Example 4 - Performing Derating Analysis of a System
Background
Most equipment failures are precipitated by stress. When the applied stress exceeds the inherent strength of the part, either a serious degradation or a failure will occur. To assure reliability, equipment must be designed to endure stress over time without failure. In addition, design stress parameters must be identified and controlled while parts and materials that can withstand these stresses must be selected. Derating standards are used to help you select and use parts and materials so that the applied stress is less than rated for a specific application.
For this example, assume that you wish to analyze the effect of ambient temperature on a computer system created using the MIL-HDBK-217F standard.
[Download Lambda Predict 3 Example File (*.lp3)] Important: The sample data set is not intended to be realistic.
Analysis
Step 1: After creating the system, Lambda Predict looks like this.
The computer system as set up in Lambda Predict.
When a derating standard is applied to a system, each component in the system is automatically evaluated to determine whether it meets the derating requirements defined for its component type in the standard. Any components of types not specified in the derating standard are not considered in the derating.
Step 2: Choose Tools > Apply Derating Standard. The Select Derating Standard window displays.
For your system, you decide to use the NAVSEA-TE000-AB-GTP-010 standard.
Step 3: Select the NAVSEA standard and then click OK. Notice that the icons in the System Hierarchy panel have changed to show the derating status of each component, as shown next.
The computer system with the NAVSEA derating standard applied.
The status of each component with regard to derating is indicated in the system hierarchy via the component icon:
A green icon
indicates that the component meets the derating requirements defined in the standard
(i.e. the stress level and temperature fall within the nominal case curve).
A light blue
icon indicates that the component does not meet the derating requirements, but it is not
overstressed (i.e. the stress level and temperature fall outside the nominal case curve, but
within the worst case curve).
A red icon
indicates that the component is overstressed according to the derating requirements
(i.e. the stress level and temperature fall outside the worst case curve or, in the case
of a component with only a nominal case curve defined, outside the nominal case curve).
A dark blue icon is
the standard component icon; when it appears within a system that has a derating standard
applied, it indicates that the component is not included in the derating standard and
derating has therefore not been applied to the component.
Notice that the CAP-NP-0603 capacitor displays in light blue, indicating that the component, while not overstressed now, is close to the stress limit.
Step 4: Select the CAP-NP-0603 capacitor and then choose Tools > View Component Derating. The Component Derating window displays, as shown next.
The initial derating analysis of the CAP-NP-0603 capacitor.
The blue point on the plot shows where the capacitor stands with respect to the requirements of the derating standard. The red line represents the worst case points and the green line represents the nominal points. Note that the capacitor currently falls between the nominal and worst case lines. In other words, it does not meet the derating requirements, but is not actually overstressed. You can see from the shape of the plot that altering the ambient temperature will not address this problem. Instead, the voltage stress must be reduced.
Step 5: Position the cursor at the intersection of the green line and the Y-axis. The location of the cursor, (0, 0.4000), will appear as a tool tip. This gives an idea of the appropriate voltage stress for this component in the current environment. Close the window.
Step 6: In the System Hierarchy panel, with the CAP-NP-0603 capacitor still selected, change the Voltage Stress field to 0.39 in the Properties panel. Press ENTER or click outside the field to accept your change. Notice that in the System Hierarchy panel, the CAP-NP-0603 capacitor component’s icon has changed to green with two straight lines, indicating that the component now meets the derating requirements.
Step 7: Select the CAP-NP-0603 capacitor and then choose Tools > View Component Derating. Note the difference in the Component Derating window as compared to the earlier version.
The revised derating analysis of the CAP-NP-0603 capacitor.
