# Let SPICE Be Your MOSFET Power Dissipation Calculator

### Key Takeaways

• MOSFETs have non-zero ON-state resistance, so they dissipate some power during operation.

• Although MOSFET current will saturate during operation, the voltage drop across the device will not, possibly leading to burnout.

• SPICE simulations can be used as a MOSFET power dissipation calculator so that you can calculate heat generated in the junction.

These MOSFETs do not have perfect conductivity and will dissipate some power during operation

MOSFETs are fundamental transistors that have replaced BJTs in many applications. These components provide switching action with highly efficient power transfer, and they can be operated at high speed/high frequency if built as smaller components. However, all MOSFETs dissipate some power during operation, as they are not perfect conductors when switched into the ON state. Once a MOSFET dissipates too much power, it will heat up and the junction can reach high temperature, leading to failure.

One problem with designing with MOSFETs is that they have a power dissipation limit, just like other components. They also have saturation current and breakdown voltage ratings, and they have a maximum junction temperature rating. These ratings are important because they define the reliability limits of your MOSFET and the maximum power/heat the component can accept before it fails. When you’re designing MOSFET circuits that will draw high power, the best MOSFET power dissipation calculator is your SPICE simulator.

## Power Dissipation in MOSFETs

There are many factors that determine the power dissipation in a MOSFET. Some of the main factors are internal to the structure of the MOSFET, but the external circuit components will also determine the electrical behavior and power dissipation. Unfortunately, this means there is no MOSFET power dissipation calculator because the power dissipated depends on the external components in your circuit. Instead, SPICE simulations need to be performed to determine the power dissipation using a MOSFET circuit model for a real component.

Consider the MOSFET amplifier shown below. In this circuit, the voltage source on the left side of the circuit modulates the MOSFET between the ON and OFF states continuously. This then causes the circuit to draw power from the 15 V DC source (V2) through R4 and C2. Because the loop from R2 to the gate electrode creates some DC offset, the output will have some DC offset, which will be filtered by the capacitor C2. The power is then delivered to the load R21.

Example MOSFET amplifier circuit with output AC coupling and a 10 kOhm load

The power dissipation in the MOSFET is defined by a few factors:

• ON-state resistance in the MOSFET junction, which will determine the drain-source voltage drop VDS; typical values are in the mOhm range at full modulation.
• Current limiting resistors connected to the source and drain, which will drop some voltage and keep the current below the saturation limit.
• The voltage applied to the gate; the gate voltage and the current limiting resistors collectively determine whether the MOSFET is operating in the linear regime.

Once you know the current in the MOSFET and VDS, simply multiply them to get the power dissipation. If a MOSFET is being used as an AC amplifier, you want to look at the peak power or average power while also paying attention to the phase angle and power factor. Once you’ve determined the power dissipation in your MOSFET, you can use the thermal resistance specification from the datasheet to determine whether the component will overheat.

## Using SPICE as a MOSFET Power Dissipation Calculator

The following process can be used in a SPICE simulator to determine the power dissipation in your MOSFET circuit:

1. Set up your schematic with DC sources on the drain-source loop and gate electrode.
2. Run a DC sweep on the gate electrode to calculate and graph a load line (important for AC circuits).
3. Take drain-source voltage, current, and power measurements using probes at each DC value for the gate voltage.
4. Use the junction thermal resistance entry from the datasheet to calculate the expected temperature rise during operation.
5. Compare the power dissipation and temperature with the absolute maximum values in the MOSFET’s datasheet.

Fast changes in the current draw through a MOSFET—such as those that might occur in a switching load or electromechanical load—can cause the MOSFET to fail due to very fast overheating. Some areas where this is known to occur include:

• High power switching regulators
• Motors driven with half-bridge or full-bridge arrangements
• Lighting systems
• High current pulse or waveform generators

MOSFET datasheets will include transient or ESD limits, where the absolute maximum peak current, voltage, and/or power draw during a fast switching event are specified over a given time interval. If you’re working with AC systems or switching systems, you also want to run a transient analysis to determine the power dissipation in the MOSFET and to check that its power or temperature limits are not violated. If this does happen, one solution is generally to put MOSFETs in parallel to provide the required power. You would expect the power dissipation to be divided evenly among the parallel arrangement, but this should still be checked with a SPICE simulation to ensure each MOSFET operates within its absolute maximum electrical and thermal ratings.