27.1.1 Resistor Smoke Parameters

Smoke Analysis calculates and displays the maximum power dissipation, maximum body temperature, and maximum voltage drop across a resistor.

A current flowing through a resistor will generate heat resulting in an increase in temperature that will affect the normal operation of the resistor. The maximum specified power rating for a resistor relates to the heat that can be dissipated up to a specified ambient temperature limit above which the operation of the resistor will degrade or fail. Beyond the temperature limit, the power that can be dissipated as heat decreases as a percentage of the maximum power rating. Fig. 27.4 shows the Derating Power curve for a resistor used in Smoke analysis.

The maximum power rating (POWER) for a resistor is specified over its normal operating ambient temperature range up to its rated temperature, TKNEE. Above this temperature, the power rating decreases at a rate given by the SLOPE, eventually falling to zero at the maximum temperature rating for a resistor, MAX_TEMP.

For example, a 0.25 W resistor is capable of dissipating a maximum power of 0.25 W up to a typical ambient temperature of 70°C. Exceeding the 0.25 W power rating of the resistor will result in excess heat that cannot be efficiently dissipated resulting in an increase in resistor temperature. Beyond 70°C, the effective power dissipation will decrease up to the maximum temperature typically 150–200°C at which point the resistor will fail. Premature resistor failure can be avoided by increasing the power rating of the resistor, increasing airflow across the resistor, using an equivalent parallel combination of resistors, using ventilated cases or to avoid placing resistors close to heat sources such as power transistors. Table 27.1 summarizes the resistor Smoke Analysis parameters.

In Smoke analysis, the resistor derating curve is defined by the KNEE temperature and the MAX temperature both of which are related by the SLOPE of the power derating curve. You can enter the SLOPE parameter value using the Variables symbol from which the KNEE value will be calculated. Alternatively, you can add the TKNEE property to the resistor using the Property Editor. See Example 1 at the end of the chapter.

27.1.2 Inductor Smoke Parameters

Smoke Analysis displays the maximum DC current rating, maximum dielectric strength, maximum power loss due to equivalent series resistance (ESR) and inductor temperature rise.

Inductors are made from insulated coils of wire wound in air or wound on a ferrite magnetic core. When current flows through the wire windings, average real power is dissipated in the form of heat associated with the inherent DC resistance of the wire windings that make up the coils. Exceeding the inductor maximum DC current ratings can lead to saturation effects. The self-heating losses due to winding resistance will also cause a temperature rise in the inductor resulting in the wire insulation material to fail. Similarly, if the inductor maximum voltage ratings are exceeded, the insulation can also break down resulting in windings to short circuit and overheat. For core wound inductors and transformers, the current flowing through the inductor windings will generate heat resulting in an increase in temperature that will affect the magnetic characteristics of ferrite core materials. Core saturation levels decrease with an increase in temperature resulting in inductance values to decrease.

Subsequently, manufacturers specify the maximum inductor current rating (DC) and maximum voltage rating over a maximum ambient temperature range for reliable operation. Smoke analysis does not calculate the derating of inductor current with temperature.

The power dissipated by the inherent DC resistance of the inductor is attributed to the average current flowing through the inductor resulting in a temperature rise above ambient. Manufacturers specify the average current as an Irms current that causes a maximum allowable rise in temperature by a specified value above the ambient temperature. Hence, the maximum ambient temperature and maximum allowable temperature rise define the inductor maximum temperature MAX_TEMP.

Inductors also specify a rated saturation current (Isat) that is associated with the saturation of the inductor core resulting in the inductance value to decrease. Smoke analysis supports four parameters associated with inductors: DC current for saturation, temperature rise, breakdown voltage, and MAX current. See Example 2 at the end of the chapter. Table 27.2 lists the inductor Smoke parameters.

The ESR property is not available on the standard inductor from the analog.olb library. You have to use the L_t from the analog.olb.

Tip

When you run Smoke analysis, the Parameter names are shown in short form. To see the complete description, rmb > Parameter Descriptions as shown in Fig. 27.5.

27.1.3 Capacitor Smoke Parameters

Smoke Analysis calculates and displays the maximum rated voltage, maximum ripple current, maximum reverse voltage, maximum ESR power dissipation and capacitor temperature rise.

Applied voltage as well as temperature affects the reliability of a capacitor and its initial power on performance. The range of ambient temperature over which a capacitor can continuously operate is known as the category temperature range and is defined by upper and lower temperature limits. The rated temperature (upper category limit) is defined as the maximum ambient temperature that a capacitor can operate continuously without exceeding the rated voltage. Exceeding the capacitors maximum voltage rating will cause the dielectric to break down. Subsequently, manufacturers provide voltage derating guidelines to improve the long-term reliability of their capacitors. Fig. 27.6 shows the voltage-temperature derating curve for a capacitor used in Smoke analysis.

