2022.12.21

Points of this article

・When a transient increase in power consumption is anticipated, the peak T_{J} in the transient state is sought.

・The transient thermal resistance is used for the thermal resistance when determining the temperature rise in a transient state.

・The peak T_{J} in the transient state should be checked to confirm that it does not exceed T_{J MAX}.

Up until now, examples of calculations to estimate T_{J} when power consumption is constant have been presented. Here, however, a method of calculation and example are described for conditions in which there is a transient increase in power consumption.

As the IC employed in the example, an LDO linear regulator is again used, the BD450M2EFJ-C. As the circuit conditions we consider a case in which the input voltage V_{IN} undergoes transient fluctuation, as in Fig. 1. The steady-state input voltage is 13.5 V, but in this example the input voltage reaches 35 V for 3 seconds in a 60 second cycle.

As a result of transient fluctuation of the input voltage, the power consumption also fluctuates in a transient manner, and consequently it can easily be imagined that T_{J} will fluctuate as well. In such cases, the transient thermal resistance is used to calculate T_{J} by adding the transient temperature rise to the steady-state T_{J}.

Strictly speaking, T_{J} rises (heat is generated) from the time power is input, and stabilizes after a certain time. The normal thermal resistance θ_{JA} is the value obtained by dividing the steady-state heat generation by the power consumption. On the other hand, the transient thermal resistance has a time-based parameter. In the example of Fig. 1, the transient thermal resistance is the value obtained by dividing the heat generation when V_{IN} has changed from 13.5 V to 35 V and 3 seconds have elapsed by the changed power consumption.

Fig. 2 shows a transient thermal resistance example. From the graph, the longer the time in the transient state (the longer the pulse width), the greater is the transient thermal resistance Z_{TH}, and we see that when about 300 seconds have elapsed, the thermal resistance becomes constant.

Transient thermal resistance is often provided in graph form, and a value can be read off from the transient state time (pulse width). From the graph, the transient thermal resistance Z_{TH} for the case of 3 seconds/cycle or a duty of 5% (green line) can be read off as 21°C/W. The steady-state value of 40°C/W is the value provided as θ_{JA} for the package.

As stated above, T_{J} is determined by adding the transient temperature rise calculated using the transient thermal resistance to the steady-state T_{J}. As the procedure used, first the steady-state and the transient power consumption values are calculated, and then the thermal resistance in each case is used to calculate the temperature rise (heat generation) for each. Then T_{A} is added to the sum of the heat generation values for the steady state and for the transient state to determine the transient T_{J}. Let’s try performing the calculations.

We calculate the power consumption P1 for steady-state V_{IN}=13.5 V, V_{OUT}=5.0 V, I_{OUT}=90 mA, and I_{CC}=40 μA (typ).

The power consumption P2 in the transient state with V_{IN}=35 V is calculated. Keep in mind that P2 is a value that includes the steady-state power consumption P1=0.77 W.

Using the steady-state thermal resistance θ_{JA}=40℃/W and the transient thermal resistance Z_{TH} (3s)=21℃/W for 3 seconds/5% duty, the respective T_{J} temperature increases are calculated. The transient temperature rise is calculated from the power consumption resulting by subtracting the steady-state power consumption P1 from P2.

Steady-state temperature increase

Transient temperature rise

These temperature increases are added to find the overall temperature increase.

Overall temperature increase

Finally, the ambient temperature T_{A} in the transient state is added to determine T_{J}. Here T_{A} is assumed to be 65°C.

In the transient state:

With this, we can find the maximum T_{J} under the condition that V_{IN} of 13.5V transiently rises to 35V for 3s in a 60s period. Fig. 3 illustrates the temperature rise for the transient change in V_{IN}. The temperature increase waveform is the V_{IN} integrated waveform because the transient thermal resistance characteristic shown in Fig. 2 has a time-based parameter.

The purpose of the T_{J} calculations up to this point are to facilitate an understanding of the T_{J} peak when there is a transient increase in power consumption. Apart from this, there is also the approach of approximating T_{J} from the average power consumption. The following is a calculation example for the same conditions as those used above. As the thermal resistance, the steady-state thermal resistance θ_{JA}=40°C/W is employed.

Average power consumption ※0.05 is from Duty＝5%.

Average temperature rise

In the above calculations, the temperature rise during transient behavior is 71.3°C, and so we see that in this case, calculation using an average power consumption is not appropriate.

The most important thing to confirm in thermal calculations is whether or not T_{J} exceeds the absolute maximum rating T_{J MAX}. A surge above the absolute maximum rating is not allowed, even for a moment, and so the transient peak temperature must be determined.

If the transient state is sufficiently long (setting aside whether it should then be called “transient”), for example in the example of Fig. 2 in which the pulse width exceeds 300 seconds, the transient thermal resistance equals the steady-state thermal resistance. Hence it is sufficient to simply determine T_{J} under maximum power consumption for the steady-state thermal resistance, and confirm that this is less than T_{J MAX}.

Downloadable materials, including lecture materials from ROHM-sponsored seminars and a selection guide for DC-DC converters, are now available.

- About Thermal Design
- Changes in Engineering Trends and Thermal Design
- A Mutual Understanding of Thermal Design
- Fundamentals of Thermal Resistance and Heat Dissipation: Heat Transmission and Heat Dissipation Paths
- Fundamentals of Thermal Resistance and Heat Dissipation: About Thermal Resistance
- Fundamentals of Thermal Resistance and Heat Dissipation : Thermal Resistance in Conduction
- Fundamentals of Thermal Resistance and Heat Dissipation : Thermal Resistance in Convection
- Fundamentals of Thermal Resistance and Heat Dissipation : Thermal Resistance in Emission
- Thermal Resistance Data: JEDEC Standards, Thermal Resistance Measurement Environments, and Circuit Boards
- Thermal Resistance Data: Actual Data Example
- Thermal Resistance Data: Definitions of Thermal Resistance, Thermal Characterization Parameters
- Thermal Resistance Data: θJA and ΨJT in Estimation of TJ: Part 1
- Thermal Resistance Data: θJA and ΨJT in Estimation of TJ: Part 2
- Estimating TJ: Basic Calculation Equations
- Estimating TJ: Calculation Example Using θJA
- Estimating TJ: Calculation Example Using ΨJT
- Estimation of Heat Dissipation Area in Surface Mounting and Points to be Noted