There are some factors to consider in calculating MOSFET power dissipation. When the power dissipation is known, it is possible to compute the junction temperature of the MOSFET. Important to take that a MOSFET installed with no heatsink has different computation strategy compared to the one with a heatsink.
Small current or low power MOSFETs are usually mounted to PCB directly. There is no heatsink involve. High current and power MOSFETs are mounted on a heatsink. Small power heatsink is usually use to drive small signal circuits while power MOSFETs are often seen in power amplifiers, switching converters and power supplies to name some.
1. Small Signal MOSFETs Power Dissipation and Junction Temperature Calculation
Equation 1:
This is the equation to use in solving the MOSFET actual power dissipation during operation. This equation is however only applicable for a MOSFET that work as a driver or a power switch but not operating in high switching frequency like in switching converters, inverters, and power supply units. In these applications, power dissipation due to switching action will be considered as well. This is often called as switching losses.
Below equation is called conduction loss.
Pdiss_MOSFET = Idrain X Idrain X RDSon
Where;
Idrain – drain current. This is dependent to actual application (this is not the drain current rating of the device as shown in the datasheet.
RDSon – drain to source on state resistance. This can be derived from the datasheet.
Equation 2:
Pdiss_MOSFET = (Tjactual – Tambient) / Rthja
Where;
Tjactual – actual measured junction temperature.
Tambient – ambient temperature near the MOSFET
Rthja – thermal resistance from junction to ambient
Both Equation 1 and 2 will give the same result. However, Equation 2 involves the actual junction temperature which cannot be easily get measured.
Equation 3:
This equation is same as Equation 2. However, this is for the maximum power dissipation the MOSFET can handle or its power capability.
Pdiss_MOSFET_max = (Tjmax – Tambient) / Rthja
Where;
Tjmax – allowable maximum junction temperature of the MOSFET. This is specified in the datasheet.
Tambient – ambient temperature near the MOSFET
Rthja – thermal resistance from junction to ambient
Equation 4:
This is just a rewrite version of Equation 2 and 3. Pdiss_MOSFET will follow same Equation 1 above.
Tjactual = Pdiss_MOSFET X Rthja + Tambient
2. High Power MOSFETs Power Dissipation and Junction Temperature Calculation
Equation 5:
Pdiss_MOSFET = Idrain X Idrain X RDSon
Where;
Idrain – drain current. This is dependent to actual application (this is not the drain current rating of the device as shown in the datasheet.
RDSon – drain to source on state resistance. Datasheet specify this.
Equation 6:
Pdiss_MOSFET = (Tjactual – Tambient) / Rthja
Where;
Tjactual – actual measured junction temperature.
Tambient – ambient temperature near the MOSFET
Rthja – thermal resistance from junction to ambient.
Both Equation 5 and 6 will give the same result. However, Equation 2 involves the actual junction temperature which is not easy to measure. This equation is also valid if a power MOSFET is not mounted to a heatsink.
Equation 7:
Actual MOSFET power dissipation. This is valid when the MOSFET is attached to a heat sink. This is the same to Equation 6 that will give the MOSFET actual power dissipation. However, you need to measure the actual junction temperature and ambient temperature.
Pdiss_MOSFET = (Tjactual – Tcactual) /(Rthjc + Rthchs + Rthhsa)
Where;
Tjactual – actual junction temperature.
Tcactual – actual case temperature
Rthjc – thermal resistance from junction to case. This is provided in the datasheet.
Rthchs – thermal resistance from case to heat sink. This is the Rth of the element bonding the case and
the heatsink.
Rthhsa – this is the thermal resistance from heat sink to air. Basically, this is the thermal resistance of
the heat sink used.
Equation 8:
This is the same as Equation 7. However, this is widely use to get the maximum power dissipation the MOSFET can handle. In other words, the power capability of the MOSFET.
Pdiss_MOSFET = (Tjmax – Tcactual) /(Rthjc + Rthchs + Rthhsa)
Where;
Tjmax – maximum junction temperature of the MOSFET per datasheet.
Tcactual – actual case temperature
Rthjc – thermal resistance from junction to case. This is provided in the datasheet.
Rthchs – thermal resistance from case to heat sink. This is the Rth of the element bonding the case and
the heatsink.
Rthhsa – this is the thermal resistance from heat sink to air. Basically, this is the thermal resistance of
the heat sink used.
Equation 9:
This is just a re-write version of Equation 8. This will give the actual junction temperature.
Tjactual = Pdiss_MOSFET X (Rthjc + Rthchs + Rthhsa) + Tcactual
3. How to Get the RDSon
Manufacturers specify RDSon or the drain to source on-state resistance in the datasheet. This is often under Static Characteristics. It is a static parameter because it is a fix value. Table 7 below is an example. This is taken from BUK6Y61-60P from nexperia. To check nexperia products that may suit your design, visit them at https://www.nexperia.com/.
From the above table, the RDSon is given at the typical and maximum unit. There are also test conditions where RDSon is derived.
What to Use? How to Select?
If the purpose is to get the near exact value of the power dissipation for whatever purpose like in getting the efficiency, then use the typical value of RDSon. Also consider the actual circuit parameters. For instance, the applied voltage to the gate is 10V with a typical drain current of around 4.7A and the junction temperature is around 25’C, then select 48 milliohm. Take note that the circuit parameters in the above table are negative because the device is a P channel MOSFET.
If the purpose is to properly size the MOSFET by evaluating its power stress, then use maximum value.
You may do the same selection as above in terms of the circuit parameters. But since you are aiming to select a rugged MOSFET, then directly consider the maximum value of the RDSon from the table above which is 130 milliohms. On the datasheet, there is also a graph like below. Sometimes this will give the value of the RDSon directly versus the junction temperature and sometimes will give the normalized value of the RDSon which is the case of the below graph. The graph is more accurate method in getting the RDSon.
More articles related to RDSon:
How to Use MOSFET RDSon Data from the Datasheet
MOSFET RDSon Temperature Coefficient Usage and Interpretation
4. Summary and Conclusion
There are several equations above to solve MOSFET power dissipations and junction temperature. Correct application of each equation will result to a near exact result. I said near exact because everything is not perfect at all. There are several variables in actual scenario. Like for instance in determining the RDSon, there is already inaccuracy on it. The determination of the other parameters as well. However, to ensure that a MOSFET will not fail, always consider the worst case in selecting the part. For application like requires accuracy, use the typical values.
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