Post History
Homework problem A MOSFET has a power dissipation of $P_{d} = 10\text{W}$. The MOSFET is mounted on a heatsink. There is an isolation pad between the MOSFET case and the heatsink. We are given t...
#2: Post edited
- > A MOSFET has a power dissipation of $P_{d} = 10\text{W}$. The MOSFET is mounted on a heatsink. There is an isolation pad between the MOSFET case and the heatsink. We are given the thermal resistances: -
- >
- > Junction to case: $R_{th,jc}= 1.7 \: \frac{°\text{C}}{\text{W}}$
- >
- > Isolation pad: $R_{th,cs}= 1.3 \: \frac{°\text{C}}{\text{W}}$
- >
- > Heatsink to ambient air: $R_{th,sa}= 2.0 \: \frac{°\text{C}}{\text{W}}$
- >
- > The ambient temperature around the MOSFET is 25°C. What is the temperature of the MOSFET case?
- **My attempt**
- The scenario must look something like this: -
- ![Image alt text](https://electrical.codidact.com/uploads/rXQ96htJtGs3ob1fiMihNpyr)
- Here I have represented the thermal resistances as resistors, the dissipated power as a current source, and the temperatures as potentials. From this I calculate the following: -
- $$P_d = \frac{T_c-T_a}{R_{th,cs}+R_{th,sa}} \Leftrightarrow$$
- $$T_c = P_d(R_{th,cs}+R_{th,sa})+T_a = (10\cdot 3.3)°C + 25°C = 58°C$$
- So the case must be 58°C.
- Is my thought process/calculations correct or have I misunderstood something? Moreover, are calculations like these useful/used when designing circuits or are they only good for "back of the envelope" calculations?
- **Homework problem**
- > A MOSFET has a power dissipation of $P_{d} = 10\text{W}$. The MOSFET is mounted on a heatsink. There is an isolation pad between the MOSFET case and the heatsink. We are given the thermal resistances: -
- >
- > Junction to case: $R_{th,jc}= 1.7 \: \frac{°\text{C}}{\text{W}}$
- >
- > Isolation pad: $R_{th,cs}= 1.3 \: \frac{°\text{C}}{\text{W}}$
- >
- > Heatsink to ambient air: $R_{th,sa}= 2.0 \: \frac{°\text{C}}{\text{W}}$
- >
- > The ambient temperature around the MOSFET is 25°C. What is the temperature of the MOSFET case?
- **My attempt**
- The scenario must look something like this: -
- ![Image alt text](https://electrical.codidact.com/uploads/rXQ96htJtGs3ob1fiMihNpyr)
- Here I have represented the thermal resistances as resistors, the dissipated power as a current source, and the temperatures as potentials. From this I calculate the following: -
- $$P_d = \frac{T_c-T_a}{R_{th,cs}+R_{th,sa}} \Leftrightarrow$$
- $$T_c = P_d(R_{th,cs}+R_{th,sa})+T_a = (10\cdot 3.3)°C + 25°C = 58°C$$
- So the case must be 58°C.
- Is my thought process/calculations correct or have I misunderstood something? Moreover, are calculations like these useful/used when designing circuits or are they only good for "back of the envelope" calculations?
#1: Initial revision
Case temperature of MOSFET
> A MOSFET has a power dissipation of $P_{d} = 10\text{W}$. The MOSFET is mounted on a heatsink. There is an isolation pad between the MOSFET case and the heatsink. We are given the thermal resistances: - > > Junction to case: $R_{th,jc}= 1.7 \: \frac{°\text{C}}{\text{W}}$ > > Isolation pad: $R_{th,cs}= 1.3 \: \frac{°\text{C}}{\text{W}}$ > > Heatsink to ambient air: $R_{th,sa}= 2.0 \: \frac{°\text{C}}{\text{W}}$ > > The ambient temperature around the MOSFET is 25°C. What is the temperature of the MOSFET case? **My attempt** The scenario must look something like this: - ![Image alt text](https://electrical.codidact.com/uploads/rXQ96htJtGs3ob1fiMihNpyr) Here I have represented the thermal resistances as resistors, the dissipated power as a current source, and the temperatures as potentials. From this I calculate the following: - $$P_d = \frac{T_c-T_a}{R_{th,cs}+R_{th,sa}} \Leftrightarrow$$ $$T_c = P_d(R_{th,cs}+R_{th,sa})+T_a = (10\cdot 3.3)°C + 25°C = 58°C$$ So the case must be 58°C. Is my thought process/calculations correct or have I misunderstood something? Moreover, are calculations like these useful/used when designing circuits or are they only good for "back of the envelope" calculations?