The rated VOLTAGE is the maximum peak DC operating voltage that can be applied to a capacitor for continuous operation, also known as the working voltage that appears printed on the capacitor. The rated temperature, KNEE is the maximum ambient temperature over which the capacitor can continuously operate at the rated VOLTAGE. The maximum ripple current is the AC rms current that flows in and out of a capacitor and it is this current that produces heat and hence power dissipation (PDML) in the ESR. Smoke analysis supports electrolytic and nonelectrolytic capacitors. The CVN parameter is used to report on short reverse transient voltages. The maximum ambient temperature at which the capacitor can operate is given by MAX_TEMP. Table 27.3 summarizes the capacitor parameters used in Smoke analysis.

The rated temperature is different for capacitors with different dielectrics. Electrolytic aluminum and tantalum capacitors have a working voltage dependence on temperature compared to nonelectrolytics such as ceramic, polyester, and other plastic film capacitors. Capacitance and leakage current are also dependent on temperature. Smoke analysis supports electrolytic and nonelectrolytic capacitors and will test for reverse transient voltages. The CVN parameter defines the negative voltage ratings.

27.2.1 Bipolar Transistors

Bipolar transistors used in power amplifiers or power switching circuits should be operated within their safe operating area in order to avoid excessive power dissipation that can lead to premature failure. Manufacturers provide derating curves that plot maximum continuous power dissipation against collector junction temperature also known as case temperature. For silicon transistors, the range is normally specified from room temperature at 25°C to 150°C.

The power derating curve for a transistor is shown in Fig. 27.7. The transistor case temperature is related to the collector-base junction temperature (TJ). The transistor junction thermal (ΘJA) resistance is composed of the junction-to-case thermal resistance (RJC) that is a fixed value and the case-to-ambient thermal resistance (RCA). The case-to-ambient thermal resistance can be decreased by using a heatsink that effectively increases the surface area of the transistor case. In the case of power transistors, the collector-base junctions have large areas in order to dissipate heat quickly away from the junction. Table 27.4 lists the Smoke analysis parameters used for bipolar (BJT) transistors.

Semiconductor smoke analysis parameters are defined in the semiconductor PSpice models. The smoke parameters can be viewed using the PSpice Model Editor that can be accessed either from the circuit diagram or from the Start Menu. In the circuit diagram you highlight the transistor part and rmb > PSpice Model Editor. See Example 4 at the end of the chapter. Fig. 27.8 shows the Smoke parameters and their values for a Q2N3904 transistor.

The other semiconductor types and their associated Smoke parameters can be found in the online PSpice Advanced Analysis User Guide.

1. Create a new PSpice project called for example Smoke_Resistor and in the Create PSpice Project window, select simple_aa.opj from the pull-down menu (Fig. 27.14).

2. Delete the AA Variables part. Draw the resistor circuit in Fig. 27.15.

3. Double click on the resistor to open the Property Editor and add the following Smoke Limit parameters values:

MAX_TEMP = 155

SLOPE = 0.01176

POWER = 0.25

VOLTAGE = 400

No need to enter the SI units.

To display the Smoke limits, highlight each Smoke parameters by holding down the control key and clicking on each property. Select Display (or rmb > Display) and in the Display Properties window, select Name and Value as shown in Fig. 27.16.

4. Set up a transient analysis (PSpice > New Simulation Profile) and leave the default Run To Time as 1000 ns.

5. Place a Power marker on the body (middle) of the resistor. PSpice > Markers > Power Dissipation or click on the icon .

6. Run the simulation.

7. The Probe window in PSpice window will appear displaying a power dissipation trace of 136.170 mW.

8. Go back to Capture and select, PSpice > Advanced Analysis > Smoke.

9. PSpice Advanced Analysis will open up showing the results of the Smoke Analysis. Invalid values will be displayed as gray bar lines. For example, only average and peak values are calculated for TB, the RMS value is invalid. Right mouse click anywhere and select Hide Invalid Values. Fig. 27.17 shows the results of the Smoke Analysis with a power dissipation calculated as 136.1702 mV.

% Max is given by: 100 × 136.1702 m/240.3004 m = 57%.

