How to Derate Inductors based on Temperature, and load.
| Current Derating Factor @ Application Ambient Temperature | |||||
| Military | Commercial | ||||
| Load Type | 0oC to 85oC | Above 85oC | 0oC to 85oC | Above 85oC | |
| Resistive | 75% of rated Resistive load |
60% of rated Resistive load |
75% of rated Resistive load |
60% of rated Resistive load |
|
| Inductive / Motor |
75% of rated Inductive load |
60% of rated Inductive load |
40% of rated Resistive load |
30% of rated Resistive load |
|
| Capacitive / Lamp |
75% of rated Capacitive load |
60% of rated Capacitive load |
25% of rated Resistive load |
20% of rated Resistive load |
|
The Derating Factor shown above is a general engineering approach to safe guarding a design.
Normally the derating factor is the safest approach, much tighter than allowing for the 10% rule.
Inductors are also derated by reducing the maximum operating temperature
based on the insulation class and reducing the operating voltage. Of course the package style also has a lot to do with derating requirements.
Through-hole and surface mount devices have very different derating requirements. Surface Mount devices are close to the printed wiring board and may not feel the effect of any forced air applied to the system, as the air may be blocked by components that may have a higher package type which will block any air flow.
Leaded Inductors are larger and have more surface area to dissipate heat while also being above the PWB to catch air flow.
The inductor derating table above should be taken as a general guideline and not a design rule. Check the devices data sheet to determine any required temperature derating.
Basically these design recommendations are saying one of two things, either reduce the load requirements on the part or get a bigger part that can handle the current expected in the circuit with added head-room.
Example of Load types;
Inductive loads = relays, solenoids and motors
Resistive loads = resistors, incandescent lights, heaters
