%

The actual junction temperature is the target value or the value that you can measure compute. The maximum junction temperature is the device maximum rating and in the example MOSFET it is 175’C.

How to Compute the Actual Junction Temperature

The actual junction temperature could be computed using the equations below

Power Rating (de-rated) = ( TJmax – TC ) / RthJC

Or

Power Rating (de-rated) = ( TJmax – TA ) / RthJA

Re-arranging to express the TJmax

TJmax = Power Rating (de-rated) x RthJC + TC

Or

TJmax = Power Rating (de-rated) x RthJA + TA

The Power Rating (de-rated) must be the computed power dissipation (actual value) while the TJmax is simply the corresponding computed junction temperature.

Supposing the computed power dissipation is 2.016W at 50’C ambient temperature, then the corresponding junction temperature is

TJmax = Power Rating (de-rated) x RthJA + TA

TJmax = 2.016W x 62 K/W + 50’C = 175’C

The thermal stress is

Thermal Stress = ( Actual Junction Temperature / Maximum Junction Temperature ) x 100%

Thermal Stress = ( 175’C / 175’C ) x 100% = 100%

The thermal stress is 100% which is no good. The 2.016W actual power dissipation results in 100% thermal stress because it is the maximum power the MOSFET can handle as well at 50’C ambient. Thus, both power dissipation and thermal stress are interrelated to each other.

Summarizing the Key Points About Important MOSFET Design Parameter to Take Note

1. The major parameters to consider are gate voltage; both VGS threshold and the VGS limit, drain to source on-state resistance, current stress, voltage stress, power stress and thermal stress.

2. To ensure turning on the MOSFET, the circuit VGS level must be higher than the maximum required VGS with margin.

3. The drain to source on-state resistance RDSon is dependent on the other parameters defined in datasheet.

4. Current stress is the ratio of the actual current to the current capacity of the MOSFET. A good limit is 70% to prolong the life of MOSFET.

5. Voltage stress is the ratio of the actual voltage to the voltage capacity of MOSFET. A good limit is 70%-80% to prolong the life of MOSFET.

6. Power stress is the ratio of the actual application power dissipation to the power rating of MOSFET. A good limit is 70% to prolong the life of MOSFET.

7. Thermal stress is the ratio of the actual device temperature reading to the maximum allowable temperature specified in the datasheet. It is interrelated to the power stress and a good limit is 70% to prolong the life of MOSFET.

8. All the important MOSFET design parameter are specified in the datasheet. However, as a designer, you must know the actual operating conditions of your circuit to select the right value.

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