10. Using Eq. (27.3) the resistor body temperature is calculated as TB = 73.3163°C. Obviously Smoke Analysis uses a greater number of significant digits in the calculations. A more accurate “hand” calculation would be to use 1/SLOPE = 85 rather than use SLOPE = 0.01176. This gives the same result as Smoke Analysis as 73.2978°C.

11. The resistor body temperature of 73.2978°C is greater than the TKNEE temperature of 70°C and so the power dissipation is dynamically derated. The value of the derating factor (% Derating) is given by Eq. (27.8) as:

$\mathrm{PDF}=\text{SLOPE}\left(MAX\_\text{TEMP}-\mathrm{TB}\right)$

For a more accurate calculation:

$\mathrm{PDF}=\frac{\left(155-73.2978\right)}{85}=0.9612=96.12\%$

Therefore using Eq. (23.9), the power dissipation (Max Derating) is given by

$\begin{array}{c}\hfill \text{Pdis}=\mathrm{PDF}\times \text{POWER}\hfill \\ \hfill \text{Pdis}=0.9612\times 0.25=0.2403\mathrm{W}\hfill \\ \hfill \text{Pdis}=240.3\mathrm{mW}\hfill \end{array}$

This is the value of PDM shown in Fig. 27.17.

12. Right mouse click anywhere in Smoke Analysis and select Derating > Standard Derating and run the simulation by clicking on the play button.

13. You should see two red bars that indicate that the derated values for PDM (RMAX) have been exceeded by 104% Fig. 27.18. The resistor is not operating within the defined safe operating limits.

14. In Capture, change the value of POWER to 0.5.

15. Re-run the transient simulation and then run Smoke Analysis. You will see that using a 0.5 W resistor, the power dissipation of the resistor (Fig. 27.19) meets the defined safe operating limits of operation. In fact you could even use a 0.4 W resistor.

Exercise 2

1. Create a new PSpice project called Smoke_Inductor and in the Create PSpice Project window, select simple_aa.opj from the pull-down menu (Fig. 27.22).

2. Draw the inductor circuit in Fig. 27.23. Use an L_t inductor from the analog library as this has the ESR property attached. Place a VSIN source from the source library.

3. Double click on the resistor and in the Property Editor for POWER, set RMAX to 20 W. To display the power value 20 W, click on Display > Name and Value. Close the Property Editor.

4. Double click on the inductor to open the Property Editor and add the following Smoke Limit parameters values:

DC_RESISTANCE = 0.088

MAX_TEMP = 125

RTH = 78.91

POWER = 0.5

Highlight each Smoke parameters by holding down the control key and clicking on each property. Select Display and in the Display Properties Window select Name and Value (see Fig. 27.24).

5. Set up a transient analysis with a Run To Time of 4 ms.

6. Place a Current marker on pin1 of the inductor that is marked with the dot symbol. Run the simulation.

7. The Probe window in PSpice window will appear displaying the inductor current with a max value (LI) of 1.4942 A.

8. Go back to Capture and select, PSpice > Advanced Analysis > Smoke.

9. PSpice Advanced Analysis will open up showing the results of the Smoke Analysis for the resistor R1 and inductor L1. Invalid values will be displayed as gray bars. For example, only average and peak values are calculated for TJL. Right mouse click anywhere and select Hide Invalid Values (Fig. 27.25).

You should see the results in Fig. 27.26.

10. It is always a good idea to have a look at the Smoke Analysis log file. View > Log File > Smoke as shown in Fig. 27.27.

11. The log file in Fig. 27.27 reports undefined CV parameter for R1 and an undefined parameter LIDC for L1. A warning message reports on TKNEE being less than the simulation temperature and to check the values for the SLOPE and MAX_TEMP. These values were not added on the resistor and so default values were used.

12. In Capture double click on the resistor to open the Property Editor. Set the limit of MAX_TEMP (RTMAX) to 155 and the VOLTAGE limit (RVMAX) to 200. Do not close the Property Editor.

Rather than calculate the SLOPE, you can set the TKNEE limit. Select New Property and add TKNEE as shown in Fig. 27.28.

13. Re-run the simulation in Capture.

14. Re-run Advanced Analysis and look at the Smoke Analysis log file (Fig. 27.29).

15. Select the inductor and set the DC_Current limit (DC) to 1 A. As shown in Table 27.2, Smoke Analysis displays the DC_Current as LIDC.

16. Re-run the PSpice simulation in Capture.

17. Re-run Advanced Analysis.

18. The results for the resistor and inductor are displayed. Right mouse click anywhere and select Component Filter. The default wildcard setting (⁎) displays all components. As shown in Fig. 27.30, enter L1 and click on OK.

