A Derating file is a table of maximum stress values allowed for each part type. Derating files are used to set an upper limit for electrical and thermal stresses on each part type. By keeping devices operating at or below the maximum derated level, reliability can be improved. When Relex performs a
calculation, it verifies that a part is not over the derating limits that you specify. On reports, you may choose to list only the parts that are over your derating limits, and those over Relex’s overstress limits.
Relex provides a built-in system for derating. The built-in derating feature functions as follows:
If you do not specify a Derating file on the Prediction/General Files tab of File Properties, Relex uses its built-in derating and overstress checks. If a particular part has a stress ratio over 100%, or the temperature is over the rated temperature for the part, Relex indicates that the part is overstressed.
If you do specify a Derating file on the Prediction/General Files tab of File Properties, Relex uses its built-in derating and overstress checks for all parts that are not specified in that Derating file. In this case, if a particular part has a stress ratio over 100%, or if the temperature is over the rated temperature for the part, Relex indicates that the part is overstressed.
this case, even if a part has specified derating levels assigned in the Derating file, Relex still indicates the part is being overstressed if it has a stress ratio over 100%, or if the temperature is over the rated temperature for the part.
In addition to built in derating limits, Relex also includes additional Derating files that have been created for your convenience. Any one of these files may be enabled for a Project file. The supplied Derating files are based on a number of derating standards. A list of the supplied Derating files is included in the Docs directory on your Relex CD.
Figure 7 - . Sample Derating File There are four easy steps to enabling derating analysis:
1. Create a Derating file.
2. Enable the Derating file in the File Properties.
3. Assign additional derating data as required.
4. Perform a calculation or generate a report.
Creating a Derating File
1. Select File>New>Prediction>Derating from the main menu.
2. In the File name field enter the name of the new Derating file and press <Create>.
3. Upon creating the new file, a File Information window appears. This window allows you to specify descriptive information about the file that you are creating. In addition to the general descriptive information, you are asked to select a Calculation Model on the Derating File Information tab. This refers to the reliability prediction model to be used for the Derating file. Your choices are MIL-HDBK-217 or Telcordia (Bellcore). Once this model is selected, it cannot be changed, so make your decision carefully. Enter the information, and press <OK> when complete.
4. The Derating file window appears. The Derating Definition table is designed for the entry of all part types that are to be included in the Derating file. To be absolutely complete, an entry is to be included for every type of part available. However, for the part types that do not have an entry, the Relex built-in derating values are used. The table consists of the following fields of information:
the preset choice list.
Subcategory The subcategory of the part being entered into the Derating file. Select a
subcategory from the preset choice list. The list of available subcategories is based on the category that was selected in the Category field. One option on the choice list is labeled All. This option refers to all subcategories available for the selected category. Note that you may select several specific subcategories for any category, and then select a subcategory of All. For all subcategories that are specifically added, derating is based on the specified parameters. All applies to all subcategories that have not been specifically defined.
Type The type of part being entered into the Derating file. Select a type from the preset choice list. The list of available types is based on the category and subcategories that were selected in the previous fields. One option on the choice list is labeled All.
This option refers to all types available for the selected category and subcategory.
Note that you may select several specific types for any subcategory, and then select a type of All. For all types that were specifically added, derating is based on the specified parameters. All applies to all types that have not been specifically defined.
Model The model of the part being entered into the Derating file if it is different than the model of the Derating file itself. This option provides the ability to enter parts for both MIL-HDBK-217 and Telcordia (Bellcore) in the same Derating file. The model selected in this field affects the list of available parameters in the Parameter field.
Parameter The parameter of the part being entered into the Derating file. Select a parameter from the preset choice list. The list of available parameters is based on the entries in all previous fields. In a Derating file, a part of a specific
category/subcategory/type/model/parameter combination may only be entered once.
Duplicates of the same combination are not accepted.
Remarks General remarks regarding the entry.
5. After entering the information in the Derating Definition window, complete the information requested in the Derating Data window. The information requested varies depending upon the category and subcategory. The data requested may include the following:
Option Description
None No derating is specified for the selected minimum or maximum rating.
Constant Derated Value A constant derated value is entered. For instance, for temperature derating, if a value of 80 degrees is entered, the constant derated value for that part is 80 degrees. A Nominal and Worst Case value may be
specified. When a Project file is created, and the Derating file is enabled, you may select whether the nominal or worst case values are to be used for derating analysis.
Increment Minimum
Rating An increment value is entered. This Increment value is added to the specified minimum value for the part (this value is specified on the Prediction Data tab for the part during part entry). A Nominal and Worst Case value may be specified. When a Project file is created, and the Derating file is enabled, you may select whether the nominal or worst case values are to be used for derating analysis.
