MOSFET power dissipation is the sum of the conduction loss and the switching losses. These power losses are very different from each other. Conduction loss is the static loss wherein the main contributor is the drain to source on-state resistance of the MOSFET as well as the drain current. While switching losses are dynamic losses that are dependent on the switching activities of the MOSFET. Conduction loss is explained in detail here. On the other hand, check out here the explanation of MOSFET switching losses. Here’s how to calculate MOSFET power dissipation.
How to Calculate MOSFET Power Dissipation when the MOSFET is not Working in Switching Application
If MOSFET is not working in switching applications, the power dissipation is just the conduction loss. Conduction loss is primarily due to the drain to source on-state resistance (RDSon) and the current flowing to the drain. The RDSon is a parameter that depends on other conditions. To understand more about RDSon, check the article How to Get MOSFET Correct RDSon Value.
How to Calculate MOSFET Power Dissipation when the MOSFET is Working in Switching Application
When a MOSFET is used in switching converters and the likes, the power dissipation is the sum of the conduction loss and the switching losses.
Power Dissipation total = Conduction Loss + Switching Losses
Conduction loss is
Conduction loss = ID2 x RDSon
Where ID is the drain current while RDSon is the drain to source on-state resistance. Check out how to get the correct RDSon value here.
Switching losses are composed of the total gate charge loss, COSS loss and rise and fall time loss.
Switching loss = Total Gate Charge loss + COSS loss + rise and fall time loss
Total Gate Charge Loss
Total gate charge loss = ½ x Qtotal x V x Fsw
Where;
Qtotal is the total gate charge, V is the voltage applied to the gate-source while Fsw is the switching frequency
COSS Loss
COSS Loss = ½ x COSS x V2 x Fsw
Where;
COSS is the output capacitance of the MOSFET, V is the drain to source voltage and Fsw is the switching frequency
Rise and Fall Time Loss
rise and fall times loss = ½ x (trise + tfall) x Irms x V x Fsw
Where;
trise is the rise time, tfall is the fall time, Irms is the drain rms current, V is the drain to source voltage and Fsw is the switching frequency.
Switching losses are dependent on the switching frequency. The higher the switching frequency, the higher the switching loss. For a very detailed explanation about switching losses, read the article “How to compute MOSFET switching losses”.
How to Compute MOSFET Power Dissipation based on a specific temperature
What is discussed above is the actual power dissipation of the MOSFET based on the actual application. The discussion below, however, will give a guide on how to calculate the MOSFET power dissipation with respect to a specific temperature. Sometimes, this is called the de-rated power dissipation which is the capability of MOSFET. Above the typical temperature of reference, the power dissipation capacity of the MOSFET will start to de-rate.
De-rated Power Dissipation = ( TJmax – TAmax ) / RthJA
Or
De-rated Power Dissipation = ( TJmax – TCmax ) / RthJC
Where;
TJmax is the maximum junction temperature rating of the MOSFET, TAmax is the maximum operating ambient temperature, TCmax is the maximum operating case temperature, RthJA is the thermal resistance from junction to ambient and lastly RthJC is the thermal resistance from junction to case. These parameters are all given in the MOSFET datasheet.
The first equation is used when the MOSFET is not mounted to a heatsink while the second equation is used when the MOSFET is mounted on a heatsink. If you are interested in knowing what the parameters are to consider in designing circuits with MOSFET, read this.
Summarizing What we Learn So Far
These are the important points to remember on how to calculate MOSFET power dissipation.
1. Power dissipation is composed of two types of losses. First is conduction loss and the second one is switching loss.
2. Conduction loss is due to the conduction of the MOSFET (when the MOSFET is at on-state) which is mainly depends to the RDSon and the drain current.
3. Switching loss is broken down into three main parts; total gate charge loss, COSS loss and rise and fall time loss.
4. For a MOSFET operating in a non-switching circuit, only the conduction loss makes up the power dissipation.
5. Switching losses are dependent on the dynamic parameters and the switching frequency.
6. The MOSFET power rating or capability will start to de-rate above the reference typical temperature.
7. All the parameters needed on how to calculate MOSFET power dissipation are provided in the datasheet but there is a need to know the actual circuit operating conditions to select the correct value to use.
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