How to Select and Size a Switching Power MOSFET

Power and motion control are the two biggest areas where switching gate drivers are used to toggle power MOSFETs. Not all power MOSFETs are created equal, and the use of these components is broadening past silicon to include SiC and GaN. The selection of a switching MOSFET is an important step in ensuring your power converter does not blow when loads become high. The last thing anyone needs is a burning MOSFET.

To properly select a MOSFET for a switching application, there are a few parameters to examine from the data sheet. Once selected, a simple MOSFET model can be used to examine the expected power dissipation in the MOSFET during operation. You can also look at switching waveforms to see whether a MOSFET will be over-stressed. Here is how the selection process works for switching power MOSFETs.

PWM Driven Power MOSFET Specifications

In order to select the correct MOSFET for your switching circuit, you will need to examine whether it is drawing too much current at excessively high voltage. Typically, the voltage is controlled or regulated to a constant value, and we are more worried about peak current draw during switching. This is often the case in high current power supplies running at low voltage, but the peak current issue also matters in current-mode converters.

The three specifications you will need from a data sheet are:

• Peak pulse drain current rating

• Average drain current rating

• Duty cycle of the PWM waveform

With these four specifications, there is a simple process to size the MOSFET for power delivery.

Start With Average Power Delivery

First, determine the power delivery requirements to the load and work backwards. This applies in H-bridges, half bridges, or single switching MOSFET circuits. In any of these cases, it should be quite simple to determine the power delivered to the load.

Under the assumption that the ON-state resistance of the FETs is very small, we can assume that all of the voltage is dropped onto the load component. Therefore, the current delivered to the load is just the output voltage divided by the load impedance.

I = V/Z

Typically the load is taken as a real impedance and we would have a real current. For an inductive load, the current will be pulsating and so an average would need to be determined, which can be estimated as the average value between high and low currents.

If you are using a switching converter, the inductor will of course modify the voltage delivered to the load by stepping it up or down from the source. In either case, you will know the currents delivered to the load because the output voltage will be known. This current value is the average current.

Use Load Current to Get the Pulse Current

The pulse mode current is the true current flow in the MOSFET during switching. In this case, using the average current, the pulse current can be calculated with the following formula:

Ipulse = Iavg/D

where D is the PWM duty cycle. This pulse value and the average value should be compared against the MOSFET ratings. An example from a MOSFET datasheet is shown below.

Absolute maximum ratings for the CSD19538 MOSFET.

Another design direction is to determine the maximum duty cycle for the switching controller based on the MOSFET current ratings. As long as the MOSFET ratings correspond to a peak current that is larger than the PWM driver's maximum PWM duty cycle, then the MOSFET will most likely be safe to use. In current-mode control, we would have the same calculation and ratings to consider, but the drain-source voltage would be varying based on the switching duty cycle.

What About Thermal?

One should notice that the rating given above is in terms of a thermal resistance value and pulse duration. This is because these factors will determine the expected heating of the package during a pulsing event. In some datasheets, you may actually find three possible groups of ratings:

• Package-limited

• Silicon-limited, which is sometimes used interchangeably with a package rating

• Temperature-limited

The temperature limitation rating is malleable, while the package and silicon limited ratings are not. If the design can be pushed to higher temperatures, then higher currents in the MOSFETs could be acceptable. The expected temperature of the MOSFETs participating in the switching action will be a function of the package thermal resistance, which can be quite large. Typical values can range from 20-30 degree C/W for MOSFET packages. 

When you’re ready to design and simulate MOSFET circuits in a PCB, make sure you use the best PCB design features in OrCAD from Cadence. If you’re ready to take even more control over net logic and board layout, you can graduate to Allegro PCB Designer for a more advanced toolset and additional simulation options for systems analysis. Only Cadence offers a comprehensive set of circuit, IC, and PCB design tools for any application and any level of complexity.

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