Option Description
Case value may be specified. When a Project file is created, and the Derating file is enabled, you may select whether the nominal or worst case values are to be used for derating analysis.
Standard Derating
Curve A Standard Derating Curve is based on three main points: the stress ratio (a value between 0 and 1), the knee temperature (the temperature at which derating begins), and the maximum temperature (the temperature at which the derated stress ratio is 0). The Standard Derating Curve allows you to specify these three parameters for the curve. Derating is then based on the curve that is produced. Nominal and Worst Case values may be specified for each of the three parameters. When a Project file is created, and the Derating file is enabled, you may select whether the nominal or worst case values are to be used for derating analysis.
For some parameters, the Standard Derating Curve option has an additional “Calculate Maximum Derated Temperature per AS-4613 Semiconductor Derating Curve” checkbox. When enabled, the maximum temperature for the curve is calculated based on the stress ratio, knee temperature, and maximum temperature specified for the part according to the AS-4613 derating standard. If this checkbox is disabled, the derating curve is based on the entered values.
If the derating curve is calculated per AS-4613, you are to enter the maximum temperature rating of the device. The maximum temperature for the nominal and worst case conditions are also to be the same. Relex then calculates the maximum derated temperature dependent upon the stress ratio at 0 degrees (Celsius), the knee temperature, and the maximum temperature.
As values are added for the Standard Derating Curve, a sample curve is displayed to the right of the fields of data. This curve is based on the entered information. As the entries change, the diagram changes.
User-defined Derating
Curve If your derating curve contains more than 3 points, then you are to select User-defined Derating Curve. Stress and temperature values may be entered for up to 10 points to define the derating curve. Enter a stress ratio between 0 and 1, and a temperature (in degrees Celsius) for each point on your curve. A curve may be plotted for Nominal and Worst Case. When a Project file is created, and the Derating file is enabled, you may select whether the Nominal or Worst Case values are to be used for the Project file.
As values are added for the User-defined Derating Curve, a sample curve is displayed to the right of the fields of data. This curve is based on the entered information in the tables. As the entries change, the diagram changes.
6. Once all information has been entered into the Derating file, save and close the Derating file.
Using a Derating File
Enabling the Derating File in the File Properties
On the Prediction/General Files tab, enter the desired Derating file. To select a Derating file, press
<Browse> to display a list of Derating files. Choose your new file by selecting it with your cursor and pressing <Open>.
Once a Derating file has been selected, you are to select the option for Derating Calculations, which is located under the Derating field. You may select from the derating calculation options of Nominal or Worst Case. If nominal is selected, the nominal values in the Derating file are used. If worst case is selected, the worst case values in the Derating file are used. Save the changes to File Properties by pressing <OK>.
Assigning Additional Derating Data as Required
When a Derating file is enabled in a Project file, additional parameters of information may be included on the Prediction Data tab for each part. These additional fields are displayed on the right side of the Prediction Data tab in a box marked Derating Parameters. These additional parameters, used for derating purposes only, may include:
Field Description
Minimum Temperature Rating
Temperature (in degrees Celsius) below which the device should not be operated. This value is used for parts that use a minimum constant derated value or an increment minimum rating for derating. The increment value specified in the Derating file is added to the value entered in this field to determine the derating limit for the part.
Knee Temperature Temperature (in degrees Celsius) at which derating begins. If a value is entered in this field, it is used to override the knee temperature specified for the standard derating curve in the Derating file.
Maximum Temperature Rating
Temperature (in degrees Celsius) above which the device should not be operated. This value is used for parts that use a maximum constant derated value or a decrement maximum rating for derating. The decrement value specified in the Derating file is subtracted from the value entered in this field to determine the derating limit for the part.
Power Rating Rated power of the device in watts. It is used in derating calculations to verify that the device is being operated at or below the derating requirements.
Note: The fields outlined above can be used to specify a derating curve for each part type that has not been specified in that Derating file. To specify the derating curve, first enter an operating power for the part (or an operating voltage for capacitors) on the Prediction Data tab for the part. Then assign the information in the above fields for the part. Relex then uses this information to perform a derating calculation for the part based on a standard derating curve.
Performing a Calculation and/or Generating a Report
After you have created a Derating file and enabled it in the File Properties, Relex automatically uses your derating information to perform a derating analysis. When you perform a calculation or generate a report, Relex compares all data regarding the part to the information stored in the active Derating file.
After calculating, the failure rate value of all overstressed parts is displayed in red on the Parts Table.
You may also view Pi Factors on the part Prediction Data tab. If the part is overstressed, a checkmark appears on the window to the right of the calculated failure rate. You may also generate reports that include only the parts that are overstressed.