Your calculations are OK in principle. In practice, the detailed construction of most types of components makes it difficult to identify the relevant heat capacities at the different timescales correctly.

Take for example the construction of two types of resistor. One is metal film on ceramic, the other is wire-wound. In the first, the mass of the resistive element is a small fraction of the total mass of the device. With a us pulse, only the metal film gets hots, there is little heat transfer to the substrate. With a 1s pulse, the entire resistor body is available to absorb heat, and there is little transfer to the PCB. With a 100s pulse, the area of PCB around the resistor absorbs heat.

With a wire-wound resistor, the mass of the resistive element can be most of the resistor weight. Whether the pulse is 1 us or 1 s long, heat is absorbed uniformly throughout the wire, and they are often specified with very high pulse powers.

Semiconductors are often supplied with a SOA (Safe Operating Area) graph in the data sheet, which gives the maximum voltage and current at various pulse lengths that the device can withstand. This encompasses all the considerations of how fast heat spreads from the active areas to the support and heat-sinking areas, temperature balance between parts, the different time constants of bond wires etc etc.

Here is an example SOA graph from an Analog Devices technical article for use of MOSFETs ...

As you can see, while it's generally the shorter the pulse, the more power the device can take, the detail is very complicated.

Your calculations are OK in principle. In practice, the detailed construction of most types of components makes it difficult to identify the relevant heat capacities at the different timescales correctly.

Take for example the construction of two types of resistor. One is metal film on ceramic, the other is wire-wound. In the first, the mass of the resistive element is a tiny fraction of the total mass of the device. With a 1 us pulse, only the metal film gets hots, there is little heat transfer to the substrate. With a 1s pulse, the entire resistor body is available to absorb heat, and the effective heat capacity is orders of magnitude higher. With a 10s pulse, the area of PCB around the resistor absorbs heat. Any longer length pulse, we are approaching the continuous power dissipation limit.

With a wire-wound resistor, the mass of the resistive element will be a much larger fraction of the resistor weight, giving a much higher usable heat capacity. Whether the pulse is 1 us or 1 s long, heat is absorbed uniformly throughout the wire. Wire-wound resistors are often given a pulse power specification in the data sheet.

Semiconductors are often supplied with a SOA (Safe Operating Area) graph in the data sheet, which gives the maximum voltage and current at various pulse lengths that the device can withstand. This encompasses all the considerations of how fast heat spreads from the active areas to the support and heat-sinking areas, temperature balance between parts, the different time constants of bond wires etc etc.

Here is an example SOA graph from an Analog Devices technical article for use of MOSFETs ...

As you can see, while it's generally the shorter the pulse, the more power the device can take, the detail is very complicated.

Your calculations are OK in principle. In practice, the detailed construction of most types of components makes it difficult to identify the relevant heat capacities at the different timescales correctly.

Take for example the construction of two types of resistor. One is metal film on ceramic, the other is wire-wound. In the first, the mass of the resistive element is a small fraction of the total mass of the device. With a us pulse, only the metal film gets hots, there is little heat transfer to the substrate. With a 1s pulse, the entire resistor body is available to absorb heat, and there is little transfer to the PCB. With a 100s pulse, the area of PCB around the resistor absorbs heat.

With a wire-wound resistor, the mass of the resistive element can be most of the resistor weight. Whether the pulse is 1 us or 1 s long, heat is absorbed uniformly throughout the wire, and they are often specified with very high pulse powers.

Semiconductors are often supplied with a SOA (Safe Operating Area) graph in the data sheet, which gives the maximum voltage and current at various pulse lengths that the device can withstand. This encompasses all the considerations of how fast heat spreads from the active areas to the support and heat-sinking areas.

Your calculations are OK in principle. In practice, the detailed construction of most types of components makes it difficult to identify the relevant heat capacities at the different timescales correctly.

Take for example the construction of two types of resistor. One is metal film on ceramic, the other is wire-wound. In the first, the mass of the resistive element is a small fraction of the total mass of the device. With a us pulse, only the metal film gets hots, there is little heat transfer to the substrate. With a 1s pulse, the entire resistor body is available to absorb heat, and there is little transfer to the PCB. With a 100s pulse, the area of PCB around the resistor absorbs heat.

With a wire-wound resistor, the mass of the resistive element can be most of the resistor weight. Whether the pulse is 1 us or 1 s long, heat is absorbed uniformly throughout the wire, and they are often specified with very high pulse powers.

Semiconductors are often supplied with a SOA (Safe Operating Area) graph in the data sheet, which gives the maximum voltage and current at various pulse lengths that the device can withstand. This encompasses all the considerations of how fast heat spreads from the active areas to the support and heat-sinking areas, temperature balance between parts, the different time constants of bond wires etc etc.

Here is an example SOA graph from an Analog Devices technical article for use of MOSFETs ...

As you can see, while it's generally the shorter the pulse, the more power the device can take, the detail is very complicated.