19. Fig. 27.31 shows the results of the Smoke Analysis for the inductor only.

The current LIrms is 1.0604 A

Therefore from Eq. 27.12 the approximate value for Pdis is

$\text{Pdis}={\left(1.0606\right)}^{2}0.088=0.09895=98.95\mathrm{mW}$

The average inductor temperature is given by

$\mathrm{TJL}=\text{Tambient}+\left[{\left(\text{LIrms}\right)}^2\times \mathrm{ESR}\times \mathrm{RTH}\right]$

$\mathrm{TJL}=27+\left[{\left(1.060\mathrm{\mu }\right)}^2\times 0.088\times 78.91\right]={34.8083}^{°}\mathrm{C}$

However, the values are shown with no derating.

20. In Smoke Analysis, select Edit > Profile Settings and in the Profile Settings window, click on the New (Insert) icon as shown in Fig. 27.32. Click on three ellipses (…) and browse to the default standard.drt derating file in the; < install path > oolspspicelibrary. For example:

                    C:CadenceSPB_17.2 oolspspicellbrary

21. Click on the Edit Derate File to display the Derate Types and Derating Factors (Fig. 27.33). Maximum power dissipation (PDM) is derated to 0.55 and voltage rating (RV) is 0.8.

22. Select the inductor (IND) and you will see that LI, LV, and LIDC all have a derating factor of 0.9.

23. Close both the Edit Derate File and Profile Settings windows.

24. Right mouse click anywhere in Smoke Analysis and select Component Filter and enter ⁎ to select all components.

25. Again, right mouse click anywhere in Smoke Analysis but this time select Derating > Standard Rating. Run Smoke analysis again. You will see two red lines indicating that the derated resistor power has been exceeded.

26. Change the resistor power in Capture to 30 W and re-run the PSpice simulation and then the Smoke Analysis. The final Smoke Analysis is shown in Fig. 27.34.

Exercise 3

The voltage tripler circuit in Fig. 27.35 is used to demonstrate how Smoke analysis can be used to improve circuit performance by detecting components that do not meet the derated MOCs.

1. Create a new project called, for example, voltage_tripler. After you name the project, the Create PSpice Project window appears. In the pull-down menu select the simple_aa.opj and click on OK (Fig. 27.36).

2. Draw the voltage_tripler circuit in Fig. 27.35 Use a C_t capacitor from the analog library as this has the ESR property attached.

3. Add SMOKE parameter values to the Variables part as shown in Fig. 27.35.

4. Set up a transient analysis with a Run To Time of 50 ms.

5. Place a differential voltage marker on node A and B, PSpice > Markers > Voltage Differential. When you place the first V + marker, the second V − marker appears automatically. Alternatively, click on the icon .

6. Run the simulation.

7. The Probe window in PSpice window will appear displaying the differential voltage between nodes A and B.

8. In PSpice, select View > Measurement Results and in the Measurement Results window, check on the box for Max(V(A)-V(B)) that will display a value of 73.25649 V, see Fig. 27.37.

9. Go back to Capture and select, PSpice > Advanced Analysis > Smoke.

10. PSpice Advanced Analysis will open up showing the results of the Smoke Analysis for the voltage_tripler circuit. Invalid values will be displayed as gray bar lines. For example, only average and peak values are calculated for TJL. Right mouse click anywhere and select Hide Invalid Values Fig. 27.38.

11. Right mouse click anywhere again and select Component Filter and enter C⁎ as shown in Fig. 27.39.

12. Fig. 27.40 shows the results of the Smoke Analysis for the voltage_tripler circuit.

The capacitor ripple current is given by PDML (rms) for capacitor C1 and is shown as 43.5728 μA. Therefore from Eq. (27.20) the approximate value for Pdis is

$\text{Pdis}={\left(43.5728\times {10}^{-6}\times 0.159\right)}^2=0.302\mathrm{nW}$

This small power dissipation is attributed to the small ESR value and is not going to change the capacitor temperature significantly. What is of more interest is the voltage rating for the capacitors. The yellow bar lines indicate that the voltage ratings are within 90% of their maximum ratings.

13. Right mouse click anywhere and select Derating > Standard Derating and run the Smoke analysis simulation.

14. You should see the results as shown in Fig. 27.41 where the red bars indicate that the derated MOCs have been exceeded.

15. Capacitors with a higher working voltage of 63 V can be used instead. Globally change the value of CMAX from 50 to 63 V and run the PSpice simulation and then the Smoke analysis simulation. You should see that the derated MOCs for the capacitors have not been exceeded.